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 PHYSICS Vol.32 No.3 Jun. 2012 Recent progress on two-photon absorbing organic materials Wang Yao-Chuan1, Yan Yong-Li2, Li Bo3, Qian Shi-Xiong*4 1. Physics Department, Dalian Maritime University, Dalian, 116026, China 2. Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China 3. Key Laboratory of Polar Materials and Devices, Ministry of Education, East China Normal University, Shanghai 200241, China 4. Physics Department, Fudan University, Shanghai 200433, China In this article, we review recent studies on two-photon absorbing organic materials. We introduce the research progresses on the nonlinear optical properties of two-photon absorption (TPA) and two- photon excited fluorescence (TPF) in organic materials with different structures including dipolar, quadrupolar, and multibranched molecules, macrocycles, and polymers. Ultrafast dynamics in these materials, as characterized by transient absorption spectroscopy, is included for a better understanding of the physical mechanisms. Moreover, we summarize recent proposed applications of the two-photon absorbing organic materials. Key words: Two-photon absorption; Two-photon excited fluorescence; Ultrafast dynamics; Nonlinear optics; Organic materials CLC number : O437 Document Code : A CONTENTS I. Introduction 135 A. Theoretical background of TPA process 136 B. The study history of TPA in organic pi-conjugated molecules 137 II. Experimentals 139 A. Nonlinear optical properties 139 1. Z-scan technique 139 2. Two-photon excited fluorescence 140 B. Ultrafast dynamics 140 1. Transient absorption technique 140 2. Time-resolved fluorescence 140 III. Structure/Properties and Ultrafast dynamics 141 A. Dipolar and quadrupolar molecules with strong TPF response 141 B. Multibranched Structures 145 1. Triphenylamine 145 2. S-triazine 146 3. Alkylpyridinium 148 4. Tricyanobenzene 149 C. Porphyrins 150 1. Calculation 150 2. Substituent porphyrin 150 3. Porphyrin based polymers 152 D. Polymers 152 E. Macrocycles 155 IV. Applications 156 A. Unique advantages of TPA 156 B. TPF microscopy 157 C. Optical limiting 157 Received date: 2011-11-16 *sxqian@fudan.ac.cn D. Frequency-upconversion lasing 158 E. Microfabrication 159 F. Three-dimensional optical data storage 160 V. Conclusions 160 Acknowledgments 161 References 161 I. INTRODUCTION Two-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 Figure 1. The concept of the TPA process was theoretically proposed by Go¨ppert-Mayer in 1931. 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 light field, making the experimental demonstration very difficult at that time prior to the invention of laser. In 1960s, Kaiser 文章编号: 1000-0542(2012)03-0135-30 135 136 Wang Yao Chuan et al.: Recent progress on two-photon absorbing organic materials FIG. 1. Energy diagrams illustrating the processes of one- photon absorption and TPA and Garrett observed the blue fluorescence (FL) emis- sion from a CaF2:Eu2+ crystal excited by the focused Ruby laser pulses at a wavelength of 694.3 nm.[2] In 1963, Peticola and Rieckhoff observed 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 for flu- 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- trinsic fluorophores which absorb UV or visible light. With these advantages, TPA has been implanted for applications in optical limiting,[4∼9] two-photon ex- cited fluorescence imaging,[10∼15] two-photon poly- merization induced 3D 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 various field 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 Figure 2, the medium can usually be considered as bounded charged particles. Under an applied electric field, the positive charges and nega- tive charges would move to the opposite directions, respectively, and the electric field induces an electric polarization in the medium. The electric-dipole mo- ments caused by these small movements can be de- scribed as µind = −ex, where e is electron charge, and x is the displacement induced by electric field. FIG. 2. Oscillator model of the interaction between light and matter The overall polarization is µind = −Neex, where Ne is the electron density in the medium. Under a weak electric field, the polarization is linearly proportional to the electric field Pind(E) = χ(1)E, where χ(1) is the linear electric susceptibility. The incident light field is in fact a combination of electric and magnetic fields oscillating