Porphyrin photochemistry in inorganicorganic hybrid materials

Journal of Photochemistry and Photobiology C:Photochemistry Reviews

7(2006)104–126

Review

Porphyrin photochemistry in inorganic/organic hybrid materials:Clays, layered semiconductors,nanotubes,and mesoporous materials Shinsuke Takagi a,?,Miharu Eguchi a,b,Donald A.Tryk a,Haruo Inoue a,c

a Department of Applied Chemistry,Faculty of Urban Environmental Sciences,Tokyo Metropolitan University,

Minami-ohsawa1-1,Hachiohji,Tokyo192-0397,Japan

b Japan Society for the Promotion of Science for Young Scientist,Japan

c SORST,JST(Japan Science an

d Technology),Japan

Received24February2006;received in revised form31March2006;accepted10April2006

Available online23October2006

Abstract

Porphyrin derivatives are known as useful functional dyes.Porphyrin derivatives exhibit various properties in complexes with inorganic host materials that are much different from those in homogeneous solutions.In this paper,the structure and photochemical properties of porphyrins in inorganic host materials such as clays,layered semiconductors,nanotubes,and mesoporous materials are described.The photochemical properties, including the absorption properties and excited lifetimes,are much affected by the complex formation with inorganic materials.Aggregation phenomena,structural perturbations,and selected chemical reactions such as metalation and protonation affect the photochemical properties of porphyrins accommodated in inorganic host materials.The combination of porphyrin derivatives and inorganic materials should be promising for the construction of novel hybrid materials.Inorganic materials can act as novel environments for photochemical reactions.The utilization of inorganic materials for photochemical reactions is also described.

?2006Elsevier B.V.All rights reserved.

Keywords:Porphyrin;Clay minerals;Layered materials;Semiconductor;Nanotube;Mesoporous;Photochemistry;Electron transfer;Energy transfer;Aggregation

Contents

1.Introduction (105)

2.Porphyrin–clay complexes (105)

2.1.Anionic clays (106)

2.1.1.Structures and photochemical properties of porphyrin–anionic clay complexes (106)

2.1.2.Photochemical reactions in porphyrin–anionic clay complexes (110)

2.2.Cationic clays (111)

2.2.1.Structure and photochemical properties of porphyrin–cationic clay complexes (111)

2.2.2.Photochemical reactions in porphyrin–cationic clay complexes (114)

3.Porphyrin-layered metal oxide semiconductor complexes (115)

3.1.Structural and photochemical properties of porphyrin-layered metal oxide semiconductor complexes (115)

3.2.Photochemical reactions in porphyrin-layered metal oxide semiconductor complexes (117)

4.Porphyrin–nanotube complexes (118)

5.Other porphyrin–inorganic host material complexes (120)

6.Concluding remarks (122)

Acknowledgements (122)

Appendix A (122)

References (124)

?Corresponding author.

E-mail addresses:takagi-shinsuke@c.metro-u.ac.jp(S.Takagi),inoue-haruo@c.metro-u.ac.jp(H.Inoue).

1389-5567/$20.00?2006Elsevier B.V.All rights reserved.

doi:10.1016/j.jphotochemrev.2006.04.002

S.Takagi et al./Journal of Photochemistry and Photobiology C:Photochemistry Reviews7(2006)104–126

105

Shinsuke Takagi received his BS and MS degrees in

applied chemistry from Tokyo Metropolitan University

in1991and1993.After enrolling in the doctoral pro-

gram,he joined the research staff of the Department of

Industrial Chemistry,Tokyo Metropolitan University,

in1995.He was invited as a visiting scientist in Profes-

sor V.Ramamurthy’s group(Tulane University,USA)

from1999to2000under the auspices of the US-Japan

Bilateral Program.He received his PhD degree from

Tokyo Metropolitan University under the supervision of

Professor Haruo Inoue.He is currently a Associate Pro-fessor of the Department of Applied Chemistry,Tokyo Metropolitan University. He received the Excellent Lecture Award for Young Scientists from The Chem-ical Society of Japan in2002,the Excellent Lecture Award for Young Scientists from The Clay Science Society of Japan in2003,and an international prize,the APA Prize for Young Scientists from The Asian and Oceanian Photochemistry Association,in2004.His research was selected for a“Frontier in Chemistry”for 2005in the Chemical Society of Japan membership journal Kagaku-to Kogyo (Chemistry and Industry).His research interests include photochemistry of porphyrins,photochemical reactions in chemical reaction micro-environments

provided by micelles,reversed micelles,vesicles,zeolites,and clay

minerals.

Miharu Eguchi received her BS and MS in applied

chemistry from Tokyo Metropolitan University in2002

and2004.She was a research fellow of the JSPS(DC1)

from2004to2006.She received her PhD degree from

Tokyo Metropolitan University under the supervision

of Professor Haruo Inoue in2006.She is currently a

research fellow of the JSPS in Professor Thomas E.

Mallouk’s group(Pennsylvania State University).She

received the Excellent Lecture Award from The Chem-

ical Society of Japan in2005and an international prize,

the Poster Prize at the International Conference on Pho-tochemistry XXII in2005.Her research interests include controlling molecules

by using the structure and photochemical properties of clay–dye

complexes.

Donald A.Tryk graduated from the University of

Florida in1969with bachelor of science in chemistry.

He worked as an environmental chemist for the State of

New Mexico before returning to graduate school at the

University of New Mexico,graduating in1980with a

doctorate in chemistry.From there,he went on to Case

Western Reserve University,working in the Chemistry

Department as a senior research associate,principally

with the late Professor Ernest Yeager.In1995,he moved

to the University of Tokyo,working?rst as a research

associate and then as a special associate professor in the group of Professor Akira Fujishima until2001,when he accepted a position as a research associate at Tokyo Metropolitan University in the group of Professor Haruo Inoue.In2003,he took a position as visiting professor in the Chemistry Department at the University of Puerto Rico,working with Professor Carlos Cabrera and returned to Tokyo Metropolitan University in2005,where he is a visiting professor in the Department of Applied Chemistry.His research interests include analytical electrochemistry,electrocatalysis,photoelectrochemistry,and photocatalysis.He is particularly interested in the use of diamond as an electrode material,as well as the development of biomimetic electrocatalysts for redox

reactions such as those involving dioxygen,dihydrogen,and carbon

dioxide.

Haruo Inoue was born in1947in Japan and graduated from the University of Tokyo in1969.After?nish-ing his doctoral program at the University of Tokyo, he joined the faculty of the Department of Applied Chemistry at Tokyo Metropolitan University in1972. He received the Japanese Photochemistry Association Award in1997.Currently,he is a full professor of applied chemistry at Tokyo Metropolitan University and serves as a Dean of the Faculty of Urban Envi-ronmental Sciences.He also serves as a Vice President of the Chemical Society of Japan(2004–2006)and as President of the Japanese Photochemistry Association(2006–2007).He has been on the editorial boards of the Journal of Photochemistry and Photobiology A: Chemistry,the Journal of Photochemistry and Photobiology C:Photochemistry Reviews,and Research on Chemical Intermediates.His major research interests include photochemistry,energy coupling among chemical reactions,selective energy?ow in solution,anisotropic control of chemical reactions in the excited state,nano-layered compounds,metal complexes,and arti?cial photosynthesis. He is a project leader of Core Research on Evolutional Science and Technol-ogy(CREST),under the auspices of Japan Science and Technology(JST),on the research subject“Construction of Arti?cial Photosynthesis with Water as an Electron Source”.

1.Introduction

Porphyrin derivatives play highly important roles in vari-ous?elds of science,including chemistry,physics,geology, and biology.Researchers around the world have studied por-phyrins intensively from diverse viewpoints[1–4].Their unique properties have attracted great interest.For example,porphyrins generally have intense?–?*absorption bands in the visible region.They exhibit a wide variety of redox properties.Fur-thermore,since it is possible to control the photochemical and electrochemical properties by modi?cation of the substituents and selection of the central metal,the porphyrin molecule can very likely expand its role in biological,chemical,and physical research.

Intrinsically,porphyrins are biogenic compounds and are responsible for various biological processes.In many biological systems,the involvement of a porphyrin is crucial for its func-tionality.For example,porphyrins are beautifully arranged in a circular assembly in the photosynthetic light-harvesting system. The combination of porphyrins and appropriate host materials produces the regulated structure and excellent functionality in biological systems.In enzymes,the structures of the proteins surrounding the porphyrin derivatives are crucial for their func-tions.One important role of the chemist could be to try to mimic the functionality of excellent biological systems,such as enzy-matic and photosynthetic systems.

The research on host–guest chemistry involving porphyrins promises to open a window onto a new?eld of chemistry. Recently,organic/inorganic hybrids containing porphyrins have been the subject of intensive investigations to explore their novel properties and functionalities.In the present review,porphyrin complexes with inorganic host materials are focused on from the viewpoint of photochemistry.The structures and photochemical properties of porphyrin complexes with inorganic host materi-als,especially for layered materials,are described in the present paper.Two-dimensional interlayer spaces of?exible height have an advantage for incorporating large guest molecules,which cannot be incorporated into rigid cage-like materials with small window size.Several reviews describing porphyrin–inorganic materials hybrids are available[5–14];here,the photochemical aspects of the porphyrin in the inorganic materials are focused upon.

