Donor–Acceptor Stenhouse Adducts: Exploring the Effects of Ionic Character

Abstract

The effects of solution-state dielectric and intermolecular interactions on the degree of charges eparation provide ar oute to understanding the switchingp roperties and concentration dependence of donor-acceptorS tenhouse adducts (DASAs). Through solvatochromic analysis of the open-form DASA in conjunction with X-ray diffraction and computational theory,w eh ave analyzed the ionic character of as eries of DASAs. First-and third-generation architectures lead to ah igher zwitterionic resonance contribution of the open form and az witterionic closed form, whereas the second-generation architecture possesses al ess charge-separatedo penf orm and neutral closed form. This can be correlated with equilibrium controla nd photoswitching solvent compatibility. As ar esult of the high contribution of the zwitterionic resonance forms of first-and third-generation DASAs, we were ablet oc ontrol their switching kinetics by meanso fi on concentration,w hereas second-generation DASAs were less affected. Importantly,t hese results show how the previously reported concentration dependenceo f DASAs is not universal,a nd that DASAs with am ore hybrid structure in the open form can achievep hotoswitching at high concentrations.

Molecular photoswitches in aqueous environments

“…The stronger the zwitterionic character of the respective open and closed isomers, the more pronounced are the concentration effects. 494,499 Some first generation DASA derivatives, such as 3.41 and 3.42 in Fig. 36B, could not be photoisomerized in any of the media studied so far.…”

“…490,491 Apolar solvents induce a bathochromic shift, while in polar-protic solvents l max is hypsochromically shifted. 491,494 Also, the type of donor moiety influences the position of the absorption maximum strongly. Dialkylamine-based DASAs exhibit their maximum around 500 nm, while DASAs bearing (cyclic) anilines absorb up to 700 nm, hence, reaching the NIR region of the electromagnetic spectrum.…”

“…This red-shift is particularly pronounced in derivatives bearing electron-donating substituents in ortho or para positions on the aromatic donor. 489,494,495 DASAs are characterized by strong inverse photochromism, with the less coloured, cyclic form hardly showing any absorbance above 300 nm. Upon irradiation of the open form, the p-p* transition of the push-pull substituted triene rapidly bleaches, and new transition bands attributed to the formation of the cyclic form arise.…”

“…The stronger the zwitterionic character of the respective open and closed isomers, the more pronounced are the concentration effects. 494,499 …”

The Missing Piece: Concentration Dependence of Donor‐Acceptor Stenhouse Adduct (DASA) Reactivity

“…Both aforementioned studies suggest the thermal back‐isomerization (from the cyclized to the linear form) is much faster at high concentrations, avoiding light penetration and subsequent photoswitching in deeper layers of a DASA containing solution [20,24] . A consensus is yet to be reached, as it has been recently argued that the concentration limitation is not a universal property of DASA photoswitches and an alternative DASA with different donor moieties has been proposed to overcome such limitations [27,28] . The complexity of the concentration effect on DASA photoswitches motivated the present work in the context of the DASA reaction with a thiol in a thiol‐Michael addition (Scheme 2).…”