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 , 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  of conventional push-pull
character and a pyridinium-phenoxide series  of zwitterionic character. The results agree qualitatively with semiempirical
simulations based on the AM1 Hamiltonian  used with the COSMO solvation model . 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.
Polymers doped with non-linear optical (NLO) molecules are key materials in the elaboration of organic NLO devices. In this field, there is an ongoing need for chromophores with large dipole moments and optical non linearities. Here, we consider pyridinium phenoxides, a type of zwitterionic biphenyl-like molecule. A combination of mathematical modelling and some preliminary experimental measurements indicate that the NLO properties of these molecules depend on the twist angle existing between the two aromatic rings. In order to corroborate this structure/activity relationship, different sterically hindered pyridinium phenoxides were synthesized using the Suzuki coupling reaction involving a boronic ester and an aryl halide. We analyze the solvatochromism of the substituted zwitterions in details, determine the chemical equilibrium of protonation and perfom nonlinear optical measurements which are interpreted with the help of semi-empirical calculations.
Our present understanding of the dependence of non linear optical (NLO) properties of charge transfer compounds on their dielectric environment is generally summarized as a set of universal response functions versus the ground state ionicity. Experimentally, these behaviors are recovered by piece-wise assembling measurements performed in solvents of increasing polarities on series of chromophores with varying acceptor and donor groups and/or conjugation paths. In this work, we will take advantage of the recent success in synthesizing pyridinium phenoxides with or without tert-butyl oxygen protection groups as well as with or without steric methyl groups to modify the twisting of the central diaryl bond. Thus, we are in a position to sweep over a wide range of zwitterionic character with essentially the same chromophore. This opens up a great opportunity to investigate the relationships between geometrical structure and NLO properties, to examine the validity of current formalisms, and to test numerical simulations at the semi empirical and density functional levels. In this work, we have carried out an experimental and theoretical study combining UV visible, IR, Raman, and Hyper-Rayleigh spectroscopies to extract information concerning the geometry, the electronic structure, and the NLO response of our compounds. In particular, we show that the steric effect is sufficient to push the chromophore to the full zwitterionic limit. More generally, the approach we followed here shows great potential in probing chromophore-environment interactions.
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.