有机双光子吸收材料的研究进展

第32卷第3期2012年6月物理学进展PROGRESS IN PHYSICSVol.32No.3Jun.2012Recent progress on two-photon absorbing organic materialsWang Yao-Chuan1,Yan Yong-Li2,Li Bo3,Qian Shi-Xiong*41.Physics Department,Dalian Maritime University,Dalian,116026,China2.Beijing National Laboratory for Molecular Sciences(BNLMS),Key Laboratory of Photochemistry,Institute of Chemistry,Chinese Academy of Sciences,Beijing100190,China3.Key Laboratory of Polar Materials and Devices,Ministry of Education,East China Normal University,Shanghai200241,China4.Physics Department,Fudan University,Shanghai200433,ChinaIn this article,we review recent studies on two-photon absorbing organic materials.We introducethe research progresses on the nonlinear optical properties of two-photon absorption(TPA)and two-photon excitedfluorescence(TPF)in organic materials with different structures including dipolar,quadrupolar,and multibranched molecules,macrocycles,and polymers.Ultrafast dynamics in thesematerials,as characterized by transient absorption spectroscopy,is included for a better understandingof the physical mechanisms.Moreover,we summarize recent proposed applications of the two-photonabsorbing organic materials.Key words:Two-photon absorption;Two-photon excitedfluorescence;Ultrafast dynamics;Nonlinearoptics;Organic materialsCLC number:O437Document Code:ACONTENTSI.Introduction135A.Theoretical background of TPA process136B.The study history of TPA in organicπ-conjugatedmolecules137 II.Experimentals139A.Nonlinear optical properties1391.Z-scan technique1392.Two-photon excitedfluorescence140B.Ultrafast dynamics1401.Transient absorption technique1402.Time-resolvedfluorescence140 III.Structure/Properties and Ultrafast dynamics141A.Dipolar and quadrupolar molecules with strongTPF response141B.Multibranched Structures1451.Triphenylamine1452.S-triazine1463.Alkylpyridinium1484.Tricyanobenzene149C.Porphyrins1501.Calculation1502.Substituent porphyrin1503.Porphyrin based polymers152D.Polymers152E.Macrocycles155 IV.Applications156A.Unique advantages of TPA156B.TPF microscopy157C.Optical limiting157Received date:2011-11-16*sxqian@D.Frequency-upconversion lasing158E.Microfabrication159F.Three-dimensional optical data storage160 V.Conclusions160 Acknowledgments161 References161I.INTRODUCTIONTwo-photon absorption(TPA)process is a type of third-order nonlinear optical(NLO)phenom-ena,where two photons are simultaneously absorbed through a virtual state in a sample.[1]In the case of degenerated TPA process,two photons with the same photon energy resonant to half of the energy gap in-duce transitions to excited state in a sample.The pro-cess of TPA is schematically illustrated in compared to one-photon absorption in Figure1.The concept of the TPA process was theoretically proposed by G¨o ppert-Mayer in1931.In her doctoral dissertation,she predicted that a TPA process would lead to a transition from a ground state to a higher en-ergy state of a molecule.However,as a NLO process with simultaneous absorption of two photons,TPA process requires intense incident lightfield,making the experimental demonstration very difficult at that time prior to the invention of laser.In1960s,Kaiser文章编号:1000-0542(2012)03-0135-30135136Wang Yao Chuan et al.:Recent progress on two-photon absorbing organic materialsFIG.1.Energy diagrams illustrating the processes of one-photon absorption and TPAand Garrett observed the bluefluorescence(FL)emis-sion from a CaF2:Eu2+crystal excited by the focused Ruby laser pulses at a wavelength of694.3nm.[2]In 1963,Peticola and Rieckhoffobserved TPA in a dilute organic solution.[3]TPA process has several unique advantages over the conventional one-photon absorption for many re-searches and applications.Firstly,the quadratic de-pendence of TPA on the incident light intensity per-mits a highly-confined spatial excitation,leading to improved three-dimensional(3D)resolution forflu-orescence imaging.Secondly,benefiting from lower scattering at longer incident wavelength,TPA pro-cess can enhance the penetration depth in scatter-ing and absorbing media.Thirdly,longer excitation wavelength can reduce the photobleaching and cel-lular auto-fluorescence from proteins and other in-trinsicfluorophores which absorb UV or visible light. With these advantages,TPA has been implanted for applications in optical limiting,[4∼9]two-photon ex-citedfluorescence imaging,[10∼15]two-photon poly-merization induced3D microfabrication,[16∼27]pho-todynamic therapy,[28]optical data storage,[29∼31]up-conversion lasing,[32,33]and so on.These techniques have made significant impacts in variousfield includ-ing physics,chemistry,biology,material science and information technology.After the observation of large TPA parameters of