Tunability of optical nonlinear response through twisting of conjugation paths in push-pull biphenyl compounds

Abstract

Charge separated molecules with strong zwitterionic character make especially interesting species for quadratic Non Linear Optical (NLO) doped materials because of their large dipole moments and rather large quadratic hyperpolarizabilities. Furthermore, their sensitivity to the dielectric environment brands them as good candidates for checking the validity of formalisms relating geometrical structure and NLO properties. We have carried out an experimental and theoretical study on two phenoxide pyridinium derivatives with different inter ring twist angle, combining UV-visible, IR and NLO spectroscopies supplemented by semi-empirical and density functional calculations. Experimentally, changing the polarity of the solvent allows us to explore only a small range of twist angles. However, changing the pyridinium nitrogen atom from para to meta position gives essentially the same compound but with a larger twist angle. Changes in solvent polarity and position substitution give rise to a combined enhancement of the quadratic response by an order of magnitude, showing experimentally the possibility of tuning NLO properties of chromophores to be used as doping agents in NLO applications.