- Email: [email protected]
Transient spectroscopy of Frenkel excitons in α-hexathiophene single crystals
25 August 2000
Chemical Physics Letters 326 Ž2000. 558–566 www.elsevier.nlrlocatercplett
Transient spectroscopy of Frenkel excitons in a-hexathiophene single crystals S.V. Frolov ) , Ch. Kloc, S. Berg, G.A. Thomas, B. Batlogg Bell Laboratories, Lucent Technologies, 600 Mountain AÕenue, Murray Hill, NJ 07974, USA Received 15 February 2000; in final form 11 July 2000
Abstract Photoexcitation dynamics in pure single molecular crystals of a-hexathiophene are studied using ultrafast photomodulation spectroscopy in the spectral range from 0.15 to 2.3 eV. Frenkel excitons are uniquely described by transient photoinduced absorption and stimulated emission bands at 1.3 and ; 2 eV, respectively. The transient spectra are interpreted using exciton states identified by one- and two-photon absorption. Exciton dynamics in pure crystals dramatically differs from that in polycrystalline films, which is attributed to a much lower defect density in single crystals. Intramolecular internal conversion rate is found to be ; 150 fs for excitation energies up to 3.6 eV. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Recent use of conjugated organic materials in light-emitting devices w1,2x and thin-film transistors w3–5x has stimulated new studies of their electronic structure and basic optical properties. Particular attention has been given to oligothiophenes w6x, due to their relatively high charge carrier mobilites w7,8x and ability to form large single crystals w9,10x. Scrupulous measurements of absorption and emission spectra w11,12x have provided fruitful insights into the electronic structure of a-hexathiophene Ž a6T. crystals. However, non-linear optical measurements and, in particular, transient ultrafast spectroscopy have not been thoroughly used to study these or any other molecular crystals. Ultrafast dynamics in polycrys) Corresponding author. Fax: q1-908-582-3260; e-mail: [email protected]
talline a6T films, on the other hand, have been studied in depth w13–15x. In this study we show that ultrafast photophysics of pure single crystals significantly differs from that of polycrystalline films and reveals processes associated with intrinsic electronic relaxation in organic semiconductors. It is well established that the lowest excited states of most molecular crystals are adequately described by Frenkel excitons and characterized by localized wavefunctions with negligible overlap between individual molecules w16,17x. Although the theory of molecular excitons has been developed rather extensively w16,18,19x, previous experiments have mostly concentrated on doped molecular crystals w20x, due to the lack of high purity materials and the apparatus for ultrafast measurements. In doped crystals, however, the dynamics of host excitons are obscured by exciton trapping on guest molecules. Only in pure crystals one may observe intrinsic exciton dynamics,
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 8 2 3 - X
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including intra- and intermolecular phonon-assisted relaxation, intermolecular charge transfer, exciton migration, trapping at defects, self-localization due to lattice relaxation, radiative recombination and intersystem crossing w17x. Learning what roles each of these processes play in the excited state relaxation is necessary for understanding the nature of electronic states in molecular crystals, which in turn has been the subject of great interest and debate in the past 30 years w17,21x. We study optical emission and absorption of a6T single crystals using cw absorption and emission spectroscopies, and femto- and picosecond photoexcitation dynamics using ultrafast transient photomodulation ŽPM. spectroscopy. Whereas steady state photoluminescence ŽPL. w22,23x generally contains contributions from traps and impurities, psec transient PM spectra are less affected by the latter and may exclusively characterize the lowest excitons. We measure the transient spectrum and dynamics of optical gain due to Frenkel excitons, which previously has been shown to result in pronounced stimulated emission at high excitation intensities w24,25x. We find that primary excitations in a6T crystals, unlike those in films w13x, are characterized by a single narrow photoinduced absorption band at 1.25–1.30 eV. Using this band as a unique probe of Frenkel excitons, we find that the lowest excitons are produced within the first 150 fs following excitation with photon energies up to 3.6 eV. The subsequent relaxation is determined by radiative recombination, exciton traping and phonon-assisted relaxation. Exciton trapping at deep traps is found insignificant on the picosecond time scale due to the low trap density in pure single crystals. We also show that internal conversion is the primary relaxation channel between different excited states.
2. Experimental As described elsewhere w10x, a6T single crystals are grown in the form of free-standing platelets with thickness from 1 to 10 mm. A pump-and-probe technique is used to obtain time-resolved PM spectra by measuring DT Ž t .rT versus probe photon energy " v , where T is the probe transmission, DT is the change in T due to the pump pulse and t is the time
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delay between the pump and probe pulses. Photoinduced absorption ŽPA. leads to negative DT, whereas probe-induced stimulated emission ŽSE. result in positive DT. Two electronically synchronized Ti:Sapphire mode-locked lasers ŽSpectra-Physics., one of which pumps a tunable optical parametric oscillator, are used to produce 100 fs pump and probe pulses. Photoexcitation density and DTrT are below 2 = 10 16 cmy3 and 10y4 , respectively. All measurements are done at room temperature in air or flowing N2 ; in either case we do not observe any signal degradation due to photo-oxidation.
3. Results and discussion Previously, it has been shown that a6T molecules crystallize in two similar crystallographic phases: the low ŽLT. and high ŽHT. temperature phases w10,26x. Both have been thoroughly studied w27x and found to be herringbone structures with monoclinic unit cells containing four a6T rigid rod-like molecules. The latter lie in the plane formed by the a and c crystal axes; the b crystal axis is thus almost perpendicular to the long axes Ž L. of a6T molecules. Since the crystal growth direction corresponds to the a axis, one can readily measure polarized absorption for light polarizations parallel to b and c crystal axes, respectively. Each molecular excited state of a single a6T molecule is split into four components w11x and corresponding exciton states can be classified according to the irreducible representations of a crystal unit cell group ŽFig. 1a, inset. w16x. The unit cell group of the a6T crystal is C 2h and the corresponding representations are a g , a u , bg and b u w6x. The same excited state classification holds for the molecular excited states ŽA g , A u , B g and B u ., since the symmetry group of a6T single molecules is also C 2h w6x. The site symmetry in the crystal is C 1 w6x, therefore A g , A u , B g and B u molecular states are somewhat intermixed. One-photon transitions in a system with the inversion center are allowed only between different parity states, e.g., a g and b u . In contrast, two-photon transitions occur only between same parity states, e.g., a g and bg . Absorption spectra shown in Fig. 1a can be used to describe a u states with weak b-polarized, and b u states with strong ac-polarized transitions
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Fig. 1. Ža. Polarized absorption spectra of a6T single crystals in HT Žsolid line. and LT Žbroken line. phases Ž b and c are the crystal axes.; the arrow indicates the nA g state. The inset shows energy levels in the a6T single molecule and crystal; vibronic sublevels are not shown. Žb. PL spectra of a6T single HT Žsolid line. and LT Žbroken line. crystals Ž0 through III indicate vibronic sidebands.; the SE spectrum Žopen circles. at t s 5 ps is given for comparison Žsee text..
