The Role of Substituents on Functionalized 1,10-Phenanthroline in Controlling the Emission Properties of Cationic Iridium(III) Complexes of Interest for Electroluminescent Devices

The photophysical and electrochemical properties of the novel complexes [Ir(ppy)2(5-X-1,10-phen)][PF6] (ppy = 2-phenylpyridine, phen = phenanthroline, X = NMe2, NO2), [Ir(pq)2(5-X-1,10-phen)][PF6] (pq = 2-phenylquinoline, X = H, Me, NMe2, NO2), [Ir(ppy)2(4-Me,7-Me-1,10-phen)][PF6], [Ir(ppy)2(5-Me,6-Me-1,10-phen)][PF6], [Ir(ppy)2(2-Me,9-Me-1,10-phen)][PF6], and [Ir(pq)2(4-Ph,7-Ph-1,10-phen)][PF6] have been investigated and compared with those of the known reference complexes [Ir(ppy)2(4-Me or 5-H or 5-Me-1,10-phen)][PF6] and [Ir(ppy)2(4-Ph,7-Ph-1,10-phen)][PF6], showing how the nature and number of the phenanthroline substituents tune the color of the emission, its quantum yield, and the emission lifetime. It turns out that the quantum yield is strongly dependent on the nonradiative decay. The geometry, ground state, electronic structure, and excited electronic states of the investigated complexes have been calculated on the basis of density functional theory (DFT) and time-dependent DFT approaches, thus substantiating the electrochemical measurements and providing insight into the electronic origin of the absorption spectra and of the lowest excited states involved in the light emission process. These results provide useful guidelines for further tailoring of the photophysical properties of ionic Ir(III) complexes.