Macrocyclic systems with photoswitchable functions

Abstract

Photochromism is a reversible transformation of a single chemical species between two states, the absorption spectra of which are clearly different, the transition in at least one direction being induced by electromagnetic radiation [1]. The widest and most important group of the photochromic system is based on electrocyclic reactions [2,3]; a few have been commercially successful (polymer-based photochromic eyewear, novelty items and security printing inks). Several other photochromic systems based on E,Zisomerization, cycloaddition reaction, electron or proton transfer have potential industrial applications [4]. Photochromic macrocyclic compounds constitute a new class of artificial receptors in which the recognition of metal cations induces a conformational change in the receptor framework accompanied by signalling (coloration). In addition, since binding of cations is sensitive to the ligand environment, the binding constant can be controlled effectively by employing photochromic systems which change in the light [5,6]. This review surveys our studies devoted to the photoswitchable molecular receptors based on photochromic crown ethers. Photochromic systems described in the review may be classified into three groups according to the reaction types: E,Z-isomerization, [2+2]-photocycloaddition reactions and electrocyclic reaction. It has proved the groups to be an especially suitable basis for photochromic systems, and promising for the industrial applications. It is known from literature that several reversible photochemical reactions, such as geometric isomerism of azobenzene [7], electrocyclic reaction of dihydroindolizines, fulgides and diarylethylenes with heterocyclic groups [8-10], dimerization of anthracene [11], and photochromic reaction of spirocompounds [12] have been also employed to provide photocontrol over metal-ion binding ability of crown ethers. © 2005 Springer. Printed in the Netherlands.