Synthesis of dual-emissive organometallic complexes containing heterogeneous metal elements

We present the dual-emissive dye containing heterogeneous organometallic units. The diad ligand composed of two diketonate moieties linked by fluorene was synthesized. By introducing boron and platinum elements, the emissive molecules were obtained. From optical measurements, it was confirmed that the boron and the platinum-containing diads showed the emissions from the charge transfer and the triplet π–π^∗ ligand centered transitions, respectively. Next, to observe these fluorescence and phosphorescence properties from the dye molecule, the diad composed of boron and platinum diketonates linked by the fluorene unit was prepared. Finally, the dual emission containing fluorescence from the boron complex and phosphorescence from the platinum complex was simultaneously observed.     

Tuning the Photoisomerization of a N^C‐Chelate Organoboron Compound with a Metal?Acetylide Unit

To examine the impact of metal moieties that have different triplet energies on the photoisomerization of B(ppy)Mes₂ compounds (ppy=2‐phenyl pyridine, Mes=mesityl), three metal‐functionalized B(ppy)Mes₂ compounds, Re‐B , Au‐B , and Pt‐B , have been synthesized and fully characterized. The metal moieties in these three compounds are Re(CO)₃(tert‐Bu₂bpy)(C?C), Au(PPh₃)(C?C), and trans‐Pt(PPh₃)₂(C?C)₂, respectively, which are connected to the ppy chelate through the alkyne linker. Our investigation has established that the Re^I unit completely quenches the photoisomerization of the boron unit because of a low‐lying intraligand charge transfer/MLCT triplet state. The Au^I unit, albeit with a triplet energy that is much higher than that of B(ppy)Mes₂, upon conjugation with the ppy chelate unit, substantially increases the contribution of the π→π* transition, localized on the conjugated chelate backbone in the lowest triplet state, thereby leading to a decrease in the photoisomerization quantum efficiency (QE) of the boron chromophore when excited at 365 nm. At higher excitation energies, the photoisomerization QE of Au‐B is comparable to that of the silyl–alkyne‐functionalized B(ppy)Mes₂ ( TIPS‐B ), which was attributable to a triplet‐state‐sensitization effect by the Au^I unit. The Pt^II unit links two B(ppy)Mes₂ together in Pt‐B , thereby extending the π‐conjugation through both chelate backbones and leading to a very low QE of the photoisomerization. In addition, only one boron unit in Pt‐B undergoes photoisomerization. The isomerization of the second boron unit is quenched by an intramolecular energy transfer of the excitation energy to the low‐energy absorption band of the isomerized boron unit. TD‐DFT computations and spectroscopic studies of the three metal‐containing boron compounds confirm that the photoisomerization of the B(ppy)Mes₂ chromophore proceeds through a triplet photoactive state and that metal units with suitable triplet energies can be used to tune this system.     

Metal binding to bipyridine-modified PNA

Substitution of natural nucleobases in PNA oligomers with ligands is a strategy for directing metal ion incorporation to specific locations within a PNA duplex. In this study, we have synthesized PNA oligomers that contain up to three adjacent bipyridine ligands and examined the interaction with Ni2+ and Cu2+ of these oligomers and of duplexes formed from them. Variable-temperature UV spectroscopy showed that duplexes containing one terminal pair of bipyridine ligands are more stable upon metal binding than their nonmodified counterparts. While binding of one metal ion to duplexes that contain two adjacent bipyridine pairs makes the duplexes more stable, additional metal ions lower the duplex stability, with electrostatic repulsions being, most likely, an important contributor to the destabilization. UV titrations showed that the presence of several bipyridine ligands in close proximity of each other in PNA oligomers exerts a chelate effect. A supramolecular chelate effect occurs when several bipyridines are brought next to each other by hybridization of PNA duplexes. EPR spectroscopy studies indicate that even when two Cu2+ ions coordinate to a PNA duplex in which two bipyridine pairs are next to each other, the two metal-ligand complexes that form in the duplex are far enough from each other that the dipolar coupling is very weak. EXAFS and XANES show that the Ni2+-bipyridine bond lengths are typical for [Ni(bipy)2]2+ and [Ni(bipy)3]2+ complexes.     

