Intramolecular charge transfer in 4-cyano-(4 '-methylthio)diphenylacetylene

The photophysical properties of 4-cyano-(4'-methylthio)diphenylacetylene have been examined by quantum chemical methods. It is concluded that the lowest-energy, excited singlet state in a polar solvent like water corresponds to intramolecular charge transfer from the S-based donor to the benzonitrile unit. This excited state adopts a twisted geometry, in marked contrast to the planar ground state. Because of the structural change that follows from UV excitation there is a substantial Stokes shift, the magnitude of which depends on the polarity of the surrounding solvent. Indeed, formation of the twisted intramolecular charge-transfer state involves a significant increase in dipole moment. The charge transfer state decays via a combination of fluorescence, charge recombination and intersystem crossing. The calculations reveal values for the accompanying nuclear and solvent reorganisation energies and assign relative values for the electronic coupling matrix elements associated with formation of the ground and triplet states. Overall, the computed results remain in good agreement with the relevant experimental data.