Spectroscopic and excited-state properties of luminescent rhenium(I) N-heterocyclic carbene complexes containing aromatic diimine ligands

Rhenium(I) N-heterocyclic carbene complexes of the type [HNCH₂CH₂NHCRe(L-L)(CO)₃]⁺ have been prepared, where L-L = 4,4′-dimethoxy-2,2′-bipyridine (2), 4,4′-bis(tert-butyl)-2,2′-bipyridine (3), 2,2′-bipyridine (4), 4,4′-dichloro-2,2′-bipyridine (5), 4,4′-bis(carbomethoxy)-2,2′-bipyridine (6), 5-phenyl-1,10-phenanthroline (7), and o-phenylenebis(diphenylphosphine) (8). The molecular structures of 4, 6, and 8 have been determined by X-ray analyses and show Re-C(carbene) bond distances of 2.171(7), 2.163(4), and 2.199(6) Å, respectively. HF-SCF and MP₂ calculations on the model compound [HNCH₂CH₂NHCRe(NHCHCHNH)-(CO)₃]⁺ (4m) show that the HOMO is nonbonding d(Re) and the LUMO is mainly π*(diimine) with partial p_z(carbene) character. CIS calculations on the excited state of optimized 4m suggest that the lowest energy absorption originates from a HOMO to LUMO spin-forbidden transition. Complexes 2-7 are emissive at room temperature and 77 K. The room-temperature and 77 K luminescence data of 2-6 are consistent with emission from a ³-MLCT state. The nature of the emission of 7 at room temperature is also ³MLCT but changes to IL at 77 K. Complex 8 does not emit at room temperature, but at 77 K, the IL π(pdpp) → π(pdpp) emission is observed. The combination of detailed spectroscopic studies and theoretical calculations reveal that the emitting state at room temperature is ³[d(Re) → π*-(diimine)], with the latter exhibiting partial σ*(carbene) parentage. The excited-state energies and redox potentials can be tuned using diimine ligands with varying electron-donating/accepting abilities.