Phosphorescent iridium complexes based on 2-phenylimidazo[1,2-a]pyridine ligands: tuning of emission color toward the blue region and application to polymer light-emitting devices

A series of new blue-phosphorescent iridium(III) complexes 1-14 with ligands of 2-phenylimidazo[1,2-a]pyridine (pip) derivatives were successfully prepared, and their electrochemical, photophysical, and electroluminescent (EL) properties were systematically investigated. It was found that the emission maxima are significantly dependent on the substituents on the phenyl ring in the range of 489-550 nm. For instance, electron-withdrawing groups such as F and CF3 shift the emission maxima to shorter wavelengths by lowering the HOMO levels (complexes 4-8), whereas the extended pi-conjugation leads to bathochromic shifts (2, 3). To obtain further information about the frontier orbital, substitution effects on the imidazole part were also investigated here, and it was found that electron-withdrawing or -donating substituents on the imidazole ring affected the emission maxima (9, 557 nm; 10, 525 nm). These results including their oxidation potentials suggest that the HOMO of the pip-based complex is a mixture of Ir-d, phenyl-pi, and imidazole-pi orbitals. From this viewpoint, combination of electron-withdrawing substituents on the phenyl ring with the use of another ancillary ligand enabled further blue shifts (13, 468, 499 nm; 14, 464, 494 nm). This new system based on pip is one of the rare examples of iridium complexes whose emissions can be tuned to the blue region. Preliminary polymer light-emitting devices (PLEDs) employing the Ir complexes were fabricated, and the devices showed moderate EL efficiencies.     

FT-IR spectra, vibrational assignments, and density functional calculations of imidazo[1,2-a]pyridine molecule and its Zn(II) halide complexes

The geometry, frequency, and intensity of the vibrational bands of imidazo[1,2-a]pyridine (which is abbreviated as impy) were obtained by the density functional theory (DFT) calculations with BLYP, B3LYP, and B3PW91 functionals and 6-31G(d) basis set. The optimized geometric bond lengths and bond angles are in good agreement with the available X-ray data. The infrared spectrum of imidazo[1,2-a]pyridine was computed by the DFT method in order to reproduce the vibrational wavenumbers and intensities with an accuracy, which allows reliable vibrational assignments. Total energy distribution and isotopic shifts have been calculated in order to help for the perfect assignment of the vibrational modes. The zinc halide complexes Zn(impy)₂X₂ [X = Cl, Br, and I] have also been synthesized. The compounds were characterized using the elemental analysis, FT-IR spectra, and quantum chemical calculations. The geometry optimization of Zn(impy)₂X₂ yields distorted tetrahedral environment around Zn ion.     

丁吡吗啉与其苯基类似物的电子结构特征比较

通过化学合成获得丁吡吗啉及其苯基类似物,并对其杀菌活性进行了比较.借助X射线晶体衍射方法,对丁吡吗啉的结构进行了解析.进一步选择6-31G(2df,2pd)基组,利用密度泛函理论B3LYP方法对丁吡吗啉及其苯基类似物的空间几何结构进行优化,借助前线分子轨道、Mulliken电荷、自然键轨道(NBO)分析、表观静电势等对丁吡吗啉及其苯基类似物的电子结构与其杀菌活性相关性进行了理论探讨.结果表明吡啶环取代苯环后,一方面吡啶环上的N原子是一个负电中心,有利于与受体分子间形成氢键等相互作用;另一方面,吡啶环又是一个缺电子的芳环,与苯环相比,在与受体的π-π相互作用中能起到更好的电子接受体的作用.这两种因素使得丁吡吗啉更容易与受体结合,因而活性更高.     

Preparation of 2-aryl and 2-aryloxymethyl imidazo[1,2-a]pyridines and related compounds

A series of substituted 2-aryl imidazo[1,2-a]pyridines has been prepared in which a variety of substituents are introduced on the 4′-position of the phenyl ring and on the 3, 5 , 6 or 7 position of the heterocyclic ring. Most examples have acetamido, bromo, cyano, or formyl substituents at the 4′-position. Analogous imidazo-[2,1-b]fhiazoles and imidazo[1,2-a]pyrimidines have also been prepared. Another series of compounds consisting of 4′-formylphenoxymethyl derivatives of imidazole, the three positional isomers of pyridine, thiazole, benzimidazole and ring-substituted imidazo[1,2-a]pyridines has been prepared. 2-(4′-Formylphenylethenyl) derivatives of imidazole and imidazo[1,2-a]pyridine were also prepared.     

