Interchromophoric communication– when darkness falls: This study investigates the electronic communication between various hole-transporting (HT) ligands and a rhenium(phenanthroline) centre to which they are coordinated. It is found that when the conjugation within the HT ligand is increased, the energy of a non-emissive HT ligand electronic transition is sufficiently lowered so that it interacts with, and partially deactivates, emission from the rhenium(phenanthroline) moiety (see figure).
Spectroscopic, electrochemical and density functional theory (DFT) methods have been employed to investigate a group of [Re(CO)3(HT)(phen)]+ complexes (phen=1,10-phenanthroline), and in particular the level of electronic communication between various hole-transporting (HT) ligands and the rhenium centre. Here, the HT ligand consists of a coordinating pyridine connected to dimethylaniline group through a single-, double- or triple-bond-connecting system. Electronic absorption, resonance Raman, and steady-state emission spectroscopy combined with lifetime studies and DFT calculations suggest that multiple dπ(Re)→π*(phen) metal-to-ligand charge transfers (MLCTs) exist for each complex, two of which significantly absorb at about 340 and 385 nm, and one that emits at approximately 540 nm. In the complexes containing more-conjugated HT ligands, non-emissive intraligand transitions (IL(HT)) exist with energies between the ground and MLCT excited states. The overlap of these IL(HT) transitions and the absorbing MLCT of lowest energy deactivates emission resulting from about 385 nm excitation, and lowers the quantum yield and excited-state lifetimes of these complexes. Cyclic voltammetry experiments indicate that throughout the series investigated, the highest occupied molecular orbital (HOMO) of each complex is centred on the HT ligand, while the occupied molecular orbitals localised on the rhenium are lower in energy.