Conformer aggregates exhibit dual wavelength emissions on chiral binaphthyl-based triphenylethylenes and acetone detection.

The study on structure-property relationship has been a significant focus in the field of organic molecular luminescence. In the present work, three chiral binaphthyl-based triphenylethylene (HTPE) derivatives were prepared through condensation reactions. Despite their similar structures, these compounds exhibited distinct luminescent properties. Diphenylmethane-derived HTPE displayed dual-state emissions, characterized by dual-wavelength emissions which were insensitive to the polarity of solvents. The dual emissions in solution state could be attributed to the different locally excited (LE) excitons. However, upon aggregation, two stable conformers were generated, probably leading to different emission peaks. In contrast, dibenzocycloheptadiene-derived HTPE aggregates showed only a single emission peak. Surprisingly, fluorene-derived HTPE exhibited obvious luminescence in neither solution nor aggregate states due to inherent π-π interactions. These conclusions were substantiated by X-ray analysis, spectroscopic analysis, and theory calculations. Application studies demonstrated that fluorescence on/off switches could be achieved through exposure to acetone. More importantly, trace amounts of acetone could be detected using luminescent materials in both organic and aqueous phases with a detection limit of 0.08 %. Thus, this work not only presents a strategy for designing chiral triphenylethylene fluorophores but also provides valuable information for dual wavelength emissions resulting from two stable conformations.

Visible-light Excited and Highly Photostable Organic Fluorophore with dual-state Emission.

Organic fluorophores with dual-state emission (DSE) are rare or difficult to observe because most of them display either aggregation-induced emission (AIE) or aggregation-caused quenching (ACQ). Amazing works have been accomplished, yet most of the DSE compounds were excited by UV light which limits their wide application in bioimaging. In this work, we achieved a visible-light excited DSE fluorophore and realized its imaging in SKOV-3 cells and zebrafish. The naphtho[2',3':4,5]imidazo[1,2-a]pyridine (NIP) core ensures its emission in dilute solution. Meanwhile, the twisted phenyl ring blocks fluorescence quenching induced by the π-π stacking and leads to the emission of the solid. The fluorescence intensity is steady even after 6 h of continuous intense sunlight. More importantly, photostability of NIP in cells is much better than commercial dye (mitochondrial green).

Designing Dual-State and Aggregation-Induced Emissive Luminogens from Lignocellulosic Biosourced Molecules.

Utilizing lignocellulosic biosourced platforms, we synthesized novel cyanostilbene (CS) derivatives featuring the 3,4-dimethoxyphenyl moiety. These derivatives were investigated for their emission properties in both solution and solid states. The two simple CS derivatives exhibit very weak luminescence in solution but significant luminescence in the solid state, indicating distinct Aggregation-Induced Emission (AIE) characteristic. Furthermore, combining these two CS units, without conjugation and with quasi perpendicular orientation, results in a Dual-State Emission (DSE) fluorophore showing luminescence both in solution and solid states. X-ray crystallography studies on the solid-state compounds reveal a structure-emission relationship, demonstrating that the colour emission correlates with the conformations adopted by the molecules in the solid state, which influence the type of stacking.

Achieving Controllable Thermochromic Fluorescence via Synergistic Intramolecular Charge Transfer and Molecular Packing.

Thermochromic fluorescent materials (TFMs) have attracted significant attention due to their unique fluorescent colorimetric response to temperature. However, existing TFMs still suffer from weak stimulus responsiveness, broad temperature response ranges, uncontrollable emission color changes, and low quantum yields. In this study, we address these issues by designing and synthesizing three diketone-boron complexes with distinct emission wavelengths (NWPU-(2-4)). Utilizing a molecular engineering strategy to manipulate intramolecular charge transfer transitions and molecular packing modes, our synthesized complexes exhibit efficient fluorescence emission in both solution and solid states. Moreover, their emission wavelengths are highly sensitive to environmental polarity. By incorporating these compounds into thermosensitive matrices of long-chain alkanes, we produced TFMs with varied fluorescence emission peak variation ranges. Notably, the TFM based on NWPU-4, owing to its strong charge transfer transitions and dense J-aggregate packing configuration, not only exhibits intense fluorescence emission spanning the deep red to near-infrared spectrum but also displays a remarkable 90 nm broad range of thermochromic properties. Ultimately, it was successfully applied to programmable, thermally controlled, multi-level information encryption.

