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

- New symmetrical aza-BODIPY dyes with particular physical properties have been designed and successfully synthesized. Aza-BODIPY dyes were symmetrically substituted with aryl groups on the pyrrole rings. Among substituents investigated were triphenylamine, terthiophene, benzothiadiazole and thiophene.
- Substituents on aza-BODIPY dyes created additional conjugation and shifts to significantly longer wavelengths were realised in both absorption and emission spectra.
- A remarkable red shift in absorption and emission was observed for aza-BODIPY dye (8f) substituted with electron-deficient groups (benzothiadiazole groups). Dye 8f showed an absorption maximum at 855 nm, which represents the highest absorption maximum known for an aza-BODIPY dye.
- 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.
- It has been demonstrated how absorption and emission of aza-BODIPYs can be fine-tuned by manipulating the intramolecular-charge-transfer (ICT) between variously electronic donating and withdrawing substituents in the aza-BODIPY structure.

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 aza-BODIPYs 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.