A Family of Symmetrical and Unsymmetrical Aza-Dipyrromethenes and Aza-BODIPYs with Through-Bond and Face-To-Face π-Interaction between Termini.

The dipyrromethenes (DPMs) and their borylated adducts (BODIPYs) constitute a class of versatile chromophores that has become one of the most widely studied over recent decades. They combine excellent photochemistry properties with opportunity for synthetic manipulation and tuning. We report here a related class of aza-dibenzodipyrromethenes and show that they present an interesting architecture where the core adopts a helical arrangement that places terminal aryl functional groups directly on top of each other in close, π-stacked arrangement. Complexation by reaction with boron trifluoride, demonstrated to be significantly improved by addition of trimethylsilyl chloride, induces a stereochemical inversion that destroys the helix, flattens the system, and switches on fluorescence. A wide range of terminal aromatic fragments can be easily introduced. Unsymmetrical derivatives can be made conveniently by simple mixed condensations, but a controlled, rational approach is also described whereby the ability of one component to homocondense (the more reactive partner) is removed by conversion to its corresponding tosylate or triflate. The two approaches have been investigated through successful introduction of complementary electron rich, electron poor, and π-extended functional termini.