17715
J. Phys. Chem. 1995, 99, 17715-17723
Spectroscopic, Kinetic, and Thermodynamic Deuterium Isotope Effects in the HexarnethylbenzenelTetracyanoethylene Charge-Transfer Complex Kristen Kulinowski; Ian R. Gould? Nancy S. Ferris? and Anne B. Myers*?+ Department of Chemistry and Center for Photoinduced Charge Transfer, University of Rochester, Rochester, New York 14627-0216, and Research Laboratories, Eastman Kodak Corporation, Rochester, New York 14650 Received: July 31, 1 9 9 P
The charge-transfer absorption and fluorescence spectra, resonance Raman spectra, and equilibrium constants and molar absorptivities of the complexes between tetracyanoethylene and hexamethylbenzene (hlg-HMB) or perdeuteriated hexamethylbenzene (dlg-HMB) are compared. The enthalpies and entropies of complexation and the absorptivities in C C 4 solution are the same for the two isotopes to within an experimental uncertainty of about &lo%. The vibrations that carry significant intensity in the resonance Raman spectra are only slightly shifted by perdeuteriation of the HMB, suggesting that hydrogen motions are only weakly coupled to the charge-transfer transition. However, the fluorescence quantum yields in both C C 4 and cyclohexane solvents indicate that perdeuteriation decreases the rate of nonradiative return electron transfer by a factor of about 1.6, implying more significant participation of modes involving hydrogen motion. Perdeuteriation shifts the absorption spectra about 120 cm-’ to the blue while having little effect on the fluorescence spectra. Complexes of HMB, durene, and p-xylene with tetracyanobenzene as acceptor similarly exhibit negligible isotope effects on the fluorescence band shapes but significant (factors of 1.3-2.1) effects on the fluorescence yields. Calculations on HMB/TCNE within the harmonic approximation are unable to reproduce the isotope effects on both the spectra and the kinetics with a common set of parameters. Anharmonicities of the CH (CD) stretches may play an important role as is thought to be the case in other radiationless transition processes.
Introduction Electron transfer in the “Marcus inverted region” (Figure 1) is generally treated as a nonadiabatic electronic radiationless transition which can be described by a Fermi Golden Rule expression involving the product of an electronic matrix element and a Franck-Condon weighted density of states. Chargetransfer optical absorption and emission spectra are described by very similar expressions consisting of an electronic transition dipole matrix element multiplying a vibrational Franck-Condon part. In a system which may undergo return electron transfer either radiatively (by charge-recombination fluorescence) or nonradiatively, the vibrational contribution to the nonradiative rate is the same as that for emission of a hypothetical “zerofrequency” photon. The relationships between optical spectroscopic band shapes and rates of nonradiative electron transfer have become a topic of considerable interest,’-6 particularly with the comparatively recent understanding that Raman intensities excited on resonance with a charge-transfer transition can provide mode-specific information about the vibrations whose reorganization determines both the band shape of the optical transition and the dependence of the nonradiative return electrontransfer rate on thermodynamic driving f o r ~ e . ~ , ’ - ’ ~ Isotope effects on the nonradiative return electron-transfer rate should also constitute a nonspectroscopic probe of the molecular vibrations that are coupled to the charge-transfer transition. 15,17-21 Since the electronic coupling matrix element, which depends on the distance and relative orientation of donor and acceptor, is presumably isotope-independent,any isotopic sensitivity of the return electron-transferrate should reflect the ~~
~
* To whom inquiries
should be addressed.
’ University of Rochester.
Eastman Kodak Corporation and Center for Photoinduced Charge Transfer. Abstract published in Advance ACS Abstracts, December 1, 1995. @
0022-3654/95/2099-17715$09.00/0
Nuclear Configuration
Figure 1. Processes of optical absorption (a), resonance Raman scattering (b), relaxed fluorescence (c), and nonradiative back electron transfer (d) for a charge-transfer complex in the Marcus inverted region.
contribution of vibrations involving the substituted atoms to the Franck-Condon envelope of the electronic transition; that is, only those atoms whose motions contribute to the internal reorganization energy should exhibit an isotope dependence of the rate. Several years ago, Gould and Farid observed that return electron-transfer rates in a number of complexes between methyl-substituted benzene donors and cyanoanthracene acceptors are substantially reduced by deuterium substitution of the methyl hydrogens of the donor but essentially unaffected by substitution of the ring hydrogens.” This was interpreted to mean that stretching and/or bending vibrations involving the methyl hydrogens are coupled to the electron-transferreaction, perhaps through a hyperconjugative mechanism, while motions of the phenyl ring hydrogens are not. These observations led
0 1995 American Chemical Society
Kulinowski et al.
