Modification of Luminescent Iridium(III) Polypyridine Complexes with Discrete Poly(ethylene glycol) (PEG) Pendants: Synthesis,Emissive Behavior,Intracellular Uptake,and PEGylation Properties

We report the synthesis, characterization, and photophysical properties of a new class of luminescent cyclometalated iridium(III) polypyridine poly(ethylene glycol) (PEG) complexes [Ir(N^C)₂(N^N)](PF₆) (HN^C=Hppy (2‐phenylpyridine), N^N=bpy? CONH? PEG1 (bpy=2,2′‐bipyridine; 1 a ), bpy? CONH? PEG3 ( 1 b ); HN^C=Hpq (2‐phenylquinoline), N^N=bpy? CONH? PEG1 ( 2 a ), bpy? CONH? PEG3 ( 2 b ); HN^C=Hpba (4‐(2‐pyridyl)benzaldehyde), N^N=bpy? CONH? PEG1 ( 3 )) and their PEG‐free counterparts (N^N=bpy? CONH? Et, HN^C=Hppy ( 1 c ); HN^C=Hpq ( 2 c )). The cytotoxicity and cellular uptake of these complexes have been investigated by the MTT assay, ICPMS, laser‐scanning confocal microscopy, and flow cytometry. The results showed that the complexes supported by the water‐soluble PEG can act as biological probes and labels with considerably reduced cytotoxicity. Because the aldehyde groups of complex 3 are reactive toward primary amines, the complex has been utilized as the first luminescent PEGylation reagent. Bovine serum albumin (BSA) and poly(ethyleneimine) (PEI) have been PEGylated with this complex, and the resulting conjugates have been isolated, purified, and their photophysical properties studied. The DNA‐binding and gene‐delivery properties of the luminescent PEI conjugate 3 ‐PEI have also been investigated.     

New luminescent cyclometalated iridium(III) diimine complexes as biological labeling reagents

We report the synthesis, characterization, and photophysical and electrochemical properties of thirty luminescent cyclometalated iridium(III) diimine complexes [Ir(N-C)(2)(N-N)](PF(6)) (HN-C = 2-phenylpyridine, Hppy; 2-(4-methylphenyl)pyridine, Hmppy; 3-methyl-1-phenylpyrazole, Hmppz; 7,8-benzoquinoline, Hbzq; 2-phenylquinoline, Hpq; N-N = 4-amino-2,2'-bipyridine, bpy-NH(2); 4-isothiocyanato-2,2'-bipyridine, bpy-ITC; 4-iodoacetamido-2,2'-bipyridine, bpy-IAA; 5-amino-1,10-phenanthroline, phen-NH(2); 5-isothiocyanato-1,10-phenanthroline, phen-ITC; 5-iodoacetamido-1,10-phenanthroline, phen-IAA). The X-ray crystal structure of [Ir(mppz)(2)(bpy-NH(2))](PF(6)) has also been investigated. Upon irradiation, all the complexes display intense and long-lived luminescence under ambient conditions and in 77-K glass. On the basis of the photophysical and electrochemical data, the emission of most of these complexes is assigned to an excited state of predominantly triplet metal-to-ligand charge-transfer ((3)MLCT) (dpi(Ir) --> pi(N-N)) character. In some cases, triplet intraligand ((3)IL) (pi --> pi)(N-N or N-C(-)) excited states have also been identified. In view of the specific reactivity of the isothiocyanate and iodoacetamide moieties toward the primary amine and sulfhydryl groups, respectively, we have labeled various biological molecules with a selection of these luminescent iridium(III) complexes. The photophysical properties of the luminescent conjugates have been investigated. In addition, a heterogeneous assay for digoxin has also been designed on the basis of the recognition of biotinylated anti-digoxin by avidin labeled with one of the luminescent iridium(III) complexes.     

