Hydrostatic Pressure‐Controlled Ratiometric Luminescence Responses of a Dibenzo[ a,j ]phenazine‐Cored Mechanoluminophore

Synthetic progress of organic thermally activated delayed fluorescence emitters via C-H activation and functionalization

Thermally activated delayed fluorescence (TADF) emitters have become increasingly prominent due to their promising applications across various fields, prompting a continuous demand for developing reliable synthetic methods to access them. This review aims to highlight the progress made in the last decade in synthesizing organic TADF compounds through C-H bond activation and functionalization. The review begins with a brief introduction to the basic features and design principles of TADF emitters. It then provides an overview of the advantages and concise development of C-H bond transformations in constructing TADF emitters. Subsequently, it summarizes both transition-metal-catalyzed and non-transition-metal-promoted C-H bond transformations used for the synthesis of TADF emitters. Finally, the review gives an outlook on further challenges and potential directions in this field.

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.

Pressure-Responsive Polymer Chemosensors for Hydrostatic-Pressure-Signal Detection: Poly-l-Lysine-Pyrene Conjugates

Stimulus-responsive polymer materials are an attractive alternative to conventional supramolecular and polymer assemblies for applications in sensing, imaging, and drug-delivery systems. Herein, we synthesized a series of pyrene-labeled α- and ε-poly-l-lysine conjugates with varying degrees of substitution (DSs). Hydrostatic-pressure-UV/vis, fluorescence, and excitation spectroscopies and fluorescence lifetime measurements revealed ground-state conformers and excited-state ensembles emitting fluorescence with variable intensities. The polylysine-based chemosensors demonstrated diverse ratiometric responses to hydrostatic pressure through adjustments in polar solvents, DSs, and polymer backbones. Additionally, the fluorescence chemosensor exhibited a promising glum value of 3.2 × 10-3, indicating potential applications in chiral fluorescent materials. This study offers valuable insights into the development of smart hydrostatic-pressure-responsive polymer materials.

15N CP/MAS NMR as a Tool for the Mechanistic Study of Mechanical Stimuli-Responsive Materials: Evidence for the Conformational Change of an Emissive Dimethylacridane Derivative

Mechanochromic luminescent molecules are currently attracting considerable attention because of their promising technological applications, but understanding their mechanism of action is challenging and is thus hindering our deeper understanding of these materials. The conformational change of 9,9'-dimethyl-9,10-dihydroacridane derivative 1 was examined using solid-state ¹⁵N nuclear magnetic resonance (NMR) spectroscopic techniques without using a specifically ¹⁵N-labeled compound. A difference between the two conformers was clearly observed in the measurements and was assigned to the ⟨pl⟩ and ⟨bf⟩ spatial structures. The results were supported by quantum chemical calculations on ¹⁵N NMR chemical shifts of each isomer. The technique presented here can clearly identify the structural changes caused by crushing a powder sample. Such structural changes are difficult to determine using X-ray diffraction (XRD) measurements.

Solution-State Hydrostatic Pressure Chemistry: Application to Molecular, Supramolecular, Polymer, and Biological Systems

ConspectusPressure (P), as one of the most inherent state quantities, has become an academic subject of study and has attracted attention for a long time for the minute control of reaction equilibria and rates, not only in the gas phase, based on the gas state equation, but also in the solution state. In the latter case, the pressure applied to the solutions is classified as hydrostatic pressure, which is a type of isotropic mechanical force. For instance, deep-sea organisms are exposed to hydrostatic pressure environments of up to 100 MPa, implying that hydrostatic pressurization plays a role in homeostatic functions at physiological levels. The pressure control of such complicated biological behavior can be addressed by thermodynamics or kinetics. In fact, the spontaneity (ΔG) of a reaction that is governed by weak interactions (approximately 10 kcal/mol), such as electrostatic, van der Waals, hydrophobic, hydrogen bonding, and π-π stacking, is determined by the exquisite balance of enthalpy (ΔH) and entropy changes (ΔS), in accordance with the fundamental thermodynamic equation ΔG = ΔH - TΔS. The mutually correlated ΔH-ΔS relationship is known as the enthalpy-entropy compensation law, in which a more negative enthalpic change (more exothermic) causes further entropic loss based on a more negative entropy change. Namely, changing the temperature (T) as the state quantity, except for P, is highly likely to be equal to controlling the entropy term. The solution-state entropy term is relatively vague, mainly based on solvation, and thus unpredictable, even using high-cost quantum mechanical calculations because of the vast number of solvation molecules. Hence, such entropy control is not always feasible and must be demonstrated on a trial-and-error basis. Furthermore, the above-mentioned equation can be rearranged as ΔG = ΔF + PΔV, enabling us to control solution-state reactions by simply changing P as hydrostatic pressure based on the volume change (ΔV). The volume term is strongly relevant to conformational changes, solvation changes, and molecular recognition upon complexation and thus is relatively predictable, that is, volumetrically compact or not, compared to the complicated entropy term. These extrathermodynamic and kinetic observations prompted us to use hydrostatic pressure as a controlling factor over a long period. Hydrostatic pressure chemistry in the solution phase has developed over the past six decades and then converged and passed the fields of mechanochemistry and mechanobiology, which are new but challenging and current hot topics in multidisciplinary science. In this Account, we fully summarize our achievements in solution-state hydrostatic pressure chemistry for smart/functional molecular, supramolecular, polymer, and biological systems. We hope that the phenomena, mechanistic outcomes, and methodologies that we introduced herein for hydrostatic-pressure-controlling dynamics can provide guidance for both theoretical and experimental chemists working in supramolecular and (bio)macromolecular chemistry, mechanoscience, materials science, and technology.

