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 (t)Bu 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.