Chemiluminescence of Coelenterazine and Fluorescence of Coelenteramide: A Systematic Theoretical Study

A systematic investigation of the structural and spectroscopic properties of coelenteramide has been performed at the TD-CAM-B3LYP/6-31+G(d,p) level of theory, including various fluorescence and chemiluminescence states. The influence of geometric conformations, solvent polarity, protonation state, and the covalent character of the O-H bond of the hydroxyphenyl moiety were carefully studied. Striking differences in geometries and electronic structures among the states responsible for light emission were characterized. All fluorescence states can be described as a limited charge transfer process for a planar amide moiety. However, the chemiluminescence state is characterized by a much larger charge transfer that takes place over a longer distance. Moreover, the chemiluminescent coelenteramide structure exhibits an amide moiety that is no longer planar, in agreement with recent, more accurate ab initio results [Roca-Sanjuán et al. J. Chem. Theory Comput.2011, 7, 4060]. Because the chemiluminescence state appears to be completely dark, a new mechanism is tentatively introduced for this process.