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Femtosecond intramolecular proton transfer in photoexcited mono- and dienol derivatives of bipyridine
6 August 1999
Chemical Physics Letters 309 Ž1999. 19–28 www.elsevier.nlrlocatercplett
Femtosecond intramolecular proton transfer in photoexcited mono- and dienol derivatives of bipyridine D. Marks a , H. Zhang a , P. Borowicz b, A. Grabowska b, M. Glasbeek a
a,)
Laboratory for Physical Chemistry, UniÕersity of Amsterdam, Nieuwe Achtergracht 129, NL-1018 WS Amsterdam, The Netherlands b Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44 r 52, 01-224 Warsaw, Poland Received 29 March 1999; in final form 4 June 1999
Abstract A femtosecond study of the intramolecular proton transfer dynamics in photoexcited mono- and dienol derivative compounds of pyridine is reported. It is shown that when a strongly electron-withdrawing substituent is attached to w2,2X-bipyridylx-3,3X-diol, the excited-state proton transfer of this compound is changed from a double-proton transfer to a single-proton transfer. The excited-state single-proton transfer time in the title compounds is - 150 fs. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Ultrafast excited-state intramolecular proton transfer ŽESIPT. is characteristic of numerous enol–keto tautomerization reactions in the condensed phase w1–8x. Both single- and double-proton transfer processes have been studied. For w2,2X-bipyridylx-3,3Xdiol, henceforth abbreviated as BPŽOH. 2 , the mechanism of the double-proton transfer process in the excited state has been extensively studied, experimentally w7,9–16x and theoretically w17,18x. Following short-pulse optical excitation, concerted Žonestep. and consecutive Žtwo-step. double-proton transfer reactions were found to occur concurrently w7,12x. The concerted process takes place within 100 fs. In the consecutive process, the first proton is transferred
) Corresponding author. Fax: q31 20 525 6994; e-mail: [email protected]
in - 100 fs, the second proton follows in a few picoseconds. In this Letter, we investigate the ESIPT dynamics in two derivative compounds of BPŽOH. 2 . The first of these Žsee also Scheme 1. is 6-Me-BPOH Ž6methyl-w2,2X-bipyridylx-3X-ol.; the second compound is BPŽOH . 2 -6-COOMe Žmethyl-3,3X-dihydroxyw2,2X-bipyridylx-6-carboxylate.. 6-Me-BPOH serves as a model compound for studying the dynamics of a single-proton transfer in the excited state of a bipyridyl ring system. In the case of BPŽOH. 2-6COOMe we examine the influence of an electrophilic substituent on the intramolecular doubleproton transfer dynamics in BPŽOH. 2 . Both were already introduced to the literature among six new derivatives of BPŽOH. 2 and BPOH w19x. The properties of their ground states were studied experimentally and by ab initio calculations. The main aim of this work was to study the influence of electrondonating and -withdrawing substituents, introduced
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 6 5 9 - 4
D. Marks et al.r Chemical Physics Letters 309 (1999) 19–28
20
Scheme 1.
next to nitrogen atom, on the phototautomerization mechanism of the molecule. The steady-state absorption and fluorescence spectra in inert solvents were also determined and compared. Previously, Bulska et al. w20x have reported the emission spectrum of w2,2X-bipyridylx-3-ol ŽBPOH.. Compared to BPŽOH. 2 it appeared that the fluorescence quantum yield for BPOH is smaller by two orders of magnitude. INDOrS calculations suggested that the keto tautomer of BPOH has a nonfluorescent S 2 Žn, p ) . state very close to the lower fluorescent S 1Ž p, p ) . state w21x. The low fluorescence quantum yield for BPOH then results from effective coupling between the fluorescent and nonfluorescent excited singlet states w20x. Compared to BPOH, 6-Me-BPOH turns out to possess a much higher fluorescence efficiency w19x. As a result, the time dependence of the fluorescence of this molecule could be investigated using the femtosecond fluorescence upconversion technique w7x. The fluorescence transients provide information concerning the dynamics of the intramolecular single-proton transfer process in the excited state Žvide infra.. Also for the second derivative compound, BPŽOH. 2-6-COOMe, fluorescence upconversion transients are reported. It is discussed that for BPŽOH. 2-6-COOMe only a single- Žand not a double-. proton transfer occurs due to the presence of the electrophilic COOCH 3 substituent.
2. Experimental Synthesis of both studied compounds was described elsewhere w22x. Throughout the present Letter
the names of compounds are as in Ref. w19x. Spectrograde quality cyclohexane ŽMerck. and acetonitrile ŽMerck. were used as solvents without further purification. Two pulsed-laser setups were used for the measurement of the fluorescence transients: a regeneratively amplified Ti:sapphire laser system with upconversion detection for the time span 150 fs–100 ps, and a picosecond laser system with time-correlated single-photon-counting ŽSPC. detection for the time range 15 ps–5 ns. The systems have been described in detail previously w12,23x. In the femtosecond laser system, excitation was accomplished by laser pulses Ž; 100 fs. from an OPA system in the range of 300–380 nm Ž; 0.1 mJrpulse.. The photo-induced fluorescence was detected after upconversion, in a BBO crystal, with a gating pulse Žat 800 nm.. The time elapsed between the excitation pulse and the gating pulse could be controlled by means of an optical delay line, with a variable optical path length, for the gating pulses. Photodetection was limited to the upconverted beam polarized at a magic angle with respect to the excitation beam. In this way the probed fluorescence decay did not reflect contributions of rotational diffusion motions of the solute in the liquid. In the picosecond fluorescence setup w23x, photoexcitation was at 322 nm by means of picosecond pulses Ž4 mW, ; 1 ps pulse duration.. The fluorescence emitted from the sample in a direction perpendicular to the excitation beam was focused on to the entrance slit of a monochromator outfitted with a multi-channel plate photodetector. A linear polarizer was inserted in the detection pathway to detect at magic-angle conditions. The instrumental response time was ; 17 ps ŽFWHM..
