Overruling the energy gap law: fast triplet formation in 6-azauracil

The photophysical properties of 6-azauracil were studied by means of ab initio quantum chemical methods. On the basis of our calculations we propose here the following mechanism for the lack of fluorescence and the high triplet quantum yield that was observed experimentally after irradiation of this compound with UV light [Kobayashi et al., J. Phys. Chem. A, 2008, 112, 13308]. Multiple potential energy surface crossings between excited singlet states of pi -> pi* and n -> pi* character lead to an ultrafast transfer of the S-2 ((1)pi -> pi*) population to the lower-lying S-1 ((1)n -> pi*) state. This state acts as a doorway state from which the T-1 ((3)pi -> pi*) state is formed approximately within 125 ps in the isolated 6-azauracil and within 30 ps in acetonitrile solution according to our calculations. The enhancement of the S-1 (sic) T-1 intersystem crossing in acetonitrile solution is noteworthy as it goes along with an increased adiabatic energy gap between the interacting states. Blue shift of the S-1 potential energy surface by about 0.2 eV in this polar, aprotic environment places the intersection between the S-1 and T-1 potentials close to the S-1 minimum, thus increasing the overlap of the vibrational wavefunctions and consequently speeding up the spin-forbidden nonradiative transition.