Anthraquinodimethane-Based Molecular Switches Tethered by Four-Arm Star-like Polymers

Molecular switches that reversibly change their structures and physical properties are important for applications such as sensing and information processing at molecular scales. In order to avoid the intermolecular aggregation that is often detrimental to the stimuli-responses of molecular switches, previous studies of molecular switches have been often conducted in dilute solutions which are difficult for applications in solid-state devices. Here we report molecular design and synthesis that integrates anthraquinodimethane as molecular switching units into polymers with amenable processibility in solid states. Optical and electron spin resonance characterizations indicate that the four-arm polymers of poly(ϵ-caprolactone) or poly(D,L-lactide) tethered from anthraquinodimethane slow down the dynamics of the conformational switching between the folded and the twisted conformations, enhance the photoluminescence in solid states and impart materials with a small energy gap from singlet ground state to thermally accessible triplet state.

Synthesis and Properties of Boron-Containing Heteromerous Bistricyclic Aromatic Enes: Structural Effects on Thermodynamic Stability and Photoreactivity

Overcrowded bistricyclic aromatic enes (BAEs) have several conformations such as twisted and anti-folded conformers, and their stereochemistry and chromism have been studied in earnest. In this study, boron-containing heteromerous BAEs having various tricyclic structures were synthesized and their photophysical properties investigated. Single-crystal X-ray analysis revealed that the introduction of a rigid fluorene unit resulted in a twisted conformer, whereas the introduction of flexible units such as thioxanthene and 9,9-dimethyl-9,10-dihydroanthracene units resulted in an anti-folded conformer. The absorption spectra of the heteromerous BAEs were dependent on the introduced tricyclic structures, suggesting the immense impact of the tricyclic structures on the electronic structures of BAEs. DFT calculations revealed the large effect of the flexibility of the tricyclic structures on the thermodynamic stability of the conformers. In addition, the boron-containing heteromerous BAEs underwent photocyclization reactions, indicating their potential application as precursors of polyaromatic hydrocarbons and helical aromatic materials.

Tetrafluorenofulvalene as a sterically frustrated open-shell alkene

Electronic and steric effects are known to greatly influence the structure, characteristics and reactivity of organic compounds. A typical π bond is weakened by oxidation (corresponding to the removal of electrons from bonding orbitals), by reduction (through addition of electrons to antibonding orbitals) and by unpairing of the bonding electrons, such as in the triplet state. Here we describe tetrafluorenofulvalene (TFF), a twisted, open-shell alkene for which these general rules do not hold. Through the synthesis, experimental characterization and computational analysis of its charged species spanning seven redox states, the central alkene bond in TFF is shown to become substantially stronger in the tri- and tetraanion, generated by chemical reduction. Furthermore, although its triplet state contains a weaker alkene bond than the singlet, in the quintet state its bond order increases substantially, yielding a flatter structure. This behaviour originates from the doubly bifurcated topology of the underlying spin system and can be rationalized by the balancing effects of benzenoid aromaticity and spin pairing.

Sterically Frustrated Aromatic Enes with Various Colors Originating from Multiple Folded and Twisted Conformations in Crystal Polymorphs

Overcrowded ethylenes composed of 10-methyleneanthrone and two bulky aromatic rings contain a twisted carbon-carbon double (C=C) bond as well as a folded anthrone unit. As such, they are unique frustrated aromatic enes (FAEs). Various colored crystals of these FAEs, obtained in different solvents, correspond to multiple metastable conformations of the FAEs with various twist and fold angles of the C=C bond, as well as various dihedral angles of attached aryl units with respect to the C=C bond. The relationships between color and these parameters associated with conformational features around the C=C bond were elucidated in experimental and computational studies. Owing to the fact that they are separated by small energy barriers, the variously colored conformations in the FAE crystal change in response to various external stimuli, such as mechanical grinding, hydrostatic pressure and thermal heating.

Tunable Solid-State Thermochromism: Alkyl Chain Length-Dependent Conformational Isomerization of Bianthrones

The introduction of linear alkoxy chains into bianthrone promoted the aggregation of the minor twisted conformer and made it compete with the major folded conformer in the solid-state. Rapid evaporation approach to the methoxy- and n -butoxy-substituted derivatives provided powder mixtures of folded and twisted conformers as metastable solids. Upon heating the powders, different lengths of alkyl chains converged to different conformers via solid-state thermal reaction with an apparent color change. Monitoring by FT-IR spectroscopy and powder XRD ensured that these thermal reactions involve only conformational isomerization without pyrolysis. In addition, single crystal X-ray structure analysis revealed that twisted conformers with longer alkyl chains are stabilized in the solid-state by the synergistic effect of their inter-chain hydrophobic interactions and π-π stacking interactions.

Synthesis of gem-Dimethylcyclopentane-Fused Arenes with Various Topologies via TBD-Mediated Dehydro-Diels-Alder Reaction

The development of efficient methods for the synthesis of substituted polycyclic arenes with various topologies is in high demand due to their excellent electrical and optical properties. In this work, a series of gem-dimethylcyclopentane-fused arenes with more than ten topologies were synthesized via a 1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD)-mediated dehydro-Diels-Alder reaction with moderate to good yields. The introduction of the near-planar gem-dimethylcyclopentane moiety not only impacts the molecular conjugative system but also regulates the intermolecular π-π interactions and crystal packing, which are critical for the photoelectric performance of arenes. The photophysical properties, molecular geometry, molecular packing of these compounds, and electrochemical properties were investigated by UV-vis absorption, fluorescence emission spectra, DFT calculations, single-crystal X-ray structure analysis, and cyclic voltammetry study.

