Distinct doorway states lead to triplet excited state generation in epigenetic RNA nucleosides.

-Hydroxymethyladenosine (hmA) and -formyladenosine (fA) are two important intermediates during the demethylation process of -methyladenosine (mA), which has been proven to show epigenetic function in mRNA. However, there is no knowledge about how the chemical integrity and stability could be altered when these two nucleosides are exposed to ultraviolet (UV) radiation. Herein, we report the first study on excited state dynamics of hmA and fA in solutions by using femtosecond time-resolved spectroscopy and quantum chemistry calculations. Surprisingly, triplet excited species are clearly identified in both hmA and fA after UV excitation, which is in sharp contrast to the 10 level triplet yield of adenosine scaffolds. Moreover, the doorway states leading to triplet states are found to be an intramolecular charge transfer state and a lower-lying dark nπ* state in hmA and fA, respectively. These discoveries pave the way to further study their effects on RNA strands and provide insight for understanding RNA photochemistry.