J. Phys. Chem. 1991, 95, 1918-1923
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Time-Resolved Spectroscopy on Ultrafast Proton Transfer In 2 4 2’-Hydroxy-5‘-methylphenyl) benzotriazole In Liquld and Polymer Environments M. Wiechmann, H. Port,* 3. Physikalisches Institut der Universitiit Stuttgart, 0-7000 Stuttgart 80, Federal Republic of Germany
W. Frey, F. Liirmer, and T. Elsjisser Physik Department E 1 1 der Technischen Universitat Munchen, 0-8000 Munchen 2, Federal Republic of Germany (Received: June 4 , 1990; In Final Form: November 9, 1990) The intramolecular proton transfer in 242’-hydroxy-5’-methylphenyl)benmtriamle(TIN) is studied in different environments by stationary as well as pice and femtosecond spectroscopy. Two ground-state conformers with a relative concentration depending on the specific solvent are distinguished from the spectra and the picosecond kinetics. The main species in nonpolar solution and in the polymer shows a weak fluorescence in the red with subpicosecond buildup and decay times. This emission is indicative of excited-state proton transfer resulting in the formation of a planar keto-type tautomer. Transient absorption measured on a femtosecond time scale gives evidence of the ultrafast keto-enol back-reaction occurring in the electronic ground state with time “ a n t s between 500 fs and 1.2 ps. In addition, a species of nonplanar geometry in the electronic ground state is detected showing emission in the blue spectral range. The fraction of the latter tautomers lies between less in nonpolar solvents and up to 60% in polar solution. than
1. Introduction Organic compounds undergoing intramolecular proton transfer in electronically excited states have been investigated for a long time by a variety of spectroscopic techniques.’ In this type of reaction, the change of the proton location within the molecular geometry results in drastic changes of the vibrational and electronic spectra of the molecules. For instance, the emission spectrum of the new species is Stokes-shifted by several thousands of wavenumbers relative to the absorption band of the original tautomer. Proton-transfer times shorter than a few picoseconds have been estimated for numerous molecules from the rise time of the fluorescence attributed to the new tautomer.2-10 In many cases, the detailed reaction dynamics and the influence of the environment on the reaction mechanism are not well understood. In the present work the photoinduced reaction cycle of 242’hydroxy-5’-methylpheny1)benzotriazole (trade name TINUVIN P or TIN) comprising the enol (I) and keto-type (11) tautomer is studied. TIN has received increasing attention because of its particularly high efficiency as UV photostabilizer of polymers. The absorption and emission properties of T I N have been investigated extensively in room-temperature solutions, in a variety of low-temperature glasses, and in crystals.11-22 enol keto type
(1)
(11)
( I ) (a) Weller, A. Natunvissenschafren 1955,42, 175. (b) Weller, A. Z. Elcktrochem. 1956, 60, 1144. (2) Elsiisser, T.; Kaiser, W. Chem. Phys. L r r r . 1986, 128, 231. (3) Barbara, P. F.; Brus, L. E.; Rentzepis, P. M. J . Am. Chem. Soc. 1980, 102, 2786, 5361. (4) Flom, S. R.; Barbara, P. F. Chem. Phys. Lori. 1983, 94, 488. (5) Shizuka, H.; Machii, M.; Higaki, Y.;Tanaka, M.; Tanaka, 1. J . Phys. Chem. 1985,89, 320. (6) Woolfe, G. J.; Melzig, M.; Schneider, S.;Dbrr, F. Chem. Phys. 1983, 77, 213. (7) (a) Elsiisser, T.; Schmetzer, B. Chem. Phys. Lrrr. 1987, 140, 293. (b) Elstisser, T.; Schmetzer, B.; Lipp, M.; Btiuerle, R. J. Chem. Phys. Lrrr. 1988, 148, 112. (8) Smith, K. K.;Kaufmann, K.J. J . Phys. Chrm. 1978,82, 2286. (9) OConnor, D. B.; Scott, G. W.; Coulter, D. R.; Gupta, A.; Webb, S. P.; Yeh, S.W.; Clark, J. H. Chem. Phys. Lea. 1985, 121, 417. (IO) Dick, B.; Ernsting, N. P. J . Phys. Chem. 1987, 91, 4261, ( I 1) Heller, H. 1. Eur. Polym. J . Suppl. 1969, 105. (12) (a) Heller, H. J.; Blattmann, H. R. Pure Appl. Chrm. 1972,30, 145. (b) Heller, H . J.; Blattmann, H. R. Pure Appl. Chem. 1974, 36, 141.
