- Email: [email protected]
Relaxation paths of charge transfer excitons
Journal of Luminescence 87}89 (2000) 121}125
Invited Paper
Relaxation paths of charge transfer excitons L. Valkunas *, V. Gulbinas , S. Jursenas Institute of Physics, A. Gostauto 12, Vilnius 2600, Lithuania IMSAR, Vilnius University, Naugarduko 24, Vilnius 2006, Lithuania
Abstract Relaxation dynamics of the charge transfer (CT) excitons in molecular solids is considered with respect to a competitive channel of the charge carrier photogeneration. Examples of the ultrafast charge separation from the nonrelaxed low-energy CT states in molecular crystals and in polymers are presented, providing a basis to assume that such a carrier photogeneration mechanism is a general feature of molecular systems with low-energy CT states. 2000 Elsevier Science B.V. All rights reserved. Keywords: Molecular solids; Charge transfer excitons; Photogeneration; Relaxation
1. Introduction Optically attainable charge transfer (CT) states usually play an essential role in most electronic processes initiated by excitation of various photosensitive polymers, organic "lms and crystals [1,2]. In addition to Frenkel excitons, which are responsible for the electronic optical transitions, the CT states are considered as being responsible for the photoconducting properties of the sensitized polymers [3] and molecular crystals [4,5]. Owing to their distinct photoconductivity, the sensitized polymers and molecular solids especially those, which are built up by polar organic molecules and contain well-pronounced electron donating and accepting moieties, seem to be promising materials for optoelectronics. However, the detailed mechanism of the relaxation paths of CT-excitations, such as the initial charge separation, self-trapping processes of the CT-excitons, etc., are still under debate especially in the light of recent observations obtained with fs/ps time resolution. According to the present understanding of the charge carrier photogeneration in organic photoconductors at least two steps can be distinguished in this complex process. Immediately after the light quantum absorption, Coulombically bound electron}hole pair characterized
* Corresponding author. Fax: 00370-2-617070. E-mail address: [email protected] (L. Valkunas)
by a particular distance of separation is generated [3,4,6]. This step is very fast, it proceeds on the subpicosecond time scale, while the second step, which describes the subsequent charge separation by overcoming the Coulomb barrier and/or the geminate recombination of the charges is much slower [1}3,7]. The latter process is quite adequately described by the Onsager theory, however, the initial step of the charge pair photogeneration is not completely clear yet. It is currently accepted that low-energy charge transfer (CT) states responsible for the longest wavelength absorption, play an essential role by initiating the "rst step of the separation process. However, the transfer time and the mean transfer distance related to this "rst step are not well de"ned and usually are considered as "tting parameters by describing subsequent events of the charge carrier generation. Excited states of polar molecular crystals built-up by #at dipolar organic molecules often have a well-expressed CT-state origin, therefore, such systems can be considered as good model systems to study the relaxation paths of the CT-excitations. Our recent investigations of the exciton states of N,N-dimethylaminobenzylidene 1,3indandione (DMABI) solids have shown that the lowest absorption bands correspond to intermolecular CT states that resulted from a well-expressed stacked crystal structure with alternating donor and acceptor fragments [8,9]. These CT-states result in an e$cient photoconductivity of the DMABI crystals and "lms bearing out essentially a similar symbatic relationship with the longest
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 2 4 4 - 6
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L. Valkunas et al. / Journal of Luminescence 87}89 (2000) 121}125
wavelength part of the absorption spectra [2,10,11]. Static dipole moments, which are inherent in the separate molecule, and a dimeric arrangement of the molecules in the stack of the crystal structure predetermine the formation of the localised exciton states caused by a strong exciton}phonon coupling (g"1.87 according to estimations for the DMABI crystals [8,9]). Thus, peculiarities of the absorption and luminescence spectra at elevated temperatures can be attributed to the coexistence of free and self-trapped excitons [8]. In this work, we discuss the relaxation paths of the CT states. A special emphasis is laid on the generation mechanism of the charge carriers in systems with optically active CT states. Our experimental data obtained in phthalocyanines, sensitised polymers and crystals of polar molecules create the basis for this discussion. It is noteworthy that the threshold of the photoconductivity coincides with the long-wavelength absorption band edge and, moreover, the photoconductuvity is almost wavelength independent at higher photon energies for all systems under consideration.
