Two new cyclometalated iridium complexes with non-conjugated triphenylene moieties attached to the ancillary ligand, namely YF3 and YF4, are reported. Both iridium complexes presented intense bluish-green emission both in solution (ΦPL ∼ 0.37–0.5) and in the solid state. A high hole mobility of 0.001 cm2 V−1 s−1 was obtained for YF4 annealed film, evidenced by the space-charge-limited-current method. This result is among the highest hole mobilities reported for a cyclometalated iridium complex. Organic light-emitting diodes employing these iridium complexes as dopants were fabricated. The influence of the annealing temperature of both the emitting layer and the host matrix on the device performance was explored. Interestingly, YF4-based device with an annealing temperature of 60 °C shows the best performance with a maximum current efficiency of 15.6 cd A−1 and an external quantum efficiency of 6.8% compared to 1.5 cd A−1 and 0.8% for the non-annealed device.

Two new cyclometalated iridium complexes with non-conjugated triphenylene moieties attached to the ancillary ligand, namely YF3 and YF4, are reported. Both iridium complexes presented intense bluish-green emission both in solution (ΦPL ∼ 0.37–0.5) and in the solid state. A high hole mobility of 0.001 cm2 V−1 s−1 was obtained for YF4 annealed film, evidenced by the space-charge-limited-current method. This result is among the highest hole mobilities reported for a cyclometalated iridium complex. Organic light-emitting diodes employing these iridium complexes as dopants were fabricated. The influence of the annealing temperature of both the emitting layer and the host matrix on the device performance was explored. Interestingly, YF4-based device with an annealing temperature of 60 °C shows the best performance with a maximum current efficiency of 15.6 cd A−1 and an external quantum efficiency of 6.8% compared to 1.5 cd A−1 and 0.8% for the non-annealed device.

The use of electron-withdrawing substituents on the orthometalated phenyl ring is a common strategy to blue shift the emission of cyclometalated iridium complexes by stabilizing the highest occupied molecular orbital (HOMO), that is, increasing the oxidation potential of the complex. However, for application in blue organic light-emitting diodes (OLEDs), this approach imposes host materials with a deep HOMO, which negatively impacts the injection of charges, and hence the performance of the devices. In this context, we report new iridium complexes with an electron-donating substituent on the cyclometalated ligand to blue shift the emission while keeping a relatively low oxidation potential. As a result, bluish-green OLEDs based on host materials with shallow HOMOs (TCTA = 4,4′,4′′-tri(N-carbazolyl)-triphenylamine) display a higher performance than devices using FIrpic in the same architecture. The improvements are primarily attributed to the lower turn-on voltage (2.8 to 3 V) compared to those of FIrpic-device (3.6 V). White OLED was then prepared with a maximum brightness of 20226 cd m−2 and current efficiency of 20.4 cd A−2 (at 100 cd m−2). Interestingly, a very small efficiency roll-off of about 1% at 1000 cd m−2 and high color stability were achieved. At a luminance level of 5000 cd m−2 the roll-off efficiency was still below 20%. The introduction of electron-donating substituents on a 2-phenylpyridine scaffold to obtain blue emitters with low oxidation potentials provides an alternative to strategies based on replacing the pyridine with imidazole, carbene, and pyrazole.

•Tow D-π-A-π-A dyes bearing 5-phenyl-5H-dibenzo-[b,f]azepine units were synthesized.•The effect of donor size on the property of sensitizer was systematically studied.•The dyes show panchromatic absorption between 300 and 800 nm in solution and neat film.•PCE of 4.38% was achieved for DSSCs based on I3−/I− electrolyte without co-sensitizer.Two D–π-A–π-A organic dyes (YC-1 and YC-2) with 5-phenyl-5H-dibenzo[b,f]azepine derivatives as donor, thiophene as π bridge, and isoindigo and cyanoacrylic acid as acceptors were prepared. YC-1 and YC-2 show a panchromatic absorption between 300 nm and 800 nm both in solution and neat film. The photovoltaic performances of both dyes were evaluated in dye-sensitized solar cells based on iodide/triiodide electrolyte without any co-sensitizer. The YC-1 based device displays better device performance with open-circuit photocurrent density of 12.12 mA cm−2, open-circuit voltage of 0.53 V, and fill factor of 68.9%, corresponding to overall conversion efficiency (η) of 4.38%. The inferior performance of device based on YC-2 (η = 1.46%) is ascribed to short electron lifetime as evidenced from electrochemical impedance spectroscopy measurement. This research provided a potential promising donor unit for organic dyes and revealed the influence of donor size in organic dyes for photovoltaic performances.