Luminescent Molecular Octopuses with a Polyhedral Oligomeric Silsesquioxane (POSS) Core and Iridium(III) Polypyridine Arms: Synthesis,Aggregation Induced Emission,Cellular Uptake,and Bioimaging Studies

A new class of luminescent molecular hybrids in which eight cyclometalated iridium(III) polypyridine complexes are grafted onto a polyhedral oligomeric silsesquioxane (POSS) unit [POSS-{Ir(N^C)₂(py-im)}₈](PF₆)₈ [py-im=pyridine imine; HN^C=N-phenylpyrazole (Hppz) ( 1 a ), 2-phenylpyridine (Hppy) ( 2 a ), 2-phenylquinoline (Hpq) ( 3 a )] were synthesized and characterized. On photoexcitation, the complexes showed intense and long-lived orange-red to red emission in fluid solutions at room temperature and in low-temperature glasses. The photophysical properties including aggregation-induced emission and biological properties of these complexes were studied and compared with those of their POSS-free counterparts [Ir(N^C)₂(py-im)](PF₆) [HN^C=Hppz ( 1 b ), Hppy ( 2 b ), Hpq ( 3 b )]. The (photo)cytotoxicity of the complexes was examined by the MTT assay, and their cellular uptake and intracellular localization were investigated by inductively coupled plasma-mass spectrometry and laser-scanning confocal microscopy.     

Synthesis, photophysical and electrochemical properties, and protein-binding studies of luminescent cyclometalated iridium(III) bipyridine estradiol conjugates

A new series of luminescent cyclometalated iridium(III) bipyridine estradiol conjugates [Ir(N-C)2(N-N)](PF6) (N-N = 5-(4-(17alpha-ethynylestradiolyl)phenyl)-2,2'-bipyridine, bpy-est, HN-C = 2-phenylpyridine, Hppy (1 a), 1-phenylpyrazole, Hppz (2 a), 7,8-benzoquinoline, Hbzq (3 a), 2-phenylquinoline, Hpq (4 a), 2-((1,1'-biphenyl)-4-yl)benzothiazole, Hbsb (5 a); N-N = 4-(N-(6-(4-(17alpha-ethynylestradiolyl)benzoylamino)hexyl)aminocarbonyl)-4'-methyl-2,2'-bipyridine, bpy-C6-est, HN-C = Hppy (1 b), Hppz (2 b), Hbzq (3 b), Hpq (4 b), Hbsb (5 b)) was synthesized, characterized, and their photophysical and electrochemical properties studied. Upon photoexcitation, all the complexes displayed intense and long-lived emission in fluid solutions at 298 K and in low-temperature glass. The emission of complexes 1 a-3 a and 1 b-3 b was assigned to a triplet metal-to-ligand charge-transfer ((3)MLCT) (dpi(Ir)-->pi*(bpy-est and N-C-)) state mixed with some triplet intraligand ((3)IL) (pi-->pi*) (N-C- and N-N) character. However, the emissive states of the pq- and bsb- complexes 4 a, 4 b, 5 a, and 5 b showed substantial (3)IL (pi-->pi*) (pq-/bsb-) character. The lipophilicity of all the complexes was determined by reversed-phase HPLC. Upon binding to estrogen receptor alpha, all of these iridium(III) estradiol conjugates exhibited emission enhancement and lifetime extension, rendering them a novel series of luminescent probes for this receptor.     

Cyclometalated Iridium(III)–Polyamine Complexes with Intense and Long‐Lived Multicolor Phosphorescence: Synthesis,Crystal Structure,Photophysical Behavior,Cellular Uptake,and Transfection Properties