the dyes,the organic conjugated systems with TPA properties attracted great attentions.Design and syn-thesis strategies have been rapidly developed,and a series of organic conjugated materials with superior TPA properties have been synsynthesized.As the TPA parameters of the organic materials are very crucial to many applications,exploring the organic conjugate materials with large TPA cross-section and strong TPF has become one of the hottest topics in the past decades.A.Theoretical background of TPA process Theoretically,one can describe TPA process with a dipole model of the intense light-matter interaction. As shown in Figure2,the medium can usually be considered as bounded charged particles.Under an applied electricfield,the positive charges and nega-tive charges would move to the opposite directions, respectively,and the electricfield induces an electric polarization in the medium.The electric-dipole mo-ments caused by these small movements can be de-scribed asµind=−ex,where e is electron charge,and x is the displacement induced by electricfield.FIG.2.Oscillator model of the interaction between light and matterThe overall polarization isµind=−N e ex,where N e is the electron density in the medium.Under a weak electricfield,the polarization is linearly proportional to the electricfield P ind(E)=χ(1)E,whereχ(1)is the linear electric susceptibility.The incident lightfield is in fact a combination of electric and magneticfields oscillating sinusoidally at optical ually,the effect of the magnetic field on nonlinear optics is pretty weak and would be neglected later.The motion of charged particles in a dielectric medium in response to an optical electric field is oscillatory.In the oscillator model,motion of the electrons is dominant,since the mass of nuclei is orders heavier than that of electron.The motion of the electron in the electricfield can be described by a simple mechanical spring model,which is governed by the equation of motion for an oscillator.d2xd t2+2Γd xd t+ω20x=−emE(t)(1)Where x is the displacement from the mean position,ω0is the resonance frequency,Γis the damping con-stant,t is the time,and m is the mass of electron.Wang Yao Chuan et al.:Recent progress on two-photon absorbing organic materials137 Considering a sinusoidal optical electricfield,E(t)=E0cos(ωt)=12E0exp(−iωt)+exp(iωt)(2)whereωis the optical frequency.Substituting equa-tion(2)into(1),the solution can be described asx=−eE0me−iωtω20−2iΓω−ω2+c.c.(3)c.c.is complex conjugate.Then the electric polariza-tion can be described asP=−Nex=Ne2m1ω20−2iΓω−ω2E(ω)e−iωt+c.c.(4)If the appliedfield becomes intense enough,the equa-tion of motion should include anharmonic terms. Thus,the description of equation(4)must be mod-ified tod2x d t2+2Γd xd t+ω20x+(ax2+bx3+···)=−emE(t)(5)Hence,there is no longer an exact harmonic solution for the equation.If the anharmonicity is much smaller in comparison to the linear term,the solution can be approximated as a power series in E.In general,the electric polarization in the medium could be expanded as following:P=χ(1)E+χ(2)EE+χ(3)EEE+ (6)whereχ(2)andχ(3)are the second-order and third-order nonlinear optical susceptibility,respectively.In principle,high-order NLO effects would only happen when the intensity of light electricfield is compara-ble to the bound electricfield inside an atom or a molecule.Under the excitation of intensefield in-duced by the laser beam,the NLO effects might be strong enough to be observed.In addition to this, if the frequency of the light lies near to the intrinsic frequency of the oscillating dipoles,there would be a resonant enhancement of the NLO effects.Thus,it is possible to observe high-order NLO effects under relatively low laser excitation with the resonance en-hancement.The TPA process is an effect arising from such resonant enhancement.The TPA process is related to the imaginary part of the third-order nonlinear optical susceptibility.Dur-ing the two-photon process,energy transfers from the field to medium,known as nonlinear dissipative pro-cess.The rate of energy transfer can be described as[1]:d W d t =<E·P>=12ωIm(E·P)(7)E and P are the electricfield and the electric polar-ization vector,respectively.The bracket indicates a time average over several cycles of thefield.For the degenerate TPA process,the rate of energy absorbed in a medium can be written as:d Wd t=8πωn2c2I2Im(χ(3))(8)where I=EE∗nc/8π,n is the refractive index,c is the velocity of light.It can be clearly seen that the rate of absorption of energy is quadratically dependent on the excitation intensity,while this dependence is linear in one-photon absorption process.Thus,it is one criterion to check whether a process is TPA or not. The TPA