3. Results 3.1. 6-Me-BPOH Fig. 1 shows the CW absorption and emission spectra as measured for 6-Me-BPOH dissolved in cyclohexane and acetonitrile. The absorption spectrum of the 6-Me-BPOH molecule in both solvents is similar, with the absorption band maximum at 30 670
D. Marks et al.r Chemical Physics Letters 309 (1999) 19–28
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detection is at the blue side of the spectrum, however, an additional fast decay component appears. Now the transients are best fitted to a bi-exponential decay function of the form, yt
I Ž l , t . s a1 Ž l . exp
Fig. 1. Steady-state absorption and emission spectra. Emission spectra: Ža. BPŽOH. 2 -6-COOMe in acetonitrile; Žb. 6-Me-BPOH in acetonitrile; Žc. BPŽOH. 2 -6-COOMe in cyclohexane; Žd. 6Me-BPOH in cyclohexane; and Že. BPŽOH. 2 in cyclohexane. Absorption spectra: Žf. BPŽOH. 2 ; Žg. BPŽOH. 2 -6-COOMe; and Žh. 6-Me-BPOH, dissolved in cyclohexane.
and 31 490 cmy1 in cyclohexane and acetonitrile, respectively. The band maximum in the emission spectrum of 6-Me-BPOH shows a red-shift when a more polar solvent is used. The emission band maximum is at 18 220 cmy1 when 6-Me-BPOH is dissolved in cyclohexane and 17 150 cmy1 for the solute dissolved in acetonitrile. The fluorescence transients measured with the time-correlated single-photon-counting fluorescence setup could be fitted to a single-exponential decay function convoluted with the system response function. The fluorescent-state lifetime of 6-Me-BPOH in cyclohexane is found as 190 ps; the lifetime of the emissive state is 12 ps for the solute dissolved in acetonitrile. Thus the fluorescence lifetime is sensitive to the nature of the solvent. The transients obtained in the femtosecond fluorescence upconversion measurements exhibit a strong dependence on the detection wavelength. Transients at a few selected detection wavelengths for 6-MeBPOH dissolved in cyclohexane are presented in Fig. 2a. When detecting at the red part of the emission band Ž l ) 580 nm. the fluorescence consists of a sharp instantaneous rise followed by a decay. These transients could still be fitted, after deconvolution with the system response function, to a single-exponential decay function. The decay time is equal to that obtained in the picosecond measurements. When
ž / t1
yt
q a2 Ž l . exp
ž / t2
,
in which t 1 is the same as above for detection wavelengths higher than 580 nm. The faster decay time, t 2 , is 0.5 ps. Best fittings are plotted as solid curves in Fig. 2a. It is noted that at all detection wavelengths within the emission band only decaying components are observed. Upconversion transients observed for 6-Me-BPOH dissolved in acetonitrile at a few selected wavelengths are presented in Fig. 2b. The features are similar as for the solute in cyclohexane. The bi-exponential fit now yields t 1 s 12 ps, and t 2 s 1.0 ps. Again only decaying components are observed. Using the OPA system, the excitation wavelength was varied in the range from 310 to 360 nm in the fluorescence transient measurements. No effect of the excitation wavelength on the fluorescence kinetics was found.
3.2. BP(OH)2-6-COOMe Fig. 1 includes CW absorption and emission spectra as measured for BPŽOH. 2-6-COOMe dissolved in acetonitrile and cyclohexane. The absorption spectrum is little dependent on the solvent used. The absorption band maximum is at 28 950 and 29 620 cmy1 , in cyclohexane and acetonitrile, respectively. The steady-state emission shows a strong resemblance with the emission of 6-Me-BPOH. As for the 6-Me-BPOH emission, the emission band maximum in the spectrum of BPŽOH. 2-6-COOMe is shifted when changing solvent, with the maximum at 18 530 and 17 300 cmy1 , in cyclohexane and acetonitrile, respectively. For BPŽOH. 2-6-COOMe fluorescence transients were measured as before. The transients of the picosecond measurements fitted, after deconvolution with the response function of the SPC setup, a
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Fig. 2. Fluorescence upconversion transients of 6-Me-BPOH dissolved in: Ža. cyclohexane and Žb. acetonitrile. In the 70 ps time window of the figure, the system response function would appear as a d-function and thus this function has been left out.
single-exponential decay. The decay time, corresponding to the fluorescence lifetime, is dependent on the solvent. The decay time is 170 ps for BPŽOH. 2-6-COOMe dissolved in cyclohexane and 21 ps for the molecule dissolved in acetonitrile.
As for 6-Me-BPOH, the fluorescence upconversion transients Žobtained in the femtosecond upconversion experiments. display a significant dependence on the detection wavelength Žsee Fig. 3a and b.. When detection is at the red part of the spectrum
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Fig. 3. Fluorescence upconversion transients of BPŽOH. 2 -6-COOMe dissolved in: Ža. cyclohexane and Žb. acetonitrile. In the 70 ps time window of the figure, the system response function would appear as a d-function and thus this function has been left out.
Ž l ) 580 nm., the transients consist of an instantaneous rise followed by a decay. After deconvolution with the system response function, these transients fitted a single-exponential decay function. The decay times are the same as those obtained in the picosec-
ond experiments. When detection is at the high-energy part of the emission band, a second, much faster, decay component is present. The transients were fitted to the bi-exponential decay function given above. t 1 has the same value as determined in the
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D. Marks et al.r Chemical Physics Letters 309 (1999) 19–28
picosecond measurements. The additional decay component has a decay time, t 2 , of 3.0 ps when the molecule dissolved in cyclohexane and 2.6 ps when it is dissolved in acetonitrile. The experiments were repeated using a series of excitation wavelengths in the range from 320 to 360 nm. No variation in the temporal characteristics of the fluorescence of BPŽOH. 2-6-COOMe with the excitation wavelength was observed.
