Intramolecular charge transfer tuning of azo dyes: Spectroscopic characteristic and third-order nonlinear optical properties

Five novel azo dyes molecules with different intramolecular charge transfer (ICT) characters were synthesized and characterized. Their thermal stabilities were studied with thermogravimetric analysis (TGA). Their linear optical properties were investigated by Ultraviolet-visible (UV-Vis) and Fluorescence spectroscopy. Delta-Self consistent-field density functional theoretical (SCF-DFT or simply Delta SCF) calculation and optical method were used to analyze the ICT strength of the azo Dyes 1, 2, 3, 4, 5, Disperse Red 1 (DR 1), DR 17 and DR 13. The fundamental gap, energy gaps (E-g) between HOMO and LUMO, optical gap falls down with the increase of the ICT strength. The third-order nonlinear optical (NLO) properties of azo dyes were investigated by Z-scan technique at the condition of 440 nm, 190 femtosecond, 20 Hz. The relationship between the third-order NLO properties and ICT character was discussed in detail. Going from dyes 1 -> 2 -> 3 -> 4 -> 5 -> DR 1 -> DR 17 -> DR 13, the E-g values decrease and the ICT strength increase. The third-order nonlinear susceptibility chi((3)) and molecular second hyperpolarizability gamma of the azo dyes are up to the order of 10(-8) esu and 10(-2)7 esu, respectively. The chi((3)) and gamma increase with the ICT strength. The Figures of Merit W >> 1 and T << 1, meet the requirement of all-optical materials: W > 1 and T < 1. The results reveal that the studied azo dyes may be promising candidates for optical devices and this work may provide a reference for synthesis of organic optical materials.