MOLECULAR HYPERPOLARIZABILITIES FRONT VIBRATIONAL SPECTROSCOPY - POLYENOVANILLINS

In this work we have studied the vibrational (relaxation) contribution s beta(r) and gamma(r) to molecular nonlinear optical (nlo) properties of polyenovanillins, a class of push-pull molecules with increasing n umber n of conjugated CC bonds (n = 1, 3, 4, 5). Both experiments and ab initio calculations have been carried out. The model adopted expres ses the nuclear relaxation contributions in terms of vibrational spect roscopic observables, namely frequencies and infrared and Raman intens ities. A relevant observation is that the electronic hyperpolarizabili ty beta(e), measured from EFISH experiments or calculated by ab initio methods, turns out to be very similar to the experimental or ab initi o vibrational beta(r). The same behavior has already been found (both for beta and gamma) in other polyconjugated low band-gap materials wit h large electron-phonon coupling. Moreover, the spectroscopic approach has the advantage of allowing useful insight into the role played by structural parameters in the nlo processes.