A new class of phosphorescent cyclometalated iridium(III)–polyamine complexes [{Ir(N^C)₂}_n(bPEI)](PF₆)_n (bPEI=branched poly(ethyleneimine), average M_w=25 kDa, n=15.6–27.4; HN^C=2‐phenylpyridine Hppy ( 1 a ), 2‐((1,1′‐biphenyl)‐4‐yl)pyridine Hpppy ( 2 a ), 2‐phenylquinoline Hpq ( 3 a ), 2‐phenylbenzothiazole Hbt ( 4 a ), 2‐(1‐naphthyl)benzothiazole Hbsn ( 5 a )) and [Ir(N^C)₂(en)](PF₆) (en=ethylenediamine; HN^C=Hppy ( 1 b ), Hpppy ( 2 b ), Hpq ( 3 b ), Hbt ( 4 b ), Hbsn ( 5 b )) have been synthesized and characterized. The X‐ray crystal structure of complex 5 b was also determined. All of these complexes showed a reversible iridium(IV/III) oxidation couple at +1.01 to +1.26 V and a quasi‐reversible ligand‐based reduction couple at ?1.54 to ?2.08 V (versus SCE). Upon photoexcitation, the complexes displayed intense and long‐lived green to orange–red emission in fluid solutions at room temperature and in low‐temperature glass. Lipophilicity measurements indicated that bPEI played a dominant role in the polar nature of complexes 1 a – 5 a , thus rendering them very soluble in aqueous solutions. Inductively coupled plasma–mass spectrometry (ICP‐MS) and confocal laser scanning microscopy (CLSM) data indicated that an energy‐requiring process, such as endocytosis, was involved in the cellular uptake of all of the complexes. In addition, the cytotoxicity of the complexes toward human cervix epithelioid carcinoma (HeLa) and human embryonic kidney 293T (HEK293T) cell‐lines has been evaluated by the 3‐(4,5‐dimethyl‐2‐thiazolyl)‐2,5‐diphenyltetrazolium bromide (MTT) assay. The DNA‐binding properties of complex 5 a have been investigated by gel‐retardation assays and the polyplexes that were formed from this complex with plasmid DNA (pDNA) were studied by zeta‐potential measurements and particle‐size estimation. Furthermore, complex 5 a was grafted with poly(ethylene glycol) (PEG, average M_w=2 kDa) to different extents, thereby yielding the phosphorescent copolymers PEG_12.3‐g‐5 a , PEG_25.4‐g‐5 a , and PEG_62.1‐g‐5 a . Interestingly, these copolymers showed enhanced transfection activity, as revealed by in vitro transfection experiments with tissue‐culture‐based luciferase assays.     

Cyclometalated iridium(III) diimine bis(biotin) complexes as the first luminescent biotin-based cross-linkers for avidin

Four luminescent cyclometalated iridium(III) diimine complexes [Ir(N-C)2(N-N)](PF6) (HN-C = 2-(4-(N-((2-biotinamido)ethyl)aminomethyl)phenyl)pyridine, Hppy-4-CH2NHC2NH-biotin, N-N = 3,4,7,8-tetramethyl-1,10-phenanthroline, Me4-phen (1a); N-N = 4,7-diphenyl-1,10-phenanthroline, Ph2-phen (2a); HN-C = 2-(4-(N-((6-biotinamido)hexyl)aminomethyl)phenyl)pyridine, Hppy-4-CH2NHC6NH-biotin, N-N = Me4-phen (1b); N-N = Ph2-phen (2b)), each containing two biotin units, have been synthesized and characterized. The photophysical and electrochemical properties of these complexes have been investigated. Photoexcitation of these iridium(III) diimine bis(biotin) complexes in fluid solutions at 298 K and in alcohol glass at 77 K resulted in intense and long-lived luminescence. The emission is assigned to a triplet metal-to-ligand charge-transfer (3MLCT) (d pi(Ir) --> pi*(N-N)) excited state. The emissive states of complexes 1a,b are probably mixed with some 3IL (pi --> pi*) (Me4-phen) character. The interactions of these iridium(III) diimine bis(biotin) complexes with avidin have been studied by 4'-hydroxyazobenzene-2-carboxylic acid (HABA) assays and emission titrations. The potential for these complexes to act as cross-linkers for avidin has been examined by resonance-energy transfer- (RET-) based emission quenching experiments, microscopy studies using avidin-conjugated microspheres, and HPLC analysis.     

Effect of axially projected oligothiophene pendants and nitro-functionalized diimine ligands on the lowest excited state in cationic Ir(III) bis-cyclometalates

