Experimental and Computational Analysis of Para-Hydroxy Methylcinnamate following Photoexcitation.

Jack Dalton,Gareth W Richings,Scott Habershon,Jack M Woolley,Temitope T Abiola,Vasilios G Stavros

doi:10.3390/molecules26247621

Abstract

Para-hydroxy methylcinnamate is part of the cinnamate family of molecules. Experimental and computational studies have suggested conflicting non-radiative decay routes after photoexcitation to its S1(ππ*) state. One non-radiative decay route involves intersystem crossing mediated by an optically dark singlet state, whilst the other involves direct intersystem crossing to a triplet state. Furthermore, irrespective of the decay mechanism, the lifetime of the initially populated S1(ππ*) state is yet to be accurately measured. In this study, we use time-resolved ion-yield and photoelectron spectroscopies to precisely determine the S1(ππ*) lifetime for the s-cis conformer of para-hydroxy methylcinnamate, combined with time-dependent density functional theory to determine the major non-radiative decay route. We find the S1(ππ*) state lifetime of s-cis para-hydroxy methylcinnamate to be ∼2.5 picoseconds, and the major non-radiative decay route to follow the [1ππ*→1nπ*→3ππ*→S0] pathway. These results also concur with previous photodynamical studies on structurally similar molecules, such as para-coumaric acid and methylcinnamate.

Highlights

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]
A probe beam at 240 nm (41,667 cm−1 ) ensures that ionisation is only optically accessible from the S1 state, since the first two ionisation levels are of π−1 character and there is insufficient energy to reach the D2 (n−1 ) ionisation state from any possible nπ* states [28]
We can reasonably assume that the excited state lifetimes extracted from the time-resolved ionyield (TR-IY) transient using a 240 nm probe in Figure 3b are directly related to the S1 state lifetime

Summary

Introduction

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. PYP was first discovered in the Ectothiorhodospira halophila bacterium [6,7,8] and utilizes the photoisomerization of para-coumaric acid (p-CA) to facilitate a negative phototactic response towards blue light, protecting the bacterium from potentially harmful photodamage [9,10,11,12]. The decay dynamics of p-HMC following photoexcitation to the

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