Localization of a charge transfer excited state in molecular crystals: A direct confirmation by femtosecond diffuse reflectance spectroscopy

Transient absorption spectra of the durene-pyromellitic dianhydride (PMDA) charge transfer (CT) crystal, PMDA-doped durene crystal and its molten state were measured by femtosecond diffuse reflectance spectroscopy, and electronic structure and dynamics of the excited state in the CT crystal are discussed. It is directly confirmed that the excited CT singlet state is almost localized (self-trapped) in one donor(D)-acceptor(A) unit within a few picoseconds after excitation. The electronic structure of the excited state is quite polar but it is not an electron transfer state from D to A.