from the ground state. The lowest absorption edge at 2.3 eV corresponds to the a u state, whereas the second edge at 2.6 eV – to the b u state w10x. These are believed to be the two Davidov’s components of
the B u state, which has the transition dipole moment oriented along the L axis w11,12x. However, the spectra are complicated by additional transitions due to strong electron–phonon coupling, which mixes
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the vibronic sidebands of the a u and b u states w11,16x. The c-polarized absorbance spectrum saturates Ždue to the crystal thickness. at " v ) 2.6 eV, therefore only b-polarized transitions at 3.6 and 4.0 eV can be identified. We first use photoluminescence ŽPL. to study the lowest excited states. Fig. 1b shows the PL spectra obtained from the excited side of the crystals. Vibronic sidebands I–III, corresponding to optical transitions with simultaneous phonon emission Ž0 n , where n is the number of optical phonons., seemingly do not obey the Franck–Condon coupling mechanism generally used to describe dipole-allowed transitions w6x. Instead of an expected strong 0 0 PL sideband, only a weak shoulder is observed at the PL origin Ž " v f 2.3 eV.. This suggests that the a u exciton is indeed the lowest excited state, since the radiative transition from the latter is only weakly allowed. However, the a u state may borrow oscillator strength from the b u state via intermolecular vibrational mixing; as a result, its vibronic sidebands appear in the PL spectrum as strong ac-polarized transitions w11,16x. Furthermore, PL sidebands I, II and III in Fig. 1b are not equidistant
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Ž0.15 vs. 0.18 eV., indicating that perhaps more than one excited state contributes to the observed PL. Such a behavior has indeed been observed in previous studies of a6T films and crystals w22x, which revealed complex behavior of the PL spectrum at low temperatures and identified several shallow X traps and aggregate states w23x. To further understand the nature of excited states in a6T crystals, we measure their transient PM spectra shown in Fig. 2 for t s 5 ps. The crystals are excited at 2.58 eV; the pump and probe polarizations are parallel to the c axis. A pronounced short-lived PA band is observed at " v s 1.25 eV Ž1.3 eV. for the LT ŽHT. crystal phase. In a recent study of 6T films w28x, a PA band at " v s 1.3 eV has also been observed, but attributed to disordered or amorphous parts of the films. Fig. 3a compares the PA decay for probe polarizations parallel to b and c axes and shows that PA at " v s 1.3 eV is c-polarized. The probe pulse experiences net optical gain Žpositive DT . and consequently SE is observed at " v ) 1.85 eV, as shown in Fig. 2. Transient PA occurs simultaneously with SE and thus lowers the net gain experienced by the probe. This PA is obvious in case
Fig. 2. Transient PM spectra of the LT Žopen circles. and HT Žsolid squares. a6T single crystals at t s 5 ps; the LT spectrum is offset for clarity.
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Fig. 3. Ža. DTrT decay at " v s 1.3 eV in the LT crystal for the probe polarization parallel to c Žsolid line. and b Žbroken line.; the inset shows the rise of the PA signal Žrrc . at " v s 1.3 eV. Žb. DTrT decay at " v s 2.1 eV in the HT a6T single crystal for the probe polarization rrc Žsolid line. and rrb Žbroken line.; the inset shows the SE onset Žrrc . at " v s 2.1 eV.
of the LT crystal, where the SE component is weaker. SE is c-polarized and its decay is well correlated with the PA decay, as shown in Fig. 3b Žhere the PA contribution is evident for the b-polarized probe.. Fig. 1b compares the SE spectrum of the HT a6T crystal with the corresponding PL spectrum Žthe SE spectrum is arbitrarily offset to compensate for the
transient PA contribution., indicating that SE and PL share the same origin, i.e. the a u exciton at EŽa u . s 2.3 eV. Since the psec dynamics of SE and PA are well correlated, we can attribute the whole PM spectrum at t s 5 ps to the a u exciton. PA onset is instantaneous ŽFig. 3a, inset. and limited only by the experimental resolution Ž150 fs.,
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suggesting an ultrafast electronic relaxation from the initially excited state Žat " v s 2.6–3.2 eV. to the lowest a u exciton. However, SE onset ŽFig. 3b, inset. is slower than that of PA and occurs in about 0.5 ps at excitation energy " v s 3.2 eV. We attribute this discrepancy to exciton thermalization within the vibrational manifold of the a u exciton after initial ultrafast relaxation. Alternatively, primary excited states might first decay into the dipole-forbidden a g state, which then relaxes into the a u state; however, this is an unlikely scenario, because of the near-degeneracy between the a g and a u excitons. The ultrafast relaxation to the lowest excited state Žin our case the a u exciton. is common for many complex molecular systems w29x. The majority of these systems obey Vavilov–Kasha’s rule; w30x, the PL excitation spectrum of a6T crystals is practically flat between " v s 2.4 and 3.2 eV. The non-radiative relaxation between excited states of the same spin multiplicity Žsinglets in our case. has been termed internal conversion w30x. Internal conversion in single molecules containing many closely spaced energy levels is the fastest relaxation channel for most excited states, except perhaps the lowest one. This situation is apparently preserved in crystals, where intermolecular interactions are weak and, subsequently, the ultrafast dynamics are determined by intramolecular interactions. The PM spectra do not change significantly from 1 ps to 0.5 ns, which suggests that after initial rapid relaxation the nature of excited states does not change on the picosecond time scale. The lowest excitons have several relaxation pathways, including radiative recombination, internal conversion, intersystem crossing into the triplet manifold, exciton diffusion with subsequent trapping at defects and impurities. Bimolecular Auger-type processes become important only at high excitation densities Ž; 10 18 cmy3 .. Neither radiative or nonradiative transitions to the ground state affect the PM spectrum shape. The intersystem crossing and exciton trapping, on the other hand, do change the PM spectrum w31x. Shallow trapping also does not affect the PM spectra, since the smallest observable energy shift in the PM spectra of Fig. 2 is on the order of 0.1 eV. In contrast, exciton localization at deep traps may have an observable effect, especially when such localization is followed by exciton dissociation. We there-
