Laser spectroscopic study on sinapic acid and its hydrated complex in a cold gas phase molecular beam
Introduction
Sinapic acid (SA), shown in Scheme 1, is one of the hydroxyl cinnamic acids. This molecule is known as an effective MALDI (matrix assisted laser desorption ionization) matrix [1], [2] for protein mass spectroscopy, because it has a large absorption cross section in the UV region and releases a proton. In nature, SA and feluric acid (FA, Scheme 1) exist in the cell wall as cell-wall polymers. The polymers are formed through cross-linking by photochemical reaction or radical coupling reactions [3], [4], [5], [6]. In addition to the role of building blocks of polymer in the cell wall, many attentions are also paid to the photochemical behavior of the derivative of cinnamic acids experimentally [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23] and theoretically [9], [24], [25], [26], [27], [28], [29], [30]. In the electronic ground state (S0), cinnamic acids have the trans (E) conformation along the propenyl double bond. Upon the UV-B irradiation in the 280–315 nm region, the molecules are excited to the S1 (ππ∗) state and return back to either trans (E) or cis (Z) isomer [7], [8], [9], [10], [11], [12], [20]. The relaxation processes are very effective so that almost all the absorbed UV energy is translated finally to thermal energy. Because of this property, cinnamic acids are thought as good candidates for commercial sunscreen reagents [31], [32]. The photochemistry of cinnamic acids and their derivatives in the gas phase has been extensively studied by several groups [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. Buma and coworkers reported the S1-S0 electronic spectra of several para-substituted cinnamates [12], [13], [14], [15]. They found that after the photoexcitation to S1 (ππ∗), the molecule relaxes very rapidly to some transient states having a lifetime of ∼30 ns. They suggested that the transient state is probably the 1nπ∗ state. Zwier and coworkers studied the S1(ππ∗)-S0 electronic spectra of jet-cooled SA, sinapoyl malate (SM) and their ester derivatives [16]. They reported that SM shows a broad structureless absorption spectrum among the species having similar complexity of the structure even under jet-cooled condition. They tentatively described the broadening of the UV spectrum to the coupling between the S1 and 1nπ∗ states. Stavros and coworkers measured the S1-S0 electronic spectrum and excited state lifetime of ethyl ferulate (EF) by ultrafast pump-probe spectroscopy [17]. From the time profile, they proposed a stepwise decay involving S2 (nπ∗) and T1 (3ππ∗) states as the possible decay process. Stavros and coworkers also carried out ultrafast pump-probe spectroscopy for SA and sinapoyl malate (SM) in solution [18]. They observed the transient S1 → Sn absorption spectra after exciting SA at ∼320 nm (∼31300 cm−1). The transient absorption showed multi-exponential decay described by three time constants. The three constants were attributed to the stepwise processes involving vibrational relaxation (τ1 = ∼100 fs) within S1 (1ππ∗), internal conversion (IC) to the 21ππ∗ state via S1/21ππ∗ conical intersection (CI) (τ2 = ∼1 ps), and IC to S0 (τ3 = 12–22 ps), which is followed by isomerization. Ebata and coworkers also have been studying the nonradiative decay (NRD) route of the derivatives of methylcinnamate (MC) in the gas phase [19], [20], [21], [22], [23]. Very recently, they reported the NRD leading to the isomerization of para-methoxy methylcinnamate (p-MMC) [22] and para-hydroxy methylcinnamate (p-HMC) [23] occurs via [trans-S1 → 1nπ∗ → T1 → cis-S0] stepwise scheme, while in ortho- and meta-hydroxy methylcinnamate (o-, m-HMC), the isomerization occurs via [trans-S1 → twisting along the C = C double bond to 90° on S1 → cis-S0] scheme [23].
Though the NRD process of bare cinnamic acids and cinnamates has been extensively studied so far, little is known about the effect of hydration. Most of NRD initiated from the UV excitation to the S1 (ππ∗) state involves the internal conversion (IC) to 1nπ∗ excited state via conical intersection (CI), and this state is strongly affected by the environment such as hydrogen (H)-bonding. For example, Ebata et al. reported that H-bonding of a water to the OH group of p-HMC elongates the S1 lifetime by two orders of magnitudes [19], while the H-bonding of the water to the carbonyl oxygen of p-MMC shortens the S1 lifetime, suggesting the above two NRD channels are competing in these molecules [20].
In the present study, we report the electronic and vibrational spectra of SA and its complex with water under the cold gas phase condition. Since cinnamic acids and their derivatives are nonvolatile, either thermal heating or laser ablation is necessary to vaporize and introduce in a supersonic beam. However, upon heating cinnamic acids easily decompose by thermal decarboxylation [33]. Zwier and coworkers successfully measured the S1-S0 electronic spectrum of jet-cooled SA by using laser ablation technique [16]. However, none of the electronic spectrum of the hydrated complex of SA has been reported. It is of great interest to investigate the hydrated structure of SA and the effect of the hydration on its photochemical process including trans-cis isomerization. In the present study we measure the UV spectra of SA and SA-H2O 1:1 complex by resonance enhanced two-photon ionization (R2PI) and the infrared spectra by IR-UV double resonance (DR) spectroscopy. By using a newly developed laser ablation combined with a channel nozzle supersonic beam setup, well resolved R2PI spectra of SA and SA-H2O complex were obtained. The structures of SA and SA-H2O 1:1 are discussed by the comparison of the observed UV and IR spectra with those of calculated ones at DFT level. The S1 lifetime of SA is also measured by picosecond pump-probe spectroscopy, and the obtained lifetime is compared with those of other cinnamate derivatives.
Section snippets
Experimental and computational
Fig. 1 shows a schematic experimental setup. The mixture of solid powder of SA and carbon black is put on the side of a graphite disk with a diameter of 80 mm. The disk is attached at the side of a channel nozzle and is rotated by a stepping motor. The channel nozzle is beak-shaped and an ablation laser beam (1.064 μm) passes through the slit of the channel nozzle and irradiates the sample powder on the graphite disk to vaporize. The pulsed Ar carrier gas at a backing pressure of 15 atm is
S1-S0. electronic spectra
Fig. 2(a) and (b) show the LIF and R2PI spectrum of the S1-S0 transition of jet-cooled SA. The R2PI spectrum of SA is previously reported by Zwier and coworkers [16], and the present spectrum is essentially the same with their spectrum. On the other hand, to the best of our knowledge, the LIF detection is the first report of the jet-cooled spectrum by combining laser ablation method. The signal-to-noise ratio of the LIF spectrum as good as R2PI detection. According to the study of Zwier and
Acknowledgements
T. E. acknowledges the financial support from the Institute for Quantum Chemical Exploration. K. Y. is grateful for the financial supports from Building of Consortia for the Development of Human Resources in Science and Technology, MEXT. This work was partly supported by the JSPS KAKENHI Grant Number JP16H04098 (Y. I.). Part of the calculations in this paper was carried out by using a supercomputer at Okazaki Research Facilities (Research Center for Computational Science).
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