One-Step Access to Ultra-Bright Macrocyclic Oligo-BODIPYs Featuring Long-Lived Cyclic Excitons.

The controlled organization of chromophores into defined assemblies with collective photophysical properties is a key concept for natural and artificial photonic systems. A template-free, one-step synthesis affords a series of methylene-bridged macrocyclic oligo-BODIPYs from readily available BODIPY precursors. Brønsted acid catalysis with paraformaldehyde as a C1 synthon links multiple BODIPY cores into β-connected linear oligomers and macrocycles comprising up to twelve non-conjugated chromophores. Optical spectroscopy and quantum chemical calculations provide initial insights into the photophysical mechanisms: steady-state and time-resolved spectroscopy reveal pronounced excitonic coupling across the chromophore arrays, which is confirmed by quantum chemical investigations at the simplified time-dependent density functional theory level. The linear oligomers display typical J-type characteristics, while the macrocyclic systems exhibit distinct multiband emission and unusual size-dependent photophysical properties. Distinct excitonic regimes seem to coexist in the macrocyclic oligomers. Intense absorption bands are consistent with localized J-coupled chromophore segments within the ring, while fluorescence can occur from J-coupled segments and a lower-lying cyclic exciton state with strongly reduced oscillator strength. In summary, our work establishes cyclic oligo-BODIPYs as a versatile platform for investigating topology-dependent excitonic phenomena, and also for developing functional superchromophores with tunable photophysical properties.