Quadratic Optical Nonlinearities of N‐Methyl and N‐Aryl Pyridinium Salts

Four series of dyes with dimethylamino electron donor groups and N‐R‐pyridinium (R = methyl Me, phenyl Ph, 2,4‐dinitrophenyl 2,4‐DNPh, or 2‐pyrimidyl 2‐Pym) electron acceptors are studied as their hexafluorophosphate salts. The intramolecular charge‐transfer (ICT) energies (E_max) of these compounds decrease within each of the series in the order R = Me > Ph > 2,4‐DNPh > 2‐Pym, as the electron‐accepting ability of the pyridinium ring increases. Hyper‐Rayleigh scattering with femtosecond 1300 or 800 nm lasers yields fluorescence‐free first hyperpolarizabilities β, and static first hyperpolarizabilities β₀[H] are obtained via the two‐state model. Dipole moment changes Δμ₁₂ for the ICT transitions obtained from Stark spectroscopy afford β₀[S] values by using β₀ = 3Δμ₁₂(μ₁₂)²/2(E_max)² (μ₁₂ = transition dipole moment). The β₀[S] data show that the combination of pyridyl N‐arylation with conjugation extension affords large increases in β₀. The β₀[H] data generally agree with this conclusion, but resonance effects may explain some apparent anomalies. X‐ray structural studies on various salts reveal that the use of tosylate anions is not a generally applicable approach to engineering noncentrosymmetric structures of pyridinium salts. However, trans‐N‐phenyl‐4‐(4‐dimethylaminophenyl‐4‐buta‐1,3‐dienyl)pyridinium hexafluorophosphate adopts the polar space group Cc, and shows a very large powder second harmonic generation efficiency from a 1907 nm laser, which is similar to that of the well‐studied material trans‐4′‐(dimethylamino)‐N‐methyl‐4‐stilbazolium tosylate (DAST).     

Functional Pyrimidinyl Pyrazolate Pt(II) Complexes: Role of Nitrogen Atom in Tuning the Solid‐State Stacking and Photophysics

Pt(II) metal complexes are known to exhibit strong solid‐state aggregation and are promising for realization of efficient emission in fabrication of organic light emitting diodes (OLED) with nondoped emitter layer. Four pyrimidine–pyrazolate based chelates, together with four isomeric Pt(II) metal complexes, namely: [Pt(pm2z)₂], [Pt(tpm2z)₂], [Pt(pm4z)₂], and [Pt(tpm4z)₂], are isolated and systematically investigated for their structure–property relationships for practical OLED applications. Detailed single molecular and aggregated structures are revealed by photophysical and mechanochromic measurements, grazing‐incidence X‐ray diffraction, and theoretical approaches. These results suggest that these Pt(II) emitters pack like a deck of playing cards under vacuum deposition, and their emission energy is not only affected by the single molecular designs, but notably influenced by their intermolecular packing interaction, i.e., Pt···Pt separations that are arranged in the order: [Pt(tpm4z)₂] > [Pt(pm4z)₂] > [Pt(tpm2z)₂] > [Pt(pm2z)₂]. Nondoped OLED with emission ranging from green to red are prepared, to which the best performances are recorded for [Pt(tpm2z)₂], giving maximum external quantum efficiency (EQE) of 27.5% at 10³ cd m^?2, maximum luminance of 2.5 × 10⁵ cd m^?2 at 17 V, and with stable CIE_x,y of (0.56, 0.44).     

Efficient Photocatalytic Nitrogen Fixation: Enhanced Polarization,Activation, and Cleavage by Asymmetrical Electron Donation to NN Bond

Photocatalytic nitrogen (N₂) fixation suffers from low efficiency due to the difficult activation of the strongly nonpolar N?N bond. In this study, a Ru–Co bimetal center is constructed at the interface of Ru/CoS_x with S‐vacancy on graphitic carbon nitride nanosheets (Ru‐Vs‐CoS/CN). Upon adsorption, the two N atoms in N₂ are bridged to the Ru–Co center, and the asymmetrical electron donation from Ru and Co atoms to N₂ adsorbate highly polarized N?N bond to double bond order. The plasmonic electric‐field‐enhancement effect enables the Ru/CoS_x interface to boost the generation of energetic electrons. The Schottky barrier between Ru and CoS_x endows the interface with electron transfer from CoS_x to Ru. The Ru‐end bound N at the Ru–Co center is preferentially hydrogenated. As a result, the Ru‐Vs‐CoS/CN photocatalyst shows an NH₃ production rate of up to 0.438 mmol g^?1 h^?1, reaching a high apparent quantum efficiency of 1.28% at 400 nm and solar‐to‐ammonia efficiency of 0.042% in pure water under AM1.5G light irradiation.     