4. Discussion 4.1. 6-Me-BPOH We applied the method of spectral reconstruction w24,25x to study the temporal behavior of the emission spectra of 6-Me-BPOH from its observed fluorescence upconversion transients. The data points as calculated from the transients for 6-Me-BPOH in cyclohexane are presented as black squares in Fig. 4a. After ; 2 ps, the reconstructed spectrum has reached its final form and the same spectrum as in the CW experiment is obtained. For the latter band emission, the low-energy part nicely fitted a log-normal line shape function. We label this band as band I. The reconstructed emission spectra at early times Žup to a few ps., are a superposition of the aforementioned band I and an additional fast decaying band Žband II.. Band II has its band maximum at 20 020 cmy1 for the solute in cyclohexane. The drawn curves in Fig. 4a are the resultants of the log-normal shaped dotted curves Žband I. and the log-normal shaped dashed curves Žband II.. The fluorescence in the red part of the spectrum shows a single-exponential decay, with a time constant of 190 ps for 6-MeBPOH dissolved in cyclohexane. This time constant is thus characteristic for the decay of band I. The typical decay time of band II is 0.5 ps in cyclohexane, as obtained from the fittings of the transients in the blue side of the emission band. Table 1 summarizes the results for 6-Me-BPOH dissolved in cyclohexane. A similar analysis of the upconversion results for 6-Me-BPOH dissolved in acetonitrile was performed.
Fig. 4b shows the points reconstructed from the experimental data and the simulated log-normal fits for bands I and II. The position of the band maxima and the emission band lifetimes are summarized in Table 1. Previously, Bulska et al. w20x have studied the spectroscopy of BPOH in 3-methylpentane solution. The large Stokes shift of ; 13 000 cmy1 was discussed to be characteristic of an ESIPT process in which a keto tautomer is formed. A very low quantum yield Ž; 10y3 . for the fluorescence of BPOH was found w20,21x probably caused by coupling to torsional motions that could give rise to an enhanced radiationless decay out of the excited state. An increase by a factor of ; 35 in the fluorescence quantum yield of BPOH was measured when lowering the temperature to 77 K. The temperature effect may be explained from the closeness of two almost degenerate excited states, as predicted by INDOrS calculations w20,21x, the lower of which is the S 1Ž p p ) . state with the higher oscillator strength Ž0.382., and the somewhat higher-lying S 2 Žnp ) . state, with an almost negligible oscillator strength Ž0.002.. Our measurements for 6-Me-BPOH show the presence of two emissive excited states almost 2 000 cmy1 apart. This energy separation is quite comparable to the value predicted in the INDO calculations for BPOH w20x. Also, it seems that for 6-Me-BPOH, the higher of the two emissive states is less emissive than the lower one. Recently, we have shown that methylation of BPŽOH. 2 has negligible effect on the excited state charge distribution and hence also on the proton transfer dynamics in the excited state of this molecule w26x. Likewise, the excited states of BPOH and 6-Me-BPOH should be very similar. We thus attribute the short-living Ž; 1 ps. emission to arise from the S 2 Žnp ) . state, ; 2 000 cmy1 above the lower S 1Ž p p ) . state. The latter has a solvent-dependent lifetime Ž190 ps in cyclohexane, 12 ps in acetonitrile.. The short lifetime of ; 1 ps for S 2 might at first sight be attributed to a rapid population relaxation from S 2 to S 1. From the simulation of the fluorescence transients starting from a simple kinetic scheme in which Ž1. S 2 decays partially into S 1 and partially into S 0 and Ž2. S 1 is fed from S 2 and decays into S 0 , it is readily shown, after substitution of the experimental time constants, that the transients detected for
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Fig. 4. Reconstructed fluorescence spectra of 6-Me-BPOH dissolved in: Ža. cyclohexane and Žb. acetonitrile. The delay times following the excitation pulse are indicated in the panels. The filled squares represent the data points obtained in the spectral reconstruction procedure. The solid curve is the best fit to the reconstructed spectra. This curve is a sum of two separate bands: band I, represented by the dotted curve, relates to emission from a S1Ž p, p ) . state; and band II, represented by a dashed curve, is related to emission from a S 2 Žn, p ) . state.
l ) 550 nm are expected to exhibit a rise component with a time constant of ; 1 ps. Although such a fast rise should be experimentally resolvable, it could not be observed. We infer from the absence of the rapid
rise component the existence of a relaxation channel for S 2 additional to the S 2 to S 1 internal conversion process. Possibly the S 2 state is strongly coupled to the ground state, thus providing an additional chan-
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Table 1 Spectral properties of 6-Me-BPOH and BPŽOH. 2 -6-COOMe in various solvents Absorption Žcmy1 .
Emission band I
band II
max. Žcmy1 .
decay time Žps.
max. Žcmy1 .
decay time Žps.
6-Me-BPOH in cyclohexane in 3-methylpentane w19x in acetonitrile
30 670 30 700 31 490
18 220 16 940 a 17 150
190 – 12
20 020 – 19 200
0.5 – 1.0
BPŽOH. 2-6-COOMe in cyclohexane in 3-methylpentane w19x in acetonitrile
28 950 29 000 29 620
18 530 17 370 a 17 300
170 – 21
20 100 – 19 600
3.0 – 2.6
a
Peak of the steady-state emission bands.
