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

Abstract

We performed UV spectroscopy for p-coumaric acid (pCA), ferulic acid (FA), and caffeic acid (CafA) under jet-cooled gas-phase conditions by using a laser-ablation source. These molecules showed the S1(1ππ*)–S0 absorption in the 31 500–33 500 cm–1 region. Both pCA and FA exhibited sharp vibronic bands, while CafA showed only a broad feature. The decay time profile of the 1ππ* state was measured by picosecond pump–probe spectroscopy, and the transient state produced through the nonradiative decay (NRD) from 1ππ* and its time profile were measured by nanosecond UV–deep UV pump–probe spectroscopy. The transient state was observed for pCA and FA and assigned to the T1 state, and we concluded that the NRD process of 1ππ* is S1(1ππ*) → 1nπ* → T1(3ππ*), 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 T1 state was not detected in CafA, and its NRD route is suggested to be S1(1ππ*) → 1πσ* → 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 1ππ* energy and suppresses the NRD process for all the species.

Excited-state dynamics of isolated and (micro)solvated methyl sinapate: the bright and shady sides of a natural sunscreen

“…Since the ionisation energy of the lower-lying electronically excited states is higher than from the initially excited state, they will be more efficiently ionised when ionisation takes place with 193 nm instead of ∼ 320 nm. Such a situation is quite similar to what has been observed for cinnamates and coumarates for which it was concluded that after excitation of the bright 1 ππ* state internal conversion to a dark 1 nπ * state and intersystem crossing to the triplet manifold occurred [15,26,30,33,37,46]. We thus conclude that internal conversion pathways to other electronically excited states also play a significant role for MS.…”

Elucidating the photoprotective properties of natural UV screening agents: ZEKE–PFI spectroscopy of methyl sinapate

“…Our calculations find that MS adopts a perpendicular geometry of the vinyl double bond in its lowest excited triplet state 57 -similar to the other cinnamates and hydroxycinnamates that have been studied so far 23,43,46,49,70,73 -and predicts that this state has an adiabatic excitation energy of 2.47 eV at the M05-2X/6-31+G(d) level. This would imply an adiabatic ionization energy of T 1 of about 4.95 eV.…”

“…Compared with methyl-4-hydroxycinnamate for which an ionization threshold of 65 154 cm À1 (8.078 eV) has been determined, 18 this implies that the substitution with the two methoxy groups ortho to the hydroxyl group lowers the ionization potential by more than 0.6 eV. For p-coumaric acid and ferulic acid ionization thresholds have been determined of 65 200 and 63 560 cm À1 (8.084 and 7.892 eV), 70 respectively. These thresholds suggest that substitution with one methoxy group leads to a lowering of the ionization potential by 1640 cm À1 (0.203 eV).…”

Experimental and Computational Analysis of Para-Hydroxy Methylcinnamate following Photoexcitation

“…It therefore stands to reason that they can be used as starting models for unravelling the NRD dynamics of p-HMC. Upon photoexcitation of p-CA to the 1 ππ* state in the gas-phase, the NRD process shown in Figure 2a dominates, with the observed lifetime of the 1 ππ* and 3 ππ* states measured as ≤10 picoseconds (ps) and 20 ± 4 nanoseconds (ns), respectively [5]. For MC, the same NRD process dominates with the 1 ππ*, 1 nπ* and 3 ππ* states having been experimentally observed.…”

“…Cinnamates and coumaric acids have been widely studied to elucidate their remarkable light absorbing properties, driven by efficient non-radiative decay (NRD) pathways [1][2][3][4][5]. In nature, these chromophores are embedded in proteins to enable vitally important processes to occur.…”