Unravelling the photoisomerization mechanism via environment–induced tuning of reactivity – PhotoMecha

Double-bond photoisomerization has long been exploited in man-tailored molecular photo-switches or light-driven molecular motors for opto-mechanical energy conversion and photocontrol of molecular functions. In such applications, a high photo-isomerization quantum yield (QY) is desirable to enhance the molecular device efficiency. However, no chemical design criterion has been rationalized to date for maximizing the photo-isomerization QY of synthetic molecules. Natural evolution instead has developed photosensory proteins, in which photoisomerization triggers biological functions with outstanding efficacy. A paradigmatic example is the rhodopsin (Rho) protein, the pigment for vision. In the PhotoMecha project, we will explore the mechanism of ultrafast C=C double bond photoisomerization of small synthetic, biomimetic photo-switches. These compounds have been designed to mimic the electronic structure – i.e. the potential energy surfaces (PESs) - of biological chromophores and shown to undergo an ultrafast C=C double bond photoisomerization similar to that observed in Rho. Like most photoisomerizing compounds, the QY of the biomimetic switches does however not exceed 30%, while it is 67% in Rho.
Many theoretical predictions agree, that the PESs landscapes, their so-called “conical” intersection (CInt) and the vibrational motions at the CInt control the overall photoreaction QY. Here, we propose to use the unique set of biomimetic compounds as models to unravel the physico-chemical parameters that control the excited state decay at the CInt and thus the QY. Our strategy will be to engineer their microenvironment in a rational way, in order to fine-tune their photoreactivity. The example of Rho already demonstrates that such a tuning is possible and has a critical influence on the photoreaction dynamics and QY. Hence, we will extend the investigation of the existing biomimetic compounds (i) in the gas phase (ii) in solvents of various polarities and (iii) encapsulated in appropriate supramolecular cages. State-of-the-art experimental and theoretical approaches will be jointly employed, including ultrafast UV-Vis spectroscopy (Partner P1 = IPCMS Strasbourg), time-resolved XUV photoelectron spectroscopy (Partner P3 = ILM, Lyon), synthetic supramolecular chemistry (Partner P2=IPCMS, Strasbourg) and quantum chemical modelling (partners P4=ICS Strasbourg and P5=U. Siena, Italy).