nel to the standard route of depopulation via the S 1 level. The short lifetime of the S 1 state, on the pico- to subnanosecond time scale, is indicative that nonradiative decay dominates the depopulation to the ground state. The solvent dependence of the lifetime is indicative that the radiationless decay is mediated by the solvent. In the more polar solvent the dark process is faster. Within the time resolution of our experiments Ž; 150 fs., only the excited state dynamics for 6Me-BPOH in its keto tautomer is probed, i.e., the data are typical of the molecule after the excited state proton transfer. No response of the primary molecule, in its excited enol state, is observed. We thus conclude a very fast excited-state single-proton transfer, namely within the pulse duration time of 150 fs. In this regard the result is similar to that previously found for the formation of the excited monoketo tautomer of BPŽOH. 2 w7x. There too the formation time is within 150 fs. As mentioned in Section 3, the fluorescence transients showed no dependence on the excitation wavelength. This is in contrast to the observations for BPŽOH. 2 in which case the transients could be affected when changing the excitation wavelength w16x. For the BPŽOH. 2 system these results were discussed to be representative of an activation energy barrier of ; 600 cmy1 for the dienol-to-monoketo reaction in the excited state w12,16x. Apparently, such an activation barrier cannot be resolved in the case of 6-Me-
BPOH where only a single proton is transferred in the excited state. 4.2. BP(OH)2-6-COOMe As is seen from Fig. 1, the emission spectra of BPŽOH. 2-6-COOMe and 6-Me-BPOH are quite similar but red-shifted with respect to the BPŽOH. 2 emission. The band emissions of the BPŽOH. 2-6-COOMe and 6-Me-BPOH molecules also show the same solvent shift when the polarity of the aprotic solvent is changed Žred-shift in more polar solvent, cf. Fig. 1.. As in the case of 6-Me-BPOH, we have reconstructed from the measured fluorescence transients for BPŽOH. 2-6-COOMe its emission spectrum at several times after the pulsed excitation. The results for BPŽOH. 2-6-COOMe, dissolved in cyclohexane, are presented as black squares for a series of delay times in Fig. 4a. The steady-state emission is obtained ; 20 ps following laser excitation. At earlier times, the emission appears to consist of a superposition of two bands. This is illustrated by the dotted Žband I. and dashed Žband II. log-normal shaped curves in Fig. 5a. Likewise, in Fig. 5b we present the results for the reconstructed spectra of BPŽOH. 2-6COOMe dissolved in acetonitrile. Table 1 comprises the results for the band positions and lifetimes. As follows from Table 1, the similarity of the excited state properties of the BPŽOH. 2-6-COOMe and 6-Me-BPOH molecules is striking. In particular,
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Fig. 5. Reconstructed fluorescence spectra of BPŽOH. 2 -6-COOMe dissolved in Ža. cyclohexane and Žb. acetonitrile. The delay times following the excitation pulse are indicated in the panels. The filled squares represent the data points obtained in the spectral reconstruction procedure. The solid curve is the best fit to the reconstructed spectra. This curve is a sum of two separate bands: band I, represented by the dotted curve, relates to emission from a S1Ž p, p ) . state; and band II, represented by a dashed curve, is related to emission from a S 2 Žn, p ) . state.
position and lifetimes of emission bands I and II for both molecules are comparable. This feature leads us to conclude that also in the excited BPŽOH. 2-6-
COOMe molecule a single-proton transfer occurs. This implies that for BPŽOH. 2-6-COOMe in the excited state only a monoketo tautomer is formed.
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Similar to the case of 6-Me-BPOH, in BPŽOH. 2-6COOMe the emission may arise from S 1Ž p p ) . and S 2 Žnp ) . singlet states. The influence of the substitutional –COOCH 3 group probably is such that the transfer of a second proton, as occurs in the case of excited BPŽOH. 2 w7x or methylated BPŽOH. 2 w26x, is hampered. It is to be expected that the –COOCH 3 ester group due to its electrophilic character more strongly affects the electronic charge distribution of the molecule in the excited state than methyl substitution. More specifically, due to the electrophilic nature of the –COOCH 3 group, the electronic charge at the nearby nitrogen atom in the pyridyl ring to which the ester group is attached, will be reduced, as shown in Ref. w19x. This, in turn, will reduce the proton affinity of that nitrogen site, thus decreasing the driving force for the second proton transfer. As was done for 6-Me-BPOH, we have also simulated for BPŽOH. 2-6-COOMe the fluorescence transients expected within the three-level S 0 , S 1 and S 2 scheme for detection wavelengths higher than 620 nm. The simulations show the presence of a rise component with a typical time of 3 ps. This component could not be verified experimentally. Thus, similar to 6-Me-BPOH, for BPŽOH. 2-6-COOMe a population relaxation process in addition to the S 2 ™ S 1 internal conversion is inferred. It is noted that also for BPŽOH. 2-6-COOMe the lifetime of the S 1 state is strongly solvent-dependent. It is likely therefore that the lifetime of this S 1 state is also limited by a solvent-mediated population relaxation process. In summary, from femtosecond fluorescence upconversion experiments it is concluded that the two bipyridine derivatives compounds, 6-Me-BPOH and BPŽOH. 2-6-COOMe, have almost identical excitedstate dynamics. For both molecules a short-living fluorescence from the S 2 state has been inferred. The results show that by a proper choice of the bipyridine substituent, one may control the intramolecular proton transfer mechanism Žsingle- vs. double-proton transfer. in the bipyridyl-diol aromatic systems.
Acknowledgements The research was financially supported by the Council for Chemical Sciences of the Netherlands Organization for Scientific Research ŽCW-NWO..
The discussions with Professor Ł. Kaczmarek, who synthesized both studied compounds, are gratefully acknowledged. References w1x M. Wiechmann, H. Port, W. Frey, F. Laermer, T. Elsaesser, J. Phys. Chem. 95 Ž1991. 1918. w2x W. Frey, T. Elsaesser, Chem. Phys. Lett. 189 Ž1992. 565. w3x J.L. Herek, S. Pedersen, L. Banares, A.H. Zewail, J. Chem. ˜ Phys. 97 Ž1992. 9046. w4x A. Douhal, F. Lahmani, A.H. Zewail, Chem. Phys. 207 Ž1996. 477. w5x C. Chudoba, S. Lutgen, T. Jentzsch, E. Riedle, M. Woerner, T. Elsaesser, Chem. Phys. Lett. 240 Ž1995. 35. w6x C. Chudoba, E. Riedle, M. Pfeiffer, T. Elsaesser, Chem. Phys. Lett. 263 Ž1996. 622. w7x H. Zhang, P. van der Meulen, M. Glasbeek, Chem. Phys. Lett. 253 Ž1996. 97. w8x M. Chachisvillis, T. Fiebig, A. Douhal, A.H. Zewail, J. Phys. Chem. 102 Ž1998. 669. w9x H. Bulska, Chem. Phys. Lett. 98 Ž1983. 298. w10x P. Borowicz, A. Grabowska, R. Wortmann, W. Liptay, J. Lumin. 52 Ž1992. 265. w11x R. Wortmann, K. Elich, S. Lebus, W. Liptay, P. Borowicz, A. Grabowska, J. Chem. Phys. 96 Ž1992. 9724. w12x P. Prosposito, D. Marks, H. Zhang, M. Glasbeek, J. Phys. Chem. A 102 Ž1998. 8894. w13x F.W. Langkilde, A. Mordzinski, R. Wilbrandt, Chem. Phys. Lett. 190 Ž1992. 305. w14x A. Mordzinski, K. Kownacki, A. Les, ´ N.A. Oyler, L. Adamowicz, F.W. Langkilde, R. Wilbrandt, J. Phys. Chem. 98 Ž1994. 5212. w15x D. Marks, H. Zhang, M. Glasbeek, P. Borowicz, A. Grabowska, Chem. Phys. Lett. 275 Ž1997. 370. w16x D. Marks, P. Prosposito, H. Zhang, M. Glasbeek, Chem. Phys. Lett. 289 Ž1998. 535. w17x A.L. Sobolewski, L. Adamowicz, Chem. Phys. Lett. 252 Ž1996. 33. w18x V. Barone, C. Adamo, Chem. Phys. Lett. 241 Ž1995. 1. w19x P. Borowicz, A. Grabowska, A. Les, ´ Ł. Kaczmarek, B. Zagrodzki, Chem. Phys. Lett. 291 Ž1998. 351. w20x H. Bulska, A. Grabowska, Z.R. Grabowski, J. Lumin. 35 Ž1986. 189. w21x Ł. Kaczmarek, R. Balicki, J. Lipkowski, P. Borowicz, A. Grabowska, J. Chem. Soc., Perkin Trans. 2 Ž1994. 1603. w22x Ł. Kaczmarek, B. Zagrodski, B. Kamienski, M. Pietrzak, W. Schiff, A. Les Žto be published.. w23x E.R. Middelhoek, P. van der Meulen, J.W. Verhoeven, M. Glasbeek, Chem. Phys. 198 Ž1995. 373. w24x H. Zhang, A.M. Jonkman, P. van der Meulen, M. Glasbeek, Chem. Phys. Lett. 224 Ž1994. 551. w25x M. Maroncelli, G.R. Fleming, J. Chem. Phys. 86 Ž1987. 6221. w26x D. Marks, H. Zhang, M. Glasbeek, J. Lumin. 76,77 Ž1998. 52.