Fig. 1. Fluorescence intensity and electric photoresponse of Y-form TiOPc as a function of the delay time between two excitation pulses of 10 ps duration, 539 nm wavelength and 3 mJ/cm intensity.
2. Ultrafast charge separation Y-form of titanyl phthalocyanine (TiOPc) is a typical example where a very fast initial phase of the charge carrier generation from the excited state has been discovered [4]. It is worthwhile to mention that this form of phthalocyanine is one of the most photosensitive organic materials in the near-infrared region. Low-energy absorption band that appears in Y-form TiOPC is attributed to the states of a well-expressed CT character [12]. To study the dynamics of the charge separation in the "lm of TiOPc dispersed in polyvinylbutyral matrix a competition between this process and the exciton}exciton annihilation was considered. The dependences of the luminescence intensity and of the electric photoresponse were measured as a function of the separation time between two excitation pulses of 10 ps duration. The corresponding data are shown in Fig. 1. It is clearly seen that the luminescence intensity is reduced by the exciton}exciton annihilation if the excitation pulses were separated by a time shorter than the lifetime of the excited state, which was much longer than the pulse duration. However, the in#uence of the exciton}exciton annihilation on the electric photoresponce was enhanced only when the excitation pulses were overlapping in time. The di!erence in the results can be considered as an indication that the mean time of the charge separation is much shorter than the excited state lifetime. Moreover, this time is evidently much shorter than the duration of the excitation pulse (10 ps in our case). Investigations by means of femtosecond time-resolved pump} probe absorption of phthalocyanine solids revealed
Fig. 2. Modi"ed Noolandi}Hong}Popovic scheme of the charge carrier generation. CT* indicates a nonrelaxed CT state.
that equilibration of the excited state takes place on a subpicosecond time scale [13]. On the basis of these data the short charge carrier separation time can be related with this subpicosecond excited state dynamics. Fig. 2 shows the charge carrier photogeneration scheme, which summarizes our experimental results. This scheme is based on the Noolandi}Hong}Popovic model [14,15], however, being modi"ed by assuming that the carrier generation proceeds from the nonrelaxed CT state. Poly-N-epoxypropylcarbazole (PEPCz) sensitized with trinitro#uorenone (TNF) molecules is an example of another widely used and intensively investigated class of photoconductors [3,7]. Weak CT complexes of carbazolyl chromophores and the TNF molecules are responsible for the presence of the CT bands in the absorption spectrum at longer wavelengths than the absorption band of the TNF molecules and the carbazolyl
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3. Self-trapping of the CT excitons
Fig. 3. (a) Transient absorption kinetics stimulated by excitation at 527 nm and probed at j "800 nm of PEPCz "lm doped with 10 mol% TNF measured with a femtosecond time resolution for parallel (circles) and perpendicular (squares) polarizations of the excitation and probe light. (b) Time dependence of the transient anisotropy calculated from the polarized kinetics.
chromophores themselves. Since this system is photosensitive at excitation to the lowest energy CT band, and, thus, the excess energy of the excitation has no in#uence on the charge photogeneration, it was proposed that the charge separation occurs during the entire lifetime of the excited state [16]. However, our investigations showed that this is not the case. We used polarised pump}probe absorption spectroscopy to monitor the orientation of the cation radicals of the carbazolyl chromophores. Polarised excitation light was used to create the orientationally anisotropic distribution of the initial population of holes located on carbazolyls in the CT complexes. Using probe pulses polarized parallel and perpendicular to the polarization of the excitation light the time evolution of the created anisotropy can be monitored. Partial anisotropy decay was observed on the subpicosecond time scale (see Fig. 3), indicating that a fraction of the holes have changed their initial positions, which can be attributed to generation of the charge pairs with larger separation distances. The remaining anisotropy is caused by non-separated holes, which decay with a mean time constant of 150 ps corresponding to the lifetime of the CT state. Thus, the charge separation took place from the non-relaxed excited states, which is in accordance with the relaxation scheme presented in Fig. 2. This conclusion is supported by other experimental data of the geminate recombination, based on the modelling of the transient absorption and luminescence kinetics [7], which also cannot be understood without assumption of the distant initial charge separation.