The So-SIabsorption spectrum consists of two bands with maxima at about 300 and 345 nm, the latter being characteristic of planar tautomers forming an intramolecular hydrogen bond. A strongly Stokes-shifted emission in the red spectral range has been observed in the solid state. In liquid solutions only a blue emission component with solvent-dependent quantum yield and lifetime has been detected. The absence of the red keto emission in solution and the influence of the solvent on the parameters of the blue fluorescence lead to the development of several controversial and often speculative models of the proton transfer in TIN. Very recently, we have given a preliminary account of results on the photophysics of TIN in solution.22 The red emission of the keto-type tautomer (11) has been detected in nonpolar solvents at room temperature. Applying femtosecond spectroscopy, the first direct measurement of the transfer rate of the proton in TIN has been made, and time constants of 100 and 500 fs were found for the enol-keto and keto-enol transformation, respectively. In this paper, we report on detailed investigations of TIN in different solvents and in polymer matrix by various spectroscopic methods. The short-lived keto emission is detected in all cases and distinguished from the blue fluorescence. The complete reaction cycle of TIN is monitored on a subpicosecond time scale, and the mechanisms of the proton-transfer reaction are analyzed. (13) Otterstedt, J.-E. A. J . Chcm. Phys. 1973, 58, 5716. (14) (a) Werner, T.; Kramer, H. E.A.; Kiistcr, B.; Herlinger, H. Angew. Makromol. Chem. 1976, 54, 15. (b) Werner, T. J. Phys. Chem. 1979,83, 320. (c) Werner, T.; Wbssner, G.; Kramer, H. E. A. Photodegradation and Phorosrabilirarion ojCoatings; ACS Symposium Series 151; Pappas, S. P., Winslow, F. H., Eds.; American Chemical Society: Washington, DC, 1981; P 1. (IS) (a) Wbssner, G.; Gbller, G.; Kollat, P.; Stezowski, J. J.; Hauser, M.; Klein, U. A. K.; Kramer, H. E. A. J . Phys. Chem. 1984, 88. 5544. (b) Wbsner, G.; Gbller, G.; Rieker, J.; Hoier, H.; Stezowski, J. J.; Daltrozzo, E.; Neureitcr, M.; Kramer, H. E. A. J . Phys. Chem. 1985, 89, 3629. (16) Kramer, H. E. A. Farbe Lack 1986, 92, 919. (17) Gbller, G.; Rieker, J.; Maier, A.; Stezowski, J. J.; Daltrozzo, E.; Neureiter, M.; Port, H.; Wiechmann, M.; Kramer, H . E. A. J . Phys. Chem. 1988, 92, 1452. (18) (a) Huston, A. L.; Mcrritt, C. D.; Scott, G. W. Picosecond Phenomena / I ; Hochstrasser, R. M., Kaiser, W., Shank, C. V., Eds.; Springer: Berlin. 1980; p 232. (b) Huston, A. L.; Scott, G. W.; Gupta, A. J . Chem. Phys. 1982, 76, 4978. (c) Huston, A. L.; Scott, G. W. J . Phys. Chem. 1987, 91, 1408. (19) Bocian, D. F.;Huston, A. L.; Scott, G.W. J . Chem. Phys. 1983, 79, 5802. (20) (a) Lee, M.; Yardley, J. T.; Hochstrasser, R. M. J . Phys. Chem. 1987, 91,4621. (b) Kim, Y. R.; Yardley, J. T.; Hochstrasser, R. M. Chem. Phys. 1989, 136, 31 I . (21) Ghiggino, K. P.; Scully, A. D.; Lcavcr, 1. H. J . Phys. Chem. 1986, 90, 5089. (22) Wiechmann, M.; Port, H.; Liirmer, F.; Frey, W.; Elsiisser, T.Chem. Phys. Lerr. 1990, 165, 28.
