Peripheral Fusion of Carbon-based Aromatic Rings to B4N4-Heteropentalene Leading to Close π-Stacking in the Solid State.

π-Electronic molecules with a BN-heterocyclic and carbon-based aromatic hybrid ring system (h-CBN) are interesting in that they potentially exhibit synergistic properties arising from the two different π-systems. Here we report the synthesis and properties of a h-CBN-type molecule (1) having a bicyclic B4N4-heteropentalene core fused with extended aromatic rings. This molecule exhibits excellent chemical stability despite the absence of bulky substituents for kinetic protection, which in turn provides effective stacking of the π-system upon crystallization. Depending on the crystallization solvent, 1 forms two polymorphs, i.e., the α- and ß-phases. While both phases have one-dimensional columnar structures, the π-stacking geometries associated with the transfer integrals of the frontier orbitals are different, resulting in a twofold difference in the electrical conducting properties. We also found that upon thermal vacuum deposition, 1 gives an amorphous film, which serves as a host material for a red phosphorescent OLED device (maximum external quantum efficiency: 15.5 and 13.3% at 0.1 and 2.5 mA, respectively).

Lead(II)-Based Coordination Polymer Exhibiting Reversible Color Switching and Selective CO2 Photoreduction Properties.

Herein, we report a new photofunctional Pb-S-based coordination polymer (CP) with the formula [Pb(ATAT)(OAc)]n (ATAT = 3-amino-5-mercapto-1,2,4-triazole, OAc = acetate, CP1). Apart from its photoactive one-dimensional (1D) (-Pb-S-)n chain, CP1 is also composed of another 1D (-Pb-O-)n chain that originates from the coordination with acetate. The coordinated acetate can be exchanged with water (H2O) or dimethyl sulfoxide (DMSO), leading to the formation of a CP1-H2O or CP1-DMSO structure that exhibits a distinct change in optical properties, including a white-to-yellow color change. The structural transformation of CP1 to CP1-H2O and CP1-DMSO, and its subsequent recovery to the original CP1 structure could be controlled by the presence or absence of acetic acid vapor; the transformation was completely reversible. CP1 absorbed light with wavelengths shorter than 390 nm, with an estimated bandgap of 3.18 eV. Density functional theory calculations indicated that the valence band of CP1 is mainly formed by N and S orbitals originating from the ATAT unit, whereas the conduction band is composed of the Pb orbitals. Even without any modification, such as the incorporation of a molecular catalyst, CP1 reduced CO2 into formate under UV light with >99% selectivity.

Photo-Responsive Hydrogen-Bonded Molecular Networks Capable of Retaining Crystalline Periodicity after Isomerization.

The molecular conformation, crystalline morphology, and properties of photochromic organic crystals can be controlled through photoirradiation, making them promising candidates for functional organic materials. However, photochromic porous molecular crystals with a networked framework structure are rare due to the difficulty in maintaining space that allows for photo-induced molecular motion in the crystalline state. This study describes a photo-responsive single crystal based on hydrogen-bonded (H-bonded) network of dihydrodimethylbenzo[e]pyrene derivative 4BDHP. A crystal composed of H-bonded undulate layers, 4BDHP-2, underwent photo-isomerization in the crystalline state due to loose stacking of the layers. Particularly, enantio-pure crystal (S,S)-4BDHP-2 allowed to reveal the structure of the photoisomerized crystal, in which the closed form (4BDHP) and open form (4CPD) were arranged alternately with keeping crystalline periodicity, although side reactions were also implied. The present proof-of-concept system of a photochromic framework that retains crystalline periodicity after photo-isomerization may provide new light-driven porous functional materials.

Expanded [2,1][n]Carbohelicenes with 15- and 17-Benzene Rings.

Expanded carbohelicenes with structures fused to 15- and 17-benzene were successfully synthesized. Establishing a new synthetic strategy is crucial to realize the development of longer expanded [2,1][n]helicenes with a kekulene-like projection drawing structure. This article describes the sequential integration of the π-elongating Wittig reaction of functionalized phenanthrene units and ring-fusing Yamamoto coupling for the synthesis of [2,1][15]helicenes and [2,1][17]helicenes. X-ray crystallographic structures, photophysical properties, and density functional theory (DFT) calculations revealed the unique characteristics of the synthesized expanded helicenes. Furthermore, because of the high enantiomerization barrier derived from a wide-range intrahelix π-π interaction, the optical resolution of [2,1][17]helicene was successfully achieved, and chiroptical properties such as circular dichroism and circularly polarized luminescence were elucidated for the first time as enantiomers of pristine [2,1][n]helicene core.