The novel terthiophene (3T) oligomer 6 and a series of cationic Ir(III) bis-cyclometalates [Ir(C^N)(2)(N^N)]PF(6) 9-12 were prepared. The synthesis, characterization, electrochemical, and photophysical properties are reported. The cyclometalating ligands (C^N) are 2-phenylpyridinato (ppy) or the 3T oligomer (3T-ppy), asymmetrically capped in the 5 and 5" positions with the ppy and mesityl groups. The diimine ligands (N^N) are 2,2'-bipyridine (bpy) or 4-NO(2)-bipyridine (4-NO(2)-bpy). Hybrid metal-organic complexes 11 and 12 bear 3T-pendants ligated through the ppy cap, 10 and 12 contain NO(2) functionalized diimines, whereas 9 contains neither. Structural characterization of 10 by single crystal X-ray diffraction confirms the presence of the NO(2) substituent and pseudo-octahedral coordination geometry about the Ir(III) ion. Cyclic voltammetry highlights the large electron withdrawing effect of the NO(2) substituent, providing an 850 mV shift toward lower potentials for the first diimine centered reduction of 10 and 12. Strong overlap of the intense π → π* absorptions of the 3T-pendants with Ir(III) charge transfer bands is evident in complexes of 11 and 12, precluding the possibility for selective excitation of either chromophore. Photoexcitation (λ(ex) = 400 nm) of the series affords strong luminescence from the 3T oligomer 6 and the unsubstituted 9, with φ(em) = 0.11. In stark contrast the NO(2) and 3T functionalized complexes 10-12 display near total quenching of luminescence. Computations of the ground and excited state electronic structure using density functional theory (DFT) and time-dependent DFT (TD-DFT) indicate that both the NO(2) and 3T substituents play an important role in excited state deactivation of complexes 10-12. A substantial electronic contribution of the NO(2) substituent results in stabilization of the diimine based molecular orbital (MO) and offers an efficient nonradiative decay pathway for the excited state. Spin-orbit coupling effects of the Ir(III) ion lead to efficient population of the low lying, nonluminescent, triplet states centered on the 3T-pendants.     

The influence of triplet energy levels of bridging ligands on energy transfer processes in Ir(III)/Eu(III) dyads

A series of N^N,O^O-bridging ligands based on substituted 1-(pyridin-2-yl)-3-methyl-5-pyrazolone and their corresponding heteroleptic iridium(III) complexes as well as Ir-Eu bimetallic complexes were synthesized and fully characterized. The influence of the triplet energy levels of the bridging ligands on the energy transfer (ET) process from the Ir(III) complexes to Eu(III) ions in solution was investigated at 77 K in Ir(III)/Eu(III) dyads. Photophysical experiment results show the bridging ligands play an important role in the ET process. Only when the triplet energy level of the bridging ligand was lower than the triplet metal-to-ligand charge transfer ((3)MLCT) energy level of the Ir moiety, was pure emission from the Eu(III) ion observed, implying complete ET took place from the Ir moiety to the Eu(III) ion.     

Synthesis,photophysical and electrochemical properties,and biological labeling studies of cyclometalated iridium(III) bis(pyridylbenzaldehyde) complexes: novel luminescent cross-linkers for biomolecules

We report the synthesis, characterization, photophysical, and electrochemical properties of a series of luminescent cyclometalated iridium(III) complexes containing two aldehyde functional groups [Ir(pba)(2)(N-N)](PF(6)) (Hpba=4-(2-pyridyl)benzaldehyde; N-N=2,2'-bipyridine, bpy (1), 1,10-phenanthroline, phen (2), 3,4,7,8-tetramethyl-1,10-phenanthroline, 3,4,7,8-Me(4)-phen (3), 4,7-diphenyl-1,10-phenanthroline, 4,7-Ph(2)-phen (4)). The X-ray crystal structure of complex 1 has been investigated. Upon photoexcitation, complexes 1-4 exhibit intense and long-lived emission in fluid solutions at 298 K and in low-temperature glass. The luminescence is assigned to a triplet intra-ligand ((3)IL) excited state associated with the pba(-) ligand, probably with mixing of some triplet metal-to-ligand charge-transfer ((3)MLCT) (dpi(Ir)-->pi*(pba(-))) character. Since each of these complexes possesses two aldehyde groups, which can react with the primary amine groups of biomolecules to form stable secondary amines after reductive amination, we have investigated the possibility of these complexes as novel luminescent cross-linkers for biological substrates. L-Alanine has been labeled with complexes 1-4 to give the luminescent bioconjugates 1-(Ala)(2)-4-(Ala)(2). These conjugates show strong photoluminescence with long emission lifetimes under ambient conditions. On the basis of the emission energy trend, the excited state of these luminescent bioconjugates is likely to bear a high parentage of (3)MLCT (dpi(Ir)-->pi*(N-N)) character. In addition, the glycoprotein avidin (Av) has also been conjugated with complexes 1-4 to give the bioconjugates 1-Av-4-Av. Upon photoexcitation, these bioconjugates also display intense and long-lived (3)MLCT (dpi(Ir)-->pi*(N-N)) emission in aqueous buffer at 298 K. Furthermore, a heterogeneous competitive assay for biotin has been developed using 2-Av and biotinylated microspheres. We have shown that complexes 1-4 represent a new class of multicolor luminescent cross-linkers for biomolecular species.     

