3190
J . Phys. Chem. 1991, 95, 3190-3195
Proton-Transfer and Solvent Polarlzatlon Dynamics in 3-Hydroxyflavone G.A. Brucker,+ T. C. Swinney, and D. F. Kelley* Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 (Received: July 31, 1990) The excited-state intramolecular proton-transfer (ESIPT) dynamics of 3-hydroxyflavone (3HF) and 3-deuteroxyflavone(3DF) in nitrile solvents has been studied by using static and picosecond emission spectroscopies. ESIPT rates were determined for 3HF and 3DF in acetonitrile over a range of temperatures. Room temperatures rates of 1.7 X l o l l s-I and apparent activation energies of -2.9 kcal/mol were obtained. Little or no kinetic isotope effect was observed. ESIPT rates for 3 HF in benzonitrile were also determined over a range of temperatures. The room temperature rate was 3.1 X 1OlI s-I and the apparent activation energy was -2.0 kcal/mol. These results are analyzed in terms of a simple model in which solvent reorganization is needed to facilitate ESIPT, and once the appropriate solvent configuration is reached, proton tunneling occurs very rapidly. From this analysis we conclude the following. (1) There is a significant solvent polarization induced barrier in acetonitrile. (2) The ESIPT rate in acetonitrile is partially limited by the rate of barrier crossing, and partially limited by the rate of solvent reorganization. (3) There is little or no solvent-induced barrier in benzonitrile. (4) The ESIPT rate in benzonitrile is entirely limited by the rate of solvent reorganization. Introduction Proton-transfer reactions are of great importance in many areas of chemistry. The role of solvent dynamics in these reactions is of particular importance and has been extensively studied. Molecular systems which exhibit excited-state intramolecular proton transfer (ESIPT) are well suited to the study of solvent-mediated proton-transfer dynamics, because of their well-defined geometries and spectroscopic accesibility. Two types of solvent interactions can be imagined: specific hydrogen-bonding interactions and longer range solvent polarization interactions. Solute-solvent hydrogen-bonding interactions can disrupt an intramolecular hydrogen bond across which proton transfer occurs, and thereby dramatically alter the reaction dynamics.14 In the absence of solute-solvent hydrogen bonding, the solvent polarization can also affect the ESIPT dynamics. Intramolecular proton transfer alters the charge distribution of the molecule and is therefore coupled to the orientational motions of the surrounding polar solvent. Due to this change in charge distribution the equilibrium solvent configuration may be very different before and after ESIPT. One can imagine an ESIPT reaction which is exothermic overall; i.e., free energy of the reactant is higher than that of the product, when both reactant and product are in their respective equilibrium solvation. However, due to the change in charge distribution, this ESIPT may be energetically unfavorable with the solvent held in its prereaction equilibrium configuration. Under these circumstances a solvent fluctuation is needed to make the reaction possible. Once an appropriate (energetically uphill) solvent configuration is achieved the ESIPT reaction can proceed via proton tunneling. The ESIPT reaction therefore has an apparent solvent-induced barrier. Several theoretical studies have recently been published, based on the above idea^.^,^ These theories make predictions about proton-transfer rates, activation energies, kinetic isotope effects, etc. The results reported in this paper provide an experimental test of various aspects of these theories. The solute-solvent systems chosen for study is 3-hydroxyflavone (3HF) in nitrile solvents. These solvents are ideal for the study of solvent polarization effects on ESIPT rates. The nitriles (specifically alkanenitriles and benzonitrile) are highly polar, yet only very weakly hydrogen bonding. Complications due to scission of the intramolecular hydrogen bond are not expected. 3Hydroxyflavone exhibits ESIPT and has been extensively studied in liquid and frozen solution^,^,^.^** in rare gas mat rice^,'.^ and in the gas phase.I0 The structure of this molecule and the proton-transfer reaction are shown in Scheme I. The dynamics of the 3HF ESIPT reaction in nonpolar aprotic environments are fairly simple and have recently become well *Author to whom correspondence should be addressed. 'Present address: Department of Chemistry, University of Southern California, Los Angeles, CA 90089.
0022-3654/91/2095-3 190$02.50/0
SCHEME I
A
normal
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-1'
-1
L ESlPT
L
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~nderstood.~ Static spectroscopic studies of matrix-isolated 3HF suggest that there is an energetic barrier to proton transfer, which is larger than the zero-point vibrational energies.' Consistent with these results, the gas-phase (molecular jet) spectrum shows Lorentzian line shapes with widths corresponding to an ESIPT time of 1-2 ps.l0 The molecular jet spectra prove the existence of a barrier to proton transfer and show that tunneling is facile even in the vibrationless molecule. Time-resolved studies have also shown that excited-state proton transfer is very fast, less than a few picoseconds.',9 These very fast ESIPT rates are dramatically altered by the presence of a hydrogen-bonding environment, which can interrupt the intramolecular hydrogen bond. The ESIPT dynamics have been studied in 3HF'(ROH),, n = 1,2, hydrogen-bonded clusters which were isolated in IO K argon matrices. The results show that ESIPT occurs and is rapid in the n = 1 ( I ) Brucker, G. A.; Kelley, D. F. J . Phys. Chem. 1987, 91, 2856. (2) Brucker, G. A,; Kelley, D. F. J . Phys. Chem. 1987, 91, 2862. ( 3 ) McMorrow, D.; Kasha, M. J . Phys. Chem. 1984, 88, 2235. (4) Strandiord. A. J. G.: Barbara. P. F. J. Phvs. Chem. 1985.89. 2355.
