Subphthalocyanines: Novel Targets for Remarkable Second-Order Optical Nonlinearities

Abstract

The design and synthesis of new molecular and supramolecular systems for nonlinear optics (NLO) is currently of great scientific and technological interest.1 Until now investigations on second-order nonlinear optical properties of organic molecules have concentrated mainly on one-dimensional dipolar systems. Recently, theoretical and experimental considerations have suggested the possibility of developing new synthetic approaches to molecules with nonlinearities of so-called “multipolar” origin.2-5 In this communication we show that subphthalocyanines (SubPcs), π-conjugated phthalocyanine-related compounds, present very large second-order molecular polarizabilities (â) and emerge as novel targets for second-order nonlinear optical applications. Thus, for example the trinitroSubPc 1, described here for the first time, has shown experimental values of 〈â2〉1/2(2ω) ) 2000 × 10-30 (static 〈â2(0)〉1/2 ) 407 × 10-30 esu) by hyper-Rayleigh scattering (HRS) experiments. These second-order polarizabilities are mostly associated with the octupolar contribution and are comparable to those found in the most efficient linear (or dipolar type) compounds, such as polyenes,6 and to the best octupolar molecules described until now. Phthalocyanines (Pcs) are two-dimensional macrocyclic 18π-electron conjugated systems with a great variety of technological applications.7 In the last years Pcs8 and related analogues9 have emerged also as a novel class of molecular materials for third-order NLO applications. SubPcs 1-3 (Figure 1) are Pc-related macrocyclic compounds formed by three coupled isoindole moieties having a delocalized 14-π-electron system.10 Despite their cone-shaped structure,11 these compounds have an aromatic nature.12 Few axially and peripherally substituted boron SubPcs10,13-15 and dimers of SubPcs12 have been described. Their NLO properties remain unexplored, except for a very recent study on the third-harmonic generation (THG) spectroscopy of an evaporated thin film of SubPc 2,16 where values of o(3), three times larger than those corresponding to Pcs in the same wavelength range, were measured. In that case, the possibility of “cascading mechanisms” related to important second-order NLO properties was suggested. Therefore, the main objective of this work has been to evaluate the potential of these new molecules for second-order NLO applications. In this work we follow a 2D strategy for the design of SubPcbased NLO systems. Thus, besides the nonsubstituted SubPc 2, we prepared SubPc 1, with three surrounding strong acceptor substituents (NO2) forming a trigonal arrangement (Figure 2), and SubPc 3, having three peripheral donor groups (tert-Bu). Compounds 210,16 and 312 were prepared as previously described. Trinitro-SubPc 1 was synthesized for the first time in a similar way, by condensation of 4-nitro-1,2-dicyanobenzene in the pres-