Gating the photochromism of an azobenzene by strong host–guest interactions in a divalent pseudo[2]rotaxane

Abstract

The ability of an E-configured azobenzene guest to undergo photoisomerisation is controlled by the presence of a complementary host. Addition of base/acid allowed for a weakening/strengthening of the interactions in the divalent pseudo[2]rotaxane complex and hence could switch on/off photochromic activity.

Synthesis and photoisomerization of an azobenzene-containing tetrapyrrolic macrocycle

“…[26,27] However, it is known that attachment of the azobenzene components to the chromophoric molecules reduces its transcis conversion and quantum yield in the photoisomerization. [28][29][30] Furthermore, the isomerization also highly depends on its structural restrictions as well as the electronic properties of the attached substituent groups. For these reasons, it is challenging to synthesize a photo-responsive cyclic oligopyrrole through a procedure incorporating an azobenzene component into a highly restricted -conjugated system.…”

Allosteric and Chelate Cooperativity in Divalent Crown Ether/Ammonium Complexes with Strong Binding Enhancement

“…With increasing methanol content,t he Gibbs binding energy initially becomes larger (in terms of DG) until an optimum is reached for volume fractions of methanol of about 0.33 for G C 2@H2 and 0.5 for G O 2@H2 (Figure 4). Complex G O 2@H2 has am aximum binding constant of K = 1.0 10 5 m À1 ,w hich is in the range of those of previouslys tudied crown ether/ammonium pseudorotaxanes with rigid spacers, [23,37] and G C 2@H2 even exhibits ab inding constant one order of magnitude higher than that (K = 2.4 10 6 m À1 ), althought he spacer lengths of both guests are virtually the same. Beyondt heseo ptima, increasing the methanolc ontent leads to weaker binding again.T wo counterbalancing effectssolvent-dependent ion pairinga nd ammonium-crown ether hydrogen bonding-rationalize this behavior.I on pairing is important in the free guests,b ut is negligible for their crown ether complexes.…”

“…It is the factor in the association constant of the divalent complex that accounts for the intramolecular ring closure in the second binding step (Figure 1, complex a). However,E Mc annot be measured directly.T oq uantify EM, ad ouble-mutant cycle (DMC) analysis [18,23,24,[32][33][34][35][36][37][38] (Figure 1, top) can be used (for ad etailed description including the determination of statistical factors, see the Supporting Information). Briefly,m utation 1, which corresponds to acut through the host spacer,leads from a to b.This step contains the effects on divalent binding that are due to the host spacer.…”

“…We investigated the association processeso fd ivalent primary ammonium ions G O 2 and G C 2 (Scheme 1) with the divalent [18]crown-6 dimer H2 by isothermal titration calorimetry (ITC). To quantify allosteric andc helate cooperativity, [13,14] double-mutant cycle (DMC) analyses [18,23,[32][33][34][35][36][37][38] for both host-guest complexes were performed. Althought he structurald ifferences of the two guests G O 2 and G C 2 are small( two ether oxygen atoms in G O 2 insteado ft wo isoelectronic methylene groups in G C 2), they differ significantly with respect to chelate cooperativity.S tudies by Gibsone tal., [39][40][41] Stoddarte tal.…”

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

“…Conformational changes in the protein following the cis–trans photoisomerization of the retinal Schiff‐base initiates the pathway leading to a nerve signal to the brain . This adaptability of protein structures to accommodate chromophore molecules in their different states represents a level of sophistication that is difficult to achieve with artificial systems …”