Controlling Interchromophore Coupling in Diamantane-Linked Pentacene Dimers To Create a "Binary" Pair.

Two isomeric pentacene dimers, each linked by a diamantane spacer, have been synthesized. These dimers are designed to provide experimental evidence to support quantum mechanical calculations, which predict the substitution pattern on the carbon-rich diethynyldiamantane spacer to be decisive in controlling the interpentacene coupling. Intramolecular singlet fission (i-SF) serves as a probe for the existence and strength of the electronic coupling between the two pentacenes, with transient absorption spectroscopy as the method of choice to characterize i-SF. 4,9-Substitution of diamantane provides a pentacene dimer (4,9-dimer) in which the two chromophores are completely decoupled and that, following photoexcitation, deactivates to the ground state analogous to a monomeric pentacene chromophore. Conversely, 1,6-substitution provides a pentacene dimer (1,6-dimer) that exhibits sufficiently strong coupling to drive i-SF, resulting in correlated triplet M(T1T1) yields close to unity and free triplet (T1 + T1) yields of ca. 50%. Thus, the diamantane spacer effectively switches "on" or "off" the coupling between the chromophores, based on the substitution pattern. The binary control of diamantane contrasts other known molecular spacers designed only to modulate the coupling strength between two pentacenes.

Tetracene Dimers: A Platform for Intramolecular Down- and Up-conversion.

Photon energy conversion can be accomplished in many different ways, including the two opposing manners, down-conversion (i.e., singlet fission, SF) and up-conversion (i.e., triplet-triplet annihilation up-conversion, TTA-UC). Both processes have the potential to help overcome the detailed balance limit of single-junction solar cells. Tetracene, in which the energies of the lowest singlet excited state and twice the triplet excited state are comparable, exhibits both down- and up-conversion. Here, we have designed meta-diethynylphenylene- and 1,3-diethynyladamantyl-linked tetracene dimers, which feature different electronic coupling, to characterize the interplay between intramolecular SF (intra-SF) and intramolecular TTA-UC (intra-TTA-UC) via steady-state and time-resolved absorption and fluorescence spectroscopy. Furthermore, we have used Pd-phthalocyanine as a sensitizer to enable intra-TTA-UC in the two dimers via indirect photoexcitation in the near-infrared part of the solar spectrum. The work is rounded off by temperature-dependent measurements, which outline key aspects of how thermal effects impact intra-SF and intra-TTA-UC in different dimers.

Sensitized Singlet Fission in Rigidly Linked Axial and Peripheral Pentacene-Subphthalocyanine Conjugates.

The goal of harnessing the theoretical potential of singlet fission (SF), a process in which one singlet excited state is split into two triplet excited states, has become a central challenge in solar energy research. Covalently linked dimers provide crucial models for understanding the role of chromophore arrangement and coupling in SF. Sensitizers can be integrated into these systems to expand the absorption bandwidth through which SF can be accessed. Here, we define the role of the sensitizer-chromophore geometry in a sensitized SF model system. To this end, two conjugates have been synthesized consisting of a pentacene dimer (SF motif) connected via a rigid alkynyl bridge to a subphthalocyanine (the sensitizer motif) in either an axial or a peripheral arrangement. Steady-state and time-resolved photophysical measurements are used to confirm that both conjugates operate as per design, displaying near unity energy transfer efficiencies and high triplet quantum yields from SF. Decisively, energy transfer between the subphthalocyanine and pentacene dimer occurs ca. 26 times faster in the peripheral conjugate, even though the two chromophores are ca. 3 Å farther apart than in the axial conjugate. Following a theoretical evaluation of the dipolar coupling, Vdip2, and the orientation factor, κ2, of both the axial (Vdip2 = 140 cm-2; κ2 = 0.08) and the peripheral (Vdip2 = 724 cm-2; κ2 = 1.46) arrangements, we establish that this rate acceleration is due to a more favorable (nearly co-planar) relative orientation of the transition dipole moments of the subphthalocyanine and pentacenes in the peripheral constellation.

Nickel-Catalyzed O-Arylation of Primary or Secondary Aliphatic Alcohols with (Hetero)aryl Chlorides: A Comparison of Ni(I) and Ni(II) Precatalysts.

