On the possibility of excimer state formation in homodimers of the pyrimidine bases.

A detailed investigation of the ground state and the first excited state of the pyrimidine bases of uracil, cytosine, thymine and their stacked homodimers is carried out by the Extended Hückel method. The first exicted state is shown to be the (see article)-state. The excitation is localized mainly on the C5-C6 bond region. The energy dependence of the ground state and the first excited state of the homodimers on the mutual position of bases is considered. For every dimer a minimum on the exicted state potential curve is found. This fact creates the possibility for the formation of excimer. The obtained data give some reason for the suggestion about participation of the excimer state as a precursor of photodimer.