Absorption, phosphorescence and Raman spectra of IrQ(ppy)₂ organometallic compound

The absorption and photoluminescence (PL) spectra, PL decays, Raman spectrum, cyclic voltammetry (CV) and nuclear magnetic resonance of heteroleptic Ir-compound IrQ(ppy)₂ compound with two phenylpyridine (ppy) ligands and one quinoline (Q) ligand have been investigated experimentally and theoretically. Two very weak absorption bands due to the transitions to the triplet states are found at about 560 and 595 nm in IrQ(ppy)₂ doped in CH₂Cl₂ solution. IrQ(ppy)₂ exhibits a dual emission of red and green phosphorescence bands. The red emission intensity is much higher than the green one in IrQ(ppy)₂ powder, but much lower than the green one in lightly IrQ(ppy)₂-doped CH₂Cl₂ solution and PMMA film. The intensity ratio of the red emission to the green emission, however, is observed to increase with increasing the IrQ(ppy)₂ concentration in CH₂Cl₂ solution and PMMA film. The enhancement of the red emission is suggested to be caused by the Forester energy transfer from Ir-ppy component to Ir–Q components between two neighboring IrQ(ppy)₂ molecules. The HOMO energy is estimated to be −4.865 eV from the CV measurement, which is close to the HOMO energy of −4.844 eV calculated using the time dependent density function theory (TD-DFT). The LUMO energy is estimated as −2.856 eV from the HOMO energy and the long-wavelength absorption edge found at 617 nm in the absorption spectrum. The absorption spectrum of IrQ(ppy)₂ is calculated by the TD-DFT. Discussion is given on a deviation of the calculated spectrum from the measured spectrum. - Graphical abstract: Display Omitted - Highlights: • IrQ(ppy)₂ has red and green emissions of different ratio between film and solution. • Intensity ratio of red to green emissions increases with IrQ(ppy)₂ concentration. • Enhancement of red emission is due to energy transfer in two neighboring IrQ(ppy)₂. • Lowest-energy absorption band due to the S₀ → T₁ transition was found at 595 nm. • 5d-Ir orbital and π-obital of quinoline (Q) form HOMO, while π^∗ orbitals of Q LUMO