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

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.

Synthesis, structures and properties of two donor-acceptor acridone-based compounds

Two donor-acceptor acridone-based compounds, namely, 2-{10-[4-(diphenylamino)phenyl]acridin-9-ylidene}malononitrile (TPA-AD-DCN), C34H22N4, and 2-{10-[4-(9H-carbazol-9-yl)phenyl]acridin-9-ylidene}malononitrile (CzPh-AD-DCN), C34H20N4, have been synthesized in high yield and their structures determined. TPA-AD-DCN and CzPh-AD-DCN crystallized in the centrosymmetric space groups P-1 and P21/c, respectively. Both molecules adopt a `butterfly-like' configuration of the common part of the structure and differences occur within the substituents on the acridine N atom. A Hirshfeld surface analysis showed that the H...H and C...H/H...C contacts constitute a high percentage of the intermolecular interactions. The optical and electrochemical properties, as well as theoretical calculations, of TPA-AD-DCN and CzPh-AD-DCN support the structural characterization of these materials. As crystallization-induced emission materials, TPA-AD-DCN and CzPh-AD-DCN are anticipated to be of potential use in the construction of promising optoelectronic materials.

Precise peripheral design enables propeller-like squaraine dye with highly sensitive and wide-range piezochromism

Piezochromic fluorescent (PCF) materials that feature high sensitivity and wide-range switching are attractive in intelligent optoelectronic applications but their fabrication remains a significant challenge. Here we present a propeller-like squaraine dye SQ-NMe₂ decorated with four peripheral dimethylamines acting as electron donors and spatial obstacles. This precise peripheral design is expected to loosen the molecular packing pattern and facilitate more substantial intramolecular charge transfer (ICT) switching caused by conformational planarization under mechanical stimuli. As such, the pristine SQ-NMe₂ microcrystal exhibits significant fluorescence changes from yellow (λ_em = 554 nm) to orange (λ_em = 590 nm) upon slight mechanical grinding and further to deep red (λ_em = 648 nm) upon heavy mechanical grinding. Single-crystal X-ray diffraction structural analysis of two SQ-NMe₂ polymorphs provides direct evidence to illustrate the design concept of such a piezochromic molecule. The piezochromic behavior of SQ-NMe₂ microcrystals is sensitive, high-contrast, and easily reversible, enabling cryptographic applications.

9,9-Bis[4-(N-aryl)phenyl]methylidene-xanthylidene Derivatives Displaying Mechano-, Crystallo-, and Thermochromism

Tetraphenylethylene (TPE) derivatives bearing a xanthene moiety are of interest because they have novel optical properties. 9,9-Bis[4-(N,N-diphenylamino)phenyl] and 9,9-bis[4-(9-carbazolyl)-phenyl]methylidene-xanthylidenes 3 and 4 were synthesized using Suzuki-Miyaura coupling of 9,9-dibromomethylidene-xanthylidene with the corresponding boronic acids. Diphenylamino derivative 3 exhibits mechanochromism and mechanofluorochromism (MC and MFC) reflected in absorption and fluorescence color changes. In contrast, carbazolyl derivative 4 displays thermo- and crystallo-chromism in addition to MC and MFC in the solid state. Powder X-ray diffraction and single crystal X-ray crystallographic analysis reveal that the solid state photophysical properties of these substances are governed by conformational changes rather by the creation of planar π-conjugation extended geometries.

Inkjet printing of mechanochromic fluorenylidene-acridane

In mechanochromic material research, a serious problem is that mechanical treatment cannot be applied to the materials because of their responsiveness to stimuli. Inkjet printing is a useful solution deposition method for electronics, but materials must be processed to be suitable for an inkjet printer. Fluorenylidene-acridane (FA) exhibits ground-state mechanochromism with visual color changes and responds not only to mechanical pressure but also to alcohol. Alcohol inhibits the color change induced by mechanical stimulation because the mechanochromism of FA is based on a conformational change in its molecular structure. This phenomenon suggests that the mechanochromism of FA can be controlled using alcohol. For use in inkjet printing, minute particles of FA obtained by bead milling in ethanol were investigated for uniformity and size by scanning electron microscopy and gas adsorption measurement. Also, ink containing FA particles was prepared and examined for physical properties such as viscosity and surface tension. It was confirmed that the inkjet-printed pattern demonstrated visual color changes between yellow and green in response to mechanical pressure and alcohol. This report describing the control of mechanochromism and its specific application is expected to contribute to broadening the mechanochromic materials research field.

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.

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.

