J. Phys. Chem. 1983, 8 7 , 1184-1188
1184
Excited-State Intramolecular Proton Transfer and Vibrational Relaxation in 2-( 2-Hydroxyphenyl) benzothiazole K. Dlng, S. J. Courtney, A. J. Strandjord, S. Flom, D. Frledrich, and P. F. Barbara’ Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 (Received: October 8, 1982; I n Final Form: November 23, 1982)
The time- and wavelength-resolved electronic fluorescence of 2-(2-hydroxyphenyl)benzothiazolein argon at 12 K has been investigated under high-resolution conditions. The 475-nm emission band is produced 10l1s-l) actually exceeds vibrational relaxation (kVR N 101’s-l) by at least an order of magnitude, see below. The implication of these results on the understanding of the mechanism for excited-state intramolecular proton transfer of HBT and related molecules will be discussed below.
-.
-
(1)(a) K. Choi and M. R. Topp, Chem. Phys. Lett., 69,441(1980);(b) K. Choi, B. P. Boczar, and M. R. Topp, ibid., 57,415 (1981). (2)(a) B. I. Greene, R. M. Hochstrasser, and R. B.Weisman, J. Chem. Phys., 70,1247 (1979);(b) G.Beddard, T. Doust, and M. Windsor in “Picosecond Phenomena 11”, C. V. Shank, R. Hochstrasser, and W. Kaiser, Ed., Spinger, Berlin, 1980. (3) P. F. Barbara, P. M. Rentzepis, and L. E. Brus, J . Chem. Phys., 72,6802(1980). (4)P.F.Barbara, S. D. Rand, and P. M. Rentzepis, J. Am. Chem. SOC., 103,2156 (1981). (5) (a) P.F. Barbara, P. M. Rentzepis, and L. E. Brus, J . Am. Chem. Soc.. 102.2786 (1980);(b) P. F.Barbara, L. E. Brus, and P. M. Rentzepis, ibid., 102,5631 (1980).
Experimental Section All fluorescence measurements were made with frontsurface collection at normal (goo)incidence and excitation at 30’ incidence. For the fluorescence excitation experiments (-2-nm resolution) (Figure 2), a 250-W xenon lamp (Hanovia) coupled to a 0.25-m Jarrel Ash monochromator was used for excitation. The fluorescence was monitored (521 nm) with a 25-m HR320 monochromator (Instruments SA) coupled to a 1P28 (RCA) photomultiplier tube (PMT). The photon flux of the excitation source was monitored with a “quantum counter”, Le., an optically dense dye solution viewed by a 1P28 PMT. The excitation spectrum in Figure 2 represents a ratio of fluorescence output to excitation photon flux determined by an analog ratiometer. The nontime-resolved fluorescence spectra in Figures 2 and 5a were recorded with an HR320 polychromator coupled to a photodiode array detector, PDA (Model 1412, Princeton Applied Research, Photocathode S20). The sample was excited at 355 nm with a Nd:YAG laser, see below. The nontime-resolved fluorescence spectra are corrected for the channel-dependent gain of the PDA but not for the overall wavelength-dependent sensitivity of our apparatus. The time-resolved kinetic emission traces were recorded with a picosecond emission spectrometer that employs a passively mode-locked TEMm Nd:YAG laser with associated optoelectronics and nonlinear optics (Quantel, Inc.) to produce -3O-ps, 355-nm excitation pulses of 1mJ in energy at a spot size on the matrix of 50 mm2 and a repetition rate of 101s. Time resolution of the emission from the matrix is accomplished by Type I1 sum-frequency mixing (upconversion) with the 1064-nm (30 ps) laser fundamenta11b$6 whose relative arrival time at the Type I1 KDP crystal (l-cm cube) is varied by a computer-controlled time delay stage which varies the optical path length of the 355-nm excitation pulse. The “upconverted” light intensity (Aut-' = Xfl-’ + 1064-1 nm) from the KDP crystal is measured as a function of delay stage length to extract the fluorescence time dependence. By adjusting the relative convergence of 1064-nm and fluorescence light in the KDP crystal, we are able to ”upconvert” fluorescence over a -75-nm region with the Type I1 crystal tilt angle fixed. For kinetic traces, e.g., Figure 3, at a particular wavelength region (- 15 nm wide), we monitor the upconverted light with a 1P28 PMT. Data collection is fully automated to enhance accuracy and the signal-to-noise ratio by (i) signal averaging over -25 laser pulses, (ii) correcting the data for laser energy fluctuations, and (iii) rejecting data from poorly mode-locked pulses. The experimental digital
0022-3654/83/2087-1184$01.50/00 1983 American Chemical Society
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Vibrational Relaxation in 2-(2-Hydroxyphenyl)benzothiazole
The Journal of Physical Chemistry, Vol. 87, No. 7, 1983 1185
490 nm
355 nm
Figure 2. Nontime-resohred fluorescence spectra of HBT in argon at 12 K. The left trace is an excitation spectrum, while the right trace is a wavelength-resohred emission spectrum, see Experimental Section for details.