Supramolecular Recognition within a Nanosized "Buckytrap" That Exhibits Substantial Photoconductivity.

We report here a nanosized "buckytrap", 1, constructed from two bis-zinc(II) expanded-TTF (exTTF) porphyrin subunits. Two forms, 1a and 1b, differing in the axial ligands, H2O vs tetrahydrofuran (THF), were isolated and characterized. Discrete host-guest inclusion complexes are formed upon treatment with fullerenes as inferred from a single-crystal X-ray structural analyses of 1a with C70. The fullerene is found to be encapsulated within the inner pseudohexagonal cavity of 1a. In contrast, the corresponding free-base derivative (2) was found to form infinite ball-and-socket type supramolecular organic frameworks (3D-SOFs) with fullerenes, (2•C60)n or (2•C70)n. This difference is ascribed to the fact that in 1a and 1b the axial positions are blocked by a H2O or THF ligand. Emission spectroscopic studies supported a 1:1 host-guest binding stoichiometry, allowing association constants of (2.0 ± 0.5) × 104 M-1 and (4.3 ± 0.9) × 104 M-1 to be calculated for C60 and C70, respectively. Flash-photolysis time-resolved microwave conductivity (FP-TRMC) studies of solid films of the Zn-complex 1a revealed that the intrinsic charge carrier transport, i.e., pseudo-photoconductivity (ϕ∑µ), increases upon fullerene inclusion (e.g., ϕ∑µ = 1.53 × 10-4 cm2 V-1 s-1 for C60⊂(1a)2 and ϕ∑µ = 1.45 × 10-4 cm2 V-1 s-1 for C70⊂(1a)2 vs ϕ∑µ = 2.49 × 10-5 cm2 V-1 s-1 for 1a) at 298 K. These findings provide support for the notion that controlling the nature of self-assembly supramolecular constructs formed from exTTF-porphyrin dimers through metalation or choice of fullerene can be used to regulate key functional features, including photoconductivity.

3,11-Diaminodibenzo[a,j]phenazine: Synthesis, Properties, and Applications to Tröger's Base-Forming Ladder Polymerization.

A new class of diamino-substituted π-extended phenazine compound was synthesized, and its photophysical properties were investigated. The U-shaped diaminophenazine displayed photoluminescence in solution with moderate quantum yield. The diamino aromatic compound was found applicable to the poly-condensation with formaldehyde to form Tröger's base ladder polymer. The obtained microporous ladder polymer features high CO2 adsorption selectivity against N2 , most likely due to the presence of basic nitrogen atoms in the phenazine rings.

Inversion of Diaza[5]Helicenes Through an N-N Bond Breaking Pathway.

1,1',10,10'-Biphenothiazine and its S,S,S',S'-tetroxide are diaza[5]helicenes with N-N linkages. Kinetic experiments on racemization together with DFT calculations revealed that they undergo inversion through the N-N bond breaking pathway rather than the general conformational pathway. In these diaza[5]helicenes with this inversion mechanism, the reduction of electronic repulsion in the N-N bond by modification of S to SO2 at the outer position of the helix led to a significantly higher inversion barrier, 35.3 kcal/mol, compared to [5]helicene. 1,1',10,10'-Biphenothiazine S,S,S',S'-tetroxide was highly resistant to acid-mediated N-N bond breaking and racemization under acidic conditions.

Thermodynamic Control of Intramolecular Singlet Fission and Exciton Transport in Linear Tetracene Oligomers.

