Approaches to the design and synthesis of near-infrared absorbing dyes

Abstract

Dyes with absorption bands in the near-infrared (i.e. at wavelengths longer than about 700 nm) were for many years regarded as curiosities, having few practical applications other than as sensitisers in photographic emulsions. However, with the advent of lasers this situation changed dramatically. Many lasers, particularly those suitable for applications of a more routine nature, emit near infrared (NIR) radiation, and if this coherent light is to be used in a practical way, then appropriately absorbing dyes can provide the important link between the laser emission and the observable effect. The simplest case is where the infrared dye (IR) acts as an absorbing species, and may therefore be used for spectrophoto-metric analysis (e.g., medical diagnostic systems), or for machine readable codes (e.g., security printing). The fact that wavelengths well outside the visible region are employed has many advantages in such situations. If the dye fluoresces after light absorption, then the emission will be even further into the IR region and thus further removed from possible background fluorescence. Such dyes, although rare, have particular value in spectrophotometric analysis in medical and biochemical assays and probes.