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Spectroscopic and Excited-State Properties of Luminescent Rhenium(I) N-Heterocyclic Carbene Complexes Containing Aromatic Diimine Ligands

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journal contribution
posted on 25.03.1998, 00:00 by Wen-Mei Xue, Michael Chi-Wang Chan, Zhong-Min Su, Kung-Kai Cheung, Shiuh-Tzung Liu, Chi-Ming Che
Rhenium(I) N-heterocyclic carbene complexes of the 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 (4m) show that the HOMO is nonbonding d(Re) and the LUMO is mainly π*(diimine) with partial pz(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 27 are emissive at room temperature and 77 K. The room-temperature and 77 K luminescence data of 26 are consistent with emission from a 3MLCT 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.