Design, Synthesis and Characterization of BODIPY based 1H-Tetrazole Ligands

Four novel fluorescence active ligands (1-4) consisting of a 1H-tetrazol-1-yl moiety as coordinating unit and a 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) derivative as fluorophore, bridged via alkyl (-(CH2)n-, n = 1‑3) or benzyl (-CH2-C6H4-) spacers were designed. Successful synthesis is demonstrated by multinuclear NMR spectroscopy, as well as powder and single crystal XRD analysis. The methylene bridged ligand 2 (4,4-difluoro-1,3,5,7-tetramethyl-8-[(1H-tetrazol-1-yl)methyl]-4-bora-3a,4a-diaza-s-indacene) crystallizes in different polymorphs and solvatomorphs, in contrast to the other three ligands, which show no polymorphism under identical conditions. Photophysical studies revealed high fluorescence quantum yields (69 – 95%) in solution for the -(CH2)2- bridged ligand 3 (4,4-difluoro-1,3,5,7-tetramethyl-8-[(1H-tetrazol-1-yl)ethyl]-4-bora-3a,4a-diaza-s-indacene) and the -(CH2)3- bridged ligand 4 (4,4-difluoro-1,3,5,7-tetramethyl-8-[(1H-tetrazol-1-yl)propyl]-4-bora-3a,4a-diaza-s-indacene). Non-radiative decay due to rotational motion of the 1H-tetrazol-1-yl- and/or -CH2-C6H4- moiety for 2 and 1 (4,4-difluoro-1,3,5,7-tetramethyl-8-[4-((1H-tetrazol-1-yl)methyl)phenyl]-4-bora-3a,4a-diaza-s-indacene) respectively leads to reduced quantum yields of ≥ 35%. Complete fluorescence quenching upon aggregation is prevented by installation of the sterically demanding 1H-tetrazol-1-yl moiety and a spacer in meso-position of the BODIPY core to elongate the intermolecular distances between two adjacent BODIPY cores. Detailed photophysical and crystallographic investigations are supported by theoretical calculations.