Excited state dynamics in DNA base monomers: The effects of solvent and chemical modification on ultrafast internal conversion

The excited state dynamics of four of the five naturally occurring nucleic acid bases and several of their derivatives were studied by femtosecond transient absorption spectroscopy. Two tautomers of Adenine were distinguished in solution, with the canonical 9H tautomer having a 1pipi* lifetime of ≈200 fs. The 7H tautomer, that we determined accounts for 22 +/- 4% of the population, has a forty-fold longer lifetime. The 1pipi* lifetimes of 1-, 3-, and 9-methyladenine were found to be the same as the 9H tautomer. The lifetime of 7-methyladenine was ≈4 ps, half the lifetime of the 7H tautomer. Neither the lifetimes nor the relative populations of the tautomers changed drastically with solvent. The effects of protonation on the excited state lifetimes were also studied. These results were interpreted in light of several proposals for the excited state decay mechanism.; A new intermediate state between the excited 1pipi* state and the ground state was observed to be specific to the pyrimidine bases uracil, thymine, cytosine and their derivatives. This state, assigned to the lowest energy 1npi* state, absorbs between ∼300 and 450 nm and displays a lifetime that is sensitive to solvent and varies strongly between pyrimidines, with the lifetime much longer in the nucleotides than in the bases.; UV probe experiments showed that 50 to 90% of the excited population decays via direct internal conversion to the ground state. The remaining 10--50% ends up in the 1npi* state where it either decays directly to the ground state or intersystem crosses to the triplet state. The triplet yield for 1-cyclohexyluracil was measured as a function of solvent. The triplet yield was found to increase as the solvent hydrogen bond donating ability decreased. Triplet state formation was observed to be complete within ∼10 ps. These observations in the pyrimidines lead to a new picture of the excited state dynamics of these molecules in solution.