Isomerism tunes the diradical character of difluorenopyrroles at constant Hückel-level anti-aromaticity.

A new diradical based on diindenocarbazole or difluorenopyrrole was synthesized and experimentally characterized by optical, electrochemical, and magnetic techniques, as well as quantum chemical calculations. The isomerism of these structures tunes the diradical character and the associated properties, representing a unique case of such important modulation. A full study of the electronic structure was carried out considering the perturbative interactions between different canonical forms as well as the anti-aromatic character of the molecular cores. Such a study reveals how we can tune diradical character simply by reorganizing the bonding patterns at constant chemical costs (composition).

Site-selective radical reactions of kinetically stable open-shell singlet diradicaloid difluorenoheteroles with tributyltin hydride and azo-based radical initiators.

We have demonstrated site-selective radical reactions of the kinetically stable open-shell singlet diradicaloids difluoreno[3,4-b:4',3'-d]thiophene (DFTh) and difluoreno[3,4-b:4',3'-d]furan (DFFu) with tributyltin hydride (HSn(n-Bu)3) and azo-based radical initiators. Treatment of these diradicaloids with HSn(n-Bu)3 induces hydrogenation at the ipso-carbon in the five-membered rings, while treatment with 2,2'-azobis(isobutyronitrile) (AIBN) induces substitution at the carbon atoms in the peripheral six-membered rings. We have also developed one-pot substitution/hydrogenation reactions of DFTh/DFFu with various azo-based radical initiators and HSn(n-Bu)3. The resulting products can be converted into substituted DFTh/DFFu derivatives via dehydrogenation. Theoretical calculations unveiled a detailed mechanism of the radical reactions of DFTh/DFFu with HSn(n-Bu)3 and with AIBN, and that the site-selectivity of these radical reactions is controlled by the balance of the spin density and the steric hindrance in DFTh/DFFu.

A double-helical S,C-bridged tetraphenyl-para-phenylenediamine and its persistent radical cation.

A structurally constrained, double-helical S,C-bridged tetraphenyl-para-phenylenediamine (TPPD) has been synthesized. The stable radical cation of the S,C-bridged TPPD was generated by chemical oxidation, and the electron spin was found to be delocalized over the entire π-conjugated framework. The excellent conformational stability of the neutral molecule facilitated the separation of its enantiomers.

Medium Diradical Character, Small Hole and Electron Reorganization Energies and Ambipolar Transistors in Difluorenoheteroles.

Four difluorenoheteroles having a central quinoidal core with the heteroring varying as furan, thiophene, its dioxide derivative and pyrrole have shown to be medium character diradicals. Solid-state structures, optical, photophysical, magnetic, and electrochemical properties have been discussed in terms of diradical character, variation of aromatic character and captodative effects (electron affinity). Organic field-effect transistors (OFETs) have been prepared, showing balanced hole and electron mobilities of the order of 10-3  cm2 V-1 s-1 or ambipolar charge transport which is first inferred from their redox amphoterism. Quantum chemical calculations show that the electrical behavior is originated from the medium diradical character which produces similar reorganization energies for hole and electron transports. The vision of a diradical as simultaneously bearing pseudo-hole and pseudo-electron defects might justify the reduced values of reorganization energies for both regimes. Structure-function relationships between diradical and ambipolar electrical behavior are revealed.

Augmented Self-Association by Electrostatic Forces in Thienopyrrole-Fused Thiadiazoles that Contain an Ester instead of an Ether Linker.

During the self-assembly of π-conjugated molecules, linkers and substituents can potentially add supportive noncovalent intermolecular interactions to π-stacking interactions. Here, we report the self-assembly behavior of thienopyrrole-fused thiadiazole (TPT) fluorescent dyes that possess ester or ether linkers and dodecyloxy side chains in solution and the condensed phase. A comparison of the self-association behavior of the ester- and ether-bridged compounds in solution using detailed UV-vis, fluorescence, and NMR spectroscopic studies revealed that the subtle replacement of the ether linkers by ester linkers leads to a distinct increase in the association constant (ca. 3-4 fold) and the enthalpic contribution (ca. 3 kcal mol-1 ). Theoretical calculations suggest that the ester linkers, which are in close proximity to one another due to the π-stacking interactions, induce attractive electrostatic forces and augment self-association. The self-assembly of TPT dyes into well-defined 1D clusters with high aspect ratios was observed, and their morphologies and crystallinity were investigated using SEM and X-ray diffraction analyses. TPTs with ester linkers exhibit a columnar liquid crystalline mesophase in the condensed phase.

S,C,C- and O,C,C-Bridged Triarylamines and Their Persistent Radical Cations.