Conjugating Coumarin with Tetraphenylethylene to Achieve Dual-State Emission for Reversible Mechanofluorochromism and Live Cell Imaging.

Dual-state emission luminogens (DSEgens) are receiving research interest in the construction of multifunctional materials due to their inherent advantage of high emission efficiency in both the molecularly dispersed solution state and the solid state. However, it remains challenging in synthesizing DSEgens via a delicate manipulation of the molecular structures. This work presents an example of bright DSEgen synthesis by tuning the molecular electronic structures and conformations. Three coumarin-tetraphenylethylene (TPE) molecules with a donor-acceptor electronic structure and highly twisting conformations have been synthesized. While compound resulting from direct conjugation of coumarin with a TPE unit shows aggregation-induced emission, compound with an N,N-diaminoethyl modification on the 7-position of coumarin and compound with a further phenyl linker between coumarin and TPE units feature strong dual-state emission. Benefiting from their strong solid emission and twisting conformations, these fluorophores display reversible mechanofluorochromism. Finally, applications for rewritable information storage in the solid state and live-cell imaging in the solution state were demonstrated.

Design Concepts for Solution and Solid-State Emitters - A Modern Viewpoint on Classical and Non-Classical Approaches.

For a long time, luminescence phenomena were strictly distinguished between the emission of isolated molecules in dilute solutions or close-packed structures such as in powders or aggregates. This changed with the breakthrough observation of dual-state efficient materials, which led to a rapid boost of publications examining the influence of structural features to achieve balanced emission with disregarded molecular surroundings. Some first general structural design concepts have already been proposed based on reoccurring patterns and pivotal motifs. However, we have found another way to classify these solution and solid-state emitters (SSSEs). Hence, this minireview aims to present an overview of published structural features of SSSEs while shining light on design concepts from a more generalized perspective. Since SSSEs are believed to bridge the gap of hitherto known aggregation-sensitive compound classes, we hope to give future scientists a versatile tool in hand to efficiently design novel luminescent materials.

Resonance-Enhanced Emission Effects toward Dual-State Emissive Bright Red and Near-Infrared Emitters.

Resonance-enhanced emission (REE) effect was discovered and lead to a novel dye family of hydrostyryl pyridinium derivatives in our recent work. Herein, the REE effect was employed to design a red and near-infrared dual-state emissive fluorophore family of SW-OH-NO2 derivatives which were easily synthesized by coupling an electron-withdrawing group (W) onto nitro(hydroxyl)styryl (S-OH-NO2 ) through a C=C double bond as π-bridge. The deprotonation of a phenolic hydroxyl group promoted by a nitro group and the electron-withdrawing group (W) on the other side of the π-bridge triggered resonance, resulting in significantly red-shifted emission. All the resultant SW-OH-NO2 compounds showed excellent dual-state emission behavior. Remarkably, hydrostyryl quinolinium (SQ-OH-NO2 ) is one of the smallest NIR emitter molecular skeleton (λem =725 nm, MW<400) and showed dual-state emission characteristics and obvious viscosity-depended fluorescent behaviors. In addition to constructing electron donor-acceptor structures and prolonging π-bridges, the REE effect promises a reliable strategy toward novel fluorophores with small size, long emissive wavelength, and dual-emission characteristics, and importantly, feasible industrial manufactures and applications due to their easy and low-cost synthesis strategy.

Lighting Up Fluorescence: Precise Recognition of Halogenated Solvents Through Effective Fluorescence Detection Using Chalcone Derivatives as a D-A-D-A-type Fluorescent Chemosensor.

In this paper, we report a D-A-D-A-type fluorescence sensor, FX, composed of triphenylamine and pyrazine units as electron donors, pyridine units, and α-ß unsaturated carbon-based structures as electron acceptors. FX exhibits typical ICT characteristics. As a dual-emission material, FX undergoes acid-base-induced color changes and displays mechanofluorochromic properties in the solid state. In solution, FX, as an AIE material, shows significant fluorescence enhancement behavior in most halogenated solvents. Notably, it achieves a high quantum yield of 0.672 in a chloroform solution. We utilized this phenomenon to quantitatively detect chloroform through fluorescence titration analysis, with a detection limit of 0.061%. Additionally, we developed a test paper to verify the practical applicability of the sensor for detecting halogenated solvents. The fluorescence enhancement behavior was confirmed through DFT calculations. The results indicate that FX is not only a multifunctional dual-state emission material but also provides valuable references for the fluorescence detection of halogenated solvents.

