Analysis and extraction of molecular hyperpolarizability from on and off resonance hyper-Rayleigh scattering measurements in solution : Ferrocenyl donor-organic acceptor and -(P=N)- linkage based donor acceptor molecules as case studies

Abstract

Knowledge of accurate second?order nonlinearity or first hyperpolarizability (?) of organic and organometallic molecules is important in a variety of applications. The second?order response from a molecule in solution is measured by a technique of two? photon Rayleigh or hyper?Rayleigh scattering (HRS). In this method a laser beam is incident on the solution containing the molecule under investigation and second?harmonic scattered light in a perpendicular direction is monitored. As a consequence, other two? or multiphoton processes that lead to light emission in the vicinity of the second?harmonic wavelength compete with the second?harmonic scattering process and often lead to higher and inaccurate ? values. One way to avoid such competing processes is to make measurements at very low frequencies (close to zero). But this is not trivial. Another way to extract this intrinsic hyperpolarizability, ??, from the measured value of ? at any frequency will be by utilizing the well?known two?state model. If the absorption maximum of the compounds is away from resonance, this model produces reliable values of ??. However, most molecules show a broad absorption band in the UV–visible region and for such molecules a model recently developed by Berkovic et al. suggests how to obtain reliable ?? values by taking into consideration the full width of the absorption band. In this work, the first hyperpolarizability (?) for two sets of chromophores using on? and off?resonance HRS has been measured. The two chromophores are ferrocenyl donor– organic acceptor and phosphine–imine linked donor–acceptor types of molecules. The dispersion behavior of the chromophores was investigated by carrying out measurements at 1064 and 1907 nm. Measurements at 1907 nm enabled us to determine the first hyperpolarizability of the chromophores very close to their ?? values. The two?photon scattering technique has also been employed to measure ? of the complex between hexamethylbenzene and chloranil at 1907 nm in solution.