J. Phys. Chem. 1994, 98, 12198-12205
12198
Excited-State Proton (or Hydrogen Atom) Transfer in Jet-Cooled 242'-Hydroxyphenyl)-5-phenyloxazole Abderrazzak Douhal,*1?*$ Franqoise Lahmani,*$+ Anne Zehnacker-RentienJ and Francisco Amat-Guerris Luboratoire de Photophysique MolCculaire du C.N.R.S. Bat. 213, UniversitC Paris-Sud, 91405 Orsay Cedex, France; Facultad de Quimicas, San Lucas 3, Universidad de Castilla-La Mancha, 45007 Toledo, Spain; and Instituto de Quimica Orghnica, CSIC, Juan de la Cierva 3, 28006 Madrid, Spain Received: July 7, 1994; In Final Form: September 9, 1994@
The fluorescence excitation and dispersed emission spectra of 2-(2'-hydroxyphenyl)-5-phenyloxazole (HPPO) and its OH deuterated derivative (DPPO) have been investigated under gas-phase isolated conditions in a supersonic jet. While the excitation spectrum of HPPO is composed of broad overlapping bands, the deuteration of the OH group induces a drastic narrowing of the vibronic structure. The main vibronic bands of HPPO are well reproduced by Lorentzian line shapes corresponding to a homogeneous width ranging from 24 to 30 cm-', while those of DPPO range from 3.6 to 9.7 cm-'. The dispersed fluorescence of both compounds exhibits an identical large Stokes shift. The results are interpreted in terms of the occurrence of a fast ('4.5 x 10l2 s-') excited-state intramolecular proton-tunneling-transfer reaction in the enol form producing a keto tautomer through an asymmetric potential energy surface with a small energy barrier.
1. Introduction The great importance of hydrogen bonding and proton transfer in chemistry, physics, and biology leads to intensive studies on excited-state intramolecular proton (or hydrogen atom) transfer (ESIPT) reactions in a large variety of aromatic molecules.' Since the work of Zewail et al. on methyl salicylate (MS) in a cooled jet,2 several reports have been published in this field using supersonic jet fluorescence spectroscopy and providing important information on the potential energy surfaces (PES) and on the mechanisms of the displacement of the proton (or hydrogen atom) in the excited state of isolated molecule^.^ Among the systems studied by this technique are MS,2,4 o-hydr~xybenzaldehyde,~ salicylamide (SAM),5 3-hydroxyflavone (3HF),6,71-hydroxy-2-acetonaphthone (HAN),3tropolone (TRP),* 1,4- and 1&dihydroxyanthraquinone (DHAQ),9 and 2,5-bis(2'-benzoxazolyl)hydroquinone (BBXHQ).'O On the grounds of the symmetry of the molecule, Barbara et al. have made a qualitative classification of the PES involved in these reactions." According to a simplified one-coordinate description, the ESIPT process can take place with (i) a barrierless transition (single-minimum distorted curve, MS), (ii) tunneling through a low-energy barrier (two minima and asymmetric PES, HAN, and BBXHQ), and (iii) "deep" tunneling through a highenergy barrier (two minima and symmetric PES, TRP, and DHAQ). Very recently, we have reported on the ESIPT reaction in the jet-cooled molecule of HAN, interpreting the process in terms of the movement of the proton through a small energy barrier between two closely lying local potential minima.3 However, the ESIPT cannot be explained by a single separate reaction coordinate involving the proton motion. It should instead be described by a multidimensional coordinate, in agreement with a global skeletal rearrangement of the molecule in the excited state.
* To whom
correspondence should be addressed at Universitt Paris-
Sud. t Universitt Paris-Sud.
* Universidad de Castilla-La Mancha. @
Instituto de Quim'ca Orginica, CSIC. Abstract published in Advance ACS Abstracts, October 15, 1994.
