Visible‐Light‐Driven Hydrogen Production and Polymerization using Triarylboron‐Functionalized Iridium(III) Complexes

The development of novel iridium(III) complexes has continued as an important area of research owing to their highly tunable photophysical properties and versatile applications. In this report, three heteroleptic dimesitylboron‐containing iridium(III) complexes, [Ir(p‐B‐ppy)₂(N^N)]⁺ {p‐B‐ppy=2‐(4‐dimesitylborylphenyl)pyridine; N^N=dipyrido[3,2‐a:2′,3′‐c]phenazine (dppz) ( 1 ), dipyrido[3,2‐d:2′,3′‐f]quinoxaline (dpq) ( 2 ), and 1,10‐phenanthroline (phen) ( 3 )}, were prepared and fully characterized electrochemically, photophysically, and computationally. Altering the conjugated length of the N^N ligands allowed us to tailor the photophysical properties of these complexes, especially their luminescence wavelength, which could be adjusted from λ=583 to 631 nm in CH₂Cl₂. All three complexes were evaluated as visible‐light‐absorbing sensitizers for the photogeneration of hydrogen from water and as photocatalysts for the photopolymerization of methyl methacrylate. The results showed that all of them were active in both photochemical reactions. High activity for the photosensitizer (over 1158 turnover numbers with 1 ) was observed, and the system generated hydrogen even after 20 h. Additionally, poly(methyl methacrylate) with a relatively narrow molecular‐weight distribution was obtained if an initiator (i.e., ethyl α‐bromophenylacetate) was used. The living character of the photoinduced polymerization was confirmed on the basis of successful chain‐extension experiments.     

Red Phosphorescent Bis‐Cyclometalated Iridium Complexes with Fluorine‐, Phenyl‐, and Fluorophenyl‐Substituted 2‐Arylquinoline Ligands

Red phosphorescent iridium(III) complexes based on fluorine‐, phenyl‐, and fluorophenyl‐substituted 2‐arylquinoline ligands were designed and synthesized. To investigate their electrophosphorescent properties, devices were fabricated with the following structure: indium tin oxide (ITO)/4,4′,4′′‐tris[2‐naphthyl(phenyl)amino]triphenylamine (2‐TNATA)/4,4′‐bis[N‐(1‐naphthyl)‐N‐phenylamino]biphenyl (NPB)/4,4′‐bis(N‐carbazolyl)‐1,1′‐biphenyl (CBP): 8 % iridium (III) complexes/bathocuproine (BCP)/tris(8‐hydroxyquinolinato)aluminum (Alq₃)/8‐hydroxyquinoline lithium (Liq)/Al. All devices, which use these materials showed efficient red emissions. In particular, a device exhibited a saturated red emission with a maximum luminance, external quantum efficiency, and luminous efficiency of 14200 cd m^?2, 8.44 %, and 6.58 cd A^?1 at 20 mA cm^?2, respectively. The CIE (x, y) coordinates of this device are (0.67, 0.33) at 12.0 V.     

Theoretical studies of ground and excited electronic states in a series of heteroleptic iridium complexes using density functional theory

A series of new heteroleptic iridium(III) complexes [Ir(C^?N)₂(N^?N)]PF₆ ( 1 ‐ 6 ) (each with two cyclometalating C^?N ligands and one neutral N^?N ancillary ligand, where C^?N = 2‐phenylpyridine (ppy), 5‐methyl‐2‐(4‐fluoro)phenylpyridine (F‐mppy), and N^?N = 2,2′‐dipyridyl (bpy), 1,10‐phenanthroline (phen), 4,4′‐diphenyl‐2,2′‐dipyridy (dphphen) were found to have rich photophysical properties. Theoretical calculations are employed for studying the photophysical and electrochemical properties. All complexes are investigated using density functional theory. Excited singlet and triplet states are examined using time‐dependent density functional theory. The low‐lying excited‐state geometries are optimized at the ab initio configuration interaction singles level. Then, the excited‐state properties are investigated in detail, including absorption and emission properties, photoactivation processes. The excited state of complexes is complicated and contains triplet metal‐to‐ligand charge transfer, triplet ligand‐to‐ligand charge transfer simultaneously. Importantly, the absorption spectra and emission maxima can be tuned significantly by changing the N^?N ligands and C^?N ligands. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Long‐Lived Room‐Temperature Deep‐Red‐Emissive Intraligand Triplet Excited State of Naphthalimide in Cyclometalated IrIII Complexes and its Application in Triplet‐Triplet Annihilation‐Based Upconversion

