Reduction of the Fluorescence Transition Dipole Moment by Excitation Localization in a Vibronically Coupled Squaraine Dimer

A rigidly bridged squaraine dimer serves as a model compound to study exciton interactions between two chromophores without interfering with conformational or other stereochemical isomers. We describe the synthesis as well as steady state and fs- and ps-time-resolved optical spectroscopic data. The spectra are interpreted using a vibronic coupling model, which considers a single vibrational mode that produces a shallow excited state surface with two minima. These two minima cause symmetry breaking of the excited state, which leads to a partial localization of excitation. The localization of the wave function causes a reduced fluorescence transition moment, although both the absorption and the emission spectra display exchange narrowing typical of excitonically coupled chromophores.