0022-365419412098-12198$04.5010
We report in this paper on the fluorescence excitation and dispersed fluorescence spectra of jet-cooled 2-(2'-hydroxyphenyl)-5-phenyloxazole (HPPO) (Scheme 1) and of the corresponding OH-deuterated molecule (DPPO). We also report for comparison the fluorescence spectra of the o-methylated derivative, 2-(2'-methoxyphenyl)-5-phenyloxazole (OMePPO), where the ESIPT reaction cannot take place. Recently, Emsting et al. have reported a study of 2-(2'-hydroxyphenyl)benzoxazole (HB0),12 a molecule which is similar to HPPO. The fluorescence excitation spectrum of jet-cooled HBO is broad and does not show any vibrational structure that could provide information and details on the mechanisms and PES of this reaction. The study of HPPO in solution shows a resolved 0-0 UV transition as well as the occurrence of a fast ESIPT rea~ti0n.I~Observation of a resolved excitation spectrum and of deuterium isotope effect in the isolated molecule may provide better insight into the ESIPT reaction of aromatic molecules having the oxazole ring, such as BBXHQ and HBO. 2. Experimental Section HPPO and OMePPO were synthesized and purified as will be described e1~ewhere.l~The corresponding OH-deuterated compound, DPPO, was obtained in the jet apparatus by passing the carrier gas (He) containing D20 through the sample housing containing HPPO. The efficiency of the deuteration was very high, as checked by the fluorescence excitation spectrum of DPPO in the jet. The fluorescence experiments have been performed on a continuous jet apparatus using He (1-3 atm) as the carrier gas. The sample of HPPO or DPPO was heated to 373 K before the expansion. The excitation source was provided by a Nd:YAG pumped dye laser (frequency-doubled LDS 698 and DCM pumped at 532 nm). The LIF excitation spectra were measured through appropriate filters. The dispersed fluorescence was observed by using a 60 cm Jobin-Yvon monochromator and detected with a RCA photomultiplier (bandwidth Av = 20 cm-l). IR spectra have been recorded by using a Perkin-Elmer 681 spectrophotometer.
0 1994 American Chemical Society
Proton (or Hydrogen Atom) Transfer in HPPO and DPPO
SCHEME 1 p
HPPO
3
0
.H-0
J. Phys. Chem., VoE. 98, No. 47, I994 12199
Figure 4 shows the dispersed fluorescence spectrum resulting from the excitation of the electronic origin band of HPPO. This spectrum is independent of the excitation wavelength and consists of a structureless and largely Stokes-shiftedband (8900 cm-' between the origin of the excitation and the maximum of the fluorescence spectra). The maximum of the spectrum (,Ima = 480 nm) is 10 nm blue-shifted from that in cyclohexane at room temperature, and the fwhm is 2750 cm-'. We did not observe any normal resonance fluorescence that could be ascribed to the radiative deactivation of the initially excited enol form. The dispersed fluorescenceband of the deuterated compound, DPPO, obtained by excitation of the 0-0 band (not shown) has the same large Stokes shift and is very similar to that of HPPO. Again, no resonance fluorescence was observed.
