posted on 2023-12-28, 12:36authored byChenjian Lin, James P. O’Connor, Brian T. Phelan, Ryan M. Young, Michael R. Wasielewski
Photoexcitation
of molecular electron donor and/or acceptor chromophore
aggregates can greatly affect their charge-transfer dynamics. Excitonic
coupling not only alters the energy landscape in the excited state
but may also open new photophysical pathways, such as symmetry-breaking
charge separation (SB-CS). Here, we investigate the impact of excitonic
coupling on a covalent donor–acceptor–acceptor system
comprising a perylene donor (Per) and two perylenediimide (PDI) acceptor
chromophores in which the three components are π-stacked in
a geometry that is slipped along their long axes (Per-PDI2). Following selective photoexcitation of
PDI, femtosecond transient absorption data for Per-PDI2 is compared to that for the single-donor,
single-acceptor Per-PDI system, and the PDI2 dimer, which both have the same interchromophore
geometry as Per-PDI2. The data
show that electron transfer from Per to the lower exciton state of
the PDI dimer is slower than that of the single PDI acceptor system.
This is due to the lower free energy of the reaction for charge separation
because of the electronic stabilization afforded by the excitonic
coupling between the PDIs. While PDI2 was shown previously to undergo ultrafast SB-CS, the strong
π–π electronic interaction of Per with the adjacent
PDI in Per-PDI2 breaks the electronic
symmetry of the PDI dimer, resulting in the oxidation of Per rather
than SB-CS. These results show that the electronic coupling between
molecules designed to accept charges produced by SB-CS in molecular
dimers and the chromophores comprising the dimer must be balanced
to favor SB-CS.