Solvent-dependent ultrafast optical response of conjugated push-pull chromophores

Two new difluoroboron beta-carbonyl cyclic ketonate complexes C2B and DC2B were investigated using several spectroscopic methods. Relative to the absorption spectra, the fluorescence spectra were more affected by the polarity of the solvent. Also, compound C2B showed a more pronounced Stokes' shift after solvent polarity increased. Transient absorption measurements then demonstrated the relaxation behaviour of the excited state compound molecule. The kinetic results showed that the excited state C2B in tetrahydrofuran (THF) can return from the intramolecular charge-transfer (ICT) state and the initial excited state to the ground state. The kinetic relaxation pathway after THF was replaced by dimethyl sulfoxide became single. When the carbazole unit was introduced, DC2B also exhibited an ICT state but there was no significant difference in the excited state relaxation path after solvent polarity was changed. The results indicated that C2B is more susceptible to solvent polarity regulation. The global fit results revealed that an increase in the solvent polarity prolonged the lifetime of the ICT state of compound C2B and had the opposite effect on compound DC2B. These results provide guidance for understanding the relationship between solvent polarity and the designing and synthesizing advanced compound materials.