2,4-二硫代尿嘧啶的紫外吸收光谱和共振拉曼光谱

硫代嘧啶碱基是光动力疗法潜在的重要光敏剂,其最低单重激发态的光物理研究已有广泛报道。然而,其较高激发态的跃迁性质和反应动力学研究较为稀少。因此,本文采用共振拉曼光谱和密度泛函理论计算方法研究2,4-二硫代尿嘧啶的紫外光谱和几个较高单重激发态的短时结构动力学。首先,基于共振拉曼光谱强度与电子吸收带振子强度f的关系,将紫外光谱去卷积成四个吸收带,分别为358 nm(f=0.0336)中等强度吸收带(A带),338 nm(f=0.1491)、301 nm(f=0.1795)和278 nm(f=0.3532)强而宽的吸收带(B、C和D带)。这一结果既吻合密度泛函理论计算结果,又符合共振拉曼光谱强度模式对紫外光谱带的预期。据此,去卷积得到的四个吸收带被分别指认为S0→S2跃迁、S0→S6跃迁、S0→S7跃迁和S_0→S_8跃迁。同时,分别对B,C和D带共振拉曼光谱进行了详细的指认,获得了短时动力学信息。结果表明,S_8态短时动力学的显著特征是在Franck-Condon区域或附近发生了S8(ππ~*)/S(nπ~*)势能面交叉引发的、伴随超快结构扭转的非绝热过程。S7和S6态短时动力学的主要特征是反应坐标的多维性,它们分别沿C_5C_6/C_2S_8/C_4S_(10)/N_2C_3+C_4N_3H_9/N_1C_2N_3/C_2N_1C_6/C_6N_1H_7/C_5C_6H_(12)和C_5C_6/N_3C_2/C_4S_(10)/C_2S_8+C_6N_1H_7/C_5C_6H_(12)/C_5C_6N_1/C_5C_6H_(12)/C_2N_1C_6/N_1C_2N_3/C_4N_3H_9/N_1C_2N_3等内坐标演化。

UV Absorption and Resonance Raman Spectra of 2, 4-Dithiouracil

2, 4-Dithiouracil is potentially an important photosensitizer for use in photodynamic therapy. Its photophysics when populated in the lowest excited state has been studied extensively. However, its higher light absorbing excited states and the corresponding reaction dynamics have not been investigated sufficiently. Herein, the resonance Raman spectroscopy and density functional theory were adopted to clarify the electronic transitions associated with the UV absorptions in the far-UV region and the short-time structural dynamics corresponding to the higher light absorbing excited states. The UV absorption spectrum in acetonitrile was deconvoluted into four bands:the moderate intense absorption band at 358 nm (f=0.0336) (A band), the intense broad absorption bands at 338 nm (f=0.1491), 301 nm (f=0.1795), and 278 nm (f=0.3532) (B, C, and D bands) respectively, on the basis of the relationship between the resonance Raman intensities and the oscillator strength f. The result was consistent with the predictions made using the time-dependent density functional theory calculations and the resonance Raman intensity patterns. Thus, the four bands resulted from the deconvolution are assigned as the S₀→S₂, S₀→S₆, S₀→S₇ and S₀→S₈ transitions, respectively. The resonance Raman spectra of the corresponding B, C, and D bands are assigned and the qualitative short-time structural dynamics are obtained. The major character in the short-time structural dynamics of 2, 4-dithiouracil in the S₈ excited state is that a non-adiabatic process via S₈(ππ^*)/S(nπ^*) curve-crossing, accompanied with ultrafast structural distortion, takes place in or near the Franck-Condon region, while the major character in the short-time structural dynamics in the S₇ and S₆ excited state appears in the multidimensional reaction coordinates, which are mostly along the C₅C₆/C₂S₈/C₄S₁₀/N₂C₃ bond lengths + C₄N₃H₉/N₁C₂N₃/C₂N₁C₆/C₆N₁H₇/C₅C₆H₁₂ bond angles for the S₇ excited state and the C₅C₆/N₃C₂/C₄S₁₀/C₂S₈ bond lengths + C₆N₁H₇/C₅C₆H₁₂/C₅C₆N₁/C₅C₆H₁₂/C₂N₁C₆/N₁C₂N₃/C₄N₃H₉/N₁C₂N₃ bond angles for the S₆ excited state.