A spectroscopic and computational study on the effects of methyl and phenyl substituted phenanthroline ligands on the electronic structure of Re(I) tricarbonyl complexes containing 2,6-dimethylphenylisocyanide

Abstract

[Re(CO)3(CNx)(L)]+, where CNx = 2,6-dimethylphenylisocyanide, forms complexes with L = 1,10-phenanthroline (1), 4-methyl-1,10-phenanthroline (2), 4,7-dimethyl-1,10-phenanthroline (3), 3,4,7,8-tetramethyl-1,10-phenanthroline (4), 2,9-dimethyl-1,10-phenanthroline (5) and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (6). The metal–ligand-to-ligand charge transfer transition (MLLCT) absorption bands follow the series: 3 (27800 cm−1) > 1, 2, 4 and 5 (27500 cm−1) > 6 (26600 cm−1). Density functional theory (DFT) geometry optimizations reveal elongated Re–N (L) distances of 2.28 and 2.27 Å for 5 and 6, respectively, compared to 2.23 Å for 1–4. The reversible reduction potentials (E1/2(red)) of 1–4 are linearly dependent on the B3LYP calculated LUMO energies. Time-dependent (TD) DFT and conductor-like polarizable continuum model (CPCM) calculated singlet excited states deviate by 700 cm−1 or less from the experimental absorption maxima and aid in the spectral assignments. The 3MLLCT emitting state energies are within 900 cm−1 of the experimental 77 K emission energies for 1–6. The 77 K emission energies, E1/2(red), and the room temperature emission quantum yields (LUMOem) decrease in the order 1 > 2 > 3 > 4 whereas ELUMO and the room temperature emission energies follow the opposite trend. The emission lifetimes (τem) decrease in the order 3 > 4 > 2 > 1 > 5 with 3 having the highest emission lifetime values of 26.9 µs at room temperature and 384 µs at 77 K and complex 5 having the lowest emission lifetimes of 4.6 µs at room temperature and 61 µs and 77 K.