A pyrimidine-like nickel(II) DNA base pair

4-(2'-Pyridyl)-pyrimidinone deoxyriboside is synthesized and characterized as a DNA metallo base-pair; this novel nucleoside forms a self-pair in the presence of Ni(II) and stabilizes double helical DNA to the same extent as a G.C pair.     

Luminescent cyclometalated gold(III) complexes

Many luminescent gold(I) compounds are known, but in the vast majority of gold(III) complexes reported until recently, room temperature emission in fluid solution does not occur. As for other d(8) and d(6) metals, the key to obtaining gold(III) compounds with favorable luminescence properties seems to be the use of cyclometalating ligands that ensure very strong ligand fields. Recent progress in this emerging research field is discussed, and where appropriate, comparison to isoelectronic platinum(II) complexes and their photophysical properties is made.     

A novel silver(i)-mediated DNA base pair

We have generated a novel silver(I)-mediated unnatural DNA base pair consisting of two 2,6-bis(ethylthiomethyl)pyridine nucleobases SPy. This metallo-base pair has a remarkably high pairing stability and selectivity which rivals that of the natural base pairs dA:dT and dC:dG. UV-melting experiments revealed that the dSPy:dSPy self-pair can replace natural base pairs at multiple sites and still form stable DNA duplexes.     

Electrochemiluminescence of cyclometalated iridium (III) complexes

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C-H activation of 2,4,6-triphenylphosphinine: unprecedented formation of cyclometalated [(P^C)Ir(III)] and [(P^C)Rh(III)] complexes

An unprecedented C-H activation of 2,4,6-triphenylphosphinine by Ir(III) and Rh(III) has been observed. Time-dependent (31)P{(1)H} NMR spectroscopy gave insight into the cyclometalation reaction and the corresponding coordination compounds were characterized by means of X-ray crystallography. In contrast, 2,4,6-triphenylpyridine does not show any ortho-metalation, demonstrating a remarkable difference in reactivity between these two structurally related aromatic heterocycles.     

Long-range electron and hole transport through DNA with tethered cyclometalated iridium(III) complexes

A cyclometalated complex of Ir(III) is covalently tethered to DNA oligonucleotides and serves as both a photooxidant and photoreductant in the study of DNA-mediated hole transport (HT) and electron transport (ET). Spectroscopic and melting temperature studies support intercalation of the tethered complex into the DNA duplex through the functionalized dppz ligand. Using these tethered assemblies, ET and HT is initiated in DNA by the same photoredox probe. Cyclopropylamine substituted bases, N4-cyclopropylcytosine (CPC) and N2-cyclopropylguanine (CPG) are used as kinetically fast electron and hole traps to probe the resulting electron migration processes after direct irradiation of the tethered Ir assembly. Oxidation of CPG and CPC is promoted efficiently by HT from photoexcited Ir(III) when the modified bases are positioned in the purine strands of the A-tract. In contrast, when CPC is embedded in a pyrimidine tract, ET to yield reductive decomposition is observed. Thus, the Ir(III)-tethered DNA assembly containing cyclopropyl-modified bases provides a unique model system to explore the two DNA-mediated electron migration processes using the same photoredox probe and the same DNA bridge.     

Synthesis of facial cyclometalated iridium(III) complexes triggered by tripodal ligands

The tripodal ligands composed of the 1,3,5-trisubstituted cyclohexyl moiety as a molecular scaffold and 2-phenylpyridyl moieties as a coordination site were designed. The homoleptic cyclometalated fac-Ir(C^N)(3) complexes could be obtained by the reaction of IrCl(3)·nH(2)O with the designed tripodal ligands. The single crystal X-ray structure determination confirmed the fac configuration and a distorted octahedral geometry with three intramolecular cyclometalated 2-phenylpyridyl ligands surrounding the iridium metal center. Also, the cyclohexyl scaffold was found to serve as a flexible scaffold to induce the fac configuration. The thus-obtained homoleptic cyclometalated fac-Ir(C^N)(3) complexes exhibited a broad emission band in the emission spectra at 298 K.     

Structure of a copper-mediated base pair in DNA.