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0022-3654/91/2095-1918$02.50/0(8 1991 American Chemical Society
Ultrafast Proton Transfer in TIN
2. Experimental Section Steady-state emission spectra are recorded by using a mercury arc (HB0200) for excitation and a 0.55-m monochromator with a RCA 31034 photomultiplier for detection. Excitation spectra are measured with a xenon arc (XBOI 50) combined with a 0.25-m monochromator. The light intensities are corrected with a calibrated photodiode. The spectra are corrected for the spectral response of the apparatus. Time-resolved single-photon counting (SPC)was applied in the picosecond experiments. A cavity-dumped dye laser system synchronously pumped by a mode-locked Nd:YAG laser (Coherent Antares) serves for excitation. The dye laser output was frequency-doubled to generate excitation pulses at 328 nm. The fluorescence light is spectrally selected in a 0.5-m double monochromator with subtractive dispersion and detected by a cooled fast MCP photomultiplier. The SPC electronics consists of commercially available amplifiers (Avantek), constant fraction discriminators (Canberra), a time-to-amplitude converter (Ortec), and a home-built multichannel analyzer including a high-speed 16-bit analog-to-digital converter and a sequencer. The spectral correction on quantum flux includes the polarization dependence of the monochromator. The time response (fwhm) of the detection system is approximately 30 ps, while preserving excellent dynamics and sensitivity. To eliminate the influence of rotational depolarization on the fluorescence transients, all measurements are performed under magic angle (54.7') conditions. The system response is obtained by Raman scattering in I-octanol. A pump and probe technique is used in the femtosecond experiments. Intense pulses are generated in a colliding-pulse mode-locked (CPM) dye laser in conjunction with a six-pass dye amplifier, which is pumped by a XeCl excimer laser. Excitation pulses at 310 nm are obtained by frequency-doubling part of the intense red pulses. The second part of the output of the dye amplifier generates a femtosecond white light continuum between 400 and 800 nm by self-phase modulation in a jet of ethylene glycol. A tunable dichroic filter selects a narrow component of the continuum (bandwidth IO nm) which serves as the probe pulse. The temporal resolution of the system is about 70 fs. A detailed description of the apparatus has been given elsewhere.23 TIN and the 2'-methoxy derivative (MeTIN) were produced by Ciba-Geigy. The samples were purified by repeated recrystallization from n-heptane. Spectrograde solvents were used in all measurements. The sample thickness was 0.03 cm for the femtosecond and 0.4 cm for the picosecond measurements. The concentration of the samples lies in the range from IO-' to IO4 M for the different measurements. Doped polystyrene films were produced by solving T I N and polystyrene granulate (BASF) in methylene chloride and subsequent evaporation of the solvent. The thickness of the films was around 0.03 cm, achieving an optical density (OD) of N 1 at 310 nm.
3. Experimental Results In this section, we first present the steady-state spectra and picosecond emission measurements of T I N in different solvents. Subsequently, the results for T I N in polystyrene and the femtosecond experiments are summarized. All measurements were performed at room temperature. Infrared Absorption Spectra. The infrared absorption spectra of the 0-H stretching vibration of TIN were recorded in different solvents. In Figure I , results for TIN dissolved in C2CI4and in DMSO are presented. T I N in C2CI4shows an 0-H stretching frequency of approximately 3200 cm-', which is low compared to that of a free 0 - H group. This fact and the large spectral bandwidth of 200 cm-' point to the Occurrence of a strong intramolecular hydrogen bond between the 0-H group and a nitrogen atom in the triazole moiety favoring a planar geometry of the TIN molecules. The H bridge reduces the force constant of the stretching vibration and leads-via anharmonic coupling (23) (a) Lllrmer, F.; Eldsacr, T.; Kaiser, W. Chem. Phys. Lcrr. 1989, IS6, 381. (b) Urmer, F.; Israel, W.; EIsilaser, T. J . Opt. Soc. Am. B 1990, 7, 1604.
The Journal of Physical Chemistry, Vol. 95, No. 5, 1991 1919
TIN in DMSO-d6
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Figure 1. Infrared absorption spectra of TIN dissolved in C2C14and DMSO,energy region of the 0-H stretching vibration. 450
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Figure 2. Absorption spectra of TIN in C2Cl4 (a), TIN in DMSO (b), and MeTlN in C2CI4(c).
of the fast stretching motion and low-frequency degrees of freedom-to the broadening of the band.24 The 0-H frequency of TIN dissolved in DMSO is considerably higher (3500 cm-I), demonstrating the weakening of the H bridge by the more polar surrounding. Electronic Absorprion Spectra. Figure 2 summarizes absorption spectra of TIN in different solvents. The solid line represents the absorption spectrum of TIN in C2C1! consisting of two distinct bands around 345 and 300 nm, respectively. The long-wavelength absorption band is characteristic for a planar chromophore comprising the whole molecular structure whereas the short-wavelength band is commonly attributed to electronic excitation located in the triazole moiety. Correspondingly, the absorption spectrum of the nonplanar MeTIN dissolved in C2CI4only shows the latter band (dashed line c in Figure 2). The absorption spectra of TIN in acetonitrile and I-octanol are close to the result for C2CI4. In DMSO, however, the strength of the longwavelength component is reduced substantially (dashed line b in Figure 2), giving evidence for a smaller (relative) concentration of the planar species. A crude fitting of the absorption spectrum with two Gaussians leads to a concentration ratio of 0.4/0.6 for planar and nonplanar tautomers. This estimate is in agreement with values given in the literature (0.44/0.56 in ref 15b and 0.34/0.66 in ref 21). (24) Hofacker, G. L.; Marechal, Y.; Ratner, M. A. The Hydrogen Bond, Schuster, P., Zundel, G., Sandorfy, C., Eds.; North-Holland: Amsterdam, 1976; p 295.