Synthesis of 5-iodopyrrolo[1,2-a]quinolines and indolo[1,2-a]quinolines via iodine-mediated electrophilic and regioselective 6-endo-dig ring closure

The endo-cyclic ring closure of 1-(2-(substituted ethynyl)phenyl)-1H-pyrroles 3a-t and 1-(2-(substituted ethynyl)phenyl)-H-indole 4a-o mediated by Lewis acid (I(2)) under mild conditions afforded substituted 5-iodopyrrolo[1,2-a]quinolines 5a-t and 5-iodoindolo[1,2-a]quinolines 6a-o in good to excellent yields. The reaction shows selective C-C bond formation on the more electrophilic alkynyl carbon, resulting in the regioselective 6-endo-dig-cyclized product. Iodo derivatives of pyrrolo- and indoloquinolines allow functional group diversification on the quinoline nucleus, which proves to be highly advantageous for structural and biological activity assessments.     

Effect of hydroxylic solvent on the fluorescence behavior of some bioactive 9-oxo-imidazo[1,2-a]purine derivatives

The spectral and photophysical behavior of four fluorescent 9-oxo-imidazo[1,2-a]purine derivatives containing pyridyl, pyridylphenyl, phenyl, and biphenylyl substituents at the C(6) position of the tricyclic skeleton is described. The studies were performed in several aprotic and protic organic solvents using absorption spectroscopy as well as steady-state and time-resolved fluorescence spectroscopy. The results are also presented of TDDFT calculations on singlet-singlet excitation energies and oscillator strengths for two models of 9-oxo-imidazo[1,2-a]purine, with phenyl or pyridyl substituents, both in the gas phase and in methanol solution. While the derivatives with aryl substituents did not show any significant dependence of their static and dynamic fluorescence properties on the nature of the solvent, the compounds containing a pyridine residue exhibited a remarkable reduction of their fluorescence quantum yields and lifetimes in the alcoholic solutions. The solute-solvent hydrogen-bonding interaction in the first excited singlet state is responsible for the fast radiationless decay rates determined for pyridyl- and pyridylphenyl-substituted compounds in protic solvents. The results of experimental and theoretical studies show that the hydrogen of the alcohols' hydroxyl group and the nitrogen atom of the pyridine moiety are involved in the interaction. The fluorescence-quenching experiments performed for the pyridyl-substituted 9-oxo-imidazo[1,2-a]purine derivative using trifluoroethanol, methanol, and butanol as quenchers revealed that the quenching efficiencies, expressed by the Stern-Volmer quenching constants, correlate with the H-bond donating abilities of the alcohols. The quenching is a dynamic process, and the H-bonded complex formed is nonfluorescent. The experimentally determined and the calculated values of the dipole moment change associated with the electronic excitation indicate that the excited S(1) states of all of the molecules studied in this work have an intramolecular charge-transfer character and that electronic charge is transferred to the C(6) substituent upon excitation. Thus, the ability of the pyridyl substituent nitrogen atom to act as an H-bond acceptor in the excited S(1) state is enhanced. The 6-pyridyl-9-oxo-imidazo[1,2-a]purine presents a novel fluorophore, which, besides its medical applications, may be useful as a sensor of hydroxyl groups in microorganized systems.     

