Mechanically triggered reversible stepwise tricolor switching and thermochromism of anthracene-o-carborane dyad

Influence of Intermolecular Structural Effects on Radiative Efficiency in Xanthene-Based Carboranyl Luminophores

Two o-carboranes with (i) 9,9-dimethyl-9H-xanthene and (ii) spiro[fluorene-9,9'-xanthene] moieties (XTC and sXTC, respectively) were prepared and characterized. Single X-ray crystallography analysis revealed the presence of intermolecular hydrogen bonds in XTC crystals. Although both compounds did not exhibit emission in tetrahydrofuran solutions at 298 K, intense bluish emission was observed in the solid states and frozen tetrahydrofuran solutions at 77 K. According to the results of theoretical calculations, this emission originated from an intramolecular charge transfer (ICT) transition with the o-carborane moiety. The absolute quantum efficiency (Φem) of the ICT-based emission in the film state equaled 49% for XTC and 20% for sXTC but was as high as 90% for the crystals of both compounds. The crystal structures of XTC and sXTC revealed that the o-carboranyl-appended phenyl plane was orthogonal (85-89°) to the carbon-carbon bonding axis in the o-carborane, indicating the existence of a strong exo-π-interaction, which was identified as the structural basis for the ICT-based transition. These results implied that the intermolecular structural effect of XTC in the randomly aggregated solid state (film) helped maintain the above orthogonality and, hence, the high efficiency from the ICT radiative mechanism. Thus, we concluded that the ICT radiative efficiency of o-carboranyl luminophores in the aggregated solid state can be controlled by specific intermolecular interactions and that the molecular geometric design inducing this feature can be important for developing highly efficient carboranyl luminophores.

Synthesis, Photophysical and Electronic Properties of a D-π-A Julolidine-Like Pyrenyl-o-Carborane

We synthesized 2-(1-1,2-dicarbadodecaboranyl(12))-6,6,12,12-tetramethyl-7,8,11,12-tetrahydro-6H,10H-phenaleno[1,9-fg]pyrido[3,2,1-ij]quinoline (4), a julolidine-like pyrenyl-o-carborane, with pyrene substituted at the 2,7-positions on the HOMO/LUMO nodal plane. Using solid state molecular structures, photophysical data, cyclic voltammetry, DFT and LR-TDDFT calculations, we compare o-carborane and B(Mes)2 (Mes=2,4,6-Me3C6H2) as acceptor groups. Whereas the π-acceptor strength of B(Mes)2 is sufficient to drop the pyrene LUMO+1 below the LUMO, the carborane does not do this. We confirm the π-donor strength of the julolidine-like moiety, however, which raises the pyrene HOMO-1 above the HOMO. In contrast to the analogous pyrene-2-yl-o-carborane, 2-(1-1,2-dicarbadodecaboranyl(12))-pyrene VI, which exhibits dual fluorescence, because the rate of internal conversion between locally-excited (LE) and charge transfer (CT) (from the pyrene to the carborane) states is faster than the radiative decay rate, leading to a thermodynamic equilibrium between the 2 states, 4 shows only single fluorescence, as the CT state involving the carborane as the acceptor moiety in not kinetically accessible, so a more localized CT emission involving the julolidine-like pyrene moiety is observed.

Solid-state excimer emission of o-carborane derivatives and applications for stimuli-responsive luminescent materials

o-Carborane is an icosahedral cluster containing boron and has been paid attention as a versatile platform for constructing stimuli-responsive materials with solid-state luminescence properties by suppressing aggregation-caused quenching (ACQ), which is the emission annihilation process observed in condensed states, such as concentrated solutions, solids and films. Non-specific intermolecular interactions in the condensed states resulting in ACQ are disturbed by the spherical boron cluster. More recently, it has been demonstrated from several reports that o-carborane is capable of assisting solid-state excimer emission in aryl-modified o-carboranes. Moreover, some of these materials provide unique stimuli-responsiveness. In the commodity design for solid-state luminescent materials, o-carboranes are responsible for disturbing intermolecular interactions and subsequently presenting solid-state luminescence. On the other hand, excimer emission, which is inducible from the intermolecular photophysical process, has been discovered in the solid state based on o-carborane materials. In this manuscript, recent progress in stimuli-responsive luminescent materials via excimer formation will be described. Moreover, as the further potential of o-carborane materials, recent findings on the photo-salient effect which can be induced through excimer formation followed by photoreaction will also be explained.

The Photosalient Effect and Thermochromic Luminescence Based on o-Carborane-Assisted π-Stacking in the Crystalline State

Herein, we report the unique multiple-stimuli responsiveness of anthracene-tethered o-carborane derivatives. We designed and synthesized anthracene derivatives with different substitution positions and numbers of the o-carborane units. Two compounds had characteristic crystal structures involving the columnar π-stacking structures of the anthracene units. From the analysis of crystalline-state structure-property relationships, it was revealed that the crystals exhibited the photosalient effect accompanied by photochemical [4+4] cycloaddition reactions and temperature-dependent photophysical dual-emission properties including excimer emission of anthracene. Those properties were considered as non-radiative and radiative deactivation pathways through the excimer formation in the excited state and the formation of excimer species was facilitated by the π-stacking structure of anthracene units. Moreover, we found unusual temperature dependency on the occurrence of the photosalient effect. According to the data from variable temperature X-ray crystallography, a strong correlation between lattice shrinkage and strain accumulation is suggested.

