Structure–Photoluminescence Quenching Relationships of Iridium(III)–Tris(phenylpyridine) Complexes

Abstract

Keywords: Iridium / Luminescence / Photoluminescence / Electrochemistry / Oxygen sensitivity / Density functional calculationsThe synthesis, structural, photophysical, theoretical, and electrochemical characterization of four tris(2-phenylpyridine)-based Ir III complexes are reported. The complexes were functionalized on the pyridine or on the phenyl rings with amide moieties substituted with a tris(ethyl)amine or ethyl groups, thereby yielding a family of compounds with hemicaged or open (without a capping unit but with similar functional groups on the ligand) structure. Within the context of the parent tris(2-phenylpyridine) and the full-cage iridium(III) complexes, structure-photoluminescence quenching relationships (SPQR) of the four complexes have been investigated. Luminescence quenching by oxygen has been studied with Stern-Volmer plots and through evaluation of the thermodynamic parameters involved in the quenching process. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations have been performed on the IntroductionHere we report the evidence for a structure-related shielding effect of the oxygen quenching of the luminescence of iridium(III)-tris(phenylpyridine) derivatives with hemicaged (i.e., with a capping unit) or open (i.e., without a capping unit) ligand structures. Iridium(III)-based luminophores have been the object of extensive studies in the last decades, especially due to their wide spectrum of possible applications.[1] The possibility of tuning the emission wavelength and the generally good emission properties make iridium(III) complexes ideal luminophores for the realization of organic light-emitting diodes (OLEDs), [2][3][4] [ and, more recently, also for several biomedical applications. [5][6][7][8] Moreover, the interaction of the excited states of transition-metal complexes with dioxygen (which results in the quenching of luminescence and the generation of highly reactive species like singlet oxygen and superoxide radical anion) has received increasing interest due to the possible realization of diagnostic and therapeutic agents. [9][10][11][12][13][14] Despite the studies directed towards understanding the theoretical relationship between structure and optical properties, [15,16] little is known about the structure-photoluminescence quenching relationship (SPQR) of these complexes. Transition-metal complexes with a cagelike ligand structure show a remarkable decrease in oxygen quenching. For instance, an Ru II caged complex shows an 80 % decrease in oxygen quenching with respect to [Ru(bpy) 3 ] 2+ (bpy = 2,2Ј-bipyridine), [17] and an Ir III caged complex (ii, Scheme 1) that shows a similar decrease in oxygen quenching compared to the archetypical [Ir(ppy) 3 ] (ppy = 2-phenylpyridine) (i, Scheme 1); this has been recently described by our group.[18] However, the origin of this quenching decrease is not yet clear. Moreover, many parameters (e.g., triplet energy, oxidation potential) are different from the caged to the respective reference complexes {i.e., [Ru(bpy) ...

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