Time resolved resonance raman studies on photo-induced electron transfer reaction

Abstract

Time-Resolved Resonance Raman Studies on Proton-Induced Electron Transfer Reaction from Triplet 2-Methoxy-Naphthalene to Benzophenone In this chapter, the effect of solvent polarity on the mechanism and the nature of ion-pair intermediates formed in photo-induced electron transfer processes has been investigated using transient absorption and TR³ spectroscopy. Hydrogen transfer has been found to compete with the electron transfer mechanism. In nonpolar media, the formation of ketyl radical due to hydrogen transfer from TMB to FL is favored. As the polarity of the solvent increases, the hydrogen transfer mechanism switches over to the electron transfer mechanism. The nature of primary ion-pair intermediates generated in different solvent polarities has been investigated from the structural changes in the acceptor following charge transfer in the ion-pair. In medium-polar media, contact ion-pairs of exciplex nature are more favored, and this species eventually decays via intra-ion-pair proton transfer. In polar solvents, a complete ionic nature of the solvent-separated ion-pair has been observed. Transient intermediates formed in proton-induced electron transfer reactions between 3ROMe and the benzophenone system have been studied by TR³ technique. Comparison of TR³ spectra of 3ROMe in the presence of BP with those obtained in its absence has shown that the interaction between ROMe and BP in the triplet exciplex is very weak. The transient intermediates generated from electron transfer reactions are ROMe•? and DFBPH•. In polar solvent CH?CN–H?O mixture, the triplet radical ion-pair intermediate between ROMe•? and DFBPH• dissociates rapidly into solvated ions and was not detected within the nanosecond time resolution of our experimental setup. The TR³ spectra of ROMe•? show a pronounced effect from the substituent methoxy group in terms of an increase in the number of totally symmetric bands compared to the radical cation of naphthalene. The structural parameters obtained from DFT calculations for 3ROMe have shown that the effect of the methoxy group results in the localization of the antibonding ?* electron density more on the ring connected to the methoxy group and effectively more localized on the C7–C8 bond. The experimental TR³ bands of ROMe have been assigned to the calculated vibrational frequencies obtained from DFT calculations. The TR³ spectra and structure of ROMe have shown that the overall structure and nature of TR³ spectra are very similar to the triplet excited state of naphthalene. Analysis made on the structure of ROMe•? from density functional calculations has shown that the structure of the naphthalene unit in ROMe•? deviates from the D?h structure found in the cation radical of naphthalene. The ordering of C–C bond lengths in the naphthalene unit shows a quinonoidal structure, whereas the structure of the naphthalene unit of 3ROMe is close to the parent naphthalene triplet. TR³ spectroscopy combined with density functional calculations has been used to study the effect of asymmetric substitution of methyl groups on the structure and vibrational spectra of the radical anion of benzoquinone. Compared to the ground state, the radical anion of MBQ and 2,6-DMBQ shows reverse ordering of the bond length between the proximal and distal carbonyl bonds. The proximal carbonyl bond shows an increase in bond length compared to the distal. Bond length changes of the C=O bond upon reduction to the radical anion reveal that the ?* antibonding character is more localized on the proximal carbonyl bond compared to the distal bond. The hydrogen-bonding interaction between the methyl hydrogen and the proximal carbonyl group is evident from the polarization of the spin density from the proximal carbonyl oxygen to the carbon atom bonded to it. Assignments of the experimental TR³ bands with the calculated ones have been made on the basis of isotopic shift and PED analysis. The additional second methyl substituent in 2,6-DMBQ•? results in more shift in the vibrational frequency for the symmetric stretch compared to the asymmetric C=O stretch. From the shift in the vibrational frequency upon (¹?O) isotopic substitution of both the carbonyl oxygen atoms, it has been found that the additional methyl substituent does not change the isotopic shift for the asymmetric C=O stretch, whereas the isotopic shift for symmetric C=O stretch is affected. Therefore, it has been suggested that the second methyl substituent in the case of 2,6-DMBQ•? affects the coupling of the C=O modes with other internal coordinates more for symmetric C=O stretch compared to asymmetric C=O stretch. The results obtained from the site-specific isotopic labeling of the carbonyl oxygen and carbon atoms have shown that the C=O modes for symmetric C=O stretch are less coupled to each other and are predominantly from the proximal carbonyl group, and the contribution from the proximal C=O mode increases considerably in the case of 2,6-DMBQ•?. The isotopic shift, which reveals the presence of asymmetry of the methylated quinones in the isolated state, might be considerable for these kinds of quinones when reconstituted in the RC because of strong interaction with the protein. So, comparison of isotopic shift and vibrational spectra of the asymmetrically substituted quinone in in vivo and in vitro will help to understand the interaction of the quinones with the protein in the photosynthetic reaction center.