New method for the annelation of a pyridine ring on derivatives of imidazole and benzimidazole

The interaction of (Z)-1,3-diaryl-4-bromo-2-buten-1-ones with 1-substituted (benz)imidazoles in benzene gave (Z)-1-R-3-(2,4-diaryl-4-oxo-2-butenyl)-1H-imidazolium bromides and (Z)-1-R-3-(2,4-diaryl-4-oxo-2-butenyl)-1H-benzimidazolium bromides which readily cyclize in the presence of base to form derivatives of 7,9-diarylpyrido[1,2-a]benzimidazole and 6,8-diarylpyrimidazo[1,2-a]pyridine. The effects of the nature of substituents in the benzene ring of the diarylbutenones and the substituent at N(1) in the (benz)imidazoles on the alkylation and cyclization reactions has been studied. The optimum conditions for the synthesis of the 5-R-4-hydroxy-2,4-diphenyl-4,5-dihydro-1H-pyrido[1,2-a]benz-imidazol-10-ium, 5-R-2,4-diaryl-4-hydroxy-4,5-dihydro-3H-pyrido[1,2-a]benzimidazol-10-ium, and 5-R-2,4-diaryl-5H-pyrido[1,2-a]benzimidazol-10-ium have been found.     

Substituent effect on the photophysical properties, electrochemical properties and electroluminescence performance of orange-emitting iridium complexes

A series of bis(2-phenylbenzothiozolato-N,C(2'))iridium(acetylacetonate) [(bt)(2)Ir(acac)] derivatives, 1-4, were synthesized. Different substituents (CF(3), F, CH(3), OCH(3)) were introduced in the benzothiazole ring to study the substituent effect on the photophysical, electrochemical properties and electroluminescent performance of the complexes, and finally to select high-performance phosphors for use in organic light-emitting diodes (OLEDs). All complexes 1-4 and (bt)(2)Ir(acac) are orange-emitting with tiny spectral difference, despite the variation of the substituent. However, the phosphorescent quantum yield increases with the electron-withdrawing ability of the substituent. This is in contrast to the previous observation that the substituent in the phenyl ring bonded to the metal center of (bt)(2)Ir(acac) not only affected the luminescent quantum efficiency but also greatly tuned the emission color of the complexes. Quantum chemical calculations revealed that the substituents in this position do not make a significant contribution to both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), which probably accounts for the fact that they do no strongly influence the bandgap and emission color of the complexes. Orange OLEDs were fabricated using 1-4 as doped emitters. The electron-withdrawing CF(3) and F groups favor improving the electroluminescence efficiency in comparison with that of the parent (bt)(2)Ir(acac), while electron-donating CH(3) and OCH(3) are not favorable for light emission. The complex 1 based OLED exhibited a maximum luminance efficiency of 54.1 cd A(-1) (a power efficiency of 24 lm W(-1) and an external quantum efficiency of 20%), which are among the best results ever reported for vacuum deposited orange OLEDs so far.     

Regioselective Cycloaddition of the Dimethyl Ester of Acetylenedicarboxylic Acid to 2,4,6-Triazidopyridines

2,4,6-Triazido-3,5-dichloropyridine was obtained in the reaction of pentachloropyridine with sodium azide. At room temperature, this azide reacts regioslectively with norbornene at the -azide group to give the corresponding 4-(3-azatricyclo[3.2.1.0]octanyl)-2,6-diazidopyridine in 88% yield. The cycloaddition of the dimethyl ester of acetylenedicarboxylic acid to this triazide proceeds at the azide groups at C₍₂₎ and C₍₆₎ in the pyridine ring to give 4-azido-2,6-di(4',5'-dimethoxycarbonyl)-1H-1,2,3-triazolopyridine. The analogous reaction of 2,4,6-triazido-3,5-dicyanopyridine with the dimethyl ester of acetylenedicarboxylic acid stops at the formation of 2,4-diazido-6-(4',5'-dimethoxycarbonyl)-1H-1,2,3-triazolopyridine. In contrast to reactions with electron-rich dipolarophiles, the cycloaddition of electron-deficient dipolarophiles to 2,4,6-triazidopyridines proceeds with thermodynamic control primarily a! t the azide groups bearing the highest orbital density in the HOMO.     

