Why does 5-mthyl-2-pyrimidinone fluoresce? An ab initio study of the photophysics of a fluorescent DNA pyrimidine analog.

The photophysical properties of gas phase 5-methyl-2-pyrimidinone, a fluorescent DNA/RNA pyrimidine analog, have been calculated using multi-reference configuration-interaction with three levels of dynamical electron correlation included. The bright state at vertical excitation is S$_{2}$ ($\pi \pi $*) at 4.42 eV. An S$_{1}$-S$_{2}$ seam originates close to vertical, and a gradient pathway from this seam leads to a global minimum on the S$_{1}$ surface. A conical intersection between S$_{1}$ and S$_{0}$ (\textit{ci}01) was found, but all levels of theory show that this conical intersection is significantly higher than the minimum, supporting fluorescence from S$_{1}$. The details for this energetic inaccessibility, and the emission energy calculated (2.36 - 2.75 eV), depend on the level of theory used. Pathways along the S$_{1}$ surface when no dynamical or intermediate dynamical correlation is included show a small barrier along with a second minimum in the S$_{1}$ path from vertical to \textit{ci}01. When higher level of correlation is included ($\sim $130,000,000 configurations) the barrier vanishes, the secondary minimum becomes global, the energy of \textit{ci}01 increases, and the fluorescence energy becomes closer to experimental. Thus the importance of dynamical correlation when studying the photophysics of the DNA/RNA bases theoretically is underscored.