Supervisors: Vas Stavros (Chemistry), Ben Robinson, Adam Burris, Ben Holt, Conor Whitehouse (Syngenta)

Background

Agrochemicals play a key role in many farming practices but, when applied to plant surfaces and exposed to sunlight, photodegradation is one key mechanism by which their efficiency can be reduced. Photoprotectants are found extensively in nature, protecting a variety of different organisms from damage caused by solar radiation. They are also added to synthetic systems, including sunscreens and polymers, to protect humans and materials from the harmful effects of sunlight. The concept of using sunscreens for agrochemicals is not new, but their successful use in commercial products is limited. This is due to: (1) the complexity associated with finding photoprotectants that can be formulated with, and provide efficient protection to, agrochemicals with different physical properties and mechanisms of photo-degradation; and (2) the laborious and inefficient screening approaches often used to attempt to identify suitable photoprotectants. The development of more effective methods to identify photoprotectants could help towards increasing efficiency in the use of agrochemicals. In this project, we will investigate if ultrafast laser spectroscopy techniques can be developed and used to identify and design photoprotectants for a set of model agrochemical compounds based on mechanistic studies of short time scale light induced processes.

 

Programme of work

The proposed research focuses on studying both the early, femtoseconds to nanoseconds (fs-ns; 10^-15-10^-9 seconds) and late, microseconds to seconds (ms-s; 10^-6-seconds) time consequences of ultraviolet radiation (UVR) interaction with a selection of model agrochemicals and a range of potential ‘plant-inspired’ UVR- filters by time-resolved spectroscopy. Such ultrafast spectroscopy measurements have shown considerable promise, notably spectroscopy-linked measurements on plant-inspired sunscreens [1-3]. Based on these studies, we hypothesize that time-resolved spectroscopy, whereby we track the energy dissipation mechanisms of UVR-filters in real time, allied with existing theory, holds the key to establishing a rigorous understanding of the energy dissipation mechanisms operating in these molecules as well as their fidelity for ground state recovery so that they are available for numerous absorption/recovery cycles. Such studies will allow us to quantify the effects of: solvent; pH; structure (ie modification of molecular structure); blend (mixture of UVR filter and agrochemical); and deposition on waxy cuticle – see recent similar publications from Stavros group on the UVR-filter deposited on model skin [3-4]. The latter studies on the waxy cuticle will, vitally, enable us to garner an unprecedented understanding of the photochemistry (or photostability) of these UVR-filters, in as-close to real-use conditions as possible. Importantly, such a ‘structure-dynamics-function’ approach may enable us to establish ‘innovative design rules’ for next generation UVR filters and formulations which tackle challenges such as increased photostability and optimal delivery of photo protectant and agrochemical to the leaf surface, as well as causing minimal damage to the environment.

 

Methodology

In the first instance, we will use ultrafast spectroscopy corroborated with high level time-dependent density functional theory (TD-DFT) through our theory collaborators at Warwick to track the energy flow following photoexcitation of the model agrochemicals and UVR-filters.

Sinapate derivatives have previously shown highly efficient relaxation mechanisms through trans/cis isomerisation leading to efficient repopulation of the electronic ground state; this is a key property as the sinapate can then readily absorb more light. In this proposal, one option would be to extend these systems to look at polymeric sinapates based on sinapic acid. Solutions will be buffered to better mimic the leaf environment. Our studies will focus on: firstly, in bulk solution; and secondly, deposited on the waxy cuticle surface of the leaf. To explore these two environments, transient absorption spectra (TAS) in a pH buffered system will be applied, as outlined in Figure 1. The excitation pulse will be centred at the peak absorbance (say) of the UVR-filter (see Figure 1) and our detection pulse will track the energy flow within the UVR-filter as a function of time delay (Dt) between our excitation and detection pulses. Solutions of candidate UVR-filters (including agrochemical) will be flown through a cuvette or deposited on a glass substrate with a waxy cuticle spin coated onto it. As we have shown previously, these experimental findings correlate well with our TD-DFT calculations in terms of vertical excitation energies and barriers along reaction pathways.

 

Skills to be learned

• Training in state-of-the-art ultrafast spectroscopy including transient electronic and vibrational absorption spectroscopies, respectively TEAS and TVAS (at UoW)
• Basic characterisation studies of UVR-filters including UV-Vis, FTIR and NMR (at UoW)
• Electronic-structure calculations with assistance from our theory collaborators (at UoW)
• Training in agrochemical research and formulation (at Syngenta)

This position has now been filled.