A comparative experimental and computational study examining the interplay of the ancillary ligand structure and Ni oxidation state in the Ni-catalyzed C(sp2)-O cross-coupling of (hetero)aryl chlorides and primary or secondary aliphatic alcohols is presented, focusing on PAd-DalPhos (L1)-, CyPAd-DalPhos (L2)-, PAd2-DalPhos (L3)-, and DPPF (L4)-ligated [(L)NiCl]n (n = 1 or 2) and (L)Ni(o-tol)Cl precatalysts. Both L1 and L2 were found to outperform the other ligands examined, with the latter proving to be superior overall. While Ni(II) precatalysts generally outperformed Ni(I) species, in some instances the catalytic abilities of Ni(I) precatalysts were competitive with those of Ni(II). Density-functional theory calculations indicate the favorability of a Ni(0)/Ni(II) catalytic cycle featuring turnover-limiting C-O bond reductive elimination over a Ni(I)/Ni(III) cycle involving turnover-limiting C-Cl oxidative addition.

Monodentate Alkoxy/Aryloxy Anchored Yttrium Dihydrides; Anticipated Outcomes and Surprises.

The synthesis, characterization, and solid-state structure of bulky alkoxy- and aryloxy-supported yttrium polynuclear hydrides are reported. Hydrogenolysis of the supertrityl alkoxy anchored yttrium dialkyl, Y(OTr*)(CH2SiMe3)2(THF)2 (1) (Tr* = tris(3,5-di-tert-butylphenyl)methyl), resulted in the clean conversion to the tetranuclear dihydride, [Y(OTr*)H2(THF)]4 (1a). X-ray analysis revealed a highly symmetrical structure (4̅ site symmetry) with the four Y atoms located on the corners of a compressed tetrahedron, each bonded to an OTr* and tetrahydrofuran (THF) ligand and the cluster held together by four face-capping, µ3-H, and four edge-bridging, µ2-H, hydrides. DFT calculations on the full system with and without THF, but also on model systems, clearly show that the structural preference for complex 1a is controlled by the presence and coordination of THF molecules. Contrary to the exclusive formation of the tetranuclear dihydride, hydrogenolysis of the bulky aryloxy yttrium dialkyl, Y(OAr*)(CH2SiMe3)2(THF)2 (2) (Ar* = 3,5-di-tert-butylphenyl) gave a mixture of the analogous tetranuclear 2a and trinuclear, [Y3(OAr*)4H5(THF)4], polyhydride, 2b. Similar results, i.e., a mixture of tetra-/tri-nuclear products, were obtained from hydrogenolysis of the even bulkier Y(OArAd2,Me)(CH2SiMe3)2(THF)2 compound. Experimental conditions were established to optimize the production of either the tetra- or trinuclear products. X-ray structure of 2b revealed a triangular array of three yttrium atoms with two face-capping µ3-H and three edge-bridging µ2-H hydrides, with one yttrium bonded to two aryloxy ligands while the other two have a complement of one aryloxy and two THF ligands; the solid-state structure is close to being C2 symmetric, with the C2 axis running through the unique Y and unique µ2-H hydride. As opposed to 2a, which shows distinct 1H NMR resonances for µ3/ µ2-H (δ = 5.83/6.35 ppm, respectively), no hydride signals for 2b were observed at room temperature, indicating hydride exchange on the NMR time scale. Their presence and assignment were secured at -40 °C from 1H SST (spin saturation) experiment.

Frustrated Lewis Pair Adduct of Atomic P(-1) as a Source of Phosphinidenes (PR), Diphosphorus (P2 ), and Indium Phosphide.

We report phosphinidenes (PR) stabilized by an intramolecular frustrated Lewis pair (FLP) chelate. These adducts include the parent phosphinidene, PH, which is accessed via thermolysis of coordinated HPCO. The reported FLP-PH species acts as a springboard to other phosphorus-containing compounds, such as FLP-adducts of diphosphorus (P2 ) and InP3 . Our new adducts participate in thermal- or light-induced phosphinidene elimination (of both PH and PR, R=organic group), transfer P2 units to an organic substrate, and yield the useful semiconductor InP at only 110 °C from solution.

