Evaluation of Hyperpolarizability from the Solvatochromic Method: Thiophene Containing Push–Pull Cationic Dyes as a Case Study

We report here a successful attempt to test a solvatochromic method to estimate the hyperpolarizability (β) of cationic push-pull chromophores. This represents a simple method, alternative to the sophisticated spectroscopic techniques often employed, which can be easily and quickly applied through equipment commonly available in a typical chemistry laboratory. The case study taken into consideration consists of nine donor-π-acceptor derivatives exhibiting the rarely observed negative solvatochromism. In these dyes the electron acceptors are positively charged methylpyridinium or quinolinium rings and the electron donors are electron rich thiophene rings eventually coupled with the strongly electron donating dibuthylamino group or piperidine. The obtained β values are enhanced in this molecular series upon increasing molecular dimensionality and conjugation as well as by increasing the donor/acceptor strength. The highest hyperpolarizability is estimated for the chromophore bearing methyl quinolinum and piperidine where the most efficient photoinduced intramolecular charge transfer is also revealed by means of state of the art femtosecond transient absorption measurements.