Electronic responses of donor acceptor substituted twisted biphenyls

Abstract

Donor-acceptor biphenyl derivatives are particularly interesting model compounds presenting a charge transfer absorption band because the extent of conjugation, and therefore the amount of charge transfer between the substituents, may be varied by a controllable structural feature, namely the torsion angle between the two phenyl rings. This feature has become even more interesting since chromophores of strong zwitterionic character, synthesized with a twist of nearly 90°, have been shown to exhibit unprecedented quadratic responses [1], over an order of magnitude above that of the best conventional push-pull chromophores. In this context, we have investigated both experimentally and theoretically, two biphenyl based systems with varying inter-aryl angles: a nitro-piperidinyl series [2] of conventional push-pull character and a pyridinium-phenoxide series [3] of zwitterionic character. The results agree qualitatively with semiempirical simulations based on the AM1 Hamiltonian [4] used with the COSMO solvation model [5]. For the first series, the decrease in quadratic response with increasing dihedral angle indicates that oscillator strength loss is the dominant factor. In the second series, the corresponding increase in quadratic response points to the change in dipole moment upon excitation as the leading effect. Here, we will analyze to what extent the more ab initio electronic structure calculations based on the density functional theory may provide more quantitative results in spite of the problems they face in the description of charge transfer systems.