An NHC-Stabilized H2 GeBH2 Precursor for the Preparation of Cationic Group 13/14/15 Hydride Chains.

The synthesis, characterization and reactivity studies of the NHC-stabilized complex IDipp ⋅ GeH2 BH2 OTf (1) (IDipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) are reported. Nucleophilic substitution of the triflate (OTf) group in 1 by phosphine or arsine donors provides access to the cationic group 13/14/15 chains [IDipp ⋅ GeH2 BH2 ERR1 R2 ]+ (2 E=P; R, R1 =H; R2 =t Bu; 3 E=P; R=H; R1 , R2 =Ph; 4 a E=P; R, R1 , R2 =Ph; 4 b E=As; R, R1 , R2 =Ph). These novel cationic chains were characterized by X-ray crystallography, NMR spectroscopy and mass spectrometry. Moreover, the formation of the parent complexes [IDipp ⋅ GeH2 BH2 PH3 ][OTf] (5) and [IDipp ⋅ GeH3 ][OTf] (6) were achieved by reaction of 1 with PH3 . Accompanying DFT computations give insight into the stability of the formed chains with respect to their decomposition.

The Effects of Ring Strain on Cyclic Tetraaryl[5]cumulenes.

Cyclic tetraaryl[5]cumulenes (1 a-f) have been synthesized and studied as a function of increasing ring strain. The magnitude of ring strain is approximated by the extent of bending of the cumulenic core as assessed by a combination of X-ray crystallographic analysis and DFT calculations. Trends are observed in 13 C NMR, UV-vis, and Raman spectra associated with ring strain, but the effects are small. In particular, the experimental HOMO-LUMO gap is not appreciably affected by bending of the [5]cumulene framework from ca. 174° (λmax =504 nm) in 1 a to ca. 178° (λmax =494 nm) in 1 f.

Pentacenones as Divergent Intermediates to Unsymmetrically Substituted Pentacenes: Synthesis and Crystallographic Analysis.

We report a general method for the desymmetrization of 6,13-pentacenequinone to access ethynylated pentacene ketones, namely, 13-hydroxy-13-(ethynylated)pentacene-6(13H)-ones. These pentacene ketones ("pentacenones") serve as divergent intermediates to unsymmetrically 6,13-disubstituted pentacenes, commonly used for studying singlet fission processes and charge transport phenomena in organic field effect transistors. We report a synthetic method to access pentacenones, which utilizes a precipitation/crystallization from the crude mixture to enable facile purification on a multigram scale. X-ray crystallographic analysis of the pentacenones reveals key noncovalent interactions that contribute to the crystallization, specifically, hydrogen bonding between the ketone and alcohol functional groups as well as π-π-stacking and dipole-dipole interactions.

Frustrated Lewis Pair Chelation and Reactivity of Complexed Parent Iminoborane and Aminoborane.

An intramolecular phosphine-borane frustrated Lewis pair (FLP) chelate, iPr2P(C6H4)BCy2 or PB (Cy = cyclohexyl), was used to coordinate aminoborane (H2BNH2) and iminoborane (HBNH) units via donor-acceptor stabilization. Attempts to induce dehydrogenation from these B-N adducts with known metal catalysts (or pre-catalysts) have been unsuccessful thus far, and related observations were noted with an H2BNH2 complex supported by a modified FLP chelate bearing a geometrically constrained bicyclic 9-borabicyclo(3.3.1)nonane (BBN) unit. Treatment of the iminoborane adduct [PB{HBNH}] with a chlorinating agent led to ligand activation via B-C bond cleavage instead of the expected H/Cl exchange at boron to give [PB{ClBNH}]. Nucleophilic attack at the boron center in [PB{HBNH}] was observed upon addition of BnK (Bn = benzyl), yielding the amidoborate complex [PB{H(Bn)BNH}{K(THF)2}].

Diagnosis and Follow-up of Incidental Liver Lesions in Children.