Bluish-Green BMes(2) -Functionalized Pt(II) Complexes for High Efficiency PhOLEDs: Impact of the BMes(2) Location on Emission Color

New phosphorescent Pt(II) compounds based on dimesitylboron (BMes(2) )-functionalized 2-phenylpyridyl (ppy) N,C-chelate ligands and an acetylacetonato ancillary ligand have been achieved. We have found that BMes(2) substitution at the 4'-position of the phenyl ring can blue-shift the phosphorescent emission energy of the Pt(II) compound by approximately 50?nm, compared to the 5'-BMes(2) substituted analogue, without substantial loss of luminescent quantum efficiencies. The emission color of the 4'-BMes(2) substituted Pt(II) compound, Pt(Bppy)(acac) (1) can be further tuned by the introduction of a substituent group at the 3'-position of the phenyl ring. A methyl substituent red-shifts the emission energy of 1 by approximately 10?nm whereas a fluoro substituent blue-shifts the emission energy by about 6?nm. Using this strategy, three bright blue-green phosphorescent Pt(II) compounds 1, 2 and 3 with emission energy at 481, 492, and 475?nm and Φ(PL) =0.43, 0.26 and 0.25, respectively, have been achieved. In addition, we have examined the impact of BMes(2) substitution on 3,5-dipyridylbenzene (dpb) N,C,N-chelate Pt(II) compounds by synthesizing compound 4, Pt(Bdpb)Cl, which has a BMes(2) group at the 4'-position of the benzene ring. Compound 4 has a phosphorescent emission band at 485?nm and Φ(PL) =0.70. Highly efficient blue-green electroluminescent (EL) devices with a double-layer structure and compounds 1, 3 or 4 as the phosphorescent dopant have been fabricated. At 100?cd?m(-2) luminance, EL devices based on 1, 3 and 4 with an external quantum efficiency of 4.7, 6.5 and 13.4?%, respectively, have been achieved.     

7,7,8,8-tetracyanoquinodimethane-based molecular dopants for p-type doping of OLEDs: a theoretical investigation

The array of organic conductivity dopants used for organic light-emitting devices (OLEDs) to reduce the operating voltage and improve power efficiency is extremely limited. Here we report a comparative theoretical study between newly proposed analogues and the standard state-of-the-art conductivity dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ). We used density functional theory to determine the bond lengths, bond angles, and electronic properties, such as the energy of the highest occupied molecular orbital (E(HOMO)) and the lowest unoccupied molecular orbital (E(LUMO)) states. The ground state structures of the proposed molecules were optimized at the B3LYP/6-31G* level. The results show that substitution of one or two fluorine groups in the F4-TCNQ core with a substituted phenyl ring or other electron-withdrawing moieties, will not substantially affect the geometry of the molecule or its electronic ability to accept electrons. The most significant finding was that the phenyl substitutions onto the TCNQ core are nearly perpendicular to the TCNQ plane, and thus there is no electronic communication between the two rings. This is extremely important, as such extension of the π conjugated system would negatively affect the E(LUMO) and thus the electron affinity of the molecule.     

Molecular Orbital Controlling Donor Moiety for High‐Efficiency Thermally Activated Delayed Fluorescent Emitters

A new donor moiety, 7,7,13,13‐tetramethyl‐7,13‐dihydro‐5H‐indeno[1,2‐b]acridine (IAc), was developed to control the highest occupied molecular orbital (HOMO) dispersion of thermally activated delayed fluorescent (TADF) emitters. The IAc unit expanded the HOMO dispersion of the emitters and increased the quantum efficiency of the TADF devices up to 20.9 %.     

Combined experimental and theoretical studies on the photophysical properties of cycloparaphenylenes

We studied the UV-vis absorption and fluorescence in solution/solid states of [n]cycloparaphenylene ([n]CPP: n = 9, 12, 14, 15, and 16), and conducted theoretical studies to better understand the experimental results. The representative experimental findings include (i) the most intense absorption maxima (λ(abs1)) display remarkably close values (338-339 nm), (ii) the longest-wavelength absorption maxima (λ(abs2)) are blue-shifted with increasing the ring size (395 → 365 nm), (iii) the emission maxima (λ(em)) are blue-shifted with increasing the ring size (494 → 438 nm for longest-wavelength maxima), (iv) the fluorescent quantum yields (Φ(F)) in solution are high (0.73-0.90), (v) the fluorescence lifetimes (τ(s)) of [9]- and [12]CPP are 10.6 and 2.2 ns, respectively, and (vi) the Φ(F) values slightly increase in polymer matrix but significantly decrease in the crystalline state. According to TD-DFT calculations, the longest-wavelength absorption (λ(abs2)) corresponds to a forbidden HOMO → LUMO transition and the most intense absorption (λ(abs1)) corresponds to degenerate HOMO - 1 → LUMO and HOMO → LUMO + 1 transitions with high oscillator strength. The interesting and counterintuitive optical properties of CPPs (constant λ(abs1) and blue shift of λ(abs2)) could be ascribed mainly to the ring-size effect in frontier molecular orbitals (in particular the increase of the HOMO-LUMO gap as the number of benzene rings increases). On the basis of comparative calculations using hypothetical model geometries, we conclude that the unique behavior of HOMO and LUMO of CPPs is due mainly to their lack of a conjugation length dependence in combination with a significant bending effect (particularly to HOMO) and a torsion effect (particularly to LUMO).     

