Mechanism for ultrafast internal conversion of adenine

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 ${\mathrm{n\pi }}^{*}$ and ${\mathrm{\pi \sigma }}^{*}$ 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 $\text{∼1\hspace{0.17em}}\mathrm{ps}$ at the excitation energy of $\text{37\hspace{0.17em}500\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}.$ These results provide compelling evidence that the ${\mathrm{n\pi }}^{*}$ state, rather than the ${\mathrm{\pi \sigma }}^{*}$ state, plays the role of a doorway state for internal conversion via a conical intersection to the ground state.