2
1 2
- (2-HYDROXYPHENLY)
BENZOTHIAZOLE PHOTOTAUTOMERIZ AT ION
Figure 1. A schematic representation of the photodynamic mechanism of HBT in nonpolar solvents.
time-dependent fluorescence traces (points in Figure 3) were fit (solid lines in the figures) by an iterative convolution of the experimentally determined instrument response function with biexponential decay (Figure 3a) or appearance (Figure 3b) kinetics, see below for further details. The wavelength-resolved (-0.5 nm) emission spectra, at specific delay times from excitation, are determined by recording the spectrum of the upconverted light. This approach has been described in detail by Topp and coworkers.lbs6 We record the spectrum in our apparatus with the PDA and the HR320 polychromator (2400 grove/" grating, blazed in the ultraviolet). The PDA is operated at -40 "C to reduce background noise. The matrix apparatus is very similar to that described by Brus and Co-workers.' The matrix in our apparatus is prepared on a copper or sapphire flat that is temperature regulated by a displex helium closed cycle refrigerator (Air Products). Typical conditions for deposition were as follows: (1) Deposition rate was -40 cm3/min, (ii) the HBT was held at ambient temperature in the argon stream, (iii) the matrix holder was regulated at 30 K during deposition, and (iv) after deposition and during the experiment the matrix was regulated at 12 K. No detectable emission was observed prior to deposition. The emission spectrum was identical before and after prolonged irradiation. HBT was purchased from Polysciences, Inc. and used without purification except for the sublimation that is inherent to the deposition technique.
Results and Discussion Basic Photochemical Model. The photochemistry and spectroscopy of HBT, its derivatives, and the closely related N-salicylideneanilines have been studied extensively?big.12 Depending upon solvent polarity,12btemperature? (6)L.A. Hallidy and M. R. Topp, Chem. Phys. Lett., 46,8 (1977). (7)A. Baca, R. Rossetti, and L. E. Brus, J. Chem. Phys., 70,4475 (1979). (8)(a) M. D.Cohen and G. M. J. Schmidt, J. Phys. Chem., 66,2442 (1962);(b) M. D.Cohen, Y. Hirshberg, and G. M. J. Schmidt, J . Chem. SOC.,2051 (1964);(c) M. D.Cohen and S. Flavian, J. Chem. B, 317 (1967). (9)D. L. Williams and A. Heller, J. Phys. Chem., 74, 4473 (1970). (10)(a) M. Ottolenghi and D. S. McClure, J . Am. Chem. Phys., 46, 4613 (1967);(b) R. Potashnik and M. Ottolenghi, ibid., 51,3671(1969); (c) T. Rosenfeld, M. Ottolenghi, and A. Y. Meyer, Mol. Photochem., 5, 39 (1973). (11) (a) R. S. Becker and W. F. Richey, J. Am. Chem. SOC.,89,1298 (1967);(b) W. F. Richey and R. S. Becker, J. Chem. Phys., 49, 2092 (1968).
