Design and study of photochemical actuators based on phosphalkenes
Title: Design and study of photochemical actuators based on phosphalkenes
DNr: NAISS 2024/22-844
Project Type: NAISS Small Compute
Principal Investigator: Jorn Steen <>
Affiliation: Uppsala universitet
Duration: 2024-06-07 – 2025-07-01
Classification: 10405


Photochemically triggered double bond isomerization is a fundamental reaction that allows controlling the geometry of unsaturated compounds at the nanoscale. This reaction is often employed in natural and artificial photoactuators: molecules that can interconvert between a stable and a metastable form by means of a light stimulus that triggers the motion of the unsaturated bond. The photochemical isomerization of C=P bonds is known in the literature and is mostly used to isomerize compounds later to be applied as ligands for metal complexes. However, the majority of the examples report very unselective switching between the two isomers (yielding mostly 1:1 mixtures upon irradiation) due to the choice of substituents, and they are overshadowed in terms of number of studies and applications by the C=C, C=N and N=N photoactuators. To the best of our knowledge, there is no thorough study focusing on the design of air-stable photochemical actuators based on C=P bonds as well as on the photophysical, photochemical, and electrochemical properties of the different isomers. In this study, we aim to lay the groundwork for the application of these systems as active components of air-stable molecular machines driven by orthogonal stimuli, by computational design of various phosphalkene molecules and characterisation of the photo- and electrochemical and thermal properties of the C=P bonds. We have previously investigated a simple fluorenone-based phosphalkene, focusing on the mechanism of isomerisation in the ground and excited state, and have used these computations to explain experimental ultrafast transient absorption spectroscopy data. We aim to build upon this work by broadening the scope to various substituted fluorenone-based phosphalkenes, including asymmetric analogues that potentially show unidirectional rotation. Furthermore, following an approach previously utilized for C=N switches, we have computationally designed a prototypical structure that potentially could overcome the limitations typical of the reported C=P scaffolds. While we kept the classic super-mesityl (Mes*) group on the P atom to increase the kinetic stability of the C=P bond, we have also introduced a hemithioindigo half in the switching core. This feature allows for the thermodynamic differentiation of the E/Z isomers, which have a computed ΔG of ca. 4 kcal/mol, favoring the Z. This property will allow the preferential thermal population of one of the two isomers. The different steric encumbrance in the hemithioindigo half grants different optical properties to the two isomers: Z is predicted to absorb at 409 nm while E has a maximum at 468 nm. Consequently, they could be selectively addressed using different wavelengths. The thermal barrier of rotation is predicted to be 33.8 kcal/mol, strongly hinting towards a bistable system that can be addressed selectively by different light stimuli. With these promising results we aim to continue our detailed investigation into the nature and role of the excited (and oxidation) states of phosphalkenes as well as the thermal mechanisms of isomerisation.