A doubly pyrene-grafted bis-cyclometallated iridium complex with engineered electronically excited states demonstrates reversible electronic energy transfer between adjacent chromophores giving rise to extremely long-lived red luminescence in solution (τ = 480 μs). Time-resolved spectroscopic studies afforded determination of pertinent photophysical parameters including rates of energy transfer and energy distribution between constituent chromophores in the equilibrated excited molecule (ca. 98% on the organic chromophores). Incorporation into a nanostructured metal–oxide matrix (AP200/19) gave highly sensitive O2 sensing films, as the detection sensitivity was 200–300% higher than with the commonly used PtTFPP and approaches the sensitivity of the best O2-sensing dyes reported to date.

To demonstrate that incompatible pendent chains can be used as a strategy to control the morphology of blends of immiscible materials, we have developed a novel triphenylene-based amphiphile-like mesogen with hydrophobic (alkyl) chains and hydrophilic (2-(2-methoxyethoxy)ethoxy) pendent chains, named TP6Gall hereafter. TP6Gall is a room-temperature liquid crystal presenting a cubic phase with a clearing point of 30 °C. Blends of TP6Gall in various amounts with an equimolar mixture of the hydrophobic 2,3,6,7,10,11-hexahexyloxytriphenylene (TP6) and hydrophilic 2,3,6,7,10,11-hexa(2-(2-methoxyethoxy)ethoxy)triphenylene (TP6EO2M) have been studied by polarised optical microscopy (POM), differential scanning calorimetry (DSC) and small-angle X-ray scattering (SAXS). Without TP6Gall, the TP6:TP6EO2M mixture exhibits large incompatible domains of pure TP6 and pure TP6EO2M. As the quantity of TP6Gall increases in the blend, the size of the domains decreases significantly. Ultimately, when the proportion of TP6Gall reaches 50 mol% of the blend, μm-featured interpenetrated networks of crystalline TP6EO2M and of TP6 mixed with TP6Gall are obtained. Interestingly, a single liquid phase is obtained above the clearing point of the blend. Furthermore, no macrophase separation is observed upon multiple temperature cycles between room-temperature and the temperature above the clearing point of the blend and the interpenetrated network is reliably reformed upon cooling.

Luminescent liquid crystal (LC) materials have attracted significant interest for organic optoelectronic applications, especially for linearly polarised emission, because of their combination of ordered alignment and luminescence property. Since the first demonstration of polarised organic light-emitting diodes (OLEDs) in 1995, remarkable progress has been made with polarised electroluminescence because of the continuous advances in the design of suitable LC materials. In this Review, we summarize luminescent LC materials with representative examples based on fluorescent materials, phosphorescent materials, and lanthanide complexes. The structure–property relationships for polarised emission and, when available, electroluminescence of these materials are presented and discussed, with a focus on phosphorescent metallomesogens. We also present the rationale behind the design and development of luminescent LC materials for high efficiency polarised OLEDs, along with the challenges ahead to achieve efficient devices.

The complex [Cu(xantphos)(dmp)][PF6] (dmp = 2,9-dimethyl-1,10-phenanthroline) in a nanostructured metal oxyde matrix shows better sensitivity to oxygen (KSV = 9.74 ± 0.87 kPa−1 between 0 and 1 kPa pO2 and 5.59 ± 0.15 kPa−1 between 0 and 10 kPa pO2) than cyclometallated iridium complexes in the same conditions.

