Kenji Sakota, Akihiko Hara and Hiroshi Sekiya*
Department of Chemistry, Faculty of Sciences, Graduate School of Molecular Chemistry, Faculty of Science, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka, 812-8581, Japan. E-mail: hsekiscc@mbox.nc.kyushu-u.ac.jp
First published on 27th November 2003
Fluorescence excitation and hole-burning spectra have been measured for deuterated 7–azaindole dimers [(7AI)2] in a free jet in order to investigate their excited-state double-proton transfer (ESDPT) dynamics. Only one transition system is observed in the S1–S0 region of the excitation spectrum of (7AI)2-dd, where two hydrogen atoms of the NH groups are deuterated. Two transition systems are observed in the spectrum of (7AI)2-hd in which one of the hydrogen atom of the NH groups is deuterated. The two systems have been ascribed to the S1–S0 transitions of (7AI)2-h*d and (7AI)2-hd*. In these molecules one monomer moiety, 7AI-h or 7AI-d, is excited in the S1 state. The separation of the two electronic origins has been determined to be 21 cm−1. In contrast to (7AI)2-hd, two monomer moieties must be simultaneously excited in the S1(1Bu) states of (7AI)2-dd and (7AI)2-hh. These findings can be consistently explained by considering that (7AI)2-dd and (7AI)2-hh in the S1 state have C2h symmetry, whereas (7AI)2-h*d and (7AI)2-hd* have Cs symmetry. The bandwidth for one quantum of the intermolecular stretching vibration of (7AI)2–h*d (4.1 cm−1) in the excitation spectrum is greater than 2.4 cm−1 for the stretching vibration of (7AI)2-hd*, indicating that the rate of the ESDPT reaction depends significantly on the excited site. These results support a concerted mechanism for proton transfer in (7AI)2-dd and (7AI)2-hh. We will discuss the reason for the observation of bi-exponential decays detected by photo-excitation of vibronic bands of (7AI)2 in a molecular beam with a femtoseond laser (A. Douhal, S. K. Kim and A. H. Zewail, Nature, 1995, 378, 260) on the basis of the symmetry of the 7AI dimers and vibrational mode-specific proton transfer.
Recent studies of (7AI)2 have concentrated on the ESDPT mechanism, i.e., whether the two protons move simultaneously (concerted mechanism) or stepwise (sequential mechanism). In Scheme 1 a concerted double proton transfer (DPT) and a sequential single-proton transfer (SPT) is shown. Douhal et al. observed femtosecond transients of the (7AI)2 ions and also the ions of deuterated species in the gas phase.8 Bi-exponential decays with two time constants, 200∼650 fs and 1.6–3.3 ps, were observed as a function of the vibrational excess energy in the region ΔE=0–1.5 kcal mol−1.8 The fast decay component was attributed to proton transfer from the initial state to an intermediate state, and the slow component to proton transfer from the intermediate state to the tautomer. Similar bi-exponential decays were observed in deuterated (7AI)2-hd and (7AI)2-dd, where hd indicates that one 7AI moiety is N-deuterated, while dd indicates that both moieties are N-deuterated. On the basis of these results, a sequential mechanism was proposed for the ESDPT reaction. Essentially the same conclusion was obtained from a femtosecond time-resolved Coulomb explosion study.13
Scheme 1 |
The mechanism of ESDPT proposed from time-resolved study of (7AI)2 in solution is inconsistent with that in the gas phase. Two groups observed bi-exponential decay profiles with time constants of ∼0.2 ps and ∼1.1 ps by photo-excitation of (7AI)2 in solution.12,14,17 Fiebig et al.17 ascribed the two decay constants to the sequential mechanism. However, Takeuchi and Tahara12c measured decay profiles from higher energy region to the red-edge of the dimer absorption (218–313 nm). They showed that the fast component is due to internal conversion from the 1Lb-type state to the 1La-type state, while the slow component is due to the simultaneous double-proton transfer reaction. This assignment was confirmed by observing a single exponential decay profile on exciting the red-edge of the absorption. The results of Takeuchi and Tahara supported the concerted mechanism.12
The geometry of undeuterated (7AI)2-hh in the electronic excited state and the potential energy surfaces have been calculated to investigate the mechanism of ESDPT.9,16,21–23 Douhal et al. carried out CIS geometry optimization and showed that the most stable structure in the S1 state has Cs symmetry, and that one monomer moiety is excited in the S1 state.21 The locally excited nature of (7AI)2-hh is suggested to be important for the sequential proton transfer. On the other hand, Catalan et al. proposed that (7AI)2-hh has C2h symmetry, therefore two 7AI moieties must be simultaneously excited.22 In contrast to the Cs-symmetry potential energy surface obtained by Douhal et al.21 no local minimum exists on the potential energy surface obtained from CIS calculations by Catalan et al.22 Thus, theoretical results suggest that the investigation of the symmetry of the excited state is crucial to understand the ESDPT dynamics of (7AI)2.