Base-base interactions in the minor groove of double-stranded DNA

A method has been developed for the synthesis of bisheaded nucleosides with thymine and adenine base moieties. We have demonstrated that, when incorporated in oligonucleotides, extrahelical A-T base interactions are possible when the bisheaded nucleosides are positioned in opposite strands of the duplex and are separated from each other by one regular base pair.     

Palladium cluster catalysts supported on chelate resin-metal complexes

The dry beads of chelate resin-metal complexes have been prepared from metal ions and the chelate resin containing iminodiacetic acid moieties. The surface area of the chelate resin can be increased both by washing with an organic solvent miscible with water and by complexing with multi-valent cations. Palladium clusters are supported on the chelate resin-metal complexes by two methods, in which the order is reversed between “complexing of metal ions” and “supporting of palladium clusters”. The supported palladium clusters catalyze the hydrogenation of C=C bonds, and the catalytic activity greatly depends on the metal ions used for the complexation. In the case of typical metal ions, the complexing of metal ions after supporting of palladium clusters makes the surface area of the resin increase, but makes the catalytic activity decrease compared with the reverse order. In the case of lanthanoid ions, on the other hand, the same order makes both the surface area and the catalytic activity increase.     

Oligonucleotide based artificial nuclease (OBAN) systems. Bulge size dependence and positioning of catalytic group in cleavage of RNA-bulges

Three zinc ion dependent oligonucleotide based artificial nucleases (OBANs) have been synthesized. These consist of 2'-O-methyloligoribonucleosides connected to 5-amino-2,9-dimethylphenanthroline via a urea function to a linker extending either from C-5 of deoxyuridine or from the 2'-position of uridine moieties. Both types of linkers are placed centrally in the modified sequence and in addition one OBAN carries the C-5 modified dU as an additional nucleoside unit at the 5'-end. All three OBANs are shown to cleave target oligoribonucleotides selectively. The target RNA's are varied to form differently sized bulges (0-5 nucleotides (nt)) and the different OBANs have different preferences for which sizes are preferentially cleaved. The OBAN with the centrally positioned C-5 linked zinc chelate preferentially cleaves 3 and 4-nt bulges, the OBAN with the 2'-linked chelate has a preference for slightly smaller bulges and the OBAN with a 5'-end chelate is more efficient the larger the bulge is. In addition the OBAN with the centrally positioned C-5 linked zinc chelate is shown to be a real enzyme, capable of turnover of substrate and displaying Michaelis-Menten behaviour. The main differences in efficiency of cleavage between the different OBAN-RNA substrate combinations are likely to be due to proximity factors i.e. the positioning of a catalytic group relative to cleaved phosphodiester functions. The model systems investigated partially display the importance of catalytic group positioning and should be useful in future development of more efficient OBANs.     

Kinetics of photochemical reactions of a new biphotochromic compound upon photolysis with light of different wavelengths

Using the method of microsecond flash photolysis with UV and visible light, the spectral and kinetic characteristics of intermediate products of photolysis in toluene and methanol solutions of a new biphotochromic compound have been studied, in which two photochromic moieties, spironaphthoxazine and azomethine, are linked to each other in such a way that there is conjugation between π-electronic systems of the moieties in the ground state of the molecule. Two intermediate products have detected, whose relative efficiency of formation substantially depends on the solvent and the wavelength of excitation light.     