FIrpic is the most investigated bis-cyclometallated iridium complex in particular in the context of organic light emitting diodes (OLEDs) because of its attractive sky-blue emission, high emission efficiency, and suitable energy levels. In this Perspective we review the synthesis, structural characterisations, and key properties of this emitter. We also survey the theoretical studies and summarise a series of selected monochromatic electroluminescent devices using FIrpic as the emitting dopant. Finally we highlight important shortcomings of FIrpic as an emitter for OLEDs. Despite the large body of work dedicated to this material, it is manifest that the understanding of photophysical and electrochemical processes are only broadly understood mainly because of the different environment in which these properties are measured, i.e., isolated molecules in solvent vs. device.

•A–D–A–D–A dye based on DPP and BODIPY units has been synthesized and characterized.•Novel DPP-BODIPY dye possesses quasi-planar structure and low LUMO level.•Absorption from 300 nm to 1000 nm is observed both in solution and neat film.•Near-infrared emission at 804 nm and ambipolar transporting properties are revealed.A novel near-infrared dye of DPP-BODIPY with a diketopyrrolopyrrole (DPP) central unit and two 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) flanking units has been synthesized and characterized. Its quasi-planar structure was confirmed by X-ray diffraction and panchromatic absorption from 300 nm to 1000 nm was exhibited both in solution and in solid state. Near-infrared emission at about 804 nm and low LUMO energy level at −3.93 eV are observed. Furthermore, ambipolar charge transporting properties with hole mobility of 1.5×10–5 cm2 V−1 S−1 and electron mobility of 2×10–6 cm2 V−1 S−1 were obtained in thin films.

There is presently a lack of efficient and stable blue emitters for light-emitting electrochemical cells (LEECs), which limits the development of white light emitting systems for lighting. Cyclometalated iridium complexes as blue emitters tend to show low photoluminescence efficiency due to significant ligand-centred character of the radiative transition. The most common strategy to blue-shift the emission is to use fluorine substituents on the cyclometalating ligand, such as 2,4-difluorophenylpyridine, dFppy, which has been shown to decrease the stability of the emitter in operating devices. Herein we report a series of four new charged cyclometalated iridium complexes using methoxy- and methyl-substituted 2,3′-bipyridine as the main ligands. The combination of donor groups and the use of a cyclometalated pyridine has been recently reported for neutral complexes and found electronically equivalent to dFppy. We describe the photophysical and electrochemical properties of the complexes in solution and use DFT and TDDFT calculations to gain insights into their properties. The complexes exhibit bluish-green emission with onsets around 450 nm, which correspond to the maximum emission at 77 K. Furthermore, photoluminescence quantum yields in solution are all above 40%, with the brightest in the series at 66%. Finally, LEECs were prepared using these complexes as the emissive material to evaluate the performance of this particular design. Compared to previously reported devices with fluorine-containing emitters, the emitted colours are slightly red-shifted due to methyl substituents on the coordinating pyridine of the main ligand and overall device performances, unfortunately including the stability of devices, are similar to those previously reported. Interestingly within the series of complexes there appears to be a positive effect of the methoxy-substituents on the stability of the devices. The poor stability is therefore attributed to the combination of cyclometalated pyridine and methoxy groups.

A cyclometalated iridium complex is reported where the core complex comprises naphthylpyridine as the main ligand and the ancillary 2,2′-bipyridine ligand is attached to a pyrene unit by a short alkyl bridge. To obtain the complex with satisfactory purity, it was necessary to modify the standard synthesis (direct reaction of the ancillary ligand with the chloro-bridged iridium dimer) to a method harnessing an intermediate tetramethylheptanolate-based complex, which was subjected to acid-promoted removal of the ancillary ligand and subsequent complexation. The photophysical behavior of the bichromophoric complex and a model complex without the pendant pyrene were studied using steady-state and time-resolved spectroscopies. Reversible electronic energy transfer (REET) is demonstrated, uniquely with an emissive cyclometalated iridium center and an adjacent organic chromophore. After excited-state equilibration is established (5 ns) as a result of REET, extremely long luminescence lifetimes of up to 225 μs result, compared to 8.3 μs for the model complex, without diminishing the emission quantum yield. As a result, remarkably high oxygen sensitivity is observed in both solution and polymeric matrices.

