Excited-State Properties of Heteroleptic Iridium(III) Complexes Bearing Aromatic Hydrocarbons with Extended Cores

The synthesis, complete structural characterization, electrochemistry, and excited-state dynamics of a series of four bis-heteroleptic iridium(III) charge-transfer complexes composed of a single acac-functionalized and two ortho-metalated 2-phenylpyridine ligands. The formed iodophenyl complex (2) was used as a metallosynthon to introduce extended-core ethynyltolyl (3), ethynylpyrene (4), and ethynylperylene (5) residues into these structures projecting from the acac ancillary ligand. Static and dynamic photoluminescence along with ultrafast and conventional transient absorption measurements in conjunction with cyclic voltammetry were employed to elucidate the nature of the intramolecular energy-transfer processes occurring in the excited states of polychromophores 4 and 5 and are directly compared with those of model complexes 2 and 3. Upon charge-transfer excitation of these molecules, the long-lived triplet-state metal-to-ligand charge-transfer ((MLCT)-M-3)-based photoluminescence readily observed in 2 and 3 (tau = 1 mu s) is nearly quantitatively quenched, resulting from production of the associated triplet intraligand ((IL)-I-3) excited states in 4 and 5 through intramolecular triplet-triplet energy transfer. The respective formation of the extended-core (3*)pyrenyl and (3*)perylenyl-localized excited states in 4 and 5 is confirmed by their ultrafast excited-state evolution, which ultimately generates features associated with these 3IL excited states and their greatly extended excited-state lifetimes with respect to the parent complexes 2 and 3.