Exciplex Formation and Excited State Deactivation of Difluoroborondipyrromethene (Bodipy) Dyads

Two series of geometrically‐related dyads are discussed based on the difluoroborondipyrromethene (Bodipy) unit, and incorporating covalently attached hydroquinone/quinone groups. These units are anchored directly, or via a phenylene spacer, to the Bodipy core at the meso position in one series ( BD‐MHQ , BD‐MQ , BD‐MPHQ , BD‐MPQ ), but for the second series the attachment site is the 2‐position ( BD‐SHQ , BD‐SQ , BD‐SPHQ , BD‐SPQ ). The compounds show various levels of fluorescence depending on the oxidation state of the appended group and the substitution pattern. In non‐polar solvents such as toluene, diethyl ether and dichlorobenzene, the S₁ state deactivation of the Bodipy unit in BD‐SPQ and BD‐MPQ is dominated by ^{1, 3}exciplex formation, which has not been reported for Bodipy derivatives so far. In the latter molecule, the decay of the exciplex is divided between population of the Bodipy triplet state (13 %–21 %) and ground state reformation. This partitioning is not seen for the side‐on substituted derivative, BD‐SPQ , and only ground state reformation is observed following decay of the exciplex. This difference in behavior is explained by the radical‐pair inter‐system‐crossing mechanism, which more effectively operates in BD‐MPQ because of the orthogonality of the donor‐acceptor units. In the more polar solvent CH₃CN all the quinone derivatives show fast formation of the charge‐separated state (k_CS) followed by slower charge recombination (k_CR). The ratio k_CS/k_CR≤80.     

Intermediates Involved in the Oxidation of Nitrogen Monoxide: Photochemistry of the cis‐N2O2⋅O2 complex and of sym‐N2O4 in Solid Ne Matrices

Pure sym‐N₂O₄ isolated in solid Ne was obtained by passing cold neon gas over solid N₂O₄ at ?115 °C and quenching the resulting gaseous mixture at 6.3 K. Filtered UV irradiation (260–400 nm) converts sym‐N₂O₄ into trans‐ONONO₂, a weakly interacting (NO₂)₂ radical pair, and traces of the cis‐N₂O₂?O₂ complex. Besides the weakly bound ON?O₂ complex, cis‐N₂O₂?O₂ was also obtained by co‐deposition of NO and O₂ in solid Ne at 6.3 K, and both complexes were characterised by their matrix IR spectra. Concomitantly formed cis‐N₂O₂ dissociated on exposure to filtered IR irradiation (400–8000 cm^?1), and the cis‐N₂O₂?O₂ complex rearranged to sym‐N₂O₄ and trans‐ONONO₂. Experiments using ¹⁸O₂ in place of ¹⁶O₂ revealed a non‐concerted conversion of cis‐N₂O₂?O₂ into these species, and gave access to four selectively di‐¹⁸O‐substituted trans‐ONONO₂ isotopomers. No isotopic scrambling occurred. The IR spectra of sym‐N₂O₄ and of trans‐ONONO₂ in solid Ne were recorded. IR fundamentals of trans‐ONONO₂ were assigned based on experimental ^16/18O isotopic shifts and guided by DFT calculations. Previously reported contradictory measurements on cis‐ and trans‐ONONO₂ are discussed. Dinitroso peroxide, ONOONO, a proposed intermediate in the IR photoinduced rearrangement of cis‐N₂O₂?O₂ to the various N₂O₄ species, was not detected. Its absence in the photolysis products indicates a low barrier (≤10 kJ mol^?1) for its exothermic O? O bond homolysis into a (NO₂)₂ radical pair.     

1H,13C and 19F NMR data of N‐substituted 6‐(4‐methoxyphenyl)‐7H‐pyrrolo[2,3‐d]pyrimidin‐4‐amines in DMSO‐d6

Chemical shift assignment of seven N‐substituted 6‐(4‐methoxyphenyl)‐7H‐pyrrolo[2, 3‐d]pyrimidin‐4‐amines, six of which are fluorinated, have been performed based on ¹H, ¹³C, ¹⁹F, and 2D COSY, HMBC and HSQC experiments. Copyright © 2009 John Wiley & Sons, Ltd.     

