Theoretical Study Of Nonlinear Optical Properties Of Squaraine Derivatives

In this work, we calculated the first hyperpolarizabilities of squaric acid homologues, the 1,3-substitued squaric acid derivatives and the squaraine molecules by ab initio and semiempirical methods. The emphasis was put on the relationship between the first hyperpolarizabilities and the electronic structures of these molecules. We optimized 16 squaric acid homologues by RHF/6-311+G(d,p), MP2/6-311+G(d,p) and B3LYP/6-311+G(d,p) methods. All the optimized structures were characterized by the analysis of the harmonic virational frequencies obtained from the diagonalization of force constant matrices. We calculated the first hyperpolarizabilities of these molecules at the same theoretical levels. The geometry of the molecules, basis set and electron correlation, were taken into account. We find no significant differences between the first hyperpolarizabilities according to different molecular geometries based on different optimization methods. Efficient and effective basis set should be adopted in order to get accurate results and to save the computational time as possible as we can. The basis set 6-311+G(d,p) was accepted for there are no significant differences in contrast to the results gotten by more complex basis sets 6-311+G(3d,2p), 6-311+G(3df,2p). Electron correlation is an important effect that should be considered as there is usually a considerable increase of the first hyperpolarizabilities once it is considered. In this article, we adopted the second order M?ller-Plesset perturbation method (MP2) and Becke's three-parameter exchange functional along with the Lee-Yang-Parr nonlocal correlation functional method (B3LYP) to calculate the first hyperpolarizabilities of squaric acid homologues. There are great differences between the values calculated by HF and MP2 methods and no significant differences between that of HF and B3LYP. And the differences of the first hyperpolarizabilities of different molecules are more significant within the MP2 results As we know the HF values are often inconsistent with the experimental observations, it is necessary to taken into account the correlation effect. Frequency dispersion and solvent dependence should be considered if we contrast the calculated values with experimental observations. The frequency dispersion effect was considered by the TDHF treatment implemented in the GAMESS procedure. It is easy to find that as the external fieldbecomes strong, there is a significant increase of the first hyerpolarizabilities especially for the SHG process. However, it should be noted that, some calculated values are too large. More work should be done about this part. The solvent effect was considered by the Onsager model implemented in the GAUSSIAN 98 program. We can distinguish the differences between the results based on different solvents. After we have considered all the above factors, it is worthy to point out that heavy atom effect, which has been discovered within furan homologues, still take effect for squaric acid homologues. This it is a good strategy to improve the first hyperpolarizabilities of organic molecules.An important species of suqaric acid derivatives are 1,3-substitued molecules, in which, the donors are linked to the 1,3 position of squaric ring. For the general, the donors are connected to the squaric acid ring by a benzene ring. As the substitutes connected on both sides are donors, the asymmetry of the charge distribution can be ascribed to the different donor capacity of the donor substitutes. In order to increase the charge transfer in the molecules as well as the NLO properties, we designed a series of small molecules, in which, at the diagonal sides of squaric ring, the donors and acceptors are connected to the ring directively. We contrasted the results of 1,3-DA and 1,3-DD substituted squaric acid derivatives and we find no significant differences between them. The first hyperpolarizabilities increase as stronger donors are added to the squaric ring.We also calculated the first hyperpolarizabilities of unsymmetrical squaraine mole...