Novel quadruple fluorescence properties of two benzoylthiourea isomers

Two benzoylthiourea isomers, N-2-flurobenzoy-N'-4- (N,N-dimethyl) amidophenylthiourea (2FBDAPT) and N-4-fluro-benzoy-N'-4- (N,N-dimethyl) amidophenylthiourea (4FBDAPT) were determined by fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR) and X-ray diffraction. It was found that intra- and intermolecular hydrogen bonds played an important role in determining their conformations. Electronic spectra of the two compounds were investigated by UV absorption and steady-state fluorescence methods. The intermolecular hydrogen bond between the title compounds and methanol molecules caused the long wavelength absorption bands in methanol to weaken and vanish indeed. Quadruple fluorescence bands in ultraviolet and visible region were observed in the studied solvents upon the variable excitation wavelength. As same as Azumaya's suggestions for benzanilide (BA), F4 fluorescence bands with the maximum wavelength (λ_max) between 546 nm and 622 nm were characteristic of TICT fluorescence. F3 bands of λ_max from 434 nm to 483 nm were explained by the ESIPT model of the S1 state of the thiol tautomer to the S1 state of the keto tautomer. The new proposition was that F2 bands with λ_max at about 365 nm were attributed to ESIPT from the S1 state of the thiol tautomer to the S0 state of the enol tautomer. And F1 fluorescence emissions with λ_max at about 310 nm originated from the local S1 transitions of the enol tautomer. All experimental results were supported by MP2, CASSCF and CASPT2 quantum chemical calculations.