Influence of a Substituted Methyl on the Photoresponsive Third-Order Nonlinear-Optical Properties Based on Azobenzene Metal Complexes.

For studying the effect of a substituted group on the photoresponsive third-order nonlinear-optical (NLO) properties, photosensitive azobenzene derivative H2L1 was first selected to construct metal complexes {[Zn2(L1)2(H2O)3]·2DMA)}n (1) and {[Cd(L1)(4,4'-bpy)H2O]·H2O}n (2). Then H2L2 with a substituted methyl on the azobenzene ring was used to construct complexes {[Zn(L2)(4,4'-bpy)(H2O)]}n (3) and {[Cd(L2)(4,4'-bpy)(H2O)]}n (4). When the azobenzene moiety of the complexes is trans, the NLO behaviors of the complexes are the same. However, after the azobenzene moiety is excited by ultraviolet (UV) light to change from trans to cis, the substituted methyl increases the repulsion between two azobenzene rings in 3 and 4, thereby affecting their NLO behaviors. Therefore, the nonlinearity of the two types of complexes is different after UV irradiation. Density functional theory calculations support this result. The substituted methyl has a significant influence on the nonlinear absorption behaviors of 3 and 4. This work not only reports the examples of photoresponsive NLO materials based on metal complexes but also provides a new idea to deeply explore NLO properties.