The presence of static molecular dipole moments in the ground and excited states of the molecules evidently in#uences the strength of intermolecular interactions and enhances the exciton}phonon coupling. These interactions in polar crystals manifest themselves in the exciton spectral properties, which remarkably di!er from those of the molecules in solutions [9,17]. The steady-state absorption spectra of the DMABI "lm and that of the solution in n-heptane (10\ mol/l) as shown in Fig. 4 demonstrate the formation of the crystal-type bands F and C. These bands resemble the spectral dependence of the photocurrent quantum e$ciency. The transition dipole moments of the crystal-type bands are orientated parallel to the molecular stack, therefore, they can be related to the intermolecular CT-states [9]. It is worth noting that the crystal-type bands are situated more than 0.4 eV below the molecular band, which is a direct consequence of the polar nature of the DMABI molecules. Essential changes of the dipole moments of the molecules by their excitation and the extremely dipolar nature of the CT exciton itself cause the existence of strong intermolecular Coulomb forces, which create pronounced changes in the geometry of the molecular arrangement in the vicinity of the excited molecule. Such a strong local lattice deformation, leading to a strong exciton}phonon coupling, results in the self-trapping of the CT excitons. Luminescence spectrum of the DMABI "lm displays a typical picture of coexistence of the free and self-trapped excitons [8,9]. A broad Gaussian band with a signi"cant Stokes shift (0.43 eV) dominates in the luminescence spectra. A weak, narrow 0}0 optical transition at 2.187 eV in the high-energy wing of this band, being in resonance with the F absorption line, is distinguished. The potential energy scheme that can be
Fig. 4. Absorption spectra of the DMABI "lm, deposited in vacuum on a quartz substrate, and of the DMABI solution in n-heptane. Points show the spectral dependence of the photocurrent e$ciency measured for the DMABI "lm.
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applied by describing the exciton spectrum of the DMABI "lm is shown in Fig. 5. Here, two con"gurational coordinates are attributed to two di!erent selftrapped excitons corresponding to two bands which are seen in the emission spectra of the DMABI solids [9]. The shallow-trapped exciton band S is supposed to ! have originated from the dimers, formed by monomers in the stack, while band S! of the deeper-trapped excitons can be attributed to the dimers in the face-to-face packed molecules within the stacks [9]. The excitation kinetics for such systems (where the exciton}phonon interaction is strong) is, in general, rather complex. Initially, after the optical generation, the exciton may be delocalised and behaves like a wave spreading through the crystal. However, due to strong interaction with phonons, the exciton becomes localised and can be self-trapped in the potential well associated with the local lattice distortion induced by the exciton itself. In order to monitor the possible relaxation paths of the excitation, time evolution of the luminescence spectra after excitation of the DMABI "lm into the middle of the exciton band at 2.539 eV was studied with the fs time resolution (see Fig. 6) [18,19]. The bands of self-trapped excitons S and S were resolved already at the pres! ! ence of the excitation pulse. A very short rise time of the luminescence from the self-trapped exciton states (taking place within )100 fs) and the absence of the luminescence from the F band permit us to conclude that the self-trapping process is extremely fast. A rapid, highly non-exponential decay of the luminescence signal is observed after formation of the trapped CT exciton states. Two stages in the exciton relaxation kinetics is distinguished. The "rst stage, taking place on a subpicosecond time scale, is related to the "nal localisation of the trapped excitons. Such a "nal localisation is monitored by a rapid red-shift and a narrowing of the self-trapped exciton luminescence band (see Fig. 6). The
Fig. 5. The scheme of the adiabatic potential energy for the self-trapped excitons in DMABI. The possible relaxation routes are indicated by arrows.