Synthesis of New Europium Complexes and Their Application in Electroluminescent Devices

Several substituted phenanthrolines (L = pyrazino[2,3‐f][1,10]phenanthroline (PyPhen), 2‐methylpyrazino[2,3‐f][1,10]phenanthroline (MPP), dipyrido[3,2‐a:2′,3′‐c]phenazine (DPPz), 11‐methyldipyrido[3,2‐a:2′,3′‐c]phenazine (MDPz), 11,12‐dimethyldipyrido[3,2‐a:2′,3′‐c]phenazine (DDPz), and benzo[i]dipyrido[3,2‐a:2,3‐c]phenazine (BDPz)) were successfully prepared and europium complexes Eu(TTA)₃L (Eu‐L) based on these ligands were synthesized from EuCl₃, 2‐thenoyltrifluoroacetone (TTA) and L in good yields. Irradiation at the absorption band between 320–390 nm of all these europium complexes, except Eu‐BDPz, in solution or in the solid state leads to the emission of a sharp red band at ～ 612 nm, a characteristic Eu³⁺ emission due to the transition ⁵D₀ → ⁷F₂. No emission from the ligands was found. The result indicates that complete energy transfer from the ligand to the center Eu³⁺ ion occurs for these europium complexes. In contrast, the photoluminescence spectrum of Eu‐BDPz exhibits a strong emission at around 550 nm from the coordinated BDPz ligand and a weak emission at 612 nm from the central europium ion. Incomplete energy transfer from the ligand to the central Eu³⁺ ion was observed for the first time. Several electroluminescent devices ( A – I ) using Eu‐PyPhen, Eu‐MPP, Eu‐DPPz, and Eu‐DDPz as dopant emitters with the device configuration: TPD or NPB (50 nm)/Eu:CBP (1.7–7 %, 30 nm)/BCP (20–30 nm)/Alq (25–35 nm) (where TPD: 4,4′‐bis[N‐(p‐tolyl)‐N‐phenylamino]biphenyl; NPB: 4,4′‐bis[1‐naphthylphenylamino]biphenyl; CBP: 4,4′‐N,N′‐dicarbazole biphenyl; BCP: 2,9‐dimethyl‐4,7‐diphenyl‐1,10‐phenanthroline; Alq: tris[8‐hydroxyquinoline]aluminum) were fabricated. Some of these devices emit saturated red light and are the only europium complex‐based devices that show a brightness of more than 1000 cd m^–2.     

Bright and Efficient,Non‐Doped,Phosphorescent Organic Red‐Light‐Emitting Diodes

Ir^(III) metal complexes with formula [(nazo)₂Ir(Fppz)] ( 1 ), [(nazo)₂Ir(Bppz)] ( 2 ), and [(nazo)₂Ir(Fptz)] ( 3 ) [(nazo)H = 4‐phenyl quinazoline, (Fppz)H = 3‐trifluoromethyl‐5‐(2‐pyridyl) pyrazole, (Bppz)H = 3‐t‐butyl‐5‐(2‐pyridyl) pyrazole, and (Fptz)H = 3‐trifluoromethyl‐5‐(2‐pyridyl) triazole] were synthesized, among which the exact configuration of 1 was confirmed using single‐crystal X‐ray diffraction analysis. These complexes exhibited bright red phosphorescence with relatively short lifetimes of 0.4–1.05 μs in both solution and the solid‐state at room temperature. Non‐doped organic light‐emitting diodes (OLEDs) were fabricated using complexes 1 and 2 in the absence of a host matrix. Saturated red electroluminescence was observed at λ_max = 626 nm (host‐emitter complex 1 ) and 652 nm (host‐emitter complex 2 ), which corresponds to coordinates (0.66,0.34) and (0.69,0.31), respectively, on the 1931 Commission Internationale de l'Eclairage (CIE) chromaticity diagram. The non‐doped devices employing complex 1 showed electroluminance as high as 5780 cd m^–2, an external quantum efficiency of 5.5 % at 8 V, and a current density of 20 mA cm^–2. The short phosphorescence lifetime of 1 in the solid state, coupled with its modest π–π stacking interactions, appear to be the determining factors for its unusual success as a non‐doped host‐emitter.     

