Project description:Polycyclic aromatic hydrocarbons (PAHs) that contain both five- and seven-membered rings are rare, and those where these rings are annulated to each other and build azulene units have, to date, mainly been generated in minute amounts on surfaces. Herein, a rational approach to synthesize soluble contorted PAHs containing two embedded azulene units in the bulk is presented. By stepwise detachment of tert-butyl groups, a series of three azulene embedded PAHs with different degrees of contortion has been made to study the impact of curvature on aromaticity and conjugation. Furthermore, the azulene PAHs showed high fluorescence quantum yields in the NIR regime.
Project description:A contorted polycyclic aromatic hydrocarbon (PAH) in the shape of a monkey saddle has been synthesized in three steps from a readily available truxene precursor. The monkey saddle PAH is consisting of three five-, seven six-, and three eight-membered rings and has been unambiguously characterized by single-crystal X-ray diffraction. Owing to the three biaryl axes the monkey saddle PAH is inherently chiral. The inversion of the two enantiomeric structures into each other preferably occurs through a twisting of peripheral rings rather than by a fully planar intermediate, as has been calculated by DFT methods. Enantiomers were separated by chiral HPLC and inversion barriers determined by variable temperature circular dichroism spectroscopy, supporting the twisting mechanism.
Project description:Conjugated nanohoops allow to investigate the effect of radial conjugation and bending on the involved π-systems. They can possess unexpected optoelectronic properties and their radially oriented π-system makes them attractive for host-guest chemistry. Bending the π-subsystems can lead to chiral hoops. Herein, we report the stereoselective synthesis of two enantiomers of chiral conjugated nanohoops by incorporating dibenzo[a,e]pentalenes (DBPs), which are generated in the last synthetic step from enantiomerically pure diketone precursors. Owing to its bent shape, this diketone unit was used as the only bent precursor and novel "corner unit" in the synthesis of the hoops. The [6]DBP[4]Ph-hoops contain six antiaromatic DBP units and four bridging phenylene groups. The small HOMO-LUMO gap and ambipolar electrochemical character of the DBP units is reflected in the optoelectronic properties of the hoop. Electronic circular dichroism spectra and MD simulations showed that the chiral hoop did not racemize even when heated to 110 °C. Due to its large diameter, it was able to accommodate two C60 molecules, as binding studies indicate.
Project description:Nanographenes have kindled considerable interest in the fields of materials science and supramolecular chemistry as a result of their unique self-assembling and optoelectronic properties. Encapsulating the contorted nanographenes inside artificial receptors, however, remains challenging. Herein, we report the design and synthesis of a trigonal prismatic hexacationic cage, which has a large cavity and adopts a relatively flexible conformation. It serves as a receptor, not only for planar coronene, but also for contorted nanographene derivatives with diameters of approximately 15 Å and thicknesses of 7 Å. A comprehensive investigation of the host-guest interactions in the solid, solution and gaseous states by experimentation and theoretical calculations reveals collectively an induced-fit binding mechanism with high binding affinities between the cage and the nanographenes. Notably, the photostability of the nanographenes is improved significantly by the ultrafast deactivation of their excited states within the cage. Encapsulating the contorted nanographenes inside the cage provides a noncovalent strategy for regulating their photoreactivity.
Project description:The first example of a chiral halogen-bond donor with a sp3-hybridized carbon-iodine moiety in a fluorobissulfonyl scaffold is described. The binaphthyl backbone was designed as a chiral source and the chiral halogen-bond donor (R)-1 was synthesized from (R)-1,1'-binaphthol in 11 steps. An NMR titration experiment demonstrated that (R)-1 worked as a halogen-bond donor. The Mukaiyama aldol reaction and quinoline reduction were examined using (R)-1 as a catalyst to evaluate the asymmetric induction.