This work reports the synthesis, crystal structures, and electronic properties of structurally constrained S,C,C- and O,C,C-bridged triarylamine derivatives and their persistent radical cations. O,C,C-Bridged triphenylamines and a dinaphthylphenylamine were obtained through a straightforward synthetic protocol. Similar to a previously reported S,C,C-bridged triphenylamine, the O,C,C-bridged triarylamines were easily oxidized to afford the corresponding radical cations, which were obtained as hexachloroantimonate salts. X-ray crystallographic analyses showed almost planar structures for these O,C,C-bridged triarylamine radical cations, which represent new members of the family of planar triarylamine radical cations without substituents on the aryl rings. Detailed investigations of the electronic properties of the S,C,C- and O,C,C-bridged triarylamine radical cations demonstrated that the spin and positive charge are sufficiently delocalized over the planar triarylamine scaffolds. The results provide the following insights into the effects of the bridging unit (sulfur vs oxygen) and the dibenzo-annulation on the spin delocalization in the bridged triarylamine radical cations: (1) An effective decrease of the spin density on the nitrogen atom is observed for the sulfur bridge relative to the oxygen bridge; and (2) a moderate decrease of the spin density on the oxygen atom rather than the nitrogen atom is induced by the dibenzo-annulation.

Open-shell singlet diradicaloid difluoreno[4,3-b:3',4'-d]furan and its radical cation and dianion.

Difluoreno[4,3-b:3',4'-d]furan (DFFu) was prepared via a short, scalable, multi-step synthesis, and characterized by crystallography, spectroscopic measurements, and theoretical calculations. Even though the results revealed an open-shell singlet diradical character, DFFu is stable under ambient conditions. The radical cationic and dianionic species of DFFu were also synthesized and characterized.

Synthesis, Structures, and Properties of Neutral and Radical Cationic S,C,C-Bridged Triphenylamines.

A structurally constrained S,C,C-bridged triphenylamine was synthesized, and the corresponding radical cation was obtained as a hexachloroantimonate by chemical oxidation. An X-ray crystallographic analysis revealed an almost planar structure for this radical cation, which thus represents the first example of a planar, para-unsubstituted triphenylamine radical cation analogue with a sulfur bridge. The electronic properties of the radical cation were examined by UV-vis-NIR and ESR spectroscopy as well as DFT calculations.

Controlling the Emergence and Shift Direction of Mechanochromic Luminescence Color of a Pyridine-Terminated Compound.

In this study, mechanochromic luminescence was induced in a complex of mechano-inactive compounds. Dye/acid complexes containing the same π-conjugated backbones were prepared. While the luminophore showed blue and red shifts in photoluminescence spectra when combined with different acids by grinding, it exhibited slight mechanoresponsiveness itself. Also, compounds with similar molecular backbones to the dye/acid complex were synthesized to clarify the color change mechanism. The compounds showed both blue and red shifts in photoluminescence and diffuse reflectance spectra upon grinding, indicating that mechanochromic luminescence in the hydrogen-bonded complex is like its monomeric analogue and that aggregation structure plays an important role in mechanoresponsive behavior rather than the π-conjugated structure. It was shown that a color change can be mechanically induced by imitating the solid-state aggregation structure of other mechanoresponsive compounds without synthetic modification.

2,4,5,7,9,10-Hexaethynylpyrenes: Synthesis, Properties, and Self-Assembly.

A series of 2,4,5,7,9,10-hexaethynylpyrenes was synthesized using 2,7,9,10-tetrabromopyrene-4,5-dione as the key intermediate. The effects of the position and number of the ethynyl groups on the physicochemical properties of the corresponding pyrenes were clarified by comparison with 4,5,9,10-tetraethynylpyrene and 2,7-diethynylpyrene derivatives. The prepared hexaethynylpyrenes that bear benzene moieties self-assemble via π-π stacking in solution and/or the condensed phase.

Synthesis and Properties of a Decacyclene Monoimide and a Naphthalimide Derivative as Three-Dimensional Acceptor-Donor-Acceptor Systems.