In the present work, we have measured the electronic spectra of deuterated (7AI)2. The vibronic transitions of deuterated species are superimposed with those of (7AI)2-hh. The application of UV–UV hole-burning spectroscopy enabled us to separate the transitions of different species. We have determined the symmetry of the S1 states of the three isotopomers (7AI)2-dd, (7AI)2-hh, and (7AI)2-hd. It has been found that the ESDPT rate depends significantly on the excited site of (7AI)2-hd, in which one of the two monomer moieties, 7AI-h or 7AI-d, could be selectively excited. We will provide a new interpretation for the bi-exponential decay behavior detected by Douhal et al.8 on the basis of the vibrationally resolved hole-burning spectra. The results in the present work are consistent with the concerted mechanism for (7AI)2–dd and (7AI)2–hh.
Fig. 1 Fluorescence excitation spectrum of a mixture of (7AI)2-hh marked with the open squares and deuterated dimers in a free jet obtained by monitoring visible emission (a). The vibronic bands marked with the open-circles are due to the bands of the 7AI dimers which have C–D bond(s). UV–UV hole-burning spectra measured by probing the vibronic band at 32293 cm−1 (b) and the band at 32472 cm−1 (c). The positions of these bands are indicated with the arrows in Fig. 1a. |
Fig. 2 Hole-burning spectrum of deuterated (7AI)2 obtained by probing the vibronic band at 32472 cm−1. The spectrum has been assigned to (7AI)2-dd (see text). |
An enlarged feature of the hole-burning spectrum in Fig. 1c is shown in Fig. 2. The vibronic pattern in the hole-burning spectrum in Fig. 2 is very similar to that in the excitation spectrum of (7AI)2-hh,5 although the bandwidths for vibronic bands in Fig. 2 are much narrower than those for (7AI)2–hh. By analogy with the excitation spectrum of (7AI)2-hh, the 98 cm−1 and 128 cm−1 vibrations are assigned to the intermolecular bending (β) and stretching (σ) fundamentals, respectively. The spectrum consists of a progression of the intermolecular stretching vibration and a progression of the combination bands due to the intermolecular stretching and bending vibrations.
Fig. 3 shows the hole-burning spectrum measured by probing the band at 32293 cm−1 that is marked with an arrow in Fig. 1a. Fig. 3 is an enlarged feature of the hole-burning spectrum in Fig. 1b. The vibronic pattern in the hole-burning spectrum in Fig. 3 is apparently very different from that in Fig. 2. The lowest frequency band (A) at 32293 cm−1 is assigned to the electronic origin of the S1–S0 transition. A prominent feature in the spectrum is the observation of a transition at 32314 cm−1 (B), which is located only 21 cm−1 above band A. It is clear that the two transition systems are simultaneously observed in the hole-burning spectrum. The intermolecular bending (β) fundamentals are the same (96 cm−1) for the two systems within the resolution of the spectrum, while those of the intermolecular stretching (σ) (121 and 123 cm−1) are slightly different between the two systems. The vibrational assignment for the two systems is summarized in Table 1 together with the bandwidths measured from the fluorescence excitation spectrum. The excitation spectrum of Fig. 1a is shown in Fig. 4 on an enlarged scale to distinguish the band positions and the widths of vibronic bands of undeuterated and deuterated species. Previously, the bands at 32293 cm−1 and 32314 cm−1 labeled A and B were assigned to the S1–S0 origins of (7AI)2-hd and (7AI)2-dd, respectively.5b However, the hole-burning spectrum in Fig. 3 clearly shows that these two bands originate from the same electronic ground state.