17716 J. Phys. Chem., Vol. 99, No. 50, 1995 us to initiate resonance Raman studies on a related chargetransfer system, hexamethylbenzeneltetracyanoethylene (HMBI TCNE), in the hope of elucidating the types of hydrogen motions inv01ved.I~ However, the results appeared only to confuse the issue, since the modes having large deuterium isotope shifts exhibit little or no resonance Raman intensity, implying they have negligible reorganization energies. In particular, there is no discernible resonance enhancement of CH (or CD) stretching modes. While the HMBRCNE complex was not studied by Gould et al., more recent measurements by Hilinski's group,22 as well as our own fluorescence yield measurements (vide infix), show that return electron transfer in this system exhibits an isotope effect similar to that observed with the cyanoanthracene acceptors. Here we present further studies of isotope effects in the HMB/ TCNE complex in the hope of shedding some light on these seemingly inconsistent observations. The equilibrium constant for complexation is measured as a function of temperature for both the normal and perdeuteriated donors to obtain the enthalpies and entropies of complexation and the molar extinction coefficients for both isotopic species. Absolute resonance Raman excitation profiles of the perdeuteriated complex (dl 8HMBRCNE), together with its optical absorption spectrum and charge-recombination fluorescence band shape, are obtained and analyzed to deduce all the molecular vibrational reorganization energies for comparison with the corresponding analysis of the hig-HMB complex.23 The deuterium isotope effect on the fluorescence quantum yield, which presumably reflects the return electron-transfer rate, is measured in two different solvents and compared with the isotope effect on the rate predicted from the spectroscopic parameters. The perdeuteriation effects on the fluorescence spectra and quantum yields of HMB, durene, and p-xylene with tetracyanobenzene as acceptor are also examined. The unexpected isotope independence of the HMB/ TCNE nominal intermolecular donor-acceptor stretching vibration at 165 cm-I is reexamined. Finally, we attempt to rationalize the very small isotope effects on the spectra with the significant isotope effects on the nonradiative return electrontransfer rates. Experimental and Computational Methods The methods used to obtain the absorption, fluorescence, and resonance Raman spectra, as well as the equilibrium constants and molar extinction coefficients as a function of temperature, have been described in detail in a previous p ~ b l i c a t i o n .The ~~ enthalpy and entropy of complexation, AH and AS, were estimated from the temperature-dependent equilibrium coefficients through the van't Hoff formulation, In K = -AH/RT
+ ASIR
In the solutions used for our Raman experiments, the concentration ratio of uncomplexed donor (HMB) to complex (HMB/ TCNE) is about a factor of 10, and while only the complex Raman lines exhibit any resonance enhancement, the strongest nonresonant Raman lines of the donor (the CH or CD stretches) have nonnegligible intensities compared to the Raman lines of the complex. We therefore measured the Raman spectrum of the uncomplexed donor separately and subtracted its contribution to the spectrum. In addition, while no reabsorption corrections to the relative Raman intensities were made in our previous work,I3 more careful consideration reveals that these corrections are not entirely negligible. Approximate reabsorption corrections were applied to both the hl8 and d]8 complex data by assuming that the Raman-scattered light escapes the sample through an estimated path length of 1 mm. This assumption
TABLE 1: Benesi-Hildebrand Analysis of h18- and dls-HMB/TCNE Complex EK
E
(M-I cm-')
K (M-I)
(104 M-2 cm-])
T("C)
hl8
dix
his
dis
his
dis
15 20 25 30 35
5370 % 240 5120% 150 4830 % 110 4 7 4 0 f 150 4510% 170
4640 i 310 4640% 390 4420 % 4 3 0 4200f450 4080f470
232 k 12 193 f 7 163 f 4 134f4 115 f 4
276 f 23 210*21 176 5 20 1 5 0 % 18 125 f 16
125 99 79 64 52
128 97 78 63 51
was consistent with the observed differences in the relative intensities of solvent lines in the solutions containing the complex compared with solutions of donor only24and resulted in corrections to the reported cross sections of a maximum of 10- 15% at most excitation wavelengths. Fluorescence lifetimes were measured by time-correlated single photon counting as described elsewhere.25 Excitation was obtained from a Nd:YLF-pumped dye laser cavity dumped at 1.9 MHz. The excitation and detection wavelengths were 380 and 750 nm, respectively. Hexamethylbenzene (hi E-HMB),perdeuteriated hexamethylbenzene (dl E-HMB),and tetracyanoethylene (TCNE), all from Aldrich, were used without further purification for the equilibrium constant and resonance Raman experiments. The fluorescence measurements required further purification of the hexamethylbenzenes to remove small quantities of one or more long-lived impurities. This was accomplished through chromatography on a silica gel column with HPLC grade hexane (Aldrich) as the eluting solvent. Tetracyanobenzene (TCB) was purified by silica gel column chromatography with CH2C12 as the eluting solvent, following by recrystallization twice from CHC13. Durene (Aldrich), dl4-durene (Aldrich), p-xylene (Aldrich), and dlo-p-xylene (MSD Isotopes) were all used as received. The calculations of absorption and fluorescence spectra, absolute resonance Raman excitation profiles, and return electron-transfer rates were performed using the time-dependent wave packet method as described p r e v i ~ u s l y .The ~ ~ effects of all solvent or solute motions too low in frequency to be observed directly in the resonance Raman spectrum (
