Study Of Excited-State Structure And Non-radiative Dynamics Of Thioacetamide And Its Derivatives

There has caused widely attention of the photophysical and photochemical processes ofamide compounds in recent years, because of containing the amide group (-CO-NH-), whichsubstitute the oxygen in carbon-oxygen double bond of the sulfur, it is also the most obviouslyfeature of thioacetamide and its derivatives. The most critical characteristic of excited statedynamics of these compounds is stretching of C=S and twisting of C-N. Therefore the protontransfer and isomerization may take place easily in such compounds, and it makes meaningful tostudy the process of their non-radiative dynamics.In thioacetamide and its derivatives, Ï€â†'Ï€*transitions occured between the Ï€ orbital and Ï€*orbital of C=S, while coupling with the transitions of corresponding nâ†'Ryd in C-N. It makes S2and S3electonic excited state played an important role in the process of the photophysical andphotochemical. In this work, we selected the thioacetamide and its methylamino groupsubstituted N-methylthiourea and dimethyl substituted N,N-dimethylthioacetamide as ourresearch system. We hane taken the Resonance Raman and related auxiliary technique,combining with the time-dependent density functional theory (TD-DFT) calculation method andCASSCF to study excited state structural dynamics information of these compounds. Weexplored geometry of each excited state potential energy surface crossing point and decaychannels of non-radiative, and we have acquired main conclusions as follows:(1) We obtained the UV, FT-IR, FT-Raman and the Resonance Raman spectra ofthioacetamide (TAA) in three different polar solvent as acetonitrile, methanol, and water withdifferent excitation wavelength. A/B-band absorptions were identified both electronic transitionsof Ï€H-1â†'Ï€L*and nâ†'Ryd1. The main vibration modes of Resonance Raman spectra include6(C1N4stretch/H8N4H9scissor),10(C1N4stretch),11(C1N4H8/C1N4H9bend/C1C2stretch),14(C1N4H8/C1N4H9bend/C1C2/C1S3stretch),15(C1C2/C1S3stretch),19(C1C2S3bend).Besides, the stable geometry, electronic transitions and vibrational frequencies wereobtained by FT-Raman and result of calculation with TD-DFT method, and then investigatedsolvent on the influence of short-term dynamics informations in Franck-Condon region under different excited state. The Raman evidence of electronic coupling between S2and S3state wasfound by means of fitting calculation combining with quantum wave packet theory of ResonanceRaman spectra intensity, and analysis with the vibration-electronic coupling theory. Calculatingof Raman intensity and the time-dependent wave-packet modeling to explain the reasons for thevariances between the theoretical and experimental values. CASSCF(6,5)/6-31G(d) thecalculation method was employed to acquire structure of excited states and potential energysurfaces of TAA, and also use B3LYP-TD/6-311++G(df, pd) calculated energy of TAA and itsmain thiol tautomers in different excited states. And discussed photoisomerization process of themolecule excited to S2,FCstate relaxed to ground state and decay paths of non-radiative.(2) We obtained the UV, FT-IR, FT-Raman and the Resonance Raman spectra ofN-methylthiourea (MMTU) in three different polar solvent as acetonitrile, methanol, and waterwith different excitation wavelength. A/B-band absorptions were identified both electronictransitions of Ï€H-1â†'Ï€L*and nâ†'Ryd1. The main vibration modes of Resonance Raman spectrainclude8(C1N4stretch/N4H8in-plane bend),10(H9C5H11torsion/C5H10out-of-plane bend),12(methyl umbrella/C1N4/C1N3stretch),13(N4H8in-plane bend/H6N3H7rock/C1S2stretch),14(methyl rock/N4H8in-plane bend),15(methyl out-of-plane bend),16(N3C1N4sym stretch/H6N3H7rock/C5N4stretch),18(C1C2stretch/N3C1N4scissor).Besides, the stable geometry, electronic transitions and vibrational frequencies wereobtained by FT-Raman and result of calculation with TD-DFT method, and then investigatedsolvent on the influence of short-term dynamics informations in Franck-Condon region underdifferent excited state. The Raman evidence of electronic coupling between S2and S3state wasfound. The different between TAA and MMTU is only cis-structure exist in the pure solid, butboth cis and trans structure exist in solution. CASSCF(6,5)/6-31G(d) the calculation method wasemployed to acquire structure of cis and trans structure and potential energy surface crossingpoints of MMTU. And we discussed photoisomerization process of the molecule excited to S2,FCstate relaxed to ground state and decay paths of non-radiative.(3) We obtained the UV absorption, FT-IR, FT-Raman and the Resonance Raman spectra ofN,N-dimethylthioacetamide (DMTAA) in three different polar solvent as acetonitrile, methanoland water with different excitation wavelength. A-band absorptions were identified electronictransitions of Ï€H-1â†'Ï€L*and B-band absorptions were identified electronic transitions of nâ†'Ryd1. The main vibration modes of Resonance Raman spectra include7(H10C5H11scissor/C1N4stretch/N4C6H13bend),8(H10C5H11scissor/C5H12bend),12(C6-methyl umbrella/H10C5H11scissor/C1N4stretch),14(C1N4C5/C1N4C6bend/H7C2H9/H14C6H15wag),16(C1C2stretch/C1S3bend/C5-methyl wag),20(C5N4C6sym stretch/C1C2/C1S3/C1N4stretch).Besides, the stable geometry, electronic transitions and vibrational frequencies wereobtained by FT-IR and result of calculation with TD-DFT method, and then investigated solventon the influence of short-term dynamics informations in Franck-Condon region under differentexcited state. The Raman evidence of electronic coupling between S2and S3state, S3and S4state, S3and S5state was found by means of fitting calculation combining with quantum wavepacket theory of Resonance Raman spectra intensity, and analysis with the vibration-electroniccoupling theory. CASSCF(6,5)/6-31G(d) the calculation method was employed to acquirestructure of excited states and potential energy surfaces of DMTAA. And we discussedphotoisomerization process of the molecule excited to S3,FCstate relaxed to ground state anddecay paths of non-radiative.(4) Comparison of the effect of substituents on the solvent effects and excited state structuredynamics of three different structural compounds in this system.