Ortho-Carborane Decorated Multi-Resonance TADF Emitters: Preserving Local Excited State and High Efficiency in OLEDs

A novel class of o-carboranyl luminophores, 2CB-BuDABNA (1) and 3CB-BuDABNA (2) is reported, in which o-carborane moieties are incorporated at the periphery of the B,N-doped multi-resonance thermally activated delayed fluorescence (MR-TADF) core. Both compounds maintain the inherent local emission characteristics of their MR-emitting core, exhibiting intense MR-TADF with high photoluminescence quantum yields in toluene and rigid states. In contrast, the presence of the dark lowest-energy charge transfer state, induced by cage rotation in THF, is suggested to be responsible for emission quenching in a polar solvent. Despite the different arrangement of the cage on the DABNA core, both 1 and 2 show red-shifted emissions compared to the parent compound BuDABNA (3). By utilizing 1 as the emitter, high-efficiency blue organic light-emitting diodes (OLEDs) are achieved with a remarkable maximum external quantum efficiency of 25%, representing the highest reported efficiency for OLEDs employing an o-carboranyl luminophore as the emitter.

Stimuli-fluorochromic smart organic materials

Smart materials based on stimuli-fluorochromic π-conjugated solids (SFCSs) have aroused significant interest due to their versatile and exciting properties, leading to advanced applications. In this review, we highlight the recent developments in SFCS-based smart materials, expanding beyond organometallic compounds and light-responsive organic luminescent materials, with a discussion on the design strategies, exciting properties and stimuli-fluorochromic mechanisms along with their potential applications in the exciting fields of encryption, sensors, data storage, display, green printing, etc. The review comprehensively covers single-component and multi-component SFCSs as well as their stimuli-fluorochromic behaviors under external stimuli. We also provide insights into current achievements, limitations, and major challenges as well as future opportunities, aiming to inspire further investigation in this field in the near future. We expect this review to inspire more innovative research on SFCSs and their advanced applications so as to promote further development of smart materials and devices.

Mechanofluorochromic Properties of 1,4-Diphenylanthracene Derivatives with Hypsochromic Shift

Several types of 1,4-diphenylanthracene derivatives 1-4 were prepared, and their photophysical properties were observed in the solid and solution states. Interestingly, the CN-group-substituted 1,4-diphenylanthracene derivative 2 was found to exhibit a higher fluorescence quantum yield (ϕf = 0.71) in the solid state than in the solution state, probably due to the formation of an intermolecular Ar-CN⋯H-Ar hydrogen bond and antiparallel type locked packing structure in the solid state. Furthermore, for some derivatives, an increase in the fluorescence quantum yield was observed in the PMMA film (1 wt%) over both the solid state and the solution state. More interestingly, some of the 1,4-diphenylanthracene derivatives exhibited unusual mechanofluorochromic properties with a "hypsochromic shift" in luminous color depending on the substituents of the phenyl group, and with the derivatives having CF3, OMe, CN, and two F substituents (1d-1f, 2-4) showing a significant luminous color change with a "hypsochromic shift" after grinding. However, no change in the luminous color was observed for the derivatives having H, Me, and one F substituent (1a-1c), and especially for some of the CN-substituted derivatives, a reversible luminous color change with a "hypsochromic shift" was observed, probably due to the formation of an antiparallel type packing structure. These "hypsochromic" anthracene derivatives could probably be utilized as new mechanofluorochromic materials.

One stimulus-induced two-step photophysical response with high contrast and tunable switching time for dynamic displaying

One stimulus-induced two-step photophysical response, especially with tunable switching time, is a great challenge for organic chromophores. Herein, a polymorphic material 2,7-DCF could undergo in situ two sequential dual-channel responses upon dichloromethane fuming. Both the appearance color and the fluorescence change from red to yellow to deep red with high contrast. The first step corresponds to a fast amorphous-to-crystalline transformation, while the second is a slow solid-state cocrystallization process. Based on single crystal structures and theoretical calculations, such distinct color changes are mainly attributed to conformation twisting and the electron coupling with incorporated solvent molecule through C-H⋅⋅⋅O interaction. Importantly, the second slow photophysical response could be drastically sped up by seeding strategy, or be totally inhibited. Such characteristics pave a way for the potential applications in dynamic anti-counterfeiting and data encryption. Based on the two-step transformation, polymorph 2,7-DCF-a could achieve a successive four-level response to external stimuli. In contrast, polymorph 2,7-DCF-d exhibits a stepwise hypsochromic fluorescence shift over 100 nm. This study would significantly promote the development of stimuli-sensitive systems from "one stimulus, one-step response" to "one stimulus, two or multi-step response".

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.

