2P149 Subtituent effect on the excited-state double proton transfer of7-azaindole dimer in gas phase○ Xuan Zhang, Yusuke Komoto, Kenji Sakota, Teruo Shinmyozu,
1112Masayuki Nakagaki,1 Hiroshi Sekiya1
1

Department of Chemistry, Faculty of Sciences, Kyushu University, 2 Institute for Material Chemistry andEngineering, Kyushu University

The excited state double-proton transfer (ESDPT) in a model base pair, 7-azaindole dimer(7AI2), has attracted much attention to its mechanism of reaction during a past decade.
Recently, the ESDPT reactions in the substituted 7AI2 dimers have been extensively studied
in the condensed phase, however the effect of the substitution on ESDPT has not been
understood. The measurement of electronic spectrum in a supersonic free jet expansion
provides information on vibronic-mode specific ESDPT. Previously, we have measured thefluorescence excitation (FE) spectrum of jet-cooled 3-methyl-7-azaindole dimer (3MAI2) andshowed that the methyl substitution has little effect on the ESDPT rate. Here we will report
the electronic spectra of 4-chloro-7-azaindole dimer (4CAI2) in a supersonic jet expansion toinvestigate the effect of Cl substitution on ESDPT. The Cl atom is an electron-withdrawing
group and has a heavy mass. Therefore, the substitution of Cl may provide either electroniceffect or mass effect on ESDPT. Spectroscopic measurement in the gas phase combined withquantum mechanical calculations may allow us to discuss the effect of Cl substitution on theESDPT reaction.
Figure 1 displays the FE spectrum of 4CAI2-hh near the origin. The vibronic pattern in
the FE spectrum is similar to that of 7AI2-hh, suggesting that 4CAI2 undergoes ESDPT. We
noted that the bandwidths in the FE spectrum of 4CAI2-hh are 2.0, 1.3, 3.0 and 5.0 cm−1 forthe origin, intermolecular bending (β1, β2), and stretching (σ) vibrations, which aresignificantly narrower than the corresponding bandwidths of 2.7, 3.0, 10.0 cm−1 for the origin,β, and σ of 7AI2-hh, respectively.

The reduced bandwidth indicates that ESDPT is

suppressed in 4CAI2. Dual fluorescence has been detected in the dispersed fluorescence (DF)
spectrum of 4CAI2-hh excited to the origin (Figure 1). The observation of red-shifted
tautomer fluorescence clearly evidences the occurrence of ESDPT in 4CAI2, whereas theobservation of UV fluorescence of the normal dimer, that is absent in the DF spectrum of7AI2-hh, reveals that the ESDPT rate may be decreased in 4CAI2, in consistent with thenarrower bandwidths in the FE spectrum.

We carried out ab initio and DFT calculations to obtain the geometries, binding energies,
and the energy difference between the normal dimer and the transition state for 4CAI2 andalso 7AI2 for comparison. The geometries of the two dimers in S1 were obtained with CIS
calculations. The results indicate that the geometries of the two normal dimers are very
similar both in the S0 and S1 states, and the potential energy barriers are also similar between
the two dimers, suggesting that the potential curve along the one-dimensional ESDPT
coordinate does not change significantly upon the Cl substitution.

However, the normal

coordinates of β2 and σ (Figure 2), are found to be substantially altered by the substitution ofthe Cl atom. The displacements of the NH stretching occur linearly along the NH···N bond inthe σ mode of 7AI2 and promote ESDPT. But the σ mode of 4CAI2 involves the NH
displacements similar to the β mode of 7AI2. Thus, the reduced ESDPT rate in 4CAI2 isreasonably explained by the multidimensional nature of ESDPT; the motions of the heavy
atoms couple with those of the NH stretching motions due to the introduction of the heavy Cl
atom and change the ESDPT reaction path, which may be responsible for the reduced ESDPTrate in 4CAI2.
4CAI2-hh0−0σβ1

2β1 β1+σβ232200

2σ3σ2β1+σ β1+2σβ2+β1 β2+σ32300
32400−1
Wavenumber / cm

β2+β1+σ β2+2σ32500

Fluorescence Intensity

Fluorescence Intensity

4CAI2-hh0−0 exc.

×1020000

24000
28000
−1
Wavenumber / cm

Figure 1 The FE (left) and DF (right) spectra of 4CAI2-hh

7AI2 β 2

7AI2 σ

4CAI2 β 24CAI2 σFigure 2 The normal coordinates of S1 state of 7AI2-hh and 4CAI2-hh

32000