SrPd4B and BaPd4B: New Type of Crystal Structure and Physical Properties

Two new intermetallic alkaline‐earth palladium borides, SrPd₄B and BaPd₄B were synthesised and their physical properties were investigated. The crystal structure of SrPd₄B was solved from powder X‐ray diffraction data: new structure type, space group Pnma, a = 6.0014(1) Å, b = 5.5041(1) Å, c = 11.8723(2) Å, R_I = 0.065, R_P = 0.093. BaPd₄B is isostructural with a = 6.0883(1) Å, b = 5.6066(1) Å, c = 12.0050(2) Å, R_I = 0.062, R_P = 0.097. The relationship of this structure type with the series of derivatives of the CaCu₅ type is discussed. Calculated electronic band structures for palladium, Pd₃B, SrPd₅, SrPd₄B and SrPd₃B are compared. The role of boron and strontium for the electronic properties is discussed in detail. SrPd₄B shows metallic behaviour with a DOS(E_F) ≈? 1.7 eV^–1 · f.u.^–1 at the Fermi level. Magnetic properties, electrical resistivity and specific heat capacity measurements reveal that the two compounds are diamagnetic metallic conductors with low electronic density of states, in agreement, with the electronic structure calculations.     

Synthesis,Cytotoxicity and Antibacterial Studies of Novel Symmetrically and Nonsymmetrically 4‐(Methoxycarbonyl)benzyl‐Substituted N‐Heterocyclic Carbene–Silver Acetate Complexes

From the reaction of 1H‐imidazole ( 1a ), 4,5‐dichloro‐1H‐imidazole ( 1b ), 1H‐benzimidazole ( 1c ), 1‐methyl‐1H‐imidazole ( 1d ), and 1‐methyl‐1H‐benzimidazole ( 1f ) with methyl 4‐(bromomethyl)benzoate ( 2 ), symmetrically and nonsymmetrically 4‐(methoxycarbonyl)benzyl‐substituted N‐heterocyclic carbene (NHC) precursors, 3a – 3f , were synthesized. These NHC precursors were then reacted with silver(I) acetate (AgOAc) to yield the NHC–silver acetate complexes (acetato‐κO){1,3‐bis[4‐(methoxycarbonyl)benzyl]imidazol‐2‐ylidene}silver ( 4a ), (acetato‐κO){4,5‐dichloro‐1,3‐bis[4‐(methoxycarbonyl)benzyl]‐2,3‐dihydro‐1H‐imidazol‐2‐yl}silver ( 4b ), (acetato‐κO){1,3‐bis[4‐(methoxycarbonyl)benzyl]‐2,3‐dihydro‐1H‐benzimidazol‐2‐yl}silver ( 4c ), (acetato‐κO){1‐[4‐(methoxycarbonyl)benzyl]‐3‐methyl‐2,3‐dihydro‐1H‐imidazol‐2‐yl}silver ( 4d ), (acetato‐κO){4,5‐dichloro‐1‐[4‐(methoxycarbonyl)benzyl]‐3‐methyl‐2,3‐dihydro‐1H‐imidazol‐2‐yl}silver ( 4e ), and (acetato‐κO){1‐[4‐(methoxycarbonyl)benzyl]‐3‐methyl‐2,3‐dihydro‐1H‐benzimidazol‐2‐yl}silver ( 4f ), respectively. The three NHC–AgOAc complexes 4a, 4c , and 4d were characterized by single‐crystal X‐ray diffraction. All compounds studied in this work were preliminarily screened for their antimicrobial activities in vitro against Gram‐positive bacteria Staphylococcus aureus, and Gram‐negative bacteria Escherichia coli using the qualitative disk‐diffusion method. All NHC–AgOAc complexes exhibited weak‐to‐medium antibacterial activity with areas of clearance ranging from 4 to 7 mm at the highest amount used, while the NHC precursors showed significantly lower activity. In addition, NHC–AgOAc complexes 4a and 4b , and 4d – 4f exhibited in preliminary cytotoxicity tests on the human renal‐cancer cell line Caki‐1 medium‐to‐high cytotoxicities with IC₅₀ values ranging from 3.3±0.4 to 68.3±1 μM .     

