| The excited electronic states of DNA and RNA bases play an important role in the UV photodamage of nucleic acids. More and more people have paid attention to them.These excited states sometimes decay to deleterious photoproducts, which can lead to mutations and interfere with the normal cellular processing of DNA.Nucleic acids's photostability is closely related to the transient process of internal conversion (IC). Over the past two decades, much effort has been made in the study of the relaxation dynamics of excited-state nucleic acid bases. The lifetimes of the first electric dipole-allowed state (S1) have been observed to be generally short (around 1 ps), and fast decay to the ground state via IC has been proposed. However, the decay mechanism is far from certain. As a very efficient and sophisticated means, Resonance Raman spectral technique has a unique advantage on the research of the conical intersection and vibronic coupling in the Franck-Condon region, and on other aspects.In this paper, the short-time dynamics of photo-induced Charge-Transfer(CT) of adenine, 6-N,N-dimethyladenine, 9-methyladenine and adenosion in various kinds of solutions(water , methanol, acetonitrile and tetrahydrofuran) have been investigated by the Resonance Raman spectra in combination with DFT calculation. We also discussed the influential factors to tune the photochemical reactions. Main contributions of the present work are summarized as follows,(1) We obtained the absorption spectrum of adenine in different kinds of solutions which has a CT band near 261nm and 207nm respectively, seeming very similar to each solution. Resonance Raman spectrums were obtained for adenine in different solutions with 204.2, 223.1, 252.7, 266and 273.9nm excitation wavelength in resonance with the CT-band absorption spectrum. Combined with density functional theory calculations, we studied the A-, and B-band electronic excitations and the excited state structural dynamics of adenine.π'π* transition is the main part of the A-band absorption. While amount of this transition decreases a little in B-band absorption, as the n'Ryd andπ'Ryd transitions, where Ryd denotes the diffuse Rydberg orbital, increase a little. The fundamental vibrations of C8H bend+ C8=N7 stretch + NH2 scissor modeν9, N9-H/C8-H/C2-H bend + purine ring deformation modeν13, C8-H/N9-H bend+N7=C8 stretch+the 6-membered ring breathν16 and their overtones and combination bands occupy most of the A-band Resonance Raman intensities, indicating that the 1ππ*excited state structural dynamics of adenine is mainly along theν13,ν9 andν16 reaction coordinates. The majority of the B-band Resonance Raman intensities is dominated by the fundamental vibrations ofν7,ν11,ν13,ν23 and their overtones and combination bands. accompanied by a moderate degree of the motion along byν9,ν16 andν19.(2) The A-, and B-band electronic excitations and the excited state structural dynamics of 6-N,N-dimethyladenine (DMA) were studied by using the resonance Raman spectroscopy and density functional theory calculations.πH'π*L transition is the main part of the A-band absorption and its calculated oscillator strength occupies 79% of the A-band absorption. n'Ryd andπH'Ryd transitions play important roles in the B-band electronic transitions, and their calculated oscillator strengths possess about 62% of the B-band absorption, while the oscillator strength forπH'π*L transition, which predominates the A-band electronic transitions, only occupies about 33% of the B-band absorption. The fundamental vibrations of purine ring deformation stretch + C8H/N9H bend modeν23 and the 5 member ring deformation stretch + C8-H bend modeν13 and their overtones and combination bands occupy most of the A-band Resonance Raman intensities, indicating that the 1πHπ*L excited state structural dynamics of DMA is mainly along theν23 andν13 reaction coordinates. The majority of the B-band resonance Raman intensities is dominated by the fundamental vibrations ofν10,ν29,ν21,ν26,ν40, and their overtones and combination bands. This suggests the B-band excited state structural dynamics of DMA is mostly along the purine ring deformation, C6N10 stretch, N9-H/C8-H/C2-H bend and N(CH3)2 antisymmetry stretch reaction coordination. The appearance ofν26 andν12 in the A-band Resonance Raman spectra is correlated to the Franck-Condon region 1nπ*/1ππ* conical intersection, while the activation ofν21 in the B-band Resonance Raman spectra is correlated to the Franck-Condon region 1ππ*/1πσΝ9Η* conical intersection.(3) The A-, and B-band electronic excitations and the excited state structural dynamics of 9-methyladenine were studied by using the Resonance Raman spectroscopy and density functional theory calculations.π'π* transition is the main part of the A-band absorption. n'π* transition occupies 5%, increasing a little compared with adenine. The A-band excited state structural dynamics of adenine is mainly along the purine ring deformation + C2-H/C8-H bend + CH3 rock modeν19 reaction coordinates. The majority of the B-band Resonance Raman intensities is dominated by the fundamental vibrations of NH2 scissor + purine ring deformation + C2-H bendν10 andν19. In addition, we found 3 new modes in the Resonance Raman spectra of B-band. They areν29,ν24 andν25. All of them are related to moderate degree of motion along CH3 rock mode. In the A-band, the relatively intensities ofν11 andν13 are changed to some exent by different solvents. We thought the differnce is something to do with the effect of hydrogen bind between the solvent and 9-methyladenine.(4) The A-, and B-band electronic excitations and the excited state structural dynamics of adenosine were studied by using the Resonance Raman spectroscopy and density functional theory calculations. The A-band excited state structural dynamics of adenine is mainly along the purine ring deformation + C'-H bend + NH2 scissor + C6-N10 stretch modeν27, C8-H bend + the 5-membered ring deformation + N9-C1'stretch +C1'–H bend modeν17, and the NH2 scissor + purine ring deformation +C2-H bend modeν16. The majority of the B-band Resonance Raman intensities is dominated by the fundamental vibrations ofν16 andν27. |
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