Thinking Outside the “Blue Box”: Induced Fit within a Unique Self-Assembled Polycationic Cyclophane

Abstract

We present herein the development of a new polycationic molecular receptor, inspired by the ubiquitous cyclobis­(paraquat-p-phenylene)­cyclophane (“blue box”). Our analogue, the “white box”, has been easily self-assembled on a preparative scale in water, using a template-assisted process by acyl hydrazone bonding of complementary bis­(pyridinium)­xylylene tweezers, followed by kinetic trapping of the empty receptor. The obtained macrocycle was found to display a marked pH responsiveness in water, because of an abnormal acidity of the amide protons within its structure. Consequently, and because of the concurrence of rotational isomerism under acidic conditions (fixed at higher pH values), the compound was found to display a dual behavior as a conformationally locked/flexible molecular host, being able to recognize appropriate aromatic substrates, in a lock and key or induced fit fashion, by a conjunction of π–π, C–H···π, and, crucially, the hydrophobic effect.

Thinking outside the “Blue Box”: from molecular to supramolecular pH-responsiveness

“…Following our previously reported protocol for the synthesis of W 4+ , 18 we successfully accomplished the quantitative selfassembly in aqueous media of the inclusion complex 2,7-DHN3R 4+ (DHN ¼ dihydroxynaphthalene). This was achieved by performing the TFA-d (10%) catalysed reaction between equimolar 1.5 mM D 2 O solutions of the complementary tweezers 1$2Br and 2b$2Br, 19 and 1.5 eq.…”

“…S117 †), was found to be optimal for the establishment of p-p interactions as reported for similar systems. 12,13,18,21…”

“…14 Recently, we and others have reported the use of imine-based dynamic covalent chemistry 15 for the aqueous self-assembly of constitutionally dynamic cyclophanes, wholly organic compounds which can act as binding parts not only in host:guest aggregates, 16 but as well within self-threading catenanes and knots. 17 In our particular case, 18 we have described a new conformationally exible host, the "white box" (W 4+ , Scheme 1), an acyl hydrazone-based analogue of the well-known redox-responsive receptor "blue box" (cyclobis(paraquat-p-phenylene)cyclophane) developed by Stoddart and co-workers. 10 Our macrocycle shows not only the expected constitutional dynamism in water caused by the imine bonds, but also accessible stimuli-responsiveness induced by the unusual acidity of the amide protons within W 4+ (pK a ¼ 6.5).…”

Untying the Photophysics of Quinolinium‐Based Molecular Knots and Links

“…The first set of signals corresponds to am ixture of the quickly interconverting trans isomers( transsyn and trans-anti), while the second set of signals corresponds to am ixture of the quickly interconverting cis isomers (cis-syn and cis-anti). [54] The results of this conformational analysisa re transferablet o the more complex knots andlinks.The energetic difference between the four major isomers cis-syn, cis-anti, trans-syn,a nd trans-anti being relatively low,t he isomerization readily occurs in the non-interlocked macrocycle Mac and the loosely braided Hopf link HL.I ndeed, the occurrence of the syn/anti isomerization and the presence of severali nterconverting conformations have been previouslyd etected for HL in 1D and 2D 1 HNMR spectra. The relative ratio between the cis and the trans conformationsc ould be accurately determined from the 1 HNMR spectrum and is approximately 1:4.…”

“…[54] When pH increases, the main CT-absorption band of the protonated speciesdisappears and anew band, attributed to the deprotonated zwitterionic species,a ppears at longer wavelength. [54] When pH increases, the main CT-absorption band of the protonated speciesdisappears and anew band, attributed to the deprotonated zwitterionic species,a ppears at longer wavelength.…”