A method involving the Diels-Alder (DA) cycloaddition of diacenaphtheno[1,2-b;1',2'-d]thiophenes (DATs) with N-alkylacenaphthylene-5,6-dicarboximides (AIs) was developed to synthesize decacyclene monoimides (DCMIs). The reactions generate the corresponding 1:2 adducts (BAIAs) as major products together with 1:1 adducts (the DCMIs). The molecular structure of BAIAb (N-octyl derivative) was unambiguously assigned as the bis-adduct having an endo,endo spatial disposition of the two acenaphthylene-5,6-dicarboximide moieties by using X-ray crystallographic analysis. Relative to the absorption spectrum of decacyclene triimide (DCTIa, N-2-ethylhexyl derivative), that of the analogous N-2-ethylhexyl-substituted monoadduct, DCMIa, is bathochromically shifted despite the fact that it possesses a less delocalized π-electron system. DCMIa does not fluoresce in various organic solvents, whereas DCTIa emits yellow fluorescence in CH2 Cl2 with a low quantum yield (ΦSN ). Moreover, DCMIa in CDCl3 displays concentration-dependent 1 H NMR spectroscopy behavior, which suggests that it self-aggregates with an association constant (Ka ) of (193±50) m-1 at 20 °C. Despite the presence of four bulky tert-butyl groups in DCMIa, its Ka value for aggregate formation is comparable to that of DCTIa [(495±42) m-1 ], which does not contain tert-butyl substituents. Spectroscopic studies with the bis-adduct BAIAa (N-2-ethylhexyl derivative) show that it displays remarkable solvatofluorochromism corresponding to an emission maximum shift (ΔλEM ) of 100 nm. The results of density functional theory calculations on BAIAc (N-methyl derivative) demonstrate that a considerable spatial separation exists between the HOMO and LUMO coefficient distributions, which indicates that the ground-to-excited state transition of the novel three-dimensional acceptor-donor-acceptor BAIAa system should have intramolecular charge-transfer character.

Crystal structures of 1-hy-droxy-4-prop-yloxy-9,10-anthra-quinone and its acetyl derivative.

1-Hy-droxy-4-prop-yloxy-9,10-anthra-quinone, C17H14O4, (I), and its acetyl derivative, 4-acet-yloxy-4-prop-yloxy-9,10-anthra-quinone, C19H16O5, (II), were synthesized from the commercially available dye quinizarin. In both compounds, the anthra-quinone frameworks are close to planarity. There is a large difference in the conformation of the prop-yloxy group; the mol-ecule of (I) adopts a gauche conformation [O-C-C-C = -64.4 (2)°], although the mol-ecule of (II) takes a trans-planar conformation (zigzag) [O-C-C-C = 176.1 (3)°]. In the mol-ecule of (I), there is an intra-molecular O-H⋯O hydrogen bond. In both crystals, the mol-ecules are linked by C-H ⋯O hydrogen bonds. A difference in the mol-ecular arrangements of (I) and (II) is found along the stacking directions.

Isolation and spectral characterization of thermally generated multi-Z-isomers of lycopene and the theoretically preferred pathway to di-Z-isomers.

Lycopene has a large number of geometric isomers caused by E/Z isomerization at arbitrary sites within the 11 conjugated double bonds, offering varying characteristics related to features such as antioxidant capacity and bioavailability. However, the geometric structures of only a few lycopene Z-isomers have been thoroughly identified from natural sources. In this study, seven multi-Z-isomers of lycopene, (9Z,13'Z)-, (5Z,13Z,9'Z)-, (9Z,9'Z)-, (5Z,13'Z)-, (5Z,9'Z)-, (5Z,9Z,5'Z)-, and (5Z,9Z)-lycopene, were obtained from tomato samples by thermal isomerization, and then isolated by elaborate chromatography, and fully assigned using proton nuclear magnetic resonance. Moreover, the theoretically preferred pathway from (all-E)-lycopene to di-Z-isomers was examined with a computational approach using a Gaussian program. Fine-tuning of the HPLC separation conditions led to the discovery of novel multi-Z-isomers, and whose formation was supported by advanced theoretical calculations.

Chiral Pyridinium Phosphoramide as a Dual Brønsted Acid Catalyst for Enantioselective Diels-Alder Reaction.

Chiral pyridinium phosphoramide 1·HX was designed to be a new class of chiral Brønsted acid catalyst in which both the pyridinium proton and the adjacent imide-like proton activated by the electron-withdrawing pyridinium moiety could work cooperatively as strong dual proton donors. The potential of 1·HX was shown in the enantioselective Diels-Alder reactions of 1-amino dienes with various dienophiles including N-unsubstituted maleimide, which has yet to be successfully used in an asymmetric Diels-Alder reaction.

Isolation and characterization of (15Z)-lycopene thermally generated from a natural source.

(15Z)-Lycopene was prepared by thermal isomerization of (all-E)-lycopene derived from tomatoes, and isolated by using a series of chromatographies. The fine red crystalline powder of (15Z)-lycopene was obtained from 556 mg of (all-E)-lycopene with a yield of 0.6 mg (purity: reversed-phase HPLC, 97.2%; normal-phase HPLC, ≥99.9%), and (1)H and (13)C NMR spectra of the isomer were fully assigned. More refined computational analyses that considered differences in the energy levels of the conformers involved in isomerization have also determined the stabilities of (15Z)-lycopene and other geometric isomers, along with the activation energies during isomerization from the all-E form. The fine control of conditions for HPLC separation and an advanced theoretical insight into geometric isomerization have led to the discovery of the 15Z-isomer generated from a natural source.