2.Porphyrin–clay complexes

Clay minerals are well known as multi-layered inorganic materials that provide quasi-two-dimensional spaces,which,

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104–126

Fig.1.(a)The unit structure of saponite clay,(b)AFM image of clay particles,and (c)AFM image of the surface of clay particle [17].Reprinted with permission.

because of their well-de?ned dimensions,should be interesting from the viewpoint of micro-environments for chemical reac-tions [5–14].The surface of the clay sheet is very ?at,even at an atomic level.Thus,the clay will act as an ideal host material to construct regulated structures of guest molecules.Intercala-tion of organic molecules into the clay layer structure can afford very interesting inorganic/organic hybrid compounds.Because of the photochemical interest,complexes between clay minerals and dyes are expected to provide new functions and versatility.By means of the isomorphous-replacement of elements in the layer,the clay surface possesses positive or negative charge.In natural clays,the charge density is widely variable.Thus,vari-ous ionic molecules can be reversibly inserted into the interlayer space through general ion-exchange phenomena.Depending on the purpose,it is possible to choose the best clay minerals based on the exchangeable charge and structure.Furthermore,it is possible to use both natural and chemically synthesized clay minerals.In the case of synthetic clay minerals,the composition is controllable and can be well characterized.Recently,synthetic clay minerals have attracted increasing interest,especially for their applications in photochemical reactions.The characteristic point of clay minerals is the ?exibility of the interlayer distance between clay sheets.The interlayer space has a swelling ability with solvents such as water.Thus,it is possible to incorporate many different types of guest molecules,including relatively large molecules such as porphyrins and phthalocyanines.The clay minerals are classi?ed by the structure and exchangeable charge capacity [10].Basically,they are divided into two types,anionic and cationic clays.2.1.Anionic clays

Though there are many kinds of anionic clays [5,8–14],smectite group clays are frequently used.Smectite clays possess layers consisting of a 2:1pair of octahedral and tetrahedral sheets.The layer thickness is typically 0.96nm.There are two types of clays,synthetic and natural.Since natural clays sometimes contain iron in the structure,they have a color and can quench the excited state of a guest molecule.The synthetic clays,then,should be appropriate for photochemical research.The clay sheets form large secondary particles in the powder state.Under appropriate conditions,the stacked clay sheets can swell or even exfoliate perfectly in the solution.Though it depends on the dispersion degree and the particle size,solutions containing clay sheets can be quite transparent.The

structure of saponite,which is one of the typical synthetic smec-tites,is shown in Fig.1(a).The chemical formula is expressed as [(Si 7.20Al 0.80)(Mg 5.97Al 0.03)O 20(OH)4]?0.77(Na 0.49Mg 0.14)+0.77.The isomorphous substitution of Si by Al in the tetrahedral layer produces anionic charge in the structure.The unit structure can be extended widely in two dimensions.In this case,the average inter-charge distance on the clay surface is 1.2nm in the hexagonal array.Since the charge densities of smectite clays are not as high as those in the mica group,it is relatively easy to swell and exfoliate them in aqueous solution [15,16].An atomic force microscopic (AFM)image of clay particles and a highly magni?ed view of the surface are shown in Fig.1(b and c)[17].In this case,the particle size is 20–50nm.As can be seen in Fig.1(c),the surface of the clay particle is very ?at,even at the atomic level over a wide area.This character is suitable for constructing regulated structures of guest molecules on the clay surface.

2.1.1.Structures and photochemical properties of porphyrin–anionic clay complexes

The research on the formation of complexes with por-phyrins and clays has been reported since the 1970s by geologists and chemists [18–22].Neutral porphyrins such as tetraphenylporphyrin (TPP)and tetra(4-pyridyl)porphyrin (TPyP)were used in the early stages of these studies.Since typical smectite clays possess anionic charges in the struc-ture,cationic guest molecules can form stable complexes by means of electrostatic interactions.Thus,porphyrin deriva-tives having cationic moieties such as pyridinium or anilin-ium groups can form stable complexes with clays.Recently,cationic porphyrins such as tetrakis(1-methyl-pyridinium-4-yl)porphyrin (TMPyP)and tetrakis(N ,N ,N -trimethyl-anilinium-4-yl)porphyrin (TMAP),which form stable complexes,have been used frequently.It has been reported that the photochemical properties,e.g.,the absorption characteristics,of porphyrins are much affected by complex formation with clays.The absorp-tion spectra of H 2TMPyP and ZnTMPyP without and with clay (synthetic saponite)in aqueous solution are shown in Fig.2[23].Depending on the sample preparation procedure,two types of complexes,exfoliated (b)and intercalated (c)can form.In the exfoliated complex,clay sheets are exfoliated,and the porphyrin molecules adsorb on the clay surface.In the intercalated com-plex,clay sheets are stacked,and porphyrin molecules are inter-calated between the clay sheets.Upon complex formation with the clay,the porphyrin molecule exhibits a relatively large blue

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Fig.2.Absorption spectra of cationic porphyrins with clay and without clay in the Soret band region in aqueous solution(H2TMPyP-a(H2TMPyP without clay),H2TMPyP-b(H2TMPyP exfoliated complex),H2TMPyP-c(H2TMPyP intercalated complex),ZnTMPyP-a(ZnTMPyP without clay),ZnTMPyP-b (ZnTMPyP exfoliated complex),and ZnTMPyP-c(ZnTMPyP intercalated com-plex)).The clay used was a synthetic saponite[23].Reprinted with permission. shift in the absorption spectrum.Blue shifts of approximately30 and60nm were observed for exfoliated and intercalated com-plexes,respectively.The free-base and Zn porphyrins exhibit similar spectral shifts.The spectral pattern in the Q-band region is not consistent with protonation of the porphyrin;this indicates that protonation is not the reason for the spectral shift.Cast ?lms,in which porphyrin molecules are intercalated,exhibit absorption spectra similar to those for intercalated complexes in water.According to X-ray diffraction(XRD)measurements, porphyrin molecules should adsorb on the clay surface with par-allel orientation with respect to the clay surface.When TMAP, in which the cationic moiety is bulky,was used as the cationic porphyrin,the spectral shifts were much smaller than those in TMPyP[23,24].A number of researchers explain these spec-tral shifts upon complex formation with clay as being,more or less,due to the structural perturbation of the porphyrin on the clay surface[23–28].Speci?cally,enhanced?-conjugation and the electron-withdrawing effect of the pyridinium group,due to a?attening of the TMPyP on the clay,induce the spectral change.By?attening,one means that the four cationic tetram-ethylpyridinium moieties become parallel to the porphyrin ring. However,the possibility of ring distortion of the porphyrin in the clay complex was suggested by Raman spectroscopy[29]. Ring distortion could induce the red shift[30–34].A red shift of MnTMPyP bound to montmorillonite was interpreted as the result of a?-interaction between the complex aromatic ring and the oxygen-atom planes of the aluminosilicates[35].The rea-sons for the drastic spectral change of porphyrin in the clay complex remain somewhat under discussion.In any case,the adsorption structure and orientation pattern of the porphyrin on the clay surface should be crucial for the photochemical

prop-Fig.3.Arrangement of CoTMPyP in the interlayer space of(a)hectorite and (b)?uorohectorite and of(c)CoPcTs(Co phthalocyanine tetrasulfonate)in the interlayer space of layered double hydroxide[37].Reprinted with permission. erties of the porphyrin,although the electronic interactions with the clay sheet and the conventional polar effect might also be considered to some extent[25,29].

It is known that the properties of host materials and guest molecules affect the adsorption structure of porphyrins in the interlayer space.The adsorption structure of the porphyrin molecule on the clay surface or in the interlayer space was investigated by X-ray diffraction[23,36–41],electron spin res-onance(ESR)[38,39],dichroic absorption measurements[42], and dichroic absorption measurements on a waveguide[43]. Generally,the orientation angle of the porphyrin with respect to the clay surface tends to increase as the charge density of the clay increases[37–41].When hectorite clay(charge exchangeable capacity(CEC)=0.7meq g?1)or saponite clay (CEC=0.997meq g?1)were used,the orientation of the por-phyrin was parallel to the clay surface.When?uorohectorite (CEC=1.9meq g?1(0.27nm2per charge[39])was adopted, the orientation angle of the porphyrin derivative was estimated to be27?–35?[37–39]with respect to the clay surface,as shown in Fig.3.In?uorohectorite clay,the aggregation of porphyrin was presumed,according to the absorption spectrum[28].In the case of a layered double hydroxide(LDH,see Section2.2, on cationic clays),which has a much higher ion exchangeable capacity,the orientation of the porphyrin derivative is estimated to be perpendicular.ESR experiments indicate that the orienta-tion can also change in response to atmospheric changes such as humidity[38].

