Solvent-dependent Excited State Structure Dynamics Of 2,5 Substituted 1,3,4-thiadiazole

In this paper, solvent-dependent excited state structural dynamics in the Franck-Condon region of 5-methylthio-1, 3, 4-tiadiazole-2-tione(MTTN), 5-amino-1, 3, 4-thiadiazole-2-thione(AMT), 2-amino-1, 3, 4-thiadiazole(ATD) and 2, 5-dimethyl 1, 3, 4-thiadiazole(DMETD) in acetonitrile, methanol and water has been investigated by the resonance Raman spectra combined with density functional theoretical(DFT) Calc.ulations. The innovation of this paper is concentrated on the hydrogen bonds between solute and solvent molecules for MTTN and AMT are key factor to influence and control the excited sate structural dynamics. Here, the important results are as follows:(1) The ultraviolet absorption spectra of MTTN and AMT have been obtained in acetonitrile,methanol and water. Molecular orbitals, transition energies and oscillator strength of MTTN and AMT are Calc.ulated in gas phase and different solvent models by time-dependent density functional theoretical(TD-DFT). The results indicated the electronic transition was Ï€â†'Ï€* in the first band and the maximum absorption wavelength located at 319.9 nm in solvent. The resonance Raman spectra of MTTN and AMT in acetonitrile, methanol and water with different excitation wavelengths were assigned by FT-Raman, FT-IR, 488 and 1064 nm normal Raman spectra combined the experiment with methanol-d4 and deuteroxide. The results confirmed that the MTTN and AMT in CH3 CN, CH3 OH and H2 O have hydrogen bond(H-bond)MTTN(solvent)n and AMT(solvent)n clusters that produce significantly different Raman intensity patterns in different solvents. Combined with the normal Raman assignment, most resonance Raman spectra were assigned to the vibration modes of the H-bond cluster, repspectively, 1:1 in CH3 CN, 1:3 for MTTN and 1:4 for AMT in CH3 OH and H2 O. The theoretically-predicted frequencies and intensities in different surrounding environments enabled reliable assignments of Raman bands. The intermolecular >NH???O and >NH???N hydrogen bond interactions are key constituents of stable thione structures of MTTN and AMT. This underlines the significant structural differences of MTTN and AMT in CH3 CN, CH3 OH and H2 O. Hydrogen bond perturbation of MTTN may reveal important insights about the intermolecular excited state proton transfer(ESPT) reaction mechanisms in the Franck-Condon region structural dynamics of the thioneâ†'thiol tautomer in CH3 OH and H2 O.(2) The ultraviolet absorption band and resonance Raman spectra of 2-amino-1, 3,4-thiadiazole(ATD) are obtained at different excitation wavelengths in acetonitrile, methanol and water. TD-DFT Calc.ulations were also done to determine the molecular orbitals, transition energies and oscillator strength of ATD in gas phase and solvent. The maximum absorption wavelength is 252.7 nm corresponding to Ï€â†'Ï€* transition. The resonance Raman spectra areidentified by FT-Raman and FT-IR in solid. The results showed that the S1 excited state structure dynamics mainly along the reaction coordinates as follows: C1N6 stretch mode, C1H4 in plane bend, N7H8H9 in plane bend/C2N5 + N5N6 stretch mode, C1S3C2 + C2N7H9 methyl wag/C1N6N5 twist.(3)The ultraviolet absorption spectra and resonance Raman spectra of 2, 5-dimethyl-1, 3, 4-thiadiazole were obtained in acetonitrile, methanol and water at different excitation wavelengths to probe the short-time structural dynamics. At the theory of TD-B3LYP/6-311++G(d, p) level,the main band absorption is assigned as the ?â†'?* transition, relative to the A absorption band in gas phase and solvent. The resonance Raman spectra were assigned to the fundamentals,overtones and combination bands of about seven vibrational modes ?8, ?12, ?15, ?17, ?22, ?23and?27in the Franck-Condon region, predominately in the C=N and N-N stretch mode. The S2?S0photophysics process along the C=N and N-N reaction coordinates were proposed.