keto form 4. Discussion
3. Results The fluorescence excitation spectrum of jet-cooled HPPO shown in Figure l a consists of diffuse bands starting from an origin centered at 29 852 cm-'. A dominant progression appears, with a constant band separation of 68 cm-'. At about 400 cm-' above the origin, no more fluorescence signal is observed, indicating the onset of a fast nonradiative decay process. By adding D20 to the He carrier gas, a much narrower structured spectrum appears, replacing the broad structure observed in pure HPPO (Figure lb). This new spectrum is stable for several hours after stopping the flux of D20 and thus can be safely attributed to the OH-deuterated compound DPPO that results from a very efficient deuteration of HPPO. The same spectrum is obtained when CD30D is used instead of D20. The strong 0-0 transition is blue-shifted by 75 cm-' with respect to that of HPPO. The spectral position of vibronic bands can be more accurately determined due to the better resolved structure of the spectrum of DPPO (Table 1). The main progression involves the same 68 cm-' interval as observed in HPPO. Other weak progressions built on the 68 cm-' vibration start at 88 and 164 cm-I from the origin. Interestingly, this later series presents a Franck-Condon distribution different from that starting from the origin of the transition. For comparison, the excitation spectrum of the methylated derivative (OMePPO), where the labile hydrogen atom of the OH group has been replaced by a CH3 group, has been also investigated (Figure 2a). The spectrum starts from the origin at 30 376 cm-' and exhibits considerable complex vibrational structures. The main vibrational frequencies are reported in Table 1. For comparison, the 0-0 transition of the parent molecule 2,5-diphenyloxazole (PPO) is located at 31 376 cm-'.14 The dispersed emission spectrum (Figure 2b) resulting from the excitation of the 0-0 transition is a typical resonance fluorescence spectrum from which the main ground-state vibrational frequencies active in the S1 SO transition can be deduced (Table 1). The contour of the vibronic features of DPPO is characteristic of a Lorentzian line shape, as is apparent from the extended wings of the bands (Figure 3). For comparison, the rotational contour of the origin of OMePPO, recorded in identical conditions, shows a narrower width. In the same manner, the excitation spectrum of HPPO can be fitted with a calculated spectrum built with adjustable Lorentzian line shapes, for the 68 cm-' progression starting from the origin and from the 164 cm-' vibronic band (Figure 3b). The fwhm obtained from the fitting of the experimental contours of the HPPO and DPPO spectra with Lorentzian functions are collected in Table 2.
-
4.1. Dispersed Emission Spectra. The fluorescence spectrum obtained by exciting the 0-0 transition of jet-cooled HPPO and DPPO starts at 6500 cm-' from the 0-0 transition of the enol form and is displaced by more than one quantum of the ground-state OH stretching band (3050 cm-') with respect to the 0-0 transition. The observation of this strong red-shifted and broad fluorescence band indicates the occurrence of an ESIPT reaction, involving a proton (or hydrogen atom) transfer from the phenol group to the nitrogen atom of the oxazole moiety (Scheme 1) as has already been observed in intramolecular hydrogen-bonded molecules of similar structure. The formation of a keto form upon electronic excitation of this kind of molecule has been confirmed by time-resolved IR spectroscopy in the case of 2-(2'-hydro~yphenyl)benzothiazole.~~ For comparison in OMePPO, a molecule which cannot undergo a proton-transfer process, the dispersed emission shows wellresolved vibronic structure starting from the 0-0 transition. For HPPO and DPPO no such resonance fluorescence is detected. This indicates that the proton (or hydrogen atom) displacement in HPPO occurs on a time scale much shorter than the radiative decay time of the normal enol form. The energy difference of 6500 cm-' between the 0-0 transition enol form and the onset of the fluorescence of the keto form represents the sum of the keto-enol energy gaps in both ground and excited states and thus does not allow one to know the exothermicity of the ESIPT reaction in S1 state. However, because of the similarity of HPPO and HBO, it may be assumed that the energy difference between the enol and the keto forms in SI state is of the same order. In the case of HBO this energy gap has been determined experimentally to be 2100 cm-' from the absorption and emission data of the keto and enol forms in both the singlet and triplet states.16 4.2. Excitation Spectra. Origin of the transition. The 0-0 electronic transition of jet-cooled HPPO is about 830 cm-' higher in energy than in cyclohexane at room temperat~re.'~ A similar shift has been observed in MS.4 The red shift on going from the gas to the condensed phase is due to the larger solvent stabilization of the excited state relative to the ground state in the condensed medium. The high intensity of the band of the absorption spectrum in solution allows its assignment to the lowest n-n* transition of the conjugated system having an intramolecular hydrogen bond between the phenol group and the oxazole moiety. The energy of the 0-0 transition of HPPO (29 852 cm-I ) is lower than that of OMePPO (30 376 cm-' ) by 524 cm-l (that is, 1.5 kcal mol-'). Crude calculations, obtained with the PCMODEL program from Serena Software17 using the MMX force field derived from MM2, give a coplanar
Douhal et al.
