J. Phys. Chem. 1989, 93, 5179-5183
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timization of assemblies for the storage and conversion of solar energy.
fine-tuning the dynamics of the electron-transfer system. When we compare the results in acetonitrile and butyl acetate solution, we see that in the latter medium more usable chemical energy is stored 100 times as long as in the former with only a 10% less quantum yield. This could be of interest in the design and op-
Acknowledgment. We gratefully acknowledge the financial support by the Deutsche Forschungsgemeinschaft.
Intra- and Intermolecular Proton Transfer in 3-Hydroxyflavone/Ammonia Complexes G. A. Brucker and D. F . Kelley* Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 (Received: December 16, 1988)
Excited-state intra- and intermolecular proton-transfer processes have been studied in 10 K matrix-isolated 3-hydroxyflavone (NH,),, n = 1, 2 , ..., complexes. Both static and time-resolved (picosecond) spectroscopies have been used. The results indicate that the n = 1 complex exhibits intramolecular excited-state proton transfer (ESPT) while the n = 2 and probably n = 3 complexes exhibit intermolecular ESPT. Higher order complexes exist as ground-state ion pairs. The ESPT times are = l o and < I O ps for the n = 1 and n = 2 complexes, respectively.
SCHEME I
Introduction The study of excited-state proton-transfer (ESPT) processes in isolated solute-solvent complexes is one of the most important recent developments in the field of acid-base chemistry. Microsolvent clusters, with one or more solvent molecules complexed to the solute molecule of interest, have been isolated in both ultracold molecular beams and rare-gas matrices, and their proton-transfer dynamics have been analyzed as a function of the number of solvent molecules they contain. The results of these studies provide new insight into how solvation and proton-transfer processes evolve from the limit of simple molecular complexes to that of the bulk solution. , ESPT reactions require the simultaneous presence of a proton donor and a proton acceptor in the same molecule or molecular cluster. I n all cases, the energetic driving force for the reaction is provided by a change in the acid-base properties of the donor and/or the acceptor with electronic excitation.' As a consequence, these acid-base reactions can be photoinitiated. In general, the proton-transfer reaction results in a large change in the electronic configuration of the molecule or cluster, which in turn changes its emission spectrum. Such changes in emission spectra are often used to identify ESPT processes. Recently, several molecular jet and matrix isolation studies have been reported on the intermolecular ESPT reactions in molecular clusters of aromatic alcohols complexed with ammonia. Intermolecular ESPT has been observed in gas-phase and matrixisolated a-naphthol/NH3 complexes and matrix-isolated P-naphthol/NH, c o m p l e x e ~ . ~ -Evidence ~ for ESPT has also been observed in gas-phase phenol/NH3 complexes.6 3-Hydroxyflavone (3HF), like most other aromatic alcohols, is a very weak acid in its ground state and becomes quite acidic in its SI excited state. It also has a basic carbonyl group and can undergo intramolecular ESPT. The mechanism of intramolecular ESPT in 3 H F is indicated in Scheme I. Considerable work has been done in recent years to elucidate the solvent and temperature dependences of the solution-phase reaction.' Other work has ~~
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(7) (a) Strandjord, A. J. G.; Courtney, S. H.; Friedrich, D. M.; Barbara, P. F. J . Phys. Chem. 1983,87, 1125. (b) Strandjord, A. J. G.; Barbara, P. F. Chem. Phys. Lett. 1983, 98, 21. (c) Strandjord, A. J. G.; Barbara, P. F. J . Phys. Chem. 1985,89,2255. (d) Sengupta, P. K.; Kasha, M. Chem. Phys. Lett. 1979,68, 382. ( e ) McMorrow, D.; Kasha, M. J . Phys. Chem. 1984,88, 2235. (f') McMorrow, D.; Dzugan, T. P.; Aartsma, T. J . Chem. Phys. Lett. 1984, 103, 492. (g) McMorrow, D.;Kasha, M. Proc. N a f l . Acad. Sci. U.S.A. 1984, 81, 3375.