Stable and selective DNA base pairing by metal coordination was recently demonstrated with nucleotides containing complementary pyridine-2,6-dicarboxylate (Dipic) and pyridine (Py) bases (Meggers, E.; Holland, P. L.; Tolman; W. B.; Romesberg, F. E.; Schultz, P. G. J. Am. Chem. Soc. 2000, 122, 10714-10715). To understand the structural consequences of introducing this novel base pair into DNA we have solved the crystal structure of a duplex containing the metallo-base pair. The structure shows that the bases pair as designed, but in a Z-DNA conformation. The structure also provides a structural explanation for the B- to Z-DNA transition in this duplex. Further solution studies demonstrate that the metallo-base pair is compatible with Z- or B-DNA conformations, depending on the duplex sequence.     

Efficient electrogenerated chemiluminescence from cyclometalated iridium(III) complexes

Very efficient electrogenerated chemiluminescence (ECL) phenomena were realized by deliberately tuning electron-transfer reactions from electrochemically generated electron donor to metal complex radical cations. By controlling the relative positions of HOMO and LUMO levels (oxidation potential and reduction potential) of Ir(III) complexes, we could obtain 77 times higher ECL from iridium(III) complexes in the presence of TPA than that of the Ru(bpy)32+/TPA system. This high ECL efficiency of new Ir(III) complexes can be used in many interesting applications such as sensors and luminescent devices.     

Phosphorescent iridium(III) complexes with nonconjugated cyclometalated ligands

A series of blue phosphorescent iridium(III) complexes 1-4 with nonconjugated N-benzylpyrazole ligands were synthesized and their structural, electrochemical, and photophysical properties were investigated. Complexes 1-4 exhibit phosphorescence with yields of 5-45 % in degassed CH2Cl2. Of the compounds, 1 showed emission that was nearly true blue at 460 nm with a lack of vibronic progression. These photophysical data clearly demonstrate that the methylene spacer of the cyclometalated N-benzylpyrazole chelate effectively interrupts the pi conjugation upon reacting with a third L X chelating chromophore. This gives a feasible synthesis for the blue phosphorescent complexes with a sufficiently large energy gap. In another approach, these complexes were investigated for their suitability for the host material in phosphorescent OLEDs. The device was synthesized by using 1 as the host for the green-emitting [Ir(ppy)3] dopant, which exhibits an external quantum conversion efficiency (EQE) of up to 11.4 % photons per electron (and 36.6 cdA(-1)), with 1931 Commission Internationale de L'Eclairage (CIE) coordinates of (0.30, 0.59), a peak power efficiency of 21.7 lmW(-1), and a maximum brightness of 32000 cdm(-2) at 14.5 V. At the practical brightness of 100 cdm(-2), the efficiency remains above 11 % and 18 lmW(-1), demonstrating its great potential as the host material for phosphorescent organic light-emitting diodes.     

AT base pair anions versus (9-methyl-A)(1-methyl-T) base pair anions

The anionic base pairs of adenine and thymine, (AT)(-), and 9-methyladenine and 1-methylthymine, (MAMT)(-), have been investigated both theoretically and experimentally in a complementary, synergistic study. Calculations on (AT)(-) found that it had undergone a barrier-free proton transfer (BFPT) similar to that seen in other dimer anion systems and that its structural configuration was neither Watson-Crick (WC) nor Hoogsteen (HS). The vertical detachment energy (VDE) of (AT)(-) was determined by anion photoelectron spectroscopy and found to be in agreement with the VDE value predicted by theory for the BFPT mechanism. An AT pair in DNA is structurally immobilized into the WC configuration, in part, by being bonded to the sugars of the double helix. This circumstance was mimicked by methylating the sites on both A and T where these sugars would have been tied, viz., 9-methyladenine and 1-methylthymine. Calculations found no BFPT in (MAMT)(-) and a resulting (MAMT)(-) configuration that was either HS or WC, with the configurations differing in stability by ca. 2 kcal/mol. The photoelectron spectrum of (MAMT)(-) occurred at a completely different electron binding energy than had (AT)(-). Moreover, the VDE value of (MAMT)(-) was in agreement with that predicted by theory. The configuration of (MAMT)(-) and its lack of electron-induced proton transfer are inter-related. While there may be other pathways for electron-induced DNA alterations, BFPT in the WC/HS configurations of (AT)(-) is not feasible.     