properties of a molecule is often evaluated in a term of TPA cross-section:d n pd t=σ2NF2(9)where d n p/d t is the number of photons absorbed in unit time,σ2is TPA cross-section,and the unit commonly used is GM(1GM=10−50cm4s photon−1molecule−1),N is the number of absorbing molecules per unit volume,F=I/hνis the photon flux.As d W/d t=d n p/d t·hν(h is Planck constant), we can get:σ2=16π2hν2n2c2NIm(χ(3))(10)B.The study history of TPA in organicπ-conjugated moleculesAbout thirty years ago,the study of TPA was mainly focused on the inorganic crystals and semi-conductors.However,due to the limited number of inorganic materials,the magnitude of TPA response in most of inorganic materials is relatively weak and the response time is relatively long.After many or-ganic materials were successfully synthesized,NLO re-sponse of organic material arising from the delocal-ization ofπ-electrons was found much faster.What is more important is that the optical nonlinearity of these organic materials was found very large.Formed by covalent bonds,the organic materials have excel-lent manufacture processability,relatively high pho-todamage threshold,and high mechanical intensity. Moreover,the structure of organic molecules can be designed,modified and optimized conveniently with relatively low cost.Thus,organic materials have been intensively investigated in the decades.Practical applications of TPA materials benefit from the availability of materials with large TPA cross-section and high FL quantum yield,which can reduce the exposure time and lower the photonflux required to generate the two-photon effect.To fully explore the potential applications of TPA materials,138Wang Yao Chuan et al.:Recent progress on two-photon absorbing organic materialsintensive research efforts have been devoted to the fabrication of molecules with large TPA cross-section and fast response.Theoretical calculation and exper-imental investigation have been done to study the structure-dependent TPA property in organic ma-terial.These results have been used to guide the synthesis of new materials.Till now a huge num-ber of organic conjugated systems with quite large TPA cross-section have been reported,including dipo-lar molecules,quadrupolar,octupolar multibranched structure,macrocycle,polymer,and so on.[34,35] Organic molecules with TPA in visible and near in-frared(NIR)range didn’t attract much attention un-til the late1990s when the measured values of the TPA cross-section increased significantly.In1998, Perry and co-workers reported a series of quadrupo-lar molecules with large TPA cross-section value.[36] Prasad et al.calculated the TPA cross-sections of a series of symmetrically substituted distyrene deriva-tives,and the results indicated that the TPA cross-sections of modified distyrene derivatives(length-ening the conjugated length,substituting different pull/push electron groups,modifying and increasing the repeat unit)can be enhanced obviously.The electron donor(D)increased the delocalization of the electron cloud,and the charge redistribution inside the molecule after the photo-excitation would enhance the transition moment from S1to S2,the intensity of pull/push electron groups,conjugation length as well as the symmetry property of the molecule.These fac-tors will increase TPA cross-section.[37]In the earlier stage,the study on TPA materi-als was mainly focused on electron push/pull dipolar molecule systems,with the symmetrical or asymmet-rical connection of D and electron acceptor(A)via π-bridge,forming D-π-D,D-π-A and A-π-A struc-tures.In1998,Prasad’s group synthesized a series of asymmetrical molecules with D-π-A structure and systematically studied the effect of the planarity,the intensity of D/A group,conjugation length,length of the modified side-chain as well as the solvent envi-ronment on the TPA property.[37]Marder and Perry reported a series of molecules with D-π-D and A-π-A structure,and studied their TPA cross-sections.[7] In2000,Kim and co-workers compared a series of molecules with D-π-D and D-π-A structures,and the results indicated that the TPA cross-section of ante-rior molecule is much larger than the latter case.[38] Due to the good planarity and high FL quantum yield of thefluorene derivatives,Belfield et al.syn-thesized a series offluorene derivatives.[39∼43]Since then,many new push/pull or push/push systems were synthesized,utilizing anthracene,fluorene,oligoflu-orenes,diphenyl,dithienothiophene,fused aromatic rings and ethynylene.Effective intramolecular charge transfer(ICT)processes occur in these rge cross-section(the maximum is2600GM)and rel-ative high FL quantum yield were observed,while chromophores containing triple bonds displayed mod-est value of TPA cross-section.