Improvement in Radiative Efficiency Via Intramolecular Charge Transfer in ortho-Carboranyl Luminophores Modified with Functionalized Biphenyls

In this study, we found that the electronic effects of the functional groups on aromatic units attached to o-carboranyl species can enhance the efficiency of intramolecular charge transfer (ICT)-based radiative decay processes. Six o-carboranyl-based luminophores having attached functionalized biphenyl groups with CF₃, F, H, CH₃, C(CH₃)₃, and OCH₃ substituents were prepared and fully characterized by multinuclear magnetic resonance spectroscopy. In addition, their molecular structures were determined by single-crystal X-ray diffractometry, which revealed that the distortion of the biphenyl rings and the geometries around the o-carborane cages were similar. All compounds exhibited ICT-based emissions in the rigid state (solution at 77 K and film). Intriguingly, the quantum efficiencies (Φ_em) of five compounds (that of the group with CF₃ could not be measured because of its extremely weak emissions) in the film state increased gradually as the electron-donating power of the terminal functional group modifying the biphenyl moiety increased. Furthermore, the nonradiative decay constants (k_nr) for the group with OCH₃ were estimated to be one-tenth of those for the group with F, whereas the radiative decay constants (k_r) for the five compounds were similar. The dipole moments (μ) calculated for the optimized first excited state (S₁) structures gradually increased, from that of the group with CF₃ to that of the group with OCH₃, implying that the inhomogeneity of the molecular charge distribution was enhanced by electron donation. The electron-rich environment formed as a result of electron donation led to efficient charge transfer to the excited state. Both experimental and theoretical findings revealed that the electronic environment of the aromatic moiety in o-carboranyl luminophores can be controlled to accelerate or interrupt the ICT process in the radiative decay of excited states.

Tuning of the height of energy barrier between locally-excited and charge transfer states by altering the fusing position of o-carborane in phenylnaphthalene

We synthesized two types of the regioisomers fused by a phenylnaphthalene ring with variable connection points to the o-carborane scaffold. In this paper, we describe their photoluminescence (PL) properties and detailed photochemical mechanisms. According to the series of optical measurements, interestingly, they showed different PL characters in terms of wavelength and the dual-emission character despite that they have the common aromatic unit. Variable-temperature PL measurements and quantum chemical calculations suggested that the substitution position of aryl groups to o-carborane plays an important role in determining the energy barrier to the intramolecular charge-transfer (ICT) state at the S₁ state. Finally, it is revealed that the relative position of the C-C bond of o-carborane and the aryl center should be responsible for the photophysical events of aryl-o-carboranes.

Iridium-catalyzed selective amination of B(4)-H for the synthesis of o-carborane-fused indolines

An iridium-catalyzed selective amination of B(4)-H via dehydrogenative cross-coupling of B-H/N-H bonds for the synthesis of o-carborane-fused indolines has been developed for the first time. Various types of unprecedented o-carborane-fused indolines have been synthesized, which would be potential candidates for applications in drug discovery, pharmaceutical chemistry and functional materials. This work offers a valuable reference for the designing and synthesis of o-carborane-fused heterocycles.

Solvates of a dianisyl-substituted donor–acceptor-type benzothiadiazole: mechanochromic, vapochromic, and acid-responsive multicolor luminescence

Solvates of a donor–acceptor-type fluorophore exhibited multicolor luminescence that can respond to mechanical stimuli, solvent and acid vapors.

Relationship between the Molecular Geometry and the Radiative Efficiency in Naphthyl-Based Bis-Ortho-Carboranyl Luminophores

The efficiency of intramolecular charge transfer (ICT)-based emission on π-aromatic-group-appended closo-ortho-carboranyl luminophores is known to be affected by structural fluctuations and molecular geometry, but investigation of this relationship has been in progress to date. In this study, four naphthyl-based bis-o-carboranyl compounds, in which hydrogen (15CH and 26CH) or trimethysilyl groups (15CS and 26CS) were appended at the o-carborane cage, were synthesized and fully characterized. All the compounds barely displayed an emissive trace in solution at 298 K; however, 15CH and 26CH distinctly exhibited a dual emissive pattern in rigid states (in solution at 77 K and in films), attributed to locally excited (LE) and ICT-based emission, while 15CS and 26CS showed strong ICT-based greenish emission. Intriguingly, the molecular structures of the four compounds, analyzed by single X-ray crystallography, showed that the C-C bond axis of the o-carborane cage in the trimethysilyl group-appended compounds 15CS and 26CS were more orthogonal to the plane of the appended naphthyl group than those in 15CH and 26CH. These features indicate that 15CS and 26CS present an efficient ICT transition based on strong exo-π-interaction, resulting in a higher quantum efficiency (Φ_em) for ICT-based radiative decay than those of 15CH and 26CH. Moreover, the 26CS structure revealed most orthogonal geometry, resulting in the highest Φ_em and lowest k_nr values for the ICT-based emission. Consequently, all the findings verified that efficient ICT-based radiative decay of aromatic group-appended o-carboranyl luminophores could be achieved by the formation of a specific geometry between the o-carborane cage and the aromatic plane.

Cu(OTf)2/NBS-Mediated Tandem Reaction of 1-Cinnamyl Alcohol-o-Carboranes via Ring Opening of Oxetane with Arenes: An Alternative Approach for the Synthesis of C-Alkenyl-o-Carboranes

The Cu(OTf)₂/NBS-mediated tandem reaction of 1-cinnamyl alcohol-o-carboranes for the synthesis of C-alkenyl-o-carboranes has been developed. Mechanism studies demonstrated that the Cu(OTf)₂-promoted ring opening of oxetane with electron-rich arenes as soft nucleophiles was involved and was the key step for the transformation. This work provided an alternative strategy for the synthesis of C-alkenyl-o-carboranes, which has an important reference for the synthesis of o-carborane derivatives with diversity.

