The Excited Stated Structural Dynamics Study Of1,3,4-thiadiazole Derivative By Resonance Raman Spectrum And Density Functional Calculation

In this paper, we select4kinds of1,3,4-thiadiazole derivatives with similarstructure as our research system:5-mercapto-1,3,4-thiadiazole-2-thione (MTT);1,3,4-thiadiazole-2-thione(McT);5-methyl-1,3,4-thiadiazole-2-thione (McMT);2-Amino-5-methylthio-1,3,4-thiadiazole(5-AMTT). Resonance Raman spectro-scopy and density functional theoretical calculation are used to explore theexcited state structural dynamics information of these compounds in the Franck-Condon region. The innovation of the paper is that5-mercapto-1,3,4-thiadiazole-2-thione exist as different tautomeric structures on266nm excitation wavelengthunder high and low energy in acetonitrile solvent by Resonance Ramanspectroscopy and density functional theoretical calculation. Through theFranck-Condon region excited state structural dynamics of MTT compared withMcT, McMT and5-AMTT, we have concluded the influences of the substituenteffect and solvent effect on the excited state structural dynamics. The mainconclusions are summarized as follows:(1) We have obtained the ultraviolet absorption spectrum of MTT inacetonitrile and methanol solvents. Combining with density functionalcalculations， we found the A-band of its absorption spectra were assigned toπH π*Ltransition. We found the ultraviolet absorption spectrum of MTT wasdifferent,and the obvious blue shift has been found in acetonitrile solvent. Wehave described the vibration spectrum of thiol-thiol, thiol-thione and thione-thioneform of MTT, according to the experiment FT-IR and FT-Raman in solid,combinated with DFT.The Resonance Raman spectra were acquired for MTT inacetonitrile solvent with266nm excitation wavelength under high and low energy,combinated with DFT, we found MTT exist as thiol-thione form under the low energy and thione-thione form under the high energy in acetonitrile solvent.Thepotential energy surface tapered cross and Excited State Intramolecular ProtonTransfer (ESIPT) Mechanism computed results conform to our experimentalresults. The Resonance Raman spectra were acquired for MTT in methanolsolvent with266nm excitation wavelength, compare with the high energy of266nm excitation wavelength in acetonitrile solvent,we found MTT exist as thione-thione form in methanol solvent.The Resonance Raman spectra were acquired forMTT in acetonitrile solvent can be assigned to the fundamentals, overtones andcombination bands of seven Franck-Condon active vibrational modes: N4-H9stretch vibration (υ1),S6-H8stretch vibration (υ2),N4-H9in plane bend and C2=N3stretch vibration (υ3), N4-H9in-plane bend and N3-N4=C5asymmetric Stretchvibration(υ4), H9-N4-C5=S7asymmetric Stretch vibration(υ5),S1-C5-S7asymmetricStretch vibration (υ8), C2-S1-C5symmetric Stretch vibration (υ11). In acetonitrilesolvent an interesting phenomenon appear in different excitation wavelength,we found the vibrational mode υ8gradually increase, the vibrational mode υ3gradually diminished, while the combination bands of υ8+υ3gradually diminishedas the wavelength from282.4nm to341.5nm, this indicates that the excitedstated structural dynamics is mainly along the C2=N3stretch, as the wavelengthgradually increase, the excited stated structural dynamics is along the S1-C5-S7asymmetric Stretch vibration.(2) We have obtained the ultraviolet absorption spectrum of McT inacetonitrile, methanol and water solvents. Combining with DFT，we found themain band of its absorption spectra were assigned to πH π*Ltransition. Theobvious blue shift has been found in aqueous solvent.we have described thevibration spectrum of McT, According to the experiment FT-IR and FT-Raman insolid, combinated with quantum chemistry calculation. The Resonance Ramanspectra were acquired for McT in different solvents with different excitationwavelengths. Most of the resonance Raman intensity of the resonance Ramanspectrum of McT in acetonitrile solvent can be assigned to the fundamentals,overtones and combination bands of nine Franck-Condon active vibrational modes: N3-H8in-plane bend and C5=N4Stretch vibration (υ3), N3-H8in-planebend and N3-C2Stretch vibration (υ4), C5-H7in-plane bend (υ5), H8-N3-C2=S6asymmetric Stretch vibration and C5-H7in-plane bend(υ6), N3-N4Stretchvibration(υ7), S1-C2-S6and C2-N3-N4asymmetric Stretch vibration(υ8), N3-N4=C5and S1-C5-H7scissor vibration+N4=C5-S1symmetric Stretch vibration(υ9), C2-S1-C5asymmetric Stretch vibration(υ10) and C2-S1-C5symmetric Stretch vibration(υ11).The vibrational mode υ4stronger than υ3in acetonitrile solvent; while the oppositephenomenon appear in methanol and water solvents, the vibrational mode υ3stronger than υ4. This indicates that the impact of protic and aprotic solvent aredifferent on the excited state short-term dynamic information in Franck-Condonregion.(3) We have obtained the ultraviolet absorption spectrum of McMT inacetonitrile and methanol solvents. Combining with DFT，we found the Aband ofits absorption spectra were assigned to πH π*Ltransition. The Resonance Ramanspectra were acquired for McMT in acetonitrile and methanol solvents with273.9nm,282.4nm,299.1nm,309.1nm and319.9nm excitation wavelengths.Researches show that the excited stated structural dynamics is mainly along theC5=N4Stretch vibration,C2S1C5symmetric and symmetric Stretch vibration,CH3scissor stretch in Franck-Condon region.(4) We have obtained the ultraviolet absorption spectrum of5-AMTT inacetonitrile, methanol and water solvents. Combining with DFT，we found themain band of its absorption spectra were assigned to πH π*Ltransition. TheResonance Raman spectra were acquired for5-AMTT in different solvents withdifferent excitation wavelengths. Researches show that the excited statedstructural dynamics is mainly along the C5=N4Stretch vibration, the C-H of CH3and NH2stretch in Franck-Condon region.