Mechanofluorochromic carbon dots under grinding stimulation

The exploration of new hotspot nanomaterials to acquire mechanofluorochromic (MFC) properties has drawn substantial interest. However, previously reported MFC nanomaterials have required external pressures on the level of gigapascals, and observing distinct reversible MFC phenomena in nanomaterials under low-pressure conditions is still a challenge. Herein, a kind of reversible MFC-carbon dots (CDs) under low pressure has been reported for the first time. The MFC-CDs exhibited an apparent solid-state fluorescence color change, with emission shifting from green to blue via anisotropic grinding, owing to the alteration of hydrogen bonds and stacking structure among the CDs. Notably, these MFC-CDs exhibited a reversible fluorescence resulting from their being treated with acid vapors. This reversibility was indicated from X-ray diffraction analysis to be due to recovery of the crystalline state. The results highlighted the relationship between reversible MFC properties and structure, and showed the utility of these MFC-CDs as security films for further applications.

Equilibrium and thermodynamic studies of chromic overcrowded fluorenylidene-acridanes with modified fluorene moieties

Chromic materials, an important class of stimuli-responsive materials, have aroused extensive attention in recent years. Normally, their color is based on changes in morphology. Few examples of chromic material based on conformational isomerization, such as in overcrowded alkenes, have been reported previously. Furthemore, experimental thermodynamic studies of overcrowded bistricyclic aromatic enes have not been carried out to our knowledge. Here, we show that N-phenyl-substituted fluorenylidene-acridanes, with a properly modified fluorene moiety, performs chromisms originating from conformational changes. Thermodynamic studies determine equilibrium constants, changes in enthalpy, entropy, and free energy in solution, enabling in-depth understanding of the equilibrium behavior of overcrowded alkenes and providing useful information for designing functional chromic compounds. Single-crystal X-ray diffraction analysis of fluorenylidene-acridanes in this work clearly shows well-tuned charge transfer from the acridane to the fluorene moiety. Various chromic behaviors such as mechanochromism, thermochromism, solvatochromism, vapochromism, and proton-induced chromism also support understanding of conformational isomerism.

Of Twists and Curves: Electronics, Photophysics, and Upcoming Applications of Non-Planar Conjugated Organic Molecules

Non-planar conjugated organic molecules (NPCOMs) contain π-conjugation across their length and also exhibit asymmetry in their conformation. In other words, certain molecular fragments in NPCOMs are either twisted or curved out of planarity. This conformational asymmetry in NPCOMs leads to non-uniform charge-distribution across the molecule, with important photophysical and electronic consequences such as altered thermodynamic stability, chemical reactivity, as well as materials properties. Majorly, NPCOMs can be classified as having either Fused or Rotatable architectures. NPCOMs have been the focus of significant scientific attention in the recent past due to their exciting photophysical behavior that includes intramolecular charge-transfer (ICT), thermally activated delayed fluorescence (TADF) and long-lived charge-separated states. In addition, they also have many useful materials characteristics such as biradical character, semi-conductivity, dynamic conformations, and mechanochromism. As a result, rational design of NPCOMs and mapping their structure-property correlations has become imperative. Researchers have executed conformational changes in NPCOMs through a variety of external stimuli such as pH, temperature, anions-cations, solvent, electric potential, and mechanical force in order to tailor their photophysical, optoelectronic and magnetic properties. Converging to these points, this review highlights the lucrative electronic features, photophysical traits and upcoming applications of NPCOMs by a selective survey of the recent scientific literature.

N-Substitution of acridone with electron-donating groups: crystal packing, intramolecular charge transfer and tuneable aggregation induced emission

Acridone derivatives with electron-rich triphenylamine functionalized at the amino position were synthesized and their properties were experimentally and computationally investigated. The single crystal structure analysis revealed that the π–π interaction of acridone and the formation of hydrogen bonds of carbonyl and the hydrogen atoms of the pending phenyl ring were crucial in the determination of molecular packing in the crystalline state. An intramolecular charge transfer (ICT) process was observed between acridone and triphenylamine even with reduced conjugation by the nitrogen atom of acridone. Tuneable aggregation induced emissions with blue and green fluorescence were found due to the different aggregation state and particle size, which varied according to the water content in THF. Furthermore, the size of the spacer between acridone and the appended amine was also important in adjusting the property of aggregation induced emission or aggregation caused quenching in the solid state.

Triphenylamine functionalized acridone leads to a donor–acceptor system with intramolecular charge transfer process and tuneable aggregation induced enhanced emission.

Mechanochromism, Twisted/Folded Structure Determination, and Derivatization of (N-Phenylfluorenylidene)acridane

(N-Phenylfluorenylidene)acridane (Ph-FA) compounds with electron-withdrawing and -donating substituents (H, MeO, Ph, NO₂ , Br, F) at the para position of the phenyl group were successfully synthesized by Barton-Kellogg reactions of N-aryl thioacridones and diazofluorene. By using the substituent on the nitrogen atom to alter the electronic properties, both the folded and twisted conformers of p-NO₂ -C₆ H₄ -FA could be crystallographically characterized, which enabled the charge transfer from the electron-donating acridane moiety to the electron-accepting fluorenylidene moiety to be understood. Ground-state mechanochromism, thermochromism, vapochromism, and proton-induced chromism were demonstrated between the folded and twisted conformations of the conformers. Protonation and chemical oxidation of Ph-FA gave two stable acridinium compounds, namely, the fluorenylacridinium and acridinium radical cations. The present study will contribute to the development of functional dyes and organic semiconductors.

