Electronic structure and spectral properties of indole based fluorescent styryl dyes: Comprehensive study on linear and non-linear optical properties by DFT/TDDFT method

Abstract

The photophysics, linear and non-linear optical properties of indole based fluorescent styryl dyes are investigated with density functional theory and time-dependent density functional theory. The performance of seven global hybrid functionals - B3LYP, PBE0, BHHLYP, M06, M06-L, M06-2X, M06-HF and six range separated hybrid functionals - CAM-B3LYP, HISSbPBE, HSEH1PBE, ωB97, ωB97X, ωB97XD with poples [6-311++G(d,p)] and correlation consistence (cc-pVDZ, cc-pVTZ) basis sets is analyzed. The linear (vertical excitations, oscillator strengths) and non-linear optical parameters (first and second order hyperpolarizabilities) are calculated with each functional in combination with different basis sets for comparison. The vertical excitation of all the dyes are shown to have good correlation with the experimental values by the M06-2X/6-311++G(d,p) and HISSbPBE/cc-pVDZ combination with the least deviations (mean standard error) of 8.67 nm and 4.34 nm with respect to global and range separated hybrid functionals respectively. On the other hand, polarizability and hyperpolarizability values evaluated for all the dyes suggested that the dye which contain N-ethyl group as a donor and nitro group as an acceptor is found to have the largest NLO response as compared to other dyes. Mean standard error was calculated for each system with respect to global and range separated hybrid functionals to evaluate the accuracy of the DFT methods. Mean standard error of polarizability and hyperpolarizability obtained using global hybrids and range separated hybrid functionals are found to be basis sets dependent i.e. each functional and basis set combination perform differently.