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fore do not observe intersystem crossing and deep exciton trapping on the psec time scale. Previous time-resolved PL measurements at low temperature w23x have revealed a significant difference in spectral dynamics between a6T films and crystals. Therefore, it is not surprising that the transient PM spectra of a6T crystals dramatically differ from that of a6T films w13,28x. Excitons in films are quickly localized at numerous defects and aggregates and, as a result, it is impossible to measure the intrinsic Frenkel exciton spectrum. On the other hand, pure single crystals have much lower density of traps and thus free excitons can be easily identified. Both the PA and SE transients in Fig. 3 closely follow a bi-exponential decay with respective time constants of t 1 s 150 psec and t 2 s 1.2 ns. Although exciton localization at shallow traps does not affect the PM spectrum, it can change both radiative and non-radiative exciton recombination rates. Consequently, we attribute the faster decay component Žt 1 . to exciton diffusion and weak localization at shallow trapping centers. Since the PL quantum yield in a6T crystals is only about 15% w32x, the longerlived component Žt 2 . is mainly due to internal conversion and deep trapping. Two-photon absorption ŽTPA. spectrum shown in Fig. 4a are obtained by measuring the PL excitation spectrum using a combination of two-photon excitation w33x and the Z-scan technique. That the PL excitation spectrum corresponds to the TPA spectrum immediately follows from Vavilov–Kasha’s rule. In order to confirm this, we directly monitor the internal conversion between different excited states using a 3-pulse Žpump1–pump2–probe. transient PM technique w14,31x. In these measurements the first excitation pulse at " v s 2.55 eV is followed after 100 ps delay by the second excitation pulse at " v s 1.3 eV, which re-excites excitons generated by the first pump pulse. Fig. 4b shows the changes in the probe pulse transmission versus t at " v s 1.3 eV with and without the second pump. It can be seen from Fig. 4b inset that upon re-excitation to the higher excited state, a fraction of the lowest exciton population is instantaneously depleted and then rapidly recovers within 200 fs. This confirms our previous conclusion that the primary relaxation channel from upper excited states, either one- or two-pho-
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Fig. 4. Ža. TPA spectrum of the HT a6T crystal for c-polarized pulsed excitation; the inset shows the same spectrum at low energies Žlines are guides to the eye.. Žb. DTrT decay in the HT crystal probed at " v s 1.3 eV with Žsolid line. and without Žbroken line. the second pump pulse at " v s 1.3 eV. The inset shows a close-up of the DTrT decay with the second pump pulse on a shorter time scale.
ton allowed, is internal conversion and explains the ultrafast photogeneration of the a u excitons. A different behavior would be observed in the case of exciton dissociation and subsequent formation of a charge transfer ŽCT. state, as shown in Fig. 1a Žinset.. It has been demonstrated that the lowest CT state Žan ana-
log of a Wannier exciton in inorganic semiconductors. in polycrystalline a6T films is located at ECT f 2.7 eV w34x, therefore, partial relaxation into a CT state is energetically possible for " v ) EŽa u . y ECT ; 0.4 eV. The recovery of the lowest excited state after re-excitation at " v s 1.3 eV is indeed incom-
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plete, as evidenced by the long-lived PA quenching component for t ) 100 ps in Fig. 4b. This effect may be attributed to the low-yield production of the CT states. Inefficient charge photogeneration is common for all pure organic semiconductors because of the large exciton binding energy w21x. The TPA spectrum in Fig. 4a shows a smooth rise with features similar to those in Fig. 1a, i.e. different odd-parity states at 2.3, 2.6 and 3.6 eV, respectively. As a result of negligible interactions among a6T molecules along the a crystal axis w6,11x, the a g and bg components of 1 B u molecular state are nearly degenerate with its a u and b u components, respectively. Thus TPA can take place at 2.3 and 2.6 eV due to admixing of A g molecular states. We suggest that the state appearing in both Fig. 1a and Fig. 4a at " v s 3.6 eV is due to nearly degenerate a g ,a u ,b g ,b u -components of such nA g state ŽDavidov’s splitting is much smaller for even parity states than for odd parity states w16x.. A direct comparison with Fig. 2 shows that in the spectral range from 0.15 to 2.3 eV there is only one strong transition from EŽa u ,a g . s 2.3 eV to E PA s EŽa u ,a g . q 1.3 eV s 3.6 eV. This can only be attributed to acpolarized transitions between a g ,a u-components of 1 B u and b u ,bg-components of nA g , respectively. The PA lineshapes in Fig. 2 are complex and cannot be fitted to either single Lorentzian or single Gaussian curves. They consist of the dominant narrow peak at " v ; 1.3 eV, which is situated on top of a much broader PA band. This broader and weaker PA can be attributed to either vibronic sidebands of the main band or weakly allowed optical transitions to other exciton levels. If the low energy tail of the PA band is due to optical absorption with simultaneous phonon emission, its dynamics should reflect the vibrational cooling of the a u exciton. However, we find that the ultrafast dynamics at " v s 1.0, 1.3 and 1.5 eV are identical and conclude that the broader background is not due to phonon-assisted PA. The lack of pronounced vibrational sidebands similar to those in the PL spectra in Fig. 1b suggests that the Franck–Condon vibrational overlap factors, which describe the strength of exciton–phonon coupling, are small for transitions between upper excited states. In summary, we measure for the first time the transient PM spectra of Frenkel excitons in a6T single crystals and interpret them in terms of odd and
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even parity states identified by one- and two-photon absorption. We show that the analysis of the PM spectra is not complicated by contributions from trap states and vibronic sidebands, which are typical for steady state absorption and emission. We find that the lowest Frenkel excitons are characterized by a vibronically broadened stimulated emission band at ; 2 eV and a narrow absorption band at 1.3 eV, which we use to follow the exciton dynamics. We determine that intramolecular internal conversion from upper excited states to the lowest exciton occurs in 100–200 fs; subsequent exciton relaxation due to exciton trapping and internal conversion to the ground state occurs in ; 1 ns.
