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

摘要:

A set of conjugated oligo- and polytluorene-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 tiara 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 pant to the iridium ion exhibited a particularly long triplet lifetime (> 100 mu s), indicative of a (LC)-L-3 excited state. A moderately high quantum yield (similar to 0.5) was displayed by this (LC)-L-3-featured phosphor. DEL 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.

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