Tuning Emission Lifetimes of Ir(C^N)2(acac) Complexes with Oligo(phenyleneethynylene) Groups

Emissive compounds with long emission lifetimes (μs to ms) in the visible region are of interest for a range of applications, from oxygen sensing to cellular imaging. The emission behavior of Ir(ppy)₂(acac) complexes (where ppy is the 2-phenylpyridyl chelate and acac is the acetylacetonate chelate) with an oligo(para-phenyleneethynylene) (OPE3) motif containing three para-rings and two ethynyl bridges attached to acac or ppy is examined here due to the accessibility of the long-lived OPE3 triplet states. Nine Ir(ppy)₂(acac) complexes with OPE3 units are synthesized where the OPE3 motif is at the acac moiety (aOPE3), incorporated in the ppy chelate (pOPE3) or attached to ppy via a durylene link (dOPE3). The aOPE3 and dOPE3 complexes contain OPE3 units that are decoupled from the Ir(ppy)₂(acac) core by adopting perpendicular ring-ring orientations, whereas the pOPE3 complexes have OPE3 integrated into the ppy ligand to maximize electronic coupling with the Ir(ppy)₂(acac) core. While the conjugated pOPE3 complexes show emission lifetimes of 0.69-32.8 μs similar to the lifetimes of 1.00-23.1 μs for the non-OPE3 Ir(ppy)₂(acac) complexes synthesized here, the decoupled aOPE3 and dOPE3 complexes reveal long emission lifetimes of 50-625 μs. The long lifetimes found in aOPE3 and dOPE3 complexes are due to intramolecular reversible electronic energy transfer (REET) where the long-lived triplet-state metal to ligand charge transfer (³MLCT) states exchange via REET with the even longer-lived triplet-state localized OPE3 states. The proposed REET process is supported by changes observed in excitation wavelength-dependent and time-dependent emission spectra from aOPE3 and dOPE3 complexes, whereas emission spectra from pOPE3 complexes remain independent of the excitation wavelength and time due to the well-established ³MLCT states of many Ir(ppy)₂(acac) complexes. The long lifetimes, visible emission maxima (524-526 nm), and photoluminescent quantum yields of 0.44-0.60 for the dOPE3 complexes indicate the possibility of utilizing such compounds in oxygen-sensing and cellular imaging applications.

Divergent Approach for Tris-Heteroleptic Cyclometalated Iridium Complexes Using Triisopropylsilylethynyl-Substituted Synthons

Bis-heteroleptic cyclometalated iridium complexes of the form Ir(L^a)₂(acac), where L^a is a substituted 2-phenylpyridine derivative and acac is an acetylacetonato ligand, are a useful class of luminescent organometallic complexes for a range of applications. Related tris-heteroleptic complexes of the form Ir(L^a)(L^b)(acac) offer the potential advantage of greater functionality through the use of two different cyclometalated ligands but are, in general, more difficult to obtain. We report the synthesis of divergent bis- and tris-heteroleptic triisopropylsilylethynyl-substituted intermediate complexes that can be diversified using a “chemistry-on-the-complex” approach. We demonstrate the methodology through one-pot deprotection and Sonogashira cross-coupling of the intermediate complexes with para-R-aryliodides (R = H, SMe, and CN). The photophysical and electrochemical behaviors of the resultant bis- and tris-heteroleptic complexes are compared, and it is shown that the tris-heteroleptic complexes exhibit subtly different emission and redox properties to the bis-heteroleptic complexes, such as further red-shifted emission maxima and lower extinction coefficients, which can be attributed to the reduced symmetry. It is demonstrated, supported by DFT and time-dependent DFT calculations, that the charge-transfer character of the emission can be altered via variation of the terminal substituent; the introduction of an electron-withdrawing cyano group in the terminal position leads to a significant red shift, while the introduction of an SMe group can substantially increase the emission quantum yield. Most notably, this convenient synthetic approach reduces the need to perform the often challenging isolation of tris-heteroleptic complexes to a single divergent intermediate, which will simplify access to families of complexes of the form Ir(L^a)(L^b)(acac).

Exceptionally fast radiative decay of a dinuclear platinum complex through thermally activated delayed fluorescence

A novel dinuclear platinum(ii) complex featuring a ditopic, bis-tetradentate ligand has been prepared. The ligand offers each metal ion a planar O^N^C^N coordination environment, with the two metal ions bound to the nitrogen atoms of a bridging pyrimidine unit. The complex is brightly luminescent in the red region of the spectrum with a photoluminescence quantum yield of 83% in deoxygenated methylcyclohexane solution at ambient temperature, and shows a remarkably short excited state lifetime of 2.1 μs. These properties are the result of an unusually high radiative rate constant of around 4 × 105 s-1, a value which is comparable to that of the very best performing Ir(iii) complexes. This unusual behaviour is the result of efficient thermally activated reverse intersystem crossing, promoted by a small singlet-triplet energy difference of only 69 ± 3 meV. The complex was incorporated into solution-processed OLEDs achieving EQEmax = 7.4%. We believe this to be the first fully evidenced report of a Pt(ii) complex showing thermally activated delayed fluorescence (TADF) at room temperature, and indeed of a Pt(ii)-based delayed fluorescence emitter to be incorporated into an OLED.