A Multifunctional Iridium‐Carbazolyl Orange Phosphor for High‐Performance Two‐Element WOLED Exploiting Exciton‐Managed Fluorescence/Phosphorescence

By attaching a bulky, inductively electron‐withdrawing trifluoromethyl (CF₃) group on the pyridyl ring of the rigid 2‐[3‐ (N‐phenylcarbazolyl)]pyridine cyclometalated ligand, we successfully synthesized a new heteroleptic orange‐emitting phosphorescent iridium(III) complex [Ir( L ₁ )₂(acac)] 1 ( HL ₁ = 5‐trifluoromethyl‐2‐[3‐(N‐phenylcarbazolyl)]pyridine, Hacac = acetylacetone) in good yield. The structural and electronic properties of 1 were examined by X‐ray crystallography and time‐dependent DFT calculations. The influence of CF₃ substituents on the optical, electrochemical and electroluminescence (EL) properties of 1 were studied. We note that incorporation of the carbazolyl unit facilitates the hole‐transporting ability of the complex, and more importantly, attachment of CF₃ group provides an access to a highly efficient electrophosphor for the fabrication of orange phosphorescent organic light‐emitting diodes (OLEDs) with outstanding device performance. These orange OLEDs can produce a maximum current efficiency of ～40 cd A^?1, corresponding to an external quantum efficiency of ～12% ph/el (photons per electron) and a power efficiency of ～24 lm W^?1. Remarkably, high‐performance simple two‐element white OLEDs (WOLEDs) with excellent color stability can be fabricated using an orange triplet‐harvesting emitter 1 in conjunction with a blue singlet‐harvesting emitter. By using such a new system where the host singlet is resonant with the blue fluorophore singlet state and the host triplet is resonant with the orange phosphor triplet level, this white light‐emitting structure can achieve peak EL efficiencies of 26.6 cd A^?1 and 13.5 lm W^?1 that are generally superior to other two‐element all‐fluorophore or all‐phosphor OLED counterparts in terms of both color stability and emission efficiency.     

White Organic Light‐Emitting Diodes with Evenly Separated Red,Green, and Blue Colors for Efficiency/Color‐Rendition Trade‐Off Optimization

A novel yellowish‐green triplet emitter, bis(5‐(trifluoromethyl)‐2‐p‐tolylpyridine) (acetylacetonate)iridium(III) (1), was conveniently synthesized and used in the fabrication of both monochromatic and white organic light‐emitting diodes (WOLEDs). At the optimal doping concentration, monochromatic devices based on 1 exhibit a high efficiency of 63 cd A^?1 (16.3% and 36.6 lm W^?1) at a luminance of 100 cd m^?2. By combining 1 with a phosphorescent sky‐blue emitter, bis(3,5‐difluoro‐2‐(2‐pyridyl)phenyl)‐(2‐carboxypyridyl)iridium(III) (FIrPic), and a red emitter, bis(2‐benzo[b]thiophen‐2‐yl‐pyridine)(acetylacetonate)iridium(III) (Ir(btp)₂(acac)), the resulting electrophosphorescent WOLEDs show three evenly separated main peaks and give a high efficiency of 34.2 cd A^?1 (13.2% and 18.5 lm W^?1) at a luminance of 100 cd m^?2. When 1 is mixed with a deep‐blue fluorescent emitter, 4,4′‐bis(9‐ethyl‐3‐carbazovinylene)‐1,1′‐biphenyl (BCzVBi), and Ir(btp)₂(acac), the resulting hybrid WOLEDs demonstrate a high color‐rendering index of 91.2 and CIE coordinates of (0.32, 0.34). The efficient and highly color‐pure WOLEDs based on 1 with evenly separated red, green, blue peaks and a high color‐rendering index outperform those of the state‐of‐the‐art emitter, fac‐tris(2‐phenylpyridine)iridium(III) (Ir(ppy)₃), and are ideal candidates for display and lighting applications.     

Efficient red electrophosphorescence from a fluorene-based bipolar host material

We have prepared efficient red organic light-emitting diodes (OLEDs) incorporating 2,7-bis(diphenylphosphoryl)-9-[4-(N,N-diphenylamino)phenyl]-9-phenylfluorene (POAPF) as the host material doped with the osmium phosphor Os(fptz)₂(PPh₂Me)₂ (fptz = 3-trifluoromethyl-5-pyridyl-1,2,4-triazole). POAPF, which possesses bipolar functionalities, can facilitate both hole- and electron-injection from the charge transport layers to provide a balanced charge flux within the emission layer. The peak electroluminescence performance of the device reached as high as 19.9% and 34.5 lm/W – the highest values reported to date for a red phosphorescent OLED. In addition, we fabricated a POAPF-based white light OLED – containing red-[doped with Os(fptz)₂(PPh₂Me)₂] and blue-emitting {doped with iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,C^2′] picolinate, FIrpic} layers – that also exhibited satisfactory efficiencies (18.4% and 43.9 lm/W).     