A cyclometalated iridium(III) complex with enhanced phosphorescence emission in the solid state (EPESS): synthesis, characterization and its application in bioimaging

Iridium(III) complexes with intense phosphorescence in solution have been widely applied in organic light-emitting diodes, chemosensors and bioimaging. However, little attention has been paid to iridium(III) complexes showing weak phosphorescence in solution and enhanced phosphorescence emission in the solid state (EPESS). In the present study, two β-diketonate ligands with different degrees of conjugation, 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (HL1) and 1-phenyl-3-methyl-4-phenylacetyl-5-pyrazolone (HL2), have been synthesized to be used as ancillary ligands for two iridium(III) complexes, Ir(ppy)(2)(L1) and Ir(ppy)(2)(L2) (Hppy = 2-phenylpyridine). The two complexes have been characterized by single-crystal X-ray crystallography, (1)H NMR and elemental analysis. Interestingly, Ir(ppy)(2)(L1) is EPESS-active whereas Ir(ppy)(2)(L2) exhibits moderately intense emission both in solution and as a neat film, indicating that the degree of conjugation of the β-diketone ligands determines the EPESS-activity. The single-crystal X-ray analysis has indicated that there are π-π interactions between the adjacent ppy ligands in Ir(ppy)(2)(L1) but not in Ir(ppy)(2)(L2). Finally, EPESS-active Ir(ppy)(2)(L1) has been successfully embedded in polymer nanoparticles and used as a luminescent label in bioimaging.     

Control of intramolecular π-π stacking interaction in cationic iridium complexes via fluorination of pendant phenyl rings

Intramolecular π-π stacking interaction in one kind of phosphorescent cationic iridium complexes has been controlled through fluorination of the pendant phenyl rings on the ancillary ligands. Two blue-green-emitting cationic iridium complexes, [Ir(ppy)(2)(F2phpzpy)]PF(6) (2) and [Ir(ppy)(2)(F5phpzpy)]PF(6) (3), with the pendant phenyl rings on the ancillary ligands substituted with two and five fluorine atoms, respectively, have been synthesized and compared to the parent complex, [Ir(ppy)(2)(phpzpy)]PF(6) (1). Here Hppy is 2-phenylpyridine, F2phpzpy is 2-(1-(3,5-difluorophenyl)-1H-pyrazol-3-yl)pyridine, F5phpzpy is 2-(1-pentafluorophenyl-1H-pyrazol-3-yl)-pyridine, and phpzpy is 2-(1-phenyl-1H-pyrazol-3-yl)pyridine. Single crystal structures reveal that the pendant phenyl rings on the ancillary ligands stack to the phenyl rings of the ppy ligands, with dihedral angles of 21°, 18°, and 5.0° between least-squares planes for complexes 1, 2, and 3, respectively, and centroid-centroid distances of 3.75, 3.65, and 3.52 ? for complexes 1, 2, and 3, respectively, indicating progressively reinforced intramolecular π-π stacking interactions from complexes 1 to 2 and 3. Compared to complex 1, complex 3 with a significantly reinforced intramolecular face-to-face π-π stacking interaction exhibits a significantly enhanced (by 1 order of magnitude) photoluminescent efficiency in solution. Theoretical calculations reveal that in complex 3 it is unfavorable in energy for the pentafluorophenyl ring to swing by a large degree and the intramolecular π-π stacking interaction remains on the lowest triplet state.     