12200 J. Phys. Chem., Vol. 98, No. 47, 1994
** H 15 .->\ 01 +-,
C Q,
+-,
C
-
10
0 ~ ~ " " " ~ " " " " ' ~ " " ~ " " ~ 2.98 2.99 3.00 3.01 3.02 3.03 x10*
cm-1
>r .-+ 01
15
C 9) +-,
C
-
10
9)
>
n "
2.98
2.39
3-00 cm-1
3.C 1
3.02
3.C: i:'i ,3'
Figure 1. Fluorescence excitation spectra of (a) HPPO and (b) DPPO seeded in an expansion of He.
TABLE 1: Main Vibrational Frequencies (Av, cm-') in the SOand SI States of DPPO and OMePPO DPPO SI OMePPO SI OMePPO SO 0" Ob 0 68 136 (68 x 164 204 (68 x 234 (164 244 300 (164 368 (164
2)
3)
+ 68) + 136) + 204)
20 55 58 68 116 (58 x 127 146 201 (146 23 1 289 (231 550 616 (558 805 863 (805
0-0 band at 29 928 cm-'.
2)
+ 58) + 58) + 58) + 58)
60 120 (60 x 2) 246 940 1210 1520 1580 (1520 60) 1640 (1520 120)
+ +
0-0 band at 30 376 cm-I.
conformation of both aromatic and heterocyclic rings for both HPPO and OMePPO in the ground state. The energy difference mentioned above thus reflects a moderate stabilization due to
the extended electronic delocalization introduced by the intramolecular H bond in HPPO. The 75 cm-' blue shift of the 0-0 transition induced by H/D exchange indicates that electronic excitation induces a strengthening of the internal H bond as found in other H-bonded molecule^.^^^^^ Infrared spectra of HPPO and DPPO in potassium bromide show that the groundstate frequencies of OH and OD stretching are respectively 3050 and 2250 cm-'. The isotope ratio of these frequencies, v(0H)I v(0D) = 1.36, is the expected one from the harmonic approximation. Assuming that the isotope shift is mainly due to OH vibration in terms of change in zero-point energies in S1 and SO and that the ratio v(OH)Iv(OD) is conserved in the excited state, we deduce that the excited-state frequencies of these vibrations are reduced to 2455 and 1805 cm-', respectively. Unfortunately, the onset of a fast radiationless transition at 400 cm-' above the 0-0 transition did not allow us to observe any band in the region of OH and OD stretching. Vibrational structure. The excitation spectrum of DPPO reveals low-frequency bands which are indicative of the activity of large-amplitude motions. The same main frequencies at 68 and 164 cm-I are also seen in HPPO, showing that these modes
J. Phys. Chem., Vol. 98, No. 47, 1994 12201
Proton (or Hydrogen Atom) Transfer in HPPO and DPPO
a
3.05
a',
.--wA
6
0)
c
a,
4
-
c
I
,
I
I
I
I
3.06 3.07 cm-1 ,
-
J
4
I
,
~!
3.10 xi0'
b
-
.
,
3.09
-
- 4a .->
1
3.08
.
