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

Abstract

The high photostability of DNA/RNA nucleobases is attributed to the effective internal conversions of their bright 1ππ* 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 1nπ* state to the lowest triplet 3ππ* state. However, recent studies showed that ISC from the initial populated bright 1ππ* state to higher energy triplet 3nπ* 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 1nπ* state in pyrimidinones, resulting in direct ISC from the S1 (1ππ*) state to triplet T3 (3nπ*) state with much higher QYs. Meanwhile, hydrogen bonding between the carbonyl group in pyrimidinones and protic solvents can accelerate vibrational cooling of the hot S1 (1ππ*) state, leading to higher fluorescence QYs and smaller ISC rate constants. These results not only evidence the hypothesis of the direct 1ππ* → 3nπ* ISC mechanism, but also contribute to a better understanding of triplet formation in pyrimidines.

A combined experimental and computational study on the deactivation of a photo-excited 2,2′-pyridylbenzimidazole–water complex via excited-state proton transfer

“…The nucleobase in DNA and RNA is a perfect example illustrating the intrinsic properties of biomolecules to effectively quench the dissociation channels by fast internal conversion to an electronic ground state and thus protecting the genetic information. [6][7][8] Most of the biomolecules exhibiting excited-state deactivation channels have an ultrashort excited-state lifetime. The process involves proton transfer, followed by internal conversion.…”

“…The nucleobase in DNA and RNA is a perfect example illustrating the intrinsic properties of biomolecules to effectively quench the dissociation channels by fast internal conversion to an electronic ground state and thus protecting the genetic information. 6–8…”

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

“…Therefore, 5fdCyd has the potential to be an internal triplet photosensitizer which can initiate DNA photolesions. Although it has been reported that ISC in DNA nucleosides usually takes place from the singlet nπ* state to the lowest triplet ππ* state, [9] it was recently argued that ultrafast ISC can also occur from the initially populated bright 1 ππ* state to a higher energy triplet nπ* state, [10] and this mechanism may also exist in 5fdCyd. Yet, a full understanding of the ISC mechanism of 5fdCyd in solution is still missing.…”

Boosting Ultrafast Trans‐Cis Photoisomerization and Intersystem Crossing in Nanocrystals of Double‐Bond Photoswitching Molecules

“…For one thing, the NBA NCS meets two requirements for the possibility of ISC: a small energy gap between singlet and triplet state (Δ E ST ) and a large enough spin‐orbital coupling (SOC) value. [ 58 ] Therefore, vertical excitation energy levels and SOC values were calculated for AB, TS, and NBA in the solution phase (DFT optimized structures with the SMD solvation model) and crystal (structures extracted from the SC‐XRD). From Table S1 (Supporting Information), for NBA in MeCN, although the Δ E ST between S 1 →T 3 is the smallest (0.342 eV), the SOC value (1.854 cm −1 ) is limited.…”