Cyclometalated Ir^III complexes with acetylide ppy and bpy ligands were prepared (ppy=2‐phenylpyridine, bpy=2,2′‐bipyridine) in which naphthal ( Ir‐2 ) and naphthalimide (NI) were attached onto the ppy ( Ir‐3 ) and bpy ligands ( Ir‐4 ) through acetylide bonds. [Ir(ppy)₃] ( Ir‐1 ) was also prepared as a model complex. Room‐temperature phosphorescence was observed for the complexes; both neutral and cationic complexes Ir‐3 and Ir‐4 showed strong absorption in the visible range (ε=39600 M ^?1 cm^?1 at 402 nm and ε=25100 M ^?1 cm^?1 at 404 nm, respectively), long‐lived triplet excited states (τ_T=9.30 μs and 16.45 μs) and room‐temperature red emission (λ_em=640 nm, Φ_p=1.4 % and λ_em=627 nm, Φ_p=0.3 %; cf. Ir‐1 : ε=16600 M ^?1 cm^?1 at 382 nm, τ_em=1.16 μs, Φ_p=72.6 %). Ir‐3 was strongly phosphorescent in non‐polar solvent (i.e., toluene), but the emission was completely quenched in polar solvents (MeCN). Ir‐4 gave an opposite response to the solvent polarity, that is, stronger phosphorescence in polar solvents than in non‐polar solvents. Emission of Ir‐1 and Ir‐2 was not solvent‐polarity‐dependent. The T₁ excited states of Ir‐2 , Ir‐3 , and Ir‐4 were identified as mainly intraligand triplet excited states (³IL) by their small thermally induced Stokes shifts (ΔE_s), nanosecond time‐resolved transient difference absorption spectroscopy, and spin‐density analysis. The complexes were used as triplet photosensitizers for triplet‐triplet annihilation (TTA) upconversion and quantum yields of 7.1 % and 14.4 % were observed for Ir‐2 and Ir‐3 , respectively, whereas the upconversion was negligible for Ir‐1 and Ir‐4 . These results will be useful for designing visible‐light‐harvesting transition‐metal complexes and for their applications as triplet photosensitizers for photocatalysis, photovoltaics, TTA upconversion, etc.     

Tricyclometalated Iridium Complexes as Highly Stable Photosensitizers for Light‐Induced Hydrogen Evolution

The development of an efficient and stable artificial photosensitizer for visible‐light‐driven hydrogen production is highly desirable. Herein, a new series of charge‐neutral, heteroleptic tricyclometalated iridium(III) complexes, [Ir(thpy)₂(bt)] ( 1 – 4 ; thpy=2,2′‐thienylpyridine, bt=2‐phenylbenzothiazole and its derivatives), were systematically synthesized and their structural, photophysical, and electrochemical properties were established. Three solid‐state structures were studied by X‐ray crystallographic analysis. This design offers the unique opportunity to drive the metal‐to‐ligand charge‐transfer (MLCT) band to longer wavelengths for these iridium complexes. We describe new molecular platforms that are based on these neutral iridium complexes for the production of hydrogen through visible‐light‐induced photocatalysis over an extended period of time in the presence of [Co(bpy)₃]²⁺ and triethanolamine (TEOA). The maximum amount of hydrogen was obtained under constant irradiation over 72 h and the system could regenerate its activity upon the addition of cobalt‐based catalysts when hydrogen evolution ceased. Our results demonstrated that the dissociation of the [Co(bpy)₃]²⁺ catalyst contributed to the loss of catalytic activity and limited the long‐term catalytic performance of the systems. The properties of the neutral complexes are compared in detail to those of two known non‐neutral bpy‐type complexes, [Ir(thpy)₂(dtb‐bpy)]⁺ ( 5 ) and [Ir(ppy)₂(dtb‐bpy)]⁺ ( 6 ; ppy=2‐phenylpyridine, dtb‐bpy=4,4′‐di‐tert‐butyl‐2,2′‐dipyridyl). This work is expected to contribute toward the development of long‐lasting solar hydrogen‐production systems.     