An iridium(III) complex of oximated 2,2'-bipyridine as a sensitive phosphorescent sensor for hypochlorite

The designed synthesis of a sensitive phosphorescent chemosensor [Ir(ppy)(2)(L1)](PF(6)) (1) (Hppy = 2-phenylpyridine, L1 = 4'-methyl-2,2'-bipyridyl-4-carbaldehyde oxime) was carried out for selective detection of hypochlorite (ClO(-)). Complex 1 is weakly emissive in solution at ambient temperature due likely to rapid isomerization of C=N-OH as an effective non-radiative decay process. When 1 reacts with ClO(-), however, the emission is remarkably enhanced, in which the oxime in L1 is converted to a carboxylic acid in L2 (4'-methyl-2,2'-bipyridine-4-carboxylic acid). The produced complex [Ir(ppy)(2)(L2)](PF(6)) (2) exhibits bright orange-yellow luminescence originating from [5d(Ir) → π*(bpy)] (3)MLCT and [π(ppy) → π*(bpy)] (3)LLCT triplet excited states as suggested from the DFT computational studies. The selective and competitive experiments reveal that complex 1 shows high sensing selectivity and sensitivity for ClO(-) over other reactive oxygen species (ROS) and metal ions.     

1,12-Diazaperylene and 2,11-dialkylated-1,12-diazaperylene iridium(iii) complexes [Ir(C^N)(2)(N^N)]PF(6): new supramolecular assemblies

A series of new monocationic iridium(iii) complexes [Ir(C^N)(2)(N^N)]PF(6) with "large-surface"α,α'-diimin ligands N^N (dap = 1,12-diazaperylene, dmedap = 2,11-dimethyl-1,12-diazaperylene, dipdap = 2,11-diisopropyl-1,12-diazaperylene) and different cyclometalating ligands C^N (piq = 1-phenylisoquinoline, bzq = benzo[h]quinoline, ppz = 1-phenylpyrazole, thpy = 2-(2-thienyl)pyridine, ppy = 2-phenylpyridine, meppy = 2-(4-methylphenyl)pyridine, dfppy = 2-(2,4-difluorophenyl)pyridine) were synthesized. The solid structures of the complexes [Ir(piq)(2)(dap)]PF(6), [Ir(bzq)(2)(dap)]PF(6), [Ir(ppy)(2)(dipdap)]PF(6), [Ir(piq)(2)(dmedap)]PF(6), [Ir(ppy)(2)(dap)]PF(6) and [Ir(ppz)(2)(dap)]PF(6) are reported. In [Ir(piq)(2)(dap)]PF(6), the dap ligand and one of the piq ligands of each cationic complex are involved in π-π stacking interactions forming supramolecular channels running along the crystallographic c axis. In the crystalline [Ir(bzq)(2)(dap)]PF(6)π-π stacking interactions between the metal complexes lead to the formation of a 2D layer structure. In addition, CH-π interactions were found in all compounds, which are what stabilizes the solid structure. In particular, a significant number of them were found in [Ir(piq)(2)(dap)]PF(6) and [Ir(bzq)(2)(dap)]PF(6). The crystal structures of [Ir(ppy)(2)(dipdap)]PF(6) and [Ir(ppy)(2)(dmedap)]PF(6) are also presented, being the first examples of bis-cyclometalated iridium(iii) complexes with phenanthroline-type α,α'-diimin ligands bearing bulky alkyl groups in the neighbourhood of the N-donor atoms. These ligands implicate a distorted octahedral coordination geometry that in turn destabilized the Ir-N(N^N) bonds. The new iridium(iii) complexes are not luminescent. All compounds show an electrochemically irreversible anodic peak between 1.15 and 1.58 V, which is influenced by the different cyclometalated ligands. All of the new complexes show two reversible successive one-electron "large-surface" ligand-centred reductions around -0.70 V and -1.30 V. Electrospray ionisation mass spectrometry (ESI-MS) and collision induced decomposition (CID) measurements were used to investigate the stability of the new complexes. Thereby, the stability agreed well with the order of the Ir-N(N^N) bond lengths.     

Luminescent cyclometalated iridium(III) dipyridoquinoxaline indole complexes as biological probes

