摘要：

A comprehensive review of recent theoretical and experimental advances in the singlet electronic transitions, excited state structures and dynamics of nucleic acid bases (NABs) and base assemblies are presented. It is well known that NABs absorb ultraviolet radiation, but the absorbed energy is efficiently dissipated in the form of ultrafast internal conversion processes believed to occur in the subpicosecond time scale and, therefore, enabling NABs highly photostable. It is not known how much evolutionary role was played in evolving these molecules and the ultimate selection by nature as genetic materials, but it is well accepted that survival-of-fittest prevails. Recently, significant efforts have been continuously paid to understand the mechanism of electronic excitation deactivation, but universally acceptable mechanism is still elusive. However, recent investigations reveal that electronic excited state geometries of DNA bases are usually nonplanar and this structural nonplanarity may facilitate nonradiative deactivation. Investigation of excited state structures is challenging and, therefore, it is not surprising that despite the impressive theoretical and computational advances, this research area is still hampered by the methodological and computational limitations. Further, stacking has significant influence on the emission properties of molecules. The 2-aminopurine, a fluorescent adenine derivative frequently used in studying DNA dynamics, shows significant attenuations in fluorescence quantum yield when incorporated in the DNA. Theoretical and computational bottlenecks limit a thorough theoretical understanding of effect of stacking interactions on the excited state dynamics of NABs. Despite these limitations the investigations of excited state properties are progressing in the right direction and our better understanding of excited state structure and dynamics of NABs and nucleic acids may help to design preventive strategy for radiation induced illness and photostable materials.

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