Synthesis and nonlinear optical properties in the near-IR range of stilbazolium dyes.

A series of stilbazolium salts based on donor-π-acceptor (D-π-A) structure have been synthesized and fully characterized. Photophysical properties including linear absorption, one-photon excited fluorescence (OPEF), two-photon absorption (2PA) properties were systematically investigated. The results suggest that increasing electron-releasing character of the terminal group leads to a more pronounced donor-to-acceptor intramolecular charge transfer (ICT). In addition, the dyes possess the largest 2PA cross sections in the near infrared region (NIR) and display maximum two-photon absorption cross sections within the narrow wavelength range from 950 to 970nm and BL3 exhibits a large nonlinear refractive index coefficient and possesses very large values of the real part of the cubic hyperpolarizability χ((3)) at 960nm. Furthermore, the initial density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations provide reasonable explanations for their absorption spectra, meanwhile we used the Lippert-Mataga equation to evaluate the dipole moment changes of the dyes with photoexcitation, the results are corresponding with linear and nonlinear optical properties of the dyes.