and solvent vis~osity,~ the photochemical mechanism for HBT can be quite complex with as many as five distinct isomeric intermediatesginvolved in the photodynamics and two or more stable ground-state isomers present at thermal equilibrium. In nonpolar, low-temperature environments, however, the mechanism is simpler and apparently well represented by the basic photochemical mechanism first proposed by Cohen and Flavian8(see Figure 1)with certain refinements which will be discussed below. The stable ground-state isomer of HBT in low-temperature nonpolar environments is predominantly an intramolecularly hydrogen-bonded enol isomer, 1, as judged by spectral e v i d e n ~ e . ~Frank-Condon J~ optical excitation of 1 in the ultraviolet (Figure 2) yields an electronically excited enol species 1*,which rapidly rearranges (kPT > loll s-l) to an electronically excited planar keto form, 2*, which lives for several nanonseconds at low t e m ~ e r a t u r e . ~ ~ In the following discussion (i) we present new evidence in support of the Cohen and Flavian mechanism, (ii) we present data that cast doubt on the validity of a modification to this mechanism proposed recently,12and (iii) we suggest refinements to the basic mechanism that are indicated by our observations and recent reports5b of vibrational dynamics in the excited state of HBT. Nontime-Resolved Spectroscopy. We have investigated the emission and emission-excitationspectroscopy of HBT in a vapor-deposited argon matrix at 1 2 K. This environment offers the advantages of a weakly interacting solvent and a small likelihood of HBT dimer formation which, in principle, could complicate the spectral analysis. The observed emission spectrum and emission-excitation spectrum (Figure 2) are in complete agreement with the expected behavior from Figure 1. The fluorescence excitation spectrum we observe is alsmot identical with the absorption spectrum of HBT in low-temperature glasses,12band the crystalline state? where 1 is the dominant i s ~ m e r . ~The ~ ~fluorescence J~ spectrum we observed is similar to that previously observed for HBT in argon at 4 K5b and for HBT in nonpolar organic glasses.12bA single strong fluorescence band (Amm = 490 nm) is observed. We do not observe any evidence of phosphorescence or "enol" type12bemission. A variety of indirect evidence on the spectroscopy of HBT and it derivatives has been presented to assign the 490-nm band to a planar keto form, Z9 The oscillator strength in absorption and emission13J4 indicate that 1* and 2* are both (.rr-a*) excited states. Recent calculations on closely related methyl salycilate (12)(a) R. Nakagaki, T. Kobayashi, J. Nakamura, and S. Nagakura, Bull. Chem. SOC.Jpn., 50,1909(1977);(b) R. Nakagaki, T. Kobayashi, and S. Nagakura, ibid., 51, 1671-5 (1978). (13) The molar extinction coefficient of HBT (-lo4 mol-' L cm-l) is in the range usually observed for X - X * transitions in the ultraviolet. (14)A lower limit to the radiative rate constant can be estimated from the quantum yield value, 0.36,lZbin low-tem erature nonpolar environments and the fluorescence lifetime, -5 ns.g This estimate, kR N lo8 s-l, is typical for fluorescence from ~ - r in * the visible.
