Cytosine in context : A theoretical study of substituent effects on the excitation energies of 2-pyrimidinone derivatives

The ultrafast radiationless decay mechanism for cytosine has been shown to be in part dependent upon high vertical excitation, while slower fluorescence displayed in some cytosine analogs is generally linked to lower vertical excitation energies. To probe how excitation energies relate to pyrimidine structure, substituent effects on the vertical excitation energies for a number of derivatives of 2-pyrimidin-(1H)-one (2P) have been calculated using multireference configuration-interaction ab initio methods. Substitutions using groups with π electron donating, withdrawing and conjugation-extending properties at the C4 and C5 positions on the 2P system give predictive trends for the first three singlet excited-state energies. The S1 ππ* energies of 2P derivatives involving C4 substitution vary linearly with the Hammett substituent parameter σP+. Cytosine is shown to have the highest bright ππ* energy of the 2P derivatives presented, with that energy being strongly dependent on the position, orientation, and ...