Unexpected Third-Order Nonlinear Optical Responses in Two Isomeric Non-Fused Ring A-D-A Electron-Acceptor Molecules

The third-order nonlinear optical properties of NOC6F-1 and NOC6F-2, two mutually isomeric noncovalently fused-ring A-pi-D-pi-A electron acceptor molecules, are investigated using the Z-scan technique in both film and solution forms. NOC6F-1 possesses a planar structure, whereas NOC6F-2 has a large dihedral angle between its central and neighboring conjugated units. NOC6F-2 exhibits clearly superior reverse saturation absorption properties compared to NOC6F-1, both in poly(methyl methacrylate) (PMMA) films and in polar solvents such as chloroform and chlorobenzene. In particular, NOC6F-2/PMMA exhibits a nonlinear absorption coefficient (beta(eff)) of 7.2 x 10(-10) m W-1, which is almost an order of magnitude higher than that of NOC6F-1/PMMA (8.8 x 10(-11) m W-1). Density functional theory calculations reveal that NOC6F-2 has a large dipole moment and second hyperpolarizability owing to its large dihedral angle, which decisively affects its non-linear optical behavior. In addition to conjugation and charge transfer, the dipole moment is another important factor that enhances the nonlinear optical properties of organic molecules. This study provides useful guidance for the future design of high-performance organic non-linear optical materials.