(3) Knochenmuss, R.; Cheshnovsky, 0.;Leutwyler, S. Chem. Phys. Lett. 1988, 144, 317. (4) Cheshnovsky, 0.;Leutwyler, S. J Chem. Phys. 1988, 88, 4127. ( 5 ) Brucker, G. A.; Kelley, D. F. J . Chem. Phys., in press. ( 6 ) Solgadi, D.; Jouvet. C.; Tramer, A. J . Phys. Chem. 1988, 92, 3313.
0022-3654/89/2093-5179$01.50/0 ,
-
tautomer
(1) Vander Donckt, E. Prog. React. Kinef. 1970, 5, 273. Ireland, J. F.; Wyatt, P. A. H. Ado. Phys. Org. Chem. 1976, 12, 131. (2) Cheshnovsky, 0.;Leutwyler, S. Chem. Phys. Lett. 1985, 1 2 1 , 1.
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5180 The Journal of Physical Chemistry, Vol. 93,No. 13, 1989
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Figure I, Schematic diagram of the apparatus used in the time-resolved measurements. The dye laser is etalon-tuned and synchronously pumped by an active/passive mode-locked Nd-YAG laser. Two stages of dye laser amplification are used.
tions,1° we reported the results of our studies on the intramolecular ESPT dynamics of well-defined matrix-isolated complexes of 3-hydroxyflavone(3HF) with different hydrogen-bonding solvents, such as water, alcohols, and diethyl ether. As stated above, in its ground electronic state, 3 H F is a very weak acid (pKa(So) = 8.35).8 A Forster cycle determination predicts that the molecule is a much stronger acid in its first electronically excited state, pKa(Sl) = -1.5. On the basis of the results reported for other aromatic alcohols, the highly acidic excited state suggests that when 3 H F is complexed with N H 3 molecules, there should be a critical value of n, for which the 3HF(NH3), complexes exhibit intermolecular rather than intramolecular ESPT. The mechanism of intermolecular ESPT in 3HF is indicated in Scheme 11. With this prediction in mind, we recently extended our studies on the ESPT dynamics of matrix-isolated 3HF(solvent), complexes to include N H 3 as the hydrogen-bonding solvent. In this paper, we report spectra that indicate that intramolecular ESPT occurs in n = 1 complexes and that intermolecular ESPT occurs in the complexes where n = 2 and probably n = 3. Picosecond-emission kinetics are also reported. These results indicate that ESPT times are =IO and 99.998%) and was dried by passage through a liquid nitrogen/ethanol trap (-95 "C). The ammonia used was high purity and was further purified by distillation at dry-ice temperature. 3 H F (Aldrich) was purified by repeated resublimation under high vacuum. It was further dried by high vacuum, pumping for at least h prior to deposition. T h e purified, dry 3 H F was sublimed into the Ar/NH3 stream at (8) Strandjord, A. J. G.; Smith, D. E.;Barbara, P. F. J. Phys. Chem. 1985, 89,2365. (9) Brucker, G. A.; Kelley, D. F. J . Phys. Chem. 1988, 92, 3805. (IO) (a) Brucker, G. A.; Kelley, D. F. J . Phys. Chem. 1987.91, 2856. (b) Kelley, D. F.; Brucker, G. A. VItraJh Phenomena; Fleming, G. R., Siegman, A . E., Eds.; Springer-Verlag: Berlin, 1986; p 319.
Wavelength I n r r
Figure 2. Fluorescence excitation (FE) and dispersed emission (DE) spectra from 3 H F / N H 3 / A r matrices under the following annealing conditions: (A) nonannealed matrix, (B)30 K for 10 min, (C) 26 K for 10 min, and (D) 30 K for 10 min. Arrows indicate the corresponding observation and excitation wavelengths.