References w1x Proceedings of the 2nd International Conference on Electroluminescence from Molecular Materials and Related Phenomena, Sheffield, UK, 1999, Synth. Met. 111.112 Ž2000.. w2x R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. Dos Santos, J.L. Bredas, M. Logdlund, W.R. Salaneck, Nature 397 Ž1999. 121. w3x Z. Bao, A. Dodabalapur, A.J. Lovinger, Appl. Phys. Lett. 69 Ž1996. 4108. w4x A. Dodabalapur, Z. Bao, A. Makhija, J.G. Laquindanum, V.R. Raju, Y. Feng, H.E. Katz, J. Rogers, Appl. Phys. Lett. 73 Ž1998. 142. w5x H. Sirringhaus, N. Tessler, R.H. Friend, Science 280 Ž1998. 1741. w6x D. Fichou, Handbook of Oligo- and Polythiophenes, Wiley– VCH, New York, 1999. w7x L. Torsi, A. Dodabalapur, L.J. Rothberg, A.W.P. Fung, H.E. Katz, Phys. Rev. B 57 Ž1998. 2271. w8x J.H. Schon et al., Appl. Phys. Lett. 73 Ž1998. 3574. w9x G. Horowitz, B. Bachet, A. Yassar, P. Lang, F. Demanze, J.L. Fave, F. Garnier, Chem. Mater. 7 Ž1995. 1337. w10x Ch. Kloc, P.G. Simpkins, T. Siegrist, R.A. Laudise, J. Cryst. Growth 182 Ž1997. 416. w11x M. Muccini, E. Lunedei, A. Bree, G. Horowitz, F. Garnier, C. Taliani, J. Chem. Phys. 108 Ž1998. 7327. w12x M. Muccini, E. Lunedei, D. Beljonne, J. Cornil, J.L. Bredas, C. Taliani, J. Chem. Phys. 109 Ž1998. 10513. w13x G. Lanzani, S.V. Frolov, P.A. Lane, Z.V. Vardeny, M. Nisoli, S. De Silvestri, Phys. Rev. Lett. 79 Ž1997. 3066. w14x G. Cerullo, G. Lanzani, M. Muccini, C. Taliani, S. De Silvestri, Phys. Rev. Lett. 83 Ž1999. 231. w15x K. Watanabe, T. Asahi, H. Fukumura, H. Masuhara, K. Hamano, T. Kurata, J. Phys. Chem. B 102 Ž1998. 1182. w16x D.P. Craig, S.H. Walmsley, Excitons in Molecular Crystals, W.A. Benjamin, New York, 1968.
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w17x E.A. Silinsh, Organic Molecular Crystals, Their Electronic States, Springer, New York, 1980. w18x A.S. Davydov, Theory of Molecular Excitons, McGraw-Hill Book, New York, 1962. w19x V.M. Agranovich, M.D. Galanin, Electronic Excitation and Energy Transfer in Condensed Matter, North Holland Publ., Amsterdam, 1982. w20x V.M. Agranovich, R.M. Hochstrasser ŽEds.., Spectroscopy and Excitation Dynamics of Condensed Molecular Systems, North Holland Publ., Amsterdam, 1983. w21x M. Pope, C.E. Swenberg, Electronic Processes in Organic Crystals and Polymers, Oxford University Press, New York, 1999. w22x R.N. Marks, M. Muccini, E. Lunedi, R. Michel, M. Murgia, R. Zamboni, C. Taliani, G. Horowitz, F. Garnier, M. Hopmeier, M. Oestreich, R.F. Mahrt, Chem. Phys. 227 Ž1998. 49. w23x M. Hopmeier, R.N. Marks, R. Michel, M. Muccini, M. Murgia, R. Zamboni, C. Taliani, G. Horowitz, F. Garnier, M. Oestreich, R.F. Mahrt, J. Luminesc. 76r77 Ž1998. 416. w24x D. Fichou, S. Delysse, J.-M. Nunzi, Adv. Mater. 9 Ž1997. 1178.
w25x F. Garnier, G. Horowitz, P. Valat, F. Kouki, V. Wintgens, Appl. Phys. Lett. 72 Ž1998. 2087. w26x R.A. Laudise et al., J. Cryst. Growth 152 Ž1995. 241. w27x A.J. Lovinger, D.D. Davis, A. Dodabalapur, H.E. Katz, L. Torsi, Macromolecules 29 Ž1996. 4952. w28x G. Lanzani, G. Gerullo, S. Stagira, S. De Silvestri, F. Garnier, J. Chem. Phys. 111 Ž1999. 6474. w29x C.V. Shank, E.P. Ippen, O. Teschke, Chem. Phys. Lett. 45 Ž1977. 291. w30x J.B. Birks, Photophysics of Aromatic Molecules, Wiley–Interscience, London, 1970. w31x S.V. Frolov, M. Liess, P.A. Lane, W. Gellermann, Z.V. Vardeny, M. Ozaki, K. Yoshino, Phys. Rev. Lett. 78 Ž1997. 4285. w32x M. Wohlgenannt, Z.V. Vardeny, S.V. Frolov, Ch. Kloc, B. Batlogg, Synth. Metals Žin press.. w33x N. Periasamy, R. Danieli, G. Ruani, R. Zamboni, C. Taliani, Phys. Rev. Lett. 68 Ž1992. 919. w34x L.M. Blinov, S.P. Palto, G. Ruani, C. Taliani, A.A. Tevosov, S.G. Yudin, R. Zamboni, Chem. Phys. Lett. 232 Ž1995. 401.