Synthesis,Structural Characterization and Luminescence Studies of Di‐ and Trinuclear Gold(I) Alkynyl‐phosphine Complexes

Reactions of the polymer {Au^IC₂Ph}_n with polyphosphine ligands [1,4‐bis(2‐diphenylphosphino‐1H‐imidazol‐1‐yl)‐benzene (dpib), 1,3,5‐tris(4‐diphenylphosphinophenyl)benzene (tppb), 2,2′‐bis(diphenylphosphanyl)‐4,4′‐bipyridine (dpbp), and 3,6‐bis(diphenylphosphanyl)pyridazine (dppz)] afforded four gold(I) alkynyl‐polyphosphine complexes [{AuC₂Ph}₂(μ‐dpib)] ( 1 ), [{AuC₂Ph}₃(μ₃‐tppb)] ( 2 ), [{AuC₂Ph}₂(μ‐dpbp)] ( 3 ), and [{AuC₂Ph}₂(μ‐dppz)] ( 4 ) in nearly quantitative yield. The compounds obtained were characterized using elemental analysis, ESI‐MS, X‐ray crystallography, and polynuclear NMR spectroscopy. Intermolecular aurophilic interaction together with π–π and σ–π stacking build up the supramolecular 3D network of complex 3 , whereas none of these intermolecular bondings were found in the crystal structures of compounds 1 , 2 , and 4 . Complexes 1 – 4 are luminescent both in solution (CH₂Cl₂) and in solid state under laser irradiation (λ_ex = 308 nm). In solution, the diphosphine complexes 1 – 4 display dual emission corresponding to ligand centered transitions (λ_em = 360–375 nm) along with weaker contribution from MLCT excited states at ca. 490 nm. The long wavelength component of the emission plays a dominant role in the solid state luminescence spectra of complexes 1 , 3 , and 4 (460, 544, 520 nm, respectively) whereas the triphosphine complex 2 shows dual luminescence (372 and 520 nm) with considerable contribution from ligand centered excited state.     

Decoration of Dibenzofuran Using Cyanocarbazole via 6‐Position as a Molecular Design Approach for High‐Triplet‐Energy Bipolar Host Materials

In this study, two new dibenzofuran derivatives featuring one or two cyanocarbazole units, 6‐(dibenzo[b,d]furan‐4‐yl)‐9‐phenyl‐9H‐carbazole‐3‐carbonitrile ( mBFCzCN) and 6,6′‐(dibenzo[b,d]furan‐4,6‐diyl)bis(9‐phenyl‐9H‐carbazole‐3‐carbonitrile) ( dBFCzCN ), were developed as host materials for phosphorescent organic light emitting diodes (PhOLEDs). A new molecular design connecting the cyanocarbazole to the dibenzofuran using the cyanocarbazole 6‐position instead of its 9‐position was created, and the effects of number of cyanocarbazole units in the dibenzofuran building block on the photophysical and electroluminescence properties were investigated in detail. The mBFCzCN compound revealed high triplet energy (2.78 eV) than that of dBFCzCN (2.68 eV) and good bipolar charge transporting properties. The potential of these materials as hosts for blue and green PhOLEDs was investigated using bis(4,6‐(difluorophenyl)pyridinato‐N,C^2′)picolinate iridium(III) (FIrpic) and tris(2‐phenylpyridinato)iridium(III) (Ir(ppy)₃) dopants, respectively. The results indicated that the mBFCzCN with one cyanocarbazole unit showed better device performance than the dBFCzCN with two cyanocarbazole units in the blue and green devices. High external quantum efficiencies of 19.0 and 21.2 % were demonstrated in the blue and green PhOLEDs with the mBFCzCN host due to its high triplet energy and good bipolar charge transporting characteristics.     