The effect of fluorination: a crystallographic and computational study of mesogenic alkyl 4-[2-(perfluorooctyl)ethoxy]benzoates

The series of alkyl 4-[2-(perfluorooctyl)ethoxy]benzoates (F8-n) shows a systematic change of crystal structures depending on the length of the alkyl chain: separate packing of perfluorooctyl (Rf) and alkyl (Rh) chains from each other for shorter (n=2) and longer (n=11) members, alternate packing of Rf and Rh chains for middle (n=6,7) members, and an intermediate type of packing for n=4. Semiempirical MO calculations show slightly repulsive interactions between the Rf chains, and attractive ones between Rf and Rh chains and between Rh and the core of a molecular pair. It is concluded that fluorination determines the molecular shape of the crystal structures by making the chain rigid. It is confirmed that the interactions between Rf chains are small compared with those between other moieties and that they are forced to aggregate owing to the exclusion from other moieties. Thus, the effect is dependent on the geometries and intermolecular interactions of the other moieties.     

Molecular emission cavity analysis (meca)—a new flame analytical technique: Part III. The determination of boron

Two highly selective methods are described for the determination of boron by molecular emission cavity analysis. In one, boron (>0.1 p.p.m.) is extracted as its 2-ethylhexane-1,3-diol chelate into methyl isobutylketone; 5-80 ng of boron in 5 μl of the extract is determined by injection into the MECA cavity. In the other, boron (2-30 μl) is converted into volatile methyl borate, and carried into the rear of the cavity by a stream of nitrogen. In both methods, the emission intensity at 518 nm, stimulated by a hydrogen-nitrogen-oxygen flame, is measured. Of the other ions tested, none interfered in the volatilization procedure, and only fluoride ( ? 100 mg) interfered in the extraction method. The precision is ±4%.     

Mechanisms of inversion of bond configuration at the tetrahedral boron atom in five-membered chelate cycles

Mechanisms of inversion of the bond configuration at the tetrahedral boron center in five-membered chelate cycles of the 1,3,2-oxazaborolidine and 1,3,2-oxazaborolidene molecules were studied by theab initio MP2(full)/6-31G^** method. It was shown that enantiotopomerization occurs by a dissociative mechanism with the cleavage of the B←N bond and the formation of acyclic intermediates with tricoordinate planar boron atom. The calculated energy barriers to inversion of tetrahedral bond configurations at boron centers in the two chelate complexes are equal to 13.1 and 15.4 kcal mol⁻¹, respectively. In contrast to 1,3,2-oxazaborolidine, internal rotation about the B−O bond in its unsaturated analog makes an appreciable contribution to the reaction coordinate. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 250–255 February, 1999.     

Boron complexes of pyrrolyl ligands

Complexes of boron with ligands containing pyrrolyl motifs are surveyed. The ligands range from simple pyrrolyl groups to dipyrroles and linear terpyrroles. Macrocyclic ligands include tripyrroles, which encompass subphthalocyanines, subporphyrins, subtriazaporphyrins, and subtribenzoporphyins, the familiar tetrapyrroles porphyrin and corrole but also N-confused and -fused porphyrins, and expanded porphyrins containing up to eight pyrroles. The role of boron in these compounds depends on the nature of the ligand. Boron acts as a Lewis acid center in simple boron pyrrolyl compounds, and as a structure-directing and templating agent in the cyclic terpyrroles and some of the expanded porphyrins. The difluorboron dipyrrins are well-known as fluorescent dyes. Boron porphyrins and corroles are unusual in containing two coordinated boron atoms rather than the single coordinated atom usually occurring in these ligands, and the proximity of two boron atoms at close quarters in the ligand cavities gives rise to some unusual reaction and redox chemistry. The survey is organized by the number of pyrrole moieties occurring in the ligand and focuses on new and unique chemistry observed for the complexes.     

Synthesis of crowded triarylphosphines carrying functional sites

4,4′-Diphosphinobiaryls and 1,3- and 1,4-borylphosphinobenzenes carrying crowded triarylphosphine moieties were synthesized by reaction of the corresponding diarylchlorophosphine with an arylcopper(I) reagent. Intramolecular interaction of the phosphorus redox center with the other phosphorus or the boron redox center was investigated by cyclic voltammetry. 4,4′-Diphosphinobiaryls displayed two-step reversible redox waves with slight differences of the oxidation potentials due to weak interaction between two phosphorus redox centers across 4,4′-biarylene linkage. Borylphosphinobenzenes showed two step redox waves corresponding to oxidation at the phosphorus and reduction at the boron. Although significant interaction between the phosphorus and boron redox centers was not observed in the cyclic voltammograms due to large difference of the redox potentials between phosphorus and boron redox centers, an absorption due to weakly interacting phosphorus and boron was observed in the UV-Vis spectrum of the 1,4-borylphosphinobenzene.     