A pressure-induced ratiometric signalling chemosensor: a case of helical anthracenes

One of the helical anthracenes, [4]HA, in which two fused anthracene ends are spatially arranged top and bottom, exhibits a ratiometric fluorescence response due to the hydrostatic pressure-dependent intramolecular [4+4] photocyclodimerization. This ratiometric signalling comes from the formation of an intramolecular stacked species and its subsequent photoreaction upon hydrostatic pressurization. The ratiometric indexes as a function of hydrostatic pressure may enable us to quantify an unknown pressure in solutions.

Luminescent Behavior Elucidation of a Disilane-Bridged D-A-D Triad Composed of Phenothiazine and Thienopyrazine

A σ-π extended aryldisilane, comprising a thienopyrazine group as an acceptor fragment and phenothiazine groups as the donor moiety, has been prepared through the introduction of two Si-Si bridges (compound 1). X-ray diffraction analysis determined the crystal structure of 1, and experimental and theoretical approaches investigated its optical properties. Solvatochromic studies revealed the dual emission of 1 in all solvents tested. Compound 1 also exhibited fluorescence in the solid state upon excitation with a hand-held UV lamp, as well as mechanochromic luminescent properties. The packing mode in the crystal structure, variation of phenothiazine conformation, morphological changes between crystalline and amorphous phases are the major factors showing reversible fluorescence under external stimuli. A theoretical conformer study found that 1 exists in distinct conformational groups differing in Gibbs free energy by less than 3 kcal mol^-1 . The conformer in the crystalline state of 1 can promote the complete separation of the HOMO and LUMO between the phenothiazine donor and the thienopyrazine acceptor, linked by the disilane linker. HOMO-LUMO energy transition in the crystalline state is forbidden due to the lack of frontier orbital overlap. Crystalline state emission showed LUMO → HOMO-1 transition (locally excited (LE) state). In the amorphous state, the partial presence of quasi-axial conformers allows intramolecular charge-transfer type emission via energy transfer from dominant quasi-equatorial conformers. The strategy proposed in this work provides important guidance for developing stimuli-responsive materials with controlled excited states.

Delineating Conformation Control in the Photophysical Behaviour of a Molecular Donor-Acceptor-Donor Triad

Mechanochromic luminescent materials, exhibiting a change in luminescence behavior under external stimuli have emerged as one of the promising candidates for upcoming efficient OLEDs. Recently mechanochromic luminescence was reported in a donor-acceptor-donor (D-A-D) triad featuring two phenothiazine units separated by a dibenzo[a,j]phenazine motif. The triad follows different emissive routes ranging from phosphorescence to TADF based on the conformational switching of the D units. In this article, we investigate such conformation-dependent photophysical behavior of this triad through theoretical calculations. By analyzing the nature of ground state, excited state and factors determining the reverse ISC crossing rates associated with the relative orientation of the D and A units, we delineate the effect of the conformational changes on their photophysical properties. Our findings reveal that axial orientation of both the donor groups enhances the overlap between HOMO and LUMO leading to a large singlet-triplet gap ( Δ E S T ) which drives phosphorescence emission. On the contrary, the equatorial orientation of the donor groups minimizes Δ E S T to facilitate rISC making the conformers TADF active. The role of several geometric factors affecting the photophysical properties of the conformers is also highlighted. Finally, we show how to regulate the population difference among the conformers by functionalizing the triad to harvest the maximum TADF efficiency.

Hydrostatic Pressure-Controllable Chiroptical Properties of Chiral Perylene Bisimide Dyes: A Chiral Aggregation Case

Hydrostatically pressurized spectroscopic and lifetime decay analyses of optically active perylene bisimides were demonstrated in the pressure range of 0.1-320 MPa to show a π-stacked aggregation. The hydrostatic pressure-induced excitation and circular dichroism spectral changes of the fluorescence perylene dye enabled us to differentiate the slight pressure-sensitive aggregates. This work will lead to a new strategy for creating a pressure-responsive supramolecular polymerization material.

