Desolvation Induced Conformational Transition in a Cryptophane Host: Crystal To Crystal Transformation

Abstract

Cryptophanes show different conformations in solution and solid state depending upon various factors, such as the length of connecting linkers, medium, and nature of the incoming guest molecule(s). A cyclotriguaiacylenes (CTG) based cryptophane molecule was synthesized using click chemistry containing three triazole linkers and studied as well. This molecule shows two conformations, out-out crown-crown (CC), and out-in CC, in the presence or absence of guest molecule(s), as studied both in solution and solid state. The out-in CC, in which both CTG fragments are in crown conformation with one crown sitting above the other, could be obtained by slow escape of the trapped acetone molecules from out-out CC in solid state. This transformation could be obtained through a single-crystal-to-single-crystal (SCSC) transformation from a large volume out-out CC to a smaller volume out-in CC conformation which is also supported by density functional theory calculations.Cavitands are organic molecules with enforced cavity large enough to encapsulate various small molecule(s). [1] The interior cavity of cavitands have been used for stabilization of unstable molecules, [2,3] trapping of reaction intermediate(s), [1e,4] visible light photocatalysis, [5] asymmetric catalysis, [6] enantioselective recognition, [7] for stabilizing reactive intermediates, [7] as microreaction chambers, [8,9] chlorine capture with attomolar affinity, [10] modulate chiroptical properties. [11] Cryptophanes, are one such class of molecules, [12] contain two bowl shaped cyclic molecular systems (cyclotribenzylenes or its derivatives), joined together with three linkers. Changing linker size changes the free space of the cavity considerably. [13,14] Cryptophanes are well studied for entrapment of small neutral guest molecules CHCl 3 , CH 2 Cl 2 , CHFClBr, [13a] cationic organic molecules, [15,16] metal ions, [17] and gases like Xe, CFCs, CH 4 in solution phase. [13,18] These molecules are shown to bind biologically important molecules such as choline and avidin. [16] Typically, cryptophanes remain in an outout CC conformation, in which the connecting methylene groups in the cryptophane unit pointed oppositely outward. However, when flexible enough, i. e., the connecting alkyl chain contains 6 or more carbons, one of the cyclotribenzylenes (CTBs) unit flips to give an out-in CC conformation (Scheme 1).In cryptophanes, one of the factors that governs the feasibility of host-guest complexation is length of the linker. Thus, with an optimise linker length, small molecule can trigger the conversion of out-out CC to out-in CC or vice-versa. Such stimuli-responsive materials have potential applications in sensors, actuators, photoelectronic devices, information storage etc. [19] When changes occur due to escape of the trapped guest molecule(s), compresses the crystals and such systems are proposed to be helpful in designing the high-energy molecular actuators. [20] Out-out CC conformer in cryptophanes is generally found in solid state with small...

Synthesis of Cryptophanes: Recent Advances

“…[14] Factors such as length of linkers, the nature of solvent and guest molecules influence the conformational preferences in solution and the solid state [15] . The imploded conformations, particularly the out – saddle conformer, can be formed by the collapse of out – out conformers due to the evacuation of guest molecules at high temperature, particularly in solvents that are too big to occupy the cavity [15a] (although an exception of spontaneous isomerization has been reported [16] ). The formation of imploded conformations can be reversed by reflux in a solvent suitable as a guest molecule [17] .…”