Excited-State Double-Proton Transfer in the 7-Azaindole Dimer in the Gas Phase. Resolution of the Stepwise versus Concerted Mechanism Controversy and a New Paradigm

Abstract

The 7-azaindole dimer (7AI 2 ) is a prototype of double hydrogen-bonded molecules. 7AI 2 has been considered as a model DNA base pair and has attracted much attention to the mechanism of the excited-state double-proton transfer (ESDPT). Two ESDPT mechanisms, stepwise and concerted mechanisms, have been proposed so far. Great efforts have been devoted to clarify the mechanism of ESDPT using experimental and theoretical methods. However, the reaction mechanism had been controversial for more than a decade. We provide the resolution of the two mechanisms on the basis of new data obtained from electronic spectroscopy and picosecond time-resolved spectroscopy in the gas phase. The initial state of the ESDPT reaction has been well characterized by investigating the exciton resonance interaction with UV-UV hole-burning spectroscopy for various 7AI 2 isotopomers. The lowest-excited state of 7AI 2 has been classified into the weak coupling case of the exciton theory. We have concluded that the ESDPT reaction in 7AI 2 occurs via the concerted mechanism on the basis of the results of picosecond time-resolved experiments and the H/D kinetic isotope effect on ESDPT studied by measuring the vibronic-state selective dispersed fluorescence spectra. ESDPT of 7AI 2 has a ''dynamic cooperative'' nature that may arise from the coupling of the two moving protons with the reorganization of electrons. We have provided a new paradigm of ESDPT, where two quantum effects, the exciton resonance interaction and the proton tunneling, are concerned with the ESDPT reaction.Proton-transfer reactions are one of the most basic reactions, and they are important in physics, chemistry, and biology. [1][2][3][4][5][6] Spectroscopic studies on various proton-transfer systems combining with theoretical studies have provided rich information about the hydrogen-bonded structure and proton-transfer dynamics. In particular, a very detailed picture has been obtained for intramolecular single-proton transfer in polyatomic molecules, with laser spectroscopic measurements in the gas phase combining with theoretical studies. For example, a proton transfers by quantum mechanical tunneling mechanism when a potential energy barrier exists on a potential energy surface. The proton tunneling was clearly observed in the electronic and vibrational spectra of several polyatomic molecules. [6][7][8][9][10] Spectroscopic data and theoretical studies showed that the proton tunneling has a multi-dimensional nature, i.e., the motion of the moving proton couples with the coordinates of the heavy atoms that constitute the molecule. Such a multi-dimensional nature and quantum mechanical tunneling are important properties that characterize proton-transfer reactions. Now much effort is devoted to obtain a more complete picture for singleproton transfer reactions and also a new picture for more complicated multiproton-transfer reactions. There are many examples of multiproton-transfer reactions such as proton relay systems in enzymes, in hydrogen-bonded water complexes, and prot...

The answer to concerted versus step-wise controversy for the double proton transfer mechanism of 7-azaindole dimer in solution

“…The doubly hydrogen-bonded dimer of 7-azaindole is a prototypical system showing such a reaction in the photoexcited state. This dimer has served as one of the most central model systems, and hence its photochemistry and photophysics have been extensively studied for more than three decades (4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18). Spectroscopic measurements revealed that the 7-azaindole dimer exhibits a double proton transfer reaction after photoexcitation ( Fig.…”

Dynamics of proton transfer reactions of model base pairs in the ground and excited states: Revisited

“…A c c e p t e d M a n u s c r i p t 24 The profiles of the  ND bands of these dimers exhibit a relatively simple pattern similar to those of the  ND bands of the deuterated normal dimers shown in Figure 11, however, the bandwidths of the former are much wider than those of the normal dimer. As described in Section III-1, the activation energy barrier of the PTR determined is about 2300 cm -1 , which is close to the excitation energy of the  ND mode.…”

“…After these studies, several research groups argued on the mechanism of PTR for more than 6 years. Details of the debates were reviewed by Sakota and Sekiya [24]. Here, we briefly summarize these studies and the kinetic mechanism of the PTR reaction of (7-AI) 2 both in solution and in the gas phase.…”

“…Since then, for more than a decade, several experimental and theoretical groups, including Catalan' group [23], studied this reaction extensively and argued about the reaction mechanism. The details of these debates were reviewed in elsewhere [24]. The outline of these arguments is also presented in the later section together with the recent theoretical works on a reliable potential energy surface, which support the single-step mechanism [25,26].…”

Tautomerization in Condensed Phases and in Isolated Molecules

“…1H‐pyrrolo[3,2‐ h ]quinoline ( 10 ), a bifunctional HB donor‐acceptor molecule, has been investigated in a series of papers 36–45 as a chromophore complementary to 7‐azaindole (7AI). The latter has become a paradigm for excited‐state double‐proton transfer, 46–58 the reaction occurring rapidly in 7AI dimers and much more slowly in complexes with alcohols and water. Both 10 and 7AI are characterized by the presence of the same structural motifs (), the NH proton donor and the pyridine‐type nitrogen acceptor.…”