New Insights about the Photostability of DNA/RNA Bases: Triplet nπ* State Leads to Effective Intersystem Crossing in Pyrimidinones.

The high photostability of DNA/RNA nucleobases is attributed to the effective internal conversions of their bright ππ* states to the ground state through conical intersections. Intersystem crossing (ISC) from singlet to triplet excited states is a minor decay pathway in nucleobases and it is observed with ∼1-2% quantum yields (QYs) in pyrimidine bases. Presumably, ISC in pyrimidines takes place from the dark singlet nπ* state to the lowest triplet ππ* state. However, recent studies showed that ISC from the initial populated bright ππ* state to higher energy triplet nπ* states indeed occurs in the subpicosecond timescale. Such a mechanism is still poorly understood since direct observation of this pathway is challenging. Herein, excited state dynamics of three pyrimidinones, which share the same skeleton with pyrimidine bases, is investigated in different solvents. Compared to canonical pyrimidine bases, removing the oxygen atom at the C4 position revokes the low-lying dark nπ* state in pyrimidinones, resulting in direct ISC from the S (ππ*) state to triplet T (nπ*) state with much higher QYs. Meanwhile, hydrogen bonding between the carbonyl group in pyrimidinones and protic solvents can accelerate vibrational cooling of the hot S (ππ*) state, leading to higher fluorescence QYs and smaller ISC rate constants. These results not only evidence the hypothesis of the direct ππ* → nπ* ISC mechanism, but also contribute to a better understanding of triplet formation in pyrimidines.