Resonance Raman, hyper-Raman, and hyper-Rayleigh depolarization ratios and symmetry breaking in solution

Abstract

The depolarization ratios in resonance Raman, resonance hyper-Raman and resonance hyper-Rayleigh scattering have been measured for two octupolar conjugated organic chromophores that possess nominal threefold symmetry, crystal violet (CV) and 1,3,5-tris[4-[(1E)-2-(4-nitrophenyl)-ethenyl]phenyl]benzene (TPB(PV)1(NO2)). For comparison, the same quantities are measured for two dipolar donor–acceptor substituted conjugated molecules. The dipolar molecules both exhibit depolarization ratios of for resonance Raman and ρ = 1/5 for hyper-Rayleigh and hyper-Raman, consistent with the polarizability and the hyperpolarizability tensors having a single non-zero element. The resonance hyper-Rayleigh scattering from both of the octupolar molecules exhibits a depolarization ratio close to the value of 2/3 expected for D3h or D3 symmetry, but the resonance Raman and resonance hyper-Raman depolarization ratios deviate significantly from the values expected for those symmetries. In TPB(PV)1(NO2), the depolarization ratios indicate that the resonant vibrational polarizability and hyperpolarizability tensors are dominated by a single diagonal term, suggesting that each of the three conjugated chains acts as a nearly independent chromophore. The results for CV are more complicated and suggest partial but incomplete delocalization of the electronic and/or vibrational excitations among the three nominally equivalent chromophores.