The fluorescence mechanism of 5-methyl-2-pyrimidinone: an ab initio study of a fluorescent pyrimidine analog.
The photophysically important potential energy surfaces of the fluorescent pyrimidine analog 5-methyl-2-pyrimidinone have been explored using multireference configuration-interaction ab initio methods at three levels of dynamical correlation, all of which support a fluorescence mechanism. At vertical excitation S1 (dark, n(N)pi*) and S2 (bright, pipi*) are almost degenerate at 4.4 eV, with S3 (dark, n(O)pi*) at 5.1 eV. The excited system can follow the S1-S2 seam of conical intersections, accessible from the Franck-Condon region, to its minimum and then evolve from this conical intersection on the S1 (pipi*) surface to a global minimum. At lower levels of correlation, the S1 surface shows two minima separated by a barrier of up to 0.18 eV. The secondary minimum found at the lower levels of correlation becomes the global minimum with higher correlation. The S1 population at this minimum can be trapped from accessing the lowest energy S0-S1 (pipi*/gs) conical intersection by an energy gap at least 0.3-0.4 eV higher than the S1 minimum. The calculated emission energy from this minimum is 2.80 eV. Gradient pathways connecting important S1 geometries are presented, as well as other excited state conical intersections.
Kistler
Kurt A
KA
Department of Chemistry, Temple University, Philadelphia, PA, USA.
Matsika
Spiridoula
S
D005453:Fluorescence; D008958:Models, Molecular; D010777:Photochemistry; D011743:Pyrimidines; D011744:Pyrimidinones / Q000737:chemistry*
D011743:Pyrimidines; D011744:Pyrimidinones; C056905:5-methyl-2-pyrimidinone