Charge‐Neutral Amidinate‐Containing Iridium Complexes Capable of Efficient Photocatalytic Water Reduction

Two new charge‐neutral iridium complexes, [Ir(tfm‐ppy)₂(N,N′‐diisopropyl‐benzamidinate)] ( 1 ) and [Ir(tfm‐ppy)₂(N,N′‐diisopropyl‐4‐diethylamino‐3,5‐dimethyl‐benzamidinate)] ( 2 ) (tfm‐ppy=4‐trifluoromethyl‐2‐phenylpyridine) containing an amidinate ligand and two phenylpyridine ligands were designed and characterised. The photophysical properties, electrochemical behaviours and emission quenching properties of these species were investigated. In concert with the cobalt catalyst [Co(bpy)₃]²⁺, members of this new class of iridium complexes enable the photocatalytic generation of hydrogen from mixed aqueous solutions via an oxidative quenching pathway and display long‐term photostability under constant illumination over 72 h; one of these species achieved a relatively high turnover number of 1880 during this time period. In the case of complex 1 , the three‐component homogeneous photocatalytic system proved to be more efficient than a related system containing a charged complex, [Ir(tfm‐ppy)₂(dtb‐bpy)]⁺ ( 3 , dtb‐bpy=4,4′‐di‐tert‐butyl‐2,2′‐dipyridyl). In combination with a rhodium complex as a water reduction catalyst, the performances of the systems using both complexes were also evaluated, and these systems exhibited a more efficient catalytic propensity for water splitting than did the cobalt‐based systems that have been studied previously.     

Photophysical and electrochemical properties of heteroleptic tris-cyclometallated Ir(III) complexes

Some new heteroleptic tris-cyclometallated iridium(III) complexes have been synthesized and fully characterized. Among these iridium(III) complexes, bis(1-phenylpyrazolato-N,C^2′)iridium(III)[5-(2′-pyridyl)tetrazolate] (3) and bis(3-methyl-1-phenylpyrazolato-N,C^2′)iridium(III)[5-(2′-pyridyl)tetrazolate] (4) show excellent quantum yields at room temperature, the electron density being perturbed by introducing the pyridyltetrazole ligand, making k_r > k_nr. This destroys the concept of phenylpyrazole based iridium complexes.     

Neutral and cationic cyclometallated Ir(III) complexes of anthra[1,2-d]imidazole-6,11-dione-derived ligands: Syntheses, structures and spectroscopic characterisation

The syntheses of two new ligands and five new heteroleptic cyclometallated Ir(III) complexes are reported. The ligands are based upon a functionalised anthra[1,2-d]imidazole-6,11-dione core giving LH¹⁻³ incorporating a pendant pyridine, quinoline or thiophene unit respectively. Neutrally charged, octahedral complexes [Ir(ppy)₂(L¹⁻³)] are chelated by two cyclometallated phenylpyridine (ppy) ligands and a third, ancillary deprotonated ligand L¹⁻³, whilst cationic analogues could only be isolated for [Ir(ppy)₂(LH¹⁻²)][PF₆]. X-ray crystal structures for [Ir(ppy)₂(L¹)], [Ir(ppy)₂(LH¹)][PF₆] and [Ir(ppy)₂(L²)] showed the complexes adopt a distorted octahedral coordination geometry, with the anthra[1,2-d]imidazole-6,11-dione ligands coordinating in a bidentate fashion. Preliminary DFT calculations revealed that for the complexes of LH¹ and LH² the LUMO is exclusively localized on the ancillary ligand, whereas the nature of the HOMO depends on the protonation state of the ancillary ligand, often being composed of both Ir(III) and phenylpyridine character. UV-vis. and luminescence data showed that the ligands absorb into the visible region ca. 400 nm and emit ca. 560 nm, both of which are attributed to an intra-ligand CT transition within the anthra[1,2-d]imidazole-6,11-dione core. The complexes display absorption bands attributed to overlapping ligand-centred and ¹MLCT-type electronic transitions, whilst only [Ir(ppy)₂(L²)] appeared to possess typical ³MLCT behaviour (λ_em = 616 nm; τ = 96 ns in aerated MeCN). The remaining complexes were generally visibly emissive (λ_em ≈ 560-570 nm; τ < 10 ns in aerated MeCN) with very oxygen-sensitive lifetimes more indicative of ligand-centred processes.     