The structure of the porphyrin affects the adsorption struc-ture of the porphyrin in the clay complex.In the combination

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of TMPyP and synthetic saponite,TMPyP adsorbs on the clay surface with parallel orientation with respect to the clay surface. Meso-tetrakis(5-trimethylammoniopentyl)porphyrin,which has a?exible alkyl chain in the cationic portion,was used as the cationic porphyrin[36].It appears that this porphyrin adsorbs on the clay with a non-parallel orientation with respect to the surface.Since this porphyrin exhibits a blue shift upon complex formation with the clay,the formation of aggregates is presumed.

Other chemical reactions occurring in the interlayer space have also been examined.The protonation of porphyrins was observed in speci?c clays,especially in natural clays[28,29]. It has been reported that the protonated porphyrin can be inter-calated with a parallel orientation[22].The metallation of the free-base porphyrin occurred in speci?c clay minerals[44].In the case of Sn porphyrins,reversible metallation and demetalla-tion was observed in the clay complex[45].

Spectral shifts have also been observed in?uorescence spec-tra as a result of the complex formation with clay.The?uo-rescence lifetimes of porphyrins have been observed in clay complexes.The same order of lifetime was observed for syn-thetic saponite–porphyrin complexes compared to porphyrin itself in aqueous solution[23].In the case of methylviologen (MV2+),drastic enhancement of?uorescence intensity by the complex formation with clay was reported[46,47].The non-radiative deactivation was suppressed on the clay surface.Thus, the complex formation of dyes with clay minerals could be very interesting from the viewpoint of?nding novel properties of dyes and controlling the photochemical properties.The inter-layer spaces include exchangeable metal ions,neutralizing the net negative charges that are generated in the clay structure.The photochemical properties of the adsorbed dye can be controlled by utilizing the effect of exchangeable cations on the clay surface [48].The emission spectrum of naphthalene was observed in a clay complex in which the exchangeable cations were Li+,Rb+, Cs+,or Tl+.When the exchangeable cation was a light atom,?u-orescence was emitted from naphthalene.As the atomic number of the exchangeable cation increased,the?uorescence decreased and the phosphorescence increased.These observations can be explained by the heavy atom effect of the exchangeable cation on the adjacent naphthalene in the clay complex.Although sim-ilar effects have previously been observed in zeolites[49],the latter cannot incorporate large molecules.Because the interlayer space provided by the clay sheet is of?exible height,the utiliza-tion of clays as controllers for the photochemical properties of large molecules is attractive.In the case of natural clays,the excited state of the adsorbed dye is sometimes quenched by the iron contained in the clay structure[50].

Regarding the adsorption structure of the porphyrin in the clay complex,an interesting phenomenon was reported.Gen-erally,organic molecules tend to aggregate easily on inorganic surfaces.The absorption spectrum is then affected by dipole transition moment interaction in the aggregates[51].The?u-orescence also tends to be affected by self-quenching in the aggregate[52–57].In the speci?c combination of cationic por-phyrins and synthetic clays,the porphyrin molecules do not aggregate on the clay surface up to100%adsorption versus the cation exchange capacity(CEC)of the clay[23,24,26].

The Fig.4.Proposed structure of the cationic porphyrin–synthetic clay complex [23].Reprinted with permission.

average intermolecular distance was estimated to be2.4nm,as shown in Fig.4,when porphyrin molecules adsorb on the clay surface at100%adsorption versus CEC.The formation of these unique hybrids was rationalized by a size-matching of distances between the charged sites in the porphyrin molecule and those on the clay surface[23,24,26].Thus,this effect has been referred to as the“size-matching rule”.The?uorescence lifetimes can be analyzed in terms of a single time constant:those of H2TMAP (8.4×10?6M(6.7%versus CEC))were4.1ns for the exfoliated complex and3.2ns for the intercalated complex,respectively. Since the porphyrin retains a relatively long excited lifetime,it can undergo intermolecular photochemical reactions.The char-acteristic point of this clay–porphyrin complex is the utilization of the host–guest relationship to construct a regulated struc-ture,in contrast to the so-called self-organization phenomenon.

A beautiful porphyrin arrangement on the gold surface(111) has been reported[58–65].In these complexes,the guest–guest interactions,for example,van der Waals interactions or hydro-gen bonding interactions,are crucial in determining the arrange-ment of porphyrin on the surface.In clay–porphyrin complexes, there is no interaction between guest molecules.Thus,it is possi-ble to control the porphyrin arrangement by changing the nature of the host clay materials.

Further structural control of the porphyrin–clay complex was examined.Speci?cally,the adsorption orientation angle control of the porphyrin on the clay sheet was reported[66].Cationic porphyrins(TMPyP,cis-DPyP,and trans-DPyP;Fig.5)on the nano-layered compound surface in water were found to be ori-ented with the plane of the rings parallel to the silicate layer. When the solvent was change to an organic solvent,the orienta-tion angle of cis-DPyP was no longer parallel with respect to the clay surface,as shown in Fig.6.Based on dichroic measurements on a waveguide system,the orientation angle of the porphyrin ring with respect to the clay surface was estimated to be approx-imately70?.The ease of orientational change decreased in the order cis-DPyP>trans-DPyP>TMPyP.When various organic solvents were examined,the solvents with low hydrogen bond-ing ability tended to induce changes in the orientation angle of the porphyrin.

Cationic surfactants can be intercalated into the clay inter-layer space easily[67–69].By utilizing the clay–surfactant hybrid,the intercalation of anionic porphyrins and neutral phthalocyanines was reported[42,70–73].In a cationic poly?uo-rinated surfactant–anionic Sb(V)porphyrin complex,the aggre-gation of porphyrin molecules was much enhanced,as shown in Fig.7[42].The absorption and emission measurements sug-

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109

Fig.5.Structures of TMPyP,cis -DPyP,and trans -DPyP [66]

.

Fig.6.Adsorption angle control of cis -DPyP on the clay surface [66].

gest that two types of dimers (J and H dimers)are formed in the poly?uorinated surfactant/clay hybrid interlayers.It was also found that when the amount of adsorbed surfactant decreased,i.e.,when the volume of the poly?uorinated micro-cavities in the interlayer space increased,then the dimerization of Sb(V)TSPP was enhanced.The utilization of micro-cavities formed in the clay–surfactant complex as chemical reaction micro-environments would be interesting.

The incorporation of more complicated porphyrins,such as antimony(V)porphyrin,which has two axial ligands,was exam-ined [74,75].Because antimony is a pentavalent element,the net charge of Sb(V)TPP(OR)2(R OH or CH 3)is +1.The anti-mony porphyrin is known to be an interesting sensitizer having strong oxidation power [76–83].The effect of axial ligands on the complex structure was examined.When these

ligands

Fig.7.Schematic depiction of the aggregation mechanisms of Sb(V)TSPP in poly?uorinated surfactant/clay hybrid compounds [42].Reprinted with permis-sion.

are hydroxide anions,the complex exhibits a regulated layered structure,according to the XRD measurement.In contrast,the regulated layer structure disappeared in the case of the methoxy-coordinated porphyrin,as shown in Fig.8[74].The hydroxide group should play an important role in determining the structure of the complex.For antimony porphyrin,with its cationic axial ligands (3-trimethylammoniopropoxo group),aggregation was effectively suppressed [75]

.

Fig.8.Schematic diagram of the layered structure (A)and amorphous structure (B)[74].Reprinted with permission.

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In summary,the photochemical properties of the porphyrin are much affected by the complex formation with clay.The adsorption structure of the porphyrin in the complex is closely tied to the photochemical properties.Spectral changes of the por-phyrin in the clay complex can be induced by (i)polar effects,(ii)redox reactions,(iii)protonation or metallation of core nitro-gen atoms,(iv)electronic interaction with the clay,(v)electronic changes due to structural changes such as ?attening or distortion,and (vi)interactions between porphyrins (aggregation).2.1.2.Photochemical reactions in porphyrin–anionic clay complexes

As described earlier,the speci?c porphyrin molecule in the complex exhibits a suf?ciently long excited lifetime to undergo intermolecular photochemical reactions.Energy transfer reactions between porphyrin molecules [84]and metal complexes [85]on the clay surface were reported.In the energy transfer experiment,H 2TMPyP was used as the energy acceptor,and ZnTMAP was used as the energy donor.According to the sample preparation procedure,two types of complexes were prepared:(i)aqueous solutions of H 2TMPyP,and ZnTMAP were mixed,and the solution obtained was mixed with aqueous clay solution [co-adsorption (CA)complex:(SSA +(H 2TMPyP +ZnTMAP))];(ii)each complex (H 2TMPyP–clay and ZnTMAP–clay)was prepared and then the complexes were mixed [independent adsorption (IA)com-plex:((SSA +H 2TMPyP)+(SSA +ZnTMAP))],as shown in Fig.9.By analyzing the concentration effect of porphyrin

on

Fig.9.Sample preparation methods for ?uorescence measurements of SSA–H 2TMPyP–ZnTMPyP complexes [84].Reprinted with

permission.