While phosphorescent cyclometalated iridium(III) complexes have been widely studied, only correlations between oxidation potential EOX and Hammett constant σ, and between the redox gap (ΔEREDOX = EOX − ERED) and emission or absorption wavelength (λabs, λem) have been reported. We present now a quantitative model based on Hammett parameters that rationalizes the effect of the substituents on the properties of cyclometalated iridium(III) complexes. This simple model allows predicting the apparent redox potentials as well as the electrochemical gap of homoleptic complexes based on phenylpyridine ligands with good accuracy. In particular, the model accounts for the unequal effect of the substituents on both the HOMO and the LUMO energy levels. Consequently, the model is used to anticipate the emission maxima of the corresponding complexes with improved reliability. We demonstrate in a series of phenylpyridine emitters that electron-donating groups can effectively replace electron-withdrawing substituents on the orthometallated phenyl to induce a blue shift of the emission. This result is in contrast with the common approach that uses fluorine to blue shift the emission maximum. Finally, as a proof of concept, we used electron-donating substituents to design a new fluorine-free complex, referred to as EB343, matching the various properties, namely oxidation and reduction potentials, electrochemical gap and emission profile, of the standard sky-blue emitter FIrPic.

We report here a new cationic bis-cyclometallated iridium(III) complex, 1, with deep-blue emission at 440 nm and its use in Light-emitting Electrochemical Cells (LECs). The design is based on the 2′,6′-difluoro-2,3′-bipyridine skeleton as the cyclometallating ligand and a bis-imidazolium carbene-type ancillary ligand. Furthermore, bulky tert-butyl substituents are used to limit the intermolecular interactions. LECs have been driven both at constant voltage (6 V) and constant current (2.5 mA cm−2). The performances are significantly improved with the latter method, resulting overall in one of the best reported greenish-blue LECs having fast response (17 s), light intensity over 100 cd m−2 and a reasonable efficiency of almost 5 cd A−1.

Charged cyclometalated (C∧N) iridium(III) complexes with carbene-based ancillary ligands are a promising family of deep-blue phosphorescent compounds. Their emission properties are controlled primarily by the main C∧N ligands, in contrast to the classical design of charged complexes where N∧N ancillary ligands with low-energy π* orbitals, such as 2,2'-bipyridine, are generally used for this purpose. Herein we report two series of charged iridium complexes with various carbene-based ancillary ligands. In the first series the C∧N ligand is 2-phenylpyridine, whereas in the second one it is 2-(2,4-difluorophenyl)-pyridine. One bis-carbene (:C∧C:) and four different pyridine–carbene (N∧C:) chelators are used as bidentate ancillary ligands in each series. Synthesis, X-ray crystal structures, and photophysical and electrochemical properties of the two series of complexes are described. At room temperature, the :C∧C: complexes show much larger photoluminescence quantum yields (ΦPL) of ca. 30%, compared to the N∧C: analogues (around 1%). On the contrary, all of the investigated complexes are bright emitters in the solid state both at room temperature (1% poly(methyl methacrylate) matrix, ΦPL 30–60%) and at 77 K. Density functional theory calculations are used to rationalize the differences in the photophysical behavior observed upon change of the ancillary ligands. The N∧C:-type complexes possess a low-lying triplet metal-centered (3MC) state mainly deactivating the excited state through nonradiative processes; in contrast, no such state is present for the :C∧C: analogues. This finding is supported by temperature-dependent excited-state lifetime measurements made on representative N∧C: and :C∧C: complexes.

The behavior toward oxygen sensing of nanocomposites made of the aluminum oxide-hydroxide nanostructured solid support (AP200/19) and neutral blue emitting cyclometalated iridium(III) complexes was studied. The results are compared with the same dyes immobilized in polystyrene films. Since the photoluminescence of the complexes is totally quenched for oxygen concentrations just over 10%, these systems using the blue region of the visible spectrum are promising for oxygen detection at low concentration. In particular, dyes supported into the AP200/19 provide the best sensitivity to oxygen concentration, with the possibility to detect oxygen below 1% O2 in gas (0.01 bar).Keywords: cyclometalated iridium(III) complexes; nanostructured films; oxygen sensing; phosphorescence;

