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Download fileSpectroscopic and Excited-State Properties of Luminescent Rhenium(I) N-Heterocyclic Carbene Complexes Containing Aromatic Diimine Ligands
journal contribution
posted on 1998-03-25, 00:00 authored by Wen-Mei Xue, Michael Chi-Wang Chan, Zhong-Min Su, Kung-Kai Cheung, Shiuh-Tzung Liu, Chi-Ming CheRhenium(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 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 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.