Fig.10.Schematic view of intra-sheet and inter-sheet energy transfer [84].Reprinted with permission.

the energy transfer ef?ciency,two modes of energy transfer are proposed,depending on the conditions,as shown in Fig.10.The ?rst is assigned to energy transfer between porphyrins adsorbed on the same clay sheet (intra-sheet energy transfer).The second is assigned to energy transfer between porphyrins adsorbed on adjacent clay sheets (inter-sheet energy transfer).The electron transfer reaction between adsorbed porphyrins and quenchers in bulk solution was reported [17,86].The electron transfer between Ru complexes and Fe 3+in the clay structure was also reported [50].The radius of the Perrin active quench-ing sphere was estimated to be 1.4±0.1nm.The utilization of clay–porphyrin complexes for photochemical hole burning [87–89]and modi?ed electrodes for oxygen sensors [90]were investigated.The amount of irreversible broadening of the hole under temperature cycling to about ?100?C was smaller in these compounds than in a typical neutral porphyrin–polymer sys-tem.The rigid ?xation of cationic porphyrin on the clay surface suppresses the broadening of the hole.Although not a photo-chemical reaction,the hydroxylation of alkanes is known to be catalyzed ef?ciently by cationic Mn(III)porphyrins on clay [91].Even though the photochemical reactions reported thus far for the interlayer space provided by clay minerals,such as photo-chemical hydroxylation or epoxidation,are few,this should be a unique chemical reaction micro-environment.

Energy transfer between porphyrins on the clay surface and subsequent electron transfer to molecules in the bulk solution were investigated [92].In the clay–porphyrin complexes,pho-tochemical energy transfer from excited singlet zinc porphyrins to free-base porphyrins proceeded ef?ciently upon excitation at 428nm,which is the absorption maximum (λmax )of the zinc por-phyrin.Despite the mostly selective excitation of the zinc por-phyrin the ?uorescence was mainly emitted from the free-base porphyrin,as shown in Fig.11([hydroquinone (HQ)]=0M).Then,the ?uorescence of H 2TMPyP was effectively quenched by the addition of HQ,as shown in Fig.11.In Fig.11,the emission around 620nm is due to ZnTMAP,and that around 680nm is due to H 2TMPyP.Thus,the photochemical electron transfer reaction from an electron donor in solution to an excited singlet porphyrin adsorbed on the clay surface was observed as shown in Fig.12.The electron transfer from free-base porphyrin,which is an energy acceptor,to hydroquinone in a bulk solution,was observed,with a large rate constant (1.6×1010M ?1s ?1).

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111

Fig.11.Fluorescence spectra of SSA–H2TMPyP–ZnTMAP complexes excited at428nm([SSA]=6.7mg L?1,[H2TMPyP]+[ZnTMAP]=15.4% ([H2TMPyP]=[ZnTMAP])vs.CEC of the clay).The concentration of hydro-quinone was0–0.41M.

Since the electron transfer rate from HQ to the excited singlet

free-base porphyrin(energy acceptor)is larger than that to the

excited singlet zinc porphyrin(energy donor),it was concluded

that the energy transfer accelerated total electron transfer reac-

tion.This series of photochemical reactions could be a promising

candidate for the construction of an arti?cial photosynthetic

system.

2.2.Cationic clays

A representative cationic-layered material is the layered dou-

ble hydroxide(LDH)[9].The general formula of LDH is

[M1?x2+M x3+(OH)2][A n?]x/n·y H2O,where M2+and M3+are divalent and trivalent metal cations,respectively,and A n?is

an exchangeable intercalated anion,such as Cl?,CO32?,or

NO3?[93].Octahedral sheets spread two-dimensionally,and

possess cationic charge in the structure as shown in Fig.13.

The layer thickness is0.48nm,which is thinner than that for

the2:1type anionic clay.Generally,the charge density of LDH

is higher than that for anionic clays such as saponite.The typ-

ical charge density is3.6meq g?1.Thus,LDH can

incorporate Fig.13.Schematic representation of the LDH structure[101].Reprinted with permission.

porphyrins having anionic groups such as SO3?and COO?. However,they are not easy to swell or delaminate due to the very high charge density,compared to saponite clay.In particular, CO32?binds strongly to LDH sheets and prevents intercalation of guest anionic species.Recently,delamination procedures for LDH were reported by several groups[94–100].Therefore,the research on LDH–porphyrin complexes is increasing.

2.2.1.Structure and photochemical properties of

porphyrin–cationic clay complexes

The structures of LDH–porphyrin complexes are somewhat different from those of anionic clay–porphyrin complexes.The orientation of porphyrins with respect to the LDH surface was estimated by XRD[101–109]and ESR measurements[109]. The orientation of the porphyrin in these complexes is nearly perpendicular,due to the high charge density of the LDH.The inter-charge distance on the LDH is much shorter than the intra-molecular charge distance in the porphyrin.This relation-ship would affect the porphyrin orientation in the LDH.The structure of the LDH–Mn tetra(4-sulfonatophenyl)porphyrin (MnTSPP)complex is shown in Fig.14[101].When MnTSPP was intercalated into LDH(Mg/Al type,anion exchange capac-ity(AEC)=2.8meq g?1),the absorption spectrum of the por-phyrin was similar to that of MnTSPP in the solid state[102]. This suggests that TSPP is closely packed in the interlayer space.The absorption spectrum of a complex composed of LDH (Mg/Al type)and tetra(4-carboxyphenyl)porphyrin(TCPP)

is Fig.12.Schematic diagram of energy transfer on the clay surface and subsequent electron transfer reaction from an electron donor in solution[92].

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Fig.14.Schematic representation of a layered double hydroxide intercalated with an anionic metalloporphyrin (MnTSPP).Open circles,OH groups;dark circles,metal ions [101].Reprinted with permission.

shown in Fig.15[107].The Soret band becomes broad and exhibits a red shift due to the complex formation with LDH.According to the perpendicular orientation,an H-type aggre-gate is expected.A blue shift is expected in the case of an H-type aggregate.Thus,the precise structure of porphyrin aggregate is not known in this case.The ?uorescence lifetime of the TSPP in the LDH complex was about 100ps,although that of TSPP itself was 9800ps [107].This indicates the presence of an ef?cient quenching process due to TSPP aggregation in the interlayer space.

A different orientation of the porphyrin in the LDH (Zn/Al)interlayer space was reported [110].While the orientation of the para-substituted TCPP is perpendicular,that of the ortho-substituted TCPP is parallel and forms a bilayer in the

interlayer

Fig.15.Absorption spectra of (a)TPPS in water and (b)TPPS in DMSO,and (c)the TPPS intercalate suspended in ethanol [107].Reprinted with permission.

space,according to the XRD measurement,as shown in Fig.16.The absorption spectra are shown in Fig.17(1)for o -TCPP and (2)for p -TCPP.The Soret band of o -TCPP complex is sharp compared to that of p -TCPP.The p -TCPP complex exhibits a shoulder at 397nm and two peaks in the Q-band region.These kinds of spectral features are very informative in discussing the aggregate structure.The shoulder at 397nm may indicate the existence of H-type aggregates.The presence of two Q-bands usually indicates the metallation of the porphyrin.However,according to the chemical analysis,such metallation might not occur in this case.Though it is clearly useful to examine the spec-tral absorption features to understand the aggregation structure of porphyrins in LDH,the interpretation of these spectra requires further discussion.

Complex formation utilizing the delamination technique was reported [111].A schematic representation is shown in Fig.18.The steps can be described as follows:(a and b)glycinate–LDH (Mg/Al)exfoliation through the action of formamide;(c and d)precipitation and restacking of the single layers after porphyrin adsorption and drying.This type of procedure was previously reported by Hibino and Jones [98]The porphyrin used was

[tetrakis(2,6-di?uoro-3-

Fig.16.Schematic representation of the p -TCPP (a)and o -TCPP (b)oriented in the interlayer of LDH [110].Reprinted with permission.

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113

Fig.17.Absorption spectra of LDH–o-TCPP complex(1)and LDH–p-TCPP complex(2)[110].Reprinted with permission.

sulfonatophenyl)porphyrinato]iron(III)(FeTDFSPP).The Soret band of the porphyrin was signi?cantly shifted to longer wave-length by the complex formation,as shown in Fig.19.Although the detailed orientation of the porphyrin is not clear,

the Fig.19.UV–vis spectra of FeTDFSPP aqueous solution(b)and FeP–Gly–LDH in Nujol mull(a)[111].Reprinted with permission.

authors conclude that steric constraints cause the large red shift.

The photochemical properties of oxotitanium(IV)tetrakis(4-sulfonatophenyl)porphyrin(Ti=OTSPP)in LDH(Zn/Al)were investigated.[112]The interlayer distance(1.94nm)determined by XRD measurement indicates that the orientation of the por-phyrin was perpendicular with respect to the LDH surface.As shown in Fig.20,the broadening of the Soret band and blue shift of the Q-band occurred as a result of the complex forma-tion.Furthermore,the ratio of the Soret band/Q-band intensities decreased signi?cantly.The authors proposed that the

broaden-Fig.18.Schematic representation of the process of exfoliation and restacking of the LDH single layers in the presence of FeTDFSPP[111].Reprinted with permission.