An ionic tris-heteroleptic iridium complex gives green light-emitting electrochemical cells (LECs) with unprecedented performances for this part of the visible spectrum. The devices are very bright (>1000 cd m–2), efficient (∼3%), and stable (>55 h). The novel complex is prepared using a new and efficient synthetic procedure. We show that there is a mixed orbital formation originating from the two different orthometalating ligands resulting in photophysical properties that lie between those of its two bis-heteroleptic analogs. Therefore, tris-heteroleptic complexes provide new avenues for fine-tunning the emission properties and to bridge gaps between a series of bis-heteroleptic complexes.Keywords: green electroluminescence; iridium complex; light-emitting electrochemical cell; tris-heteroleptic complex;

Since the first report of black dye, ruthenium complexes with tridentate ligands have attracted attention due to their ability to harvest photons in the near infrared. Herein we review this family of sensitizers for dye-sensitized solar cell focusing on their chemical structures and properties. We briefly highlight their performance in photovoltaic devices.Graphical abstractSince the first report of black dye, ruthenium complexes with tridentate ligands have attracted attention due to their ability to harvest photons in the near infrared. Herein we review this family of sensitizers for dye-sensitized solar cell focusing on their chemical structures and properties. We briefly highlight their performance in photovoltaic devices.Download full-size image

The complex [Cu(xantphos)(dmp)][PF6] (dmp = 2,9-dimethyl-1,10-phenanthroline) in a nanostructured metal oxyde matrix shows better sensitivity to oxygen (KSV = 9.74 ± 0.87 kPa−1 between 0 and 1 kPa pO2 and 5.59 ± 0.15 kPa−1 between 0 and 10 kPa pO2) than cyclometallated iridium complexes in the same conditions.

We report here a new cationic bis-cyclometallated iridium(III) complex, 1, with deep-blue emission at 440 nm and its use in Light-emitting Electrochemical Cells (LECs). The design is based on the 2′,6′-difluoro-2,3′-bipyridine skeleton as the cyclometallating ligand and a bis-imidazolium carbene-type ancillary ligand. Furthermore, bulky tert-butyl substituents are used to limit the intermolecular interactions. LECs have been driven both at constant voltage (6 V) and constant current (2.5 mA cm−2). The performances are significantly improved with the latter method, resulting overall in one of the best reported greenish-blue LECs having fast response (17 s), light intensity over 100 cd m−2 and a reasonable efficiency of almost 5 cd A−1.

While phosphorescent cyclometalated iridium(III) complexes have been widely studied, only correlations between oxidation potential EOX and Hammett constant σ, and between the redox gap (ΔEREDOX = EOX − ERED) and emission or absorption wavelength (λabs, λem) have been reported. We present now a quantitative model based on Hammett parameters that rationalizes the effect of the substituents on the properties of cyclometalated iridium(III) complexes. This simple model allows predicting the apparent redox potentials as well as the electrochemical gap of homoleptic complexes based on phenylpyridine ligands with good accuracy. In particular, the model accounts for the unequal effect of the substituents on both the HOMO and the LUMO energy levels. Consequently, the model is used to anticipate the emission maxima of the corresponding complexes with improved reliability. We demonstrate in a series of phenylpyridine emitters that electron-donating groups can effectively replace electron-withdrawing substituents on the orthometallated phenyl to induce a blue shift of the emission. This result is in contrast with the common approach that uses fluorine to blue shift the emission maximum. Finally, as a proof of concept, we used electron-donating substituents to design a new fluorine-free complex, referred to as EB343, matching the various properties, namely oxidation and reduction potentials, electrochemical gap and emission profile, of the standard sky-blue emitter FIrPic.

FIrpic is the most investigated bis-cyclometallated iridium complex in particular in the context of organic light emitting diodes (OLEDs) because of its attractive sky-blue emission, high emission efficiency, and suitable energy levels. In this Perspective we review the synthesis, structural characterisations, and key properties of this emitter. We also survey the theoretical studies and summarise a series of selected monochromatic electroluminescent devices using FIrpic as the emitting dopant. Finally we highlight important shortcomings of FIrpic as an emitter for OLEDs. Despite the large body of work dedicated to this material, it is manifest that the understanding of photophysical and electrochemical processes are only broadly understood mainly because of the different environment in which these properties are measured, i.e., isolated molecules in solvent vs. device.

