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The Fluorescence Mechanism of 5‐Methyl‐2‐Pyrimidinone: An Ab Initio Study of a Fluorescent Pyrimidine Analog †
Author(s) -
Kistler Kurt A.,
Matsika Spiridoula
Publication year - 2006
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1562/2006-04-03-ra-866
Subject(s) - conical intersection , excited state , ab initio , fluorescence , chemistry , potential energy surface , potential energy , atomic physics , configuration interaction , degenerate energy levels , conical surface , excitation , molecular physics , physics , materials science , optics , quantum mechanics , organic chemistry , composite material
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 S 1 (dark, n N π*) and S 2 (bright, ππ*) are almost degenerate at 4.4 eV, with S 3 (dark, n O π*) at 5.1 eV. The excited system can follow the S 1 –S 2 seam of conical intersections, accessible from the Franck–Condon region, to its minimum and then evolve from this conical intersection on the S 1 (ππ*) surface to a global minimum. At lower levels of correlation, the S 1 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 S 1 population at this minimum can be trapped from accessing the lowest energy S 0 –S 1 (ππ*/gs) conical intersection by an energy gap at least 0.3–0.4 eV higher than the S 1 minimum. The calculated emission energy from this minimum is 2.80 eV. Gradient pathways connecting important S 1 geometries are presented, as well as other excited state conical intersections.