The Journal of Physical Chemistry, Vol. 87, No. 7, 1983
1186
(MS) suggest, by analogy, that electronic excitation in HBT is associated with charge transfer, which is responsible for reversing the order of stability of 1 and 2 and So and S1.15a Spectral evidence supports the charge-transfer assignment for HBT.g The absorption band of HBT exhibits a relatively sharp onset at -350 nm with a poorly resolved progression in some 1300-cm-’ mode. The emission spectrum also exhibits a sharp band edge at -475 nm with observed vibronic structure with splittings of 1400 and -270 cm-’. The sharp band edge in absorption suggests that, with respect to the optically coupled normal modes, a small to moderate excited-state displacement occurs for absorption of ground-state 1. A similar argument can be made for emission from the relaxed excited state of HBT. It is interesting that, while absorption and emission spectra exhibit “sharp band edges”, the Stokes loss is, in fact, quite large, -8OOO cm-l. In other words, the apparent origin of fluorescence is shifted significantly to lower energy than the origin for absorption. This is strong evidence that the state resulting from absorption differs greatly in electronic coordinates from the species responsible 490-nm fluorescence. This behavior is, of course, consistent with the ESIPT model. The observations just made about the spectroscopy of HBT apply to several other molecules known to undergo ESIPT, but, interestingly, do not hold for the well-known example methyl ~alicy1ate.l~ For this molecule, a coincident origin in absorption and emission is observed and the onset of emission is quite In MS, therefore, the species assumed to be “keto”-like is apparently “directly” accessible spectroscopically from the enol ground state. The spectroscopy, therefore, indicates the photophysical relaxation mechanism of MS and HBT may differ significantly, in contrast to previous suggestion.s Our data, therefore, support Scheme I in that excitation of “enol”type isomers yields “keto”-emissions as a result of an excited-state rearrangment. Proton-Transfer Kinetics. An estimate of the lower limit for the rate constant for proton transfer, kpT, can be derived from a measurement of the fluorescence kinetics of HBT and an analysis of the differential equations associated with Figure 1. It can be shown, in the usual fashion, that the time dependence of 2* should be represented by the following equation: I 2 0: (e-tl‘z - e-ti’l)
-
-
where T1-l
=
kR1
+ knRl + kpT
+ kR2 In these equations, kR1 and kR2,are the radiative rate constants and knRl and knR2 are the nonradiative rate T2-’
=
knR2
constants for l* and 2*. The values for T ~ and - ~ 72-l for HBT in argon at 1 2 K can be extracted from the picosecond emission kinetics of 2*. The analysis, which is complicated by the rapid evolution of 2* emission band shape after 2* is initially - ~T ~ of - >IO” ~ and -lo8 formed, yields values for T ~ and s-l, respectively, see Time-Resolved Spectroscopy. This agrees favorably with a previous measurement of T ~ - ‘ with (15) (a) J. Catalan, F. Toniblo, and A. U. Acuna, J. Phys. Chem., 86, 303 (1982); (b) J. Goodman and L. E. Brus, J. Am. Chem. SOC.,100,7472 (1978); (c) L. A. Hermbrook, J. E.Kenny, B. E. Kohler, and G. W. Scott, J. Chem. Phys., 75, 5201 (1981); (d) P. M. Felker, W. R. Lambert, and A. H. Zewail, ibid.,77,1603 (1982); (e) K. K. Smith and K. J. Kaufman, J. Phys. Chem., 82,2286 (1978); (0 A. Weller, B o g . React. Kinet., 1, 189 (1961).
Ding et ai.