Thiocyanate‐Free Ru(II) Sensitizers with a 4,4′‐Dicarboxyvinyl‐2,2′‐bipyridine Anchor for Dye‐Sensitized Solar Cells

A new class of thiocyanate‐free Ru(II) sensitizers with 4,4′‐dicarboxyvinyl‐2,2′‐bipyridine anchor and two trans‐oriented pyrid‐2‐yl pyrazolate (or triazolate) functional chromophores is synthesized, characterized, and evaluated in dye‐sensitized solar cells (DSCs). Despite their enhanced red response and absorptivity when compared to the parent sensitizer TFRS‐2 that possesses standard 4,4′‐dicarboxyl‐2,2′‐bipyridine anchor and shows the best conversion efficiency of η = 9.82%, the newly synthesized carboxyvinyl‐pyrazolate sensitizers, TFRS‐11 – TFRS‐13 , exhibit inferior performance characteristics in terms of short‐circuit current density (J_SC), open‐circuit voltage (V_OC), and power conversion efficiency (η), the latter being recorded to be in the range 5.60–7.62%. The reduction in device efficiencies is attributed to a combination of poor packing of these sensitizers on the TiO₂ surface and less positive ground‐state oxidation potentials, which, respectively, increase charge recombination with I₃^? in electrolytes and impede the regeneration of sensitizers by I^? anions. The latter obstacle can be circumvented in part by the replacement of the pyrazolates with triazolates, forming the TFRS‐14 sensitizer, which exhibits an improved J_SC, V_OC, and η of 16.4 mAcm^?2, 0.77 V, and 9.02%, respectively.     

Host Selection and Configuration Design of Electrophosphorescent Devices

Recent studies on electrophosphorescent polymeric devices have demonstrated that charge‐trapping‐induced direct recombination on the phosphorescent dopant is of crucial importance. In this paper, we show that the electrochemical properties of phosphorescent molecules, which reflect their carrier‐trapping ability, may be a basic design criterion for the selection of host and device configuration. The systems, consisting of a red phosphorescent [Ru(4,7‐Ph₂‐phen)₃]²⁺ dopant and two blue hosts 2‐biphenyl‐4‐yl‐5‐(4‐tert‐butyl‐phenyl)‐[1,3,4]oxadiazole (PBD) and poly(vinylcarbazole) (PVK), are intensively studied. The triplet energy level of PVK and PBD is higher than that of the [Ru(4,7‐Ph₂‐phen)₃]²⁺, and both hosts show the ability of efficient energy transfer to the dopant, however, efficient electroluminescence (EL) is only obtained in the PVK‐host system. The combined studies of photoluminescence (PL), EL, and electrochemistry for doped films demonstrate that [Ru(4,7‐Ph₂‐phen)₃]²⁺, which undergoes a multielectron trapping process as it is used as a dopant in electron‐rich (n‐type) hosts, for instance, PBD, may induce an inefficient recombination for the resulting emission. Whereas using a hole‐rich (p‐type) polymer, such as PVK, as a host and inserting both hole‐blocking and electron‐transfer layers can effectively increase the efficiency of the corresponding devices up to 8.63 Cd A^–1, because of the reduced probability of multielectron trapping at the [Ru(4,7‐Ph₂‐phen)₃]²⁺ sites.     

