Subphthalocyanines and subnaphthalocyanines:: Nonlinear quasi-planar octupolar systems with permanent polarity

The second-harmonic generation (SHG) response of unsubstituted subnaphthalocyanines (SubNcs) has been measured for the first time by electric field-induced second-harmonic generation (EFISH) (1.064 and 1.90 mum) and hyper-Rayleigh scattering (HRS) (1.064 mum) experiments. The quadratic hyperpolarizability derived from the experiments is significant (beta(HRS)(0) = 34.7 x 10(-30) esu) and similar to that also measured under the same conditions for the related unsubstituted subphthalocyanine (SubPc) molecule (beta(HRS)(0) = 38.3 x 10(-30) esu). To meaningfully discuss the nonlinear optical (NLO) data, semiempirical INDO/S calculations of the permanent dipole moments of the ground and excited states (Q band) have been performed. Moreover, the dipolar transition moments connecting the ground and degenerate excited states and the two excited states have been also determined. The calculations suggest that for those low-energy excited states responsible for the Q optical absorption band, the two molecules (SubPc and SubNc) behave very approximately as planar pi-conjugated octupoles with D-3h symmetry but having a permanent dipole moment along the perpendicular halogen-boron axis. The charge distribution along this axis is not appreciably influenced by the optical excitation, that is, light-induced charge motion only occurs inside the macrocycle without significant contribution of the apical Cl atom. The beta(HRS) and gamma(EFISH) data have been quantitatively analyzed with a three-level model, taking into account that only electronic terms contribute to the EFISH hyperpolarizability. As a consequence of this analysis, a sound rationale to describe the NLO behavior of these Pc-related compounds (SubPcs and SubNcs) has now emerged.