sinusoidally at optical frequency. Usually, 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 electric field can be described by a simple mechanical spring model, which is governed by the equation of motion for an oscillator. d2x dt2 + 2Γ dx dt + ω20x = − e m E(t) (1) Where x is the displacement from the mean position, ω0 is the resonance frequency, Γ is 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 materials 137 Considering a sinusoidal optical electric field, E(t) = E0 cos(ωt) = 1 2 E0 [ exp(−iωt)+exp(iωt)] (2) where ω is the optical frequency. Substituting equa- tion (2) into (1), the solution can be described as x = −eE0 m e−iωt ω20 − 2iΓω − ω2 + c.c. (3) c.c. is complex conjugate. Then the electric polariza- tion can be described as P = −Nex = Ne 2 m 1 ω20 − 2iΓω − ω2 E(ω)e−iωt + c.c. (4) If the applied field becomes intense enough, the equa- tion of motion should include anharmonic terms. Thus, the description of equation (4) must be mod- ified to d2x dt2 +2Γ dx dt +ω20x+(ax 2+bx3+ · · · ) = − e m E(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) where χ(2) and χ(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 electric field is compara- ble to the bound electric field inside an atom or a molecule. Under the excitation of intense field 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]: dW dt =< E · P >= 1 2 ωIm(E · P ) (7) E and P are the electric field and the electric polar- ization vector, respectively. The bracket indicates a time average over several cycles of the field. For the degenerate TPA process, the rate of energy absorbed in a medium can be written as: dW dt = 8piω n2c2 I2Im(χ(3)) (8) where I = EE∗nc/8pi, 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: dnp dt = σ2NF 2 (9) where dnp/dt is the number of photons absorbed in unit time, σ2 is TPA cross-section, and the unit commonly used is GM (1 GM=10−50 cm4s photon−1molecule−1), N is the number of absorbing molecules per unit volume, F = I/hν is the photon flux. As dW/dt = dnp/dt · hν (h is Planck constant), we can get: σ2 = 16pi2hν2 n2c2N Im(χ(3)) (10) B. The study history of TPA in organic pi-conjugated molecules About 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 of pi-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 photon flux required to generate the two-photon effect. To fully explore the potential applications of TPA materials, 138 Wang Yao Chuan et al.: Recent progress on two-photon absorbing organic materials intensive 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 late 1990s when the measured values of the TPA cross-section increased significantly. In 1998, 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 S1 to 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 pi-bridge, forming D-pi-D, D-pi-A and A-pi-A struc- tures. In 1998, Prasad’s group synthesized a series of asymmetrical molecules with D-pi-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-pi-D and A-pi- A structure, and studied their TPA cross-sections.[7] In 2000, Kim and co-workers compared a series of molecules with D-pi-D and D-pi-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 the fluorene derivatives, Belfield et al. syn- thesized a series of fluorene 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 systems. Large cross-section (the maximum is 2600 GM) 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-pi-A-pi-D and A-pi-D-pi-A structures have excellent TPA properties.[36] Mtaka reported a series of 2,1,3- benzothiadiazole-based red emitters with D-pi-A-pi-D structure, which show large TPA cross-sections and efficient red FL emissions.[50,51] Yang synthesized a series of D-pi-A-pi-D molecules with 1,4-diketo-3,6- diphenylpyrrolo[3,4-c]pyrrole as the core, and the TPA cross-section was reported to be 1200 GM.[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. In 1999, 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 (587 GM) at wavelength of 796 nm.[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 is 1:3.1:6.8). 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 the Wang Yao Chuan et al.: Recent progress on two-photon absorbing organic materials 139 cooperative 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 conjugated pi-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 TPA field. 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- resolved fluorescence (TRFL), and three-pulse pho- ton echo peak shift