Synthesis and Nonlinear Optical Properties of Tetrahedral Octupolar Phthalocyanine-Based Systems

Abstract

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.

Mesogenic, Luminescence, and Nonlinear Optical Properties of New Bipyrimidine-Based Multifunctional Octupoles

“…The octupolar properties are significantly influenced by the donor− acceptor strength, the conjugation length, and the nature of the frontier electronic states. 7 In contrast, a more limited number of octupoles with three-dimensional tetrahedral (T d ) or pseudotetrahedral (D 2d ) symmetries have been reported so far as efficient 9 Recently, a new class of three-dimensional NLO-phores of D 2d symmetry based on donor-substituted styryl bipyrimidine cores have been investigated. 10 The lone pair's interactions…”

Perturbation theory and the two-level approximation: A corollary and critique

“…For example, in particular connection with the illustrative case considered in the preceding analysis, the nonlinear susceptibility that supports second harmonic generation can certainly be supported in high-symmetry materials with neither ground nor excited state dipole, provided the point or space group supports an electric octupole -systems of D 3h or T d symmetry for example [36]. There is extensive literature on the development of such chromophores [37][38][39][40], particularly in the many pioneering studies of Zyss et al With these systems it is understood that the existence of octupolar nonlinearities points to inadequacies of the traditional two-level model, often demanding the use of models based on a more accurate threelevel representation [41].…”

A theoretical investigation of one-dimensional lithium-bonded chain: enhanced first hyperpolarizability and little red-shift

“…Furthermore, significant NLO responses have also been observed in nondipolar octupolar molecules [14,15], because the lack of a permanent dipole moment contributes to noncentrosymmetric arrangements. A third approach that can rule out of the formation of centrosymmetric arrangement is exploiting ∧-shaped molecules [16][17][18] and helical/chiral compounds [19], owing to the large off-diagonal β-tensor component.…”