Photoinduced Intramolecular Charge Transfer and Hyperpolarizability Coefficient in Push-Pull Pyridinium Salts with Increasing Strength of the Acceptor Group.

The synthesis of three push-pull cationic dyes is reported here together with a photophysical study carried out by stationary and ultrafast spectroscopies. The hyperpolarizability (β) values of the three molecules have been estimated through a simple solvatochromic method based on conventional, low-cost equipment, which had been tested previously with success in our laboratory. The investigated pyridinium salts showing strong negative solvatochromism bear the same piperidine ring as a strong electron-donor group and the same thiophenes as electron-rich π-linkers, but differ in terms of the N-substituent on the electron-acceptor pyridinium unit, namely N-methyl in compound A, N-pyrimidin-2yl in B and N-2,4-dinitrophenyl in C. The derived β values were found to increase (in the order A<B<C) along with the increasing electron-acceptor strength of the pyridinium substituent, with the highest polarizability being estimated for the derivative bearing the strongest electron-withdrawing substituent dinitrobenzene. State-of-the-art femtosecond transient absorption measurements evidenced fast kinetics and dynamics in the singlet excited state in agreement with charge transfer (CT) processes promoted by excitation, with a proper ICT state observed in the case of C. This finding confirms the efficiency of photoinduced intramolecular CT as a significant factor that affects the NLO response.