Crystal structure of 1,4-dieth-oxy-9,10-anthra-quinone.

The asymmetric unit of the title compound, C18H16O4, contains two crystallographically independent mol-ecules. The anthra-quinone ring systems are slightly bent with dihedral angles of 2.33 (8) and 13.31 (9)° between the two terminal benzene rings. In the crystal, the two independent mol-ecules adopt slipped-parallel π-overlap with an average inter-planar distance of 3.45 Å, forming a dimer; the centroid-centroid distances of the π-π inter-actions are 3.6659 (15)-3.8987 (15) Å. The mol-ecules are also linked by C-H⋯O inter-actions, forming a tape structure along the a-axis direction. The crystal packing is characterized by a dimer-herringbone pattern.

Crystal structure of 2-bromo-1,4-dihy-droxy-9,10-anthra-quinone.

In an attempt to brominate 1,4-diprop-oxy-9,10-anthra-quinone, a mixture of products, including the title compound, C14H7BrO4, was obtained. The mol-ecule is essentially planar (r.m.s. deviation = 0.029 Å) and two intra-molecular O-H⋯O hydrogen bonds occur. In the crystal, the mol-ecules are linked by weak C-H⋯O hydrogen bonds, Br⋯O contacts [3.240 (5) Å], and π-π stacking inter-actions [shortest centroid-centroid separation = 3.562 (4) Å], generating a three-dimensional network.

Photosensitized E/Z isomerization of (all-E)-lycopene aiming at practical applications.

Photoisomerization of (all-E)-lycopene to the corresponding Z-isomers was investigated under visible to middle-infrared light irradiation in the presence of several sensitizers, including edible ones. Highly purified (all-E)-lycopene from tomato paste was isomerized to Z-isomers to the extent of 46.4-57.4% after irradiation with the sensitizers for 60 min in acetone, in which a thermodynamically stable isomer of (5Z)-lycopene was predominantly generated, whereas kinetically preferable (9Z)- and (13Z)-lycopenes were dominant without sensitizer. Examination of the time course of photoisomerization demonstrated that the highest isomerization efficiency (80.4%) was attained using erythrosine as the sensitizer under 480-600 nm light irradiation in hexane for 60 min, a protocol that successfully suppressed the degradation of lycopene. (5Z)-Lycopene, reported as a more bioavailable isomer, was again predominantly produced with erythrosine and rose bengal in each solvent.

Data mining with molecular design rules identifies new class of dyes for dye-sensitised solar cells.

A major deficit in suitable dyes is stifling progress in the dye-sensitised solar cell (DSC) industry. Materials discovery strategies have afforded numerous new dyes; yet, corresponding solution-based DSC device performance has little improved upon 11% efficiency, achieved using the N719 dye over two decades ago. Research on these dyes has nevertheless revealed relationships between the molecular structure of dyes and their associated DSC efficiency. Here, such structure-property relationships have been codified in the form of molecular dye design rules, which have been judiciously sequenced in an algorithm to enable large-scale data mining of dye structures with optimal DSC performance. This affords, for the first time, a DSC-specific dye-discovery strategy that predicts new classes of dyes from surveying a representative set of chemical space. A lead material from these predictions is experimentally validated, showing DSC efficiency that is comparable to many well-known organic dyes. This demonstrates the power of this approach.

Characterization and thermal isomerization of (all-E)-lycopene.

A large amount of (all-E)-lycopene was successfully purified from tomato paste using an improved method that included a procedure to wash crystalline powder with acetone. The total yield of the pure (all-E) form was at least 30%. The melting point of (all-E)-lycopene was determined to be 176.35 °C by differential scanning calorimetry (DSC) measurements. Bathochromic shifts were observed in the absorption maxima of all solvents tested (at most a 36 nm shift for λ2 in carbon disulfide, as was observed in hexane) and were accompanied by absorbance decreases, namely, a hypochromic effect, showing a higher correlation between the position and the intensity of the main absorption bands. This bathochromic shift was dependent upon the polarizability of the solvent rather than its polarity. The structure of (all-E)-lycopene in CDCl3 and C6D6 was identified on the basis of one- and two-dimensional nuclear magnetic resonance (NMR) spectra, including (1)H and (13)C NMR, homonuclear correlation spectroscopy ((1)H-(1)H COSY), heteronuclear multiple-quantum coherence (HMQC), and heteronuclear multiple-bond connectivity (HMBC). The rate constants of the decrease in (all-E)-lycopene with hexane and benzene were calculated to be 3.19 × 10(-5) and 3.55 × 10(-5) s(-1), respectively. The equilibrium constants between (all-E) and (13Z) isomers were estimated to be 0.29 in hexane and 0.31 in benzene, respectively, from the point at which the amount of (13Z)-lycopene reached its maximum.