The dynamics of excited-state double proton transfer of model DNA base pairs, 7-azaindole dimers, is reported using femtosecond fluorescence spectroscopy. To elucidate the nature of the transfer in the condensed phase, here we examine variation of solvent polarity and viscosity, solute concentration, and isotopic fractionation. The rate of proton transfer is found to be significantly dependent on polarity and on the isotopic composition in the pair. Consistent with a stepwise mechanism, the results support the presence of an ionic intermediate species which forms on the femtosecond time scale and decays to the final tautomeric form on the picosecond time scale. We discuss the results in relation to the molecular motions involved and comment on recent claims of concerted transfer in the condensed phase. The nonconcerted mechanism is in agreement with previous isolated-molecule femtosecond dynamics and is also consistent with the most-recent high-level theoretical study on the same pair.7-azaindole ͉ femtochemistry ͉ reaction dynamics ͉ tautomerization S ince the work by Watson and Crick on the determination of DNA structure in 1953 (1), proton-translocating tautomerization of base pairs has been suggested as a cause of mutations (2). Model systems with similar pair structures can be studied by photoinducing proton transfer (3, 4), and such studies are helpful for understanding the dynamics of mutagenesis (5).In this regard, the dimers of 7-azaindole (7-AI) molecules are prototypical because they are structurally similar to the H-bonded adenine-thymine (A-T) pair with two bonds; the guanine-cytosine (G-C) base pair has three H-bonds. Following the first observation of excited-state double proton transfer in 7-AI by Taylor et al. in 1969 (6), a number of researchers have studied its characterization in the dimers and also of 7-AI monomers catalyzed by various H-bonded counterparts of protic guest molecules (ref. 7 and references therein). One of the fundamental issues, with important implications, is whether the transfer process (8-17) of the two hydrogens proceeds in a concerted manner with a single transition state, or through a stepwise pathway by forming an intermediate species, as shown in Fig. 1.The first report of the dynamics of double proton transfer in 7-AI in the isolated molecule (molecular beam) was published in 1995 (8). The pair was excited with a femtosecond pulse, and another pulse, at different time delays, was invoked to observe the mass spectra. The parent ion showed a biexponential behavior, even near the zero-point energy, and the time constants of both decays changed significantly, by a factor of more than eight, with isotopic substitution. These two time constants (650 fs and 3.3 ps) were found to decrease with the excess excitation energy, as more vibrational modes (18) The mechanism has been discussed in a number of theoretical papers. Ab initio configuration interaction singles calculations showed that the electronic excitation was localized on one moiety in the 7-AI dimer, making the t...