Chemical Physics Letters 326 Ž2000. 558–566 www.elsevier.nlrlocatercplett
Transient spectroscopy of Frenkel excitons in a-hexathiophene single crystals S.V. Frolov ) , Ch. Kloc, S. Berg, G.A. Thomas, B. Batlogg Bell Laboratories, Lucent Technologies, 600 Mountain AÕenue, Murray Hill, NJ 07974, USA Received 15 February 2000; in final form 11 July 2000
Abstract Photoexcitation dynamics in pure single molecular crystals of a-hexathiophene are studied using ultrafast photomodulation spectroscopy in the spectral range from 0.15 to 2.3 eV. Frenkel excitons are uniquely described by transient photoinduced absorption and stimulated emission bands at 1.3 and ; 2 eV, respectively. The transient spectra are interpreted using exciton states identified by one- and two-photon absorption. Exciton dynamics in pure crystals dramatically differs from that in polycrystalline films, which is attributed to a much lower defect density in single crystals. Intramolecular internal conversion rate is found to be ; 150 fs for excitation energies up to 3.6 eV. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Recent use of conjugated organic materials in light-emitting devices w1,2x and thin-film transistors w3–5x has stimulated new studies of their electronic structure and basic optical properties. Particular attention has been given to oligothiophenes w6x, due to their relatively high charge carrier mobilites w7,8x and ability to form large single crystals w9,10x. Scrupulous measurements of absorption and emission spectra w11,12x have provided fruitful insights into the electronic structure of a-hexathiophene Ž a6T. crystals. However, non-linear optical measurements and, in particular, transient ultrafast spectroscopy have not been thoroughly used to study these or any other molecular crystals. Ultrafast dynamics in polycrys) Corresponding author. Fax: q1-908-582-3260; e-mail: [email protected]
talline a6T films, on the other hand, have been studied in depth w13–15x. In this study we show that ultrafast photophysics of pure single crystals significantly differs from that of polycrystalline films and reveals processes associated with intrinsic electronic relaxation in organic semiconductors. It is well established that the lowest excited states of most molecular crystals are adequately described by Frenkel excitons and characterized by localized wavefunctions with negligible overlap between individual molecules w16,17x. Although the theory of molecular excitons has been developed rather extensively w16,18,19x, previous experiments have mostly concentrated on doped molecular crystals w20x, due to the lack of high purity materials and the apparatus for ultrafast measurements. In doped crystals, however, the dynamics of host excitons are obscured by exciton trapping on guest molecules. Only in pure crystals one may observe intrinsic exciton dynamics,
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 8 2 3 - X
S.V. FroloÕ et al.r Chemical Physics Letters 326 (2000) 558–566
including intra- and intermolecular phonon-assisted relaxation, intermolecular charge transfer, exciton migration, trapping at defects, self-localization due to lattice relaxation, radiative recombination and intersystem crossing w17x. Learning what roles each of these processes play in the excited state relaxation is necessary for understanding the nature of electronic states in molecular crystals, which in turn has been the subject of great interest and debate in the past 30 years w17,21x. We study optical emission and absorption of a6T single crystals using cw absorption and emission spectroscopies, and femto- and picosecond photoexcitation dynamics using ultrafast transient photomodulation ŽPM. spectroscopy. Whereas steady state photoluminescence ŽPL. w22,23x generally contains contributions from traps and impurities, psec transient PM spectra are less affected by the latter and may exclusively characterize the lowest excitons. We measure the transient spectrum and dynamics of optical gain due to Frenkel excitons, which previously has been shown to result in pronounced stimulated emission at high excitation intensities w24,25x. We find that primary excitations in a6T crystals, unlike those in films w13x, are characterized by a single narrow photoinduced absorption band at 1.25–1.30 eV. Using this band as a unique probe of Frenkel excitons, we find that the lowest excitons are produced within the first 150 fs following excitation with photon energies up to 3.6 eV. The subsequent relaxation is determined by radiative recombination, exciton traping and phonon-assisted relaxation. Exciton trapping at deep traps is found insignificant on the picosecond time scale due to the low trap density in pure single crystals. We also show that internal conversion is the primary relaxation channel between different excited states.
2. Experimental As described elsewhere w10x, a6T single crystals are grown in the form of free-standing platelets with thickness from 1 to 10 mm. A pump-and-probe technique is used to obtain time-resolved PM spectra by measuring DT Ž t .rT versus probe photon energy " v , where T is the probe transmission, DT is the change in T due to the pump pulse and t is the time
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delay between the pump and probe pulses. Photoinduced absorption ŽPA. leads to negative DT, whereas probe-induced stimulated emission ŽSE. result in positive DT. Two electronically synchronized Ti:Sapphire mode-locked lasers ŽSpectra-Physics., one of which pumps a tunable optical parametric oscillator, are used to produce 100 fs pump and probe pulses. Photoexcitation density and DTrT are below 2 = 10 16 cmy3 and 10y4 , respectively. All measurements are done at room temperature in air or flowing N2 ; in either case we do not observe any signal degradation due to photo-oxidation.
3. Results and discussion Previously, it has been shown that a6T molecules crystallize in two similar crystallographic phases: the low ŽLT. and high ŽHT. temperature phases w10,26x. Both have been thoroughly studied w27x and found to be herringbone structures with monoclinic unit cells containing four a6T rigid rod-like molecules. The latter lie in the plane formed by the a and c crystal axes; the b crystal axis is thus almost perpendicular to the long axes Ž L. of a6T molecules. Since the crystal growth direction corresponds to the a axis, one can readily measure polarized absorption for light polarizations parallel to b and c crystal axes, respectively. Each molecular excited state of a single a6T molecule is split into four components w11x and corresponding exciton states can be classified according to the irreducible representations of a crystal unit cell group ŽFig. 1a, inset. w16x. The unit cell group of the a6T crystal is C 2h and the corresponding representations are a g , a u , bg and b u w6x. The same excited state classification holds for the molecular excited states ŽA g , A u , B g and B u ., since the symmetry group of a6T single molecules is also C 2h w6x. The site symmetry in the crystal is C 1 w6x, therefore A g , A u , B g and B u molecular states are somewhat intermixed. One-photon transitions in a system with the inversion center are allowed only between different parity states, e.g., a g and b u . In contrast, two-photon transitions occur only between same parity states, e.g., a g and bg . Absorption spectra shown in Fig. 1a can be used to describe a u states with weak b-polarized, and b u states with strong ac-polarized transitions
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Fig. 1. Ža. Polarized absorption spectra of a6T single crystals in HT Žsolid line. and LT Žbroken line. phases Ž b and c are the crystal axes.; the arrow indicates the nA g state. The inset shows energy levels in the a6T single molecule and crystal; vibronic sublevels are not shown. Žb. PL spectra of a6T single HT Žsolid line. and LT Žbroken line. crystals Ž0 through III indicate vibronic sidebands.; the SE spectrum Žopen circles. at t s 5 ps is given for comparison Žsee text..