Dual-state dual emission from precise chemically engineered bi-ligand MOF free from encapsulation and functionalization with self-calibration model for visual detection.

Synthesis of dual-state dual emitting metal-organic frameworks (DSDE-MOFs) is uncommon and challenging. Additionally, DSDE-MOFs can fulfil the expanding need for on-site detection due to their stability and self-reference for a variety of non-analyte variables. In the present work, a novel intrinsic DSDE of chemically engineered bi-ligand Eu-based MOF (UoZ-1) was designed. The prepared UoZ-1 spherical particles were small-sized around 10-12 nm and displayed blue (425 nm) and red fluorescence (620 nm) at both states, dispersed in liquid and in solid state, when excited at 250 nm. A ratiometry platform was developed since the red emission was quenched by the addition of folic acid and the blue emission was almost remained unaffected. In the fluorometric ratiometric-mode, a dynamic linear range was recorded from 10 to 200 µM with LOD about 0.4 µM. Visual-based detection with assistance of smartphone was developed for quantification based on RGB analysis using Color Grab App. In the visual-mode, LOD as small as 2.3 µM was recorded. By utilizing the intrinsic dual-emitting UoZ-1, highly stable, recyclable, sensitive, and selective on-site visual detection of folic acid can be achieved. UoZ-1, a DSDE-MOF with no encapsulation or functionalization requirements, exhibits great potential for diverse applications.

Enhancing Dual-State Emission in Maleimide Fluorophores through Fluorocarbon Functionalisation.

Herein, a library of trifluoroethyl substituted aminomaleimide derivatives are reported with small size and enhanced emissions in both solution and solid-state. A diCH2 CF3 substituted aminochloromaleimide exhibits the most efficient dual-state emission (Φf >50 % in solution and solid-state), with reduced quenching from protic solvents. This is attributed to the reduction of electron density on the maleimide ring and suppressed π-π stacking in the solid-state. This mechanism was explored in-depth by crystallographic analysis, and modelling of the electronic distribution of HOMO-LUMO isosurfaces and NCI plots. Hence, these dual-state dyes overcome the limitations of single-state luminescence and will serve as an important step forward for this rapidly developing nascent field.

Rational Design and Facile Synthesis of Dual-State Emission Fluorophores: Expanding Functionality for the Sensitive Detection of Nitroaromatic Compounds.

Six novel benzimidazole-based D-π-A compounds 4 a-4 f were concisely synthesized by attaching different donor/acceptor units to the skeleton of 1,3-bis(1H-benzimidazol-2-yl)benzene on its 5-position through an ethynyl link. Due to the twisted conformation and effective conjugation structure, these dual-state emission (DSE) molecules show intense and multifarious photoluminescence, and their fluorescence quantum yields in solution and solid state can be up to 96.16 and 69.82 %, respectively. Especially, for excellent photostability, obvious solvatofluorochromic and extraordinary wide range of solvent compatibility, DSE molecule 4 a is a multifunctional fluorescent probe for the visual detection of nitroaromatic compounds (NACs) with the limit of detection as low as 10-7 M. The quenching mechanism has been proved as the results of photoinduced electron transfer and fluorescence resonance energy transfer processes. Importantly, probe 4 a can sensitively detect NACs not only in real water samples, but also on 4 a-coated strips and 4 a@PBAT thin films.

Dual-State Emission of 2-(Butylamino)Cinchomeronic Dinitrile Derivatives.

New representatives of 2-(butylamino)cinchomeronic dinitrile derivatives were synthesized as promising fluorophores showing dual-state emission. To characterize the influence of the length (from methyl to butyl) and the structure (both linear and branched) of the alkyl substituent at the amino nitrogen atom, the spectral fluorescence properties of all synthesized compounds were carefully studied both in solution and in solid state. The highest photoluminescence quantum yield values of 63% were noted for solutions of 2-(butylamino)-6-phenylpyridine-3,4-dicarbonitrile in DCM and 2-(butylamino)-5-methyl-6-phenylpyridine-3,4-dicarbonitrile in toluene.