are not sensitive to the deuteration of the OH group and do not involve localized motions on the OH-N hydrogen bond. Similar low-frequency bands are present in the excitation spectrum of aromatic derivatives of the five-membered heterocycles. For example, the excitation spectrum of jet-cooled 2,5-diphenylfuran and 2,5-diphenylo~azole'~(PPO) exhibits the same lowfrequency vibrational structure at 67, 111, 160, and 255 cm-' which have been assigned to motions of the phenyl rings relative to the five-membered heterocycle. Due to the planar structure of the parent molecules in both the ground and excited states and according to symmetry selection rules for non-totallysymmetric modes, the 67 cm-' band has been assigned to the Av = 2 transition of a phenyl torsional motion involving a -33 cm-' fundamental frequency. The observation of an identical frequency in HPPO and DPPO shows that the same attribution should apply to the 68 cm-' progression of the hydroxy derivative of PPO. The 164 cm-' frequency, which has its counterpart at 160 cm-' in PPO, may be similarly assigned to an out-of-plane deformation or to an in-plane bending vibration modifying the angle between the oxazole and the hydroxyphenyl
-
parts. In the case of OMePPO, the increased complexity of the vibrational structure in the torsional mode region may indicate some deviation from the planarity of the molecule due to steric hindrance brought by the presence of the OCH3 substituent. 4.3. Mechanism of the ESIPT Reaction in Jet-Cooled HPPO. The most striking difference between the excitation spectra of HPPO and DPPO is the narrowing of the bands upon deuteration (Figure 1). The main bands of the excitation spectra of HPPO and DPPO were fitted with Lorentzian functions, which shows that the broadening of the bands is homogeneous in nature. This case can be compared to jet-cooled HBO, a compound which belongs to the same family of HPPO, whose excitation spectrum1*is very broad and has been described in terms of Lorentzian bands, separated by 147 cm-', and with a common width (fwhm) of 210 cm-'. In the isolated 3-hydroxyflavone (3HF), characteristic Lorentzian line shapes of the vibronic bands have been also observed.' The lifetime z of a given vibronic state is related to the homogeneous line width for an isolated excited molecule by fWhmhom=(2ncs)-'. The
Douhal et al.
12202 J. Phys. Chem., Vol. 98, No. 47, 1994
a v=2
v = 3 bond
bond
2.0
-
1.5
-
10-
05-
"i t
b
h
15
I
i I
I
-200 - l o g
100
0
200
cm-1 Figure 3. (a) Experimental and Lorentzian-fitted contours of the main vibronic bands of DPPO. Also is shown the experimental 0-0 band of OMePPO superimposed on that of DPPO. (b) Experimental and fitted Lorentzian excitation fluorescence spectra of HPPO.
result of the fit by Lorenztian functions of the main vibronic bands has been used to estimate the lifetime of each vibronic state of the enol form and consequently the rate constant ( ~ E S I ~ T =z-l ) of the ESIPT reaction from these levels (Table 2). However, it should be noted that the deduced values are
the upper limits of the rate constant of the ESIPT reaction of HPPO (and DPPO) in these levels. A convolution of a Lorentzian shape by both a Gaussian (for the laser profile) and the estimated rotational contour of the transition (for example that of OMePPO) would be necessary for a rigorous treatment
Proton (or Hydrogen Atom) Transfer in HPPO and DPPO
J. Phys. Chem., Vol. 98, No. 47, 1994 12203 nm
TABLE 2: Fitted fwhm Values and Calculated Rate of ESIPT Reaction of the Main Vibronic Constant k ~ s m Bands in the Excitation Spectra of HPPO and DPPO
x
~~~~~
0 1 2 3
24.1 26.4 30.0
4.5 5.0 5.1
3.6 6.0 6.8 9.7
Y
0.7 1.1 1.3 1.8
of the data. However, even the 0-0 transition of DPPO exhibits a strongly different line shape, much broader than the experimental rotational contour of OMePPO (see Figure 3), and the convolution has thus been neglected. Several observations can be drawn from these fits of the experimental data: (i) The time constant of the ESIPT reaction of the jet-cooled HPPO molecule is in the subpicosecond range (for the 0-0 transition, kEsIpT = 4.5 x 1012s-' ). The fast proton-transfer reaction prevents the deactivation of the primary excited enol form through any other channel and is in agreement with the absence of any normal resonance fluorescence. (ii) Deuteration of the OH group leads to a significant decrease of this rate constant (kEsIpT = 6.5 x 10" s-l ). The isotope-dependent rate constant, which is about 6 times smaller for the deuterium than for the hydrogen, indicates that the transfer proceeds through a tunneling mechanism. (iii) The rate constant increases slowly for HPPO and more rapidly for DPPO when higher members of the 68 cm-' progression are excited. According to these findings, the