Origin of photoinduced third harmonic generation anisotropy for quinoline derivatives molecules embedded into polymethylmethacrylate polymer matrix

Substantial anisotropy of photoinduced change of third harmonic generation (THG) was observed during investigation of the 3-(4-nitrophenyl)-6-N,N-diphenylamin-1-phenyl-1H-pyrazolo [3,4-b]quinoline chromophores embedded into polymethylmethacrylate polymer matrix. Large THG decay was observed for the co-polarized pump-probe configuration, and small increase (weak pump intensity) or decrease (strong pump intensity) of THG signal for the cross-polarized pump-probe configuration. The performed molecular geometry optimization and quantum chemical evaluations have shown that the observed effect is originated from photoinduced anisotropy of electronic charge density distribution within the separate molecule contrary to the generally adopted model of the hole burning typical for the cis-trans isomerization.