from the ground state. The lowest absorption edge at 2.3 eV corresponds to the a u state, whereas the second edge at 2.6 eV – to the b u state w10x. These are believed to be the two Davidov’s components of
the B u state, which has the transition dipole moment oriented along the L axis w11,12x. However, the spectra are complicated by additional transitions due to strong electron–phonon coupling, which mixes
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the vibronic sidebands of the a u and b u states w11,16x. The c-polarized absorbance spectrum saturates Ždue to the crystal thickness. at " v ) 2.6 eV, therefore only b-polarized transitions at 3.6 and 4.0 eV can be identified. We first use photoluminescence ŽPL. to study the lowest excited states. Fig. 1b shows the PL spectra obtained from the excited side of the crystals. Vibronic sidebands I–III, corresponding to optical transitions with simultaneous phonon emission Ž0 n , where n is the number of optical phonons., seemingly do not obey the Franck–Condon coupling mechanism generally used to describe dipole-allowed transitions w6x. Instead of an expected strong 0 0 PL sideband, only a weak shoulder is observed at the PL origin Ž " v f 2.3 eV.. This suggests that the a u exciton is indeed the lowest excited state, since the radiative transition from the latter is only weakly allowed. However, the a u state may borrow oscillator strength from the b u state via intermolecular vibrational mixing; as a result, its vibronic sidebands appear in the PL spectrum as strong ac-polarized transitions w11,16x. Furthermore, PL sidebands I, II and III in Fig. 1b are not equidistant
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Ž0.15 vs. 0.18 eV., indicating that perhaps more than one excited state contributes to the observed PL. Such a behavior has indeed been observed in previous studies of a6T films and crystals w22x, which revealed complex behavior of the PL spectrum at low temperatures and identified several shallow X traps and aggregate states w23x. To further understand the nature of excited states in a6T crystals, we measure their transient PM spectra shown in Fig. 2 for t s 5 ps. The crystals are excited at 2.58 eV; the pump and probe polarizations are parallel to the c axis. A pronounced short-lived PA band is observed at " v s 1.25 eV Ž1.3 eV. for the LT ŽHT. crystal phase. In a recent study of 6T films w28x, a PA band at " v s 1.3 eV has also been observed, but attributed to disordered or amorphous parts of the films. Fig. 3a compares the PA decay for probe polarizations parallel to b and c axes and shows that PA at " v s 1.3 eV is c-polarized. The probe pulse experiences net optical gain Žpositive DT . and consequently SE is observed at " v ) 1.85 eV, as shown in Fig. 2. Transient PA occurs simultaneously with SE and thus lowers the net gain experienced by the probe. This PA is obvious in case
Fig. 2. Transient PM spectra of the LT Žopen circles. and HT Žsolid squares. a6T single crystals at t s 5 ps; the LT spectrum is offset for clarity.
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Fig. 3. Ža. DTrT decay at " v s 1.3 eV in the LT crystal for the probe polarization parallel to c Žsolid line. and b Žbroken line.; the inset shows the rise of the PA signal Žrrc . at " v s 1.3 eV. Žb. DTrT decay at " v s 2.1 eV in the HT a6T single crystal for the probe polarization rrc Žsolid line. and rrb Žbroken line.; the inset shows the SE onset Žrrc . at " v s 2.1 eV.
of the LT crystal, where the SE component is weaker. SE is c-polarized and its decay is well correlated with the PA decay, as shown in Fig. 3b Žhere the PA contribution is evident for the b-polarized probe.. Fig. 1b compares the SE spectrum of the HT a6T crystal with the corresponding PL spectrum Žthe SE spectrum is arbitrarily offset to compensate for the
transient PA contribution., indicating that SE and PL share the same origin, i.e. the a u exciton at EŽa u . s 2.3 eV. Since the psec dynamics of SE and PA are well correlated, we can attribute the whole PM spectrum at t s 5 ps to the a u exciton. PA onset is instantaneous ŽFig. 3a, inset. and limited only by the experimental resolution Ž150 fs.,
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suggesting an ultrafast electronic relaxation from the initially excited state Žat " v s 2.6–3.2 eV. to the lowest a u exciton. However, SE onset ŽFig. 3b, inset. is slower than that of PA and occurs in about 0.5 ps at excitation energy " v s 3.2 eV. We attribute this discrepancy to exciton thermalization within the vibrational manifold of the a u exciton after initial ultrafast relaxation. Alternatively, primary excited states might first decay into the dipole-forbidden a g state, which then relaxes into the a u state; however, this is an unlikely scenario, because of the near-degeneracy between the a g and a u excitons. The ultrafast relaxation to the lowest excited state Žin our case the a u exciton. is common for many complex molecular systems w29x. The majority of these systems obey Vavilov–Kasha’s rule; w30x, the PL excitation spectrum of a6T crystals is practically flat between " v s 2.4 and 3.2 eV. The non-radiative relaxation between excited states of the same spin multiplicity Žsinglets in our case. has been termed internal conversion w30x. Internal conversion in single molecules containing many closely spaced energy levels is the fastest relaxation channel for most excited states, except perhaps the lowest one. This situation is apparently preserved in crystals, where intermolecular interactions are weak and, subsequently, the ultrafast dynamics are determined by intramolecular interactions. The PM spectra do not change significantly from 1 ps to 0.5 ns, which suggests that after initial rapid relaxation the nature of excited states does not change on the picosecond time scale. The lowest excitons have several relaxation pathways, including radiative recombination, internal conversion, intersystem crossing into the triplet manifold, exciton diffusion with subsequent trapping at defects and impurities. Bimolecular Auger-type processes become important only at high excitation densities Ž; 10 18 cmy3 .. Neither radiative or nonradiative transitions to the ground state affect the PM spectrum shape. The intersystem crossing and exciton trapping, on the other hand, do change the PM spectrum w31x. Shallow trapping also does not affect the PM spectra, since the smallest observable energy shift in the PM spectra of Fig. 2 is on the order of 0.1 eV. In contrast, exciton localization at deep traps may have an observable effect, especially when such localization is followed by exciton dissociation. We there-