We newly synthesized a series of homo- and hetero-tetracene (Tc) oligomers to propose a molecular design strategy for the efficient exciton transport in linear oligomers by promoting correlated triplet pair (TT) dissociation and controlling sequential exciton trapping process of individual doubled triplet excitons (T+T) by intramolecular singlet fission. First, entropic gain effects on the number of Tc units are examined by comparing Tc-homo-oligomers [(Tc)n : n=2, 4, 6]. Then, a comparison of (Tc)n and Tc-hetero-oligomer [TcF3 -(Tc)4 -TcF3 ] reveals the vibronic coupling effect for entropic gain. Observed entropic effects on the T+T formation indicated that the exciton migration is rationalized by number of possible TT states increased both by increasing the number of Tc units and by the vibronic levels at the terminal TcF3 units. Finally, we successfully observed high-yield exciton trapping process (trapped triplet yield: ΦTrT =176 %).

Synthesis of the C70 Fragment Buckybowl, Homosumanene, and Heterahomosumanenes via Ring-Expansion Reactions from Sumanenone.

Bowl-shaped aromatic molecules, buckybowls, are attractive molecules because of the unique properties derived from their curved-π scaffolds. Doping heteroatoms into buckybowl frameworks is a powerful method to change their structural and electronical properties. Herein, we report the synthesis of C70 fragment buckybowl, homosumanene, and heterahomosumanenes having a lactone moiety and a lactam moiety via three ring-expansion reactions using sumanenone as a common intermediate. X-ray diffraction analysis of the single crystals reveals their columnar packing structure with a shallow bowl-depth. The lactam moiety is readily derivatized to give azahomosumanene derivatives, nitrogen-doped analogues of homosumanene possessing a pyridine ring at the peripheral carbon. The synthetic application of the α-phenyl azahomosumanene as a cyclometalating ligand with platinum also revealed its utility for preparing a metal complex bearing a buckybowl ligand.

Spatiotemporal Dynamics of Aggregation-Induced Emission Enhancement Controlled by Optical Manipulation.

We present spatiotemporal control of aggregation-induced emission enhancement (AIEE) of a protonated tetraphenylethene derivative by optical manipulation. A single submicrometer-sized aggregate is initially confined by laser irradiation when its fluorescence is hardly detectable. The continuous irradiation of the formed aggregate leads to sudden and rapid growth, resulting in bright yellow fluorescence emission. The fluorescence intensity at the peak wavelength of 540 nm is tremendously enhanced with growth, meaning that AIEE is activated by optical manipulation. Amazingly, the switching on/off of the activation of AIEE is arbitrarily controlled by alternating the laser power. This result means that optical manipulation increases the local concentration, which overcomes the electrostatic repulsion between the protonated molecules, namely, optical manipulation changes the aggregate structure. The dynamics and mechanism in AIEE controlled by optical manipulation will be discussed from the viewpoint of molecular conformation and association depending on the laser power.

Anisotropic Poisson Effect and Deformation-Induced Fluorescence Change of Elastic 9,10-Dibromoanthracene Single Crystals.

Elastic organic crystals have attracted considerable attention as next-generation flexible smart materials. However, the detailed information on both molecular packing change and macroscopic mechanical crystal deformations upon applied stress is still insufficient. Herein, we report that fluorescent single crystals of 9,10-dibromoanthracene are elastically bendable and stretchable, which allows a detailed investigation of the deformation behavior. We clearly observed a Poisson effect for the crystal, where the short axes (b and c-axes) of the crystal are contracted upon elongation along the long axis (a-axis). Moreover, we found that the Poisson's ratios along the b-axis and c-axis are largely different. Theoretical molecular simulation suggests that the tilting motion of the anthracene may be responsible for the large deformation along the c-axis. Spatially resolved photoluminescence (PL) measurement of the bent elastic crystals reveals that the PL spectra at the outer (elongated), central (neutral), and inner (contracted) sides are different from each other.

Triptycene Tripods for the Formation of Highly Uniform and Densely Packed Self-Assembled Monolayers with Controlled Molecular Orientation.

