Electronic States and Nonradiative Decay of Cold Gas-Phase Cinnamic Acid Derivatives Studied by Laser Spectroscopy with a Laser-Ablation Technique

We performed UV spectroscopy for p -coumaric acid ( p CA), ferulic acid (FA), and caffeic acid (CafA) under jet-cooled gas-phase conditions by using a laser-ablation source. These molecules showed the S 1 ( 1 ππ*)–S 0 absorption in the 31 500–33 500 cm –1 region. Both p CA and FA exhibited sharp vibronic bands, while CafA showed only a broad feature. The decay time profile of the 1 ππ* state was measured by picosecond pump–probe spectroscopy, and the transient state produced through the nonradiative decay (NRD) from 1 ππ* and its time profile were measured by nanosecond UV–deep UV pump–probe spectroscopy. The transient state was observed for p CA and FA and assigned to the T 1 state, and we concluded that the NRD process of 1 ππ* is S 1 ( 1 ππ*) → 1 nπ* → T 1 ( 3 ππ*), similar to those of methyl cinnamate and para -substituted cinnamates such as p -hydroxy and p -methoxy methyl cinnamate. On the other hand, the transient T 1 state was not detected in CafA, and its NRD route is suggested to be S 1 ( 1 ππ*) → 1 πσ* → H atom elimination, similar to those of phenol and catechol. The effect of a hydrogen bond on the electronic state and NRD process was investigated, and it was found that the H-bonding lowers the 1 ππ* energy and suppresses the NRD process for all the species.