Analysis of Optical Spectra of Intramolecular Charge Transfer System for Gibbs Energy Evaluation; Solvent-Independent Charge Transfer Band

We analyzed the optical spectra of an electron donor-acceptor system undergoing intramolecular charge transfer (ICT) in the electronically excited state to construct the Gibbs energy surfaces associated with the ICT process. The molecular systems examined were 3,6-bis(dimethylamino)-9-(4-cyanophenyl)carbazole (BANCC) and its electron-donor part, 3,6-bis(dimethylamino)carbazole (BAC), in a series of solvents. The spectral analyses employed a model in which the reaction coordinate is governed primarily by solvent polarization, while the vibronic structure is assumed to be independent of the solvation coordinate. Two key parameters describing the Gibbs energy surfaces, i.e., the solvent reorganization energy and the Gibbs energy for reaction, were extracted from the spectral analysis. Their solvent dependence allowed us to estimate the dipole-moment difference between the locally excited and charge-transfer (CT) states, yielding values in good agreement with density functional theory predictions. We note that BANCC exhibits a direct CT absorption band in the visible region, with only a weak dependence on solvent polarity. This unusual behavior originates from a fortuitous cancellation between the solvent reorganization energy and the Gibbs energy change.