Fig. 3 Hole-burning spectrum of a deuterated (7AI)2 obtained by probing the vibronic band at 32293 cm−1. The spectrum has been ascribed to (7AI)2-hd (see the text). |
Fig. 4 Fluorescence excitation spectrum of a mixture of (7AI)2-hd, (7AI)2–dd, and (7AI)2-hh near the electronic origins. The assignment of the intermolecular vibrations of (7AI)2-hd is given in the figure. The vibronic bands of (7AI)2-hh and (7AI)2-dd are indicated with open squares and solid circles, respectively. The electronic origin of (7AI)2-dd at 32348 cm−1 overlaps with the bending fundamental of (7AI)2-hh at 32350 cm−1. |
Species | Δν/cm−1a | Bandwidth/cm−1 | Assignmentd |
---|---|---|---|
a Relative wavenumber from the electronic origin. The number in the parentheses indicates the wavenumber of the electronic origin.b Bandwidth was not determined due to overlapping of bands.c Ref. 5(b).d β and σ denote the intermolecular bending and stretching vibrations, respectively. | |||
(7AI)2-dd | 0 (32348) | b | Origin |
98 | b | β | |
124 | 2.6 | σ | |
222 | 1.7 | β+σ | |
247 | 2.6 | 2σ | |
344 | 1.7 | β+2σ | |
368 | 5.1 | 3σ | |
465 | 2.6 | β+3σ | |
487 | 5.1 | 4σ | |
(7AI)2-h*d | 0 (32293) | 1.6 | Origin |
96 | 1.7 | β | |
121 | 4.1 | σ | |
216 | 2.8 | β+σ | |
239 | b | 2σ | |
(7AI)2-hd* | 0 (32314) | 1.6 | Origin |
96 | b | β | |
123 | 2.4 | σ | |
243 | 5.2 | 2σ | |
(7AI)2-hhc | 0 (32252) | 5 | Origin |
98 | 3 | β | |
120 | 10 | σ | |
215 | 7 | β+σ | |
240 | 3 | 2σ |
It is reasonable to assign the species observed in Figs. 2 and 3 to (7AI)2-dd and (7AI)2-hd, respectively, on the basis of the following reasons; (i) The relative blue shift of the origin band of (7AI)2-hd may be smaller than that of (7AI)2–dd, (ii) the bandwidth for the same vibrational mode may be broader for (7AI)2–hd than for (7AI)2–dd due to a smaller tunneling reduced mass for (7AI)2–hd, and (iii) two transition systems are observed in Fig. 3, whereas only one system is observed in Fig. 2 as well as in the excitation spectrum of (7AI)2–hh. The reason for the observation of the two systems in the spectrum of (7AI)2–hd will be discussed in the following section.