Iridium(III) and rhodium(III) cyclometalated complexes containing sulfur and selenium donor ligands

Treatment of [M(Buppy)₂Cl]₂ (M=Ir (1), Rh (2); BuppyH=2-(4^′-tert-butylphenyl)pyridine) with Na(Et₂NCS₂), K[S₂P(OMe)₂], and K[N(Ph₂PS)₂]₂ afforded monomeric [Ir(Buppy)₂(S^∧S)] (S^∧S=Et₂NCS₂ (3), S₂P(OMe)₂ (4), N(PPh₂S)₂ (5)) and [Rh(Buppy)₂(S^∧S)] (S^∧S=Et₂NCS₂ (6), S₂P(OMe)₂ (7), N(PPh₂S)₂ (8)), respectively. Reaction of 1 with Na[N(PPh₂Se)₂] gave [Ir(Buppy)₂{N(PPh₂Se)₂}] (9). The crystal structures of 3, 4, 7, and 8 have been determined. Treatment of 1 or 2 with AgOTf (OTf=triflate) followed by reaction with KSCN gave dinuclear [{M(Buppy)₂}₂(μ-SCN)₂] (M=Ir (10), Rh (11)), in which the SCN⁻ ligands bind to the two metal centers in a μ-S,N fashion. Interaction of 1 and 2 with [Et₄N]₂[WQ₄] gave trinuclear heterometallic complexes [{Ir(Buppy)₂}₂(μ-WQ₄)] (Q=S (12), Se (13)) and [{Rh(Buppy)₂}₂{(μ-WQ)₄}] (Q=S (14), Se (15)), respectively. Hydrolysis of 12 led to formation of [{Ir(Buppy)₂}₂{W(O)(μ-S)₂(μ₃-S)}] (16) that has been characterized by X-ray diffraction.     

Simultaneous determination of Ru(III) and Rh(III) using octadecyldithiocarbamate by first-derivative spectrophotometry

A first-derivative spectrophotometric method is described for the simultaneous determination of Ru(III) and Rh(III) using octadecyl dithiocarbamate. The complexes are insoluble in water, but easily extractable into chloroform. Quantitative determination of Ru(III) and Rh(III) is possible in the ranges 0.5-6.0 mug ml(-1) and 1.0-10.0 mug ml(-1) respectively, with a standard deviation of +/-0.10. A statistical evaluation of the experimental results is reported.     

Efficient red-emitting cyclometalated Iridium(III) complexes containing lepidine-based ligands

Heteroleptic cyclometalated iridium(III) complexes featuring lepidine-based ligands and acetyl acetone auxiliary ligand are synthesized. Multiple lowest energy absorption bands are observed for these complexes indicating substantial mixing of the singlet and triplet levels. All the complexes emit orange or red color in dichloromethane solutions with lifetimes in the range 1.6-3.7 micros. The emission in the complexes probably originates from the (3)MLCT state. The complexes are applied as emitting guests in LED devices of the structure ITO/HTL(BPAPF or NPB)/6% Ir in CBP/BCP/Alq(3)/LiF/Al. They exhibit excellent device characteristics with an orange to red EL profile.     

Blue phosphorescence from mixed cyano-isocyanide cyclometalated iridium(III) complexes

The synthesis, structure, and photophysical and electrochemical properties of cyclometalated iridium complexes with ancillary cyano and isocyanide ligands are described. In the first synthetic step, cleavage of dichloro-bridged dimers [Ir(N=C)2(mu-Cl)]2 (N=C = 2-phenylpyridine, 2-(2-fluorophenyl)pyridine, and 2-(2,4-difluorophenyl)pyridine) by isocyanide ligands gave monomeric species of the types Ir(N=C)2(RNC)(Cl) (RNC = t-butyl isocyanide, 1,1,3,3-tetramethylbutyl isocyanide, 2-morpholinoethyl isocyanide, and 2,6-dimethylphenyl isocyanide). In turn, the chloride was replaced by cyanide giving Ir(N=C)2(RNC)(CN). The X-ray structures for two of the complexes show that the trans-pyridyl/cis-phenyl geometry of the parent dimer is preserved, with the ancillary ligands positioned trans to the cyclometalated phenyls. The cyano complexes all display strong blue photoluminescence in ambient, deoxygenated solutions with the first lambdamax ranging from 441 to 458 nm, quantum yields spanning 0.60 to 0.75, and luminescent lifetimes of 12.0-21.4 mus. A lack of solvatochromism and highly structured emission indicate that the lowest energy excited state is triplet ligand centered with some admixture of singlet metal-to-ligand charge-transfer character.