ABSTRACT: Incidental liver lesions are identified in children without underlying liver disease or increased risk of hepatic malignancy in childhood. Clinical and imaging evaluation of incidental liver lesions can be complex and may require a multidisciplinary approach. This review aims to summarize the diagnostic process and follow-up of incidental liver lesions based on review of the literature, use of state-of-the-art imaging, and our institutional experience. Age at presentation, gender, alpha fetoprotein levels, tumor size, and imaging characteristics should all be taken into consideration to optimize diagnosis process. Some lesions, such as simple liver cyst, infantile hemangioma, focal nodular hyperplasia (FNH), and focal fatty lesions, have specific imaging characteristics. Recently, contrast-enhanced ultrasound (CEUS) was Food and Drug Administration (FDA)-approved for the evaluation of pediatric liver lesions. CEUS is most specific in lesions smaller than 3 cm and is most useful in the diagnosis of infantile hemangioma, FNH, and focal fatty lesions. The use of hepatobiliary contrast in MRI increases specificity in the diagnosis of FNH. Recently, lesion characteristics in MRI were found to correlate with subtypes of hepatocellular adenomas and associated risk for hemorrhage and malignant transformation. Biopsy should be considered when there are no specific imaging characteristics of a benign lesion. Surveillance with imaging and alpha fetoprotein (AFP) should be performed to confirm the stability of lesions when the diagnosis cannot be determined, and whenever biopsy is not feasible.

Redox-Active Heteroatom-Functionalized Polyacetylenes.

The discovery of metallic conductivity in polyacetylene [-HC=CH-]n upon doping represents a landmark achievement. However, the insolubility of polyacetylene and a dearth of methods for its chemical modification have limited its widespread use. Here, we employ a ring-opening metathesis polymerization (ROMP) protocol to prepare functionalized polyacetylenes (fPAs) bearing: (1) electron-deficient boryl (-BR2 ) and phosphoryl (-P(O)R2 ) side chains; (2) electron-donating amino (-NR2 ) groups, and (3) ring-fused 1,2,3-triazolium units via strain-promoted Click chemistry. These functional groups render most of the fPAs soluble and can lead to intense light absorption across the visible to near-IR region. Also, the presence of redox-active boryl and amino groups leads to opposing near-IR optical responses upon (electro)chemical reduction or oxidation. Some of the resulting fPAs show greatly enhanced air stability when compared to known polyacetylenes. Lastly, these fPAs can be cross-linked to yield network materials with the full retention of optical properties.

Lewis or Brønsted? A Rectification of the Acidic and Aromatic Nature of Boranol-Containing Naphthoid Heterocycles.

Boron-containing heterocycles are important in a variety of applications from drug discovery to materials science; therefore a clear understanding of their structure and reactivity is desirable to optimize these functions. Although the boranol (B-OH) unit of boronic acids behaves as a Lewis acid to form a tetravalent trihydroxyborate conjugate base, it has been proposed that pseudoaromatic hemiboronic acids may possess sufficient aromatic character to act as Brønsted acids and form a boron oxy conjugate base, thereby avoiding the disruption of ring aromaticity that would occur with a tetravalent boronate anion. Until now no firm evidence existed to ascertain the structure of the conjugate base and the aromatic character of the boron-containing ring of hemiboronic "naphthoid" isosteres. Here, these questions are addressed with a combination of experimental, spectroscopic, X-ray crystallographic, and computational studies of a series of model benzoxazaborine and benzodiazaborine naphthoids. Although these hemiboronic heterocycles are unambiguously shown to behave as Lewis acids in aqueous solutions, boraza derivatives possess partial aromaticity provided their nitrogen lone electron pair is sufficiently available to participate in extended delocalization. As demonstrated by dynamic exchange and crossover experiments, these heterocycles are stable in neutral aqueous medium, and their measured pKa values are consistent with the ability of the endocyclic heteroatom substituent to stabilize a partial negative charge in the conjugate base. Altogether, this study corrects previous inaccuracies and provides conclusions regarding the properties of these compounds that are important toward the methodical application of hemiboronic and other boron heterocycles in catalysis, bioconjugation, and medicinal chemistry.

A Stable Homoleptic Divinyl Tetrelene Series.

