Design and synthesis of near-infrared absorbing aza-dipyrromethenes and aza-BODIPYs

Abstract

Near-infrared (NIR) absorbing chromophores have been extensively used in organic photovoltaics, light emitting devices, photothermal therapy, and bioimaging. Aza-boron-difluoride dipyrromethenes (Aza-BODIPYs) are a class of NIR absorbing dyes derived from BODIPY dyes by the replacement of meso-carbon with a nitrogen atom (Figure 1). In aza-BODIPY, the presence of a nitrogen atom at the meso-position with a lone pair of electrons perturbs the frontier energy levels, which leads to the lowering of the HOMO-LUMO gap as compared to the parent BODIPY. The aza-BODIPY dye possesses unique properties such as absorption covering the visible-NIR region, excellent photochemical and thermal stability. It has a fully conjugated framework, making it easy to modify its structure to improve absorption and emission properties. As compared to BODIPY, aza-BODIPY dyes show red shifted absorption and emission bands. The majority of literature reports describe aza-BODIPYs with tetraaryl substitutions on positions 1, 3, 5, and 7 of the pyrrole rings. The substitution of various aryl moieties at α and β-positions of the aza-BODIPY perturbs its photophysical and electrochemical properties significantly. Several studies on the photophysical and electrochemical investigations of aza-BODPY dyes, along with their applications have been carried out over the past couple of decades.