Electronic Relaxation Dynamics of 6-Azauracil: The Effect of Ring Substitution on Intersystem Crossing

In an effort to obtain a fundamental understanding of substitution effects on the photodynamic response of the canonical nucleobases, recent spectroscopic studies have focused on derivatives with single atom substitutions. Uracil undergoes ultrafast internal conversion to the ground state facilitated by various ring distortions at the ethylenic bond. In 6-azauracil, site-specific nitrogen substitution at this carbon double-bond restricts access to ethylenic conical intersections that lead back to the ground state. Instead, intersystem crossing into the triplet manifold becomes highly efficient. This study uses time-resolved photoelectron spectroscopy to investigate the photodynamics of 6-azauracil with particular focus on the role of the lowest singlet excited state and hindrance of ethylenic deactivation coordinates in promoting intersystem crossing.