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Fig.20.UV–vis absorption spectra of the O Ti IV TSPP in aqueous solution (—),LDH (---),and powder (···)[112].Reprinted with permission.

ing of the absorption bands may be due to strong interaction between the sulfonatophenyl groups of the O Ti(IV)TSPP and the surface hydroxyl groups of the LDH.The ?uorescence spec-tra are shown in Fig.21.The ?uorescence band was analyzed to split into three peaks,620,654,and 705nm.The authors comment that the narrow spacing between the emission bands of the O Ti(IV)TSPP/LDH must be due to a decrease of per-manent dipole and a reduced polarization of O Ti(IV)TSPP in the excited state.The small band at 705nm was identical with that of O Ti(IV)TSPP measured in the solid.Thus,this band can be attributed to the aggregated O Ti (IV)TSPP.The ?uorescence lifetimes are shown in Table 1.The fast ?uores-cence decay (τ1)is due to a strong intermolecular interaction between porphyrins.The long decay component (τ2)is inter-preted as being due to the formation of an excited charge transfer (CT)state between the O Ti(IV)TSPP and the LDH frame-work.The authors state that the photo-excitation of the O Ti(IV)TSPP intercalated into LDH undergoes fast relaxation to the O Ti(IV)TSPP +-LDH ?CT state within a few picoseconds,followed by photo-induced electron transfer from O Ti(IV)TSPP +-LDH ?CT state to the LDH with a rate constant greater than 1010s ?1

.

Fig.21.Fluorescence emission spectra of the O Ti IV TSPP in methanol (—),LDH (---),and powder (···)[112].Reprinted with permission.

Table 1

Fluorescence decay times for the O Ti IV TSPP/LDH,O Ti IV TSPP in methanol and solid

Wavelength (nm)

τ1(ps)a 1

τ2(ps)

a 2

O Ti (IV)TSPP in methanol 610303660310O Ti (IV)TSPP in LDH 61026.90.811950.1965023.90.76100.50.2470029.80.73

168.3

0.27

O Ti (IV)TSPP powder

705

26.0

2.2.2.Photochemical reactions in porphyrin–cationic clay complexes

A photochemical reaction in the complex composed of LDH ([LiAl 2(OH)6]+Cl ?),ZnTCPP,and myristic acid was reported [113].In the presence of ethylenediamine tetraacetic acid (EDTA)as a sacri?cial electron donor,the interlayer ZnTCPP was capable of reducing propylviologen sulfonate (PVS)molecules in solution upon visible light irradiation.The authors proposed that ZnTCPP is in the non-aggregated form in the complex,judging from the ?uorescence measurements,even though the Soret band was broad.The yields of viologen radical were higher for the neutral viologen (PVS)than for the cationic viologen (heptylviologen).This indicates the restricted access of cations into the interlayer space.The ?uorescence of the porphyrin in the complex was effectively quenched by increas-ing the loading level of TiO x ,indicating the electron transfer from excited ZnTCPP to TiO x .Actually,the sensitization of the titanium oxide by ZnTCPP was possible,resulting in viologen radicals upon visible light excitation,as shown in Fig.22.This kind of organized system for photochemical reactions shows promise for further development.

An electron transfer between Fe(III)in the LDH struc-ture ([Mg(II)0.75Fe(III)0.25(OH)2]Cl 0.25·4H 2O)and intercalated Co(II)TSPP was reported [114].According to ESR and absorp-tion spectra,an electron transfer from Co(II)TSPP to Fe(III)was con?rmed.This is not a photochemical reaction.Upon vis-ible light irradiation of the resulting Co(III)TSPP–Fe(II)LDH complex,a back-electron transfer took place.The appearance of an ESR signal during irradiation clearly indicates the back-electron transfer.A very interesting point is the structural change of the LDH lamellar structure during these electron transfer reac-tions.XRD patterns are shown in Fig.23.When the structural Fe was divalent (b),the XRD pattern disappeared.Since the anion exchange properties in the LDH layer were lost,the reg-ular lamellar structure of LDH would be lost.This behavior is an example of the photo-control of the layer structure.Fe(III)in LDH +Co(II)TSPP →Fe(II)in LDH +Co(III)TSPP

Since the layer thickness of LDHs is thinner than those for ordinary anionic clays,it is possible to obtain the STM image [115,116].In the case of anionic clays,it is dif?cult to observe STM images due to their thickness.This feature is advantageous

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115

Fig.22.Schematic illustration of the LDH photochemical assembly (left),and photolysis of the complex in the presence of viologen and EDTA,indicating the production of the viologen cation radical (right)[113].Reprinted with

permission.

Fig.23.Powder XRD patterns of the CoTPPS/LDH composite:before (a)and after (b)intercalation,after photo-irradiation (c),and after 1week (d)[114].Reprinted with permission.

from the viewpoint of research on molecular arrangements on the LDH surface.The utilization of LDH–porphyrin complexes for catalytic reactions was reported [101,117].Also,the delami-nation techniques continue to develop [93–100].The utilization of LDHs as host materials for porphyrin derivatives appears to be promising.

3.Porphyrin-layered metal oxide semiconductor complexes

Ordinary clay minerals do not have semiconducting prop-erties,but layered metal oxide semiconductors (LMOSs)have the potential to exhibit novel photochemical properties in their intercalation compounds.The chemistry of semiconductors has been increasingly developing due to their various functional-

ities [118].Typical LMOSs are shown in Fig.24[119–125].Since LMOSs have semiconducting properties themselves,pho-tocatalytic reactions on LMOSs have been studied extensively.Upon direct irradiation with ultraviolet light,LMOSs have the capability of producing hydrogen and oxygen by water splitting [119,126–129].Since LMOSs cannot utilize visible light effec-tively,a hybridization with organic dyes is desired.The interca-lation of bulky species into the interlayer space of LMOSs is not easy due to their high layer charge density.Recently,research on the intercalation of porphyrin derivatives has been reported.Some of the layered niobate and titanium oxides can interca-late porphyrin derivatives into their two-dimensional interlayer arrays.

3.1.Structural and photochemical properties of

porphyrin-layered metal oxide semiconductor complexes The interlayer cations of LMOSs are exchangeable,as are clay minerals.Small cations or cationic molecules can be inter-calated through cation exchange [124,130–133].However,it is much more dif?cult to introduce porphyrin derivatives into the interlayer space in the case of LMOSs,compared to typical clay minerals.When the exchangeable cations are changed to protons,these layered materials act as Br?nsted acids and can intercalate alkylamines as bases [134].To introduce porphyrin molecules into the interlayer spaces of LMOSs,the utilization of an intermediate was examined,in which the interlayer space was expanded through an acid–base reaction between the proton-exchanged form of the LMOSs and an alkylamine [135].

The intercalation of porphyrin into H 2Ti 4O 9(0.113nm 2per charge)was reported,through propylammonium(PrNH 3+)–H x Ti 4O 9as intermediate [135].The composition of the complex obtained was [TMPyP]0.12H 0.52Ti 4O 9.The orientation angle of the porphyrin with respect to the layer surface was estimated to be 35?,which is smaller than that in ?uorohectorite.According to the high charge density of LMOSs,a perpendicular orien-tation of the porphyrin with respect to the layer surface was

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Fig.24.Structures of (a)K 4Nb 6O 17·3H 2O,(b)HNb 3O 8,(c)HTiNbO 5,and (d)H 2Ti 4O 9.Squares represent the NbO 6or TiO 6octahedra,and circles indicate the interlayer cations [119].Reprinted with permission.

expected.The reason for the relatively small orientation angle was explained as follows.Since the intercalation of H 2TMPyP occurs through the exchange of propylammonium ions,the amount is restricted to that contained in the intermediate guest species.The amount of propylammonium ions (0.67mol per mol [Ti 4O 9]2?)in the intermediate is smaller than the amount of protons in the original H 2Ti 4O 9.The density of the exchange-able sites for H 2TMPyP is about one-third of the proton density in H 2TiO 9.Thus,the orientation of porphyrin should be con-trolled by use of suitable intermediates.The absorption and ?uorescence spectra were measured for the resulting complex,as shown in Fig.25.According to the shape in the Q-band region,the porphyrin exists in a non-protonated form.Judging from the shape of the ?uorescence spectrum,the porphyrin exists as a non-aggregated species in the interlayer space,although it is densely packed in an inclined arrangement.The ?uorescence decay curves of the porphyrin in the complex were ?tted by two-component analysis.The ?uorescence lifetimes of the por-phyrins were measured,as shown in Table 2.These indicate that self-quenching of porphyrin occurred in the interlayer space due to the densely packed structure.Since the lifetime is similar to that of H 2TMPyP 4+4I ?,an electron transfer from the excited porphyrin to the layer would not occur in this case.

Table 2

Fluorescence decay lifetimes of H 2TMPyP 4+Sample

τ1(ns)(I 1)a τ2(ns)(I 2)a H 2TMPyP 4+–H x Ti 4O 9 6.0(0.36) 1.2(0.64)H 2TMPyP 4+4I ?

5.7(0.32) 1.2(0.68)

H 2TMPyP 4+in water 4.1H 2TMPyP 4+in MeOH 7.2H 2TMPyP 4+in glycerol

10.8

a

The relative contribution to the ?uorescence intensity.

