Photophysical And Structural Study On Nonlinear Optical Properties Of Benzoxadiazole Derivatives

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Abstract
The second-order nonlinear optical properties of donor and acceptor substituted benzoxadiazole derivatives were investigated on the basis of their photophysical properties and crystal structure. Second-order molecular hyperpolarizabilities were calculated using photophysical parameters determined by absorption and fluorescence spectra measurements. They were not commensurate with the intensity of the second harmonic radiation by the powder technique. The crystal structure determination of 4-chloro-7- nitro-2,1,3-benzoxadiazole (NBD-C1), which has a phase-matched and exceptionally large second harmonic generation (SHG) among the BFZ derivatives studied, has revealed that it belongs to the monoclinic system with space group P21 and there are two independent molecules in the unit cell arranged in a noncentrosym-metric manner. The charge-transfer angles of the two independent molecules to the crystallographic symmetry axis are 48.66° and 68.79°, whose values are both very close to the optimum value of 54.74°. The existence of two independent molecules with a favorable crystal packing as well as a high crystal density are therefore responsible for the enhancement of the SHG in NBD-Cl. A single crystal of NBD-Cl grown from solution showed remarkable anisotropy in the intensity of the SHG.

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