Fig. 6. Time-resolved luminescence spectra of the DMABI "lm. Lines represent the two Gaussian band "ts.
red-shift (up to 45 meV) and the narrowing of the S! luminescence occur with the same time constant of 800 fs, which is close to that of the fast component of the luminescence decay (500 fs). This fast initial decay of the trapped exciton population may be related to the enhanced nonradiative relaxation of nonthermalised excitons. At the initial period of time, due to the fast rate of relaxation during the self-trapping a su$cient amount of the excess energy, which is almost equal to the selftrapping energy, is conserved in some strongly coupled vibrational and/or phonon modes creating their nonequilibrium (heated) distribution. Later, this non-equilibrium local distribution due to exchange of energy with other phonon modes results in their thermalization. This increase of the local temperature can stimulate the nonradiative relaxation via the crossing point of the S state ! and the ground state as shown in Fig. 5 as well as the charge generation similar to the recent suggestion developed for the conjugated polymers [20]. In addition to the excess energy-induced charge separation, another mechanism of this process has also to take place in this system as it follows from the symbatic correspondence of the photocurrent e$ciency with the long wavelength part of the absorption spectrum (see Fig. 4). Indeed, the charge separation is also induced from the F band, where the excess energy cannot be su$cient for stimulation of this process. The statement about di!erent relaxation paths from higher excited states and from the low states in the vicinity of the F absorption band evidently is also supported by the luminescence excitation spectrum [17]. Moreover, the charge separation has to compete with the relaxation rate in order to give a reasonable value of the quantum yield of the process in comparison with the self-trapping process. Thus, according to rough estimations this process has also to be very fast, giving hundreds of femtoseconds.
L. Valkunas et al. / Journal of Luminescence 87}89 (2000) 121}125
4. Concluding remarks The mechanism of the extremely fast charge separation is not developed yet. In the case of molecular crystals (such as DMABI) a suggestion of delocalized states of free charges in the vicinity of the F band transition (by assuming the adiabatic band gap to be close to the transition energy of the F band) can be used for explanation. However, this is not the case on the carbazolylcontaining systems, where the delocalized free charge states are absent and the charge mobility is determined by the hopping mechanism between localised states. It is worth noting that ultrafast charge separation was also observed in photosynthetic reaction centres. This process is also too fast to be explained by the Marcus charge transfer theory. Thus, ultrafast charge separation seems to be a general feature of molecular systems and the general mechanism applicable to all systems has to be developed. Phonon modes of the surrounding material excited by creating CT states probably play an important role in this process.
Acknowledgements This research was partially supported by the Lithuanian State Foundation of Science and Studies.
References [1] M. Pope, C.E. Swenberg, Electron Processes in Organic Crystals, Clarendon Press, Oxford, 1982. [2] E.A. Silinsh, V. Capek, Organic Molecular Crystals, Interaction, Localization, and Transport Phenomena, AIP Press, New York, 1994.
