Engineering Triplet Formation versus Symmetry-Breaking Charge Separation in Shape-Persistent Perylene Diimide Macrocycles

Control over excited-state deactivation in multichromophoric assemblies is critical for advancing functional optoelectronic materials. While shape-persistent macrocycles provide a robust platform to suppress nonradiative relaxation via structural rigidity, their cyclic architecture often weakens electronic coupling, creating a trade-off between conformational constraint and deactivation pathway control. To address this challenge, we present two shape-persistent ortho-alkynyl-linked PDI macrocycles (Dimer and Trimer) that exhibit solvent-polarity-controlled switching between competing deactivation manifolds. By combining transient absorption spectroscopy with chemical calculations, we discover that in nonpolar toluene, ortho-alkynyl substitution activates a highly efficient, heavy-atom-free intersystem crossing pathway, with triplet yields increasing from the Dimer (∼65%) to the Trimer (∼85%). This enhancement is attributed to a higher density of near-resonant triplet states that compensates for decreased excitonic coupling. Conversely, increasing solvent polarity triggers ultrafast symmetry-breaking charge separation (SB-CS) on a picosecond time scale, which effectively outcompetes the triplet formation. These insights highlight the utility of rigid and weakly coupled molecular constructs in controlling triplet generation and SB-CS for potential applications in optoelectronic devices.