Porphyrin photochemistry in inorganicorganic hybrid materials

Journal of Photochemistry and Photobiology C:Photochemistry Reviews7(2006)104–126ReviewPorphyrin 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,ca Department of Applied Chemistry,Faculty of Urban Environmental Sciences,Tokyo Metropolitan University,Minami-ohsawa1-1,Hachiohji,Tokyo192-0397,Japanb Japan Society for the Promotion of Science for Young Scientist,Japanc SORST,JST(Japan Science and Technology),JapanReceived24February2006;received in revised form31March2006;accepted10April2006Available online23October2006AbstractPorphyrin 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;AggregationContents1.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.2006.04.002S.Takagi et al./Journal of Photochemistry and Photobiology C:Photochemistry Reviews7(2006)104–126105Shinsuke Takagi received his BS and MS degrees inapplied chemistry from Tokyo Metropolitan Universityin1991and1993.After enrolling in the doctoral pro-gram,he joined the research staff of the Department ofIndustrial 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-JapanBilateral Program.He received his PhD degree fromTokyo Metropolitan University under the supervision ofProfessor 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-environmentsprovided by micelles,reversed micelles,vesicles,zeolites,and clayminerals.Miharu Eguchi received her BS and MS in appliedchemistry from Tokyo Metropolitan University in2002and2004.She was a research fellow of the JSPS(DC1)from2004to2006.She received her PhD degree fromTokyo Metropolitan University under the supervisionof Professor Haruo Inoue in2006.She is currently aresearch fellow of the JSPS in Professor Thomas E.Mallouk’s group(Pennsylvania State University).Shereceived 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 moleculesby using the structure and photochemical properties of clay–dyecomplexes.Donald A.Tryk graduated from the University ofFlorida in1969with bachelor of science in chemistry.He worked as an environmental chemist for the State ofNew Mexico before returning to graduate school at theUniversity of New Mexico,graduating in1980with adoctorate in chemistry.From there,he went on to CaseWestern Reserve University,working in the ChemistryDepartment as a senior research associate,principallywith the late Professor Ernest Yeager.In1995,he movedto the University of Tokyo,workingfirst as a researchassociate 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 redoxreactions such as those involving dioxygen,dihydrogen,and carbondioxide.Haruo Inoue was born in1947in Japan and graduated from the University of Tokyo in1969.Afterfinish-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 energyflow in solution,anisotropic control of chemical reactions in the excited state,nano-layered compounds,metal complexes,and artificial 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 Artificial Photosynthesis with Water as an Electron Source”.1.IntroductionPorphyrin derivatives play highly important roles in vari-ousfields 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 modification 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 newfield 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 offlexible 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 complexesClay minerals are well known as multi-layered inorganic materials that provide quasi-two-dimensional spaces,which,106S.Takagi et al./Journal of Photochemistry and Photobiology C:Photochemistry Reviews 7(2006)104–126Fig.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-defined dimensions,should be interesting from the viewpoint of micro-environments for chemical reac-tions [5–14].The surface of the clay sheet is very flat,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 flexibility 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 classified by the structure and exchangeable charge capacity [10].Basically,they are divided into two types,anionic and cationic clays.2.1.Anionic claysThough 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.Thestructure 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 magnified 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 flat,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 complexesThe 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 blueS.Takagi et al./Journal of Photochemistry and Photobiology C:Photochemistry Reviews7(2006)104–126107Fig.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 films,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].Specifically,enhanced␲-conjugation and the electron-withdrawing effect of the pyridinium group,due to aflattening of the TMPyP on the clay,induce the spectral change.Byflattening,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 photochemicalprop-Fig.3.Arrangement of CoTMPyP in the interlayer space of(a)hectorite and (b)fluorohectorite 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.Whenfluorohectorite (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.Influorohectorite 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 combination108S.Takagi et al./Journal of Photochemistry and Photobiology C:Photochemistry Reviews7(2006)104–126of 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 aflexible 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 specific 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 specific 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 influorescence spec-tra as a result of the complex formation with clay.Thefluo-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 offluorescence 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 offinding 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,flu-orescence was emitted from naphthalene.As the atomic number of the exchangeable cation increased,thefluorescence 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 offlexible 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].Theflu-orescence also tends to be affected by self-quenching in the aggregate[52–57].In the specific 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”.Thefluorescence 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.Specifically,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 polyfluo-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-S.Takagi et al./Journal of Photochemistry and Photobiology C:Photochemistry Reviews 7(2006)104–126109Fig.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 polyfluorinated surfactant/clay hybrid interlayers.It was also found that when the amount of adsorbed surfactant decreased,i.e.,when the volume of the polyfluorinated 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 theseligandsFig.7.Schematic depiction of the aggregation mechanisms of Sb(V)TSPP in polyfluorinated 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.。