Chemical Physics Letters 309 Ž1999. 19–28 www.elsevier.nlrlocatercplett
Femtosecond intramolecular proton transfer in photoexcited mono- and dienol derivatives of bipyridine D. Marks a , H. Zhang a , P. Borowicz b, A. Grabowska b, M. Glasbeek a
a,)
Laboratory for Physical Chemistry, UniÕersity of Amsterdam, Nieuwe Achtergracht 129, NL-1018 WS Amsterdam, The Netherlands b Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44 r 52, 01-224 Warsaw, Poland Received 29 March 1999; in final form 4 June 1999
Abstract A femtosecond study of the intramolecular proton transfer dynamics in photoexcited mono- and dienol derivative compounds of pyridine is reported. It is shown that when a strongly electron-withdrawing substituent is attached to w2,2X-bipyridylx-3,3X-diol, the excited-state proton transfer of this compound is changed from a double-proton transfer to a single-proton transfer. The excited-state single-proton transfer time in the title compounds is - 150 fs. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Ultrafast excited-state intramolecular proton transfer ŽESIPT. is characteristic of numerous enol–keto tautomerization reactions in the condensed phase w1–8x. Both single- and double-proton transfer processes have been studied. For w2,2X-bipyridylx-3,3Xdiol, henceforth abbreviated as BPŽOH. 2 , the mechanism of the double-proton transfer process in the excited state has been extensively studied, experimentally w7,9–16x and theoretically w17,18x. Following short-pulse optical excitation, concerted Žonestep. and consecutive Žtwo-step. double-proton transfer reactions were found to occur concurrently w7,12x. The concerted process takes place within 100 fs. In the consecutive process, the first proton is transferred
) Corresponding author. Fax: q31 20 525 6994; e-mail: [email protected]
in - 100 fs, the second proton follows in a few picoseconds. In this Letter, we investigate the ESIPT dynamics in two derivative compounds of BPŽOH. 2 . The first of these Žsee also Scheme 1. is 6-Me-BPOH Ž6methyl-w2,2X-bipyridylx-3X-ol.; the second compound is BPŽOH . 2 -6-COOMe Žmethyl-3,3X-dihydroxyw2,2X-bipyridylx-6-carboxylate.. 6-Me-BPOH serves as a model compound for studying the dynamics of a single-proton transfer in the excited state of a bipyridyl ring system. In the case of BPŽOH. 2-6COOMe we examine the influence of an electrophilic substituent on the intramolecular doubleproton transfer dynamics in BPŽOH. 2 . Both were already introduced to the literature among six new derivatives of BPŽOH. 2 and BPOH w19x. The properties of their ground states were studied experimentally and by ab initio calculations. The main aim of this work was to study the influence of electrondonating and -withdrawing substituents, introduced
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 6 5 9 - 4
D. Marks et al.r Chemical Physics Letters 309 (1999) 19–28
20
Scheme 1.
next to nitrogen atom, on the phototautomerization mechanism of the molecule. The steady-state absorption and fluorescence spectra in inert solvents were also determined and compared. Previously, Bulska et al. w20x have reported the emission spectrum of w2,2X-bipyridylx-3-ol ŽBPOH.. Compared to BPŽOH. 2 it appeared that the fluorescence quantum yield for BPOH is smaller by two orders of magnitude. INDOrS calculations suggested that the keto tautomer of BPOH has a nonfluorescent S 2 Žn, p ) . state very close to the lower fluorescent S 1Ž p, p ) . state w21x. The low fluorescence quantum yield for BPOH then results from effective coupling between the fluorescent and nonfluorescent excited singlet states w20x. Compared to BPOH, 6-Me-BPOH turns out to possess a much higher fluorescence efficiency w19x. As a result, the time dependence of the fluorescence of this molecule could be investigated using the femtosecond fluorescence upconversion technique w7x. The fluorescence transients provide information concerning the dynamics of the intramolecular single-proton transfer process in the excited state Žvide infra.. Also for the second derivative compound, BPŽOH. 2-6-COOMe, fluorescence upconversion transients are reported. It is discussed that for BPŽOH. 2-6-COOMe only a single- Žand not a double-. proton transfer occurs due to the presence of the electrophilic COOCH 3 substituent.
