Enlightening Main Group Chemistry: Development and Synthesis of Novel Chromophores Based on Heavy Pnictogens

Abstract

Owing to their highly tunable photophysical properties, boron difluoride complexes of dipyrrins (BODIPY) have established themselves as versatile chromophores with widespread applications in research areas beyond synthetic chemistry. As such, BODIPY dyes have also seen use in photocatalysis but typically require incorporation of heavy atoms, e.g. bromine and iodine, in the ligand framework to access meaningful catalytic activity through population of long lived excited states. <br/>

This work describes the synthesis and application of novel dipyrrin complexes featuring heavy central atoms as a strategy to access molecular photocatalysts. First, a synthetic protocol to access the first antimony and bismuth analogues of BODIPY (SBDIPY & BIDPY) were developed and the spectroscopic properties of these compounds were elucidated. However, time-resolved UV/Vis spectroscopy revealed a temperature-dependent decomposition of the pnictogen complexes in solution, prompting a modification of the dipyrrin ligand to increase complex stability. Thus, a set of dipyrrin ligands was synthesized and evaluated, leading to second generation pnictogen dipyrrins with increased stability. Next, these chromophores were tested as photocatalysts for energy transfer and photoredox catalysis. While the broader applicability as photocatalysts was found to be limited, the most stable antimony complexes performed especially well as photosensitizers for singlet oxygen, achieving singlet oxygen quantum yields as high as 0.76. The light-governed properties of pnictogen dipyrrins were further investigated by theory (in collaboration with the Grimme group) and using cyclic voltammetry under light irradiation (in collaboration with the Kornienko group), which provided insight into a light-induced halide dissociation from a SBDIPY chromophore. <br/>

Finally, reduction and oxidation of SBDIPY complexes, as well as redox-neutral reactivities were explored as options to influence the photophysical properties of the compounds. In particular, the previously established halide lability was leveraged for molecular sensing. In this context, SBDIPY chromophores were found to be sensitive probes toward Lewis acids and Lewis bases, showcasing distinct spectroscopic changes in the presence of additives by NMR or photoluminescence spectroscopy with a higher sensitivity as compared to the analogous BODIPY dye. Several Lewis adducts were isolated and characterized by X-ray crystallography, further underlining the key reactivity associated with molecular sensing in SBDIPY chromophores.