Crucial Factors Regulating Intramolecular Charge-Transfer-Based Radiative Efficiency in ortho-Carboranyl Luminophores: Planarity between Substituted Biphenyl Rings

o-Carboranyl compounds contain specific geometries, ranging from planar to orthogonally distorted biphenyl rings. Herein, 13 o-carboranyl compounds, 1HF-13PP, were synthesized and fully characterized to determine the impact of structural formation of the aromatic group appended with the o-carborane to estimate the efficiency of their radiative decay process. All the compounds exhibited significant intramolecular charge transfer (ICT)-based emission in the crystalline state at 298 K. Remarkably, increasing the distorted dihedral angles between biphenyl rings gradually decreased the emission efficiencies. Furthermore, their radiative decay constants decreased linearly with increasing dihedral angles, which demonstrated the inversely proportional relationship between these two factors. These findings distinctly suggest that the planar or distorted geometry of substituted aryl groups can strongly affect the efficiency of the ICT-based radiative process in o-carboranyl luminophores.

Revisiting the Role of Charge Transfer in the Emission Properties of Carborane-Fluorophore Systems: A TDDFT Investigation

In this study, we performed a detailed investigation of the S1 potential energy surface (PES) of o-carborane-anthracene (o-CB-Ant) with respect to the C-C bond length on o-CB and the dihedral angle between o-CB and Ant moieties. The effects of different substituents (F, Cl, CN, and OH) on carbon- or boron-substituted o-CB, along with a π-extended acene-based fluorophore, pentacene, on the nature and energetics of S1 → S0 transitions are evaluated. Our results show the presence of a non-emissive S1 state with an almost pure charge transfer (CT) character for all systems as a result of significant C-C bond elongation (C-C = 2.50-2.56 Å) on o-CB. In the case of unsubstituted o-CB-Ant, the adiabatic energy of this CT state corresponds to the global minimum on the S1 PES, which suggests that the CT state could be involved in emission quenching. Despite large deformations on the o-CB geometry, predicted energy barriers are quite reasonable (0.3-0.4 eV), and the C-C bond elongation can even occur without a noticeable energy penalty for certain conformations. With substitution, it is shown that the dark CT state becomes even more energetically favorable when the substituent shows -M effects (e.g., -CN), whereas substituents showing +M effects (e.g., -OH) can result in an energy increase for the CT state, especially for partially stretched C-C bond lengths. It is also shown that the relative energy of the CT state on the PES depends strongly on the LUMO level of the fluorophore as this state is found to be energetically less favorable compared to other conformations when anthracene is replaced with π-extended pentacene. To our knowledge, this study shows a unique example of a detailed theoretical analysis on the PES of the S1 state in o-CB-fluorophore systems with respect to substituents or fluorophore energy levels. Our findings could guide future experimental work in emissive o-CB-fluorophore systems and their sensing/optoelectronic applications.

Thermodynamic equilibrium between locally excited and charge-transfer states through thermally activated charge transfer in 1-(pyren-2'-yl)-o-carborane

Reversible conversion between excited-states plays an important role in many photophysical phenomena. Using 1-(pyren-2'-yl)-o-carborane as a model, we studied the photoinduced reversible charge-transfer (CT) process and the thermodynamic equilibrium between the locally-excited (LE) state and CT state, by combining steady state, time-resolved, and temperature-dependent fluorescence spectroscopy, fs- and ns-transient absorption, and DFT and LR-TDDFT calculations. Our results show that the energy gaps and energy barriers between the LE, CT, and a non-emissive 'mixed' state of 1-(pyren-2'-yl)-o-carborane are very small, and all three excited states are accessible at room temperature. The internal-conversion and reverse internal-conversion between LE and CT states are significantly faster than the radiative decay, and the two states have the same lifetimes and are in thermodynamic equilibrium.

Controlling the Dual-Emission Character of Aryl-Modified o-Carboranes by Intramolecular CH⋅⋅⋅O Interaction Sites

It is still challenging to realize a dual-emission system, in which two luminescent bands simultaneously appear by photoexcitation, in solid with organic dyes due to the difficulty in regulation of electronic properties in the excited state and concentration quenching. o-Carborane is known to be a versatile platform for constructing solid-state emitters since the sphere boron cluster is favorable for suppressing intermolecular interactions and subsequently concentration quenching. Here, we show solid-state dual-emissive o-carborane derivatives. We prepared 4 types of o-carborane derivatives and found dual-emission behaviors both in solution and solid states. By regulating the rotation at the o-carborane unit with the intramolecular C_cage H⋅⋅⋅O interaction, the dual-emission intensity ratios were changed. Finally, it was demonstrated that the overall photoluminescence spectra can be estimated using the binding energy of intramolecular interactions.

o-Carborane-based fluorophores as efficient luminescent systems both as solids and as water-dispersible nanoparticles

A set of o-carborane-appended π-conjugated fluorophores and their light-emitting properties in the solid state are reported. The aggregation-induced emission enhancement (AIEE) exhibited for one of the fluorenyl derivatives paved the way to successfully preparing o-carborane-containing organic nanoparticles (NPs) homogeneously dispersed in aqueous media that maintain their luminescence properties. Notably, NPs processed as thin films also show high fluorescence efficiency, suggesting potential optical and optoelectronic applications.