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[3] A. Ruseckas, V. Gulbinas, V. SundstroK m, A. Undzenas, L. Valkunas, J. Phys. Chem. B 102 (1998) 7365. [4] V. Gulbinas, R. Jakubenas, S. Pakalnis, A. Undzenas, J. Chem. Phys. 107 (1997) 4927. [5] E.A. Silinsh, H. Inokuchi, Chem. Phys. 149 (1991) 373. [6] L.J. Rothberg, M. Yan, A.W.P. Fung, T.M. Jedju, E.W. Kwock, M.E. Galvin, Synth. Met. 84 (1997) 537. [7] D. Abramavicius, V. Gulbinas, A. Ruseckas, A. Undzenas, L. Valkunas, J. Phys. Chem. B 111 (1999) 5611. [8] S. Jursenas, A. Gruodis, G. Kodis, L. Valkunas, Adv. Mater. Opt. Electron. 6 (1996) 387. [9] S. Jursenas, A. Gruodis, G. Kodis, M. Chachisvilis, V. Gulbinas, E.A. Silinsh, L. Valkunas, J. Phys. Chem. B 102 (1998) 1086. [10] N.A. Dimond, T.K. Mukherjee, Discuss Faraday Soc. 51 (1971) 102. [11] N.S. Magomedova, O.V. Kolninov, E.G. Rukhadze, Z.V. Zvonkova, Zh. Fiz. Khim. 49 (1975) 1322. [12] T. Saito, W. Sisk, T. Kobayashi, S. Suzuki, T. Iwayanagi, J. Phys. Chem. 97 (1993) 8026. [13] V. Gulbinas, M. Chachisvilis, L. Valkunas, V. SundstroK m, J. Phys. Chem. 100 (1996) 2213. [14] J. Noolandi, K.M. Hong, J. Chem. Phys. 70 (1979) 3230. [15] Z.D. Popovic, J. Chem. Phys. 78 (1983) 1552. [16] H. Miyasaka, T. Moriyama, S. Kotani, R. Muneyasu, A. Itaya, Chem. Phys. Lett. 225 (1994) 315. [17] S. Jursenas, V. Gulbinas, A. Gruodis, K. Gerdenis, V. Kovalevskij, L. Valkunas, Phys. Chem. Chem. Phys. 1 (1999) 1715. [18] S. Jursenas, V. Gulbinas, T. Gustavsson, J.-C. Mialocq, L. Valkunas, Lith. J. Phys. 38 (1998) 53. [19] S. Jursenas, V. Gulbinas, Z. Kuprionis, R. Kananavicius, G. Kodis, T. Gustavsson, J.-C. Mialocq, L. Valkunas, Synth. Met. 7044 (1999) in press. [20] V.I. Arkhipov, E.V. Emelianova, H. BaK ssler, Phys. Rev. Lett. 82 (1999) 1321.
Invited Paper
Relaxation paths of charge transfer excitons L. Valkunas *, V. Gulbinas , S. Jursenas Institute of Physics, A. Gostauto 12, Vilnius 2600, Lithuania IMSAR, Vilnius University, Naugarduko 24, Vilnius 2006, Lithuania
Abstract Relaxation dynamics of the charge transfer (CT) excitons in molecular solids is considered with respect to a competitive channel of the charge carrier photogeneration. Examples of the ultrafast charge separation from the nonrelaxed low-energy CT states in molecular crystals and in polymers are presented, providing a basis to assume that such a carrier photogeneration mechanism is a general feature of molecular systems with low-energy CT states. 2000 Elsevier Science B.V. All rights reserved. Keywords: Molecular solids; Charge transfer excitons; Photogeneration; Relaxation
1. Introduction Optically attainable charge transfer (CT) states usually play an essential role in most electronic processes initiated by excitation of various photosensitive polymers, organic "lms and crystals [1,2]. In addition to Frenkel excitons, which are responsible for the electronic optical transitions, the CT states are considered as being responsible for the photoconducting properties of the sensitized polymers [3] and molecular crystals [4,5]. Owing to their distinct photoconductivity, the sensitized polymers and molecular solids especially those, which are built up by polar organic molecules and contain well-pronounced electron donating and accepting moieties, seem to be promising materials for optoelectronics. However, the detailed mechanism of the relaxation paths of CT-excitations, such as the initial charge separation, self-trapping processes of the CT-excitons, etc., are still under debate especially in the light of recent observations obtained with fs/ps time resolution. According to the present understanding of the charge carrier photogeneration in organic photoconductors at least two steps can be distinguished in this complex process. Immediately after the light quantum absorption, Coulombically bound electron}hole pair characterized
* Corresponding author. Fax: 00370-2-617070. E-mail address: [email protected] (L. Valkunas)
by a particular distance of separation is generated [3,4,6]. This step is very fast, it proceeds on the subpicosecond time scale, while the second step, which describes the subsequent charge separation by overcoming the Coulomb barrier and/or the geminate recombination of the charges is much slower [1}3,7]. The latter process is quite adequately described by the Onsager theory, however, the initial step of the charge pair photogeneration is not completely clear yet. It is currently accepted that low-energy charge transfer (CT) states responsible for the longest wavelength absorption, play an essential role by initiating the "rst step of the separation process. However, the transfer time and the mean transfer distance related to this "rst step are not well de"ned and usually are considered as "tting parameters by describing subsequent events of the charge carrier generation. Excited states of polar molecular crystals built-up by #at dipolar organic molecules often have a well-expressed CT-state origin, therefore, such systems can be considered as good model systems to study the relaxation paths of the CT-excitations. Our recent investigations of the exciton states of N,N-dimethylaminobenzylidene 1,3indandione (DMABI) solids have shown that the lowest absorption bands correspond to intermolecular CT states that resulted from a well-expressed stacked crystal structure with alternating donor and acceptor fragments [8,9]. These CT-states result in an e$cient photoconductivity of the DMABI crystals and "lms bearing out essentially a similar symbatic relationship with the longest