Dinuclear Metal–Organic Material for Binary Optical Recording

A diimine ligand tethered to anthracene in the 9‐position, 4′‐(9‐anthrylethyl)‐4‐methyl‐2,2′‐bipyridine (bpy‐An), was dimerized through cycloaddition photochemistry. The resultant head‐to‐tail photodimer (bpy‐PD) was used as a bridging ligand in the preparation of a new dinuclear Ru^II complex, [Ru(dmb)₂(bpy‐PD)Ru(dmb)₂]⁴⁺ (dmb = 4,4′dimethyl‐2,2′‐bipyridine). The corresponding mononuclear species containing anthracene ([Ru(dmb)₂(bpy‐An)]²⁺ was also synthesized and serves as a model compound in this study. UV photolysis (λ < 300 nm) of the strongly luminescent Ru^II dinuclear complex results in cycloreversion, generating two anthracene‐containing mononuclear species, [Ru(dmb)₂(bpy‐An)]²⁺, whose emission is largely quenched as a result of nonradiative triplet–triplet energy transfer. The photophysical and photochemical properties of the dinuclear system have been studied in CH₃CN solutions and in solid polyvinyl alcohol (PVA) thin films. The “on”–“off” luminescence switching characteristics and concomitant non‐destructive readout properties suggested that these molecules could be useful in read‐only memory (ROM) applications. In the solid state, micrometer‐sized objects were imaged using visible light, taking advantage of the luminescence contrast generated from the UV photochemical reaction. These written images were stable for at least 6 months, indicating that long‐term binary data storage is indeed feasible in these ROM metal–organic materials.     

[Cp*W(dmit)2]•: An Organometallic 15‐Electron Neutral Radical (d1) with Optical Limiting Properties in Solution and an Antiferromagnetic Ground State in the Solid State

The synthesis of the organometallic d² [Cp*W(dmit)₂]^1– complex (where Cp* is pentamethylcyclopentadienyl and dmit is 1,3‐dithiole‐2‐thione‐4,5‐dithiolate), and its oxidation to the paramagnetic d¹ [Cp*W(dmit)₂]^• species, is described and their X‐ray crystal structures given. Geometrical evolutions upon oxidation, characterized by a variable folding of the WS₂C₂ metallacycles along the S–S hinge, are rationalized by density functional theory (DFT) calculations and by comparison with the molybdenum analogs; as is also the evolution in the UV‐vis‐NIR absorption spectra. In solution, only the d¹ complexes exhibit positive optical density variations in transitory nanosecond spectroscopy after 10 ns laser pulses. A weak optical limiting effect was observed on these d¹ species, stronger in the W than in the Mo complex. In the solid state, the interacting, paramagnetic [Cp*W(dmit)₂]^• species (θ_{Curie–Weiss} = –20 K) orders antiferromagnetically below T_Néel = 4.5 K with a spin‐flop field, B_SF(W) of 8000 G. Compared with the molybdenum analog, the weaker θ_{Curie–Weiss}(W) and T_Néel(W) values, and larger B_SF(W) values reflect weaker intermolecular interactions due to a decreased spin density on the dithiolene ligands and stronger spin–orbit coupling with the W atom, as confirmed by DFT calculations on the d² and d¹ Mo and W complexes.     

Quadratic Nonlinear Optical Properties of N‐Aryl Stilbazolium Dyes

The new salts trans‐4′‐(dimethylamino)‐N‐R‐4‐stilbazolium hexafluorophosphate (R = methyl, Me 1 , phenyl, Ph 2 , 2,4‐dinitrophenyl, DNPh 3 , 2‐pyrimidyl, Pym 4 , Scheme 1) have been prepared. Their electronic absorption spectra show intense, visible intramolecular charge‐transfer bands, the energy (E_max) of which decreases in the order R = Me > Ph > DNPh > Pym. This trend arises from the steadily increasing electron deficiency of the pyridinium ring, a phenomenon also observed in cyclic voltammetric and ¹H nuclear magnetic resonance (NMR) data. Fluorescence‐free first hyperpolarizability β values of [ 1 – 4 ]PF₆ were measured by using femtosecond hyper‐Rayleigh scattering (HRS) with acetonitrile solutions and a 1300 nm laser, and static first hyperpolarizabilities β₀ were obtained by application of the two‐state model. The HRS results indicate that the N‐aryl chromophores in [ 2 – 4 ]PF₆ have considerably larger β₀ values than their N‐methyl counterpart in [ 1 ]PF₆, with a ca. 10‐fold increase in β₀ observed in moving from [ 1 ]PF₆ to [ 4 ]PF₆ (25 → 230 × 10^–30 esu). Stark (electroabsorption) spectroscopic studies in butyronitrile glasses at 77 K allowed the derivation of dipole moment changes Δμ₁₂ (10.9–14.8 D), which have been used to calculate β₀ according to the two‐state equation β₀ = 3Δμ₁₂(μ₁₂)²/2(E_max)² (μ₁₂ = transition dipole moment). With the exception of [ 1 ]PF₆, the Stark‐derived β₀ values are in reasonable agreement with those from HRS. However, the increase in β₀ in moving from [ 1 ]PF₆ to [ 4 ]PF₆ is only 2‐fold for the Stark data (90 → 185 × 10^–30 esu). The observed trend of increasing β₀ in the order [ 1 ]PF₆ < [ 3 ]PF₆ < [ 2 ]PF₆ < [ 4 ]PF₆ arises from a combination of decreasing E_max and increasing Δμ₁₂, with only a slight increase in μ₁₂ between [ 1 ]PF₆ and [ 4 ]PF₆. It is likely that the β₀ values for [ 3 ]PF₆ are lower than expected due to the steric effect of the ortho‐NO₂ group, which causes twisting of the DNPh ring out of the plane of the stilbazolium unit. A single crystal X‐ray structure shows that [ 2 ]PF₆ crystallizes in the space group Cc, with head‐to‐tail alignment and almost parallel stacking of the pseudo‐planar stilbazolium portions of the cations to form polar sheets within a polar bulk structure. [ 2 ]PF₆ is essentially isostructural with the related Schiff base salt trans‐4‐[(4‐dimethylaminophenyl)iminomethyl]‐N‐phenylpyridinium hexafluorophosphate ([ 8 ]PF₆). Second harmonic generation (SHG) studies on [ 2 ]PF₆ and [ 8 ]PF₆ using a 1907 nm laser and sieved powdered samples (53–63 μm) afforded efficiencies of 470 and 240 times that of urea, respectively. Under the same conditions, the well‐studied compound [ 1 ]p‐MeC₆H₄SO₃ gave an SHG efficiency of 550 times that of urea.     

