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The 7-azaindole-methanol 1:2 cluster [7AI(CH(3)OH)2] undergoes excited-state triple-proton/hydrogen atom transfer (ESTPT/HT) along the hydrogen-bonded network in the gas phase. The measurements of the resonance-enhanced multiphoton ionization (REMPI) spectra of 7AI(CH(3)OH)2-d(n) (n = 0-3), where subscript n indicates the number of deuterium, and the fluorescence excitation spectrum of 7AI(CH(3)OH)2-d(0) allowed us to investigate the ESTPT/HT dynamics. By comparing the intensity ratios of the vibronic bands between 7AI(CH(3)OH)2-d(0) and 7AI(CH(3)OH)2-d(3) in REMPI spectra, we obtained the lower limit of an acceleration factor (f(a)(low)) of 7AI(CH(3)OH)2-d(0), which is the ratio of the reaction rate for the excitation of a vibronic state to that of the zero-point state in S(1). The f(a)(low) values are 2.7 +/- 0.83 and 4.0 +/- 1.2 for an in-phase intermolecular stretching vibration (sigma(1)) and its overtone (2sigma(1)) observed at 181 cm(-1) and 359 cm(-1) in the excitation spectrum, respectively, while that of the vibration (nu(2)/sigma(1) or nu(3)/sigma(1)) at 228 cm(-1) is 1.1 +/- 0.83. Thus, vibrational-mode-specific ESTPT/HT occurs in the low-energy region (600 cm(-1)). The excitation of an intramolecular ring mode (nu(intra)) of 7AI at 744 cm(-1) substantially enhances the reaction rate (f(a)(low) = 4.4 +/- 0.98), but the increase of f(a)(low) is not prominent for the excitation of v(intra) + sigma(1) at 926 cm(-1) (f(a)(low) = 5.0 +/- 1.6), although the sigma(1) mode is excited. These results suggest that the ESTPT/HT reaction in 7AI(CH(3)OH)2-d(0) directly proceeds from the photoexcited states with the internal energy less than approximately 600 cm(-1), but it occurs from the isoenergetically vibrational-energy redistributed states when the internal energy is large. This shows a remarkable feature of ESTPT/HT in 7AI(CH(3)OH)2; the nature of the reaction mechanism changes from vibrational-mode specific to statistical fashion with increasing the internal energy. The hydrogen-bonded network in 7AI(CH(3)OH)2-d(0) is represented by a directed graph. This shows that ESTPT/HT is one of the simplest examples of cooperative phenomena.

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The fluorescence excitation spectra are recorded for jet-cooled dual hydrogen-bonded 3-methyl-7-azaindole dimer (3MAI)2-hh and deuterated dimers (3MAI)2-hd and (3MAI)2-dd near the electronic origin region of the S1−S0 transition, where hd and dd indicate the deuteration of an imino hydrogen and two imino hydrogens, respectively. A single origin is detected in the spectra of (3MAI)2-hh and (3MAI)2-dd, whereas two electronic origins separated by 13 cm-1 are detected in the spectrum of (3MAI)2-hd. The excited-state double-proton transfer (ESDPT) occurs in (3MAI)2-hh, while (3MAI)2-hd and (3MAI)2-dd undergo excited-state proton/deuteron transfer and excited-state double deuteron transfer, respectively. In (3MAI)2-hd, the excitation is localized on either the 3MAI-h or 3MAI-d moiety. The localization of the excitation is explained by a weak coupling of the excitonic states of (3MAI)2-hh and (3MAI)2-dd. The effective symmetry of the lowest excited state of (3MAI)2-hh and (3MAI)2-dd belongs to the C 2 h point group, while that of (3MAI)2-hd belongs to the C s point group. The vibronic patterns in the excitation spectra of the (3MAI)2 dimers is very similar to those of the 7-azaindole dimers, indicating that the methyl substitution provides little effect on the shape of the ESDPT potential.