Solvent‐Dependent Stabilization of a Charge Transfer State is the Key to Ultrafast Triplet State Formation in an Epigenetic DNA Nucleoside

2’‐Deoxy‐5‐formylcytidine (5fdCyd), a naturally occurring nucleoside found in mammalian DNA and mitochondrial RNA, exhibits important epigenetic functionality in biological processes. Because it efficiently generates triplet excited states, it is an endogenous photosensitizer capable of damaging DNA, but the intersystem crossing (ISC) mechanism responsible for ultrafast triplet state generation is poorly understood. In this study, time‐resolved mid‐IR spectroscopy and quantum mechanical calculations reveal the distinct ultrafast ISC mechanisms of 5fdCyd in water versus acetonitrile. Our experiment indicates that in water, ISC to triplet states occurs within 1 ps after 285 nm excitation. PCM‐TD‐DFT computations suggest that this ultrafast ISC is mediated by a singlet state with significant cytosine‐to‐formyl charge‐transfer (CT) character. In contrast, ISC in acetonitrile proceeds via a dark 1 nπ* state with a lifetime of ∼3 ps. CT‐induced ISC is not favored in acetonitrile because reaching the minimum of the gateway CT state is hampered by intramolecular hydrogen bonding, which enforces planarity between the aldehyde group and the aromatic group. Our study provides a comprehensive picture of the non‐radiative decay of 5fdCyd in solution and new insights into the factors governing ISC in biomolecules. We propose that the intramolecular CT state observed here is a key to the excited‐state dynamics of epigenetic nucleosides with modified exocyclic functional groups, paving the way to study their effects in DNA strands.