Experimental and Computational Analysis of Para-Hydroxy Methylcinnamate following Photoexcitation

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.

Understanding the Impact of Symmetrical Substitution on the Photodynamics of Sinapate Esters Using Gas-Phase Ultrafast Spectroscopy

“…The experimental setup has been described in detail elsewhere. ,, Briefly, a Ti:sapphire oscillator (Spectra-Physics Tsunami) and regenerative amplifier (Spectra-Physics Spitfire XP) produce ∼40 fs laser pulses at a rate of 1 kHz and centered around 800 nm. The output laser beam is ∼3 W and is subsequently split into three equal ∼1 W parts.…”

“…To gain a quantitative understanding of the photodynamics, the transients were fitted with a parallel (all dynamical processes starting at a pump–probe time delay, Δ t , of 0) exponential decay model with three forward (Δ t > 0) and one reverse (Δ t < 0) exponential decay, in accordance to a similar approach used previously in our group (further fitting details can be found in the Supporting Information). All three data sets returned time constants of τ 1 ≈ 4.5 ps, τ 2 ≈ 70 ps, and τ 3 > 500 ps.…”

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