Non-linear optical properties of organic molecules. Part 13.—Calculation of the structure and frequency-dependent hyperpolarisability of a blue azothiophene dye

Abstract

The structure of a commercial dye, 2-(2-acetamido-4-diethylaminophenylazo)-3, 5-dinitrothiophene, with potential application in non-linear optics has been calculated at the ab initio STO-3G level. The molecule is predicted to be essentially planar between donor and acceptor groups with a strong intramolecular hydrogen bond between the acetamido group and one nitrogen of the azo linkage. The hyperpolarisability of this structure calculated using a semi-empirical molecular orbital sum-over-states method is predicted to be large and arises mainly from the transition of an electron from the ground state to the first excited state. The value obtained is highly dependent on the frequency of the applied field and very substantial resonance enhancement effects are calculated near the transition energy, particularly for linear electro-optic modulation.