Cyclometalated Ir(iii) complexes towards blue-emissive dopant for organic light-emitting diodes: fundamentals of photophysics and designing strategies

The fundamental photophysics of cyclometalated Ir(iii) complexes and surveys design strategies for efficient blue phosphorescent Ir(iii) complexes are summarised.

Highly Efficient Deep-Red Organic Light-Emitting Devices Based on Asymmetric Iridium(III) Complexes with the Thianthrene 5,5,10,10-Tetraoxide Moiety

Highly efficient deep-red organic light-emitting devices (OLEDs) are indispensable for developing high-performance red-green-blue (RGB) displays and white OLEDs (WOLEDs). However, the shortage of deep-red emitters with high photoluminescence quantum yields (PLQYs) and balanced charge injection/transport abilities has severely restricted the performance of deep-red OLEDs. Herein, we design and synthesize four efficient emitters by combining the isoquinoline group with the thianthrene 5,5,10,10-tetraoxide group. Benefited from the introduction of the thianthrene 5,5,10,10-tetraoxide group, these Ir(III) complexes show improved electron-injection/-transport abilities. By enhancing the contribution of the triplet metal-to-ligand charge transfer (³MLCT) in emissions, the asymmetric configuration endows the related deep-red Ir(III) complexes with high PLQYs of 0.45-0.50 in solutions. More importantly, PLQYs of these Ir(III) complexes in doped host films increase up to 0.91, which is much higher than PLQYs reported for conventional deep-red Ir(III) complexes with impressive electroluminescent performance. As a result, solution-processed OLEDs based on these Ir(III) complexes exhibit deep-red emissions with Commission Internationale de L'Eclairage (CIE x, y) coordinates very close to the National Television System Committee (NTSC)-recommended standard red CIE coordinates of (0.67, 0.33). Furthermore, a deep-red OLED using the asymmetric Ir(III) complex SOIrOPh as the emitter shows outstanding performance with a peak external quantum efficiency (EQE) of 25.8%, which is the highest EQE reported for solution-processed deep-red OLEDs. This work sheds light on the great potential of utilizing the thianthrene 5,5,10,10-tetraoxide group to develop phosphorescent emitters for highly efficient OLEDs.

Theoretical investigation of the vibronic phosphorescence spectra and quantum yields for iridium(III) complexes with 2-(2,5,2',3',4',5',6'-heptafluoro-biphenyl-4-yl)-pyridine as the primary ligand

Cyclometalated Ir(III) complexes are widely used as phosphorescent materials in organic light-emitting diodes. In this work, the vibrationally resolved phosphorescence spectra of an experimental reported and four novel designed Ir(III) complexes with 2-(2,5,2',3',4',5',6'-heptafluoro-biphenyl-4-yl)-pyridine (HFYP) as primary ligand are investigated by theoretical calculations. The ancillary ligands are 3-(pentafluorophenyl)-pyridin-2-yl-1,2,4-triazolate (exp3), 3-(trifluoromethyl)-pyridin-2-yl-1,2,4-triazolate (5), 5-methylsulfonyl-2-oxyphenyl-2-oxazole (6), 5-trifluoromethyl-2-oxyphenyl-2-oxazole (7), 2-(3-(trifluoromethyl)-1H-1,2-diazol-5-yl)pyridine (8), respectively. Phosphorescence spectra show that there are mainly two strong peaks, which can be ascribed as low-frequency vibrational modes such as the rotation of ligand plane, and benzene ring/pyridine ring in ligand HFYP1 skeleton vibration coupled with CH in pyridine ring in plane bending vibration. The phosphorescence quantum yields were quantitatively determined by evaluating radiative decay rate constant k_r, intersystem crossing rate constatant k_ISC and temperature-dependent nonradiative decay rate constant k_nr(T). It shows that the quantum yields of compounds exp3, 5 and 8 are relative higher, while those of compounds 6 and 7 are much smaller. This is mainly caused by larger k_nr(T) of compounds 6 and 7. It is anticipated that in Ir(III) complex with HFYP primary ligand, pyridin-2-yl-1,2,4-triazolate, 1,2-diazol-5-yl-pyridine are good ancillary ligand, while 2'-oxyphenyl-2-oxazoline is not appropriate to be ancillary ligand.

Conjugated, rigidified bibenzimidazole ancillary ligands for enhanced photoluminescence quantum yields of orange/red-emitting iridium(iii) complexes

A family of six orange/red-emitting cationic iridium complexes were synthesized and their optoelectronic properties comprehensively characterized.