LWiechmann et al.
1920 The Journal of Physical Chemistry, Vol. 95, No. 5, 1991 450
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Steady-State Emission and Excitation Spectra. The emission spectra of TIN in solution, which have been reported in the lite r a t ~ r e , ' ~ - ' ~ . ' *consist . ~ ~ , ~ of ' a single broad band with maximum in the blue spectral region. Very recently, we succeeded in detecting an additional red steady-state fluorescence of TIN in C2C14 with a very low quantum yield of 10-s.22This band, which is similar to the red emission of T I N in crystals" and in low-temperature organic glasses: was attributed to the keto-type tautomer (11) created by excited-state proton transfer. In Figure 3 we present fluorescence excitation spectra of the red and blue emission components. The excitation spectrum of the red fluorescence of TIN in C2CI4(solid line in Figure 3, detection wavelength 640 nm) displays the long-wavelength absorption band of planar tautomers forming intramolecular hydrogen bonds. The blue emission is related to excitation in the short-wavelength absorption band (dotted line c, detection wavelength 435 nm). Similar excitation spectra detected in the blue emission range are found for T I N in I-octanol (dashed line b, detection wavelength 425 nm) and in DMSO (dashed line d, detection wavelength 450 nm) as well as for the nonplanar MeTIN. Time-Resolved Emission Spectra. Time-resolved emission spectra for TIN in various solvents are given in Figure 4. The spectra are measured with picosecond excitation at 328 nm. The time-dependent quantum flux detected by single-photon counting is integrated over different time intervals after excitation. The solid lines are obtained by integrating the signal at early times up to 50 ps after excitation. The dotted lines in Figure 4 represent the spectra observed during a time interval of 10 ns (quasi-CW spectra). In this way spectral components of different decay times are clearly distinguished. According to Figure 4a-2, the relative intensities of red and blue emission drastically change in the two time intervals. The red keto-type emission relatively enhanced a t early times must be short-lived as compared to the blue fluorescence component. The change of the relative intensities of the red and blue emission is particularly pronounced for TIN in the aprotic nonpolar solvent C2CI4(Figure 4a). The keto-type emission with maximum at about 750 nm (similar to the spectrum of single crystals of TIN) dominates at early times, whereas the quasi-stationary spectrum exhibits a very strong blue component. This finding illustrates the difficulty to observe the red emission with steady-state detection techniques. For TIN in polar solvents, e.g., acetonitrile and I-octanol (Figure 4b,c), the relative intensity of the red fluorescence is strongly decreased. The quantum flux of the blue fluorescence (maximum at 420 nm) is dominating already at early times. This result indicates an increased amount of nonplanar tautomers without an intramolecular hydrogen bond. A quantitative mtimate of the relative concentrations will be given in section 4. Identical spectra a t early and later times are found for TIN in DMSO (Figure 4d), showing only the short-wavelength emission. This band is broadened and shifted to somewhat longer wavelengths (maximum a t 470 nm). The red emission of the keto-type species could not be detected because of the very large
I
1
3
35000
Figure 3. Excitation spectra of TIN in various solvents detected selectively via red or blue fluorescence components (red at 640 nm (a); blue at 435 nm in C2C14(c), at 425 nm in I-octanol (b), and at 450 nm in DMSO (d)).
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Figure 4. Time-resolved emission spectra of TIN in various solvents, detected in two different time intervals after picosecond excitation, between -50 and +SO ps (solid lines) or between -50 and +IO ns (dotted lines) with respect to the maximum of the system response function. For TIN in DMSO no difference is observed for the spectra measured in the two time intervals. (For TIN in I-octanol a sharp Raman line is superimposed.)
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t [ n i l Figure 5. Picosecond fluorescence transients of TIN and MeTIN in different solvents. Blue emission of TIN in C2CI4(a), acetonitrile (b), I-octanol (c), DMSO (d), and MeTIN in CzCl4 (e). All plots include a fitting curve drawn as dotted line. Typical red emission (f), except DMSO (see text), following closely our system response function.
amount of blue emitting nonplanar tautomers (60%; see above), carrying much higher quantum yield. Picosecond Fluorescence Transients. The decay kinetics of the blue fluorescence component very strongly depends on the specific solvent used (Figure 5a-d). All transients were selectively recorded in the maximum of the blue emission band. TIN in C2CI4 (Figure 5a) shows a biexponential fluorescence decay with two largely differing time constants, a unresolvable fast one of less than 5 ps and a slower one of 1.4 ns. Similar transients are also found f w TIN in n-hexane.