Effects of molecular geometry on the efficiency of intramolecular charge transfer-based luminescence in o-carboranyl-substituted 1H-phenanthro[9,10-d]imidazoles

The distinct difference in solid-state emission efficiency between 1H-phenanthro[9,10-d]imidazole–o-carboranyl luminophores showed that the geometric orientation is a key factor for controlling intramolecular charge transfer-based radiative decay.

Luminescent polymorphic crystals: mechanoresponsive and multicolor-emissive properties

Polymorphic organic crystals that can switch their photophysical properties in response to mechanical stimuli are highlighted.

Reversible luminescence tuning behavior in a thermal-stimuli-responsive anthracene-based coordination polymer

A new coordination complex presented a series of structural changes accompanied by photo-fluorescence (PL) changes under different temperature stimuli.

Manipulation of electronic and structural effects on the solid-state emission of multiple linked anthracenyl-o-carborane dyads

A direct boost of emission efficiency is presented based on the strategy of linking multiple weak AIE-active anthracenyl-o-carborane dyads together for improving solid-state luminescence.

A triphenylamine derivative and its Cd(ii) complex with high-contrast mechanochromic luminescence and vapochromism

A triphenylamine derivative and its Cd(ii) complex exhibited predominant mechanochromism and vapochromism with high-contrast color and emission changes.

Synthesis of 3-Aryl-ortho-carboranes with Sensitive Functional Groups

A simple and efficient method was developed for the one-pot synthesis of 3-aryl derivatives of ortho-carborane with sensitive functional groups using 3-iodo-ortho-carborane and aryl zinc bromides that were generated in situ. A series of 3-aryl-ortho-carboranes, including those containing nitrile and ester groups, 3-RC₆H₄-1,2-C₂B₁₀H₁₁ (R = p-Me, p-NMe₂, p-OCH₂OMe, p-OMe, o-CN, p-CN, o-COOEt, m-COOEt, p-COOEt) was synthesized using this approach. The solid-state structures of 3-RC₆H₄-1,2-C₂B₁₀H₁₁ (R = p-OMe, o-CN, and p-CN) were determined by single crystal X-ray diffraction. The intramolecular hydrogen bonding involving the ortho-substituents of the aryl ring and the CH and BH groups of carborane was discussed.

Mechanochromic Luminescence (MCL) of Purely Organic Two-Component Dyes: Wide-Range MCL over 300 nm and Two-Step MCL by Charge-Transfer Complexation

Despite recent extensive studies on mechanochromic luminescence (MCL), rational control over the magnitude of the emission-wavelength shift in response to mechanical stimuli remains challenging. In the present study, a two-component donor-acceptor approach has been applied to create a variety of organic MCL composites that exhibit remarkable emission-wavelength switching. Dibenzofuran-based bis(1-pyrenylmethyl)diamine and typical organic fluorophores have been employed as donor and acceptor dyes, respectively. Outstanding wide-range MCL with an emission-wavelength shift of over 300 nm has been achieved by mixing the diamine with 3,4,9,10-perylenetetracarboxylic diimide. Unprecedented two-step MCL in response to mechanical stimuli of different intensity has also been realized for a two-component mixture with 9,10-anthraquinone. Fluorescence microscopy observations at the single-particle level revealed that the segregation and mixing of the two-component dyes contribute to the stimuli-responsive emission-color switching of the MCL composites.

Multiresponsive Tetra-Arylethene-Based Fluorescent Switch with Multicolored Changes: Single-Crystal Photochromism, Mechanochromism, and Acidichromism

In this article, we report a simple tetra-arylethene-based fluorescent switch TPS═C4 modified with a flexible alkyl chain by the Schiff base structure. The incorporation of C═N retains the excellent photochromic property of tetra-arylethene and endows TPS═C4 with new multiresponsiveness of mechanochromism and acidichromism and multicolor changes. TPS═C4 shows remarkable mechanochromism from a deep blue emission at 420 nm to bright blue with a new shoulder band at 450 nm, which arises from the force-induced phase transition from the crystal state to an amorphous form. Both the original crystalline powder and the ground amorphous sample exhibit interesting acidichromism, and their emission colors turn yellow (530 nm), due to the protonation of C═N. More interestingly, TPS═C4 displays fascinating photochromism in multiple states, especially in the single-crystal state. The flexible alkyl chain offers enough free space for molecular motion and facilitates single-crystal photochromism. Due to the multiresponsiveness and multicolor switch, TPS═C4 can be satisfactorily used for the multidimensional anticounterfeiting application. To the best of our knowledge, TPS═C4 is a rare multifunctional molecule with a simple structure but featuring multiresponsiveness and multicolor switch.

Stimuli-Responsive AIEgens

The unique advantages and the exciting application prospects of AIEgens have triggered booming developments in this area in recent years. Among them, stimuli-responsive AIEgens have received particular attention and impressive progress, and they have been demonstrated to show tremendous potential in many fields from physical chemistry to materials science and to biology and medicine. Here, the recent achievements of stimuli-responsive AIEgens in terms of seven most representative types of stimuli including force, light, polarity, temperature, electricity, ion, and pH, are summarized. Based on typical examples, it is illustrated how each type of systems realize the desired stimuli-responsive performance for various applications. The key work principles behind them are ultimately deciphered and figured out to offer new insights and guidelines for the design and engineering of the next-generation stimuli-responsive luminescent materials for more broad applications.

