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-pi-A(+) (Cl) and D-pi-A(+)-pi-D (C2) methylpyridinium (acceptor unit, A) derivatives bearing dibutylamino groups as strong electron donors (D) and bithiophenes as highly effective pi-rich spacers. The absorption spectra of Cl 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 Cl, 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 the relaxed Si geometry. Excited-state symmetry breaking was observed in all of the investigated solvents regardless of their polarity. Femtosecond transient absorption and fluorescence up-conversion measurements revealed that the excited-state dynamics of Cl is essentially dominated by solvent relaxation, whereas in the case of C2, two distinct excited singlet states were detected in polar solvents, where an intramolecular charge-transfer (ICT) state is efficiently produced. The main photoinduced decay pathway of both compounds was found to be internal conversion in all of the investigated media. High two-photon-absorption cross sections of 500 and 1400 GM for C1 and C2, respectively, were obtained by means of femtosecond-resolved two-photon excited fluorescence measurements, thus demonstrating the enhancement in the nonlinear optical properties of the quadrupolar compound over its dipolar counterpart, in agreement with the more efficient ICT observed in the case of C2.