Computational and spectroscopic studies of Re(I) bipyridyl complexes containing 2,6-dimethylphenylisocyanide (CNx) ligand

Density Functional Theory (DFT) calculations produce optimized geometries of the complexes [Re(CO)(3)(bpy)CI] (1), [Re(CO)(3)(bpy)(py)](CF3SO3) (2), [Re(CO)(3)(bpy)(CNx)](CF3SO3) (3), and [Re(CO)(bpy)(CNX)(3)](CF3SO3) (4), where bpy = 2,2 '-bipyridine, py = pyridine, and CNx = 2,6-dimethylphenylisocyanide in their ground and lowest-lying triplet states. The ground-state optimized geometry for the cation of [Re(CO)3(bpy)(CNx)](CF3SO3) (3) results in a Re-C (CNx) bond length of 2.10 angstrom, a Re-C (CO) bond length trans to CNx of 2.01 angstrom, and a Re-C (CO) bond length cis to CNx of 1.96 angstrom which compares favorably to the single-crystal analysis of a Re-C (CNx) bond length of 2.074(4) angstrom, a Re-C (CO) bond length trans to CNx of 1.971(4) angstrom, and Re-C (CO) bond length cis to CNx of 1.932(4) angstrom. The majority of the singlet excited-state energies calculated using Time-dependent Density Functional Theory (TDDFT) and Conductor-like Polarizable Continuum Model (CPCM) are metal-ligand-to-ligand charge transfer (MLLCT) states and are in good agreement with the UV-vis spectral energies for the complexes in ethanol. The complexes exhibit emission both at room temperature and at 77 K except 4 which is only emissive at 77 K. The 77 K emission lifetimes range from 3.9 mu s for 1 to 8.8 mu s for 3. The emissive lowest-lying triplet state is a (MLLCT)-M-3 state for complexes 1-3 but a triplet ligand-to-metal charge transfer ((LMCT)-L-3) state for complex 4. The electronic, electrochemical, thermodynamic, HOMO-LUMO, and emitting-state energy gaps as well as the emission lifetimes increase in the order 1 < 2 < 3. A (3)d-d excited-state, which is located above the (LMCT)-L-3 state, accounts for the loss of room-temperature emission for complex 4.