internal proton transfer can be described by an asymmetric potential energy surface along a reaction coordinate involving a low energy barrier. The asymmetry of the potential which accounts for the exothermicity of the reaction provides a manifold of final states in the keto excited state large enough to prevent the back reaction . Isotope effect on kEsIpT. Under the simplified assumption that the reaction coordinate for the tunneling process involves mainly the OH stretching mode, the kinetic isotope effect can thus be explained by the lowering of the zero-point energy level of the OD stretching vibration relative to the OH vibration, leading to an increase of the barrier height and of the tunneling distance. Moreover, the mass of the tunneling particle, which appears directly in the expression of kEsIm, is doubled. Since the proton transfer takes place at the zero-point energy level, the experimental rate constants can be compared in a first approximation to those deduced from the monodimensional model of the tunneling effect. According to Bell,18the rate constant is given by
k = v exp[-2da(2,~U)"~/h] where v is the bond vibrational frequency in the excited state as deduced previously (2455 cm-I for OH and 1805 cm-' for OD), a is the half-width of the energy barrier, p is the reduced mass for the proton or the deuteron, and U is the barrier energy from the zero-point level. Both height and width of the energy barrier are increased by deuteration because of the lowering of the zero-point level. Under the simplified assumption of a onedimensional harmonic oscillator for the OH or OD streching mode, deuteration increases the height of the barrier by 320 cm-' and its width by 0.019 A, as can be estimated from the coordinates of the turning point of the OH and OD harmonic oscillators. The experimental values of k~ and k~ for HPPO and DPPO have been used to calculate the energy barrier as a function of a, and the results show that the experimental data
2.4 x10'
2.3
2.2
2.1
2.0
1.9
1.8
cm- 1
Figure 4. Dispersed fluorescence spectrum of HPPO excited at the 0-0 band. Resolution is 20 cm-'. are not compatible with this simple description. Assuming for example that for HPPO the equilibrium half-width is 0.067 A, as suggested in the case of HI30'*and 0.086 A for DPPO, we find an energy barrier of 2950 and 2180 cm-' for the protonated and deuterated molecules, respectively. This estimation does not reproduce the difference of -320 cm-' expected from the zero-point energy of the OH and OD oscillators, and the discrepancy can be understood if one considers the oversimplification of the monodimensional model. Recent theoretical calculations and experimental results have suggested that the proton motion couples with other modes that act on the distance between the 0 and N atoms involved in the hydrogen In the case of HI3O,l2 which can be considered similar to HPPO as far as the proton transfer is concerned, a low-frequency (147 cm-') in-plane bending mode that modifies the skeletal angles between the two rings has been proposed as an effective mode to modulate the height and width of the tunneling barrier by reducing the N-O distance (Figure 5). Vibronic Dependence of kEsIpT. The shortening of the vibronic lifetime or the increase of the rate constant observed when exciting different members of the 68 cm-' progression may be due to a decrease of the tunneling parameters (height and width of the barrier) induced by this low-frequency oscillation in the excited state of the enol form. On the basis of its similar activity in the PPO spectrum, this low frequency has been assigned to a torsion of the phenyl nucleus which is not expected to change strongly the H translocation distance and to be directly involved in the proton-transfer process. The oscillation period corresponding to this 68 cm-' mode is 500 fs, which is in between the experimental time for H and D transfer. The vibronic dependence of the ESIPT rate is thus not conclusive on the role of the phenyl torsion as a promoting mode in the ESIPT reaction, as qualitatively shown in Figure 5. Several works have suggested the influence of the phenyl torsion in the ESIPT rate constant of 3HF.637,23However, recent femtosecond experiments did not find any evidence of this kind of coupling.24 The proton transfer is too fast for electronic change induced by phenyl motion to be important in the process. The increase of the rate constant of the proton-transfer reaction can be also explained by treating the proton-transfer dynamics according to the Fermi golden rule (FGR) approach developed by Siebrand et al. for this type of reaction.25 Under this assumption, the reaction may be described as a nonradiative transition with the excited enol form as the initial state and the
Douhal et al.
