Femtosecond Intersystem Crossing in the DNA Nucleobase Cytosine

TL;DR

This study reveals that cytosine undergoes ultrafast intersystem crossing within tens of femtoseconds, challenging traditional views and impacting understanding of DNA stability and organic photochemistry.

Contribution

It demonstrates that cytosine exhibits the fastest triplet state population via intersystem crossing, driven by state degeneracy rather than large spin-orbit couplings.

Findings

Cytosine's intersystem crossing occurs in tens of femtoseconds.

Degeneracy between singlet and triplet states enhances ISC efficiency.

Femtosecond ISC challenges conventional assumptions about spin-orbit coupling requirements.

Abstract

Ab initio molecular dynamics including non-adiabatic and spin-orbit couplings on equal footing is used to unravel the deactivation of cytosine after UV light absorption. Intersystem crossing (ISC) is found to compete directly with internal conversion in tens of femtoseconds, thus making cytosine the organic compound with the fastest triplet population calculated so far. It is found that close degeneracy between singlet and triplet states can more than compensate for very small spin-orbit couplings, leading to efficient ISC. The femtosecond nature of the intersystem crossing process highlights its importance in photochemistry and challenges the conventional view that large singlet-triplet couplings are required for an efficient population flow into triplet states. These findings are important to understand DNA photostability and the photochemistry and dynamics of organic molecules in general.