Comparison of Nonlinear Optical Properties of 1,3-substituted Squaraic Derivatives by ab initio and Semiempirical Method

The structures of 1，3-substituted squaraine derivatives Sq1 ～ Sq12 were fully optimized by ab initio HF method with 6-31G* basis set level，and the electronic structures of Sq1 ～ Sq12 were also calculated. Based on the optimized structures，the electronic spectra were obtained by the CIS / 6-31G* method，which suggested that the max absorption wavelength mainly resulted from the electronic transition from HOMO to LUMO. The second nonlinear optical coefficients（β0）were calculated using ab initio CPHF method at 6-31G* basis set level and FF / AM1,FF / PM3，FF / MNDO，FF / MINDO3 methods. A systematic comparison between the results was carried out. It indicated that the second nonlinear optical coefficients were affected dramatically by the properties of five-membered hetero-rings. β0 can be enhanced by introducing pyrrole，thiazole and oxazole. The position of five-membered rings containing two heteroatoms which were connected with four-membered squaraine rings also affected β0 .