55 "C immediately prior to condensation on the cold copper block. Typical argonlammonia mixing ratios were 2000:1, with 3 H F present in much smaller concentrations. The excitation source used in the static spectra was a 150-W Xe lamp coupled to an Oriel l/g-m double monochromator (resolution 0.5 nm). Dispersed emission spectra were collected with a 0.64-111 ISA monochromator with a 1200 grooves/mm 2 The detector was a Hamamatsu grating (resolution ~ 0 . A). R943-02 Cia-As P M T with single photon counting electronics. All spectra reported here are uncorrected. The apparatus used in the time-resolved measurements is based on an active/passive mode-locked Nd-YAG laser, synchronously pumping an etalon-tuned dye laser." The complete apparatus is depicted schematically in Figure 1 . A single pulse is extracted from the 1064-nm pulse train, amplified, frequency doubled, and used to pump the two-stage dye amplifier. The output pulse of the dye amplifier is then frequency mixed with the residual 1064-nm pulse. The final output consists of tunable UV pulses, which are about 20 ps in duration. Four excitation wavelengths, 355, 364, 365.5, and 375 nm, were used throughout the experiments. In all cases, the pulses were focused to about a l-mmdiameter spot size with energies between 20 and 30 MJ. Excitation at 355 nm was with the third-harmonic, 25-ps, 2 0 - ~ pulses J from the Nd-YAG laser. Time-resolved detection was accomplished with a Hamamatsu C979 streak camera, coupled to a PAR 1254E SIT vidicon and interfaced to a DEC LSI 11/02 computer. Wavelength selection was accomplished by a Spex 1/4-m monochromator with a 600 grooves/mm grating. The spectral band-pass was typically 4 nm. The temporal instrument response function was 35 f 2-ps (fwhm). Accurate determination of the instrument's temporal response results in actual temporal resolution considerably better than 35 PS.
Results and Discussion A. Bare Molecule 3HF. The spectroscopy and intramolecular ESPT dynamics of bare 3 H F have been described in detail in a ( 1 1) Brucker, G. A,; Young, M. A,; Kelley, D. F. Reo. Sci. Insfrum.,in
press.
Proton Transfer in 3-Hydroxyflavone/Ammonia Complexes previous paper.Ioa A brief summary of those results will be presented in this section. Figure 2A shows the dispersed emission (DE) and fluorescence excitation (FE) spectra of 3 H F in a nonannealed A r / N H 3 (2000:l) matrix, deposited at 10 K. These spectra are identical with those obtained from 3HF in a pure, dry, Ar matrix deposited at the same temperature. The origin of the dispersed emission spectrum is Stokes shifted ~ 8 0 0 cm-I 0 from that of the excitation spectrum. This emission has been assigned to the tautomeric form of the molecule, formed by intramolecular ESPT7d(Scheme I). No normal emission is detected in Figure 2A. This observation, along with the assumption of comparable oscillator strengths for the normal and tautomeric forms, indicates that proton-transfer times must be on the order of, or less than, a few picoseconds. An upper limit for this ESPT time was obtained by direct measurement of the tautomeric emission rise time with picosecondemission spectroscopy. Our time-resolved measurements showed that, in a 10 K Ar matrix, intramolecular ESPT in unsolvated 3 H F takes place in less than 10 ps. This result is in agreement with the rapid tautomeric emission rise time reported by Dick and Ernsting.I2 B. Spectra and ESPT Dynamics of the 3HF(NH3)lComplex. Codeposition of 3 H F with an argon/ammonia gas mixture (2000:l) results in the spectra shown in Figure 2A. When the 3HF/ammonia-doped argon matrices are annealed, new features appear in the FE and DE spectra. Since no change is detected when ammonia is absent, the new features are assigned to 3HF(NH,),, n = 1 , 2, ..., complexes. During the annealing process, solvent molecules diffuse and 3HF(NH3), complexes are formed sequentially, starting with monosolvates. As the concentration of monosolvates increases, disolvates are formed from them by further solvent diffusion. In the same way, more highly solvated complexes may be obtained by allowing the diffusion process to continue. The different 3HF(NH3), complexes have well-defined emission and excitation spectra. Analysis of the appearance kinetics of each spectral feature during the annealing process provides a mechanism by which the stoichiometry of the complex giving rise to that spectral feature is established. Some of