Photoredox‐Switchable Resorcin[4]arene Cavitands: Radical Control of Molecular Gripping Machinery via Hydrogen Bonding

Abstract

Semiquinones (SQ) are generated in photosynthetic organisms upon photoinduced electron transfer to quinones (Q). They are stabilized by hydrogen bonding (HB) with the neighboring residues, which alters the properties of the reaction center. We designed, synthesized, and investigated resorcin[4]arene cavitands inspired by this function of SQ in natural photosynthesis. Cavitands were equipped with alternating quinone and quinoxaline walls bearing hydrogen bond donor groups (HBD). Different HBD were analyzed that mimic natural amino acids, such as imidazole and indole, along with their analogues, pyrrole and pyrazole. Pyrroles were identified as the most promising candidates that enabled the cavitands to remain open in the Q state until strengthening of HB upon reduction to the paramagnetic SQ radical anion provided stabilization of the closed form. The SQ state was generated electrochemically and photochemically, whereas properties were studied by UV/Vis spectroelectrochemistry, transient absorption, and EPR spectroscopy. This study demonstrates a photoredox-controlled conformational switch towards a new generation of molecular grippers.

Mono‐ and Tri‐Functionalization of Trimethylresorcin[4]arenes

“…Resorcin [4]arenes in general are intensively studied tetrameric resorcinol-based structures with a broad range of useful applications in catalysis and especially in host-guest chemistry, [1] supramolecular containers, [2,3] or even molecular machines. [4] Resorcin [4]arenes offer a broad variety of bowl-or vase-like structural backbones on the road to specifically designed supramolecular structures.…”

“…Acetalized resorcin [4]arenes offer a rigid electron rich cavity, which can be used e.g. as multi-and mono-dentate bowlshaped ligands in several transition metal catalysed crosscoupling reactions.…”

“…[6,7] Furthermore, tri-dentate resorcinarene ligands with imidazole binding sites were used as biomimetic model-compounds to study various zinc and copper based enzymatic binding sites, reported by Reinaud et al [8][9][10] Molecular resorcinarene-based capsules connected by three linkers have been reported, [11,12] some of them even with a photosensitive gate, [13] as well as dimers with only one covalent bond between the resorcinarene-units. The classical protocol for their synthesis is following a five step procedure; first the formation of the resorcin [4]arene backbone by condensation of resorcinol with an aldehyde and then the electrophilic fourfold halogenation, mostly bromination, prerequisite as steric control element to ensure a satisfying yield for the fourfold acetalization with BrClCH 2 or similar methylenating agents. The classical protocol for their synthesis is following a five step procedure; first the formation of the resorcin [4]arene backbone by condensation of resorcinol with an aldehyde and then the electrophilic fourfold halogenation, mostly bromination, prerequisite as steric control element to ensure a satisfying yield for the fourfold acetalization with BrClCH 2 or similar methylenating agents.…”

“…[14,15] All these examples derive from tri-or mono-functionalized resorcinarenes. [16] Finally, the selectivity of the mono-lithiation or the tris-lithiation of the tetrahalogeno-resorcin [4]arene is a main factor for the overall efficiency of the reaction sequence, facing statistical distribution with under-or over-lithiated by-products as a yield diminishing problem. [16] Finally, the selectivity of the mono-lithiation or the tris-lithiation of the tetrahalogeno-resorcin [4]arene is a main factor for the overall efficiency of the reaction sequence, facing statistical distribution with under-or over-lithiated by-products as a yield diminishing problem.…”

“…For the further transformation of the lithiated resorcin [4]arene various electrophiles are suitable, e.g. simple protolysis, or transmetalations such as the borylation, or for the direct CC-bond formations through acylations (with chloroformates) or alkylations (with methyl iodide).…”