Tuning the Properties of Azadipyrromethene-Based Near-Infrared Dyes Using Intramolecular BO Chelation and Peripheral Substitutions.

Abstract

Tetraphenylazadipyrromethenes (ADPs) are attractive near-infrared (NIR) dyes because of their simple synthesis and exceptional optical and electronic properties. The typical BF2 and less explored intramolecular BO coordination planarize the molecule, making them promising π-conjugated materials for organic electronic applications. However, their use has been mostly limited to vacuum-deposited devices. To improve the properties, we synthesized and characterized a series of ADP complexes and used density functional theory calculations to further explain the properties. Hexyloxy solubilizing groups increase the complexes' solubility in organic solvents and enable film formation from solution. Phenylethynyls at the pyrrolic positions extend π conjugation, red-shift absorption and emission peaks, and increase the ionization potential (IP) and electron affinity. When the properties of complexes with hexyloxy and phenyethynyl substitutions are compared, the BO complex is more planar and has a smaller IP than the corresponding BF2 complex because of increased electron density on the proximal phenyls. The BO complex has an unusual combination of properties: a solution λmax of 781 nm, emission at 805 nm, a small Stokes shift, and a quantum yield of 6%. It forms transparent films with a low optical gap of 1.22 eV. This new complex is a promising candidate for transparent solar cells and NIR photodetectors.