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fore do not observe intersystem crossing and deep exciton trapping on the psec time scale. Previous time-resolved PL measurements at low temperature w23x have revealed a significant difference in spectral dynamics between a6T films and crystals. Therefore, it is not surprising that the transient PM spectra of a6T crystals dramatically differ from that of a6T films w13,28x. Excitons in films are quickly localized at numerous defects and aggregates and, as a result, it is impossible to measure the intrinsic Frenkel exciton spectrum. On the other hand, pure single crystals have much lower density of traps and thus free excitons can be easily identified. Both the PA and SE transients in Fig. 3 closely follow a bi-exponential decay with respective time constants of t 1 s 150 psec and t 2 s 1.2 ns. Although exciton localization at shallow traps does not affect the PM spectrum, it can change both radiative and non-radiative exciton recombination rates. Consequently, we attribute the faster decay component Žt 1 . to exciton diffusion and weak localization at shallow trapping centers. Since the PL quantum yield in a6T crystals is only about 15% w32x, the longerlived component Žt 2 . is mainly due to internal conversion and deep trapping. Two-photon absorption ŽTPA. spectrum shown in Fig. 4a are obtained by measuring the PL excitation spectrum using a combination of two-photon excitation w33x and the Z-scan technique. That the PL excitation spectrum corresponds to the TPA spectrum immediately follows from Vavilov–Kasha’s rule. In order to confirm this, we directly monitor the internal conversion between different excited states using a 3-pulse Žpump1–pump2–probe. transient PM technique w14,31x. In these measurements the first excitation pulse at " v s 2.55 eV is followed after 100 ps delay by the second excitation pulse at " v s 1.3 eV, which re-excites excitons generated by the first pump pulse. Fig. 4b shows the changes in the probe pulse transmission versus t at " v s 1.3 eV with and without the second pump. It can be seen from Fig. 4b inset that upon re-excitation to the higher excited state, a fraction of the lowest exciton population is instantaneously depleted and then rapidly recovers within 200 fs. This confirms our previous conclusion that the primary relaxation channel from upper excited states, either one- or two-pho-
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Fig. 4. Ža. TPA spectrum of the HT a6T crystal for c-polarized pulsed excitation; the inset shows the same spectrum at low energies Žlines are guides to the eye.. Žb. DTrT decay in the HT crystal probed at " v s 1.3 eV with Žsolid line. and without Žbroken line. the second pump pulse at " v s 1.3 eV. The inset shows a close-up of the DTrT decay with the second pump pulse on a shorter time scale.
ton allowed, is internal conversion and explains the ultrafast photogeneration of the a u excitons. A different behavior would be observed in the case of exciton dissociation and subsequent formation of a charge transfer ŽCT. state, as shown in Fig. 1a Žinset.. It has been demonstrated that the lowest CT state Žan ana-
log of a Wannier exciton in inorganic semiconductors. in polycrystalline a6T films is located at ECT f 2.7 eV w34x, therefore, partial relaxation into a CT state is energetically possible for " v ) EŽa u . y ECT ; 0.4 eV. The recovery of the lowest excited state after re-excitation at " v s 1.3 eV is indeed incom-
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plete, as evidenced by the long-lived PA quenching component for t ) 100 ps in Fig. 4b. This effect may be attributed to the low-yield production of the CT states. Inefficient charge photogeneration is common for all pure organic semiconductors because of the large exciton binding energy w21x. The TPA spectrum in Fig. 4a shows a smooth rise with features similar to those in Fig. 1a, i.e. different odd-parity states at 2.3, 2.6 and 3.6 eV, respectively. As a result of negligible interactions among a6T molecules along the a crystal axis w6,11x, the a g and bg components of 1 B u molecular state are nearly degenerate with its a u and b u components, respectively. Thus TPA can take place at 2.3 and 2.6 eV due to admixing of A g molecular states. We suggest that the state appearing in both Fig. 1a and Fig. 4a at " v s 3.6 eV is due to nearly degenerate a g ,a u ,b g ,b u -components of such nA g state ŽDavidov’s splitting is much smaller for even parity states than for odd parity states w16x.. A direct comparison with Fig. 2 shows that in the spectral range from 0.15 to 2.3 eV there is only one strong transition from EŽa u ,a g . s 2.3 eV to E PA s EŽa u ,a g . q 1.3 eV s 3.6 eV. This can only be attributed to acpolarized transitions between a g ,a u-components of 1 B u and b u ,bg-components of nA g , respectively. The PA lineshapes in Fig. 2 are complex and cannot be fitted to either single Lorentzian or single Gaussian curves. They consist of the dominant narrow peak at " v ; 1.3 eV, which is situated on top of a much broader PA band. This broader and weaker PA can be attributed to either vibronic sidebands of the main band or weakly allowed optical transitions to other exciton levels. If the low energy tail of the PA band is due to optical absorption with simultaneous phonon emission, its dynamics should reflect the vibrational cooling of the a u exciton. However, we find that the ultrafast dynamics at " v s 1.0, 1.3 and 1.5 eV are identical and conclude that the broader background is not due to phonon-assisted PA. The lack of pronounced vibrational sidebands similar to those in the PL spectra in Fig. 1b suggests that the Franck–Condon vibrational overlap factors, which describe the strength of exciton–phonon coupling, are small for transitions between upper excited states. In summary, we measure for the first time the transient PM spectra of Frenkel excitons in a6T single crystals and interpret them in terms of odd and
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even parity states identified by one- and two-photon absorption. We show that the analysis of the PM spectra is not complicated by contributions from trap states and vibronic sidebands, which are typical for steady state absorption and emission. We find that the lowest Frenkel excitons are characterized by a vibronically broadened stimulated emission band at ; 2 eV and a narrow absorption band at 1.3 eV, which we use to follow the exciton dynamics. We determine that intramolecular internal conversion from upper excited states to the lowest exciton occurs in 100–200 fs; subsequent exciton relaxation due to exciton trapping and internal conversion to the ground state occurs in ; 1 ns.