better time resolution, Le., T ~ >- 2~ X 10” s-1.5b The relative size of kpT and 71-l can be estimated by considering the fluorescence quantum yield of 2 in lowtemperature nonpolar environments which has been determined to be & = 0.36.lZb The expression for kpT in terms of the dz and the various rate constants is given by
since kR272 Ti-’
2 2
51
x 10”
S-l
then
kpT 2
loll
s-l
This value is similar to that obtained for HBT at 77 K in 1:1 methylcyclohexane/isopentane by a consideration of an upper limit to the quantum yield of the emission from 1, @l, which was below the detection limit of the fluorometer. In our experiments, we have been unable to detect any fluorescence from 1. A rough analysis of our data indicate 0.001. Since we expect, see above, that 2 that 0.36, we estimate that q51 I 0.00036. The lifetime of excited 1, 7 1 - l ~can be calculated from this value and a previous estimate for kR1,i.e., kR1 -2.3 X lo8 s-l.lZbThis simple analysis suggests that
5
71-1
= kpT I 5 x 1012 s-1
Proton transfer rates this large have been observed at cryogenic temperature for molecules related to HBT, including methyl ~alicylate,’~~ salicylideneaniline, and 2(2’-hydro~yphenyl)benzoxazole.’~~ Based on a thermal activation model, assuming a preexponential factor of ioi3 s-l, we feel that the activation energy for excited-state proton transfer would have to be lOO-ps spectrum are also present in the >490-nm region, but we have not investigated this wavelength region in detail. I t is interesting to note that the “gated” spectra are highly reproducible. This is demonstrated in the lower panel of Figure 4 where traces from two separate determinations of the 115-ps spectrum are plotted on the same intensity scale. The spectra in Figure 4 have not been corrected for the wavelength-dependent sensitivity of our apparatus in the “gated” mode. This correction, which should be identical for all time values, is, however, small as judged by a comparison of the nontime-resolved emission spectrum (Figure 5a) to the late time-gated spectrum which should be responsible for more than 99% of the nontime-resolved fluorescence (Figure 5b). The time evolution of the fluorescence spectrum of HBT shown in Figure 4 is entirely consistent with the previous proposal that 2* is intially created with excess vibrational energy that dissipates on the 10-30-ps time scale.5b The broadened and shifted high-energy edge of 2* band as compared to the relaxed (>250 ps) spectrum seems to be analogous to other examples of fluorescence band-shape time dependence to vibrational relaxation in the excitedelectronic s t a t e . l ~ We ~ ~ *conclude, ~ therefore, that 2* is, as previously suggested, created from the I* 2* process with an initial vibrational excess. Vibrational us. Proton-Transfer Dynamics. If it is assumed, as the picosecond data suggest, that the protontransfer process is not thermally activated, it may be accurate to classify this process as a special class of vibrational relaxation, VR, as proposed by Heller.Q It is interesting, in this regard, to estimate the relative time scale of PT to other types of VR in HBT. The picosecond data5b indicate that the relaxation time scale of the optically active modes of 2* is 10-15 ps. It is likely that these modes are characteristic of the overall chromophore, e.g., C=C stretches of the phenyl ring. If it is assumed that the rate of relaxation of these modes are similar for 1* and 2*, then one concludes the PT time scale is at least an order of magnitude shorter than other VR processes. This seems
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J. Phys. Chem. 1983, 87, 1188-1191
reasonable when one considers that the proton is vibrating in a highly asymmetric potential as a result of probable electron densities in 1* and that energy flow from normal modes in the region of a highly anharmonic potentials is especially rapid. This situation, interestingly, is similar, in principle, to polyatomic molecules undergoing unimolecular thermal isomerization with an energy content comparable to the “barrier height” for reaction. Other Mechanisms. It was recently proposed by Makagaki et a l l 2 that an additional intermediate, which they labeled “X” is involved in the 1* - 2* isomerization. I* -* “X” -.+ 2* This proposal resulted from an interpretation of their data of the transient spectroscopy of HBT at room temperature in 1:l methylcyclohexane/isopentane,where 2* undergoes a rapid radiationless decay.” These authors envoked the “X”intermediate to explain the discrepancy between the rapid excited-state radiationless decay indicated by transient absorption measurements (- 50 ps) and the slower decay (-500 ps) they observe from emission measurements. Recent lifetime measurements for HBT at the same temperature and in the same solvent, however, show the fluorescence lifetime of HBT to be -100 ps.5b Our observations verify this latter result.
Considering the inherent inaccuracy of the transient absorption measurement, which was apparently not analyzed by deconvolution methods of the instrument response function, it is likely that transient absorption and emissions are, in fact, in agreement within experimental error. The “X” intermediate is, therefore, an unnecessary modification to the Cohen and Flavian model.
