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Title: Remarkable effects of solvent and substitution on the photo-dynamics of cytosine: a femtosecond broadband time-resolved fluorescence and transient absorption study
Authors: Ma, C
Cheng, CCW
Chan, CTL
Chan, RCT
Kwok, WM 
Issue Date: 2015
Publisher: Royal Society of Chemistry
Source: Physical chemistry chemical physics, 2015, v. 17, no. 29, p. 19045-19057 How to cite?
Journal: Physical chemistry chemical physics 
Abstract: Cytosine (Cyt) among all the nucleic acid bases features the most complex and least understood nonradiative deactivation, a process that is crucially important for its photostability. Herein, the excited state dynamics of Cyt and a series of its N1- and C5-derivatives, including the full set of Cyt nucleosides and nucleotides in DNA and RNA and the nucleosides of 5-methyl cytosine, 5-methylcytidine and 22-deoxy-5-methylcytidine, have been investigated in water and in methanol employing femtosecond broadband time-resolved fluorescence coupled with fs transient absorption spectroscopy. The results reveal remarkable state-specific effects of the substitution and solvent in tuning distinctively the timescales and pathways of the nonradiative decays. For Cyt and the N1-derivatives, the nonradiative deactivations occur in a common two-state process through three channels, two from the light-absorbing ÀÀ state with respectively the sub-picosecond (<0.2 ps) and the picosecond (<1.5 ps) time constant, and the third is due to an optically dark nÀ state with the lifetime ranging from several to hundreds of picoseconds depending on solvents and substitutions. Compared to Cyt, the presence of the ribose or deoxyribose moiety at the N1 position of N1-derivatives facilitates the formation of the nÀ at the sub-picosecond timescale and at the same time increases its lifetime by <4-6 times in both water and methanol. In sharp contrast, the existence of the methyl group at the C5 position of the C5-derivatives eliminates completely the sub-picosecond £k£k? channel and the channel due to the n£k?, but on the other hand slows down the decay of the £k£k? state which after relaxation exhibits a single time constant of ?4.1 to ?7.6 ps depending on solvents. Varying the solvent from water to methanol accelerates only slightly the decay of the £k£k? state in all the compounds; while for Cyt and its N1-derivatives, this change of solvent also retards strongly the n
ISSN: 1463-9076
EISSN: 1463-9084
DOI: 10.1039/c5cp02624e
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