Mechanofluorochromism with Aggregation-Induced Emission (AIE) Characteristics: A Perspective Applying Isotropic and Anisotropic Force

Organic mechanofluorochromic (MFC) materials (that change their emission under anisotropic and isotropic pressure) have attracted a great attention in recent years due to their promising applications in sensing pressure, storage devices, security inks, three-dimensional (3D) printing, etc. Stimuli-responsive organic materials with aggregation-induced emission (AIE) characteristics would be an interesting class of materials to enrich the chemistry of MFC compounds. A diamond anvil cell (DAC) is a small tool that is employed to generate high and uniform pressure on materials over a small area. This article discusses the relationship between the chemical structure of AIE compounds and the change in emission properties under anisotropic (mechanical grinding) and isotropic (hydrostatic) pressure. The luminescent properties of such materials depend on the molecular rearrangement in the lattice, conformational changes, excited state transitions and weak intermolecular interactions. Hence, studying the change in luminescent property of these compounds under varying pressure will provide a deeper understanding of the excited-state properties of various emissive compounds with stress. The development of such materials and studies into the effect of pressure on their luminescence properties are summarized.

Ground- and excited-state dynamic control of an anion receptor by hydrostatic pressure

Stimulus-responsive supramolecular architectures have become an attractive alternative to conventional ones for many applications in sensing, drug-delivery and switchable memory systems. Herein, we used an anion receptor (H: host) as a hydrostatic-pressure-manipulatable fluorescence foldamer and halide anions as chiral (binaphthylammonium) and achiral (tetrabutylammonium) ion pairs (SS or RR ·X and TBA·X; X = Cl, Br), and then investigated their (chir)optical properties and molecular recognition behavior under hydrostatic pressures. The conformational changes and optical properties of H in various organic solvents were revealed by UV/vis absorption and fluorescence spectra and fluorescence lifetimes upon hydrostatic pressurization. The anion-recognition abilities of H upon interactions with SS or RR·X and TBA·X at different pressure ranges were determined by hydrostatic-pressure spectroscopy to quantitatively afford the binding constant (K anion) and apparent reaction volume changes . The results obtained indicate that hydrostatic pressure as well as solvation plays significant roles in the dynamic control of the present supramolecular system in the ground and excited states. This work will provide a new guideline for further developing hydrostatic-pressure-responsive foldamers and supramolecular materials.

Hydrostatic Pressure-Regulated Cellular Calcium Responses

Hydrostatic pressure control has attracted much attention and presents a still challenging objective from mechanobiological viewpoints. Herein, we reveal the calcium entry processes in HeLa cells by means of hydrostatic pressure spectroscopy. The steady-state fluorescence spectral data comprehensively elucidated the factors controlling the outcomes of the hydrostatic pressure-stimulated calcium entry behavior. The present work leads to a new perspective on ion regulations in living cells and an attractive alternative to conventional mechanostimuli.

Understanding and Designing Thermally Activated Delayed Fluorescence Emitters: Beyond the Energy Gap Approximation

In this article recent progress in the development of molecules exhibiting Thermally Activated Delayed Fluorescence (TADF) is discussed with a particular focus upon their application as emitters in highly efficient organic light emitting diodes (OLEDs). The key aspects controlling the desirable functional properties, e. g. fast intersystem crossing, high radiative rate and unity quantum yield, are introduced with a particular focus upon the competition between the key requirements needed to achieve high performance OLEDs. The design rules required for organic and metal organic materials are discussed, and the correlation between them outlined. Recent progress towards understanding the influence of the interaction between a molecule and its environment are explained as is the role of the mechanism for excited state formation in OLEDs. Finally, all of these aspects are combined to discuss the ability to implement high level design rules for achieving higher quality materials for commercial applications. This article highlights the significant progress that has been made in recent years, but also outlines the significant challenges which persist to achieve a full understanding of the TADF mechanism and improve the stability and performance of these materials.

Hydrostatic Pressure-Induced Spectral Variation of Reichardt's Dye: A Polarity/Pressure Dual Indicator

The famous solvatochromic Reichardt's dye was applied to quantify hydrostatic pressure in media. The UV/vis spectra of the dye in various organic solvents are shifted bathochromically or hypsochromically at the shorter- or longer-wavelength band, respectively, upon hydrostatic pressurization. The E _T value, determined by an absorption maximum, in ethyl acetate increases from 38.5 kcal mol^-1 at 0.1 MPa to 39.2 kcal mol^-1 at 300 MPa, which is mostly equal to the one in chloroform at 0.1 MPa. These spectroscopic origins were supported by the time-dependent density functional theory (TD-DFT) calculations. The concept and approach proposed in this paper, i.e., a dual indicator, should attract the attention of a broad spectrum in multidisciplinary science.

The interplay of conformations and electronic properties in N-aryl phenothiazines

Extra and intra conformations govern electronic properties of N-aryl phenothiazines as shown by combined experimental and computational structure–property relationships.

Alchemy of donor–acceptor–donor multi-photofunctional organic materials: from construction of electron-deficient azaaromatics to exploration of functions

This Feature Article overviews the recent development of multi-photofunctional organic materials based on a twisted donor-acceptor-donor scaffold, which is driven by novel synthetic methods for exotic azaromatic compounds.