New wavelength-tunable aza-dipyrromethene dyes with intense near-infrared absorption and emission

A series of new boron-difluoride chelated aza-dipyrromethene (aza-BODIPY) dyes with particular physical properties have been designed and synthesised for the purpose of exploring the effect of different substituents on the absorptions and the electronic properties of these dyes. These dyes can be further explored for their usefulness as donors in organic photovoltaic (OPV) cells. The synthesised azaBODIPYs were symmetrically substituted with aryl groups on the pyrrole rings. Among substituents investigated were triphenylamine, terthiophene, benzothiadiazole and thiophene. The photophysical properties of aza-BODIPYs were characterised by ultraviolet-visible (UV-vis) absorption and fluorescence spectroscopies. Aza-BODIPY dye with benzothiadiazole and triphenylamine substituents exhibits intense near-infrared red-shifts in absorption and emission. Time-dependent density functional theory (TD-DFT) has been used to provide a guide to the structure-property relationships and electronic structures of the aza-BODIPYs. Evidence has been found of strong intramolecular-charge-transfer (ICT) character in the excited state. It has been proven how absorption and emission of aza-BODIPYs can be fine-tuned by manipulating the ICT between variously electronic donating and withdrawing substituents in the aza-BODIPY structure. (C) 2017 Elsevier B.V. All rights reserved.