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) Angstrom, 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 pi*(diimine) with partial p(2)(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 (MLCT)-M-3 state. The nature of the emission of 7 at room temperature is also (MLCT)-M-3 but changes to IL at 77 K. Complex 8 does not emit at room temperature, but at 77 K, the IL pi(pdpp) --> pi*(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) --> pi*(diimine)], with the latter exhibiting partial sigma*(carbene) parentage. The excited-state energies and redox potentials can be tuned using diimine ligands with varying electron-donating/accepting abilities.