Highly Efficient Electron‐Transporting/Injecting and Thermally Stable Naphthyridines for Organic Electrophosphorescent Devices

A series of 1,8‐naphthyridine derivatives is synthesized and their electron‐transporting/injecting (ET/EI) properties are investigated via a multilayered electrophosphorescent organic light‐emitting device (OLED) using fac‐tris(2‐phenylpyridine)iridium [Ir(ppy)₃] as a green phosphorescent emitter doped into a 4,4′‐N,N′‐dicarbazolebiphenyl (CBP) host with 4,4′‐bis[N‐(1‐naphthyl)‐N‐phenylamino]biphenyl (a‐NPD) as the hole‐transporting layer, and poly(arylene ether sulfone) containing tetraphenylbenzidine (TPDPES) doped with tris(4‐bromophenyl)ammonium hexachloroantimonate (TBPAH) as the hole‐injecting layer. The turn‐on voltage of the device is 2.5 V using 2,7‐bis[3‐(2‐phenyl)‐1,8‐naphthyridinyl]‐9,9‐dimethylfluorene (DNPF), lower than that of 3.0 V for the device using a conventional ET material. The maximum current efficiency (CE) and power efficiency (PE) of the DNPF device are much higher than those of a conventional device. With the aid of a hole‐blocking (HB) and exciton‐blocking layer of bathocuproine (BCP), 13.2–13.7% of the maximum external quantum efficiency (EQE) and a maximum PE of 50.2–54.5 lm W^?1 are obtained using the naphthyridine derivatives; these values are comparable with or even higher than the 13.6% for conventional ET material. The naphthyridine derivatives show high thermal stabilities, glass‐transition temperatures much higher than that of aluminum(III) bis(2‐methyl‐8‐quinolinato)‐4‐phenylphenolate (BAlq), and decomposition temperatures of 510–518 °C, comparable to or even higher than those of Alq₃.     

A DCM‐Type Red‐Fluorescent Dopant for High‐Performance Organic Electroluminescent Devices

2‐(2‐tert‐Butyl‐6‐((E)‐2‐(2,6,6‐trimethyl‐2,4,5,6‐tetrahydro‐1H‐pyrrolo[3,2,1‐ij]quinolin‐8‐yl)vinyl)‐4H‐pyran‐4‐ylidene)malononitrile (DCQTB) is designed and synthesized in high yield for application as the red‐light‐emitting dopant in organic light‐emitting diodes (OLEDs). Compared with 4‐(dicyanomethylene)‐2‐tert‐butyl‐6‐(1,1,7,7,‐tetramethyljulolidyl‐9‐enyl)‐4H‐pyran (DCJTB), one of the most efficient red‐emitting dopants, DCQTB exhibits red‐shifted fluorescence but blue‐shifted absorption. The unique characteristics of DCQTB with respect to DCJTB are utilized to achieve a red OLED with improved color purity and luminous efficiency. As a result, the device that uses DCQTB as dopant, with the configuration: indium tin oxide (ITO)/N,N′‐bis(1‐naphthyl)‐N,N′‐diphenyl‐1,1′‐biphenyl‐4,4′‐diamine (NPB; 60 nm)/tris(8‐quinolinolato) aluminum (Alq₃):dopant (2.3 wt %) (7 nm)/2,9‐dimethyl‐4,7‐diphenyl‐1,10‐phenanthroline (BCP; 12 nm)/Alq₃(45 nm)/LiF(0.3 nm):Al (300 nm), shows a larger maximum luminance (L_max = 6021 cd m^–2 at 17 V), higher maximum efficiency (η_max = 4.41 cd A^–1 at 11.5 V (235.5 cd m^–2)), and better chromaticity coordinates (Commission Internationale de l'Eclairage, CIE, (x,y) = (0.65,0.35)) than a DCJTB‐based device with the same structure (L_max = 3453 cd m^–2 at 15.5 V, η_max = 3.01 cd A^–1 at 10 V (17.69 cd m^–2), and CIE (x,y) = (0.62,0.38)). The possible reasons for the red‐shifted emission but blue‐shifted absorption of DCQTB relative to DCJTB are also discussed.     