the 3 H F / N H 3 / A r matrices were deposited at 30 K. Under these conditions, the 3H F(NHs), complexes are formed during the deposition process. Variation of the A r / N H 3 ratio results in changes in the relative intensities of the different complex spectral features. Analysis of these results provides an alternative method for the determination of complex stoichiometries. In all cases, both of the methods used to determine stoichiometries provided equivalent results. Figure 2B shows the DE spectrum obtained with excitation at 365 nm, from a matrix annealed at 30 K for 10 min. This spectrum is qualitatively similar to the bare-molecule DE spectrum (Figure 2A), but it is red shifted ~ 3 5 cm-' 0 with respect to it. (The weak emission in the 495-505-nm region is due to 3HF( H 2 0 ) , complexes formed as a result of trace water impurities.) The corresponding excitation spectrum, obtained by observing ' 5 15-nm emission, overlaps the far more intense bare-molecule FE spectrum and can be seen only as a shoulder in the 360-370-nm region (Figure 2B). By means of the two methods previously discussed, these spectra are assigned to a 3HF(NH3), complex. As in the bare molecule, the DE origin of the 3HF(NH3)1complex is Stokes shifted =8000 cm-' with respect to the origin of the corresponding FE spectrum. We assign this emission spectrum to the tautomeric form of the monosolvate complex, resulting from an intramolecular ESPT reaction. Very little (barely detectable) emission is observed in the normal molecule region (375-450 nm) of the DE spectrum in Figure 2B. The very weak emission that is detected is probably due to complexes formed with trace water impurities. The absence of appreciable normal emission is interpreted in terms of relatively fast ESPT, probably occurring in 100 ps or less. An estimate for the intramolecular ESPT time in the monosolvate was obtained by picosecond-emission spectroscopy. A (12) Dick, B.; Ernsting, N. P. J. Phys. Chem. 1987, 91, 4261
The Journal of Physical Chemistry, Vol. 93, No, 13, 1989 5181
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Figure 3. Time-resolved emission (410 4 nm) kinetics from a 3HF/ NH,/Ar matrix annealed at 28 K for 15 min. Excitation was at 355 nm. Also shown is the curve that corresponds to the convolution of the instrument response function with a biphasic decay of the emission. The fast decay time component in the calculated curve is 10 ps.
direct measurement of the proton-transfer time may be obtained from the tautomeric emission rise time or, alternatively, from the decay time of the normal molecule emission. Due to the overlap of the bare molecule and monosolvate excitation spectra, it was impossible to excite the monosolvate without exciting some small amounts of the bare molecule. As a consequence, the tautomeric emission from the monosolvate was contaminated with emission from the bare molecule. However, the bare molecule exhibits no normal emission,1° and the 400-440-nm region can be observed with no contaminating emission from the bare molecule. Figure 3 shows the emission kinetics at 410 nm following excitation at 355 nm, from a matrix previously annealed at 28 K. This normal emission can be resolved into fast ("10-ps) and slow (several nanoseconds) decay components. Almost identical spectra were obtained from the normal molecule region exciting at 364 and 365.5 nm, where little bare-molecule excitation occurs. In all cases, no emission was detected in this region prior to annealing. The fast-decay component was assigned to normal emission from the ammonia monosolvate. On the basis of our previous studies, the slowly decaying emission is assigned to 3HF(H20),, n I 2, complexes, formed as a result of water impurities in the matrix.1° It must be kept in mind that the diffusion properties of water in the Ar matrix are not very different from those of ammonia. Annealing temperatures of about 30 K were required to obtain good signal to noise ratios in the time-resolved normal molecule emission spectra. At these high annealing temperatures, we expect that even small quantities of water could affect the 3 H F / N H 3 spectra. In most matrices used in this study, the presence of trace water impurities was also detected by means of steady-state spectra. The above time-resolved data indicate that ESPT in 3HF(NH3)1 complexes requires = l o ps, which is significantly slower than the ESPT reaction in the bare molecule (