When employing self-assembled monolayers (SAMs) for tuning surface and interface properties, organic molecules that enable strong binding to the substrate, large-area structural uniformity, precise alignment of functional groups, and control of their density are highly desirable. To achieve these goals, tripod systems bearing multiple bonding sites have been developed as an alternative to conventional monodentate systems. Bonding of all three sites has, however, hardly been achieved, with the consequence that structural uniformity and orientational order in tripodal SAMs are usually quite poor. To overcome that problem, we designed 1,8,13-trimercaptomethyltriptycene (T1) and 1,8,13-trimercaptotriptycene (T2) as potential tripodal SAM precursors and investigated their adsorption behavior on Au(111) combining several advanced experimental techniques and state-of-the-art theoretical simulations. Both SAMs adopt dense, nested hexagonal structures but differ in their adsorption configurations and structural uniformity. While the T2-based SAM exhibits a low degree of order and noticeable deviation from the desired tripodal anchoring, all three anchoring groups of T1 are equally bonded to the surface as thiolates, resulting in an almost upright orientation of the benzene rings and large-area structural uniformity. These superior properties are attributed to the effect of conformationally flexible methylene linkers at the anchoring groups, absent in the case of T2. Both SAMs display interesting electronic properties, and, bearing in mind that the triptycene framework can be functionalized by tail groups in various positions and with high degree of alignment, especially T1 appears as an ideal docking platform for complex and highly functional molecular films.

Terminal Functionalization with a Triptycene Motif That Dramatically Changes the Structural and Physical Properties of an Amorphous Polymer.

A surprising terminal-group effect on the structural and physical properties of an amorphous polymer is reported. We recently demonstrated that triptycene derivatives with substituents at the 1,8,13-positions show specific self-assembly behavior, enabling the formation of a well-defined "2D + 1D" structure based on nested hexagonal packing of the triptycenes. Upon terminal functionalization with a 1,8-substituted triptycene (1,8-Trip), a liquid polymer, polydimethylsiloxane (PDMS, Mn = 18-24 kDa), turned into a highly viscous solid that exhibits birefringence at 25 °C. Small-angle and wide-angle X-ray scattering measurements revealed that the resulting telechelic PDMS assembles into a 2D + 1D structure, where layers of PDMS domains, formed between 2D assemblies of the triptycene termini, stack into a 1D multilayer structure with a layer spacing of 18-20 nm. Because of this structuring, the complex viscosity of the telechelic PDMS was dramatically enhanced, providing a value 4 orders of magnitude greater than that of the original PDMS. Remarkably, the structural and physical properties of PDMS were hardly changed upon terminal functionalization with another regioisomer of triptycene (1,4-Trip), which differs only in the substitution pattern.

Induction of Single-Handed Helicity of Polyacetylenes Using Mechanically Chiral Rotaxanes as Chiral Sources.

Effective induction of preferred-handed helicity of polyacetylenes by pendant mechanically chiral rotaxanes is discussed. Polyacetylenes possessing optically active mechanically chiral rotaxanes in the side chains were synthesized by the polymerization of the corresponding enantiopure [2]rotaxane-type ethynyl monomers prepared by the chiral-phase HPLC separations. The CD Cotton effects revealed that the polyacetylenes took preferred-handed helical conformations depending on the rotaxane chirality. The preferred-handed helix was not disturbed by an additional chiral substituent on the rotaxane side chain. These results demonstrate the significance and utility of mechanically chiral rotaxanes for the effective construction of asymmetric fields.

Supramolecular Scaffold for Tailoring the Two-Dimensional Assembly of Functional Molecular Units into Organic Thin Films.

Tailoring structurally anisotropic molecular assemblies while controlling their orientation on solid substrates is an important subject for advanced technologies that use organic thin films. Here we report a supramolecular scaffold based on tripodal triptycene assemblies, which enables functional molecular units to assemble into a highly oriented, multilayered two-dimensional (2D) structure on solid substrates. The triptycene building block carries an ethynyl group and three flexible side chains at the 10- and 1,8,13-positions, respectively. These bridgehead-substituted tripodal triptycenes self-assembled on solid substrates to form a well-defined "2D hexagonal + 1D lamellar" structure, which developed parallel to the surface of the substrates. Remarkably, the assembling properties of the triptycene building blocks, particularly for a derivative with tri(oxyethylene)-containing side chains, were not impaired when the alkyne terminal was functionalized with a large molecular unit such as C60, which is comparable in diameter to the triptycene framework. Consequently, thin films with a multilayered 2D assembly of the C60 unit were obtained. Flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements revealed that the C60 film exhibits highly anisotropic charge-transport properties. Bridgehead-substituted tripodal triptycenes may provide a versatile supramolecular scaffold for tailoring the 2D assembly of molecular units into a highly oriented thin film, and in turn for exploiting the full potential of anisotropic molecular functions.