Molecular Design Approach Managing Molecular Orbital Superposition for High Efficiency without Color Shift in Thermally Activated Delayed Fluorescent Organic Light-Emitting Diodes

Molecular design principles of thermally activated delayed fluorescent (TADF) emitters having a high quantum efficiency and a color tuning capability was investigated by synthesizing three TADF emitters with donors at different positions of a benzonitrile acceptor. The position rendering a large overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) enhances the quantum efficiency of the TADF emitter. Regarding the orbital overlap, donor attachments at 2- and 6-positions of the benzonitrile were more beneficial than 3- and 5-substitutions. Moreover, an additional attachment of a weak donor at the 4-position further increased the quantum efficiency without decreasing the emission energy. Therefore, the molecular design strategy of substituting strong donors at the positions allowing a large molecular orbital overlap and an extra weak donor is a good approach to achieve both high quantum efficiency and a slightly increased emission energy.     

Substituent effect on the luminescent properties of a series of deep blue emitting mixed ligand Ir(III) complexes

The syntheses of the bright deep blue emitting mixed ligand Ir(III) complexes comprising two cyclometalating, one phosphine and one cyano, ligands are reported. In this study, a firm connection between the nature of the excited states and the physicochemical behavior of the complexes with different ligand systems is elucidated by correlating the observed crystal structures, spectroscopic properties, and electrochemical properties with the theoretical results obtained by the density functional theory (DFT) methods. The cyclometalating ligands used here are the anions of 2-(4',6'-difluorophenyl)-pyridine (F2ppy), 2-(4',6'-difluorophenyl)-4-methyl pyridine (F2ppyM), and 4-amino-2-(4',6'-difluorophenyl)-pyridine (DMAF2ppy). The phosphine ligands are PhP(O-(CH2CH2O)3-CH3)2 and Ph2P(O-(CH2CH2O)n-CH3), where Ph = phenyl and n = 1 (P1), 3 (P3), or 8 (P350). The thermal stabilities of the complexes were enhanced upon increasing the "n" value. The crystal structures of the complexes, [(DMAF2ppy)2Ir(P1)CN], (P1)DMA, and [(F2ppyM)2Ir(P3)CN], (P3)F2M, show the cyano and phosphine groups being in a cis configuration to each other and in a trans configuration to the coordinating Cring atoms. The long Ir-Cring bond lengths are ascribed to the trans effect of the strong phosphine and cyano ligands. DFT calculations indicate that the highest occupied molecular orbital (HOMO) is mainly contributed from the d-orbitals of the iridium atom and the pi-orbitals of cyclometalating and cyano ligands, whereas the lowest unoccupied molecular orbital (LUMO) spreads over only one of the cyclometalating ligands, with no contribution from phosphine ligands to both frontier orbitals. Dimethylamino substitution increases the energy of the emitting state that has more metal-to-ligand-charge-transfer (MLCT) character evidenced by the smaller vibronic progressions, smaller difference in the 1MLCT and 3MLCT absorption wavelengths, and higher extinction coefficients (epsilon) than the F2ppy and F2ppyM complexes. However, the increase in the basicity of the dimethylamino group in the DMAF2ppy complexes in the excited states leads to distortions and consequent nonradiative depopulation of the excited states, decreasing their lower photoluminescence (PL) efficiency. The effect of the substituents in the phosphine ligand is more pronounced in the electroluminescence (EL) than in the PL properties. Multilayer organic light emitting devices (OLEDs) are fabricated by doping the Ir(III) complexes in a blend of mCP (m-bis(N-carbazolyl benzene)) and polystyrene, and their device characteristics are studied. The (P3)F2M complex shows a maximum external quantum efficiency (etaex) of 2%, a maximum luminance efficiency (etaL) of 4.13 cd/A at 0.04 mA/cm2, and a maximum brightness of 7200 cd/m2 with a shift of the Commission Internationale de L'Eclairage (CIE) coordinates from (0.14, 0.15) in film PL to (0.19, 0.34) in EL.     