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 2 4 4 - 6
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wavelength part of the absorption spectra [2,10,11]. Static dipole moments, which are inherent in the separate molecule, and a dimeric arrangement of the molecules in the stack of the crystal structure predetermine the formation of the localised exciton states caused by a strong exciton}phonon coupling (g"1.87 according to estimations for the DMABI crystals [8,9]). Thus, peculiarities of the absorption and luminescence spectra at elevated temperatures can be attributed to the coexistence of free and self-trapped excitons [8]. In this work, we discuss the relaxation paths of the CT states. A special emphasis is laid on the generation mechanism of the charge carriers in systems with optically active CT states. Our experimental data obtained in phthalocyanines, sensitised polymers and crystals of polar molecules create the basis for this discussion. It is noteworthy that the threshold of the photoconductivity coincides with the long-wavelength absorption band edge and, moreover, the photoconductuvity is almost wavelength independent at higher photon energies for all systems under consideration.
Fig. 1. Fluorescence intensity and electric photoresponse of Y-form TiOPc as a function of the delay time between two excitation pulses of 10 ps duration, 539 nm wavelength and 3 mJ/cm intensity.
2. Ultrafast charge separation Y-form of titanyl phthalocyanine (TiOPc) is a typical example where a very fast initial phase of the charge carrier generation from the excited state has been discovered [4]. It is worthwhile to mention that this form of phthalocyanine is one of the most photosensitive organic materials in the near-infrared region. Low-energy absorption band that appears in Y-form TiOPC is attributed to the states of a well-expressed CT character [12]. To study the dynamics of the charge separation in the "lm of TiOPc dispersed in polyvinylbutyral matrix a competition between this process and the exciton}exciton annihilation was considered. The dependences of the luminescence intensity and of the electric photoresponse were measured as a function of the separation time between two excitation pulses of 10 ps duration. The corresponding data are shown in Fig. 1. It is clearly seen that the luminescence intensity is reduced by the exciton}exciton annihilation if the excitation pulses were separated by a time shorter than the lifetime of the excited state, which was much longer than the pulse duration. However, the in#uence of the exciton}exciton annihilation on the electric photoresponce was enhanced only when the excitation pulses were overlapping in time. The di!erence in the results can be considered as an indication that the mean time of the charge separation is much shorter than the excited state lifetime. Moreover, this time is evidently much shorter than the duration of the excitation pulse (10 ps in our case). Investigations by means of femtosecond time-resolved pump} probe absorption of phthalocyanine solids revealed
Fig. 2. Modi"ed Noolandi}Hong}Popovic scheme of the charge carrier generation. CT* indicates a nonrelaxed CT state.
that equilibration of the excited state takes place on a subpicosecond time scale [13]. On the basis of these data the short charge carrier separation time can be related with this subpicosecond excited state dynamics. Fig. 2 shows the charge carrier photogeneration scheme, which summarizes our experimental results. This scheme is based on the Noolandi}Hong}Popovic model [14,15], however, being modi"ed by assuming that the carrier generation proceeds from the nonrelaxed CT state. Poly-N-epoxypropylcarbazole (PEPCz) sensitized with trinitro#uorenone (TNF) molecules is an example of another widely used and intensively investigated class of photoconductors [3,7]. Weak CT complexes of carbazolyl chromophores and the TNF molecules are responsible for the presence of the CT bands in the absorption spectrum at longer wavelengths than the absorption band of the TNF molecules and the carbazolyl
L. Valkunas et al. / Journal of Luminescence 87}89 (2000) 121}125
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3. Self-trapping of the CT excitons
Fig. 3. (a) Transient absorption kinetics stimulated by excitation at 527 nm and probed at j "800 nm of PEPCz "lm doped with 10 mol% TNF measured with a femtosecond time resolution for parallel (circles) and perpendicular (squares) polarizations of the excitation and probe light. (b) Time dependence of the transient anisotropy calculated from the polarized kinetics.
