| Abstract
| - The second-harmonic generation (SHG) response of unsubstituted subnaphthalocyanines (SubNcs) has beenmeasured for the first time by electric field-induced second-harmonic generation (EFISH) (1.064 and 1.90μm) and hyper-Rayleigh scattering (HRS) (1.064 μm) experiments. The quadratic hyperpolarizability derivedfrom the experiments is significant (βHRS(0) = 34.7 × 10-30 esu) and similar to that also measured under thesame conditions for the related unsubstituted subphthalocyanine (SubPc) molecule (βHRS(0) = 38.3 × 10-30esu). To meaningfully discuss the nonlinear optical (NLO) data, semiempirical INDO/S calculations of thepermanent dipole moments of the ground and excited states (Q band) have been performed. Moreover, thedipolar transition moments connecting the ground and degenerate excited states and the two excited stateshave been also determined. The calculations suggest that for those low-energy excited states responsible forthe Q optical absorption band, the two molecules (SubPc and SubNc) behave very approximately as planarπ-conjugated octupoles with D3h symmetry but having a permanent dipole moment along the perpendicularhalogen−boron axis. The charge distribution along this axis is not appreciably influenced by the opticalexcitation, that is, light-induced charge motion only occurs inside the macrocycle without significant contributionof the apical Cl atom. The βHRS and γEFISH data have been quantitatively analyzed with a three-level model,taking into account that only electronic terms contribute to the EFISH hyperpolarizability. As a consequenceof this analysis, a sound rationale to describe the NLO behavior of these Pc-related compounds (SubPcs andSubNcs) has now emerged.
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