A Theoretical Study of the Cytosine Excimer State: The Role of Geometry Optimization

A study of the lowest excited dimer (excimer) singlet state of the DNA base cytosine was performed with fully optimized geometry (near the cis‐syn form) using a slightly modified version of MOPAC6 (AM1 hamiltonian) and 112 excited configurations involving an active space consisting of the four highest occupied and the two lowest empty MO of the dimer. A binding energy of 3 kcal/mol for the excimer state is demonstrated, and the excimer fluorescence is predicted to be shifted 150 nm to longer wavelengths relative to that of the monomer fluorescence. The excimer geometry has the planes nonparallel (40–45° interplanar angle) and distorted, with the C 5 ‐C 6 bonds showing quite close contact (2.4 Å). Excitation is found to be more localized in the region of the C 5 ‐C 6 bond than expected from the monomer wavefunction. No stable excimer was found if the planes were constrained to be planar, nor if the triplet state was selected. The results suggest that the excimer geometry found in this study may be a precursor to the cis‐syn cyclobutane photodimer.