Synthesis and comparison of the structure–property relationships of symmetric and asymmetric water‐soluble poly(p‐phenylene)s

Sodium salts of water‐soluble polymers poly{[2,5‐bis(3‐sulfonatopropoxy)‐1,4‐phenylene]‐alt‐[2,5‐bis(hexyloxy)‐1,4‐phenylene]} ( P1 ), poly{[2,5‐bis(3‐sulfonatopropoxy)‐1,4‐phenylene]‐alt‐[2,5‐bis(dodecyloxy)‐1,4‐phenylene]} ( P2 ), poly{[2,5‐bis(3‐sulfonatopropoxy)‐1,4‐phenylene]‐alt‐[2,5‐bis(dibenzyloxy)‐1,4‐phenylene]} ( P3 ), poly[2‐hexyloxy‐5‐(3‐sulfonatopropoxy)‐1,4‐phenylene] ( P4 ), and poly[2‐dodecyloxy‐5‐(3‐sulfonatopropoxy)‐1,4‐phenylene] ( P5 )] were synthesized with Suzuki coupling reactions and fully characterized. The first group of polymers ( P1 – P3 ) with symmetric structures gave lower absorption maxima [maximum absorption wavelength (λ_max) = 296–305 nm] and emission maxima [maximum emission wavelength (λ_em) = 361–398 nm] than asymmetric polymers P4 (λ_max = 329 nm, λ_em = 399 nm) and P5 (λ_max = 335 nm, λ_em = 401 nm). The aggregation properties of polymers P1 – P5 in different solvent mixtures were investigated, and their influence on the optical properties was examined in detail. Dynamic light scattering studies of the aggregation behavior of polymer P1 in solvents indicated the presence of aggregated species of various sizes ranging from 80 to 800 nm. The presence of alkoxy groups and 3‐sulfonatopropoxy groups on adjacent phenylene rings along the polymer backbone of the first set hindered the optimization of nonpolar interactions. The alkyl chain crystallization on one side of the polymer chain and the polar interactions on the other side allowed the polymers ( P4 and P5 ) to form a lamellar structure in the polymer lattice. Significant quenching of the polymer fluorescence upon the addition of positively charged viologen derivatives or cytochrome‐C was also observed. The quenching effect on the polymer fluorescence confirmed that the newly synthesized polymers could be used in the fabrication of biological and chemical sensors. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3763–3777, 2006     

Understanding Electrogenerated Chemiluminescence Efficiency in Blue‐Shifted Iridium(III)‐Complexes: An Experimental and Theoretical Study

Compared to tris(2‐phenylpyridine)iridium(III) ([Ir(ppy)₃]), iridium(III) complexes containing difluorophenylpyridine (df‐ppy) and/or an ancillary triazolylpyridine ligand [3‐phenyl‐1,2,4‐triazol‐5‐ylpyridinato (ptp) or 1‐benzyl‐1,2,3‐triazol‐4‐ylpyridine (ptb)] exhibit considerable hypsochromic shifts (ca. 25–60 nm), due to the significant stabilising effect of these ligands on the HOMO energy, whilst having relatively little effect on the LUMO. Despite their lower photoluminescence quantum yields compared with [Ir(ppy)₃] and [Ir(df‐ppy)₃], the iridium(III) complexes containing triazolylpyridine ligands gave greater electrogenerated chemiluminescence (ECL) intensities (using tri‐n‐propylamine (TPA) as a co‐reactant), which can in part be ascribed to the more energetically favourable reactions of the oxidised complex (M⁺) with both TPA and its neutral radical oxidation product. The calculated iridium(III) complex LUMO energies were shown to be a good predictor of the corresponding M⁺ LUMO energies, and both HOMO and LUMO levels are related to ECL efficiency. The theoretical and experimental data together show that the best strategy for the design of efficient new blue‐shifted electrochemiluminophores is to aim to stabilise the HOMO, while only moderately stabilising the LUMO, thereby increasing the energy gap but ensuring favourable thermodynamics and kinetics for the ECL reaction. Of the iridium(III) complexes examined, [Ir(df‐ppy)₂(ptb)]⁺ was most attractive as a blue‐emitter for ECL detection, featuring a large hypsochromic shift (λ_max=454 and 484 nm), superior co‐reactant ECL intensity than the archetypal homoleptic green and blue emitters: [Ir(ppy)₃] and [Ir(df‐ppy)₃] (by over 16‐fold and threefold, respectively), and greater solubility in polar solvents.     