2. Experimental Synthesis of both studied compounds was described elsewhere w22x. Throughout the present Letter
the names of compounds are as in Ref. w19x. Spectrograde quality cyclohexane ŽMerck. and acetonitrile ŽMerck. were used as solvents without further purification. Two pulsed-laser setups were used for the measurement of the fluorescence transients: a regeneratively amplified Ti:sapphire laser system with upconversion detection for the time span 150 fs–100 ps, and a picosecond laser system with time-correlated single-photon-counting ŽSPC. detection for the time range 15 ps–5 ns. The systems have been described in detail previously w12,23x. In the femtosecond laser system, excitation was accomplished by laser pulses Ž; 100 fs. from an OPA system in the range of 300–380 nm Ž; 0.1 mJrpulse.. The photo-induced fluorescence was detected after upconversion, in a BBO crystal, with a gating pulse Žat 800 nm.. The time elapsed between the excitation pulse and the gating pulse could be controlled by means of an optical delay line, with a variable optical path length, for the gating pulses. Photodetection was limited to the upconverted beam polarized at a magic angle with respect to the excitation beam. In this way the probed fluorescence decay did not reflect contributions of rotational diffusion motions of the solute in the liquid. In the picosecond fluorescence setup w23x, photoexcitation was at 322 nm by means of picosecond pulses Ž4 mW, ; 1 ps pulse duration.. The fluorescence emitted from the sample in a direction perpendicular to the excitation beam was focused on to the entrance slit of a monochromator outfitted with a multi-channel plate photodetector. A linear polarizer was inserted in the detection pathway to detect at magic-angle conditions. The instrumental response time was ; 17 ps ŽFWHM..
3. Results 3.1. 6-Me-BPOH Fig. 1 shows the CW absorption and emission spectra as measured for 6-Me-BPOH dissolved in cyclohexane and acetonitrile. The absorption spectrum of the 6-Me-BPOH molecule in both solvents is similar, with the absorption band maximum at 30 670
D. Marks et al.r Chemical Physics Letters 309 (1999) 19–28
21
detection is at the blue side of the spectrum, however, an additional fast decay component appears. Now the transients are best fitted to a bi-exponential decay function of the form, yt
I Ž l , t . s a1 Ž l . exp
Fig. 1. Steady-state absorption and emission spectra. Emission spectra: Ža. BPŽOH. 2 -6-COOMe in acetonitrile; Žb. 6-Me-BPOH in acetonitrile; Žc. BPŽOH. 2 -6-COOMe in cyclohexane; Žd. 6Me-BPOH in cyclohexane; and Že. BPŽOH. 2 in cyclohexane. Absorption spectra: Žf. BPŽOH. 2 ; Žg. BPŽOH. 2 -6-COOMe; and Žh. 6-Me-BPOH, dissolved in cyclohexane.
and 31 490 cmy1 in cyclohexane and acetonitrile, respectively. The band maximum in the emission spectrum of 6-Me-BPOH shows a red-shift when a more polar solvent is used. The emission band maximum is at 18 220 cmy1 when 6-Me-BPOH is dissolved in cyclohexane and 17 150 cmy1 for the solute dissolved in acetonitrile. The fluorescence transients measured with the time-correlated single-photon-counting fluorescence setup could be fitted to a single-exponential decay function convoluted with the system response function. The fluorescent-state lifetime of 6-Me-BPOH in cyclohexane is found as 190 ps; the lifetime of the emissive state is 12 ps for the solute dissolved in acetonitrile. Thus the fluorescence lifetime is sensitive to the nature of the solvent. The transients obtained in the femtosecond fluorescence upconversion measurements exhibit a strong dependence on the detection wavelength. Transients at a few selected detection wavelengths for 6-MeBPOH dissolved in cyclohexane are presented in Fig. 2a. When detecting at the red part of the emission band Ž l ) 580 nm. the fluorescence consists of a sharp instantaneous rise followed by a decay. These transients could still be fitted, after deconvolution with the system response function, to a single-exponential decay function. The decay time is equal to that obtained in the picosecond measurements. When
ž / t1
yt
q a2 Ž l . exp
ž / t2
,
in which t 1 is the same as above for detection wavelengths higher than 580 nm. The faster decay time, t 2 , is 0.5 ps. Best fittings are plotted as solid curves in Fig. 2a. It is noted that at all detection wavelengths within the emission band only decaying components are observed. Upconversion transients observed for 6-Me-BPOH dissolved in acetonitrile at a few selected wavelengths are presented in Fig. 2b. The features are similar as for the solute in cyclohexane. The bi-exponential fit now yields t 1 s 12 ps, and t 2 s 1.0 ps. Again only decaying components are observed. Using the OPA system, the excitation wavelength was varied in the range from 310 to 360 nm in the fluorescence transient measurements. No effect of the excitation wavelength on the fluorescence kinetics was found.
3.2. BP(OH)2-6-COOMe Fig. 1 includes CW absorption and emission spectra as measured for BPŽOH. 2-6-COOMe dissolved in acetonitrile and cyclohexane. The absorption spectrum is little dependent on the solvent used. The absorption band maximum is at 28 950 and 29 620 cmy1 , in cyclohexane and acetonitrile, respectively. The steady-state emission shows a strong resemblance with the emission of 6-Me-BPOH. As for the 6-Me-BPOH emission, the emission band maximum in the spectrum of BPŽOH. 2-6-COOMe is shifted when changing solvent, with the maximum at 18 530 and 17 300 cmy1 , in cyclohexane and acetonitrile, respectively. For BPŽOH. 2-6-COOMe fluorescence transients were measured as before. The transients of the picosecond measurements fitted, after deconvolution with the response function of the SPC setup, a
22
D. Marks et al.r Chemical Physics Letters 309 (1999) 19–28
Fig. 2. Fluorescence upconversion transients of 6-Me-BPOH dissolved in: Ža. cyclohexane and Žb. acetonitrile. In the 70 ps time window of the figure, the system response function would appear as a d-function and thus this function has been left out.
single-exponential decay. The decay time, corresponding to the fluorescence lifetime, is dependent on the solvent. The decay time is 170 ps for BPŽOH. 2-6-COOMe dissolved in cyclohexane and 21 ps for the molecule dissolved in acetonitrile.
