Ultrafast excited state dynamics of allopurinol, a modified DNA base.

The decay of electronically excited allopurinol riboside was studied through the fluorescence up-conversion technique and high level ab initio calculations. For the allopurinol system with a pyrazolic five-membered ring, we observed an ultrafast decay of the fluorescence signal in water (τ < 0.2 ps), similar to what has been observed for hypoxanthine and inosine (with an imidazolic five-membered ring). These results show that the S(1) dynamics in this type of heterocyclic systems are general and dominated by the distortion in the pyrimidinic six-membered ring with a negligible influence of the rest of the heterocycle. The measurements are consistent with the presence of a highly accessible conical intersection between the S(1) (π-π*) excited state and S(0), as calculated by MR-CIS/CASSCF computations. Our calculations show that the loss of planarity of the six-membered ring is responsible for direct access to the S(1)-S(0) degeneracy region without requiring distortions in the rest of the molecule.