Solid-phase synthesis of imidazo[1,2-a]pyridine using sodium benzenesulfinate as a traceless linker

[formula: see text] The preparation of the first library of imidazo[1,2-a]pyridine derivatives on a solid support is described. A sulfone linker strategy was applied in the synthesis. Key steps involved in the solid-phase synthetic procedure include (i) alpha-haloketone resin formation by sulfinate-->sulfone alkylation, (ii) imidazo[1,2-a]pyridine ring formation by treatment with 2-aminopyridine, (iii) sulfone anion alkylation, and (iv) traceless product release by oxidation-elimination. A library of 12 imidazo[1,2-a]pyridines was synthesized.     

Structures, electronic states, photoluminescence, and carrier transport properties of 1,1-disubstituted 2,3,4,5-tetraphenylsiloles

The excellent electroluminescent (EL) properties of 1,1-disubstituted 2,3,4,5-tetraphenylsiloles, 1-methyl-1,2,3,4,5-pentaphenylsilole (MPPS), and 1,1,2,3,4,5-hexaphenylsilole (HPS) have been found. Despite some studies devoted to these materials, very little is known about the real origin of their unique EL properties. Therefore, we investigated the structures, photoluminescence (PL), and charge carrier transport properties of 1,1-disubstituted 2,3,4,5-tetraphenylsiloles as well as the effect of substituents on these characteristics. The single crystals of the three siloles involving 1,1-dimethyl-2,3,4,5-tetraphenylsilole (DMTPS), MPPS, and HPS were grown and their crystal structures were determined by X-ray diffraction. Three siloles have nonplanar molecular structures. The substituents at 1,1-positions enhance the steric hindrance and have predominant influence on the twisted degree of phenyl groups at ring carbons. This nonplanar structure reduces the intermolecular interaction and the likelihood of excimer formation, and increases PL efficiency in the solid state. The silole films show high fluorescence quantum yields (75-85%), whereas their dilute solutions exhibit a faint emission. The electronic structures of the three siloles were investigated using quantum chemical calculations. The highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) are mainly localized on the silole ring and two phenyl groups at 2,5-positions in all cases, while the LUMOs have a significant orbital density at two exocyclic Si-C bonds. The extremely theoretical studies of luminescent properties were carried out. We calculated the nonradiative decay rate of the first excited state as well as the radiative one. It is found that the faint emission of DMTPS in solutions mainly results from the huge nonradiative decay rate. In solid states, molecular packing can remarkably restrict the intramolecular rotation of the peripheral side phenyl ring, which has a large contribution to the nonradiative transition process. This explains why the 1,1-disubstituted 2,3,4,5-tetraphenylsiloles in the thin films exhibit high fluorescence quantum yields. The charge carrier mobilities of the MPPS and HPS films were measured using a transient EL technique. We obtained a mobility of 2.1 x 10(-)(6) cm(2)/V.s in the MPPS film at an electric field of 1.2 x 10(6) V/cm. This mobility is comparable to that of Alq(3), which is one of the most extensively used electron transport materials in organic light-emitting diodes (LEDs), at the same electric field. The electron mobility of the HPS film is about approximately 1.5 times higher than that of the MPPS film. To the best of our knowledge, this kind of material is one of the most excellent emissive materials that possess both high charge carrier mobility and high PL efficiency in the solid states simultaneously. The excellent EL performances of MPPS and HPS are presumably ascribed to these characteristics.     