Azobenzene-linked porphyrin-fullerene dyads

A new group of porphyrin-fullerene dyads with an azobenzene linker was synthesized, and the photochemical and photophysical properties of these materials were investigated using steady-state and time-resolved spectroscopic methods. The electrochemical properties of these compounds were also studied in detail. The synthesis involved oxidative heterocoupling of free base tris-aryl-p-aminophenyl porphyrins with a p-aminophenylacetal, followed by deprotection to give the aldehyde, and finally Prato 1,3-dipolar azomethineylide cycloaddition to C60. The corresponding Zn(II)-porphyrin (ZnP) dyads were made by treating the free base dyads with zinc acetate. The final dyads were characterized by their 1H NMR, mass, and UV-vis spectra. 3He NMR was used to determine if the products are a mixture of cis and trans stereoisomers, or a single isomer. The data are most consistent with the isolation of only a single configurational isomer, assigned to the trans (E) configuration. The ground-state UV-vis spectra are virtually a superimposition of the spectral features of the individual components, indicating there is no interaction of the fullerene (F) and porphyrin (H2P/ZnP) moieties in the ground state. This conclusion is supported by the electrochemical data. The steady-state and time-resolved fluorescence spectra indicate that the porphyrin fluorescence in the dyads is very strongly quenched at room temperature in the three solvents studied: toluene, tetrahydrofuran (THF), and benzonitrile (BzCN). The fluorescence lifetimes of the dyads in all solvents are sharply reduced compared to those of H2P and ZnP standards. In toluene, the lifetimes of the free base dyads are 600-790 ps compared to 10.1 ns for the standard, while in THF and BzCN the dyad lifetimes are less than 100 ps. For the ZnP dyads, the fluorescence lifetimes were 10-170 ps vs 2.1-2.2 ns for the ZnP references. The mechanism of the fluorescence quenching was established using time-resolved transient absorption spectroscopy. In toluene, the quenching process is singlet-singlet energy transfer (k approximately 10(11) s-1) to give C60 singlet excited states which decay with a lifetime of 1.2 ns to give very long-lived C60 triplet states. In THF and BzCN, quenching of porphyrin singlet states occurs at a similar rate, but now by electron transfer, to give charge-separated radical pair (CSRP) states, which show transient absorption spectra very similar to those reported for other H2P-C60 and ZnP-C60 dyad systems. The lifetimes of the CSRP states are in the range 145-435 ns in THF, much shorter than for related systems with amide, alkyne, silyl, and hydrogen-bonded linkers. Thus, both forward and back electron transfer is facilitated by the azobenzene linker. Nonetheless, the charge recombination is 3-4 orders of magnitude slower than charge separation, demonstrating that for these types of donor-acceptor systems back electron transfer is occurring in the Marcus inverted region.     

Color Bricks: Building Highly Organized and Strongly Absorbing Multicomponent Arrays of Terpyridyl Perylenes on Metal Oxide Surfaces

Terpyridine‐substituted perylenes containing cyclic anhydrides in the peri position were synthesized. The anhydride group served as an anchor for assembly of the terpyridyl‐crowned chromophores as monomolecular layers on metal oxide surfaces. Further coordination with Zn²⁺ ions allowed for layer‐by‐layer formation of supramolecular assemblies of perylene imides on the solid substrates. With properly selected anchor and linker molecules it was possible to build high quality structures of greater than ten successive layers by a simple and straightforward procedure. The prepared films were stable and had a broad spectral coverage and high absorbance. To demonstrate their potential use, the synthesized dyes were employed in solid‐state dye‐sensitized solar cells, and electron injection from the perylene antennas to titanium dioxide was observed.     

Elucidation of the in vitro metabolic profile of stable isotope labeled BAL19403 by accurate mass capillary liquid chromatography/quadrupole time-of-flight mass spectrometry and isotope exchange

The in vitro metabolic pattern of BAL19403, a novel macrolide antibiotic, was investigated by capillary liquid chromatography/quadrupole time-of-flight mass spectrometry (LC/QTOF-MS) in incubations with human microsomes. For the elucidation of the metabolic pathway, BAL19403 labeled with four deuterium atoms (D4) was used, and detection of metabolites performed using mixtures of the unlabeled (H4) BAL19403 and its D4 analogue (1:1) as substrate. All metabolites appeared with similar chromatographic behavior. MS/MS spectra of BAL19403 and its metabolites are dominated by non-informative fragment ions. Therefore, the structure of the metabolites was elucidated mainly by accurate mass measurements with subsequent proposals of elemental compositions. Main biotransformations were N-demethylation, lactone ring hydrolysis, and oxidation. Additionally, N-dealkylation of the aromatic moiety was identified. This dealkylation results not only in formation of an aldehyde, according to the classical pathway, but also in formation of the corresponding alcohol and carboxylic acid. Final elucidation of their structures was possible, since this dealkylation takes place vicinal to the deuterium-labeled part of BAL19403 and interferes with D/H exchange. The degree of D/H exchange, determined by analysis of the metabolite isotopic pattern, was used to elucidate the adjacent functional group.     

Synthesis,Vibrational Spectrum and Structure of the Tetracyanoborate K[B(CN)4]·CH3CN

The new tetracyanoborate K[B(CN)₄]·CH₃CN was synthesized by dissolution of the solvent‐free K[B(CN)₄] in acetonitrile and subsequent careful crystallization. The crystal structure has been determined by single‐crystal X‐ray diffraction. It crystallizes in the orthorhombic space group P2₁2₁2₁ with Z = 4. Some comparisons with related structures are made, and the vibrational spectrum is discussed.