Exploring Excited‐State Tunability in Luminescent Tris‐cyclometalated Platinum(IV) Complexes: Synthesis of Heteroleptic Derivatives and Computational Calculations

The synthesis, structure, electrochemistry, and photophysical properties of a series of heteroleptic tris‐ cyclometalated Pt^IV complexes are reported. The complexes mer‐[Pt(C^N)₂(C′^N′)]OTf, with C^N=C‐deprotonated 2‐(2,4‐difluorophenyl)pyridine (dfppy) or 2‐phenylpyridine (ppy), and C′^N′=C‐deprotonated 2‐(2‐thienyl)pyridine (thpy) or 1‐phenylisoquinoline (piq), were obtained by reacting bis‐ cyclometalated precursors [Pt(C^N)₂Cl₂] with AgOTf (2 equiv) and an excess of the N′^C′H pro‐ligand. The complex mer‐[Pt(dfppy)₂(ppy)]OTf was obtained analogously and photoisomerized to its fac counterpart. The new complexes display long‐lived luminescence at room temperature in the blue to orange color range. The emitting states involve electronic transitions almost exclusively localized on the ligand with the lowest π–π* energy gap and have very little metal character. DFT and time‐dependent DFT (TD‐DFT) calculations on mer‐[Pt(ppy)₂(C′^N′)]⁺ (C′^N′=thpy, piq) and mer/fac‐[Pt(ppy)₃]⁺ support this assignment and provide a basis for the understanding of the luminescence of tris‐cyclometalated Pt^IV complexes. Excited states of LMCT character may become thermally accessible from the emitting state in the mer isomers containing dfppy or ppy as chromophoric ligands, leading to strong nonradiative deactivation. This effect does not operate in the fac isomers or the mer complexes containing thpy or piq, for which nonradiative deactivation originates mainly from vibrational coupling to the ground state.     

Phosphorescent Cyclometalated Iridium(III) Complexes That Contain Substituted 2‐Acetylbenzo[b]thiophen‐3‐olate Ligand for Red Organic Light‐Emitting Devices

We report the synthesis of a new class of thermally stable and strongly luminescent cyclometalated iridium(III) complexes 1 – 6 , which contain the 2‐acetylbenzo[b]thiophene‐3‐olate (bt) ligand, and their application in organic light‐emitting diodes (OLEDs). These heteroleptic iridium(III) complexes with bt as the ancillary ligand have a decomposition temperature that is 10–20 % higher and lower emission self‐quenching constants than those of their corresponding complexes with acetylacetonate (acac). The luminescent color of these iridium(III) complexes could be fine‐tuned from orange (e.g., 2‐phenyl‐6‐(trifluoromethyl)benzo[d]thiazole (cf₃bta) for 4 ) to pure red (e.g., lpt (Hlpt=4‐methyl‐2‐(thiophen‐2‐yl)quinolone) for 6 ) by varying the cyclometalating ligands (C‐deprotonated C^N). In particular, highly efficient OLEDs based on 6 as dopant (emitter) and 1,3‐bis(carbazol‐9‐yl)benzene (mCP) as host that exhibit stable red emission over a wide range of brightness with CIE chromaticity coordinates of (0.67, 0.33) well matched to the National Television System Committee (NTSC) standard have been fabricated along with an external quantum efficiency (EQE) and current efficiency of 9 % and 10 cd A^?1, respectively. A further 50 % increase in EQE (>13 %) by replacing mCP with bis[4‐(6H‐indolo[2,3‐b]quinoxalin‐6‐yl)phenyl]diphenylsilane (BIQS) as host for 6 in the red OLED is demonstrated. The performance of OLEDs fabricated with 6 (i.e., [(lpt)₂Ir(bt)]) was comparable to that of the analogous iridium(III) complex that bore acac (i.e., [(lpt)₂Ir(acac)]; 6 a in this work) [Adv. Mater.­ 2011 , 23, 2981] fabricated under similar conditions. By using ntt (Hnnt=3‐hydroxynaphtho[2,3‐b]thiophen‐2‐yl)(thiophen‐2‐yl)methanone) ligand, a substituted derivative of bt, the [(cf₃bta)₂Ir(ntt)] was prepared and found to display deep red emission at around 700 nm with a quantum yield of 12 % in mCP thin film.     