The Journal of Physical Chemistry, Vol. 95, No. 5, 1991 1921
Ultrafast Proton Transfer in T I N A [nml 500
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Figure 6. Picosecond time-resolved spectra of TIN in polystyrene (a) and fluorescence transients detected in the red (b) and blue (c) emission band (solid lines: time interval between -50 and +50 ps; dotted lines: between -50 ps and +IO ns).
A monoexponential decay with a time constant of 1.45 ns is measured for T I N in acetonitrile (Figure 5b). For T I N in 1octanol (Figure 5c) the fluorescence decay is more complex. A long-lived component with a decay time of 2 ns is due to the emission of the solvent. A second nonexponential fast decay occurs on a time scale of about 300 ps. This finding is similar to previous studies of T I N in a series of several l-alkanols.*& For T I N in DMSO (Figure 5d) the blue fluorescence decay is even faster and monoexponential with a time constant of 170 ps. MeTIN in C2CI4(Figure 5e) is measured as a reference species which does not undergo proton transfer after electronic excitation. The blue emission of MeTIN decays monoexponentially with a time constant of 1.65 ns (Figure 5e). In contrast to TIN in C2CI4 the initial fast decay component is absent in this transient. In Figure 5f we have plotted the kinetics of the red emission of TIN as observed in the picosecond single-photon-counting experiments. In all solvents (with the exception of DMSO, where the red fluorescence could not be detected), the transient is identical with the response of the apparatus setting an upper limit of the decay time of 5 ps. The same ultrafast transient we have observed also in some other solvents, e.g., n-hexane, methanol, and 1-propanol. Obviously, much higher time resolution is necessary to resolve the lifetime of the excited keto state. Time-Resolved Emission Spectra and Picosecond Transients of TIN in Polystyrene. The absorption spectrum of TIN in polystyrene is close to that of TIN in nonpolar solvents and exhibits the two maxima at 300 and 345 nm. The time-resolved emission spectra and the fluorescence kinetics are summarized in Figure 6. The time-resolved fluorescence spectra resemble the case of TIN in nonpolar solvents (cf. Figure 4): at early times the keto-type emission dominates (solid line in Figure 6a), whereas the quasi-CW spectrum (dotted line) contains an additional blue component of comparable intensity. The transient of the red emission (Figure 6b) follows the time response of the singlephoton-counting system; Le., the decay time is shorter than 50 ps. This finding is in contrast to data measured with TIN in copolymer films, where a decay time of 30 ps has been observed? The blue emission shows a complex decay kinetics consisting of a response-limited first contribution (7 < 5 ps) and two slower components with respective time constants of 500 ps and 2 ns. Femtosecond Pump and Probe Experiments. Femtosecond measurements with a time resolution of approximately 70 fs have been performed to elucidate the buildup and decay kinetics of the excited keto state as well as the subsequent keto-enol back-reaction in the electronic ground state. The sample is excited by an intense pump pulse at A,, = 310 nm. The temporal development of the keto emission is studied with weak probe pulses in the wavelength range from 600 to 700 nm. The probe light induces transitions
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Figure 7. Femtosecond kinetics of TIN and MeTIN after excitation at A,, = 310 nm in different solvents and of TIN in polystyrene. (a, d) Transient gain for the red keto emission of TIN in C2CI4and polystyrene. The gain G = in ( I l l o ) at 645 nm is plotted versus delay time between pump and probe pulses (I, and I: intensity of the probe pulses before and after interaction with the sample). (b, c, e) Transient absorption of TIN in C2CI4,DMSO, and polystyrene. The normalized change of absorption AA is plotted for A , = 515 nm. (f)Transient absorption of MeTIN in C2CI4for A, = 5 1 5 nm. The solid lines through the experimental points are calculated as explained in the text. by stimulated emission from the excited state to the ground state of the keto-type tautomer; Le., the probe pulses are amplified by interaction with the excited sample. In Figure 7a, the normalized gain G = In (I/fo)at a wavelength of X, = 645 nm is plotted versus delay time between pump and probe (points); Io and I represent the probe intensity before and after excitation of the sample, respectively. The signal rises on a time scale of about 200 fs and decays within less than 1 ps. A similar behavior is found in the whole range of A, between 600 and 700 nm. The maximum gain G amounts to several thousandths, corresponding to small signal amplification of the probe pulses. In this case the signal is proportional to ( N 3- N4),where N 3 and N4are the time-dependent populations of the keto excited and ground state, respectively. N4 is zero prior to excitation, since initially no keto tautomers exist. As a result, the gain measurement monitors the transient population N 3 of the emitting keto state. In addition to the investigation of stimulated emission, we study the transient absorption of the