Excited-State Double-Proton Transfer in the 7-Azaindole Dimer in the Gas Phase. 3. Reaction Mechanism Studied by Picosecond Time-Resolved REMPI Spectroscopy

Sakota, Kenji; Okabe, Chie; Nishi, Nobuyuki; Sekiya, Hiroshi

doi:10.1021/jp0520805

Cited by 71 publications

(83 citation statements)

References 34 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…A support for this interpretation was given by Castleman and coworkers (20), who detected 7-azaindoleϩH ϩ mass-119 species after photoexcitation of the 7-azaindole dimer in the Coulomb explosion experiment. Recently, this interpretation of the gas-phase experiments was questioned by a series of works by Sakota and Sekiya and their coworkers (39)(40)(41)(42). They measured fluorescence excitation and hole-burning spectra of the jet-cooled 7-azaindole dimer in the S 1 -S 0 region and showed that the bandwidth of each vibronic band is largely different, depending on the vibrational mode (39).…”

Section: Mechanism Of the Double Proton Transfer Reaction Of The 7-azmentioning

confidence: 99%

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

Takeuchi

Tahara

2007

Proc. Natl. Acad. Sci. U.S.A.

146

115

View full text Add to dashboard Cite

The dynamics and mechanism of the double proton transfer reaction of the 7-azaindole dimer was investigated in solution by excitation wavelength dependence in steady-state and femtosecond time-resolved fluorescence spectroscopy. Femtosecond measurements in the UV region revealed that the dynamics of the dimer fluorescence exhibits remarkable change as the excitation wavelength was scanned from 280 to 313 nm. The fluorescence showed a biexponential decay (0.2 and 1.1 ps) with 280-nm excitation, whereas it exhibited a single exponential decay (1.1 ps) with 313-nm excitation (the red-edge of the dimer absorption). This observation clearly indicates that the 0.2-ps component is irrelevant to the proton transfer. In the visible region, we found that the tautomer fluorescence rises in accordance with the decay of the dimer fluorescence with a common time constant of 1.1 ps. This finding unambiguously denies the appearance of any intermediate species in between the dimer and tautomer excited states, indicating that the double proton transfer reaction is essentially a single-step process. We conclude that the double proton transfer of the 7-azaindole dimer in solution proceeds in the concerted manner from the lowest excited state with the 1.1-ps time constant. On the basis of the experimental data obtained, we discuss the long-lasting concerted versus step-wise controversy for the double proton transfer mechanism in solution.femtosecond spectroscopy ͉ fluorescence ͉ ultrafast ͉ photochemistry E xcited-state proton transfer has been receiving a great deal of attention, because it plays crucial roles in photochemistry and photobiology (1-3). 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 (4-18). Spectroscopic measurements revealed that the 7-azaindole dimer exhibits a double proton transfer reaction after photoexcitation (Fig. 1) (4-7, 9, 10). The reactive nature of the 7-azaindole dimer in hydrocarbon solvent is manifested by its steady-state fluorescence spectrum, which shows significantly red-shifted fluorescence attributed to the tautomer fluorescence. The steady-state fluorescence spectra that were examined under various conditions provided information about photochemical properties of the relevant excited states (5, 6, 9, 10). The proton transfer time was first reported as Ͻ5 ps in 1979 by picosecond fluorescence spectroscopy (14), and then it was determined as 1.4 ps by measuring the rise of the tautomer fluorescence with femtosecond time resolution (15).A fundamental question relevant to this ultrafast double proton transfer is the cooperativity of the motion of the two protons. So far, it has been intensively argued whether the two protons move in a concerted or step-wise way. The head-on arguments on this mechanism were first made on the basis of time-resolved data for th...

show abstract

Section: Mechanism Of the Double Proton Transfer Reaction Of The 7-azmentioning

confidence: 99%

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

Takeuchi

Tahara

2007

Proc. Natl. Acad. Sci. U.S.A.

146

115

View full text Add to dashboard Cite

show abstract

“…Very recently, we extensively studied the ESDPT reaction in 7AI 2 in the gas phase with frequency resolved and time-resolved spectroscopy. [42][43][44][45] In this paper, we provide final resolution of the stepwise versus concerted mechanism controversy regarding ESDPT in 7AI 2 . The discrepancies in the previous studies and important problems to be solved are summarized in Chaps.…”

mentioning

confidence: 97%

mentioning

confidence: 99%

“…[18][19][20] The time scale of ESDPT in 7AI 2 ranges from subpicoseconds to picoseconds depending on the excitation energy of the UV light. 37,45 The appearance of strong fluorescence and a moderate time scale of the ESDPT reaction allow us to inves-tigate the ESDPT reaction dynamics in detail with laser spectroscopic methods. We consider that spectroscopic studies on 7AI 2 provide basic information about the dynamics of multiproton-transfer reactions involving quantum mechanical effects such as tunneling and interference in liquids and biological systems as well as in the gas phase.…”

mentioning

confidence: 99%

“…Since then, numerous studies have focused on the mechanisms of ESDPT in 7AI 2 in solution and matrix [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] and in the gas phase [35][36][37][38][39][40][41][42][43][44][45][46][47] by using various spectroscopic techniques. Theoretical studies have been also carried out to investigate the hydrogen-bonded structures, potential energy curves (or surface), and molecular dynamics of 7AI 2 .…”

mentioning

confidence: 99%

See 2 more Smart Citations

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

Sekiya¹,

Sakota²

2006

Bulletin of the Chemical Society of Japan

Self Cite

View full text Add to dashboard Cite

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...

show abstract

Excited‐State Proton Transfer via Hydrogen‐Bonded Dimers and Complexes in Condensed Phase

Hsieh

Jiang

Chou

2010

Hydrogen Bonding and Transfer in the Excited State

View full text Add to dashboard Cite

Proton transfer represents one of the most fundamental processes involved in chemical reactions as well as in living systems [1]. Vast numbers of scientists have been participating in relevant research, leading to thousands upon thousands of publications and numerous books on the topic. Accordingly, various types of proton transfer reaction, depending on, for example, the reaction in the ground or excited state, adiabatic versus non-adiabatic, strong and weak hydrogen bonding, acidity (proton donor) and basicity (proton acceptor), have been identified [2][3][4][5][6][7][8]. At this time, proton transfer reactions seem to have the potential for unlimited extensions and aspects in terms of fundamentals and applications. Thus, to cover the broad spectrum of proton transfer research within a limited number of pages seems to be an unachievable task. Rather than trying to achieve the impossible, this chapter focuses mainly on areas related to excited-state proton transfer (ESPT) [9] with pre-existing intramolecular hydrogen bonds. Under this constraint, the photoinduced process involving excited-state proton dissociation and/or protonation in bulk solvents or clusters, also a currently active topic that has received considerable attention [10][11][12][13][14], unfortunately cannot be addressed in this chapter.One particular focus of this chapter is bifunctional molecules possessing a proton donor group and a proton acceptor group, such that ESPT takes place via self-associated hydrogen-bonded (HB) dimers and/or solvent bridge relay. In spite of numerous explorations and elegant research on guest-molecule-assisted ESPT reactions, the results of which have provided great insight into the fundamentals as well as perspectives in applications,

show abstract

Excited-State Double-Proton Transfer in the 7-Azaindole Dimer in the Gas Phase. 3. Reaction Mechanism Studied by Picosecond Time-Resolved REMPI Spectroscopy

Cited by 71 publications

References 34 publications

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

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

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

Excited‐State Proton Transfer via Hydrogen‐Bonded Dimers and Complexes in Condensed Phase

Contact Info

Product

Resources

About