Guest Editorial: Photochromic Control of Molecular and Macroscopic Properties

The term photochromism indicates the photoinduced and reversible coloration of an ensemble of molecules. It was introduced by Hirshberg to describe the changes in color observed upon irradiation of solutions of bianthrone derivatives with light. Since his pioneering work more than six decades ago, the area of photochromism has evolved into a mature research field. Numerous families of photochromic compounds have been developed and their basic photochemical and photophysical properties are reasonably well understood. Photochromes respond to illumination at appropriate wavelengths with significant but reversible, structural changes. Thus, they act as molecular switches. The stereoelectronic transformations that accompany photoisomerization alter the absorption coefficients across the ultraviolet and visible regions of the electromagnetic spectrum, leading to noticeable changes in color. In addition, these photochemical processes are often accompanied by significant modifications of dipole moments, physical dimensions and redox potentials. In turn, these changes at the molecular level can be exploited to modulate optically macroscopic properties and regulate specific functions. Indeed, photochromic compounds have become invaluable building blocks for the construction of photoresponsive molecular assemblies and macroscopic materials with engineered properties. This special issue of the Israel Journal of Chemistry is centered on this particular aspect of photochromism and provides representative examples of photochromic control of molecular and macroscopic properties in the form of five review articles and three full papers. The five review articles highlight operating principles for (1) storage and processing of information at the molecular level with photochromic switches (Andréasson and Pischel), (2) modulation of the fluorescence of conjugated polymers with optical inputs (Harbron), (3) recording of fluorescence images with spatial resolution at the nanometer level (Belov and Bossi), (4) control of motions at the molecular and macroscopic levels with light (Jin and Rack) and (5) activation of the emission of fluorescent chromophores with photochromic auxochromes (Raymo). The three full papers report the design and properties of (1) photochromic chemosensors for mercury ions (Tian and coworkers), (2) photoresponsive polymer nanoparticles with dual-color emission (Li and coworkers) and (3) photochromic compounds with hydrophilic character (Irie and coworkers). This collection of contributions provides the nonexpert reader with a general overview of the field and illustrates the great potential of photochromic building blocks in the design and implementation of functional chemical systems. In addition, it will serve as a valuable reference for experts in the area and will stimulate the further development of photoresponsive materials with engineered functions and properties. 1 1