Prediction of Nonlinear Optical Effects for Phenyl-Substituted Nickel Dithiolenes
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H
S\S
H
Ni
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I: bis(ethylene-1,2-dithiolato)nickel The nickel-sulfur coordination bonds involve electron donation into the d orbitals of sulfur and are expected to lead to a large, nonresonant contribution to the nonlinearity [2]. Substitution of the hydrogens on the dithiolene ligand with appropriate functionalities allows tuning of the absorption band between 700 and 1500 nm, thereby enabling resonance enhancement in this wavelength range [4,5]. Also, since dithiolene compounds are used as laser Q-switch dyes [5], these materials are expected to be stable to laser radiation even at their absorbance maxima. Preliminary studies of nickel dithiolenes as near-IR dyes, which could be incorporated into polycarbonate for eye protection against laser radiation, were recently carried out [6]. Spectral shifts were calculated for a variety of nickel dithiolene derivatives [6] indicating that substituents that extend the conjugation length have the largest effect. In the case of phenyl substituents, the calculated shifts depend on the torsion angle between the plane of the phenyl ring and the dithiolene plane. Using the PM3 semi-empirical molecular orbital method, the optimal geometry of the diphenyl nickel dithiolene derivative with phenyl rings in a trans arrangement has the phenyl rings at about a 450 dihedral to the dithiolene moiety. This optimal geometry, however, 325 Mat. Res. Soc. Symp. Proc. Vol. 479 01997 Materials Research Society
is only 3.8 kcal/mol lower in energy than when the dithiolene moiety and the phenyl rings are coplanar. Steric repulsion prevents coplanarity of dithiolene and phenyl rings in the tetrasubstituted derivatives. The value of computational methods in determining structure-property relationships for nonlinear optical properties of organic compounds has long been recognized [7]. Although agreement with experiment is still a concern, theoretical results are often useful in the interpretation of experimental data, identifying compounds with large optical nonlinearities, and gaining a fundamental insight into nonlinear optical properties. While empirical and semiempirical methods offer relatively short computational times and can be applied to large molecular systems, only a limited number of atoms can be treated with more accurate ab initio methods. Semiempirical molecular orbital methods employed by Kodaka, et al. show that the hyperpolarizability, 03,varies markedly as a function of phenyl torsion in one mesomer of 2[cyano(methoxycarbonyl)methylene-4-phenyl-1,3-dithiole [1]. A sharp decrease in the hyperpolarizability is observed when the phenyl torsion forms a dihedral angle of 300 to the dithiole ring, and increases sharply when at a dihedral of 400. Kodaka, et al. presented evidence to suggest that at 300 the phenyl ring acts as an electron acceptor and as an electron donator at other dihedral values, though most strongly at 400. An unusual mesomeric effect is indicated by the nearly constant hyperpolarizability values calculated as a function of phenyl torsion for the stere
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