Zipper assembly of SHJ photosystems: focus on red naphthalenediimides, optoelectronic finetuning and topological matching

The objective of this study was to synthesize multichromophoric donor-acceptor systems with non-halogenated red (R-O) naphthalenediimides (NDIs) attached along p-oligophenyl (POP) and oligophenylethynyl (OPE) scaffolds, and to evaluate their usefulness for zipper assembly of artificial photosystems. Compared to halogenated red NDIs (R-Cl, R-Br), the HOMO of R-O is 0.2 eV higher and the HOMO/LUMO gap 0.1 eV smaller, the latter introducing a shade of pink. Consistent with higher HOMO levels, R-O zippers generate less photocurrent than R-Br zippers in their respective action spectra. R-O zippers are less sensitive to topological mismatch than R-Br zippers and thus more robust and broadly applicable. Transient absorption measurements reveal efficient electron transfer from excited OPE donors to R-O acceptors and less efficient hole injection from excited R-O donors into OPE acceptors. Both processes demonstrate compatibility with OMARG-SHJ photosystems (supramolecular n/p-heterojunctions with oriented multicolored antiparallel redox gradients). Decreasing hole transfer with decreasing HOMO energy differences further demonstrates that SHJ-type hole injection disappears gradually (rather than abruptly). Losses in photonic energy during this process can thus be minimized by optoelectronic finetuning, but eventual gains in open circuit voltages risk coming with complementary losses in short circuit current.