molecules. A strong absorption band with subpicosecond buildup and decay times is observed in the wavelength range between 480 and 570 nm. In Figure 7b-d, results for TIN in different environments are presented. The normalized absorption increase A = -In ( T / T o )probed at A,, = 5 15 nm is plotted as a function of delay time (points). T I N in C2C14(Figure 7b) shows an instantaneously rising absorption, which relaxes on a time scale of 2 ps. The decay clearly extends to longer delay times than the gain in Figure 7a. The maximum absorption increase has a value of A = 0.015. For TIN in DMSO (Figure 7c), we find an identical kinetics of transient absorption. However, the amplitude of this signal is reduced by a factor of 1.6, corresponding to the lower relative concentration of planar TIN tautomers in this solvent (cf. section 3b). This result demonstrates that the absorption signal is related to the fast protontransfer reaction of TIN. The transient gain and absorption of TIN in the polystyrene matrix builds up and decays on a similar time scale (Figure 7d,e). We conclude that the s p e d of the proton-transfer reaction remains
1922 The Journal of Physical Chemistry, Vol. 95, No. 5, 1991
essentially unchanged in the different environments. The femtosecond absorption of MeTIN in C2CI4was studied for comparison. This derivative, where proton transfer is impossible, exhibits a completely different kinetics of absorption, which rises within the temporal resolution of the experiment and persists up to our longest delay times of approximately 200 ps (Figure 7 0 . The experimental results of Figure 7 are analyzed quantitatively by numerical calculations. A consistent description is found for all observed transients (solid lines in Figure 7) using a four-level reaction scheme, which will be discussed in more detail in the next section. Good agreement between the calculation and the data points of the transient gain experiment (Figure 7a) is obtained with a rise time 71= 100 fs and a decay time 72 = 150 fs. The kientics of the transient absorption in Figure 7b,c is reproduced by an instantaneously rising Component which recovers with TI = 100 fs. This first contribution is followed by a transient with a decay time of r2 = 150 fs and by a final third component relaxing with 73 = 500 fs. The amplitude of the last component amounts to 70% of the other contributions. The complex femtosecond kinetics is due to the superposition of transient absorptions from both keto excited and ground states. For TIN in polystyrene (Figure 7d) good agreement with the experiment is achieved with r1 = 200 fs, T~ = 350 fs, and 73 = 1.2 ps. 4. Discussion
Distinction between Blue and Red Emitting Tautomers. We first discuss the properties of the blue fluorescence emission reThe data of Figures ported earlier in a series of papers.”*6~l*lo~1 2 and 3 demonstrate that the excitation spectrum of this emission markedly deviates from the absorption spectrum. The blue emitting species exhibits exclusively the absorption band around 300 nm, whereas the maximum at 345 nm is absent. This finding suggests the Occurrence of a t least two ground-state tautomers giving rise to the blue and red fluorescence. The relative concentration of molecules emitting around 400 nm was estimated to be less than l C 3 in nonpolar solvents.22 From analyzing the ratio of the blue and red emission maximum intensity in the first interval of the time-resolved spectra (solid line, Figure 4a-c), we are able to give an estimate also for TIN in acetonitrile and 1-octanol. Taking into account the different extinction at the excitation wavelength 328 nm for the blue emitting tautomers of T I N in the various solvents and assuming a similar lifetime for the keto-type state (confirmed by the femtosecond experiments), A similar number we obtain for both cases nbluc/nrd< 5 X we get by extrapolating the case of I-octanol from experimental data2’ on a series of alcohols. Therefore, we conclude that even in polar and hydroxylic solvents the major fraction of the tautomers does not belong to the blue emitting species. Some information about the nature of the blue emitting tautomers can be obtained from the reference measurements of MeTIN. In both absorption and excitation spectra of MeTIN only a short-wavelength band can be found. This observation is related to the nonplanar molecular conformation of MeTIN. Because of steric hindrance of the methyl group, the triazole moiety and the phenyl ring in MeTIN are twisted by 5 6 O relative to each MeTIN in solution reveals also only a blue band in the emission spectra (maximum at about 400 nm), quite similarly as the blue emitting species of TIN. Because of this correspondence with MeTIN in absorption and emission, we attribute that blue emitting species to a twisted conformer of TIN without an intramolecular hydrogen bond. Such.an interpretation is in agreement with ref 15b. However, there is a tendency in the literature to identify these molecules with a particular species “TIN-inter” exhibiting an intermolecular hydrogen bond between TIN and the solvent. This attribution seems to be plausible at first sight for DMSO and other hydroxylic solvents, but not mandatory in general. In acetonitrile. for instance, intermolecular hydrogen bonding with the solute is not expected to w u r at high p r ~ b a b i l i t y . ~ ~The * * ~ present experimental results on T I N in
