Laser spectroscopic study on sinapic acid and its hydrated complex in a cold gas phase molecular beam

The S₁ (ππ^∗)-S₀ electronic spectrum and the infrared spectrum in the OH stretching region of sinapic acid (SA) and its 1:1 complex with water (SA-H₂O) are observed in a supersonically cooled molecular beam. Supersonically-cooled SA and SA-H₂O complex are obtained by a combination of laser ablation with a newly developed pulsed channel nozzle. Both SA and SA-H₂O exhibit sharp electronic spectra, and measurements of UV-UV hole-burning spectra and time-dependent density functional theory (TD-DFT) calculation indicate that the observed vibronic bands belong to two conformers, syn and anti, in both SA and the SA-H₂O complex. The OH stretching vibrations are observed with IR-UV double resonance spectroscopy. A comparison of the observed IR spectrum and the DFT-calculated one indicates that SA-H₂O has the structure in which a carboxylic (COOH) group and the water form a cyclic hydrogen (H)-bond. The OH stretch of COOH is red-shifted as large as 600 cm⁻¹ in the complex. The S₁ lifetime of SA at the zero-point energy level is obtained to be 1.9 ns. This S₁ lifetime is shorter than the reported one of its ester derivative (methyl sinapate, MS (Dean et al., J. Am. Chem. Soc. 136 (2014) 1478–14795)) but is much longer than the para-substituted cinnamates, p-hydroxy methylcinnamate (p-HMC (Shimada et al., Phys. Chem. Chem. Phys. 14 (2012) 8999–9005)) and p-methoxy methylcinnamate (p-MMC (Miyazaki et al., J. Chem. Phys. 141 (2014) 244313)). A calculation at TD-DFT level indicates that this long lifetime of SA is due to that the ¹nπ^∗ state and the transition states along the trans → cis isomerization in S₁ (ππ^∗) are located much higher in energy than p-HMC and p-MMC. This may lead a main nonradiative channel of SA in the low energy region to be the multistep intersystem crossing to T₁ (³ππ^∗) via the ³nπ^∗ state.