Strandjord, A:J. d.;Courtney, S.H.; Friedrich, D.'M.; Barbara, P.F.i.Phys. Chem. 1983.87. 1125. (5) (a) Bbrgk, D.C.; Lee,S.;Hynes, J. T. Chem. Phys. Lett. 1989,162, 19. (b) Borgis, D.; Hynes, J. T. In The Enzyme CalaIysis Process; Cooper, A., Ed.; NATO AS1 Series; Plenum: New York, 1989. (6) (a) Cukier, R. I.; Morillo, M. J . Chem. Phys. 1989, 91, 857. (b) Morillo, M.; Cukier, R. I. J . Chem. Phys. 1990, 92, 4833. (7) Brucker, G. A.; Kelley, D. F. J . Phys. Chem. 1988. 92, 3805. (8) The literature on 3HF in solution in vast. For a recent review see: Barbara, P. F.; Walsh, P. K.; Brus, L. E. J . Phys. Chem. 1989, 93, 29. ( 9 ) Dick, B.; Ernsting, N. P. J . Phys. Chem. 1987, 91, 4261. (IO) Ernsting, N. P.; Dick, B. Chem. Phys. 1989, 136, 181.
0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 8, 1991 3191
Proton Transfer in 3-Hydroxyflavone
300
350
400
450
500
550
Wavelength /nm
G too
Time /ps 50
.50
Figure 1. Fluorescence excitation and dispersed emission spectra of 3HF
in acetonitrile at room temperature. Expansion of the ordinate shows the normal emission in the 360-450-nm region.
cluster but is completely inhibited in the n = 2 cluster. These results are completely consistent with the liquid and frozen solution results. Indeed, the solution results can be interpreted in terms of local hydrogen-bonding environments which are analogous to the structures of the matrix-isolated clusters. Despite the well-understood ESlPT dynamics in protic and nonpolar aprotic solvents, no studies of 3 H F ESIPT dynamics in nitrile solvents have previously been reported.
Experimental Section 3-Hydroxyflavone was purchased from Aldrich and purified by recrystallization from hexane and vacuum sublimation. Deuteration was accomplished by extracting the 3-deuteroxyflavone (3DF) in ether from D20. Following several extractions, the ether/3DF solution was transferred to a vacuum sublimation apparatus where the ether and any remaining D 2 0 were pumped off. This was followed by a resublimation at high vacuum of the remaining 3DF to ensure complete removal of all ether and D20. The purified, dry 3DF was then stored under high vacuum. Acetonitrile solvent was from J.T. Baker (spectrophotometry) and was dried with molecular sieves and CaH,. Benzonitrile and pentanenitrile were from Aldrich (Gold Label) and were dried with molecular sieves. All solvents were distilled over P20,! and handled under dry N2 atmosphere during preparation of the liquid samples. The temperature-dependent solution studies were carried out in a specially designed quartz Dewar, in which cold N2 gas was used to control the temperature of the sample. Careful control of the N2 flow into the Dewar resulted in temperatures stable within f2 O C . The light source used for the static spectra was a 150-W Xe lamp coupled to an Oriel '/s-m double monochromator (resolution -0.5 nm). Emission was collected and passed through a 0.64-m ISA monochromator (resolution -0.2 A). The detector was a Hamamatsu R943-02 Ga-As PMT with singlephoton-counting electronics. All spectra reported here are uncorrected . The. apparatus used in the time-resolved measurements was based on an active/passive mode-locked Nd:YAG laser, synchronously pumping an etalon-tuned dye laser. This laser system has been described in detail elsewhere." The output of the laser was amplified and frequency doubled to produce UV pulses which were about 20 ps in duration, with intensities of about 75 pJ between 320 and 355 nm. The UV excitation pulses were focused to a I-mm spot on the liquid sample. Time-resolved detection was accomplished with a Hamamatsu C979 streak camera, coupled to a PAR 1254 SIT vidicon, and 'interfaced to an IBM ~~
( 1 1 ) Brucker, G.A.; Young, M.A.; Kelley, D.F.Rev. Sci. Instrum. 1989, 60, 2592. (1 2) Sonsa Lopes,M.C.; Thompson, H.W.Specfruchim. Acta 1968,24A, 1367.
Figure 2. Time-resolved emission intensities from a solution of 3HF in acetonitrile at -42 O C . Emission wavelengths are (A) 385 27 nm, and (B) 552 i 32 nm. Also shown are curves calculated from the convolution of the instrument response function with (A) a fast rise time followed by a 20-ps decay, and (B) a 20-ps rise time with a 4.5-11s decay.
*
24.5t
, 3.4
3.6
3.8
4.0
4.2
(I/T) X 10 K Figure 3. Plot of In (km)vs 1/T for 3HF in acetonitrile measured by the time-resolved spectrometer (triangles). Also shown are In (km) values determined by relative quantum yield measurements (crosses), normalized to the -42 O C rate of (20 ps)-'. The solid line represents a linear least-squares fit of the quantum yield data.
PC AT compatible computer. Wavelength selection was accomplished by interference filters. The temporal instrument response function was -35 ps (fwhm). However, accurate determination of the instruments temporal response function results in actual temporal resolution of