The intercalation of porphyrin into KNb 3O 8(0.171nm 2per charge)was reported [136].The resulting composition was [TMPyP]0.14H 0.44Nb 3O 8·2.5H 2O.The absorption spec-trum was similar to that of free-base porphyrin.The inter-calation of porphyrin into K 4Nb 6O 17(0.171nm 2per charge)was also reported [137].The resulting composition was TMPyP 0.35H 0.6K 2Nb 6O 17·3H 2O.The intercalation was carried out though an n -butylammonium intermediate.The authors con-clude that the small red shift of the Soret band observed for

the

Fig.25.Visible absorption (solid line)and ?uorescence (broken line)spectra of H 2TMPyP (a)in water (transmission spectrum)and (b)in the interlayer of H 2Ti 4O 9(diffuse re?ectance spectrum).The ?uorescence spectra were obtained by excitation at 420nm [135].Reprinted with permission.

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117

Fig.26.Proposed structure of niobate–CTAB–Zn(II)TCPP4?hybrid compound [138].Reprinted with permission.

intercalated materials indicates an interaction between the por-

phyrin molecule and the host surface.

A unique method to intercalate an anionic porphyrin into

K4Nb6O17was reported[138].Cetyltrimethylammonium bro-

mide(CTAB)and ZnTCPP form an ionic association complex.

This complex was found to be easily intercalated into the lay-

ered niobate.The porphyrin was incorporated as a counter-anion

of the intercalated cationic surfactant.According to the XRD

measurement,an inclined double-layer structure of CTAB was

deduced,as shown in Fig.26.Although it is very dif?cult to spec-

ify the microscopic orientation of the porphyrin,it should exist

near the cationic group of CTAB.The observed?uorescence

lifetimes were2.3±0.2ns in K x Nb6O17–1.3CTAB–y ZnTCPP

(x=0.006,0.003,0.0015).The lifetimes were not affected by the

concentration of porphyrin in the complex.These results clearly

indicate that the porphyrin exists as a non-aggregated species

in the interlayer space.Since the control of aggregation behav-

ior is very important from the viewpoint of photochemistry,this

method to incorporate porphyrin molecules into the interlayer

space is interesting.

The intercalation of porphyrin into TiNbO5?was reported

[139,140].The resulting composition was(TMPyP)0.09H0.64

TiNbO5·1.8H2O.According to the XRD and dichroic measure-ments,the tilt angle of porphyrin was estimated to be47?.The

absorption spectrum is shown in Fig.27.The Soret band was

blue-shifted16nm by the complex formation.These results

imply that the TMPyP molecules are stacked in a nearly

par-Fig.27.Diffuse re?ectance spectrum of TMPyP–TiNbO5hybrids[140]. Reprinted with permission.

allel orientation and interact with each other in a face-to-face manner within the interlayer spaces.The unique photochemical behavior in this complex is described in the next section.

3.2.Photochemical reactions in porphyrin-layered metal oxide semiconductor complexes

The photo-induced charge separation between TMPyP and the electron acceptor methylviologen,intercalated in separate inter-layer spaces of titanoniobate,was reported[140].The multi-layer?lm,as shown in Fig.28,was prepared by successive casting of each of the colloidal suspensions,MV2+–TiNbO5?and TMPyP4+–TiNbO5?on a quartz glass plate.Irradiation of TMPyP with light of>420nm initiated the donation of an electron through the conduction bands of the TiNbO5?layers, yielding a one-electron reduced acceptor(MV+?)isolated from the one-electron oxidized TMPyP by the titanoniobate layers.It is evident from the action spectrum that the porphyrin sensitizes the electron transfer to the conduction bands of the

titanoniobate Fig.28.Schematic drawing of a multi-layer?lm containing a light-harvesting porphyrin and viologen electron acceptor layers separated from each other by semiconducting TiNbO5?sheets[140].Reprinted with permission.

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Table3

Fluorescence lifetimes of Zn(II)TCPP4?

Niobate(H)–CTAB–x ZnTCPP4?τ1(I1)a(ns)τ2(I2)a(ns)

x=0.0060.28(0.33) 1.8(0.67) x=0.0030.20(0.27) 2.3(0.73) x=0.00150.22(0.34) 2.1(0.66)

a Relative pre-exponential term of each component.

layers.The point of interest is the suppression of back-electron transfer in the complex.It is known that MV+?is highly sta-ble also in Ti4O92?[141].Thus,the electron transfer reactions utilizing LMOSs are interesting.

The photochemical injection of electrons into the K4Nb6O17 layer from anionic Zn porphyrin intercalated with cationic sur-factant was reported[138].The porphyrin molecule should exist near the cationic group of CTAB,as shown in Fig.26.The dis-tance between the porphyrin and the niobate sheet should be different from that of a complex containing a cationic porphyrin. This would affect the electron transfer behavior.Although the?uorescence decay curve can be analyzed by means of a single-component?tting for the K x Nb6O17–1.3CTAB–y ZnTCPP complex,it exhibits a two-component decay for the H x Nb6O17–1.3CTAB–y ZnTCPP complex.The shorter lifetime was about0.2ns,as shown in Table3.Since the lifetime did not depend on the concentration of porphyrin,the short life-time component is not due to self-quenching but might be due to an electron transfer from the excited singlet Zn(II)TCPP to the niobate layer.The fact that a short lifetime compo-nent was observed only in the H x Nb6O17system,which had a more positive conduction band energy(?0.265V versus NHE) than K x Nb6O17(?0.77V versus NHE),also supports an elec-tron transfer mechanism.In fact,visible light irradiation to H x Nb6O17–1.3CTAB–0.006ZnTCPP in the presence of KI as an electron donor induced hydrogen evolution.

The radical species tends to be stable,especially in K4Nb6O17 [132].Thus,the utilization of the layered niobate should be unique as a photochemical reaction micro-environment. Recently,the delamination technique has advanced markedly [142–147]and has led to the?nding of novel materials such as nanoscrolls[148–155].This interdisciplinary?eld of materials science and photochemistry continues to develop.

4.Porphyrin–nanotube complexes

As mentioned in the previous section,a variety of lamellar inorganics have been successfully delaminated.The delamina-tion technique has led not only to the intercalation of guest molecules but also the?nding of novel materials such as nano-scrolls[148–155].Recently,techniques to prepare nanotubes [156–162]or nanoribbons[163]from various materials have proliferated.The insertion of porphyrin into hexaniobate nano-scrolls was reported[149].First,the niobate was delaminated with alkylamine and was then transformed into nanoscrolls by changing the pH,according to a conventional technique.The incorporation of TMPyP was achieved by the dispersion of delaminated niobate gel into a TMPyP aqueous solution.

The Fig.29.TEM micrograph showing the multi-layers of the TMPyP–H2K2-Nb6O17tubes.The inset shows a sketch of the way in which the particle scrolls, forming the multi-wall tube[149].Reprinted with permission.

retention of the tubular morphology was con?rmed by the TEM image.The TEM image of a multi-layer tubular structure of a TMPyP–nanoscroll complex is shown in Fig.29.XRD data indicate that the porphyrin is not only adsorbed at internal and external surfaces of niobate tubes but it is also intercalated between the layers that form the scrolls.The observed inter-layer spacings(1.96and2.74nm)in the TEM image are in good agreement with those determined in the XRD pattern for the two present phases in the nanocomposite.The absorption spectrum of a water suspension of the tubular nanocomposite is shown in Fig.30.The authors state that some of the porphyrin molecules were protonated.Since the tubular morphology has interesting features such as increased surface area,more detailed character-ization as well as applications are expected.

Carbon nanotubes have attracted much attention due to their unique structure and properties[164–166].The

combi-Fig.30.Absorption spectrum of TMPyP–H2K2Nb6O17tubes in aqueous sus-pension[149].Reprinted with permission.

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Fig.31.UV–vis–NIR spectra for DMF solutions of(a)ZnPP,(b)SWCNT–ZnPP,and(c)re-solubilized SWCNT.Optical cell length, 1.0mm[168]. Reprinted with permission.

nation of porphyrin and carbon nanotubes has been examined by several groups[167–173].Single-walled carbon nanotubes (SWCNTs)–porphyrin hybrids were synthesized and character-ized[168].Zinc protoporphyin IX(ZnPP)was used.Although SWCNTs alone are insoluble in DMF,SWCNTs sonicated in ZnPP DMF solution afforded a reddish-black colored transpar-ent solution(Fig.31),which strongly suggests that ZnPP can disperse or dissolve SWCNTs.The absorption spectrum of a SWCNT–porphyrin hybrid,in the region of the Soret and Q-bands,is identical with that of ZnPP DMF solution.This result suggests that the adsorption of ZnPP does not cause any spec-tral shift.The?uorescence spectrum of a SWCNT–porphyrin hybrid is shown in Fig.32.The peak maxima of the spectra of the re-solubilized solution,appearing at588and643nm, were identical with those of ZnPP DMF solution.A signi?-cant decrease in the?uorescence intensity in the re-solubilized ZnPP–SWCNTs solution compared with that of the ZnPP-only solution was observed.This?uorescence quenching is most likely to be derived from ef?cient energy transfer from ZnPP to the nanotubes.Thus,ZnPP should exist near the SWCNTs.