Scrutinizing Design Principles toward Efficient,Long‐Term Stable Green Light‐Emitting Electrochemical Cells

Enhancing the efficiency and lifetime of light emitting electrochemical cells (LEC) is the most important challenge on the way to energy efficient lighting devices of the future. To avail this, emissive Ir(III) complexes with fluoro‐substituted cyclometallated ligands and electron donating groups (methyl and tert ‐butyl)‐substituted diimine ancillary (N^{^}N) ligands and their associated LEC devices are studied. Four different complexes of general composition [Ir(4ppy)₂(N^{^}N)][PF₆] (4Fppy = 2‐(4‐fluorophenyl)pyridine) with the N^{^}N ligand being either 2,2′‐bipyridine ( 1 ), 4.4′‐dimethyl‐2,2′‐bipyridine ( 2 ), 5.5′‐dimethyl‐2,2′‐bipyridine ( 3 ), or 4.4′‐di‐tert ‐butyl‐2,2′‐bipyridine ( 4 ) are synthesized and characterized. All complexes emit in the green region of light with emission maxima of 529–547 nm and photoluminescence quantum yields in the range of 50.6%–59.9%. LECs for electroluminescence studies are fabricated based on these complexes. The LEC based on ( 1 ) driven under pulsed current mode demonstrated the best performance, reaching a maximum luminance of 1605 cd m^?2 resulting in 16 cd A^?1 and 8.6 lm W^?1 for current and power efficiency, respectively, and device lifetime of 668 h. Compared to this, LECs based on ( 3 ) and ( 4 ) perform lower, with luminance and lifetime of 1314 cd m^?2, 45.7 h and 1193 cd m^?2, 54.9 h, respectively. Interestingly, in contrast to common belief, the fluorine content of the Ir‐iTMCs does not adversely affect the LEC performance, but rather electron donating substituents on the N^{^}N ligands are found to dramatically reduce both performance and stability of the green LECs. In light of this, design concepts for green light emitting electrochemical devices have to be reconsidered.     

Photoinduced Magnetization with a High Curie Temperature and a Large Coercive Field in a Co‐W Bimetallic Assembly

The crystal structure, magnetic properties, and temperature‐ and photoinduced phase transition of [{Co^II(4‐methylpyridine)(pyrimidine)}₂{Co^II(H₂O)₂}{W^V(CN)₈}₂]·4H₂O are described. In this compound, a temperature‐induced phase transition from the Co^II (S = 3/2)‐NC‐W^V(S = 1/2) [high‐temperature (HT)] phase to the Co^III(S = 0)‐NC‐W^IV(S = 0) [low temperature (LT)] phase is observed due to a charge‐transfer‐induced spin transition. When the LT phase is irradiated with 785 nm light, ferromagnetism with a high Curie temperature (T_C) of 48 K and a gigantic magnetic coercive field (H_c) of 27 000 Oe are observed. These T_C and H_c values are the highest in photoinduced magnetization systems. The LT phase is optically converted to the photoinduced phase, which has a similar valence state as the HT phase due to the optically induced charge‐transfer‐induced spin transition.     

