Molecular Switching in Confined Spaces: Effects of Encapsulating the DHA/VHF Photo‐Switch in Cucurbiturils

Abstract

Confinement of reactive chemical species uniquely affects chemical reactivity by restricting the physical space available and by restricting access to interactions with the solvent. In Nature, for example, confined protein binding pockets govern processes following photoisomerization reactions and the isomerizations themselves. Here we describe the first example of a dihydroazulene/vinylheptafulvene (DHA/VHF) photo-switch functioning in water, and we show how its switching behavior is strongly influenced by supramolecular interactions with a series of cucurbit[n]uril (CB) host molecules. In CB7 inclusion complexes, the kinetics of the thermal VHF-to-DHA back-reaction is accelerated, while in CB8 inclusion complexes, the kinetics is slowed down as compared to the free photo-switch. The effect of the CB encapsulation of the photo-switch can be effectively canceled by introducing a guest that binds the CB more strongly. According to DFT calculations, a stabilization of the reactive s-cis VHF conformer relative to the s-trans VHF appears to be a contributing factor responsible for the accelerated back-reaction when encapsulated in CB7.

A Visible–Near‐Infrared Light‐Responsive Host–Guest Pair with Nanomolar Affinity in Water

“…Confining photochromic molecules in nanometric cavities may profoundly impact their photochemical properties . As previously observed, 1 o can be converted into 1 c by light irradiation at 365 nm ( φ =0.04) and reverted with wavelengths >500 nm ( φ =0.0003, λ irr =550 nm) (see the Supporting Information, Figure S9) .…”

“…Confining photochromic molecules in nanometric cavities may profoundly impact their photochemical properties. [46][47][48][49] As previously observed, 1o can be converted into 1c by light irradiationa t3 65 nm (f= 0.04) and reverted withw avelengths > 500 nm (f= 0.0003, l irr = 550 nm) (see the Supporting Information, Figure S9). [50] 1 HNMR experimentss how that both species are quantitatively interconverted (see the Supporting Information,F igure S10).…”

Recent Implementations of Molecular Photoswitches into Smart Materials and Biological Systems

“…The λ max of VHF and its lifetime depends on the substituents and solvent polarity . Upon functionalization with a quaternary ammonium salt, DHA was conferred with water solubility and encapsulated in cucurbituril cages . DHA/VHF was applied in molecular electronics upon its anchoring to a silver electrode; gate dependence and reversible switching between states of conductance were observed …”

Reversible switching of arylazopyrazole within a metal–organic cage

“…Similarly, a related cage has recently been shown to induce negative photochromism in the spiropyran switch [18]. Other recent studies of photochromic systems within macrocyclic and supramolecular hosts [19] include dihydroazulene switches [20] and red-shifted azobenzenes [21–22] inside cucurbiturils and cyclodextrins. The behavior of light-responsive compounds can also be affected by their confinement to the surfaces of inorganic nanoparticles.…”