Electrochemiluminescence as emerging microscopy techniques.

The use of electrochemiluminescence (ECL), i.e., chemiluminescence triggered by electrochemical stimulus, as emitting light source for microscopy is an emerging approach with different applications ranging from the visualization of nanomaterials to cell mapping. In this trend article, we give an overview of the state of the art in this new field with the purpose to illustrate all the possible applications so far explored as well as describing the mechanism underlying this transduction technique. The results discussed here would highlight the great potential of the combination between ECL and microscopy and how this marriage can turn into an innovative approach with specific application in analytical sciences. Graphical abstract.

ß-Diketonate ancillary ligands in heteroleptic iridium complexes: a balance between synthetic advantages and photophysical troubles.

ß-Diketones are an important class of bidentate cyclometalating compounds, used in organometallic chemistry as ancillary ligands because of their wide commercial availability and easy synthesis. They are employed to finely tune the electronic, spectroscopic and physical properties of metal complexes. Heteroleptic iridium complexes often benefit from the use of ß-diketonate ligands, their properties being similar to those of the corresponding homoleptic tris-cyclometalated ones. Nevertheless, in some cases, their use results in a complete quenching of the phosphorescence. Aiming to understand the origin of this drawback, we designed a suitable class of heteroleptic complexes and studied their thermal stability (DSC/TGA). We explored the effect of the ancillary ligand in a series of Ir(iii) complexes bearing 2-phenylpyridine (ppy) as a cyclometalated ligand and acac (acetylacetonate), tta (2-thienoyltrifluoroacetonate), dtdk (1,3-di(thiophen-2-yl)propane-1,3-dionate) and BPhen (4,7-diphenyl-1,10-phenanthroline) as ancillary ligands. Through photochemical and electrochemical investigations, whose results agree with and support our density functional theory calculations, we demonstrate that ß-diketonate ligands with low triplet energy generate dark triplet excited states with negligible coupling to the ground state which indeed promote non-radiative relaxation through population of higher states.

Upper limit to the ultimate achievable emission wavelength in near-IR emitting cyclometalated iridium complexes.

Iridium complexes bearing cyclometalated (C^N) ligands are the current emitters of choice for efficient phosphorescent organic light emitting diodes (OLEDs). Homoleptic iridium complexes Ir(C^N)3 and the analogous heteroleptic ones carrying a ß-diketonate ancillary ligand (C^N)2Ir(O^O) often exhibit similar photophysical properties and device performances; the choice among them usually depends both on the yield/ease of the respective synthetic preparations as well as on the device fabrication methods (i.e. vacuum-deposition or solution-process). In our recent study we found a significant spectral red shift on going from the homoleptic to the ß-diketonate Ir(iii) derivatives. The NIR emitting complex Ir(iqbt)2dpm (λmax = 710 nm) has almost 20 nm red shifted emission compared to the homologue Ir(iqbt)3 making only the former a real NIR emitter. For comparison, we studied the Pt(iqbt)dpm complex as the suitable example to investigate metal ligand interactions. Noteworthily the Pt(iqbt)dpm emission perfectly overlaps that of the Ir(iqbt)2dpm. In this paper we provide an in-depth investigation of these systems by electrochemical and spectroscopic analyses and corroborate the results with DFT and TDDFT calculations to investigate whether the Pt(ii) complex can be used as a model system to predict how far the emission can be pushed in a Ir(iii) heteroleptic derivative bearing the same C^N ligand.

Near-IR Emitting Iridium(III) Complexes with Heteroaromatic ß-Diketonate Ancillary Ligands for Efficient Solution-Processed OLEDs: Structure-Property Correlations.

Three NIR-emitting neutral Ir(III) complexes [Ir(iqbt)2 (dpm)] (1), [Ir(iqbt)2 (tta)] (2), and [Ir(iqbt)2 (dtdk)] (3) based on the 1-(benzo[b]thiophen-2-yl)-isoquinolinate (iqtb) were synthesized and characterized (dpm=2,2,6,6-tetramethyl-3,5-heptanedionate; tta=2-thienoyltrifluoroacetonate; dtdk=1,3-di(thiophen-2-yl)propane-1,3-dionate). The compounds emit between λ=680 and 850 nm with high luminescence quantum yields (up to 16 %). By combining electrochemistry, photophysical measurements, and computational modelling, the relationship between the structure, energy levels, and properties were investigated. NIR-emitting, solution-processed phosphorescent organic light-emitting devices (PHOLEDs) were fabricated using the complexes. The devices show remarkable external quantum efficiencies (above 3 % with 1) with negligible efficiency roll-off values, exceeding the highest reported values for solution-processible NIR emitters.

The Quantum Efficiency Roll-Off Effect in Near-Infrared Organic Electroluminescent Devices with Iridium Complexes Emitters.

The electroluminescence quantum efficiency roll-off in iridium(III)-based complexes, namely Ir(iqbt)2(dpm) and Ir(iqbt)3 (iqbt = 1 (benzo[b]thiophen-2-yl)-isoquinolinate, dpm = 2,2,6,6-tetramethyl-3,5-heptanedionate) utilized as near-infrared emitters in organic light emitting diodes with remarkable external quantum efficiencies, up to circa 3%, 1.5% and 1%, are measured and analyzed. With a 5-6 weight% of emitters embedded in a host matrix, the double-layer solution-processed structure as well as analogous three-layer one extended by a hole-conducting film are investigated. The triplet-polaron, the Onsager electron-hole pair dissociation and the triplet-triplet annihilation approaches were used to reproduce the experimental data. The mutual annihilation of triplets in iridium emitters was identified as prevailingly controlling the moderate roll-off, with the interaction between those of iridium emitters and host matrixes found as being less probable. Following the fitting procedure, the relevant rate constant was estimated to be ( 0.5 - 12 ) × 10 - 12 cm3/s, values considered to be rather too high for disordered organic systems, which was assigned to the simplicity of the applied model. A coexistence of some other mechanisms is therefore inferred, ones, however, with a less significant contribution to the overall emission quenching.