12204 J. Phys. Chem., Vol. 98, No. 47, 1994
dependence. The intersystem crossing to the triplet state of the quinoid structure has been shown to occur in solutions of HPP013 and of the analogous derivative HB016 and may be responsible for the fast quenching of the fluorescence. The intersystem crossing may be enhanced by distortions of the tautomer, which is expected to deviate from planarity. Because of the lack of collisions in jet-cooled conditions, another possibility may be the dissociation of the hydrogen bond in the vibrationally excited singlet state of the keto form isoenergetic with the enol singlet state.26 According to the estimation of exothermicity of the reaction (-2000 cm-l), this would indicate an intramolecular hydrogen bond strength in the keto form of the order of 2400 cm-I. The subsequent twisting of the two parts of the molecule around the inter-ring double bond may lead to a nonemissive excited state of TICT character as suggested recently in the case of 2-(2'-hydroxypheny1)benz~thiazole.~~
5. Conclusion distance 0-H Figure 5. Schematic qualitative representation of the potential energy
surfaces for the tautomerization in the fist electronic excited state of HF'PO. The system evolves along the 0-N coordinate reducing the barrier and width and modulating the other intramolecular vibrational modes leading to a tunneling andor classical movement along the OH coordinate from the enol to the keto wells. excited keto form as the final state. The rate constant can thus be expressed by
where Uif denotes the interaction between the initial and final state, li) and If), coupled with the proton-transfer reaction and @(E)is the density of final states isoenergetic to the initial state. If we consider that the 68 cm-' vibration is not directly involved in the proton-transfer reaction, the increase of the rate constants should be related to the density of states in the excited keto form which, in turn, depends on the exothermicity of the transfer. In the case of HBO, this excess energy has been found to be -2100 cm-I.16 A similar value may be deduced in HPPO if we consider that there is no vibrational relaxation in the supersonic jet. The keto form of HPPO is probably not planar as has been calculated for the similar molecule HBO1*and has many low-frequency modes due to the skeletal torsion and bending modes as well as the hydrogen-bond deformations. The density of states may thus be quite large at -2000 cm-' excess energy and will be weakly affected by further addition of successive 68 cm-I quanta. The more rapid increase of the rate constant in the case of DPPO (Table 2) may be due to a larger density of states resulting from the decrease of the frequencies localized on the OD bond relatively to the OH one. Nonradiative Pathways in the Excited-Keto Form. The disappearance of the fluorescence above -400 cm-' excess energy marks the onset of a nonradiative channel. This quenching of the fluorescence is not observed in the methylated derivative (MeOPPO) where the excitation spectrum extends over more than 800 cm-l above the origin. This indicates that the competing nonradiative process occurs in the keto tautomer rather than in the primary excited enol form. It should be noted that a similar efficient nonradiative process has been observed in jet-cooled methyl~alicylate~ and 1-hydroxy-2-acetonaphthone3 and seems to be a general phenomenon in the excited state of intramolecular hydrogen-transfemng molecules. Several explanations can be invoked to account for this excitation energy