Project description:The enzymatic ?-C-H hydroxylation of the feedstock chemical isobutyric acid has enabled the asymmetric synthesis of a wide variety of polyketides. The analogous transition metal-catalyzed enantioselective ?-C-H functionalization of isobutyric acid-derived substrates should provide a versatile method for constructing useful building blocks with enantioenriched ?-chiral centers from this abundant C-4 skeleton. However, the desymmetrization of ubiquitous isopropyl moieties by organometallic catalysts has remained an unanswered challenge. Herein, we report the design of chiral mono-protected aminomethyl oxazoline ligands that enable desymmetrization of isopropyl groups via palladium insertion into the C(sp3)-H bonds of one of the prochiral methyl groups. We detail the enantioselective ?-arylation, -alkenylation, and -alkynylation of isobutyric acid/2-aminoisobutyric acid derivatives, which may serve as a platform for the construction of ?-chiral centers.
Project description:Intimately connected to the rule of life, chirality remains a long-time fascination in biology, chemistry, physics and materials science. Chiral structures, e.g., nucleic acid and cholesteric phase developed from chiral molecules are common in nature and synthetic soft materials. While it was recently discovered that achiral but bent-core mesogens can also form chiral helices, the assembly of chiral microstructures from achiral polymers has rarely been explored. Here, we reveal chiral emergence from achiral conjugated polymers, in which hierarchical helical structures are developed through a multistep assembly pathway. Upon increasing concentration beyond a threshold volume fraction, dispersed polymer nanofibers form lyotropic liquid crystalline (LC) mesophases with complex, chiral morphologies. Combining imaging, X-ray and spectroscopy techniques with molecular simulations, we demonstrate that this structural evolution arises from torsional polymer molecules which induce multiscale helical assembly, progressing from nano- to micron scale helical structures as the solution concentration increases. This study unveils a previously unknown complex state of matter for conjugated polymers that can pave way to a field of chiral (opto)electronics. We anticipate that hierarchical chiral helical structures can profoundly impact how conjugated polymers interact with light, transport charges, and transduce signals from biomolecular interactions and even give rise to properties unimagined before.
Project description:Degradation experiments and model studies suggested that the longer lived green fluorescence from an aggregated poly(p-phenylene ethynylene) (PPE) was due to the presence of highly emissive, low-energy, anthryl defect sites rather than the emissive conjugated polymer excimers proposed in a previous report. After elucidating the origin of the green fluorescence, additional anthryl units were purposely incorporated into the polymer to enhance the blue-to-green fluorescence color change that accompanied polymer aggregation. The improved color contrast from this anthryl-doped conjugated polymer led to the development of crude solution-state and solid-state sensors, which, upon exposure to water, exhibited a visually noticeable blue-to-green fluorescence color change.
Project description:We report an air-stable diradical dication of chiral D2-symmetric conjoined bis[5]diazahelicene with an unprecedented high-spin (triplet) ground state, singlet triplet energy gap, ΔEST = 0.3 kcal mol-1. The diradical dication possesses closed-shell (Kekulé) resonance forms with 16 π-electron perimeters. The diradical dication is monomeric in dibutyl phthalate (DBP) matrix at low temperatures, and it has a half-life of more than 2 weeks at ambient conditions in the presence of excess oxidant. A barrier of ∼35 kcal mol-1 has been experimentally determined for inversion of configuration in the neutral conjoined bis[5]diazahelicene, while the inversion barriers in its radical cation and diradical dication were predicted by the DFT computations to be within a few kcal mol-1 of that in the neutral species. Chiral HPLC resolution provides the chiral D2-symmetric conjoined bis[5]diazahelicene, enriched in (P,P)- or (M,M)-enantiomers. The enantiomerically enriched triplet diradical dication is configurationally stable for 48 h at room temperature, thus providing the lower limit for inversion barrier of configuration of 27 kcal mol-1. The enantiomers of conjoined bis[5]diazahelicene and its diradical dication show strong chirooptical properties that are comparable to [6]helicene or carbon-sulfur [7]helicene, as determined by the anisotropy factors, |g| = |Δε|/ε = 0.007 at 348 nm (neutral) and |g| = 0.005 at 385 nm (diradical dication). DFT computations of the radical cation suggest that SOMO and HOMO energy levels are near-degenerate.