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

摘要

Abstract 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.