Femtosecond spectroscopy of superfluorescent fluorenyl benzothiadiazoles with large two-photon and excited-state absorption

New symmetrical fluorene-containing derivatives with a benzothiadiazole (BTD) central core, 4,7-bis(5-(9,9-didecyl-7- (phenylsulfonyl)-9H-fluoren-2-yl)thiophen-2-yl)benzo[c][1,2,5]- thiadiazole (1) and 4,7-bis(5'-(9,9-didecyl-7-(phenylsulfonyl)-9Hfluoren- 2-yl)-2,2'-bithiophen-5-yl)benzo[c][1,2,5]thiadiazole (2), were prepared, and multifarious linear photophysical and nonlinear optical properties, potentially attractive for a number of emerging applications, were investigated. Comprehensive photophysical and photochemical characterization was performed in a number of organic solvents at room temperature, including linear, one-photon absorption (1PA), steady-state fluorescence, excitation anisotropy, and fluorescence lifetime properties. Fast relaxation processes in the excited states of 1 and 2 with characteristic times of ∼0.2-3 ps were revealed by a femtosecond transient absorption pump-probe method. Large two-photon absorption (2PA) cross sections of 1 and 2 (up to 2500-2700 GM) were obtained by an open aperture Z-scan technique. Efficient two-photon optical power limiting (OPL) behavior with a figure of merit (FOM) ≈ 15.4 and superfluorescence properties were demonstrated in solution under femtosecond excitation. The electronic structure and optical parameters of 1 and 2 were analyzed by quantum chemical calculations using the TD-DFT method. Simulated linear absorption spectra were found in good agreement with experimental data while 2PA cross sections were overestimated via computational simultation and a possible explanation is provided. Good fluorescence quantum yields, long wavelength absorption, far-red to near-IR emission, efficient OPL and superfluorescence properties, large 2PA cross sections, and extremely high photochemical stability make these new materials good candidates for emerging nonlinear optical applications, including optical sensor protection, stimulated emission depletion microscopy, and two-photon fluorescence microscopy deep tissue bioimaging.