Electronic Coupling between Two Amine Redox Sites through the 5,5′‐Positions of Metal‐Chelating 2,2′‐Bipyridines

Electron delocalization of new mixed‐valent (MV) systems with the aid of lateral metal chelation is reported. 2,2′‐Bipyridine (bpy) derivatives with one or two appended di‐p‐anisylamino groups on the 5,5′‐positions and a coordinated [Ru(bpy)₂] (bpy=2,2′‐bipyridine), [Re(CO)₃Cl], or [Ir(ppy)₂] (ppy=2‐phenylpyridine) component were prepared. The single‐crystal molecular structure of the bis‐amine ligand without metal chelation is presented. The electronic properties of these complexes were studied and compared by electrochemical and spectroscopic techniques and DFT/TDDFT calculations. Compounds with two di‐p‐anisylamino groups were oxidized by a chemical or electrochemical method and monitored by near‐infrared (NIR) absorption spectral changes. Marcus–Hush analysis of the resulting intervalence charge‐transfer transitions indicated that electron coupling of these mixed‐valent systems is enhanced by metal chelation and that the iridium complex has the largest coupling. TDDFT calculations were employed to interpret the NIR transitions of these MV systems.     

Synthesis, Photophysical and Electrochemical Characterization of the Heteroleptic Iridium Complexes with Modified Ancillary Ligand by Carrier-transporting Groups

Two heteroleptic iridium complexes with a general formulation of (piq)₂Ir(G‐pic) were synthesized and characterized by ¹H NMR, ¹³C NMR and element analysis, in which piq is 1‐phenylisoquinoline, G‐pic is picolinic acid derivative containing carrier‐transporting group by a non‐conjugated connection of 1,6‐dioxyhexane. Both (piq)₂Ir(G‐pic) complexes exhibited an enhanced UV absorption band at 310–400 nm, an increased HOMO energy level and an identical red emission peaked at 612 nm with higher fluorescence quantum efficiency (ø_f), compared to (piq)₂Ir(pic) in dichloromethane solution. Interestingly, this iridium complex containing both hole‐transporting triphenylamine and electron‐transporting oxadiazole moieties exhibited the best Ф_f of 0.58 using Ru(bpy)₃(PF6)₂ as the reference (ø_f=0.062 in acetonitrile). This work indicates that incorporating carrier‐transporting groups into ancillary ligand by a non‐conjugated connection is available for improving the optophysical properties of their iridium complexes.     

Tris‐cyclometalated Iridium(III) Complexes with Three Different Ligands: a New Example with 2‐(2,4‐Difluorophenyl)pyridine‐Based Complex

An iridium(III) complex comprising three different cyclometalated phenylpyridine‐based ligands was designed and synthesized. Interestingly, mixed‐ligand complexes could be obtained by using a simple and straightforward procedure. A tris(heteroleptic) Ir^III complex was obtained as a mixture of stereoisomers that could not be separated. Photophysical properties of the tris(heteroleptic) complex was investigated by UV/VIS absorption and luminescence spectroscopy, and compared with those of the parent homoleptic complexes. Modelling by time‐dependent density functional theory (TD‐DFT) was also performed to elucidate the nature and the location of the excited state, and to support the experimental results.     

