TRANSIENT RAMAN STUDIES ON THE STRUCTURE OF THE CHLORANIL ALKYLBENZENE TRIPLET CHARGE-TRANSFER COMPLEXES

Transient resonance Raman spectroscopy has been used to study the structure of chloranil-alkylbenzene triplet charge-transfer complexes in solution. Resonance excitation conditions were chosen so that only Raman bands due to chloranil vibrations were selectively enhanced. The Raman spectra of the complexes with weak donors were similar to that of T1 chloranil, while those of the complexes with strong donors were similar to that of the chloranil anion radical. The observed vibrational frequencies of chloranil in the triplet complexes were plotted as a function of the ionization potential of donors, and common "S-shape (or inverted S-shape)" dependence was observed for all the vibrations. This S-shape dependence was well-reproduced by a simple theory. It was concluded that the molecular structure of the chloranil moiety in these triplet complexes is intermediate between that of the free T1 state and that of the anion radical. The structure changes in accordance with the change in the electron-donating ability of donors, reflecting the change in the magnitude of the charge transfer occurring in the complexes.