Luminescent liquid crystal (LC) materials have attracted significant interest for organic optoelectronic applications, especially for linearly polarised emission, because of their combination of ordered alignment and luminescence property. Since the first demonstration of polarised organic light-emitting diodes (OLEDs) in 1995, remarkable progress has been made with polarised electroluminescence because of the continuous advances in the design of suitable LC materials. In this Review, we summarize luminescent LC materials with representative examples based on fluorescent materials, phosphorescent materials, and lanthanide complexes. The structure–property relationships for polarised emission and, when available, electroluminescence of these materials are presented and discussed, with a focus on phosphorescent metallomesogens. We also present the rationale behind the design and development of luminescent LC materials for high efficiency polarised OLEDs, along with the challenges ahead to achieve efficient devices.

The use of electron-withdrawing substituents on the orthometalated phenyl ring is a common strategy to blue shift the emission of cyclometalated iridium complexes by stabilizing the highest occupied molecular orbital (HOMO), that is, increasing the oxidation potential of the complex. However, for application in blue organic light-emitting diodes (OLEDs), this approach imposes host materials with a deep HOMO, which negatively impacts the injection of charges, and hence the performance of the devices. In this context, we report new iridium complexes with an electron-donating substituent on the cyclometalated ligand to blue shift the emission while keeping a relatively low oxidation potential. As a result, bluish-green OLEDs based on host materials with shallow HOMOs (TCTA = 4,4′,4′′-tri(N-carbazolyl)-triphenylamine) display a higher performance than devices using FIrpic in the same architecture. The improvements are primarily attributed to the lower turn-on voltage (2.8 to 3 V) compared to those of FIrpic-device (3.6 V). White OLED was then prepared with a maximum brightness of 20226 cd m−2 and current efficiency of 20.4 cd A−2 (at 100 cd m−2). Interestingly, a very small efficiency roll-off of about 1% at 1000 cd m−2 and high color stability were achieved. At a luminance level of 5000 cd m−2 the roll-off efficiency was still below 20%. The introduction of electron-donating substituents on a 2-phenylpyridine scaffold to obtain blue emitters with low oxidation potentials provides an alternative to strategies based on replacing the pyridine with imidazole, carbene, and pyrazole.

There is presently a lack of efficient and stable blue emitters for light-emitting electrochemical cells (LEECs), which limits the development of white light emitting systems for lighting. Cyclometalated iridium complexes as blue emitters tend to show low photoluminescence efficiency due to significant ligand-centred character of the radiative transition. The most common strategy to blue-shift the emission is to use fluorine substituents on the cyclometalating ligand, such as 2,4-difluorophenylpyridine, dFppy, which has been shown to decrease the stability of the emitter in operating devices. Herein we report a series of four new charged cyclometalated iridium complexes using methoxy- and methyl-substituted 2,3′-bipyridine as the main ligands. The combination of donor groups and the use of a cyclometalated pyridine has been recently reported for neutral complexes and found electronically equivalent to dFppy. We describe the photophysical and electrochemical properties of the complexes in solution and use DFT and TDDFT calculations to gain insights into their properties. The complexes exhibit bluish-green emission with onsets around 450 nm, which correspond to the maximum emission at 77 K. Furthermore, photoluminescence quantum yields in solution are all above 40%, with the brightest in the series at 66%. Finally, LEECs were prepared using these complexes as the emissive material to evaluate the performance of this particular design. Compared to previously reported devices with fluorine-containing emitters, the emitted colours are slightly red-shifted due to methyl substituents on the coordinating pyridine of the main ligand and overall device performances, unfortunately including the stability of devices, are similar to those previously reported. Interestingly within the series of complexes there appears to be a positive effect of the methoxy-substituents on the stability of the devices. The poor stability is therefore attributed to the combination of cyclometalated pyridine and methoxy groups.