2D hexagonal assembly of a dipolar rotor with a close interval of 0.8 nm using a triptycene-based supramolecular scaffold.

Controlling the rotation of carbon-carbon bonds, which is ubiquitous in organic molecules, to create functionality has been a subject of interest for a long time. In this context, it would be interesting to explore whether cooperative and collective rotation could occur if dipolar molecular rotors were aligned close together while leaving adequate space for rotation. However, it is difficult to realize such structures as bulk molecular assemblies, since molecules generally tend to assemble into the closest packing structure to maximize intermolecular forces. To tackle this question, we examined an approach using a supramolecular scaffold composed of a tripodal triptycene, which has been demonstrated to strongly promote the assembly of various molecular and polymer units into regular "2D hexagonal packing + 1D layer" structures. We found that a molecule (1) consisting of a dipolar 1,2-difluorobenzene rotor sandwiched by two 10-ethynyl-1,8,13-tridodecyloxy triptycenes, successfully self-assembles into the desired structure, where the dipolar rotor units align two-dimensionally at a close interval of approximately 0.8 nm while having a degree of freedom for rotational motion. Here we describe the self-assembly behavior of 1 in comparison with the general trend in molecular self-assembly, as well as the motility of the two-dimensionally aligned molecular rotors investigated using solid-state 19F-MAS NMR spectroscopy.

Chiral bifacial indacenodithiophene-based π-conjugated polymers with chirality-induced spin selectivity.

Novel optically active π-conjugated polymers having a chiral bifacial indacenodithiophene backbone were synthesized by Suzuki-Miyaura cross coupling polymerization with benzothiadiazole comonomer. The obtained C2-chiral polymers form amorphous thin films on HOPG and exhibit good chirality-induced spin selectivity with spin polarization of nearly 70%.

Supramolecular nanosheet formation-induced photosensitisation mechanism change of Rose Bengal dye in aqueous media.

Development of two-dimensional materials and exploration of their functionalities are significant challenges due to their potential. In this study, we successfully fabricated a supramolecular nanosheet composed of amphiphilic Rose Bengal dyes in an aqueous medium. Furthermore, we elucidated a distinct change in the photosensitisation mechanism induced by nanosheet formation.

Modification of Visible-Light-Responsive Pb2Ti2O5.4F1.2 with Metal Oxide Cocatalysts to Improve Photocatalytic O2 Evolution toward Z-Scheme Overall Water Splitting.

The development of a highly active photocatalyst for visible-light water splitting requires a high-quality semiconductor material and a cocatalyst, which promote both the migration of photogenerated charge carriers and surface redox reactions. In this work, a cocatalyst was loaded onto an oxyfluoride photocatalyst, Pb2Ti2O5.4F1.2, to improve the water oxidation activity. Among the metal oxides examined as cocatalysts, RuO2 was found to be the most suitable, and the O2 evolution activity depended on the preparation conditions for Ru/Pb2Ti2O5.4F1.2. The highest activity was obtained with RuCl3-impregnated Pb2Ti2O5.4F1.2 heated under a flow of H2 at 523 K. The H2-treated Ru/Pb2Ti2O5.4F1.2 showed an O2 evolution rate an order of magnitude higher than those for the analogues without the H2 treatment (e. g., RuO2/Pb2Ti2O5.4F1.2). Physicochemical analyses by X-ray absorption fine-structure spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and time-resolved microwave conductivity measurements indicated that the optimized photocatalyst contained partially reduced RuO2 species with a particle size of ~5 nm. These partially reduced species effectively trapped the photogenerated charge carriers and promoted the oxidation of water into O2. The optimized Ru/Pb2Ti2O5.4F1.2 could function as an O2-evolving photocatalyst in Z-scheme overall water splitting, in combination with an Ru-loaded, Rh-doped SrTiO3 photocatalyst.

Water-dispersible donor-acceptor-donor π-conjugated bolaamphiphiles enabling a humidity-responsive luminescence color change.

Novel water-dispersible donor-acceptor-donor π-conjugated bolaamphiphiles, having dibenzophenazine as the acceptor and heteroatom-bridged amphiphilic diarylamines as the donors, have been developed. The materials displayed a distinct photoluminescence color change in response to humidity in a poly(vinylalcohol) matrix.