Long-Lived Charge Transfer Excited States in HBC-Polypyridyl Complex Hybrids

The synthesis of
two bipyridine-hexa-peri-hexabenzocoronene (bpy-HBC)
ligands functionalized with either tBu
or C12H25 and their Re­(I) tricarbonyl chloride
complexes are reported and their electronic properties investigated
using spectroscopic and computational methods. The metal complexes
show unusual properties, and we observed the formation of a long-lived
excited state using time-resolved infrared spectroscopy. Depending
on the solvent, this appears to be of the form Rebpy•HBC•+ or a bpy-centered π,π* state.
TD-DFT calculations support the donor–acceptor charge transfer
character of these systems, in which HBC is the donor and bpy is the
acceptor. The ground state optical properties are dominated by the
HBC chromophore with additional distinct transitions of the complexes,
one associated with MLCT 450 nm (ε > 17 000 L mol–1 cm–1) and another with a HBC/metal
to bpy charge transfer, termed the MLLCT band (373 nm, ε = 66 000
L mol–1 cm–1). These assignments
are also supported by resonance Raman spectroscopy.