Theoretical study on stability and nonlinear optical properties of novel subphthalocyanine dimer and trimer

Theoretical calculation were performed using ab initio and density functional theory for studing the stabilities and nonlinear optical properties of novel subphthalocyanine (SubPc) dimer and trimer, in which two or three SubPcs are fused by sharing the benzene. The optimized geometries based on B3LYP/6-31G* agree well with X-ray data. The cone-shaped structure of the SubPc framework is preserved in the SubPc oligomer. Hence, for dimer cis form is just slightly more stable than trans form by 0.87 kJ/mol. For trimer, the order of stability is ct-SubPc>tt-SubPc>cc-SubPc and the energy differences among them are far less than 1 kJ/mol, demonstrating the three isomers exist stably at the same time in experiment. The UV-vis spectra were reproduced successfully using ZINDO/S-CIS method and the second order hyperpolarizabilities (γ) were evaluated by sum over states (SOS) formula. Due to the enlargement of π-conjugated systems, in SubPc oligomer the calculated Q-bands and B-bands shift to a longer wavelength, but the increment decreases from 120 to 68 nm with increasing SubPc units. The SubPc oligomer exhibits good nonlinear optical (NLO) properties. With the same input wavelength, the γ absolute values increase one order of magnitude from monomer to trimer. For example, the γ absolute values are 1.5, 7.3, 8.6, 16.9, 19.9 and 22.2 (×10−33) esu, respectively, going from monomer, cis-SubPc, trans-SubPc, cc-SubPc, ct-SubPc to tt-SubPc at zero frequency. It is notable that both the trans-SubPc and tt-SubPc have the best NLO properties among isomers.