As for 6-Me-BPOH, the fluorescence upconversion transients Žobtained in the femtosecond upconversion experiments. display a significant dependence on the detection wavelength Žsee Fig. 3a and b.. When detection is at the red part of the spectrum
D. Marks et al.r Chemical Physics Letters 309 (1999) 19–28
23
Fig. 3. Fluorescence upconversion transients of BPŽOH. 2 -6-COOMe dissolved in: Ža. cyclohexane and Žb. acetonitrile. In the 70 ps time window of the figure, the system response function would appear as a d-function and thus this function has been left out.
Ž l ) 580 nm., the transients consist of an instantaneous rise followed by a decay. After deconvolution with the system response function, these transients fitted a single-exponential decay function. The decay times are the same as those obtained in the picosec-
ond experiments. When detection is at the high-energy part of the emission band, a second, much faster, decay component is present. The transients were fitted to the bi-exponential decay function given above. t 1 has the same value as determined in the
24
D. Marks et al.r Chemical Physics Letters 309 (1999) 19–28
picosecond measurements. The additional decay component has a decay time, t 2 , of 3.0 ps when the molecule dissolved in cyclohexane and 2.6 ps when it is dissolved in acetonitrile. The experiments were repeated using a series of excitation wavelengths in the range from 320 to 360 nm. No variation in the temporal characteristics of the fluorescence of BPŽOH. 2-6-COOMe with the excitation wavelength was observed.
4. Discussion 4.1. 6-Me-BPOH We applied the method of spectral reconstruction w24,25x to study the temporal behavior of the emission spectra of 6-Me-BPOH from its observed fluorescence upconversion transients. The data points as calculated from the transients for 6-Me-BPOH in cyclohexane are presented as black squares in Fig. 4a. After ; 2 ps, the reconstructed spectrum has reached its final form and the same spectrum as in the CW experiment is obtained. For the latter band emission, the low-energy part nicely fitted a log-normal line shape function. We label this band as band I. The reconstructed emission spectra at early times Žup to a few ps., are a superposition of the aforementioned band I and an additional fast decaying band Žband II.. Band II has its band maximum at 20 020 cmy1 for the solute in cyclohexane. The drawn curves in Fig. 4a are the resultants of the log-normal shaped dotted curves Žband I. and the log-normal shaped dashed curves Žband II.. The fluorescence in the red part of the spectrum shows a single-exponential decay, with a time constant of 190 ps for 6-MeBPOH dissolved in cyclohexane. This time constant is thus characteristic for the decay of band I. The typical decay time of band II is 0.5 ps in cyclohexane, as obtained from the fittings of the transients in the blue side of the emission band. Table 1 summarizes the results for 6-Me-BPOH dissolved in cyclohexane. A similar analysis of the upconversion results for 6-Me-BPOH dissolved in acetonitrile was performed.
Fig. 4b shows the points reconstructed from the experimental data and the simulated log-normal fits for bands I and II. The position of the band maxima and the emission band lifetimes are summarized in Table 1. Previously, Bulska et al. w20x have studied the spectroscopy of BPOH in 3-methylpentane solution. The large Stokes shift of ; 13 000 cmy1 was discussed to be characteristic of an ESIPT process in which a keto tautomer is formed. A very low quantum yield Ž; 10y3 . for the fluorescence of BPOH was found w20,21x probably caused by coupling to torsional motions that could give rise to an enhanced radiationless decay out of the excited state. An increase by a factor of ; 35 in the fluorescence quantum yield of BPOH was measured when lowering the temperature to 77 K. The temperature effect may be explained from the closeness of two almost degenerate excited states, as predicted by INDOrS calculations w20,21x, the lower of which is the S 1Ž p p ) . state with the higher oscillator strength Ž0.382., and the somewhat higher-lying S 2 Žnp ) . state, with an almost negligible oscillator strength Ž0.002.. Our measurements for 6-Me-BPOH show the presence of two emissive excited states almost 2 000 cmy1 apart. This energy separation is quite comparable to the value predicted in the INDO calculations for BPOH w20x. Also, it seems that for 6-Me-BPOH, the higher of the two emissive states is less emissive than the lower one. Recently, we have shown that methylation of BPŽOH. 2 has negligible effect on the excited state charge distribution and hence also on the proton transfer dynamics in the excited state of this molecule w26x. Likewise, the excited states of BPOH and 6-Me-BPOH should be very similar. We thus attribute the short-living Ž; 1 ps. emission to arise from the S 2 Žnp ) . state, ; 2 000 cmy1 above the lower S 1Ž p p ) . state. The latter has a solvent-dependent lifetime Ž190 ps in cyclohexane, 12 ps in acetonitrile.. The short lifetime of ; 1 ps for S 2 might at first sight be attributed to a rapid population relaxation from S 2 to S 1. From the simulation of the fluorescence transients starting from a simple kinetic scheme in which Ž1. S 2 decays partially into S 1 and partially into S 0 and Ž2. S 1 is fed from S 2 and decays into S 0 , it is readily shown, after substitution of the experimental time constants, that the transients detected for
D. Marks et al.r Chemical Physics Letters 309 (1999) 19–28
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Fig. 4. Reconstructed fluorescence spectra of 6-Me-BPOH dissolved in: Ža. cyclohexane and Žb. acetonitrile. The delay times following the excitation pulse are indicated in the panels. The filled squares represent the data points obtained in the spectral reconstruction procedure. The solid curve is the best fit to the reconstructed spectra. This curve is a sum of two separate bands: band I, represented by the dotted curve, relates to emission from a S1Ž p, p ) . state; and band II, represented by a dashed curve, is related to emission from a S 2 Žn, p ) . state.
l ) 550 nm are expected to exhibit a rise component with a time constant of ; 1 ps. Although such a fast rise should be experimentally resolvable, it could not be observed. We infer from the absence of the rapid
rise component the existence of a relaxation channel for S 2 additional to the S 2 to S 1 internal conversion process. Possibly the S 2 state is strongly coupled to the ground state, thus providing an additional chan-
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Table 1 Spectral properties of 6-Me-BPOH and BPŽOH. 2 -6-COOMe in various solvents Absorption Žcmy1 .
Emission band I
band II
max. Žcmy1 .
decay time Žps.
max. Žcmy1 .
decay time Žps.
