Measurement and dispersion of the first molecular hyperpolarizability of organic molecules using hyper-Rayleigh scattering

Hyper-Rayleigh scattering, a form of second-order light scattering, has been employed to investigate the nonlinear optical (NLO) properties of charge-transfer chromophores in chloroform solution, specifically the first molecular hyperpolarizability, β. The archetype for this type of chromophore is para-nitroaniline (pNA), a molecule which has an electron-donor amino group and an electron-accepting nitro group separated by a π-conjugation moiety. These chromophores are investigated at near-infrared wavelengths. A series of azulene-based chromophores, where an azulene group served as the electron donor, yielded reasonable hyperpolarizabilities. A series of thiophene-based chromophores yielded exceptionally large first molecular hyperpolarizabilities. This is attributed to the presence of a thiophene group in the π-conjugation length which both disrupts aromatic stabilization and serves as an auxiliary acceptor. A series of hydrogen-bonding chromophores are also studied in chloroform solution; it is found that intramolecular hydrogen-bonding effects can serve to enhance the first molecular hyperpolarizability by contributing extra electron density to the donor part of the chromophore. The dispersion of the first molecular hyperpolarizability at the three infrared wavelengths of 1064 nm, 1543 nm, and 1907 nm is studied; it is found that a traditional two-level dispersion model cannot alone explain the results obtained at these three wavelengths. The dispersion model is extended to include both dephasing from the excited state and vibrational structure in the excited state; this model yields better results, although more theoretical work is needed in this area. Finally, the effects of an amorphous polymer in a chromophore/polymer/solvent ternary system are studied; it is found that the addition of polymer serves to decrease the hyper-Rayleigh scattering intensity from the NLO chromophore. It is suggested that this effect is due to intermolecular correlations between the chromophore and the polymer, which serve to retard local fluctuations of the chromophore believed responsible for the second-order scattering effect