[38,44∼49]However,the TPA cross-sections of molecules with dipolar structure are still limited by the chromophore intensity,the de-localization of electron cloud,and the ICT property. The addition of strong D and A may further optimize their optical properties as a result of the molecular symmetry in conjugated system.It was proved by Albota et al.theoretically and experimentally that the quadrupolar molecules with D-π-A-π-D and A-π-D-π-A structures have excellent TPA properties.[36]Mtaka reported a series of2,1,3-benzothiadiazole-based red emitters with D-π-A-π-D structure,which show large TPA cross-sections and efficient red FL emissions.[50,51]Yang synthesized a series of D-π-A-π-D molecules with1,4-diketo-3,6-diphenylpyrrolo[3,4-c]pyrrole as the core,and the TPA cross-section was reported to be1200GM.[52]These results indicated that increasing the quantity of the ICT by modifying the molecule structure can effec-tively enhance TPA response.The concept of octupolar multibranched structure was initially proposed by Zyss.[53]Recently,it was found that building multibranched molecules with oc-tupolar or dendriform structure can greatly improve the TPA response.In1999,Prasad reported a series of multibranched TPA dyes synthesized by coupling two and three two-photon active asymmetric D-A chro-mophores linked together with a common amine group triphenylamine as the central core.The dyes showed large TPA cross-section(587GM)at wavelength of 796nm.[54]The results of the measured TPA cross-section clearly indicate a remarkable increase of the effective TPA cross-section with increase of the num-ber of chromophore moieties,which is not propor-tional to the number density(the relative ratio of TPA cross-sections is1:3.1:6.8).These results are surprising and impressive.The cooperative enhance-ment of TPA effect in multibranched structures leads to new design criteria for the development of highly efficient two-photon materials.There are many lit-eratures reporting the multibranched or dendriform structures.[55∼58]Moreover,the TPA cross-sections of multibranched chromophores were found to scale lin-early with the number of branches in some materi-als but exhibit a small enhancement as the molecu-lar size increases in other materials.Investigations on tri-branched molecules indicate that increasing the di-mensionality of dipolar molecules is a good approach to enhance the NLO response,as certain branched compounds exhibit enhanced TPA cross-sections over the linear counterparts.While it is believed that theWang Yao Chuan et al.:Recent progress on two-photon absorbing organic materials139cooperative interaction among individual branches is the major factor,while the effective electronic delocal-ization and the increased ICT are also possible reasons for the enhancement of TPA.Recently,Kenji reported that TPA response can be enhanced by confining the organic dyes in a two-dimensional nanoscale space.[59]The reason of the enhancement was ascribed to the low-dimension and confinement of intramolecular twisty movement. Some metal porphyrin derivatives also show intense TPA.[60]Porphyrins,phthalocyanines,and other re-lated large macrocycles are characterized by a frame-work of conjugatedπ-electrons which extends in two-dimensions,representing an alternative type of build-ing block for the study of NLO properties.There are several domestic groups in China re-searching TPA in organic materias.Jiang,Yu and Tao et al.synthesized naphthyridine salt derivations,and studied the application possibility in TPA polymeriza-tion and TPF imaging of the biology cells.[24,25]They also theoretically investigated the effect of symmetri-cal/asymmetric charge transfer pattern on the TPA properties of the molecules.The results indicated that different patterns have different polarization re-sponse,and the symmetrical pattern of charge trans-fer have larger charge distribution property,which is much helpful to increase the TPA cross-section of molecules.Tian’s group[61,62]and Wang’s group[63,64] also did interesting work in the TPAfield.In East China University of Science and Technology,Tian’s group synthesized a lot of tri-branched compounds as well as polymers,showing large TPA cross-sections and relative high FL quantum yields.[65∼69]Theoret-ical calculations of TPA process have been done in Feng’s group.