Effect of the π Bridge and Acceptor on Intramolecular Charge Transfer in Push–Pull Cationic Chromophores: An Ultrafast Spectroscopic and TD‐DFT Computational Study

Three (donor–p–acceptor)+ systems with a methyl pyridinium or quinolinium as the electron-deficient group, a dimethyl amino as the electron-donor group, and an ethylene or buta- diene group as the spacer have been investigated in a joint spectroscopic and TD-DFT computational study. A negative sol- vatochromism has been revealed in the absorption spectra, which implies a solution color change, and interpreted by con- sidering the variation in the permanent dipole moment modu- lus and orientation upon photoexcitation. The fluorescence ef- ficiency decreases upon increasing solvent polarity, in agree- ment with the excited-state optimized geometries (planar in low-polarity media and twisted in high-polarity media). Femto- second transient absorption has revealed the occurrence of a fast photoinduced intramolecular charge transfer (ICT) and the molecular factors that determine an efficient ICT. Consider- ing the crucial role of the ICT in tuning the nonlinear optical (NLO) properties, these compounds can be considered promis- ing NLO materials.