High-Resolution Tuning of Iridium(III) 4′-Aryl-terpy Chromophores: A Hammett Parameter-Guided General Methodology for Systematic Property Control through Orbital Decoupling

A series of six phenyl-substituted 4'-phenyl-2,2':6',2″-terpyridine, RPhTerpy, iridium(III) complexes of the form [Ir(RPhTerpy)(ppy)Cl](PF₆), where ppy is C^N cyclometalated 2-(phenyl)pyridine, and with Hammett parameters spanning 1.69 units, were synthesized using microwave-assisted reaction procedures and characterized via physical (cyclic voltammetry, NMR, crystallography) and photophysical methods (absorption, emission, time-correlated single photon counting, Franck-Condon line shape analysis). The iridium complexes' redox potentials, electrochemically determined ground state HOMO-LUMO gap (eHLG), photophysically determined energy gap between ground- and excited-state (E₀₀, 77 K), estimated excited-state reduction potential (E(Ir*/-)), and time-dependent density functional theory predicted HOMO-LUMO gap and lowest energy transition (LET) correlate strongly to the ligands' Hammett parameters, suggesting that the Hammett parameter can be used as a convenient method to model and fine-tune the physical and photophysical characteristics for this series of Ir(III) complexes. These analyses reveal similar correlations when applied to data for emissive complexes previously reported. Experimental and computational modeling data indicate that the HOMO and LUMO are effectively decoupled, suggesting independent control over both is possible and offering a general methodology for high-resolution design of desired characteristics.