Ru(ii) and Ir(iii) phenanthroline-based photosensitisers bearing o-carborane: PDT agents with boron carriers for potential BNCT

Four novel transition metal-carborane photosensitisers were prepared by Sonogashira cross-coupling of 1-(4-ethynylbenzyl)-2-methyl-o-carborane (A-CB) with halogenated Ru(ii)- or Ir(iii)-phenanthroline complexes. The resulting boron-rich complexes with one (RuCB and IrCB) or two carborane cages (RuCB2 and IrCB2) were spectroscopically characterised, and their photophysical properties investigated. RuCB displayed the most attractive photophysical properties in solution (λ_em 635 nm, τ_T 2.53 μs, and φ_p 20.4%). Nanosecond time-resolved transient absorption studies were used to explore the ³MLCT nature of the triplet excited states, and the highest singlet oxygen quantum yields (Φ_Δ) were obtained for the mono-carborane-phenanthroline complexes (RuCB: 52% and IrCB: 25%). None of the complexes produce dark toxicity in SKBR-3 cells after incubation under photodynamic therapy (PDT) conditions. Remarkably, mono-carboranes RuCB and IrCB were the best internalised by the SKBR-3 cells, demonstrating the first examples of tris-bidentate transition metal-carborane complexes acting as triplet photosensitisers for PDT with a high photoactivity; RuCB or IrCB killed ∼50% of SKBR-3 cells at 10 μM after irradiation. Therefore, the high-boron content and the photoactive properties of these photosensitisers make them potential candidates as dual anti-cancer agents for PDT and Boron Neutron Capture Therapy (BNCT).

Alteration of intramolecular electronic transition via deboronation of carbazole-based o-carboranyl compound and intriguing 'turn-on' emissive variation

The conversion of closo-o-carborane-containing compounds to the nido-o-species via deboronation causes photophysical changes that could be used for sensing applications. 9-Methyl-9H-carbazole-based closo- (closo-Cz) and nido-o-carboranyl (nido-Cz) compounds were prepared and fully characterised by multinuclear NMR spectroscopy and elemental analysis, and the solid-state molecular structure of closo-Cz was analysed by X-ray crystallography. Although the closo-compound exhibited an emissive pattern centred at λ em = ca. 530 nm in the rigid state only (in THF at 77 K and as a film), nido-Cz demonstrated intense emission in the near-UV region (λ em = ca. 380 nm) in both solution and film states at 298 K. The positive solvatochromic effect of nido-Cz and the results of theoretical calculations for both the o-carboranyl compounds supported that these emissive features originate from intramolecular charge transfer (ICT) corresponding to the o-carborane. Furthermore, the calculations verified that the electronic role of the o-carboranyl unit changed from acceptor to donor upon deboronation from closo-Cz to nido-Cz. Investigations of the radiative decay mechanisms of closo-Cz and nido-Cz according to their quantum efficiencies (Φ em) and decay lifetimes (τ obs) suggested that the ICT-based radiative decays of closo-Cz and nido-Cz readily occur in the film (solid) and solution state, respectively. These observations implied that the emission of closo-Cz in the solution state could be drastically enhanced by deboronation to nido-Cz upon exposure to an increasing concentration of fluoride anions. Indeed, turn-on emissive features in an aqueous solution were observed upon deboronation, strongly suggesting the potential of closo-Cz as a turn-on and visually detectable chemodosimeter for fluoride ion sensing.

Ligand-Triggered Platinum(II) Metallacycle with Mechanochromic and Vapochromic Responses

Supramolecular coordination complexes with solid-state stimuli-responsive characteristics are highly desirable but are rarely reported. Herein, we describe two coordination-driven self-assembled monoanthracene or dianthracene-based hexagonal metallacycles by subtle structure modification. Notably, the dianthracene-containing hexagon 1 exhibits tricolor mechanochromic and vapochromic characteristics, while the monoanthracene-containing hexagon 4 does not show obvious changes toward mechanical force. Further studies have indicated that changes in hexagon 1, especially the ulterior anthracene of hexagon 1 in the molecular stacking through intermolecular interactions toward external stimuli, are responsible for the above behavioral differences. Furthermore, the present work also demonstrates a novel light-harvesting strategy for achieving high-contrast mechanochromic fluorescence involving solid-state energy transfer from hexagon 1 to an organic carbazole derivant 6 without mechanofluorochromism or tetraphenylethylene derivant 7 exhibiting inconspicuous mechanofluorochromism.

