Conformational Flexibility Determines Electronic Coupling and Charge Transfer Character in Single Propeller-Shaped Perylene Diimide Tetramer Arrays

We investigate the effect of molecular geometry and conformational flexibility on electronic coupling and charge transfer interactions within propeller-shaped perylene diimide (PDI) tetramer arrays differing by the number of covalent linkages to a central spirobifluorene core. Electronic spectra of tetramers with one (“floppy”) or two (“rigid”) bay covalent linkages display evidence of charge transfer character in either ground or excited states. Floppy tetramers exhibit marked red-shifted and broadened absorption features that we assign as overlapping inter-PDI charge transfer and PDI-centered π–π* transitions, whereas rigid tetramers retain features similar to single PDI molecules, albeit with broader line widths. Interestingly, both tetramers exhibit charge transfer character in their fluorescence emission, but this is most prominent in the rigid tetramer, which displays dominant long-lived excimer behavior in addition to a minority component resembling single PDI-like emission. We then use single-mole...