A novel 4‐methylene‐3‐borahomoadamantane: 1,1‐organoboration of an alkyn‐1‐ylsilane using 1‐boraadamantane

1,1‐Dibromo‐2,2‐bis(trimethylsilylethynyl)ethene ( 2 ) reacts with two equivalents of 1‐boraadamantane ( 1 ) by 1,1‐organoboration of both trimethylsilylethynyl groups to give a triene 3 bearing two 4‐methylene‐3‐borahomoadamantane moieties in terminal positions. The triene was characterized by one‐ and two‐dimensional ¹H, ¹¹B, ¹³C and ²⁹Si NMR spectroscopy in solution and X‐ray structural analysis in the solid state. The planes of the C double bond are strongly twisted against each other as a result of the bulky substituents, and the surroundings of the boron atoms deviate from the ideal trigonal planar geometry owing to the tension in the tricyclic frameworks. Copyright © 2006 John Wiley & Sons, Ltd.     

Syntheses and photophysical studies of cyclodextrin derivatives with two proximate anthracenyl groups

A series of permethylated cyclodextrin derivatives, cyclodextrin dimers doubly bridged with two anthracene moieties (An2CD2) and singly bridged with one (AnCD2) and the monomer bearing two anthracene moieties (An2CD), were newly synthesized. For An2CD2, the two isomeric forms are also identified. All compounds are soluble in both aqueous and various organic solvents. The bisanthracene systems, An2CD2 and An2CD, show the thermal equilibrium in an aqueous solution between the intramolecularly interacting (closed) and less-interacting (open) states of the anthracene moieties, which results in the temperature-dependent absorption changes. These systems also show the characteristic excimer emission that is enhanced in water and weakened in organic solvents. The excitation spectra for the monomer and excimer fluorescence are found to be quite different from each other and similar to the absorption spectra of the open and the closed forms, respectively. The observed unique parallelism between excitation and absorption spectra for the present excimer systems indicates the dual ground state-dual excitation scheme where the excitation state formed from the closed ground state mainly gives excimer. The fluorescence lifetime analyses reveal that the rates of the conversion from the excited state of the open form to that of the closed one (6.0 x 10(6) s(-1) for An2CD2-2) are largely retarded compared with that of the ethyleneoxy linked bisanthracene system (8.8 x 10(7) s(-1)).     

Four-coordinate organoboron compounds with a π-conjugated chelate ligand for optoelectronic applications

Four-coordinate organoboron compounds with a π-conjugated chelate backbone have emerged recently as highly attractive materials for a number of applications including use as emitters and electron-transport materials for organic light-emitting diodes (OLEDs) or organic field transistors, photoresponsive materials, and sensory and imaging materials. Many applications of this class of boron compounds stem from the electronic properties of the π-conjugated chelate backbone. Charge-transfer transitions from an aromatic substituent attached to the boron center of the π-conjugated chelate backbone and steric congestion have also been found to play important roles in the luminescent and photochromic properties of the four-coordinate boron compounds. This article provides an update-to-date account on the application aspects of this important class of compounds in materials science with the emphasis on OLED applications and photochromic switching.     

Boron Difluoride β-Diketonates: IV. Reaction of p-Alkyl-substituted Benzoylacetonates with N-Bromosuccinimide

Bromination of p-alkyl-substituted boron difluoride and chromium(III) benzoylacetonates with N-bromosuccinimide was studied. It was found that that the boron difluoride complexes are selectively brominated by the alkyl radical, whereas chromium toluoylacetonate, by the central carbon atom of the chelate ring.     