chromophores themselves. Since this system is photosensitive at excitation to the lowest energy CT band, and, thus, the excess energy of the excitation has no in#uence on the charge photogeneration, it was proposed that the charge separation occurs during the entire lifetime of the excited state [16]. However, our investigations showed that this is not the case. We used polarised pump}probe absorption spectroscopy to monitor the orientation of the cation radicals of the carbazolyl chromophores. Polarised excitation light was used to create the orientationally anisotropic distribution of the initial population of holes located on carbazolyls in the CT complexes. Using probe pulses polarized parallel and perpendicular to the polarization of the excitation light the time evolution of the created anisotropy can be monitored. Partial anisotropy decay was observed on the subpicosecond time scale (see Fig. 3), indicating that a fraction of the holes have changed their initial positions, which can be attributed to generation of the charge pairs with larger separation distances. The remaining anisotropy is caused by non-separated holes, which decay with a mean time constant of 150 ps corresponding to the lifetime of the CT state. Thus, the charge separation took place from the non-relaxed excited states, which is in accordance with the relaxation scheme presented in Fig. 2. This conclusion is supported by other experimental data of the geminate recombination, based on the modelling of the transient absorption and luminescence kinetics [7], which also cannot be understood without assumption of the distant initial charge separation.
The presence of static molecular dipole moments in the ground and excited states of the molecules evidently in#uences the strength of intermolecular interactions and enhances the exciton}phonon coupling. These interactions in polar crystals manifest themselves in the exciton spectral properties, which remarkably di!er from those of the molecules in solutions [9,17]. The steady-state absorption spectra of the DMABI "lm and that of the solution in n-heptane (10\ mol/l) as shown in Fig. 4 demonstrate the formation of the crystal-type bands F and C. These bands resemble the spectral dependence of the photocurrent quantum e$ciency. The transition dipole moments of the crystal-type bands are orientated parallel to the molecular stack, therefore, they can be related to the intermolecular CT-states [9]. It is worth noting that the crystal-type bands are situated more than 0.4 eV below the molecular band, which is a direct consequence of the polar nature of the DMABI molecules. Essential changes of the dipole moments of the molecules by their excitation and the extremely dipolar nature of the CT exciton itself cause the existence of strong intermolecular Coulomb forces, which create pronounced changes in the geometry of the molecular arrangement in the vicinity of the excited molecule. Such a strong local lattice deformation, leading to a strong exciton}phonon coupling, results in the self-trapping of the CT excitons. Luminescence spectrum of the DMABI "lm displays a typical picture of coexistence of the free and self-trapped excitons [8,9]. A broad Gaussian band with a signi"cant Stokes shift (0.43 eV) dominates in the luminescence spectra. A weak, narrow 0}0 optical transition at 2.187 eV in the high-energy wing of this band, being in resonance with the F absorption line, is distinguished. The potential energy scheme that can be
Fig. 4. Absorption spectra of the DMABI "lm, deposited in vacuum on a quartz substrate, and of the DMABI solution in n-heptane. Points show the spectral dependence of the photocurrent e$ciency measured for the DMABI "lm.