[70∼74]In the organic conjugated materials,the relaxation dynamic behavior of excited states are associated to the NLO response.The ultrafast dynamics informa-tion can help to understand the physics of the light-matter interactions.Ultrafast dynamics of the organic conjugated system with intense TPA response has been a hot topic in the frontier.By using femtosecond (fs)ultrafast time-resolved spectroscopy techniques, such as transient absorption(TA)spectroscopy,time-resolvedfluorescence(TRFL),and three-pulse pho-ton echo peak shift method,the excited-state relax-ation properties and the interaction between branches of multibranched molecules after the excitation has been investigated.Goodson’s group has made some pioneering contributions in thefield.[75∼79]He and co-workers investigated several multibranched organic conjugated systems.They found an important role of fast delocalization of charge,depolarization,the ICT and cooperation enhancement effect between branches in the excited state relaxation processes.It was pro-posed that the time-resolved anisotropy will show fast decay if there was strong intramolecular interactions among branches.They also found that the dimer and trimer show a shorter depolarization time in com-parison with the monomer.Ji’s group in National University of Singapore investigated excited dynam-ics of organic conjugated systems.[80,81]Gong’s group in Peking University also did some research work in thisfield.[82,83]Our group in Fudan University investi-gated NLO properties and ultrafast dynamics of many organic conjugated systems,including polymer,dipo-lar,quadrupolar,multibranched structure,modified phthalocyanines,dithienylethene materials with pho-tochromic property.[84∼90]All these results have pro-vide great important insights for better understanding the TPA organic compounds.II.EXPERIMENTALSFs laser systems are widely used to study the TPA properties of the materials due to its significant fea-tures.High power output characteristic of the fs laser pulses make it to be the most suitable candidate to be used to measure the NLO properties of novel materi-als.Ultrashort duration of the fs laser pulses provides the opportunities to detect the dynamics of the ex-cited states in the fs or picosecond(ps)domain.Var-ious optical configurations and set-ups for measuring the TPA properties and the dynamics of the materi-als were proposed by using the fs laser system.This section introduces some widely used techniques.A.Nonlinear optical properties1.Z-scantechniqueFIG.3.Experimental setup for z-scanZ-scan technique was introduced by Sheik-Bahae M. in1989[91]and is now widely used for the measurement of NLO properties due to its outstanding advantages. As a single beam technique,it can be used to evaluate140Wang Yao Chuan et al.:Recent progress on two-photon absorbing organic materialsnonlinear refractive index and nonlinear absorption by using closed-and open-aperture configurations,re-spectively.Typical setup is shown in Figure3.The input intensity in the sample is adjusted by moving the sample along the focused laser beam path.De-tector D2is used to measure the NLO absorption by detecting the whole transmission laser power,which is known as open-aperture z-scan.In the another con-figuration,a small part of the laser beam can reach the detector(D1)by using an iris to block the rest. The second configuration is defined as close-aperture configuration that can be used to measure the NLO refractive index.The results of open-aperture z-scan are the intensity dependent transmissions,and can be used to value the TPA properties.The formulas used to calculate the TPA cross-section are shown below:T nonlin=ln(1+q(z))q(z)(11)q(z)=βL effI0(1−R)/(1+(z/z0)2)(12)σ2=β/N=hνβ/N0=103hνβ/N A C(13)Whereβis the NLO absorption coefficient,and N0 is the number density of the absorption centers,N A is the Avogadro constant and C represents the solute molar concentration.2.Two-photon excitedfluorescenceFor some materials,FL can be generated by the TPA process.The wavelength of the excitation beam is longer than that of the FL emission.The distinct property of this TPF is that the FL intensity shows a quadratic dependence on the excitation intensity of the excitation beam.The TPA cross-section can be measured by TPF technique which wasfirst reported by Xu and Webb,[92]and was further developed by Rebane.