6-Me-BPOH in cyclohexane in 3-methylpentane w19x in acetonitrile
30 670 30 700 31 490
18 220 16 940 a 17 150
190 – 12
20 020 – 19 200
0.5 – 1.0
BPŽOH. 2-6-COOMe in cyclohexane in 3-methylpentane w19x in acetonitrile
28 950 29 000 29 620
18 530 17 370 a 17 300
170 – 21
20 100 – 19 600
3.0 – 2.6
a
Peak of the steady-state emission bands.
nel to the standard route of depopulation via the S 1 level. The short lifetime of the S 1 state, on the pico- to subnanosecond time scale, is indicative that nonradiative decay dominates the depopulation to the ground state. The solvent dependence of the lifetime is indicative that the radiationless decay is mediated by the solvent. In the more polar solvent the dark process is faster. Within the time resolution of our experiments Ž; 150 fs., only the excited state dynamics for 6Me-BPOH in its keto tautomer is probed, i.e., the data are typical of the molecule after the excited state proton transfer. No response of the primary molecule, in its excited enol state, is observed. We thus conclude a very fast excited-state single-proton transfer, namely within the pulse duration time of 150 fs. In this regard the result is similar to that previously found for the formation of the excited monoketo tautomer of BPŽOH. 2 w7x. There too the formation time is within 150 fs. As mentioned in Section 3, the fluorescence transients showed no dependence on the excitation wavelength. This is in contrast to the observations for BPŽOH. 2 in which case the transients could be affected when changing the excitation wavelength w16x. For the BPŽOH. 2 system these results were discussed to be representative of an activation energy barrier of ; 600 cmy1 for the dienol-to-monoketo reaction in the excited state w12,16x. Apparently, such an activation barrier cannot be resolved in the case of 6-Me-
BPOH where only a single proton is transferred in the excited state. 4.2. BP(OH)2-6-COOMe As is seen from Fig. 1, the emission spectra of BPŽOH. 2-6-COOMe and 6-Me-BPOH are quite similar but red-shifted with respect to the BPŽOH. 2 emission. The band emissions of the BPŽOH. 2-6-COOMe and 6-Me-BPOH molecules also show the same solvent shift when the polarity of the aprotic solvent is changed Žred-shift in more polar solvent, cf. Fig. 1.. As in the case of 6-Me-BPOH, we have reconstructed from the measured fluorescence transients for BPŽOH. 2-6-COOMe its emission spectrum at several times after the pulsed excitation. The results for BPŽOH. 2-6-COOMe, dissolved in cyclohexane, are presented as black squares for a series of delay times in Fig. 4a. The steady-state emission is obtained ; 20 ps following laser excitation. At earlier times, the emission appears to consist of a superposition of two bands. This is illustrated by the dotted Žband I. and dashed Žband II. log-normal shaped curves in Fig. 5a. Likewise, in Fig. 5b we present the results for the reconstructed spectra of BPŽOH. 2-6COOMe dissolved in acetonitrile. Table 1 comprises the results for the band positions and lifetimes. As follows from Table 1, the similarity of the excited state properties of the BPŽOH. 2-6-COOMe and 6-Me-BPOH molecules is striking. In particular,
D. Marks et al.r Chemical Physics Letters 309 (1999) 19–28
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Fig. 5. Reconstructed fluorescence spectra of BPŽOH. 2 -6-COOMe dissolved in Ža. cyclohexane and Žb. acetonitrile. The delay times following the excitation pulse are indicated in the panels. The filled squares represent the data points obtained in the spectral reconstruction procedure. The solid curve is the best fit to the reconstructed spectra. This curve is a sum of two separate bands: band I, represented by the dotted curve, relates to emission from a S1Ž p, p ) . state; and band II, represented by a dashed curve, is related to emission from a S 2 Žn, p ) . state.
position and lifetimes of emission bands I and II for both molecules are comparable. This feature leads us to conclude that also in the excited BPŽOH. 2-6-
COOMe molecule a single-proton transfer occurs. This implies that for BPŽOH. 2-6-COOMe in the excited state only a monoketo tautomer is formed.
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Similar to the case of 6-Me-BPOH, in BPŽOH. 2-6COOMe the emission may arise from S 1Ž p p ) . and S 2 Žnp ) . singlet states. The influence of the substitutional –COOCH 3 group probably is such that the transfer of a second proton, as occurs in the case of excited BPŽOH. 2 w7x or methylated BPŽOH. 2 w26x, is hampered. It is to be expected that the –COOCH 3 ester group due to its electrophilic character more strongly affects the electronic charge distribution of the molecule in the excited state than methyl substitution. More specifically, due to the electrophilic nature of the –COOCH 3 group, the electronic charge at the nearby nitrogen atom in the pyridyl ring to which the ester group is attached, will be reduced, as shown in Ref. w19x. This, in turn, will reduce the proton affinity of that nitrogen site, thus decreasing the driving force for the second proton transfer. As was done for 6-Me-BPOH, we have also simulated for BPŽOH. 2-6-COOMe the fluorescence transients expected within the three-level S 0 , S 1 and S 2 scheme for detection wavelengths higher than 620 nm. The simulations show the presence of a rise component with a typical time of 3 ps. This component could not be verified experimentally. Thus, similar to 6-Me-BPOH, for BPŽOH. 2-6-COOMe a population relaxation process in addition to the S 2 ™ S 1 internal conversion is inferred. It is noted that also for BPŽOH. 2-6-COOMe the lifetime of the S 1 state is strongly solvent-dependent. It is likely therefore that the lifetime of this S 1 state is also limited by a solvent-mediated population relaxation process. In summary, from femtosecond fluorescence upconversion experiments it is concluded that the two bipyridine derivatives compounds, 6-Me-BPOH and BPŽOH. 2-6-COOMe, have almost identical excitedstate dynamics. For both molecules a short-living fluorescence from the S 2 state has been inferred. The results show that by a proper choice of the bipyridine substituent, one may control the intramolecular proton transfer mechanism Žsingle- vs. double-proton transfer. in the bipyridyl-diol aromatic systems.
Acknowledgements The research was financially supported by the Council for Chemical Sciences of the Netherlands Organization for Scientific Research ŽCW-NWO..
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