Wiechmann et al. acetonitrile support this argument. The fluorescence decay time is quite long in this case and similar to the one observed for MeTIN. Consequently, there do not exist additional relaxation pathways due to coupling between solvent molecules and the hydroxylic group of TIN. Also, internal conversion could not be very effective for excited-state depopulation in the twisted conformation of TIN when compared to the extreme fast decay of the keto-type species discussed below. This is possibly caused by a decoupling of the phenyl ring from the benzotriazole moiety. DMSO shows by far the strongest solvent effect on the fluorescence properties of TIN. (i) The blue emission band is broadened by about a factor of 1.4 (fwhm) and red-shifted by 2500 cm-I as compared with that in other polar solvents. (ii) The fluorescence decay time is very short (170 ps). (iii) The fraction of nonplanar, blue emitting species (60%) is much higher than in any other solvent. These results indicate an exceptionally strong coupling between TIN and DMSO. The high polarity of DMSO could not be, as discussed in ref 21, the only reason for this behavior when compared with the nearly same value for acetonitrile. From our point of view the following aspect should be considered. In contrary to acetonitrile, DMSO is able to form intermolecular hydrogen bonds, presumably assisted by steric con~traints.’~~ Microscopically,this could be related to the location of the electric dipole along one edge of the DMSO trigonal ba~kbone,~’ allowing for a close encounter of solvent and solute. In 1-octanol the relatively short time scale of about 300 ps for the blue fluorescence decay indicates considerable solventsolute interaction. The complex multiexponential decay function suggests the existence of more than one tautomeric form contributing to the blue emission. Interactions via different intermolecular hydrogen bondszoa as well via complexation21 seem to be quite possible. In ref 20a tautomers with intramolecular proton transfer are also taken into consideration for measurements on different 1-alkanols, whereby referring to transfer rates in the order of several hundreds of picoseconds. We could not find any experimental evidence for intramolecular proton transfer on a time scale >5 ps, for none of the solvents investigated. From the presently available data we cannot answer the question about the blue emitting species in detail but rather conclude about their fractional number which even in polar solvents (except DMSO) is below a few percent. Thus, always the majority of TIN molecules actually participates in the enol-keto reaction and contributes to the red keto emission discussed in the following. The occurrence of a strong intramolecular hydrogen bond is confirmed for these “keto-type” tautomers (as we shall denote them in what follows) from infrared absorption spectra of TIN in C2C14. Another characteristic feature of keto-type tautomers is the long-wavelength electronic absorption band, which clearly is missing in the excitation spectra of the blue emitting tautomers. Red and blue emission can be well distinguished in the time-resolved emission spectra because of drastically different decay times. In all cases studied in this work picosecond time resolution is not sufficient to temporally resolve the red fluorescence transients. Both keto emission buildup and decay turn out to be subpicosecond processes, independent of solvent polarity and the presence of hydroxylic groups. Fast kinetics are observed not only in liquid solvents but also for TIN in polystyrene polymeric films. Like in nonpolar solvents the keto-type species predominates, and the same spectral features and time scales are measured. One particular aspect of the blue fluorescence transients is also equally found in C$I4 and polystyrene (Figures 5a and 6c)but has not been discussed yet. These transients at early times (not resolvable on a picosecond time scale) reveal an’additional fluorescence component not belonging to the blue emitting twisted tautomers characterized above. The estimated quantum yield of the emission is in the range between 10” and IO”. We attribute this component to the fluorescence from ~~
(25) Radnai, T.; Itoh, S.; Ohtaki, H. Bull. Chem. Soc. Jpn. 1988,61,3845. (26) Martin, M. M.; Ikeda, N.; Okada, T.; Mataga, N . J . Phys. Chem. 1982, 86,4148. (27) Itoh, S.;Ohtaki, H. 2.Narurforsch. 1987, 42A, 858.
The Journal of Physical Chemistry, Vol. 95, No. 5, 1991 1923
Ultrafast Proton Transfer in T I N
t
enol
keto
”\ Pro ton
Coord i not e
Figure 8. Schematics of the potential energy surfaces of TIN in the ground and first excited state. After ultraviolet excitation, a barrierless proton transfer occurs in the excited state with a time constant T~~ ‘Y 100 fs (200 fs). The keto tautomer is deactivated by internal conversion to its ground state with T~ N 150 fs (350 fs), followed by back-trans8 500 fs (1.2 ps). Numbers formation to the enol structure with 7 ~ N refer to TIN in C2CI4and DMSO and in brackets to polystyrene.
