Theoretical study of static electric properties of benzothiazole containing push-pull systems as potential candidates for NLO materials

The dipole moments mu, static polarizabilities (alpha), and first hyperpolarizabilities beta(vec) of benzothiazole based push-pull chromophores have been theoretically investigated by means of SCF-HF and DFT (B3LYP, B3PW91) methods with cc-pVDZ basis sets at geometries optimized with 3-21G basis sets. The main attention has been paid to hyperpolarizability beta(vec) as a microscopic quantity directly linked with the nonlinear optical (NLO) response of materials with the aim to choose the most suitable candidate(s) for NLO materials from the set of 24 molecules. The 4-dimethylamino-4-nitrostilbene (DANS) has also been included as a reference compound. The influence of various structural modifications of benzothiazole containing chromophores on mu,(alpha), and beta(vec) has been studied. These modifications involve the variation in unsaturated chain length connecting the aromatic units, different positions of electron withdrawing NO2 and CN group attached to four possible positions in benzothiazole fragment, addition of the second NO2 group, and the replacement of benzene by five-membered heteroaromatic rings (pyrrole, furan, thiophene) at N(CH3)(2) site. The structural parameters, frontier orbitals and relative stabilities of these systems are also reported. The DFT scheme exaggerates the magnitude as well as the chain length dependence of beta(vec) with respect to the HF results. Both B3LYP and B3PW91 functionals give very close results. The DFT optimized structures are characterized by larger delocalization, planarity, and polarization of substituents. The best candidates for NLO materials have been identified from the studied series based on the values.