Flexible and Transparent Oligothiophene-o-Carborane-Containing Hybrid Films for Nonlinear Optical Limiting Based on Efficient Two-Photon Absorption

Structure-property relationship for fluorophores with favorable nonlinear optical (NLO) properties are promising topics in organic chemistry and material science. Herein, a series of terthiophene-o-carborane dyads and triads covalently linked with different end-capping styles were readily synthesized and comprehensively investigated. Quantitative values of the crystal and packing structures, photophysical parameters including aggregation-induced emission (AIE) and two-photon absorption (2PA) were provided. Significant impact of carborane unit for introducing the AIE characteristic has been investigated in contrast to the parent oligothiophene. All the obtained fluorophores exhibit maximum absorption around 370 nm in THF and emit bright reddish photoluminscence with absolute fluorescence quantum yields above 16% in solid states. Intramolecular charge communication between oligothiophene and carborane plays important roles in the related NLO properties. These results are supported well by the time-dependent DFT theoretical calculations. Effective 2PA cross sections (δ_2PA = 95-355 GM@650 nm) and transition dipole moments of the derivatives are variable for different end-capping styles. Their potential applications as optical limiting materials based on the 2PA mechanism in solutions and doped PDMS films were further evaluated. Taken together, this work provides an understanding of their structure-property relationship, and flexible PDMS films as outstanding candidates for practical applications in optical limiting.

Regulation of Multicolor Fluorescence Changes Found in Donor-acceptor-type Mechanochromic Fluorescent Dyes

The regulation of multicolor fluorescence changes in mechanochromic fluorescence (MCF) remains a challenging task. Herein, we report the regulation of MCF using a donor-acceptor structure. Two crystal polymorphs, BTD-pCHO(O) and BTD-pCHO(R) produced by the introduction of formyl groups to an MCF dye, respond to a mechanical stimulus, allowing a three-color fluorescence change. Specifically, the orange-colored fluorescence of the metastable BTD-pCHO(O) polymorph changed to a deep-red color in the amorphous-like state to finally give a red color in the stable BTD-pCHO(R) polymorph. This change occurred by mechanical grinding followed by vapor fuming. The two different crystal packing patterns were selectively regulated by the electronic effect of the introduced functional groups. The two types of selectively formed crystals in BTD(F)-pCHO bearing fluorine atoms, and BTD(OMe)-pCHO bearing methoxy groups, respond to mechanical grinding, allowing for the regulation of multicolor MCL from a three-color change to two different types of two-color changes.

Achieving Organic Smart Fluorophores by Controlling the Balance between Intermolecular Interactions and External Stimuli

Organic smart fluorophores (OSFs) are highly desirable over the past decades because of their potential applications in advanced photonic devices. However, it is still difficult and challenging to obtain such materials with tunable photophysical properties and high emission efficiency based on robust construction strategies. Therefore, we proposed a simple and efficient strategy for constructing OSFs by balancing the competition between intermolecular interactions and external stimuli via molecular structure design. In this work, four pyrene derivatives (T1-Py, T4-Py, T12-Py, and S12-Py) with tunable stimuli-responsive properties were designed and synthesized. The tunable intermolecular interactions in solution states were successfully demonstrated by the molecular structure and solution concentration-dependent luminescence properties. The effect of alkyl chain length on molecular packing in solid states was investigated by polarized optical microscopy and powder and single-crystal X-ray diffraction; the results show that with the increase in molecular chain length, the molecular packing of the compounds gradually changed from π-π stacked compact mode to X-crossing stacked loose mode, which leads to different stimuli-responsive phenomena of these compounds. The strategy provided herein facilitates the construction of multistimuli-responsive (thermochromism, mechanochromism, and vapochromism) OSFs with adjustable emission color. Harnessing the heat-responsive luminescence properties and great solubility of T12-Py, the optical information anticounterfeiting based on temperature was demonstrated by printing different concentrations of T12-Py solution on filter papers. Much more, this research may provide broad implications for the design of organic smart materials based on intermolecular interactions.

Strategic molecular design of closo-ortho-carboranyl luminophores to manifest thermally activated delayed fluorescence

In this paper, we propose a strategic molecular design of closo-o-carborane-based donor-acceptor dyad system that exhibits thermally activated delayed fluorescence (TADF) in the solution state at ambient temperature. Planar 9,9-dimethyl-9H-fluorene-based compounds with closo- and nido-o-carborane cages appended at the C2-, C3-, and C4-positions of each fluorene moiety (closo-type: 2FC, 3FC, 4FC, and 4FCH, and nido-type: nido-4FC = [nido-form of 4FC]·[NBu4]) were prepared and characterized. The solid-state molecular structure of 4FC exhibited a significantly distorted fluorene plane, which suggests the existence of severe intramolecular steric hindrance. In photoluminescence measurements, 4FC exhibits a noticeable intramolecular charge transition (ICT)-based emission in all states (solution at 298 K and 77 K, and solid states); however, emissions by other closo-compounds were observed in only the rigid state (solution at 77 K and film). Furthermore, nido-4FC did not exhibit emissive traces in any state. These observations verify that all radiative decay processes correspond to ICT transitions triggered by closo-o-carborane, which acts as an electron acceptor. Relative energy barriers calculated by TD-DFT as dihedral angles around o-carborane cages change in closo-compounds, which indicates that the structural formation of 4FC is nearly fixed around its S0-optimized structure. This differs from that for other closo-compounds, wherein the free rotation of their o-carborane cages occurs easily at ambient temperature. Such rigidity in the structural geometry of 4FC results in ICT-based emission in solution at 298 K and enhancement of quantum efficiency and radiative decay constants compared to those for other closo-compounds. Furthermore, 4FC displays short-lived (∼0.5 ns) and long-lived (∼30 ns) PL decay components in solution at 298 K and in the film state, respectively, which can be attributed to prompt fluorescence and TADF, respectively. The calculated energy difference (ΔE ST) between the first excited singlet and triplet states of the closo-compounds demonstrate that the TADF characteristic of 4FC originates from a significantly small ΔE ST maintained by the rigid structural fixation around its S0-optimized structure. Furthermore, the strategic molecular design of the o-carborane-appended π-conjugated (D-A) system, which forms a rigid geometry due to severe intramolecular steric hindrance, can enhance the radiative efficiency for ICT-based emission and trigger the TADF nature.

