Solvent effect on the intramolecular charge transfer of zwitterions. Structures and quadratic hyperpolarizabilities Original Research Article

Solvent effect on intramolecular charge transfer of zwitterions was investigated by combining nuclear magnetic resonance and non-linear optical measurements. 1H nuclear magnetic resonance spectra of two phenoxide-pyridinium derivatives evidenced large chemical shifts on changing solvent polarity. In order to check if the ground state geometry is changed, spin-lattice relaxation rates for one compound were measured and the angle θ between the two aromatic rings calculated. The value of θ appears to be large and solvent independent within estimated accuracy. These results indicate that structural changes are less important than expected, and chemical shifts on rising solvent polarity result mainly from increasing donor to acceptor charge transfer in the ground state. Consequently, the benzenoid resonance form seems to dominate the ground state structure. Furthermore, the weight of this contribution increases with solvent polarity. The sign of the electric field induced second harmonic mean hyperpolarizability is negative for all three compounds. This also indicates that the benzenoid contribution dominates the ground state geometry in polar solvents. Moreover, the absolute value of the dipole quadratic hyperpolarizability product increases sharply on rising solvent polarity. This behaviour implies an enhancement of donor to acceptor charge transfer taking place in the ground state, and also of acceptor to donor charge transfer resulting from the transition ground state to first excited state.