Four luminescent cyclometalated iridium(III) dipyridoquinoxaline complexes appended with an indole moiety [Ir(N∧C)₂(N∧N)] (PF₆) (HN∧C = 2-phenylpyridine, Hppy; N∧N = 2-(N-(2-(indole-3-acetamido)ethyl)aminocarbonyl)dipyrido[3,2-f:2′,3′-h]quinoxaline, dpqC2indole (1a), N∧N = 2-(N-(6-(indole-3-acetamido)hexyl)aminocarbonyl)dipyrido[3,2-f:2′,3′-h]quinoxaline, dpqC6indole (1b); HN∧C = 7,8-benzoquinoline, Hbzq, N∧N = dpqC2indole (2a), N∧N = dpqC6indole (2b)) have been synthesized and characterized. Upon irradiation, all the complexes displayed moderately intense and long-lived luminescence under ambient conditions and in 77 K glass. On the basis of the photophysical data, the emission of the complexes has been assigned to an excited state of triplet metal-to-ligand charge-transfer (³MLCT) ((dπ(Ir) → π*(N∧N)) character. Cyclic voltammetric studies revealed indole-based and iridium-based oxidations at ca. +1.10 V and +1.24 V vs. SCE, respectively, and ligand-based reductions at ca. ?1.07 to ?2.29 V vs. SCE. The interactions of the complexes with an indole-binding protein, bovine serum albumin (BSA), have been examined by emission titrations.     

Design and synthesis of iridium bis(carbene) complexes for efficient blue electrophosphorescence

Five iridium bis(carbene) complexes, [Ir(pmi)(2)(pypz)] (1), [Ir(mpmi)(2)(pypz)] (2), [Ir(fpmi)(2)(pypz)] (3), [Ir(fpmi)(2)(pyim)] (4), and [Ir(fpmi)(2)(tfpypz)] (5) (pmi=1-phenyl-3-methylimdazolin-2-ylidene-C,C(2'); fpmi=1-(4-fluorophenyl)-3-methylimdazolin-2-ylidene-C,C(2'); mpmi=1-(4-methyl-phenyl)-3-methylimdazolin-2-ylidene-C,C(2'); pypz=2-(1H-pyrazol-5-yl)pyridinato; pyim=2-(1H-imidazol-2-yl)pyridinato; and tfpypz=2-(3-(trifluoromethyl)-1H-pyrazol-5-yl)pyridinato), were synthesized and their structures were characterized by NMR spectroscopy, mass spectroscopy and X-ray diffraction. These complexes showed phosphorescent emission with the emission maxima between 453 and 490 nm. Various spectrophotometric measurements, cyclic voltammetric studies, and density functional theory (DFT) calculations show that, unlike most of the phosphorescent cyclometalated iridium complexes, the lowest unoccupied molecular orbital (LUMO) energy and the emissive state of these iridium complexes are mainly controlled by the N,N'-heteroaromatic (N^N) ligand. Despite the fact that the LUMO levels of these complexes are mainly on the N^N ligands, the efficiencies of the electroluminescent (EL) devices are very high. For example, the EL devices using [Ir(mpmi)(2)(pypz)], [Ir(fpmi)(2)(pypz)], and [Ir(fpmi)(2)(tfpypz)] as the dopant emitters exhibited light- to deep-blue electrophosphorescence with external quantum efficiencies of 15.2, 14.1, and 7.6% and Commission Internationale d'énclairage (x,y) coordinates (CIE(x,y)) of (0.14, 0.27), (0.14, 0.18) and (0.14, 0.10), respectively.     

Synthesis, characterisation and application of iridium(III) photosensitisers for catalytic water reduction

The synthesis of novel, monocationic iridium(III) photosensitisers (Ir-PSs) with the general formula [Ir(III)(C^N)(2)(N^N)](+) (C^N: cyclometallating phenylpyridine ligand, N^N: neutral bidentate ligand) is described. The structures obtained were examined by cyclic voltammetry, UV/Vis and photoluminescence spectroscopy and X-ray analysis. All iridium complexes were tested for their ability as photosensitisers to promote homogeneously catalysed hydrogen generation from water. In the presence of [HNEt(3)][HFe(3)(CO)(11)] as a water-reduction catalyst (WRC) and triethylamine as a sacrificial reductant (SR), seven of the new iridium complexes showed activity. [Ir(6-iPr-bpy)(ppy)(2)]PF(6) (bpy: 2,2'-bipyridine, ppy: 2-phenylpyridine) turned out to be the most efficient photosensitiser. This complex was also tested in combination with other WRCs based on rhodium, platinum, cobalt and manganese. In all cases, significant hydrogen evolution took place. Maximum turnover numbers of 4550 for this Ir-PS and 2770 for the Fe WRC generated in situ from [HNEt(3)][HFe(3)(CO)(11)] and tris[3,5-bis(trifluoromethyl)phenyl]phosphine was obtained. These are the highest overall efficiencies for any Ir/Fe water-reduction system reported to date. The incident photon to hydrogen yield reaches 16.4% with the best system.     

