title: Highly effective quenching of the ultrafast radiationless decay of photoexcited pyrimidine bases by covalent modification: photophysics of 5,6-trimethylenecytosine and 5,6-trimethyleneuracil.
abstract: 5,6-Trimethylenecytosine (TMC) and 5,6-trimethyleneuracil (TMU), in which the twist of the C5-C6 bond (or the pyrimidalization of C5) is strongly hindered, do not exhibit the subpicosecond excited-state lifetime characteristic of the naturally occurring pyrimidine bases. This result demonstrates the important role the out-of-plane deformation of the six-membered ring plays in the ultrafast (subpicosecond) internal conversion of photoexcited nucleobases. The dramatically shorter fluorescence lifetime of TMU ( approximately 30 ps) relative to TMC ( approximately 1.2 ns), in aqueous solution at room temperature, is attributed to the presence in TMU of an efficient, secondary nonradiative decay channel of S(1)(pipi*) involving a low-lying (1)npi* state.
authors.affiliation: Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, K1A 0R6, Canada.
authors.forename: Marek Z
authors.initials: MZ
authors.lastname: Zgierski
authors.forename: Takashige
authors.initials: T
authors.lastname: Fujiwara
authors.forename: William G
authors.initials: WG
authors.lastname: Kofron
authors.forename: Edward C
authors.initials: EC
authors.lastname: Lim
mesh_terms: D002244:Carbon / Q000737:chemistry; D003596:Cytosine / Q000031:analogs & derivatives* / Q000737:chemistry; D005453:Fluorescence; D006863:Hydrogen-Ion Concentration; D015394:Molecular Structure; D010777:Photochemistry; D011743:Pyrimidines / Q000737:chemistry*; D013050:Spectrometry, Fluorescence; D013816:Thermodynamics; D013997:Time Factors; D014498:Uracil / Q000031:analogs & derivatives* / Q000737:chemistry
chemical_list: C523561:5,6-trimethylenecytosine; C523560:5,6-trimethyleneuracil; D011743:Pyrimidines; D014498:Uracil; D002244:Carbon; D003596:Cytosine; C030986:pyrimidine