Excited-State Intramolecular Proton Transfer Dyes with Dual-State Emission Properties: Concept, Examples and Applications.

Dual-state emissive (DSE) fluorophores are organic dyes displaying fluorescence emission both in dilute and concentrated solution and in the solid-state, as amorphous, single crystal, polycrystalline samples or thin films. This comes in contrast to the vast majority of organic fluorescent dyes which typically show intense fluorescence in solution but are quenched in concentrated media and in the solid-state owing to π-stacking interactions; a well-known phenomenon called aggregation-caused quenching (ACQ). On the contrary, molecular rotors with a significant number of free rotations have been engineered to show quenched emission in solution but strong fluorescence in the aggregated-state thanks to restriction of the intramolecular motions. This is the concept of aggregation-induced emission (AIE). DSE fluorophores have been far less explored despite the fact that they are at the crossroad of ACQ and AIE phenomena and allow targeting applications both in solution (bio-conjugation, sensing, imaging) and solid-state (organic electronics, data encryption, lasing, luminescent displays). Excited-State Intramolecular Proton Transfer (ESIPT) fluorescence is particularly suitable to engineer DSE dyes. Indeed, ESIPT fluorescence, which relies on a phototautomerism between normal and tautomeric species, is characterized by a strong emission in the solid-state along with a large Stokes' shift, an enhanced photostability and a strong sensitivity to the close environment, a feature prone to be used in bio-sensing. A drawback that needs to be overcome is their weak emission intensity in solution, owing to detrimental molecular motions in the excited-state. Several strategies have been proposed in that regard. In the past few years, a growing number of examples of DSE-ESIPT dyes have indeed emerged in the literature, enriching the database of such attractive dyes. This review aims at a brief but concise overview on the exploitation of ESIPT luminescence for the optimization of DSE dyes properties. In that perspective, a synergistic approach between organic synthesis, fluorescence spectroscopy and ab initio calculations has proven to be an efficient tool for the construction and optimization of DSE-ESIPT fluorophores.

Photophysical Switching between Aggregation-Induced Phosphorescence and Dual-State Emission by Isomeric Substitution.

It is attractive but highly challenging to achieve controllable regulation of photophysical properties of pure organic luminogens, due to distinct work mechanisms and molecular structures. Here, a strategy to regulate in a controllable way the emission behavior of luminogens is reported, according to which long-lived aggregation-induced emission (AIE) can be switched to short-lived dual-state emission (DSE) by an isomer-based substitution reaction. Three luminogens with sharply different photophysical behaviors, including aggregation-induced phosphorescence and dual-state fluorescence emission, were obtained through a substitution reaction with three isomers. Freely rotating structures are attributed to aggregation-induced phosphorescence behavior, whereas twisted rigidification of the molecule greatly contributes to its dual-state emission phenomenon. This work contributes to the controlled regulation of photophysical behaviors through simple reactions and provides a solid evidence to support the key role of the prohibition of intramolecular rotation in aggregation-induced emission process and molecular design of dual-state emitters.

Rational Design of Dual-State Emission Luminogens with Solvatochromism by Combining a Partially Shared Donor-Acceptor Pattern and Twisted Structures.

We report a general design strategy for a new class of luminogens with dual-state emission (DSEgens) that are brightly emissive in both the solution and solid state, with solvatochromism properties, by constructing a partially shared donor-acceptor pattern based on a twisted molecule. The DSEgens with bright fluorescence emission in both the solid and solution state demonstrate a unique solvatochromism behaviour depending on solvent polarity and thus may have applications in anti-counterfeiting.

A Strategy of "Self-Isolated Enhanced Emission" to Achieve Highly Emissive Dual-State Emission for Organic Luminescent Materials.

Currently, the commonly developed organic luminescent materials (OLMs) usually exhibit poor luminescent performance in aggregated solid states compared with their well-dissolved solution states, making it a tough goal to achieve the highly emissive dual-state emission. To overcome this limitation, a "self-isolated enhanced emission" (SIEE) strategy through flexible alkyl chains to suppress the emission-quenched π-π stacking in solids is proposed here and, based on this guideline, remarkable emission efficiency with photoluminescence quantum yields up to 99.72 % in solution and 77.46 % in the solid state are achieved for the SIEE constructed DBBT-C8, which is then successfully used in solid-state displays and data encryption.

