Dynamics of Photoinduced Processes in Adenine and Thymine Base Pairs

The excited-state dynamics of adenine and thymine dimers and the adenine−thymine base pair were investigated by femtosecond pump−probe ionization spectroscopy with excitation wavelengths of 250−272 nm. The base pairs showed a characteristic ultrafast decay of the initially excited ππ* state to an nπ* state (lifetime τππ* ≈ 100 fs) followed by a slower decay of the latter with τnπ* ≈ 0.9 ps for (adenine)2, τnπ* = 6−9 ps for (thymine)2, and τnπ* ≈ 2.4 ps for the adenine−thymine base pair. In the adenine dimer, a competing decay of the ππ* state via the πσ* state greatly suppressed the nπ* state signals. Similarities of the excited-state decay parameters in the isolated bases and the base pairs suggest an intramonomer relaxation mechanism in the base pairs.