Synthesis and nonlinear optical properties of tetrahedral octupolar phthalocyanine-based systems.

Two series of tetrahedral phthalocyanine-based systems presenting a central carbon or silicon atom have been synthesized and fully characterized. Ethynyl spacers connect the peripheral Pc units to the central core. Some of the structures contain four identical Pc moieties, whereas other ones bear either an electron-withdrawing or an electron releasing group in the fourth subunit. The synthetic strategy consisted in metal mediated coupling reactions between tri-tert-butylethynylphthalocyanine and the corresponding methane or silane derivatives. A second-order nonlinear optical (NLO) study, through hyper-Rayleigh scattering measurements, reveals that, by combining centrosymmetrical moieties that are not second-order NLO active by themselves, in an octupolar fashion, a large second-order NLO response is achieved, in contrast to classical octupolar combinations of donor-acceptor NLO active dipolar moieties. In particular, the C-centered tetramer exhibits a large beta(HRS) value, which is among the highest reported so far for octupolar Pc-based molecules. Interestingly, carbon-centered molecules show a better NLO response with respect to the silicon-centered ones, probably due to a different effective symmetry, largely T(d) for the C-centered compounds and D(2d) for the Si-centered systems. While other design strategies for second-order NLO effects have always fundamentally kept on relying on the old dipolar paradigm (even though the resulting molecular structure was octupolar--the most striking exponent of this is the octupolar 1,3,5-triamino-2,4,6-trinitrobenzene molecule, a simple octupolar expansion of the dipolar p-nitroaniline), we here present for the first time that the octupolar symmetry by itself, realized by four nondipolar moieties in a tetrahedral arrangement, results also in a large second-order nonlinear response.