Presented here are femtosecond pump-probe studies on the watersolvated 7-azaindole dimer, a model DNA base pair. In particular, studies are presented that further elucidate the nature of the reactive and nonreactive dimers and also provide new insights establishing that the excited state double-proton transfer in the dimer occurs in a stepwise rather than a concerted manner. A major question addressed is whether the incorporation of a water molecule with the dimer results in the formation of species that are unable to undergo excited state double-proton transfer, as suggested by a recent study reported in the literature [Nakajima, A., Hirano, M., Hasumi, R., Kaya, K., Watanabe, H., Carter, C. C., Williamson, J. M. & Miller, T. (1997) J. Phys. Chem. 101, 392-398]. In contrast to this earlier work, our present findings reveal that both reactive and nonreactive dimers can coexist in the molecular beam under the same experimental conditions and definitively show that the clustering of water does not induce the formation of the nonreactive dimer. Rather, when present with a species already determined to be a nonreactive dimer, the addition of water can actually facilitate the occurrence of the proton transfer reaction. Furthermore, on attaining a critical hydration number, the data for the nonreactive dimer suggest a solvation-induced conformational structure change leading to proton transfer on the photoexcited half of the 7-azaindole dimer. C luster science studies have long been utilized to yield unique perspectives of microscopic properties related to the bulk condensed phase (1). This approach involves the investigation of a broad spectrum of clusters ranging from isolated species to the study of fully solvated species, thus illustrating the progression from the gas phase to the condensed phase. In reference to our specific experiments as an example, we have been able to study the excited state double-proton transfer (ESDPT) of the 7-azaindole (7-Aza) dimer under conditions ranging from an isolated dimer to a state of solvation where the hydrogen-bonded dimer is clustered with as many as nine water molecules. As the number of water molecules on the nonreactive dimer increases, we begin to see evidence that the dimer molecule is behaving more as it would in a fully solvated condensed-phase environment. In support of these findings, it has been reported that clusters with as little as six waters begin to show liquid-like properties (2-4).The model DNA base pair 7-Aza has proven to be an interesting and enlightening species for study in both the gas and condensed phases. Of utmost interest is the double-proton transfer that the 7-Aza dimer undergoes on excitation to the S 1 state. This ESDPT was first observed in solution by Kasha and coworkers (5). Later, Kaya and coworkers performed extensive supersonic jet spectroscopic studies on the 7-Aza monomer, dimer, and solvated forms of these species (6-9). The first direct determination of the rates of the double-proton transfer was made in the gas phase by Zewail and c...
