The synthesis, spectroscopic characteristics and in vitro cellular uptake properties of a new class of therapeutic window photosensitiser, namely the BF₂ chelates of 3,5-diaryl-1H-pyrrol-2-yl-3,5-diarylpyrrol-2-ylidene amines (tetraarylazadipyrromethenes), are described with the aim of developing a novel class of photodynamic therapeutic agents.

The journey of azadipyrromethenes from accidental dye chemistry to a compound class with widely applicable near infrared photophysical properties is documented.

Bioresponsive NIR-fluorophores offer the possibility for continual visualization of dynamic cellular processes with added potential for direct translation to in vivo imaging. Here we show the design, synthesis and lysosome-responsive emission properties of a new NIR fluorophore. The NIR fluorescent probe design differs from typical amine functionalized lysosomotropic stains with off/on fluorescence switching controlled by a reversible phenol/phenolate interconversion. Emission from the probe is shown to be highly selective for the lysosomes in co-imaging experiments using a HeLa cell line expressing the lysosomal-associated membrane protein 1 fused to green fluorescent protein. The responsive probe is capable of real-time continuous imaging of fundamental cellular processes such as endocytosis, lysosomal trafficking and efflux in 3D and 4D. The advantage of the NIR emission allows for direct translation to in vivo tumour imaging, which is successfully demonstrated using an MDA-MB-231 subcutaneous tumour model. This bioresponsive NIR fluorophore offers significant potential for use in live cellular and in vivo imaging, for which currently there is a deficit of suitable molecular fluorescent tools.