| Nucleic acid bases are important building blocks of DNA moleculars.In recent decades, a variety of ultrafast time-resolved experimental methods and theoretical calculations were employed widely to study the photo-stability of nucleic acid bases. These studys revealed the excited state relaxation and photo-damage process of nucleic acid. Current studys showed nucleic acid bases could decay to ground state very quickly after photoexcitation controlled by ultrafast internal conversion via conical intersections. The two main relaxation pathes are as followed:S2, FC' S2/S0'S0 and S2, FC'S2/S1'S1/S0'S0. Because of this process happened within only 1 picosecond, it could restrain intersystem crossing occurrence and avoid photo-damage in the building blocks of DNA moleculars. Therefore, nucleic acid could maintain the function of genetic transcription.In contrast, photo-physics of thio-nucleobases are much more different from those of natural nucleobases. The excited state relaxations of thio-nucleobases are controlled by effective intersystem crossings. This unique photo-chemical property has been widely concerned for application in photodynamic therapy. We employed time-resolved resonance Raman spectroscopy combining advanced quantum chemistry calculation methods in order to study the structural dynamics within 50 femtoseconds of excited states. It furnished evidence to compare the effectiveness of three theoretical pathes, which were produced by CASSCF(CASPT2) calculations. These channels are via S2/S1、S2/T2 and S2/T3 conical intersections, respectively, and our work analyzed the different role of three channels in excited states relaxation of various thio-nucleobases. Several conclusions of our work are as followed:(1) We obtained the UV absorption spectra and the molar extinction coefficient of 2-TU, 4-TU and 2-TT and also Fourier transform (FT)-Raman and FT-infrared (IR) of these molecules respectively, and then identified various vibrational modes using density functional theory calculations. This work adopts time-dependent density functional theory to calculate the track and the transition oscillator strength of the excited state, and determined the main electronic transitions. The excited states S2 of 2-TU and 2-TT were caused by πH'πL* electronic transitions and 4-TU was caused by πH-1'πL* .The resonance Raman spectra of main wavelength in A-band of these molecules were acquired in our experiment. The resonance Raman spectra of 2-TU was assigned as strong peaks v6, V12 and V17, weak peaks V15, V14, V13, V11, V9, V8, V7 and V5. The resonance Raman spectra of 4-TU was assigned as strong peaks V6, v13 and v17, weak peaks V14, V13, V11, V10, V9, and V7. The resonance Raman spectra of 2-TT was assigned as strong peaks v34,v18,v16 and v7, weak peaks v17, v16, V15, v14, v3, v8 and v6.(2) Combining resonance Raman spectroscopy and time-dependent wave packet theory was used in the simulation of the absorption cross section in 2-TU and 4-TU.We obtained the dimensionless normal mode displacement by the simulation. The dimensionless normal mode displacements of 2TU were converted to the short-time structural dynamics in terms of the internal coordinates using the formula. By comparing the short-time structural dynamics and the intersection of structure which gain by quantum chemistry calculation methods, we can find out that the structures of in or near the Franck-Condon area 2-TU were closer to the structure of intersection S2S1 and S2T3. It revealed that the two intersections were more effective, thus decay by intersections S2T2 was difficult to occur.(3) We employed CASSCF (complete active space self-consistent field method) to calculate energy and structure of excited state and potential energy surface crossing point of 4-TU,2-TT. According to the changes in the structure and intensity of vibration peaks in resonance Raman spectra, we obtained the characteristic vibration modes in the potential energy surface of S2 excited state. The excited state dynamic of 4-TU and 2-TT was considered from structure and energy details respectively. In 4-TU, it revealed the crossing point S2/S1 and S2/T2 was close to S2,FC, because of their structure and energy were similar to S2 excited state. Thus the two crossing points were both effective. Eventually, we proposed two decay path way of 4-TU:S2' CI(S2/S1)'CI(S1/T1)'CI(T1/S0)'S0 and S2'CI(S2/T2)'CI(T2/T1)'CI(T1/S0)'S0.In 2-TT, our work compared two path way containing crossing points S2/S1 and S2/T2,and S2/T2 was more competitiveness in energy and structure. Therefore we found out the excited state decay path way of 2-TT:S2'CI(S2/T2)'CI(T2/T1)'CI(T1/S0)'S0. |
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