[93]To achieve this,TPF emission intensity is compared with that of a standard sample for which the TPA cross-section and the quantum yield are already known.In order to simplify the calculation,one photon quantum yield is used to instead of TPA quantum yield.One advantage of this technique is the prominent sensitivity that the TPA cross-section for the sam-ple with small TPA cross-section or low concentration can still be measured,when the change of the nonlin-ear transmission is too small to be detected by open-aperturez-scan.FIG.4.Experimental setup for TAB.Ultrafast dynamics1.Transient absorption techniqueTA can be used to study the ultrafast dynamics of the excited states,as shown in Figure4.The laser beam is divided into two beams with the power ratio around10:1(pump:probe).After passing through an optical-delay line(ODL),the modulated pump beam was focused on the sample,while the probe beam reached the same spot on the sample where the sam-ple was excited to a non-equilibrium state.After the sample,the probe beam was detected by either reflec-tion or transmission mode using a photodiode together with a lock-in amplifier.Temporal intervals between the pump beam and probe beam can be easily ad-justed through the movement of ODL.By sampling the probe intensity at the different delay times,the relaxation properties of excited states in a sample can be acquired,and the lifetime of the excited state can be calculated byfitting the experiment data with ap-propriate exponential equation.If the wavelengths of pump and probe beams are the same,this technique is called one-color pump-probe. In other case,the pump and probe beam can be set at different wavelengths.This two-color pump-probe technique would offer more information of the excited states.For further improving this technique,super-continuum can be employed as the probe beam,and a CCD equiped spectragraph can be applied as the detector.This measurement can record the signals within a broad spectral region,highly improving the experimental efficiency.2.Time-resolvedfluorescenceRegarding to the FL emission state,the decay prop-erties are quite important.The ultrashort FL life-time can be investigated by up-conversion or Kerr-gate technique in fs scale.Wang Yao Chuan et al.:Recent progress on two-photon absorbing organic materials 141FIG. 5.Experimental setup for the optical Kerr-gate methodIn the up-conversion process,the FL is studied by mixing it with an intense beam in a NLO crystal.The dynamics of FL can be investigated by measuring the temporal behavior of generated sum frequency signal. This method is very sensitive,and the main disad-vantage is that only the dynamics of one selected FL wavelength could be detected at one scan process. Figure5illustrates the schematic diagram of the optical Kerr-gate method,which is based on the op-tical Kerr effect.[85,94]By using a beam splitter,the fundamental beam is divided into two beams.The intense beam is used to pump the Kerr-material to generate photoinduced birefringence,while the sam-ple is excited by another beam to emitfluorescence. The polarization of the collected FL emission is set to be45◦with that of the pump beam by a polarizer. Both the polarized FL emission and the pump beam are focused onto the same point in the Kerr medium. After the Kerr-material,the polarization of the FL emission is changed due to the birefringence.Pass-ing through an analyzer with the cross polarization to the polarizer,TRFL signal is detected by a CCD or a photomultiplier combined with a monochromator.In this setup,the Kerr medium and two crossed polariz-ers act as a“gate”.The“gate”is triggered to open by the pump pulses within the temporal scale of ultrafast response time of the optical Kerr effect.III.STRUCTURE/PROPERTIES ANDULTRAF AST DYNAMICSA.Dipolar and quadrupolar molecules with strong TPF responseIn an early literature,a series of molecules with modified molecular structures exhibiting large TPA cross-sections were synthesized.[37]These samples were characterized in solution at wavelength of800 nm and pulse duration of8ns using a nonlinear trans-mission technique.[37]As shown in Figure6,there are two general organic structural types.Type I(D-π-A): When D and A groups are substituted at the oppo-site termini of aπ-bridge,the ICT process would lead to the appearance of a dipole moment in the ground and/or excited states,denoted as dipolar molecules. Type II(D-π-D,or A-π-A)is symmetrical in nature, consisting of aπelectron rich aromatic bridgeflanked on either side with a D or with an A group.These molecules are linearly conjugated chains with electron donating or withdrawing substituents arranged sym-metrically to the center of the molecule.With pre-served inversion center,the lowest order moment sup-ported by these molecules is ually, the values of the TPA cross-sections of these dipo-lar/quadrupolar dyes are about tens ofGM.FIG.6.Schematics of various linear chromophores clas-sified based on the substitution pattern.I:dipolar molecules;II:quadrupolar molecules(from reference[37]) FIG.7.Schematics of various linear chromophores fromreference[36]Bredas,Marder and Perry identified and articu-lated the design criteria for symmetrical structures based on the direct correlation between large changes in quadrupolar moment during photoexcitation.[36] As shown in Figure7,these quadrupolar molecules with alternating vinyl and1,4-arylene groups as the。