Oligo- and Polyfluorene-Tetheredfac-Ir(ppy)3: Substitution Effects

Abstract

A set of conjugated oligo- and polyfluorene-tethered fac-Ir(ppy)3 complexes were synthesized. In addition to steady-state absorption and emission, time-resolved emission spectroscopy was used to systematically study the correlation of photophysical properties with chemical structures. A chain length dependency study showed that both radiative and nonradiative triplet decay rates, as well as the phosphorescence quantum yield, decreased with increasing chain length of the appended oligofluorene. Notably, the complex with oligofluorene tethered to the pyridine para to phenyl ring possessed a substantially higher phosphorescence quantum efficiency and shorter lifetime than those of an isomeric complex with the oligofluorene linked to the phenyl ring para to pyridine. Nonetheless, both these two oligomer complexes exhibited an excited state of mixed MLCT (metal-to-ligand charge transfer) and LC (ligand-centered) transitions, whereas another isomeric complex having an oligofluorene appended to the phenyl ring para to the iridium ion exhibited a particularly long triplet lifetime (>100 μs), indicative of a 3LC excited state. A moderately high quantum yield (∼0.5) was displayed by this 3LC-featured phosphor. DFT calculations substantiated the proposition that the attachment of oligofluorene to Ir(ppy)3 at different positions resulted in varied molecular orbitals, with different relative contribution of MLCT to the emissive excited state. Hence, photophysical properties such as radiative decay rate, lifetime, and quantum yield, etc., were all influenced by the substitution isomerism. As these results indicated that if short lifetime and fast radiative decay were desired, among different substitution patterns appending the conjugated chain to the pyridine unit was the most favorable. Thus, star-shaped complexes with an oligo- or polyfluorene tethered to each of the three pyridine units of Ir(ppy)3 were prepared. In such a structure, the tris-cyclometalated iridium effected nearly complete intersystem crossing (ISC) in all three ligands across three fluorene units, without compromising the phosphorescence quantum yield. But the study showed that further extending the conjugated ligand resulted in partial ISC or even complete loss of capacity for ISC beyond a certain distance.