The interaction between the porphyrins and nanotubes

should

Fig.32.Fluorescence spectra(arbitrary units)of(a)ZnPP DMF solu-tion([ZnPP]=1.3?M)and(b)re-dissolved SWCNT–ZnPP DMF solution ([ZnPP]=1.3?M).Excitation,420nm[168].Reprinted with

permission.Fig.33.Schematic image of an SWCNT–anionic pyrene–porphyrin hybrid [169].Reprinted with permission.

be?–?and/or van der Waals interactions.Although there are still some unclari?ed aspects regarding the ZnPP–SWCNTs hybrids,there is much promise of unique characteristics for these hybrids.

The integration of SWCNTs,anionic pyrenes,and water-soluble porphyrins into functional nanohybrids,through a com-bination of associative van der Waals and electrostatic interac-tions,was reported[169,170].A schematic image of the hybrid is shown in Fig.33.The SWCNT portion acts as an electron acceptor in the system.When the hybrids are irradiated with visible light,a rapid intrahybrid charge separation causes the reduction of the SWCNT and,simultaneously,the oxidation of the porphyrin.According to the transient absorption mea-surements,the radical ion pairs are long-lived,with lifetimes in the microsecond range.Long-lived intracomplex charge separa-tion in polymer-wrapped carbon nanotube–porphyrin was also reported[171].

Photo-induced charge transfer in carbon nanotubes with covalently linked porphyrin antennae was reported[172].A schematic image is shown in Fig.34.The absorption and?uores-cence of this complex show that the carbon nanotubes serve as an ef?cient electron acceptor;this system provides a model for the construction of novel photovoltaic devices and light-harvesting

systems.

Fig.34.Porphyrin-grafted carbon nanotubes and photo-induced electron trans-fer[172].Reprinted with permission.

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The porphyrin–SWCNT composites were synthesized by condensation of tetraformylporphyrins and diaminopyrynes on SWCNTs[174].The Soret band of the porphyrin was broad-ened compared to that of the porphyrin itself.In the composites, the?uorescence of the porphyrin was nearly quenched.This observation indicates that there is a strong electronic interaction between SWCNT and porphyrin.

5.Other porphyrin–inorganic host material complexes

Zeolites and mesoporous materials are well-con?ned rigid inorganic host materials[175–177].Molecular systems for light-to-chemical energy conversion that make use of zeolites and mesoporous materials as host materials have been developed [175–179].The control of photochemical properties and reac-tions of incorporated molecules has been beautifully demon-strated in zeolites[175,176,180–182].The development of syn-thetic techniques for inorganic host materials such as ordered mesoporous molecular sieves,e.g.,MCM-41and similar mate-rials,provides opportunities for research on porphyrin hybrids [183,184].Zeolites possess regular periodic structures and may be thought of as a sponge-like material with channels and cages that extend periodically and regularly across its entire structure [175,176].Mesoporous materials have honeycomb structures with ordered cylindrical channels.The typical cavity size of a zeolite is0.6–1.2nm,while the diameters of MCMs and FSMs (folded sheet mesoporous materials)are2–10nm.Although it is not easy to accommodate porphyrins in conventional zeolites, materials with large pore sizes such as MCM-41easily incorpo-rate large molecules.

Itoh and Fukushima et al.reported the stabilization of chloro-phyll in mesoporous silica[185–187].The mesopores of FSM acted as nanoscale spaces,not only for the interaction between chlorophyll(Chl)molecules and the silica support but also for the interaction between the adsorbed chlorophyll molecules. These interactions contribute to photostability.An increase in the amount of chlorophyll adsorbed to the pores of FSM leads to an enhancement of the photostability,as shown in Fig.35, accompanied by a shift in the absorbance maximum to a longer wavelength.Such a shift in the absorption band is attributable to an interaction between two chlorophyll molecules in the arrange-ment shown in Fig.36.The effect on the stability of FSM is larger than that in a layered silicate support.Thus,the intermolecular interaction in FSM is crucial for the stabilizing effect.

Photochemical hydrogen evolution with a Chl–FSM hybrid was examined[186].The irradiation of Chl–FSM in the pres-ence of2-mercaptoethanol and platinum evolved hydrogen

gas Fig.35.Photostability of chlorophyll(Chl)a and Chl–FSM.Curve A:chloro-phyll a in benzene.Curves B–F:Chl–FSM conjugates dispersed in water with absorption maxima of671,672,673,674,and675nm,respectively.Each sample was illuminated,at a light intensity of200J m?2s?1,at30?C[186].Reprinted with

permission.

Fig.36.Tentative sketch of the arrangement of chlorophylls in FSM[186]. Reprinted with permission.

(Fig.37).It is clear that the chlorophyll conjugate acts as a sensitizer for hydrogen evolution.Very fast excitation energy transfer from the high energy chlorophyll,occurring in just a few picoseconds,was suggested from the time-resolved spectra of the Chl–FSM-22hybrid[187].Also,a stable cation radical of chlorophyll was detected by means of ESR measurements during the irradiation of the Chl–FSM-22hybrid.The stabi-lization of the porphyrin cation radical was also observed in the complex with MCM-41[188,189].H2TPP+?was stable in MCM-41and titanosilicate TiMCM-41,even at room temper-ature,and decayed,after30min photo-irradiation,by only5% after45h in TiCMC-41at room temperature.The aggregation behavior of TSPP dianion within a modi?ed mesoporous MCM-41was examined[190].J-aggregate formation in the MCM was presumed.The structural control of the chlorophyll aggre-gate in the con?ned space provided by inorganic hosts such

as Fig.37.Photochemical hydrogen evolution sensitized by Chl conjugate in FSM[186].

S.Takagi et al./Journal of Photochemistry and Photobiology C:Photochemistry Reviews7(2006)104–126121

FSM and MCM is very interesting from the viewpoint of pho-

tochemistry.These observations are likely to be related to the

phenomena in living photosynthetic systems.An application of

MCM-41–Pt(II)porphyrin complexes for oxygen sensing was

reported[191].This complex showed good sensitivity for oxy-

gen detection.The cationic porphyrin(TMPyP)also tends to

aggregate in mesoporous silicas(2.4,3.5,and4.2nm diameter)

[192].

Mo(O)and Fe porphyrins and fullerenes were encapsu-

lated in FSM-16[193].The mesoporous channels(2.7and

4.7nm diameter)of FSM-16act as a con?ned hydrophobic

micro-environment to accommodate isolated porphyrins and

prevent the irreversible formation of?-oxo dimer.The reversible

removal of O2bound with the Mo and Fe porphyrins proceeds

by UV irradiation.The spatial con?nement of porphyrins in

mesoporous channels to prevent the formation of?-oxo dimer

should be useful in constructing a photochemical reaction sys-

tem that makes use of the central metal of the porphyrin as a

catalyst.

The photo-induced electron transfer from the immobilized

Mn(III)porphyrin to the MCM-41framework was investigated

by ultrafast time-resolved spectroscopy[194].The absorption

spectrum of Mn(III)TPP showed a dramatic change compared

to that in benzene.This change was interpreted to be due to ?-electron interaction with surface hydroxyl groups of MCM-41.The authors concluded that Mn(III)TPP+?is generated by

irradiation,suggesting that the framework of MCM-41can be

act as an electron acceptor.

The organization of organic and inorganic components in

constructing a reaction model that mimics some of the func-

tions of natural photosynthetic assemblies has been investi-

gated.Mallouk and co-workers reported the sequential adsorp-

tion of polyanions and polycations to make a?ve-component

energy/electron transfer cascade,using anionic Zr(HPO4)2·H2O (zirconium hydrogen phosphate,?-ZrP)and HTiNbO5sheets as layer components[195].A porphyrin–viologen charge-separated state is formed in the system,and it has an excep-tionally long-lived component(τ～900?s)with the HTiNbO5 spacer.The semiconducting HTiNbO5sheets play an active role in relaying the electron from the photo-excited porphyrin to the viologen.The speci?c control of porphyrin orientations in?-ZrP was examined[196].Aminophenyl and pyridinium substituted porphyrins(TAPP and TMPyP)were intercalated into the phase of?-ZrP by exchanging the porphyrins into the p-methoxyaniline pre-intercalated compound.The morphology of the guest phases was dictated by the location of amino and pyridinium substituents residing on the meso-aromatic groups of the porphyrin.The morphology of p-TAPP derivatives con-sist of a monomolecular porphyrin layer in which the porphyrin planes are tilted nearly45?relative to the host lamellae,whereas o-?,?,?,?-TAPP derivatives predominantly assemble into a por-phyrin bilayer in which the porphyrin macrocycles lie parallel to the host sheets,according to the XRD and ESR measurements, as shown in Fig.38.Host–guest electrostatic interactions and hydrogen bonding between guest amines and host phosphates governs the porphyrin assembly structures.The combination of techniques used for the molecular orientation control and

device Fig.38.Proposed structures of the predominant guest phases formed upon inter-calation of(a)H2TMPyP and p-TAPP derivatives,(b)o-?,?,?,?-H2TAPP,and (c)o-?,?,?,?-H2TAPP[196].Reprinted with permission. construction,which included successive adsorption and LB tech-niques,is a promising approach for the construction of arti?cial photosynthetic mimics.