Unprecedented Homoleptic Bis‐Tridentate Iridium(III) Phosphors: Facile,Scaled‐Up Production,and Superior Chemical Stability

Bis‐tridentate Ir(III) metal complexes are expected to show great potential in organic light‐emitting diode (OLED) applications due to the anticipated, superb chemical and photochemical stability. Unfortunately, their exploitation has long been hampered by lack of adequate methodology and with inferior synthetic yields. This hurdle can be overcome by design of the first homoleptic, bis‐tridentate Ir(III) complex [Ir(pzpyph)(pz^Hpyph)] ( 1 ), for which the abbreviation (pzpyph)H (or pz^Hpyph) stands for the parent 2‐pyrazolyl‐6‐phenyl pyridine chelate. After that, methylation and double methylation of 1 afford the charge‐neutral Ir(III) complex [Ir(pzpyph)(pz^Mepyph)] ( 2 ) and cationic complex [Ir(pz^Mepyph)₂][PF₆] ( 3 ), while deprotonation of 1 gives formation of anionic [Ir(pzpyph)₂][NBu₄] ( 4 ), all in high yields. These bis‐tridentate Ir(III) complexes 2 – 4 are highly emitted in solution and solid states, while the charge‐neutral 2 and corresponding t ‐butyl substituted derivative [Ir(pzpy^Buph)(pz^Mepy^Buph)] ( 5 ) exhibit superior photostability versus the tris‐bidentate references [Ir(ppy)₂(acac)] and [Ir(ppy)₃] in toluene under argon, making them ideal OLED emitters. For the track record, phosphor 5 gives very small efficiency roll‐off and excellent overall efficiencies of 20.7%, 66.8 cd A^?1, and 52.8 lm W^?1 at high brightness of 1000 cd m^?2. These results are expected to inspire further studies on the bis‐tridentate Ir(III) complexes, which are judged to be more stable than their tris‐bidentate counterparts from the entropic point of view.     

A Light‐Blue Phosphorescent Dendrimer for Efficient Solution‐Processed Light‐Emitting Diodes

We describe the preparation of a dendrimer that is solution‐processible and contains 2‐ethylhexyloxy surface groups, biphenyl‐based dendrons, and a fac‐tris[2‐(2,4‐difluorophenyl)pyridyl]iridium(III ) core. The homoleptic complex is highly luminescent and the color of emission is similar to the heteroleptic iridium(III ) complex, bis[2‐(2,4‐difluorophenyl)pyridyl]picolinate iridium(III ) (FIrpic). To avoid the change in emission color that would arise from attaching a conjugated dendron to the ligand, the conjugation between the dendron and the ligand is decoupled by separating them with an ethane linkage. Bilayer devices containing a light‐emitting layer comprised of a 30 wt.‐% blend of the dendrimer in 1,3‐bis(N‐carbazolyl)benzene (mCP) and a 1,3,5‐tris(2‐N‐phenylbenzimidazolyl)benzene electron‐transport layer have external quantum and power efficiencies, respectively, of 10.4 % and 11 lm W^–1 at 100 cd m^–2 and 6.4 V. These efficiencies are higher than those reported for more complex device structures prepared via evaporation that contain FIrpic blended with mCP as the emitting layer, showing the advantage of using a dendritic structure to control processing and intermolecular interactions. The external quantum efficiency of 10.4 % corresponds to the maximum achievable efficiency based on the photoluminescence quantum yield of the emissive film and the standard out‐coupling of light from the device.     

Microporous Metal–Organic Frameworks with High Gas Sorption and Separation Capacity

The design, synthesis, and structural characterization of two microporous metal–organic framework structures, [M(bdc)(ted)_0.5]·2 DMF·0.2 H₂O (M = Zn ( 1 ), Cu ( 2 ); H₂bdc = 1,4‐benzenedicarboxylic acid; ted = triethylenediamine; DMF: N,N‐dimethylformamide) is reported. The pore characteristics and gas sorption properties of these compounds are investigated at cryogenic temperatures, room temperature, and higher temperatures by experimentally measuring argon, hydrogen, and selected hydrocarbon adsorption/desorption isotherms. These studies show that both compounds are highly porous with a pore volume of 0.65 ( 1 ) and 0.52 cm³ g^{– 1} ( 2 ). The amount of the hydrogen uptake, 2.1 wt % ( 1 ) and 1.8 wt % ( 2 ) at 77 K (1 atm; 1 atm = 101 325 Pa), places them among the group of metal–organic frameworks (MOFs) having the highest H₂ sorption capacity. [Zn(bdc)(ted)_0.5]·2 DMF·0.2 H₂O adsorbs a very large amount of hydrocarbons, including methanol, ethanol, dimethylether (DME), n‐hexane, cyclohexane, and benzene, giving the highest sorption values among all metal–organic based porous materials reported to date. In addition, these materials hold great promise for gas separation.