Enhancement of Two-Photon Absorption Parallels Intramolecular Charge-Transfer Efficiency in Quadrupolar versus Dipolar Cationic Chromophores

State-of-the-art femtosecond spectroscopies and quantum-chemical methods were used to investigate the excited-state dynamics of D−π–A+ (C1) and D−π–A+–π–D (C2) methylpyridinium (acceptor unit, A) derivatives bearing dibutylamino groups as strong electron donors (D) and bithiophenes as highly effective π-rich spacers. The absorption spectra of C1 and C2 are broad and shifted to the red side of the visible spectral range. A significant negative solvatochromism was observed for the absorption bands of the investigated salts with increasing solvent polarity that was rationalized in terms of the change in electron density upon excitation. The absorption spectra of C2 are red-shifted with respect to those of C1, whereas the emission bands of the two compounds overlap, suggesting a localization of the excitation on just one branch of the quadrupolar compound, which becomes the fluorescent portion. This is in agreement with our quantum-mechanical calculations, which predict that the symmetry of C2 is broken in th...