References w1x Proceedings of the 2nd International Conference on Electroluminescence from Molecular Materials and Related Phenomena, Sheffield, UK, 1999, Synth. Met. 111.112 Ž2000.. w2x R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. Dos Santos, J.L. Bredas, M. Logdlund, W.R. Salaneck, Nature 397 Ž1999. 121. w3x Z. Bao, A. Dodabalapur, A.J. Lovinger, Appl. Phys. Lett. 69 Ž1996. 4108. w4x A. Dodabalapur, Z. Bao, A. Makhija, J.G. Laquindanum, V.R. Raju, Y. Feng, H.E. Katz, J. Rogers, Appl. Phys. Lett. 73 Ž1998. 142. w5x H. Sirringhaus, N. Tessler, R.H. Friend, Science 280 Ž1998. 1741. w6x D. Fichou, Handbook of Oligo- and Polythiophenes, Wiley– VCH, New York, 1999. w7x L. Torsi, A. Dodabalapur, L.J. Rothberg, A.W.P. Fung, H.E. Katz, Phys. Rev. B 57 Ž1998. 2271. w8x J.H. Schon et al., Appl. Phys. Lett. 73 Ž1998. 3574. w9x G. Horowitz, B. Bachet, A. Yassar, P. Lang, F. Demanze, J.L. Fave, F. Garnier, Chem. Mater. 7 Ž1995. 1337. w10x Ch. Kloc, P.G. Simpkins, T. Siegrist, R.A. Laudise, J. Cryst. Growth 182 Ž1997. 416. w11x M. Muccini, E. Lunedei, A. Bree, G. Horowitz, F. Garnier, C. Taliani, J. Chem. Phys. 108 Ž1998. 7327. w12x M. Muccini, E. Lunedei, D. Beljonne, J. Cornil, J.L. Bredas, C. Taliani, J. Chem. Phys. 109 Ž1998. 10513. w13x G. Lanzani, S.V. Frolov, P.A. Lane, Z.V. Vardeny, M. Nisoli, S. De Silvestri, Phys. Rev. Lett. 79 Ž1997. 3066. w14x G. Cerullo, G. Lanzani, M. Muccini, C. Taliani, S. De Silvestri, Phys. Rev. Lett. 83 Ž1999. 231. w15x K. Watanabe, T. Asahi, H. Fukumura, H. Masuhara, K. Hamano, T. Kurata, J. Phys. Chem. B 102 Ž1998. 1182. w16x D.P. Craig, S.H. Walmsley, Excitons in Molecular Crystals, W.A. Benjamin, New York, 1968.
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S.V. FroloÕ et al.r Chemical Physics Letters 326 (2000) 558–566
w17x E.A. Silinsh, Organic Molecular Crystals, Their Electronic States, Springer, New York, 1980. w18x A.S. Davydov, Theory of Molecular Excitons, McGraw-Hill Book, New York, 1962. w19x V.M. Agranovich, M.D. Galanin, Electronic Excitation and Energy Transfer in Condensed Matter, North Holland Publ., Amsterdam, 1982. w20x V.M. Agranovich, R.M. Hochstrasser ŽEds.., Spectroscopy and Excitation Dynamics of Condensed Molecular Systems, North Holland Publ., Amsterdam, 1983. w21x M. Pope, C.E. Swenberg, Electronic Processes in Organic Crystals and Polymers, Oxford University Press, New York, 1999. w22x R.N. Marks, M. Muccini, E. Lunedi, R. Michel, M. Murgia, R. Zamboni, C. Taliani, G. Horowitz, F. Garnier, M. Hopmeier, M. Oestreich, R.F. Mahrt, Chem. Phys. 227 Ž1998. 49. w23x M. Hopmeier, R.N. Marks, R. Michel, M. Muccini, M. Murgia, R. Zamboni, C. Taliani, G. Horowitz, F. Garnier, M. Oestreich, R.F. Mahrt, J. Luminesc. 76r77 Ž1998. 416. w24x D. Fichou, S. Delysse, J.-M. Nunzi, Adv. Mater. 9 Ž1997. 1178.
w25x F. Garnier, G. Horowitz, P. Valat, F. Kouki, V. Wintgens, Appl. Phys. Lett. 72 Ž1998. 2087. w26x R.A. Laudise et al., J. Cryst. Growth 152 Ž1995. 241. w27x A.J. Lovinger, D.D. Davis, A. Dodabalapur, H.E. Katz, L. Torsi, Macromolecules 29 Ž1996. 4952. w28x G. Lanzani, G. Gerullo, S. Stagira, S. De Silvestri, F. Garnier, J. Chem. Phys. 111 Ž1999. 6474. w29x C.V. Shank, E.P. Ippen, O. Teschke, Chem. Phys. Lett. 45 Ž1977. 291. w30x J.B. Birks, Photophysics of Aromatic Molecules, Wiley–Interscience, London, 1970. w31x S.V. Frolov, M. Liess, P.A. Lane, W. Gellermann, Z.V. Vardeny, M. Ozaki, K. Yoshino, Phys. Rev. Lett. 78 Ž1997. 4285. w32x M. Wohlgenannt, Z.V. Vardeny, S.V. Frolov, Ch. Kloc, B. Batlogg, Synth. Metals Žin press.. w33x N. Periasamy, R. Danieli, G. Ruani, R. Zamboni, C. Taliani, Phys. Rev. Lett. 68 Ž1992. 919. w34x L.M. Blinov, S.P. Palto, G. Ruani, C. Taliani, A.A. Tevosov, S.G. Yudin, R. Zamboni, Chem. Phys. Lett. 232 Ž1995. 401.