The fluorescence excitation and dispersed emission spectra of HPPO and its OH-deuterated derivative DPPO have been studied in a supersonic jet. The 0-0 transition of HPPO is observed at 29 852 cm-' and is followed by a progression of 68 cm-' attributed to the phenyl torsion. From the width of the fwhm of the 0-0 transition, the rate constant of this reaction is estimated to be 4.5 x 10l2 s-'. The deuteration of the OH group shifts the excitation spectrum to the blue by 75 cm-' and leads to a significant narrowing of its bands. The main vibronic bands of HPPO are well fitted by Lorentzian line shapes having a homogeneous width ranging from 24.1 to 30.0 cm-'. Those of DPPO range from 3.6 to 9.7 cm-'. The observed broad fluorescence band of HPPO is largely Stokes-shifted, and its shape and position are not affected by deuteration. The results are interpreted in terms of the occurrence of an excitedstate intramolecular proton-tunneling-transfer reaction in the initially excited enol form producing a keto tautomer along a potential energy surface having a small energy barrier. An oversimplified monodimensional model does not explain the experimental results. A more sophisticated model is necessary to account for the isotopic and the excess energy effects. The onset of an efficient nonradiative channel is observed above -400 cm-' and has been tentatively assigned to distortions of the H-bonded ring favoring a fast intersystem crossing toward a quino'id triplet state or to the breakage of the internal H bond. Work is in progress on other hydroxyphenyl-oxazole derivatives, in particular hydroxyphenyl-methyloxazole, to elucidate the effect of the phenyl group on the ESIPT reaction dynamics and PES of HPPO. Acknowledgment. A.D. thanks the CNRS for giving him a temporal research associate position at the LPPM of the University of Paris-sud. References and Notes (1) For recent reviews see: Amaut, L.; Formosinho, S.J. J. Photochem. Photobiol. A: Chem. 1993; 75, 1. Formosinho, S. J.; Amaut, L. J. Photochem. Photobiol. A: Chem. 1993, 75,21. (2) Felker, P. M.; Lambert, W. R.; Zewail, A. H. J. Chem. Phys. 1982, 77,1603. (3) Douhal, A.; Lahmani, F.; Zehnacker-Rentien, A. Chem. Phys. 1993, 178,493. (4) Heimbrook, L.; Kenny, J. E.; Kohler, B. E.; Scott, G. W. J. Phys. Chem. 1983, 87,280. (5) Nishiya, T.; Yamauchi, S.;Hirota, N.; Baba, M.; Hanazaki, I. J . Phys. Chem. 1986, 90, 5730. (6) Emsting, N. P.; Dick, B. Chem. Phys. 1989, 136, 181. Muhlpfordt, A,; Bultmann, T.; Emsting, N. P.; Dick, B. Chem. Phys. 1994,181, 447. (7) Ito, A,; Fujiwara, Y.; Itoh, M. J. Chem. Phys. 1992, 96, 7474.
Proton (or Hydrogen Atom) Transfer in HPPO and DPPO (8) Tomioka, Y.; Ito, M.; Mikami, N. J. Phys. Chem. 1983, 87, 4401. (9) Smulevich, G.; Amirav, A.; Even, U.; Jortner, J. J. Chem. Phys. 1982, 73, 1. Gillispie, G. D.; Balakrishnan, N.; Vangsness, M. Chem. Phys. 1989, 136, 259. (10) Emsting, N. P.; Arthen-Engeland, Th.; Rodriguez, M. A,; Thiel, W. J. Chem. Phys. 1992, 97, 3914 and references therein. (11) Barbara, P. F.; Walsh, P. K.; Brus, L. E. J. Phys. Chem. 1989, 93, 29. (12) Arthen-Engeland, T.; Bultmann, T.; Emsting, N. P.; Rodriguez, M. A.; Thiel, W. Chem. Phys. 1992, 163, 43. (13) Douhal, A,; Amat-Guerri, F.; Acufia, A. U. To be published. (14) Mangle, E. A.; Salvi, P. R.; Babbit, R. J.; Motika,A. L.; Topp, M. R. Chem. Phys. Lett. 1987, 133, 214. (15) Elsaesser, T.; Kaiser, W. Chem. Phys. Left 1986, 128, 231. (16) Rodriguez Prieto, M. F.; Nickel, B.; Grellmann, K. H.; Mordzinski, A. Chem. Phys. Lett. 1988, 146, 387. (17) (a) The molecular mechanics calculations were carried out using the program PCMODEL, Serena Software, Bloomington, IN. See refs 17b and 17c. (b) Gajewski, J. J.; Gilbert K. E.; McKelvey, J. Advances in Molecular Modelling; Liotta, D., Ed.; JAI Press: Greenwich, CN, 1990;
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