Solvent dependence of apparent third-order hyperpolarizability in paranitroaniline

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Abstract
Optical third harmonic generation has been used to characterize solutions of paranitroaniline (pNA) in five different solvents. Results were analyzed to extract an apparent third‐order hyperpolarizability of pNA. Substantial variation of this quantity was observed, far exceeding the uncertainty of each solution determination. This unusual behavior was attributed to cascaded second‐order interactions through a second harmonic local field. The analysis is in substantial agreement with electric field induced second harmonic (EFISH) generation characterization of the second‐order hyperpolarizability of pNA when corrected via molecular orbital calculations for differences in dispersion and anisotropies. Solutions of 2‐methyl‐4‐nitroaniline were also studied and the cascading model suggests that the increase of second‐order nonlinearity over pNA is not generally as large as previous EFISH measurements suggest.