2-(1-Adamantyl)-7-methylimidazo-[1,2-α]pyridine and its reactions with N-bromosuccinimide

In reaction with equimolar amount or twice the amount of N-bromosuccinimide 2-(1-adamantyl)-7-methylimidazo[1,2-a]pyridine, obtained from 2-amino-4-methylpyridine and bromomethyl 1-adamantyl ketone, is converted into 2-(1-adamantyl)-3-bromo-7-methylimidazo[1,2-a]pyridine. With three times the amount of N-bromosuccinimide it gives 2-(1-adamantyl)-3-bromo-7-formylimidazo[1,2-a]pyridine.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1791–1793, December, 2004.     

Cationic IrIII Emitters with Near-Infrared Emission Beyond 800 nm and Their Use in Light-Emitting Electrochemical Cells

Solid-state near-infrared (NIR) light-emitting devices have recently received considerable attention as NIR light sources that can penetrate deep into human tissue and are suitable for bioimaging and labeling. In addition, solid-state NIR light-emitting electrochemical cells (LECs) have shown several promising advantages over NIR organic light-emitting devices (OLEDs). However, among the reported NIR LECs based on ionic transition-metal complexes (iTMCs), there is currently no iridium-based LEC that displays NIR electroluminescence (EL) peaks near to or above 800 nm. In this report we demonstrate a simple method for adjusting the energy gap between the highest-occupied molecular orbital (HOMO) and the lowest-unoccupied molecular orbital (LUMO) of iridium-based iTMCs to generate NIR emission. We describe a series of novel ionic iridium complexes with very small energy gaps, namely NIR1 – NIR6 , in which 2,3-diphenylbenzo[g]quinoxaline moieties mainly take charge of the HOMO energy levels and 2,2′-biquinoline, 2-(quinolin-2-yl)quinazoline, and 2,2′-bibenzo[d]thiazole moieties mainly control the LUMO energy levels. All the complexes exhibited NIR phosphorescence, with emission maxima up to 850 nm, and have been applied as components in LECs, showing a maximum external quantum efficiency (EQE) of 0.05 % in the EL devices. By using a host–guest emissive system, with the iridium complex RED as the host and the complex NIR3 or NIR6 as guest, the highest EQE of the LECs can be further enhanced to above 0.1 %.     

Theoretical studies of the modulation of polymer electronic and optical properties through the introduction of the electron-donating 3,4-ethylenedioxythiophene or electron-accepting pyridine and 1,3,4-oxadiazole moieties

One serious problem associated with polyfluorene and derivatives (PFs) as blue luminescent polymers is the significant energy barrier for hole or electron injections; thus they usually face charge injection and transport difficulties with the currently available cathode and anode materials. The incorporation of an electron-donating or -accepting unit is expected to improve the recombination of the charge carriers. In this paper, we apply quantum-chemical techniques to investigate three fluorene-based copolymers, copoly(2,5-ethylenedioxythiophene-alt-9,9'-dimethylfluorene) (PEF), copoly(2,5-pyridine-alt-9,9'-dimethylfluorene) (PPyF), and poly[(fluorene-2,7-diyl)-alt-(1,3,4-oxadiazole-2,5-diyl)] (PFO), in which Delta(H)(-)(L) [the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), when n = infinity], the lowest excitation energies (E(g)), ionization potentials (IP), electron affinities (EA), and lambda(abs) and lambda(em) are fine-tuned by the regular insertion of electron-donating unit 3,4-ethylenedioxythiophene (EDOT) or electron-withdrawing units pyridine and 1,3,4-oxadiazole. The results show that the alternate incorporation of electron-donating moiety EDOT increases the HOMO energy and thus reduces the IPs, and consequently the hole injection was greatly improved. On the other hand, even though both kinds of charge carriers will improve the electron-accepting ability, the results show that electron-withdrawing moieties greatly facilitate the electron-transporting. Especially in PFO, the highly planar structural character resulted from the strong push-pull effect between the fluorene ring and the 1,3,4-oxadiazole ring and a weak interaction between the nitrogen and oxygen atoms in 1,3,4-oxadiazole ring and the hydrogen atom of the fluorene ring, significantly lowering the LUMO energy levels and thus improve the electron-accepting and transporting properties by the low LUMO energy levels.