Michael Hymas

An oligomer of naturally occurring para -coumaric acid was found to behave as largely independent monomer-like units, utilising photoisomerisation for ultrafast non-radiative decay while displaying various ideal physical properties for sunscreens.

The varied shortcomings of existing commercial sunscreens have stimulated research into alternative UV absorbers. In recent years, these efforts have focused on natural and nature‐inspired molecules. Many organisms across taxonomic kingdoms have developed photoprotective regimes, many of which involve the use of small UV‐absorbing molecules. In this review, we summarize recent developments in evaluating the efficacy of these natural UV filter molecules. We discuss the results from time‐resolved spectroscopy experiments and computational studies which have helped to identify photophysical motifs involved in these molecules’ excited‐state relaxation and discuss the exploitation of these motifs for biomimetic design.

The photochemistry of a commercial sunflower sprout extract was compared to that of pure chlorogenic acid (3‐ O ‐caffeoylquinic acid), a key photoprotective species found in the extract. Steady‐state and time‐resolved spectroscopy experiments revealed virtually equivalent photodynamics between aqueous solutions of the sunflower extract and chlorogenic acid, namely ∼5 picosecond nonradiative deactivation following UV photoexcitation. For chlorogenic acid, this nonradiative deactivation is achieved by relaxation through a conical intersection, mediated through rotation around the caffeoyl CC double bond. These photophysical similarities tentatively justify the use of the unpurified sprout extract for use in UV photoprotective formulations and demonstrate that the environmental complexity conferred by the presence of other phytochemical constituents in the extract does not impede the relaxation mechanism of chlorogenic acid.

Derivatised aurones, based on phytochemical flavonoids, were synthesised and studied using steady‐state and ultrafast spectroscopy techniques. Aurones’ strong ultraviolet (UV) and visible absorption, in addition to being found in abundance in fruits, vegetables and flowers, makes them ideal photoprotection candidates for agrochemicals. Agrochemicals are well known to exhibit poor photostability, and the addition of photoprotectants to their formulations thus increases their longevity, and thereby environmental sustainability. The results presented here shed light on the complex ultrafast and long term photodynamics of five nature‐inspired derivatives in solution and on a wax surface, the latter designed to mimic the cuticle of a plant leaf. This study finds markedly different relaxation mechanisms between these condensed phase environments, highlighting the importance of studying photoprotectants in their real‐world environments. These findings inspire the development of safe and effective photoprotectants for agrochemicals.

We present a comprehensive computational study describing the excited state dynamics and consequent photostability of amino-cyclohexenone (ACyO), the central template of mycosporine systems, widely recognized for their photoprotection of aquatic species. Photoexcitation to the first excited electronic state (S1, 1nπ*) of ACyO is considered an optically dark transition, while photoexcitation to the second excited electronic state (S21ππ*) is an optically bright 1ππ* transition and largely responsible for UV absorption properties of this molecule. We show that following initial photoexcitation to S2, ACyO relaxes via two competing deactivation mechanisms, each mediated by an S1/S0 conical intersection, which directs the excited state population to the electronic ground state (S0). Our ab initio computational results are supported with nonadiabatic dynamics simulation results, yielding an excited state lifetime of ∼280 fs for this system in vacuo. These results explain the inherent photostability of this core structure, commonplace in a wide range of microorganisms.