Dynamics of Meso-Chiral Interconversion in a Butterfly-Shape Overcrowded Alkene Rotor Tunable by Solvent Properties

Elucidation of dynamics of molecular rotational motion is an essential part and challenging area of research. We demonstrate reversible diastereomeric interconversion of a molecular rotor composed of overcrowded butterfly-shape alkene (FDF). Its inherent dual rotatory motion (two rotors, one stator) with interconversion between two diastereomers, chiral trans-FDF and meso cis-FDF forms, has been examined in detail upon varying temperatures and solvents. The free energy profile of 180° revolution of one rotor part has a bimodal shape with unevenly positioned maxima (transition states). FDF in aromatic solvents adopts preferentially meso cis-conformation, while in non-aromatic solvents a chiral trans-conformation is more abundant owing to the solvent interactions with peripheral hexyl chains (solvophobic effect). Moderate correlations between the trans-FDF/cis-FDF ratio and solvent parameters, such as refractive index, polarizability, and viscosity were found.

Biradicaloid Behavior of a Twisted Double Bond

A compound with a highly twisted C═C bond in the intermediate bond dissociation region was isolated, and its biradicaloid nature was experimentally demonstrated. A bianthrone derivative substituted with long-chain alkoxy groups was synthesized, and its twisted conformer was assembled into crystals through hydrophobic intermolecular interactions of the introduced alkoxy chains. X-ray crystallography revealed that the C═C bond connecting the two anthrone rings has a long bond length (1.429 Å) and a large torsional angle (57.33°). In addition, ESR studies revealed that the twisted bianthrone has biradicaloid character with a small singlet-triplet energy gap of 23.8 kJ mol^-1. The thermally excited triplet species were observed in the ESR spectrum as distinct fine structures along with a ΔM_S = ±2 forbidden transition.

Thiophene-based twisted bistricyclic aromatic ene with tricoordinate boron: a new n-type semiconductor

The incorporation of tricoordinate boron into conjugated systems is of current interest in the field of organic electronics. In this study, a tricoordinate boron-embedded thiophene-based bistricyclic aromatic ene (BAE) was synthesized as a new boron-containing conjugated system. The combination of tricoordinate boron and fused thiophene rings imposed the twisted conformation in the BAE structure, resulting in the narrow energy absorption with the low-lying LUMO. Preliminary studies on the application of the highly electron-deficient boron-embedded BAE to organic field-effect transistors (OFETs) were also performed, revealing its moderately high electron mobility.

Ring-expansion approach towards extended asymmetric benzopentafulvalenes: overcrowded olefinic structure and chain length-dependent properties

A strain energy-induced ring-expansion is described to construct novel asymmetric benzopentafulvalenes precisely, which show chain length-dependent physical properties and air-stable ambipolar carrier transport.

Control over chiroptical and electronic properties of poly(9,9′-bifluorenylidene) bearing chiral side chains

The introduction of chiral side chains at peripheral positions of the 9,9′-bifluorenylidene moiety causes an efficient bias to its dynamic molecular helicity, which helps control the chiroptical and electronic properties of resultant polymers.

Aromatic polymers made by reductive polydehalogenation of oligocyclic monomers as conjugated polymers of intrinsic microporosity (C-PIMs)

Incorporation of tetrabenzohepta- or -pentafulvalene connectors into soluble, aromatic polymers results in significantly different optical spectra and intrinsic microporosity.

A fluorenylidene-acridane that becomes dark in color upon grinding - ground state mechanochromism by conformational change

We report mechanochromic color change controlled by conformational change (folded and twisted conformers) of fluorenylidene-acridanes (FAs).

We report mechanochromic color change controlled by conformational change (between folded and twisted conformers) of fluorenylidene-acridanes (FAs). FAs with four N-alkyl groups (methyl, ethyl, n-butyl and n-octyl) were synthesized via the Barton–Kellogg reaction of diazofluorene and electrophilic N-tert-butoxy carbonyl thioacridone, deprotection of the tert-butoxy carbonyl group gives fluorenylacridine, and alkylation on the nitrogen atom is done using alkyl tosylate or triflate. FAs were characterized by NMR, UV-vis absorption and photoluminescence spectroscopy, theoretical calculation, cyclic voltammetry, and powder and single-crystal X-ray analyses. The color and folded/twisted conformation of the FAs were changed by the choice of substituent on the nitrogen atom, physical state (solution or solid), and morphology (crystalline or amorphous). Grinding of N-methyl FA solids, using an agate mortar, caused the morphology to change from a crystalline to amorphous state, which induced a conformational change from the folded to the twisted conformer, and a mechanochromic color change from yellow to dark green. The reverse color change, along with a morphological and conformational change to the folded conformer, was performed by solvent vapor exposure (chloroform). The twisted and folded conformers showed ambipolar (hole/electron) and hole-only transport properties, respectively.