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.

Fluorescence response of cruciform D–π–A–π–D phenothiazine derivatives to mechanical force

Three kinds of crystals of two phenothiazine derivatives transformed into similar amorphous powders, in which the short-range π-stacking can be deduced by single-crystal structure.

Solvent viscosity-dependent isomerization equilibrium of tetramethoxy-substituted bianthrone

The designed and synthesized tetramethoxy-substituted bianthrone exists as an equilibrium mixture of the folded and twisted conformers in solution.

Optically Triggered Planarization of Boryl-Substituted Phenoxazine: Another Horizon of TADF Molecules and High-Performance OLEDs

We report the unprecedented dual properties of excited-state structural planarization and thermally activated delayed fluorescence (TADF) of 10-dimesitylboryl phenoxazine, i.e., PXZBM. Bearing a nonplanar phenoxazine moiety, PXZBM shows the lowest lying absorption onset at ∼390 nm in nonpolar solvents such as cyclohexane but reveals an anomalously large Stokes-shifted (∼14 500 cm^-1) emission maximized at 595 nm. In sharp contrast, when a phenylene spacer is added between phenoxazine and dimesitylboryl moieties of PXZBM, the 10-(4-dimesitylborylphenyl)phenoxazine PXZPBM in cyclohexane reveals a much blue-shifted emission at 470 nm despite its red-shifted absorption maximized at 420 nm (cf. PXZBM). The emission of PXZBM further reveals solvent polarity dependence, being red-shifted from 595 nm in cyclohexane to 645 nm in CH₂Cl₂. For rationalization, the steric hindrance between phenoxazine and the dimesitylboryl unit in PXZBM caused a puckered arrangement of phenoxazine at the ground state. Upon electronic excitation, as supported by the femtosecond early relaxation dynamics, spectral-temporal evolution and energetics calculated along the reaction potential energy surfaces, the diminution of N → B electron transfer reduces π-conjugation and elongates the N-B bond length, inducing the fast phenoxazine planarization with a time constant of 890 ± 100 fs. The associated charge-transfer reaction from phenoxazine (donor) to dimesitylboryl unit (acceptor) results in a further red-shifted emission in polar solvents. In stark contrast, PXZPBM shows a planar phenoxazine and undergoes excited-state charge transfer only. Despite the distinct difference in excited-state relaxation dynamics, both PXZBM and PXZPBM exhibit efficient TADF capable of producing highly efficient orange and green organic light emitting diodes with peak efficiencies of 10.9% (30.3 cd A^-1 and 18.7 lm W^-1) and 22.6% (67.7 cd A^-1 and 50.0 lm W^-1).

Developing the structure-property relationship to design solid state multi-stimuli responsive materials and their potential applications in different fields

Establishing the structure–property relationship for multi-stimuli responsive mechanochromic materials based on charge transfer luminogens.

Prediction of multi-stimuli responsive behavior in newly developed luminogens is an appealing yet challenging puzzle, since no concrete design strategy has been developed so far. In this article, we demonstrate a potent strategy to gain a deep understanding of the structure–property relationship to design multi-stimuli responsive mechanochromic materials. To achieve our goal, a variety of new isoindolinone core based charge transfer luminogens exhibiting aggregation-induced emission (AIE) have been prepared through C–H bond activation using a cost-effective ruthenium (Ru) metal catalyzed one-pot synthetic strategy. We have shown that slight tuning of the donor moiety is found to be highly effective in controlling molecular packing and metastable energy states in solid states, and thus, optical properties and multi-stimuli responsive behaviors. The flexibility and twisting of donor moieties afford a loosely bound ‘herringbone’ packing, enabling reversible transformation under multiple mechanical stimuli. The cyclized derivative of the donor exhibits a completely different packing mode (i.e., cross packing), and subsequently, does not give rise to mechanochromism. The Hirshfeld surface analysis from a single crystal infers that non-covalent interactions (specifically C–H···π and π···π) are extremely important to yield mechanochromism under external force. Correlating all solid-state behavior with the molecular structure, we conclude that the synergistic effect between the twisting and conformational flexibility of donor moieties along with numerous non-covalent interactions gives rise to multi-stimuli responsive behaviors. Finally, the newly designed molecules are found to be highly emissive in solution and potentially applicable in fluorescence thermometer construction, lighting up cells, acid–base sensors and rewritable devices.