Nature of the near‐IR band in the electronic absorption spectra of neutral bis(tetrapyrrole) rare earth(III) complexes: Time‐dependent density functional theory calculations

The nature of the near‐IR band in the electronic absorption spectra of bis(tetrapyrrole) rare earth(III) complexes Y(Pc)₂ (1), La(Pc)₂ (2), Y(Pc)(Por) (3), Y(Pc)[Pc(α‐OCH₃)₄] (4), Y(Pc)[Pc(α‐OCH₃)₈] (5), and Y(Pc)[Pc(β‐OCH₃)₈] (6) was studied on the basis of time‐dependent density functional theory (TD‐DFT) calculations. The electronic dipole moment along the z‐axis in the electronic transition of the near‐IR band in all the studied neutral bis(tetrapyrrole) yttrium(III) and lanthanum(III) double‐deckers is well explained on the basis of the composition analysis of the orbitals involved. The electronic transition in the near‐IR band causes the reversion of the orbital orientation of one tetrapyrrole ring in both homoleptic and heteroleptic bis(tetrapyrrole) rare earth complexes and induces electron transfer from the tetrapyrrole ring with lower orbital energy to the other ring in the heteroleptic bis(tetrapyrrole) rare earth(III) complexes. The near‐IR band can work as an ideal characteristic absorption band to reflect the π–π interaction between the two tetrapyrrole rings in bis(tetrapyrrole) rare earth(III) double‐decker complexes because of its peculiar electronic transition nature. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Iridium(III) Anthraquinone Complexes as Two‐Photon Phosphorescence Probes for Mitochondria Imaging and Tracking under Hypoxia

In the present study, four mitochondria‐specific and two‐photon phosphorescence iridium(III) complexes, Ir1 – Ir4 , were developed for mitochondria imaging in hypoxic tumor cells. The iridium(III) complex has two anthraquinone groups that are hypoxia‐sensitive moieties. The phosphorescence of the iridium(III) complex was quenched by the functions of the intramolecular quinone unit, and it was restored through two‐electron bioreduction under hypoxia. When the probes were reduced by reductase to hydroquinone derivative products under hypoxia, a significant enhancement in phosphorescence intensity was observed under one‐ (λ=405 nm) and two‐photon (λ=720 nm) excitation, with a two‐photon absorption cross section of 76–153 GM at λ=720 nm. More importantly, these probes possessed excellent specificity for mitochondria, which allowed imaging and tracking of the mitochondrial morphological changes in a hypoxic environment over a long period of time. Moreover, the probes can visualize hypoxic mitochondria in 3D multicellular spheroids and living zebrafish through two‐photon phosphorescence imaging.     

Greenish‐yellow‐, yellow‐, and orange‐light‐emitting iridium(III) polypyridyl complexes with poly(ε‐caprolactone)–bipyridine macroligands

A set of novel greenish‐yellow‐, yellow‐, and orange‐light‐emitting polymeric iridium(III) complexes were synthesized with the bridge‐splitting method. The respective dimeric precursor complexes, [Ir(ppy)₂‐μ‐Cl]₂ (ppy = 2‐phenylpyridine) and [Ir(ppy? CHO)₂‐μ‐Cl]₂ [ppy? CHO = 4‐(2‐pyridyl)benzaldehyde], were coordinated to 2,2′‐bipyridine carrying poly(ε‐caprolactone) tails. The resulting emissive polymers were characterized with one‐dimensional (¹H) and two‐dimensional (¹H? ¹H correlation spectroscopy) nuclear magnetic resonance and infrared spectroscopy, gel permeation chromatography, and matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry, and the successful coordination of the iridium(III) centers to the 2,2′‐bipyridine macroligand was revealed. The thermal behavior was studied with differential scanning calorimetry and correlated with atomic force microscopy. Furthermore, the quantitative coordination was verified by both the photophysical and electrochemical properties of the mononuclear iridium(III) compounds. The photoluminescence spectra showed strong emissions at 535 and 570 nm. The color shifts depended on the substituents of the cyclometallating ligands. Cyclic voltammetry gave oxidation potentials of 1.23 V and 1.46 V. Upon the excitation of the films at 365 nm, yellow light was observed, and this could allow potential applications in light‐emitting devices. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2765–2776, 2005