The synthesis of the new bulky vinyllithium reagent (Me IPr=CH)Li, (Me IPr=[(MeCNDipp)2 C]; Dipp=2,6-iPr2 C6 H3 ) is reported. This vinyllithium precursor was found to act as a general source of the anionic 2σ, 2π-electron donor ligand [Me IPr=CH]- . Furthermore, a high-yielding route to the degradation-resistant SiII precursor Me IPr⋅SiBr2 is presented. The efficacy of (Me IPr=CH)Li in synthesis was demonstrated by the generation of a complete inorganic divinyltetrelene series (Me IPrCH)2 E: (E=Si to Pb). (Me IPrCH)2 Si: represents the first two-coordinate acyclic silylene not bound by heteroatom donors, with dual electrophilic and nucleophilic character at the SiII center noted. Cyclic voltammetry shows this electron-rich silylene to be a potent reducing agent, rivalling the reducing power of the 19-electron complex cobaltocene (Cp2 Co).

Tellura(benzo)bithiophenes: Synthesis, Oligomerization, and Phosphorescence.

A series of planar π-extended Te-containing heteroacenes, termed tellura(benzo)bithiophenes, were synthesized. This new structural class of heterocycle features a tellurophene ring fused to a benzobithiophene unit with aromatic side groups (either -C6H4iPr or -C6H4OCH3) positioned at the 2- and 5-positions of the tellurophene moiety. Although attempts to enhance molecular rigidity and extend ring-framework π-delocalization in a cumenyl (-C6H4iPr)-capped tellura(benzo)bithiophene led to oxidation (and Te-C bond scission) to form a diene-one, the formation of an oligomeric tellura(benzo)bithiophene was possible via Kumada catalyst-transfer polycondensation (KCTP). Furthermore, one tellura(benzo)bithiophene derivative exhibits orange-red phosphorescence at room temperature in air when incorporated into a poly(methyl methacrylate) host; accompanying TD-DFT computations provided insight into a potential mechanism for the observed phosphorescence.

Zinc-Mediated Transmetalation as a Route to Anionic N-Heterocyclic Olefin Complexes in the p-Block.

Anionic N-heterocyclic olefins (aNHOs) are suited well for the stabilization of low-coordinate inorganic complexes, due to their steric tunability and strong σ- and π-electron donating abilities. In this study, the new two-coordinate zinc complex (MeIPrCH)2Zn (MeIPrCH = [(MeCNDipp)2C═CH]-, Dipp = 2,6-diisopropylphenyl) is shown to participate in a broad range of metathesis reactions with main group element-based halides and hydrides. In the case of the group 14 halides, Cl2E·dioxane (E = Ge and Sn), transmetalation occurs to form dinuclear propellane-shaped cations, [(MeIPrCHE)2(µ-Cl)]+, while the aNHO-capped phosphine ligand MeIPrCH-PPh2 is obtained when (MeIPrCH)2Zn is combined with ClPPh2. Lastly, ZnH2 elimination drives transmetalation between (MeIPrCH)2Zn and hydroboranes and hydroalumanes, leading to Lewis acidic aNHO-supported -boryl and -alane products.

Frustrated Lewis Pair Chelation as a Vehicle for Low-Temperature Semiconductor Element and Polymer Deposition.

The stabilization of silicon(II) and germanium(II) dihydrides by an intramolecular Frustrated Lewis Pair (FLP) ligand, PB, i Pr2 P(C6 H4 )BCy2 (Cy=cyclohexyl) is reported. The resulting hydride complexes [PB{SiH2 }] and [PB{GeH2 }] are indefinitely stable at room temperature, yet can deposit films of silicon and germanium, respectively, upon mild thermolysis in solution. Hallmarks of this work include: 1) the ability to recycle the FLP phosphine-borane ligand (PB) after element deposition, and 2) the single-source precursor [PB{SiH2 }] deposits Si films at a record low temperature from solution (110 °C). The dialkylsilicon(II) adduct [PB{SiMe2 }] was also prepared, and shown to release poly(dimethylsilane) [SiMe2 ]n upon heating. Overall, this study introduces a "closed loop" deposition strategy for semiconductors that steers materials science away from the use of harsh reagents or high temperatures.

Hierarchical Synthesis, Structure, and Photophysical Properties of Gallium- and Ruthenium-Porphyrins with Axially Bonded Azo Ligands.

