Application of time-resolved linear dichroism spectroscopy: Rapid relaxation of excited charge transfer complexes

Time-resolved linear dichroism spectroscopy has been used to study the charge-transfer complexes formed between methylated benzene donors and 1,2,4,5-tetracyanobenzene as acceptor. The angle between the charge-transfer absorption transition moment and the photochemically produced radical anion absorption transition moment, after relaxation, has been measured for several complexes. A simple theoretical model was used to interpret the measured values. This model correlated the extent of localized excitation mixed into the charge-transfer transitions with the results of time-resolved linear dichroism measurements. Furthermore, the comparison between the measured and predicted transition moment directions suggests that a minor geometry change occurs after excitation of these complexes. The observation of the relative transition moment vector of the initial state(s) for some of the complexes was also possible. In these cases, the initial transition moment vectors observed are not consistent with absorption due to the radical anion but are believed to originate from a different electronic state, presumable from an excited state of the CT complex. These results, imply that the rapid relaxation process observed for these complexes must be electronic in nature and not purely geometric.