The combination of titania nanosheets(TN)and mesoporous silica(MPS),and the electron transfer reactions in the result-ing composites were examined(Fig.39)[197–199].Composite ?lms of TMPyP hybrid incorporated in mesoporous silica and MV2+/titania nanosheet hybrid were synthesized.These com-posite thin?lms were able to initiate a one-electron reduction of the MV2+ions,accompanied by the simultaneous decomposi-tion of the TMPyP within the mesoporous silica channels.The TMPyP/MPS hybrid?lms were hybridized with TN nanosheets by electrophoretic or casting deposition.With UV irradiation of the complex,the Soret absorption band of TMPyP was

seen Fig.39.Schematic structure of the composite TMPyP/MPS and MV2+/TN tetrad hybrid?lm[197].Reprinted with permission.

122S.Takagi et al./Journal of Photochemistry and Photobiology C:Photochemistry Reviews7(2006)104–126

to decrease,with the appearance of new absorptions at ca.410 and600nm,while retaining the two isosbestic points at420and 500nm.It was seen that UV irradiation of the laminated com-posite?lms of a nano-structured hybrid of TMPyP and MV2+, which were separately adsorbed within MPS and TN,was able to induce the simultaneous decomposition of TMPyP and the for-mation of MV+?,indicating a photo-induced charge separation through the mesoporous silica and TN.Such investigations on laminated stacked?lms are unprecedented and have great poten-tial for the design and development of photofunctional devices and systems.

TN+hν→e cb?+h+

e cb?+MV2→MV2?

h++TMPyP→decomposition

6.Concluding remarks

In this review,the structures and photochemical properties of porphyrins in inorganic complexes are described.The synthetic techniques for inorganic materials have been advancing.The techniques used to analyze the nano-size structures also have been in progress.The scienti?c?eld involving the combination of porphyrins and inorganic materials will continue to develop. The understanding and mimicking of natural photosynthesis are the dreams of scientists.Plants are composed of many com-ponents,including pigments,proteins,membranes,and many others.Most of the components and organized structures in living plants are beautifully regulated.From the viewpoint of chemistry,the exploration of nano-construction techniques is essential in achieving advanced reaction systems.We chemists must?nd the way to construct multiple components with regu-lated structures.We believe that the efforts to realize such desired structures will lead to breakthroughs in science. Acknowledgements

The authors would like to thank collaborators on this subject, Dr.H.Tachibana,Dr.T.Shimada,and Dr.Z.Tong.The authors also thank Mrs.H.Miura for her generous assistance during this work.

This work has been partly supported by a Grant-in-Aid for Exploratory Research and Scienti?c Research on Priority Areas (417),and a Grant-in-Aid for Young Scientists(B)from the Ministry of Education,Culture,Sports,Science and Technology (MEXT)of Japan.

Appendix A

A.1.Exciton theory in porphyrin interaction

Porphyrin molecules can be used as spectroscopic probes since their absorption and?uorescence features are sensitive to the environment where the molecule is located.The electronic spectrum of TMPyP in aqueous solution presents?ve absorption bands in the visible region:the Soret band at around420nm and the four Q-bands at longer wavelength[1–3].Generally,the change of porphyrin absorption spectra is induced by(i)solvent effects,(ii)redox reactions,(iii)protonation or metallation of core nitrogen atoms,(iv)?-electron interaction,(v)electronic changes due to structural changes such as?attening or distortion, or(vi)interactions between porphyrins(aggregation).

(i)Solvent effects.As the polarity of the surrounding medium

increases,the Soret maximum moves to shorter wavelength in general.The degree of spectral shift is minor(within ～10nm).

(ii)Redox reactions.Porphyrins can act as both electron donors and acceptors.The radical cation or radical anion can be produced via a redox reaction.Their absorption spectra are usually much different from the original one.Since the porphyrin radicals are sometimes not stable,phlorin or chlorin type derivatives can be produced as a?nal prod-uct.Their absorption spectra are characteristic,especially in the Q-band region.The symmetry change from D2h or D4h induces a drastic change in the Q-band.

(iii)Protonation or metallation[200]of core nitrogen atoms.

Protonation and metallation induces speci?c spectral changes.The Soret maximum of the protonated form of the porphyrin is about445nm and,due to a molecule symmetry change from D2h to D4h,only two Q-bands are observed.

This spectral change due to protonation is reversibly recov-ered by the addition of alkali.

(iv)π-Electron interaction[201–203].The degree of?-electron interaction with the inorganic surface is not clear.

The interaction between the porphyrin and the oxygen plane is presumably not so large.

(v)Electronic change due to structure change such as?atten-ing or distortion.According to PM3calculations,the～30?twist of dihedral angle between the porphyrin ring and the peripheral aromatic ring from the vertical should cause a ～30nm red shift in the Soret band in the case of TMPyP

[27].Non-planarity,a ring distortion,also leads to signi?-

cantly red-shifted electronic spectra[30–34].The effects of structural changes on the absorption spectra are not simple. (vi)Interactions between porphyrins(aggregation).The spec-tral changes due to aggregation afford important informa-tion on porphyrin assembly structure.In this section,the theory related to porphyrin aggregation will be described.

The absorption spectrum depends on the interaction of tran-sition moments between molecules.The interaction between transition moments depends on the intermolecular distance and orientation relationship.Thus,the absorption spectral change due to aggregation affords important information on aggregate structure.The spectral changes due to aggregation are generally interpreted by the molecular exciton approximation developed by Kasha et al.[204,205].This model,which neglects elec-tronic overlap of?-systems,is based on the interaction between localized transition dipole moments.The coupling results in a splitting of the non-aggregated state into a higher-energy and a lower-energy contribution.The resulting transition energy(E±)

S.Takagi et al./Journal of Photochemistry and Photobiology C:Photochemistry Reviews7(2006)104–126123 is related to the energy of the non-aggregated molecule,E0(Eq.

(A.1))[206].

E±=E0+D±V(A.1)

in which D is a dispersion energy term and re?ects the change in

environment from non-aggregated to aggregated while V is the

exciton splitting energy.For a cofacial array of N chromophoric

units,this term is related to the magnitude of the transition

moment(M)and the geometry of the aggregate,as given by

Eq.(A.2).

V≈2

N?1

N

M2

R3(1?3cos

2α)

(A.2)

In Eq.(A.2),R is the center-to-center distance of two chro-

mophores in the aggregate,αthe angle between the center-to-

center vector and the transition moment,M the magnitude of

the transition moment,and N is the number of chromophoric

units.The theory predicts,from the selection rule,that when α<54.7?,the absorption band of the aggregate will be red-shifted,and whenα>54.7?,the band will be blue-shifted.In

the metal-free(free-base)porphyrin,the intense Soret or B-band

has two degenerate perpendicular transition dipole components

x and y.The lowest excited state is split into two components, giving the four characteristic Q x(0-1),Q x(0-0),Q y(0-1),Q y(0-0) absorptions.The largest energy shifts are observed in the Soret band region,which is a direct indication of strong exciton cou-pling.The magnitude of the transition moment M for the Q-band is very low,as compared with the Soret band,and only minor shifts are observed in the spectra.

In order to deduce a structural model for the arrangement

of the porphyrin macrocycles,the absorption spectra can be

interpreted in terms of a displacement along x and y axes as

shown in Fig.40.In the actual aggregated system,the trans-

formation between several forms of aggregate can take place.

The transformation of aggregate geometry by the addition of

NaCl was reported,as shown in Fig.41[207].Although the

aggregates are classi?ed into“head-to-tail”(J-aggregate)and

“card-packed”(H-aggregate)geometries,the actual

geometries Fig.40.Structural arrangements of porphyrins in aggregates:(a)αis the angle between the transition dipole moment(M)and the center-to-center vector(R),(b) face-to-face x,y:α=90?,blue shift(side view),(c)edge-to-edge,x:α<54.7?, y:α>54.7?red and blue shift(top view),and(d)head-to-tail,x,y:α<54.7?red shift(top view)[206].Reprinted with

permission.

Fig.41.Transformation of aggregate geometry by the addition of NaCl[207]. Reprinted with permission.

are in reality far more complex than indicated by these sim-pli?ed denotations.Many geometries such as“face-to-face(H-aggregate)”,“L-shaped”,“face-to-face slipped”,“edge-to-edge (J-aggregate)”,“head-to-tail”,“side-to-side”,“fully planar”,and “T-shaped”are possible[208].

The energy diagram for the H-aggregate is shown in Fig.42(a) [204].The absorption maximum shifts to shorter wavelengths in this aggregate.The energy diagram for the J-aggregate

is Fig.42.(a)Excitation band energy diagram for a molecular dimer,or a double molecule,with parallel orientation dipoles;(b)excitation band energy diagram for a molecular dimer,or a double molecule,with coplanar transition dipoles inclined to an interconnected axis by an angleθ[204].Reprinted with permission.

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