Conclusion and Summary The excitation and time-resolved fluorescence spectroscopy of HBT in argon at 12 K is highly consistent with the mechanism photochemical proposed by Cohen and Flavionsc as modified recently by Barbara et al.5b The unresolvably rapid rate of ESIPT in HBT suggests that ESIPT may be a special type of VR for this molecule. A simple analysis suggests that the rate of ESIPT exceeds the rate of other types of VR in electronically excited HBT by at least an order of magnitude. Acknowledgment. Acknowledgment is made for partial support of this research to the following: the donors of the Petroleum Research Fund, administered by the American Chemical Society, the Research Corporation, and the Graduate School of the University of Minnesota. P.F.B. thanks Professor A. Weller for helpful discussions. Registry No. 2-(2-Hydroxyphenyl)benzothiazole,3411-95-8.
Involvement of Amine Protons in n -Butylamine-Cresol Hydrogen Bonding Akram AI Awar, Mary Codd, Norman Pratt, and Ronald M. Scott’ Department of Chemistty,Eastern Michigan University, Ypsilanti, Michigan 48 197 (Received: March 12, 1982; I n Final Form: September 29, 1982)
Studies of the hydrogen bonding of cresols with n-butylamine by difference spectroscopy in the infrared range reveal participation of protons from both the phenols and the amines. A cyclic structure for the interaction is proposed.
Introduction Phenol-amine hydrogen bonding has been extensively studied, particularly by means of infrared spectrophotometry, NMR spectrometry, and calorimetry.’ Attention has been focused on the influence of phenol acidity and amine basicity, on solvents and solvation effects, and on steric hindrance. In all these investigations it has generally been assumed that the interaction consists solely of the attraction of the unshared electrons of the amine nitrogen for the phenolic proton. We have studied a variety of aspects of this system in our laboratory. In a recent calorimetric study2 it was observed that the enthalpy changes when triethylamine and 0-or p-cresol undergo hydrogen bonding were very similar. However, for the interactions between n-butylamine and p-cresol the enthalpy change was significantly larger, while with o-cresol it was sharply lower (Table I). Values for (1) (a) Fritsch, J.; Zundel, G. J. Phys. Chem. 1981, 85, 557. (b) Pimente], G. c.; McClellan, A. L. “The Hydrogen Bond”; W. H. Freeman: San Francisco, 1960. (c) Vinogradov, S. N.; and Linnell, R. H. ‘The Hydrogen Bond”; Van Nostrand-Reinhold: New York, 1970. (2) Kogowski, G.; Scott, R. M.; Filisko, F. J . Phys. Chem. 1980, 84, 2262. 0022-3654/83/2087-1188$0 1.50/0
TABLE I: Previously Reported Parametersu of
Cresol-Amine Hydrogen Bonding enthalpy change, phenol amine kcalimol o-cresol o-cresol p-cresol p-cresol a
log Keq
triethylamine
-7.35 t 0.35 1.67 -5.20 2 0.24 1.90 -7.61 t 0.36 1.78 -8.61 I0.41 1.78 Enthalpy changes are from ref 2 and log K,, values
n-butylamine triethylamine n-butylamine
from ref 3.
log K for these same reactions reveal a less dramatic pattern (Table I).3 Previous studies in this laboratory have indicated that the base strengths of these two amines 0- and p-cresol in nonpolar solvents are ~ i m i l a r . Since ~ are very similar in acid strength, and since the inert solvent cyclohexane was used in all these studies, the character of the hydrogen bond itself was suspected to be the cause of the patterns of enthalpy change observed. It is the purpose of this research to investigate this possibility. (3) Farah, L.; Wilson, D.; Giles, G.; Ohno, A.; Scott, R. M. J . Phys. Chem. 1979,83, 2455. (4) Reyes, A.; Scott, R. M. J . Phys. Chem. 1980, 84, 3600.
0 1983 American Chemical Society