the excited enol state which occurs in competition with the ultrafast intramolecular proton transfer. This emission can be detected here for the first time only since both high temporal resolution and sensitivity are available in the single-photon-counting experiments. Solutions of TIN in C2C14and polystyrene are in that respect fortunate cases of enol fluorescence detectability. In all other cases the much stronger emission of the other blue emitting tautomers completely determines the transients. Femtosecond Kinetics and Enol-Keto Reaction Scheme. In this final section we discuss a reaction scheme for TIN molecules undergoing rapid proton transfer (Figure 8). Absorption of UV light transfers the enol tautomers from ground state So to vibronic levels in the excited state SIvia a vertical transition, where the nuclear coordinates remain unchanged. A new potential energy surface of the proton is established by the subsequent redistribution of electronic charge, and the proton is driven to its new equilibrium geometry in the keto configuration. The formation time T E K of the excited keto state is directly monitored by the rise time of the femtosecond transient gain signal (Figure 7). For both nonpolar solvent and polystyrene film approximate values of about 100 and 200 fs are obtained, respectively. A similar time constant of the proton transfer in the excited state has been determined for HBT, a structurely similar benzothiazole molecule (170 fsz8). These measured transfer times can be correlated with vibrational periods of large-amplitude motions in the frequency range of about 100-200 cm-I. The extremely high speed of the change of the molecular geometry suggests an essentially barrierless energy surface in the excited state. The direct radiative decay of the enol excited state cannot compete effectively with the fast tautomerization reaction. Assuming the same radiative lifetime for the excited enol state as for the keto-type state of about 20 an upper limit for the quantum yield of enol fluorescence of about IC5is estimated. This value is in good agreement with our estimate made above from the observed fast component in the picosecond transient (Figures Sa and 6c, for TIN in C2CI4and polystyrene, respectively). One of the most remarkable features of TIN is the extremely rapid depopulation of the excited keto state SI’which is orders (28) Lirmer, F.; ElsBsscr, T.; Kaiser, W. Chem. fhys. Lcrr. 1988,148, 119.
of magnitude faster than in other proton-transfer systems, e.&, HBT. The decay of the gain signal (Figure 7) gives a time constant of T K = 72 = 150 fs for TIN in CZCI, and 350 fs in polystyrene for this process. This result corresponds to the rather small quantum yield of the TIN keto emission In contrast to HBT the keto form of T I N exhibits an essentially single-bond character between phenyl ring and benzotriazole moiety (see structure 11). This allows for low-frequency librational modes about this bond, most probably enhancing the internal and Si. A quantitative description cannot conversion between SI’ be given yet. However, this interpretation is supported by the considerably slower keto fluorescence decay time of 80 ps we ~~~~’ observe in single crystals of TIN a t room t e m p e r a t ~ r e . ~In a crystalline environment such torsional molecular motions are strongly hindered due to the dense molecular packing. It is interesting to note that in polystyrene films the deactivation of SI’ is nearly as fast as in liquid solution. The time constant for the proton back-reaction is obtained by separating the different contributions to the transient absorption signal (Figure 7). The signal analysis given in the previous section suggests that the excited singlet levels SIof the enol and SI’of the keto tautomer as well as the subsequently populated keto ground-state S,,’contribute to the measured transients. Using the time constants already determined in the transient gain experiment, we extract from the absorption change observed at delay times longer than the SI’lifetime a time constant for the back-reaction ( T =~T ~~of ) approximately 500 fs. This reaction is found not to depend on solvent polarity and is only about a factor of 2 slower in the polystyrene film. The extremely fast back-transfer of the proton in the electronic ground state suggests a potential with negligible bamer as depicted in Figure 8. Because of the rapid deactivation of the excited keto state SI’, a large amount of excess energy is transferred to the ground state S,,’.For the redistribution of approximately 20000 cm-I a high vibrational temperature of 800-1000 K for the keto ground state is estimated. The back-transfer time of 500 fs is short in relation to the cooling time of 5-50 ps23929for the vibrational system. A possible low barrier would be easily passed by the hot molecules. 5. Conclusion
We have studied the proton-transfer reactions of T I N in the ground and first excited singlet states as well as the competing processes occurring in various environments at room temperature. Two different tautomers can be distinguished in the excitation and time-resolved emission spectra. The blue emitting species are attributed to twisted ground-state tautomers not capable of intramolecular proton transfer. The predominant species is identified from its very fast decaying red keto emission. It exhibits a complete reaction cycle on a subpicosecond time scale, whose individual reaction time constants are determined from femtosecond experiments. The ultrafast reaction cycle is independent of solvent polarity and only slightly longer for TIN in a polystyrene film. It is responsible for the efficiency of TIN as a photostabilizer in polymers. Acknowledgment. We thank Professor H. C. Wolf and Professor w. Kaiser for valuable discussions and for continuous support. We are indebted to Professor H. E. A. Kramer for kindly providing the sample material and for helpful discussions. Financial isupport by the Deutsche Forschungsgemeinschaft is gratefully acknowledged. (29) Scherer, P. 0. J.; Seilmeier, A.; Kaiser, W. J . Chem. fhys. 1985.83. 3948.