Red to Near-Infrared Mechanochromism from Metal-free Polycrystals: Noncovalent Conformational Locks Facilitating Wide-Range Redshift

Piezochromic organic materials that present a large difference in fluorescence wavelength in the near-infrared region have important potential applications; however, few such metal-free luminophores have been reported. In this study, we design and prepare π-conjugated electron acceptors whose planar conformation can be locked by the noncovalent interactions. The planar fused-ring geometry can narrow the optical band gap, enhance the molecular stability and rigidity, as well as increase the radiative rate. As expected, the polymorphs Re-phase and Ni-phase emit the high-brightness fluorescence with wavelength maxima (λ_em,max ) at 615 and 727 nm, respectively. Upon full grinding, the λ_em,max of Re-phase is bathochromically shifted to 775 nm. The ground powder of Re-phase becomes metastable as a consequence of noncovalent conformational locking and that the red to near-infrared (large colour difference) mechanochromism arises from the high degree of conformational coplanarity. This strategy is both conceptually and synthetically simple and offers a promising approach to the development of organic piezochromic materials with wide-range redshift and excellent penetrability.

Influence of Electronic Environment on the Radiative Efficiency of 9-Phenyl-9H-carbazole-Based ortho-Carboranyl Luminophores

The photophysical properties of closo-ortho-carboranyl-based donor–acceptor dyads are known to be affected by the electronic environment of the carborane cage but the influence of the electronic environment of the donor moiety remains unclear. Herein, four 9-phenyl-9H-carbazole-based closo-ortho-carboranyl compounds (1F, 2P, 3M, and 4T), in which an o-carborane cage was appended at the C3-position of a 9-phenyl-9H-carbazole moiety bearing various functional groups, were synthesized and fully characterized using multinuclear nuclear magnetic resonance spectroscopy and elemental analysis. Furthermore, the solid-state molecular structures of 1F and 4T were determined by X-ray diffraction crystallography. For all the compounds, the lowest-energy absorption band exhibited a tail extending to 350 nm, attributable to the spin-allowed π–π* transition of the 9-phenyl-9H-carbazole moiety and weak intramolecular charge transfer (ICT) between the o-carborane and the carbazole group. These compounds showed intense yellowish emission (λ_em = ~540 nm) in rigid states (in tetrahydrofuran (THF) at 77 K and in films), whereas considerably weak emission was observed in THF at 298 K. Theoretical calculations on the first excited states (S₁) of the compounds suggested that the strong emission bands can be assigned to the ICT transition involving the o-carborane. Furthermore, photoluminescence experiments in THF‒water mixtures demonstrated that aggregation-induced emission was responsible for the emission in rigid states. Intriguingly, the quantum yields and radiative decay constants in the film state were gradually enhanced with the increasing electron-donating ability of the substituent on the 9-phenyl group (‒F for 1F < ‒H for 2P < ‒CH₃ for 3M < ‒C(CH₃)₃ for 4T). These features indicate that the ICT-based radiative decay process in rigid states is affected by the electronic environment of the 9-phenyl-9H-carbazole group. Consequently, the efficient ICT-based radiative decay of o-carboranyl compounds can be achieved by appending the o-carborane cage with electron-rich aromatic systems.

Impact of deboronation on the electronic characteristics of closo-o-carborane: intriguing photophysical changes in triazole-appended carboranyl luminophores

5-Phenyl-1,2,4-triazole-appended closo- (CB1 and CB2) and nido-o-carboranyl (nido-CB1 and nido-CB2) compounds were prepared and fully characterized using multinuclear NMR spectroscopy and elemental analysis. The solid-state molecular structures of both closo-compounds were analyzed by X-ray crystallography. Although the closo-compounds exhibited dual emissive patterns in the rigid state (in THF at 77 K), which were assignable to a π-π* local excitation (LE)-based emission (λ_em = ca. 380 nm) on the triazole moieties and to an intramolecular charge transfer (ICT)-based emission (ca. 460 nm) in which the o-carborane units acted as the acceptor (A), at 298 K in THF, the LE-based emission dominated. In contrast, the nido-compounds exhibited an intensive emission originating from ICT transitions in which the o-carborane units reversibly acted as the donor (D). In particular, the positive solvatochromic effects of both nido-compounds and the results of theoretical calculations for the o-carboranyl compounds supported the electronic role of the o-carboranyl unit in each compound. Investigation of the radiative decay mechanism of the closo- and nido-compounds using their quantum efficiency (Φ_em) and decay lifetime (τ_obs) suggested that the ICT-based radiative decay of nido-compounds occurred more efficiently than the LE-based decay of closo-compounds. These results implied that emission from the closo-compounds was drastically enhanced by the deboronation reaction upon exposure to an increasing concentration of fluoride anions, and finally became similar to the emission color (sky-blue) of the nido-compounds.