Subphthalocyanines and Subnaphthalocyanines: Nonlinear Quasi-Planar Octupolar Systems with Permanent Polarity Guillermo Martı ´n, Gema Rojo, and Fernando Agullo ´ -Lo ´ pez* ,† Departamento de Fı ´sica de Materiales (C-IV), UniVersidad Auto ´ noma de Madrid, 28049-Madrid, Spain Vı ´ctor R. Ferro and Jose ´ M. Garcı ´a de la Vega* ,‡ Departamento de Quı ´mica Fı ´sica Aplicada (C-XIV), UniVersidad Auto ´ noma de Madrid, 28049-Madrid, Spain M. Victoria Martı ´nez-Dı ´az and Toma ´ s Torres* ,§ Departamento de Quı ´mica Orga ´ nica (C-I), UniVersidad Auto ´ noma de Madrid, 28049-Madrid, Spain Isabelle Ledoux and Joseph Zyss Laboratoire de Photonique Quantique et Mole ´ culaire, Ecole Normale Supe ´ rieure de Cachan, 94235 Cachan, France ReceiVed: March 15, 2002; In Final Form: October 4, 2002 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 µm) and hyper-Rayleigh scattering (HRS) (1.064 µm) experiments. The quadratic hyperpolarizability derived from the experiments is significant ( HRS (0) ) 34.7 × 10 -30 esu) and similar to that also measured under the same conditions for the related unsubstituted subphthalocyanine (SubPc) molecule ( HRS (0) ) 38.3 × 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 π-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  HRS and γ 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. 1. Introduction Molecular photonics is a rapidly growing field that is reaching maturity. In particular, organic compounds and polymers are starting to be considered as efficient alternatives to inorganic materials for nonlinear optical (NLO) applications, such as electrical and all-optical modulation and switching. 1-4 Phtha- locyanines (Pcs) are a class of centrosymmetric highly conju- gated molecular systems that have been successfully used for third-order processes including four-wave mixing, third- harmonic generation, and optical limiting. 5-9 For second-order NLO devices, two main strategies 10 have been pursued to introduce the required noncentrosymmetry in Pcs: peripheral substitution of the macrocycle and intrinsic modification of the macrocycle. Another very promising approach 11-15 that com- bines both strategies is exemplified by subphthalocyanines (SubPcs). They consist of three coupled isoindole units contain- ing boron as central atom and have a delocalized 14 π-electron system. Unlike the related planar Pcs, SubPcs have a pyramid- shaped structure, as determined by X-ray experiments 16,17 and quantum chemical calculations, 18-21 that does not prevent them from having an aromatic nature. High second-harmonic genera- tion (SHG) responses have been obtained in solution through hyper-Rayleigh scattering (HRS) experiments. Moreover, ex- periments have suggested that the SHG response was mostly octupolar (corresponding to the triangular macrocycle in the “xy” plane) so the purely axial (z) component of the beta tensor,  zzz , was negligible. Therefore, octupolarity is an additional interest- ing feature of SubPcs that should be further explored. It is to be mentioned that octupolar molecules may present high SHG responses and offer a new route for optimization. 22-24 The purpose of this work is to extend the experimental SHG investigation performed on SubPcs 13 to the unsubstituted subnaphthalocyanine (SubNc) with the aim of understanding the effect of increasing the number of benzene rings. So far * To whom correspondence should be addressed. † E-mail: fal@uam.es. ‡ E-mail: delavega@uam.es. § E-mail: tomas.torres@uam.es. 13139 J. Phys. Chem. B 2002, 106, 13139-13145 10.1021/jp020711e CCC: $22.00 © 2002 American Chemical Society Published on Web 12/04/2002