Indirect atomic-absorption determination of boron by solvent extraction as tris(1,10-phenanthroline)cadmium tetrafluoroborate

An indirect atomic-absorption method for boron has been developed. Boric acid is converted into tetrafluoroborate and extracted into nitrobenzene with Tris(1,10-phenanthroline)cadmium(II). The cadmium in the extract is determined by its atomic-absorption at 228s>d8 nm. A fivefold molar excess of the cadmium chelate is necessary for the extraction from pH 4>d3-6>d0 medium. The sensitivity for boron is thus made about the same as that of cadmium, 0>d005 ppm. Metal ions that react with fluoride or phenanthroline interfere. A procedure is described for determination of boron in steel.     

Donor‐Appended N,C‐Chelate Organoboron Compounds: Influence of Donor Strength on Photochromic Behaviour

Recently, four‐coordinated N,C‐chelate organoboron compounds have been found to show many interesting photochemical transformations depending on the nature of their chelating framework. As such, the effect of substitution on the chelate ligand has been well‐established and understood, but the impact of the aryl groups attached to the boron atom remains less clear. To investigate the effect of enhanced charge‐transfer character, a series of new N,C‐chelate organoboron compounds with donor‐functionalized aryl groups have been synthesized and characterized using NMR, UV/Vis, and electrochemical methods. These compounds were found to possess bright and tunable charge‐transfer luminescence which is dependent on the donor strength of the amino substituent. In addition, some of these compounds undergo photochromic switching, producing dark isomers of various colors. This work establishes that donor‐functionalization of the aryl groups in N,C‐chelate boron compounds is an effective strategy for tuning both the photophysical and photochemical properties of such systems. The new findings also help elucidate the influence of electronic structure on the photoreactivity of N,C‐chelate organoboron compounds which appears to be as important as steric crowding around the boron atom.     

Reversible Photomodulation of Electronic Communication in a π‐Conjugated Photoswitch‐Fluorophore Molecular Dyad

The extent of electronic coupling between a boron dipyrromethene (BODIPY) fluorophore and a diarylethene (DAE) photoswitch has been modulated in a covalently linked molecular dyad by irradiation with either UV or visible light. In the open isomer, both moieties can be regarded as individual chromophores, while in the closed form the lowest electronic (S₀→S₁) transition of the dyad is slightly shifted, enabling photomodulation of its fluorescence. Transient spectroscopy confirms that the dyad behaves dramatically different in the two switching states: while in the open isomer it resembles an undisturbed BODIPY fluorophore, in the closed isomer no fluorescence occurs and instead a red‐shifted DAE behavior prevails.     

A molecular interpretation of vitreous boron oxide dynamics

The mobility of vitreous boron oxide is studied by molecular dynamics simulation. A polarization model that incorporates induced dipoles arising both from charges and from other induced dipoles on atoms with nonzero polarizability is used to simulate boron oxide glass at various temperatures above the glass transition temperature. Particle mobility is investigated through the calculation of the self-intermediate scattering function and the mean-squared displacement. The calculations clearly reveal a two-step relaxation with a plateau at intermediate times for all investigated temperatures. With respect to atomic species, boron atoms are less mobile than oxygen atoms at all temperatures within the plateau region. Through analyzing particle trajectories, it is revealed that BO(3) groups move as one unit and follow each other in a stringlike manner. Three connected BO(3) groups comprise a six-membered boroxol ring, which is shown to move in a collective manner, requiring the simultaneous movement of all ring atoms. The boroxol ring is observed to be confined, or caged, during the plateau region, and jumps to a new location at longer times. This observation is linked to the concept of strong versus fragile glass formers and the potential energy landscape. In addition to the caging feature, an overshoot or dip occurs in the plateau regions of the mean-squared displacement and self-intermediate scattering functions respectively. These features are followed by a ringing pattern, previously associated with finite size effects in other strong glass formers, which persist for the duration of the plateau region. Both features are shown to be consistent with the bending of atomic "cages" from the plane of the boroxol ring, and arise due to the displacement of atoms from local minimum energy configurations.