Electronic structure and dynamics of the excited state in CT microcrystals as revealed by femtosecond diffuse reflectance spectroscopy

Transient absorption spectra of charge-transfer (CT) crystals between methyl- or methoxy-substituted benzene derivatives and pyromellitic dianhydride (PMDA) were measured by femtosecond diffuse reflectance spectroscopy, and the electronic structure and dynamics of the excited state depending on the nature of the electron donor (D) are discussed. For such weak CT complexes, it was confirmed that excitation energy is localized in one donor-acceptor pair and the mixing of CT and locally excited (LE) configurations is important in the excited CT singlet state. The CT degree of the excited state depends on the oxidation potential of D and on the mutual configuration of D and electron acceptor (A) molecules. The second-order decay constant of the excited state was observed under usual photolysis conditions, from which a motion-limited diffusion was considered, A first-order decay of the transient absorption was obtained, when the excitation intensity was weak, and ascribed to charge recombination to the ground state, The energy gap (-Delta G) dependence of the charge recombination rate constant (k(CR)) was confirmed to give a linear relationship between Ln(k(CR)) and \-Delta B\.