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

Abstract

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.

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

“…In fact, the β 0 value of 330×10 −30 esu −1 cm 5 derived for A doubles or triples when the N ‐methyl substituent is replaced by the N ‐pyrimidin‐2yl ( B ) or N ‐dinitrophenyl ( C ), respectively (Table ). However, the β 0 values estimated for the present investigated salts resulted high if compared with analogous pyridinium‐based chromophores, and generally comparable with those of systems considered actual NLO materials ,. The significant quadratic NLO activity exhibited by A and particularly by B and C , can be explained taking into account the presence of the very strong electron‐donor piperidine and the positively charged N ‐pyrimidin‐2yl‐ and N ‐dinitrophenyl‐pyridinium as strong electron‐acceptors connected by electron‐rich diene and dithiophenes chain.…”

“…The β values have been found sizable and comparable to those of systems considered de facto NLO materials ,. The comparison between the A properties and those previously retrieved for the analogous ortho‐substituted salt (namely 1‐methyl‐2‐{( E )‐2‐[5′‐(piperidin‐1‐yl)‐2,2′‐bithiophen‐5‐yl]ethenyl}pyridinium iodide) by the same solvatochromic study, evidenced lower β values for the latter (halved β CT and β 0 reduced by ca . 40 %) if compared with those of A , owing to the increased conjugation in the para‐derivative.…”

“…The relative energies for the two rotamers of the investigated substrates pointed to a large prevalence of the s‐ trans species for the three compounds in the ground state (Table ). Different possible conformers with the two thiophenes in syn configuration are not taken into account because the anti configuration has always been found much more stabilized than the corresponding syn one in previous studies . For these reasons the quantum mechanical calculations reported below were performed on the s‐ trans conformers, expected to be largely prevalent in solution at room temperature.…”

“…These compounds showed a net negative solvatochromism for the S 0 →S 1 absorption spectrum and weak fluorescence that decreases in the order A > B > C . The absorption data in different solvents have been processed according to a reliable solvatochromic method previously developed and successfully tested on analogous cationic push‐pull chromophores . This method allowed the difference between the excited and ground state dipole moment (Δμ) as well as the dynamic (β CT ) and static (β 0 ) hyperpolarizability coefficients to be derived.…”

“…Particularly, the substitution of the N ‐methyl with N ‐aryl groups in the pyridinium ligands of metal‐organic complexes, and also in purely organic pyridinium dyes, has been found to produce an enhancement of β values . For this reason, the aim of the present work is to evaluate the effect of the increasing strength of the electron‐withdrawing group on the β coefficients of cation chromophores by applying the solvatochromic method already tested with success on analogous push‐pull systems ,…”