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'-b ipyridine (2), 4,4'-bis(tert-butyl)2,2'-bipyridine (3), 2,2'-bipyridin e (4), 4,4'-dichloro-2,2'-bipyridine (5), 4,4'-bis(carbomethoxy)2,2'-b ipyridine (6), 5-phenyl-1,10-phenanthroline (7), and o-phenylenebis(di phenylphosphine) (8). The molecular structures of 4, 6, and 8 have bee n determined by X-ray analyses and show Re-C(carbene) bond distances o f 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 mainl y pi(diimine) with partial p(2)(carbene) character. CIS calculations on the excited state of optimized 4m suggest that the lowest energy ab sorption originates from a HOMO to LUMO spin-forbidden transition. Com plexes 2-7 are emissive at room temperature and 77 K. The room-tempera ture and 77 K luminescence data of 2-6 are consistent with emission fr om a (MLCT)-M-3 state. The nature of the emission of 7 at room tempera ture 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) e mission is observed. The combination of detailed spectroscopic studies and theoretical calculations reveal that the emitting state at room t emperature is (3)[d(Re) --> pi(diimine)], with the latter exhibiting partial sigma(carbene) parentage. The excited-state energies and redo x potentials can be tuned using diimine ligands with varying electron- donating/accepting abilities.