Ψ0=|g1〉|g2〉 | (1) |
(2) |
(7AI)2-hd in the S1 state may have Cs symmetry, and two A′ states of (7AI)2-hd correlate to the Ag and Bu states of (7AI)2–hh and (7AI)2-dd. The electronic transitions into the two A′ states become allowed for one-photon absorption due to a lowering of the symmetry. This argument is consistent with the observation of the two transition systems in Fig. 3. Each monomer unit could be distinguished in (7AI)2–hd, and bands A and B should correspond to the transitions into the zero-point levels of the locally excited S1 states of (7AI)2-h*d and (7AI)2-hd*. This assignment is based on the consideration that the blue-shift of the electronic origin band of (7AI)2-hd* relative to that of (7AI)2-hh may be larger than that of (7AI)2-h*d. Thus, nature of the lowest excited state of (7AI)2-hd is quite different from those of (7AI)2-hh and (7AI)2-dd. A similar result has been obtained from the study of the 2-pyridone dimer.31,32,34
Two groups have calculated the excited-state geometry of (7AI)2-hh and the potential energy surface along the proton transfer coordinate.21,22 Catalan et al. treated the symmetry of (7AI)2 as C2h, and showed that no local minimum exists along the reaction coordinate.22 In contrast, Moreno et al. concluded that the excited-state geometry of (7AI)2 is Cs from the optimization of the stable structure.21 They obtained a local minimum suggesting the existence of an intermediate state, which is responsible for the sequential mechanism. The barrier height along the proton transfer coordinate remarkably depends on the symmetry of the excited state. The calculated barrier heights are ca. 8 and 17 kcal mol−1 for the C2h and Cs symmetry groups, respectively.21,22 Thus, controversial results have been reported about the symmetry of (7AI)2. The experimental results in this work support the C2h symmetry proposed by Catalan et al.22
In the spectrum of (7AI)2-hd, two systems have been observed. These systems have been ascribed to transitions into the S1 state of (7AI)2-h*d and (7AI)2-hd*. The widths of the electronic origin bands for (7AI)2-h*d and (7AI)2-hd* are the same (1.6 cm−1) within experimental error. The excitation of the stretching vibration (σ) increases the bandwidth for both (7AI)2-h*d and (7AI)2-hd*. It should be noted that the bandwidth for (7AI)2-h*d (4.1 cm−1) is significantly larger than that (2.4 cm−1) for (7AI)2-hd*. The difference in the bandwidth between (7AI)2-h*d and (7AI)2-hd* may originate from the localization of excitation on the monomer site, therefore the mechanism of ESDPT in (7AI)2-h*d should be different from that in (7AI)2-hh and (7AI)2-dd.
On the other hand, the widths of vibronic bands in the excitation spectrum of this work provide information on the ESDPT dynamics. For a vibrational-mode-specific ESDPT reaction occuring in (7AI)2, the decay time should depend on the excited vibrational state.5 The excitation spectra measured in this work indicate that closely spaced vibronic states have comparable Franck–Condon factors for one-photon absorption. For example, in the spectrum of (7AI)2-dd two closely spaced vibronic bands assigned to β+2σ and 3σ are detected at Δν(ν−ν00)=344 and 368 cm−1, for which the bandwidths are 1.7 and 5.1 cm−1, respectively. When these two bands are simultaneously excited with a femtosecond pump laser the observed decay profile may be fitted by a bi-exponential function. In the femtosecond transients of (7AI)2-dd excited at ΔE=1.0 kcal mol−1 (about ΔE=350 cm−1), bi-exponential decays with time constants of 3 ps and 25 ps were detected. In our experiment the bandwidth of the pump laser should be narrow enough to resolve the β+2σ and 3σ vibrations, which are separated by 24 cm−1 in the spectrum of (7AI)2-dd. However, the bandwidth of the pump laser used for the measurement of the femtosecond transients of (7AI)2 might be much larger than that needed for the excitation of a single vibronic band, since the cross correlation of the pump and probe pulses was reported to be ∼150 fs.8 Therefore, the pump laser might excite more than two vibronic bands in the experiment at ΔE=0, 1.0, and 1.5 kcal mol−1. When three or four bands are simultaneously excited the resulting decay profile is a multi-exponential function. The fast and slow components in the decay profile originate from the broad stretching and the narrow bending modes. Consequently, even for a multi-exponential decay an approximately bi-exponential decay profile may be obtained. Thus, we suggest that the bi-exponential decays detected by the femtosecond transients of (7AI)2 ions are caused by simultaneous excitation of vibronic states.
On the basis of the above discussion of the decay profiles and vibrational mode-specific proton transfer together with the symmetry of (7AI)2, this work supports the concerted mechanism for (7AI)2-hh and (7AI)2-dd. However, the mechanism for (7AI)2-hd should be different; A sequential ESDPT reaction may occur in (7AI)2-hd due to photo-excitation of the locally excited state. An experiment to measure picosecond transients in the gas phase is in progress in order to obtain the decay profiles when a single vibronic state is excited.
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