Phenylene-ethynylene macrocycles as model systems of interchromophoric interactions in pi-conjugated macromolecules

Unraveling the complex photophysics of macromolecular pi-conjugated systems requires both the development of suitable model systems to access a particular subset of a material's parameter space and the choice of matching spectroscopic techniques. We address the question of the strength of interchromophoric interactions in macromolecular systems by studying the fluorescence depolarization kinetics of a family of prototypical conjugated macrocycles. Shrinking the size of the molecular system decelerates fluorescence depolarization even though the radius of gyration decreases. Although the smaller macrocycles show faster rotational diffusion, the larger compounds exhibit an additional initial depolarization mechanism, attributed to intramolecular interchromophoric energy transfer. Comparison with fragments of the molecule illustrates that the larger macrocycles can be interpreted as bichromophoric systems, whereas the effectively parallel chromophoric elements of the smaller ring are indistinguishable in terms of polarization. The potential role of strong interchromophoric interaction is discussed. The results illustrate a subtle link between interchromophoric arrangement and ultrafast fluorescence depolarization, phenomena, which are often considered in the context of conjugated polymers: chromophoric alignment can potentially counteract the effect of polarization memory loss through energy transfer.