Emissive iridium(III) diimine complexes formed by double cyclometalation of coordinated triphenylphosphite

We report on the synthesis of a new series of iridium(III) complexes functionalized with various diimine chromophores, together with a facially coordinated dicyclometalated phosphite chelate and a monodentate anionic ancillary. This conceptual design presents a novel strategy in obtaining a new class of iridium(III) diimine complexes without employment of traditional nitrogen-containing polyaromatic cyclometalates. Additionally, we discuss the basic charactersistics of the ground and lower-lying excited states involved, as documented by crystal structural, photophysical studies, and density functional theory calculations. Fabrication of the green-emitting organic light-emitting diodes with one such dopant, [Ir(dbbpy)(tpit)NCS] (2b), where dbbpy and tpit represent di-tert-butyl-2,2'-bipyridine and dicyclometalated triphenylphosphite, respectively, was successfully made, attaining a peak external quantum efficiency (η(ext)), a luminance efficiency (η(l)), and a power efficiency (η(p)) of 14.1%, 46.6 cd A(-1), and 39.9 lm W(-1), respectively.     

Light-emitting electrochemical cells based on a supramolecularly-caged phenanthroline-based iridium complex

The complex [Ir(ppy)(2)(pphen)][PF(6)] (Hppy = 2-phenylpyridine, pphen = 2-phenyl-1,10-phenanthroline) has been prepared and evaluated as an electroluminescent component for light-emitting electrochemical cells (LECs). Like in analogous LECs using bpy-based iridium(III) complexes a significant enhancement of the device stability is observed.     

Olefin Isomerization by Iridium Pincer Catalysts. Experimental Evidence for an η(3)-Allyl Pathway and an Unconventional Mechanism Predicted by DFT Calculations

The isomerization of olefins by complexes of the pincer-ligated iridium species ((tBu)PCP)Ir ((tBu)PCP = κ(3)-C(6)H(3)-2,6-(CH(2)P(t)Bu(2))(2)) and ((tBu)POCOP)Ir ((tBu)POCOP = κ(3)-C(6)H(3)-2,6-(OP(t)Bu(2))(2)) has been investigated by computational and experimental methods. The corresponding dihydrides, (pincer)IrH(2), are known to hydrogenate olefins via initial Ir-H addition across the double bond. Such an addition is also the initial step in the mechanism most widely proposed for olefin isomerization (the "hydride addition pathway"); however, the results of kinetics experiments and DFT calculations (using both M06 and PBE functionals) indicate that this is not the operative pathway for isomerization in this case. Instead, (pincer)Ir(η(2)-olefin) species undergo isomerization via the formation of (pincer)Ir(η(3)-allyl)(H) intermediates; one example of such a species, ((tBu)POCOP)Ir(η(3)-propenyl)(H), was independently generated, spectroscopically characterized, and observed to convert to ((tBu)POCOP)Ir(η(2)-propene). Surprisingly, the DFT calculations indicate that the conversion of the η(2)-olefin complex to the η(3)-allyl hydride takes place via initial dissociation of the Ir-olefin π-bond to give a σ-complex of the allylic C-H bond; this intermediate then undergoes C-H bond oxidative cleavage to give an iridium η(1)-allyl hydride which "closes" to give the η(3)-allyl hydride. Subsequently, the η(3)-allyl group "opens" in the opposite sense to give a new η(1)-allyl (thus completing what is formally a 1,3 shift of Ir), which undergoes C-H elimination and π-coordination to give a coordinated olefin that has undergone double-bond migration.     

An iridium-pyridylpyrrolide complex exhibiting reversible binding of H2

The synthesis and characterization of a series of iridium(I) and iridium(III) complexes supported by a 3,5-bis(trifluoromethyl)-2-(2'-pyridyl)pyrrole (L(-)) ligand are reported. Compound [Ir(L)(COD)] (COD = 1,5-cyclooctadiene) demonstrates reversible binding of hydrogen to form a dihydride complex [Ir(L)(H)(2)(COD)] with no hydrogenation of COD.