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applied by describing the exciton spectrum of the DMABI "lm is shown in Fig. 5. Here, two con"gurational coordinates are attributed to two di!erent selftrapped excitons corresponding to two bands which are seen in the emission spectra of the DMABI solids [9]. The shallow-trapped exciton band S is supposed to ! have originated from the dimers, formed by monomers in the stack, while band S! of the deeper-trapped excitons can be attributed to the dimers in the face-to-face packed molecules within the stacks [9]. The excitation kinetics for such systems (where the exciton}phonon interaction is strong) is, in general, rather complex. Initially, after the optical generation, the exciton may be delocalised and behaves like a wave spreading through the crystal. However, due to strong interaction with phonons, the exciton becomes localised and can be self-trapped in the potential well associated with the local lattice distortion induced by the exciton itself. In order to monitor the possible relaxation paths of the excitation, time evolution of the luminescence spectra after excitation of the DMABI "lm into the middle of the exciton band at 2.539 eV was studied with the fs time resolution (see Fig. 6) [18,19]. The bands of self-trapped excitons S and S were resolved already at the pres! ! ence of the excitation pulse. A very short rise time of the luminescence from the self-trapped exciton states (taking place within )100 fs) and the absence of the luminescence from the F band permit us to conclude that the self-trapping process is extremely fast. A rapid, highly non-exponential decay of the luminescence signal is observed after formation of the trapped CT exciton states. Two stages in the exciton relaxation kinetics is distinguished. The "rst stage, taking place on a subpicosecond time scale, is related to the "nal localisation of the trapped excitons. Such a "nal localisation is monitored by a rapid red-shift and a narrowing of the self-trapped exciton luminescence band (see Fig. 6). The
Fig. 5. The scheme of the adiabatic potential energy for the self-trapped excitons in DMABI. The possible relaxation routes are indicated by arrows.
Fig. 6. Time-resolved luminescence spectra of the DMABI "lm. Lines represent the two Gaussian band "ts.
red-shift (up to 45 meV) and the narrowing of the S! luminescence occur with the same time constant of 800 fs, which is close to that of the fast component of the luminescence decay (500 fs). This fast initial decay of the trapped exciton population may be related to the enhanced nonradiative relaxation of nonthermalised excitons. At the initial period of time, due to the fast rate of relaxation during the self-trapping a su$cient amount of the excess energy, which is almost equal to the selftrapping energy, is conserved in some strongly coupled vibrational and/or phonon modes creating their nonequilibrium (heated) distribution. Later, this non-equilibrium local distribution due to exchange of energy with other phonon modes results in their thermalization. This increase of the local temperature can stimulate the nonradiative relaxation via the crossing point of the S state ! and the ground state as shown in Fig. 5 as well as the charge generation similar to the recent suggestion developed for the conjugated polymers [20]. In addition to the excess energy-induced charge separation, another mechanism of this process has also to take place in this system as it follows from the symbatic correspondence of the photocurrent e$ciency with the long wavelength part of the absorption spectrum (see Fig. 4). Indeed, the charge separation is also induced from the F band, where the excess energy cannot be su$cient for stimulation of this process. The statement about di!erent relaxation paths from higher excited states and from the low states in the vicinity of the F absorption band evidently is also supported by the luminescence excitation spectrum [17]. Moreover, the charge separation has to compete with the relaxation rate in order to give a reasonable value of the quantum yield of the process in comparison with the self-trapping process. Thus, according to rough estimations this process has also to be very fast, giving hundreds of femtoseconds.
L. Valkunas et al. / Journal of Luminescence 87}89 (2000) 121}125
4. Concluding remarks The mechanism of the extremely fast charge separation is not developed yet. In the case of molecular crystals (such as DMABI) a suggestion of delocalized states of free charges in the vicinity of the F band transition (by assuming the adiabatic band gap to be close to the transition energy of the F band) can be used for explanation. However, this is not the case on the carbazolylcontaining systems, where the delocalized free charge states are absent and the charge mobility is determined by the hopping mechanism between localised states. It is worth noting that ultrafast charge separation was also observed in photosynthetic reaction centres. This process is also too fast to be explained by the Marcus charge transfer theory. Thus, ultrafast charge separation seems to be a general feature of molecular systems and the general mechanism applicable to all systems has to be developed. Phonon modes of the surrounding material excited by creating CT states probably play an important role in this process.
Acknowledgements This research was partially supported by the Lithuanian State Foundation of Science and Studies.
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