The hierarchical synthesis of three porphyrin and four bisporphyrin derivatives is presented. This strategy relies on the incorporation of linkers based on azo moieties appended with pyridyl and/or acetylenic groups that facilitate axial coordination to Ga- and Ru-metalloporphyrins. These porphyrinic systems allow for a quantitative analysis of the effects of diamagnetic anisotropy (DA) by using 1 H NMR spectroscopic and X-ray crystallographic analyses. A simple power-law relationship between the proton chemical shift and the distance from the porphyrin core is experimentally outlined, which confirms previous theoretical predictions and shows that the limit of DA is about 2 nm. Photophysical properties of the azo-linked porphyrins are analyzed by UV/Vis spectroscopy, showing that significant cis-trans isomerization is not observed for azo ligands bound only to Ga-porphyrins. Incorporation of Ru-porphyrins to an azo ligand facilitates photoswitching behavior, but the process faces competition from decarbonylation of the Ru-porphyrin, and appreciable switching is only documented for GaL1Ru.

Early administration of imatinib mesylate reduces plexiform neurofibroma tumor burden with durable results after drug discontinuation in a mouse model of neurofibromatosis type 1.

BACKGROUND: Neurofibromatosis type 1 (NF1) is a common genetic disorder characterized by plexiform neurofibromas (pNF), which are thought to be congenital tumors that arise in utero and enlarge throughout life. Genetic studies in murine models delineated an indispensable role for the stem cell factor (SCF)/c-kit pathway in pNF initiation and progression. A subsequent phase 2 clinical trial using imatinib mesylate to inhibit SCF/c-kit demonstrated tumor shrinkage in a subset of preexisting pNF; however, imatinib's role on preventing pNF development has yet to be explored. PROCEDURE: We evaluated the effect of imatinib dosed at 10-100 mg/kg/day for 12 weeks to one-month-old Nf1flox/flox ;PostnCre(+) mice, prior to onset of pNF formation. To determine durability of response, we then monitored for pNF growth at later time points, comparing imatinib- with vehicle-treated mice. We assessed gross and histopathological analysis of tumor burden. RESULTS: Imatinib administered preventatively led to a significant decrease in pNF number, even at doses as low as 10 mg/kg/day. Tumor development continued to be significantly inhibited after cessation of imatinib dosed at 50 and 100 mg/kg/day. In the cohort of treated mice that underwent prolonged follow-up, the size of residual tumors was significantly reduced as compared with age-matched littermates that received vehicle control. CONCLUSIONS: Early administration of imatinib inhibits pNF genesis in vivo, and effects are sustained after discontinuation of therapy. These findings may guide clinical use of imatinib in young NF1 patients prior to the substantial development of pNF.

Insight into the Decomposition Mechanism of Donor-Acceptor Complexes of EH2 (E = Ge and Sn) and Access to Germanium Thin Films from Solution.

Electron-donating N-heterocyclic carbenes (Lewis bases, LB) and electron-accepting Lewis acids (LA) have been used in tandem to yield donor-acceptor complexes of inorganic tetrelenes LB·EH2·LA (E = Si, Ge, and Sn). Herein, we introduce the new germanium (II) dihydride adducts ImMe2·GeH2·BH3 (ImMe2 = (HCNMe)2C:) and ImiPr2Me2·GeH2·BH3 (ImiPr2Me2 = (MeCNiPr)2C:), with the former complex containing nearly 40 wt % germanium. The thermal release of bulk germanium from ImMe2·GeH2·BH3 (and its deuterated isotopologue ImMe2·GeD2·BD3) was examined in solution, and a combined kinetic and computational investigation was undertaken to probe the mechanism by which Ge is liberated. Moreover, the thermolysis of ImMe2·GeH2·BH3 in solution cleanly affords conformal nanodimensional layers of germanium as thin films of variable thicknesses (20-70 nm) on silicon wafers. We also conducted a computational investigation into potential decomposition pathways for the germanium(II)- and tin(II)-dihydride complexes NHC·EH2·BH3 (NHC = [(HCNR)2C:]; R = 2,6-iPr2C6H3 (Dipp), Me, and H; and E = Ge and Sn). Overall, this study introduces a mild and convenient solution-only protocol for the deposition of thin films of Ge, a widely used semiconductor in materials research and industry.