Covalent Control of Excitonic Interactions in Perylenediimide Trimers: A Computational Study

Covalently tethering chromophores is an emerging strategy to control the structure and function of supramolecular aggregates for organic electronic applications. In this study, we employ first-principles calculations to elucidate structure-property relationships in three perylenediimide (PDI) trimer systems: a noncovalent assembly (u-PDI₃), a stapled assembly (t-PDI₃), and a folded (foldamer) assembly (s-PDI₃) in aqueous solvent. Our results show how tethering controls the interchromophore geometry, particularly twist angles and slip displacements, which determine electronic coupling patterns. The t-PDI₃ system enforces symmetric cofacial alignment with small twist angles, producing high charge-transfer (CT) character across low-lying excited states, strong coupling (0.16-0.17 eV), and the largest exciton bandwidth. In contrast, u-PDI₃ features varying electronic transition character and coupling heterogeneity (0.07-0.15 eV), while s-PDI₃ has uniform CT character with moderate coupling strength (0.10-0.14 eV). The findings highlight how covalent tethering can be engineered to tune excitonic and CT properties in π-stacked molecular aggregates.