Clusters’ Structures And Excited State Decay Mechanism Of Nitrogen/sulfur Five-membered Heterocyclic Molecules

Due to their unique structure,the microsolvation aggregated clusters have been widely applied into the materials,chemistry and biological fields.In this paper,three nitrogen/sulfur five-membered heterocyclic molecules including 1,2,4-triazole-3-thione（3TT）,4-methyl-1,2,4-triazole-3-thione（4MTT）and 5-mercapto-1,3,4-thiadiazole-2-thione（MTT）have been investigated by FT-Raman and FT-IR,488 nm Raman combining with the density functional theory（DFT）calculation,which are further applied to determine their structures in the different environments.The innovation point is that the number of solvent molecules and hydrogen bond binding sites are determined using the explicit and implicit solvent models in different solvent environments.Combining with the nanosecond transient absorption spectrum and the time-dependent density functional theory（TD-DFT）calculations,the excited state decay mechanism in the Franck-Condon region was successfully proposed.The main research results are as follows:（1）Combining with FT-IR and FT-Raman spectroscopies and density functional theory calculation,we initially determined that the 1,2,4-triazole-3-thione in solid exist as thione dimer by the intramolecular hydrogen bonding interaction.Combining with DFT calculation,488 nm Raman spectra confirmed that the hydrogen bonds（>NH---O/N and>C=S---H）of the 3TT clusters were mainly located on the side of–（HN）₂C=S group.The total and average binding energy calculation also give rise to the number of solvent molecules.Raman spectra observed in acetonitrile,methanol and water are assigned to 3TT（CH₃CN）,3TT（CH₃OH）₂ and 3TT（H₂O）₄clusters in the corresponding solvents,respectively.It also proved that the hydrogen bond between3TT and solvent molecules should contribute to the shifts of Raman wavenumbers and UV absorption bands.According to our TD-DFT calculation,triplet species was captured by nanosecond transient absorption spectroscopy.The excited state decay from the S₂（ππ^*）was proposed that initially singlet ¹（ππ^*）state decay to the S₁（nπ^*）by intersection conversion,and then via two conical intersections points ¹（nπ^*）/³（ππ^*）and S₀/³（ππ^*）to the captured triplet species.（2）Combining with FT-IR and FT-Raman spectra,thione dimer was determined for the ground 4-methyl-1,2,4-triazole-3-thione.488 nm Raman spectra were assigned as the hydrogen bonding clusters in different solvents.Combining with DFT calculation,the hydrogen bond binding sites were located on the side of–N–HN–C=S group of 4MTT.The hydrogen bonding interaction between 4MTT and CH₃CN,CH₃OH and H₂O led to the shifts of the maximum absorption wavelength and Raman wavenumber.The clusters’structures were determined to be4MTT（CH₃CN）,4MTT（CH₃OH）₂ and 4MTT（H₂O）₂ in corresponding solvents.According to our TD-DFT calculation,triplet species captured by nanosecond transient absorption spectroscopy was derived from the excited state S₂（ππ^*）according to our calculated potential energy surface profile.（3）We obtained UV absorption spectra of 5-mercapto-1,3,4-thiadiazole-2-thione in different solvents,the main maximum absorption band isπ→π^*transition,and the clusters’structures and hydrogen bond binding site were determined by DFT calculation using explicit and implicit solvent models.The–N–HN–C=S group in MTT was bonded with H₂O and CH₃OH molecules,and>NH group was bonded with one CH₃CN.According to the total binding energy and average binding energy,the clusters’structures of MTT in acetonitrile,methanol and water initially were MTT（CH₃CN）,MTT（CH₃OH）₂ and MTT（H₂O）₂,respectively.Combining with the fluorescence and nanosecond transient absorption spectroscopies,the excited state decay mechanism was also proposed by TD-DFT calculation.