The lifetimes of the excited state of free nucleobases were measured in the gas phase for the first time. They are, respectively, 1.0 and 0.8 ps for the purine bases adenine (shown above) and guanine and 3.2, 2.4, and 6.4 ps for the pyrimidine bases cytosine, uracil, and thymine at 267 nm. The longer lifetimes of the pyrimidine bases may be associated with their higher propensity toward photodegradation, especially in the case of thymine. The ultrashort lifetime of nucleobases conventionally known in solution was found to be an intrinsic molecular property due to extremely facile internal conversion, and therefore the lifetime should be largely independent of the medium at this energy, that is, whether in vacuo, in solution, or in vivo. The evolutionary selection of nucleobases as the durable carriers of genetic information is suggested to be due to their inherent immunity from photochemical reactions.
The mechanism for the internal conversion of adenine was investigated by femtosecond pump–probe transient ionization time-of-flight mass spectrometry. In particular, we tried to determine which of the nπ* and πσ* states was responsible for the ultrafast decay of adenine by comparing the dynamics of adenine and its several derivatives with methyl or deuterium substitution. Adenine and all its derivatives studied here had more or less the same decay time of ∼1 ps at the excitation energy of 37 500 cm−1. These results provide compelling evidence that the nπ* state, rather than the πσ* state, plays the role of a doorway state for internal conversion via a conical intersection to the ground state.
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