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-molecule spectroscopy and imaging to understand
how conformational-dependent charge transfer properties influence
tetramer photophysics. Over 90% of single rigid tetramers display
telegraphic (i.e., two-level) blinking behavior with relatively short
“on” times compared to ∼60% of single floppy
tetramer transients, which tend to exhibit emission from multiple
levels. Electronic structure simulations were next performed to aid
in the assignment of electronic transitions and photophysical behavior.
Floppy tetramer canonical and natural transition orbitals reveal remarkable
similarities with significant charge transfer character in the lowest
energy excited states involving transverse PDI units and appreciable
spirobifluorene contributions in the ground electronic state. Rigid
tetramers exhibit greater electronic delocalization, and calculated
absorption transition energies show good agreement with experiment,
although excited-state interactions are less straightforward to discern
from simulations. Raman spectroscopy and polarization-dependent single-molecule
spectroscopy were also performed, supporting assignments based on
theoretical predictions and electronic spectroscopy results. Overall,
we demonstrate the importance of molecular geometry and conformational
flexibility of multichromophore arrays in determining the nature of
electronic interactions in ground and excited states, which can eventually
be harnessed to improve performance attributes at the materials level.