2362
J . Phys. Chem. 1986, 90, 2362-2366
LASER CHEMISTRY, MOLECULAR DYNAMICS, AND ENERGY TRANSFER Deuterium Isotope Effects of Excited-State and Ground-State Double-Proton-Transfer Processes of the 7-Azaindole H-Bonded Dimer in 3-Methylpentane Kunihiro Tokumura,* Yukari Watanabe, and Michiya Itoh*
Downloaded by UNIV OF NEBRASKA-LINCOLN on September 14, 2015 | http://pubs.acs.org Publication Date: May 1, 1986 | doi: 10.1021/j100402a023
Faculty of Pharmaceutical Sciences, Kanazawa University, Takara-machi, Kanazawa 920, Japan (Received: May 22, 1985; In Final Form: December 9, 1985)
It was confirmed that the excited-state tautomer, generated via photoinduced double proton transfer of the 7-azaindoleH-bonded dimer, relaxes to the unstable (19.0 p s at 298 K; 183 p s at 172 K) ground-state tautomer, which is then converted into the 7-azaindoledimer (starting species) by transferring back two protons in the dark. Thus, a cyclic scheme consisting of excited-state and ground-state double-proton-transfer processes was evidenced. For the dual fluorescence spectra due to the dimer and its phototautomer, a large deuterium isotope effect was apparently observed at 130-200 K. The ground-state doubleproton-transfer process also exhibits a significant deuterium isotope effect, which suggests quantum mechanical tunneling.
Introduction
The excited-state double proton transfer of the 7-azaindole (7AI) H-bonded dimer in 3methylpentane (MP) has first been reported by El-Bayoumi et as a model for the electronic interaction and photomutagenesis in H-bonded base pairs of DNA. Ingham et al. r e p ~ r t e da~temperature-dependent ,~ violet-green = 360 and 480 nm) of a condual fluorescence spectrum (A, centrated M P solution of 7AI a t 130-200 K, and they ascribed the violet and green fluorescence spectra to the excited-state dimer (NO*) and its phototautomer (TO*), respectively, on the basis of the similarity of the green fluorescence spectrum with that of 7-methyl-7H-pyrrolo[2,3b]pyridine. They proposed the excitedstate tautomerization (ND* TD*) passing across a double well potential surface by a thermal process and possibly by tunneling. Tautomerization with a thermal activation barrier was supported by the detection of a much slower rise (=1 ns) of To* fluorescence at 77 K compared to a very rapid rise (lo7 K) were attained by adjusting the flow rate of the cold N2-gas stream. In the transient absorption measurements, the 308-nm (XeCl) pulse of an excimer laser (Lambda Physik EMG 50E) and a steady-state xenon arc beam were used as the excitation and monitoring light sources, respectively. A shutter, operated synchronously with the excimer laser, was used for the latter. Weak transient absorptions were detected by a differential comparator (Tektronix 7A 13). In the two-step laser excitation (TSLE) fluorescence measurements, the 308-nm pulse of an excimer laser and a N2-laser-pumped dye laser (Molectron UV 12 and DL 14) were used as the first (actinic) and the second (probe) laser pulses, respectively. The delay time (