The Dual-State Luminescent Mechanism of 2,3,4,5-Tetraphenyl-1H-pyrrole.

2,3,4,5-Tetraphenyl-1H-pyrrole (TePP) was synthesized by a simple one-step reaction. The compound showed a balanced emission in both the solution and solid state with the absolute quantum yield of ΦF/THF =65.6 % and ΦF/solid =74.3 %, respectively. Temperature and viscosity variation measurements demonstrated that the phenyl group at the 1-position (N-position) of the pyrrole core can act as a rotor in pyrrole-based molecules, which can consume the excited energy and reduce the molecular emission in solution. TePP without the phenyl group at the 1-position can effectively enhance the emission in solution. Single-crystal analysis showed that the phenyl groups at the 2,5-positions of pyrrole extend the molecular conjugation and lock the conformation. The phenyl groups at the 3,4-positions with a twisted conformation prevent their molecules from close packing and are helpful for aggregated emission. A delicate balance between the twisting conformation and rigid conjugation takes advantage of both ACQ and AIE luminogens. The strategy can tune the AIE, ACQ, or solution and solid dual-state emission properties of pyrrole-based molecules by simply altering the position of phenyl groups, which provides a great opportunity to explore the luminescent mechanism in greater detail and to facilitate practical applications.

Novel Dual-Emission Emitters Featuring Phenothiazine-S-Oxide and Phenothiazine-S,S-Dioxide Motifs.

In this study, we have successfully designed and synthesized two novel dual-emission emitters featuring phenothiazine-5-oxide and phenothiazine-5,5-dioxide motifs, characterized by highly lopsided and asymmetric conformational states. Through rigorous spectral examinations and DFT calculations, the compounds exhibit distinctive ICT phenomena, coupled with efficient emission in solid states and AIEE characteristics under high water fractions in DMF/H2O mixtures. These non-planar luminogens exhibit vibrant green and blue solid-state luminescence, with fluorescence quantum yields of 24.1 % and 15.21 %, respectively. Additionally, they both emit green fluorescence in THF solution, with notable emission quantum yields (QYs) 36.4 % and 30.4 %. Comprehensive theoretical investigations unveil well-defined electron cloud density separation between the energies of HOMO/LUMO levels within the two luminogens. Notably, the targeted molecule harboring the phenothiazine-S,S-dioxide motif also demonstrates remarkable reversible mechanofluorochromic properties. Moreover, we testify their potential in applications such as solid-state rewritable information storage and live-cell imaging in solution states. Through theoretical calculations and comparative studies, we have explored the intrinsic relationship between molecular structure and performance, effectively screening and identifying new fluorescent molecules exhibiting outstanding luminescent attributes. These discoveries establish a robust theoretical and technical foundation for the synthesis and application of efficient DSE-based MFC materials, opening new avenues in the realm of advanced luminescent materials.

Red/near-infrared (NIR) difluoroboron ß-diketonate derivatives with reversible mechanochromism for cellular imaging.

Near-infrared (NIR) fluorophores have promoted the development of materials for bioimaging, but traditional NIR dyes usually suffer from aggregation-caused quenching (ACQ), impeding their applications. Herein, we propose two difluoroboron ß-diketonate complexes TBO and TBS, consisting a donor-acceptor (D-A) structure with triphenylamine (TPA) moiety as an electron donors and difluoroboron as well as furan or thiophene building block as an electron acceptor. The theoretical calculation and optical data shows that both of them have intramolecular charge transfer (ICT) characteristics. Such ICT characteristics endow them with both solvatochromism and dual-state emission (DSE) properties. In the solvent CH2Cl2, the emission wavelength of TBO ranges from 550 nm to 750 nm, with a low fluorescence quantum yield (Φ = 7.0 %). However, in the less polar solvent hexane, the emission wavelength blue-shifts, with an increased Φ reaching up to 18 %. Moreover, TBO and TBS exhibit mechanochromic characteristics and rare multi-channel fluorescence emission phenomena at solid-state. Their solid-state samples can emit fluorescence in four spectral bands with maximum emission wavelengths at 300 nm, 400 nm, 600 nm, and 770 nm under excitation at 240 nm. These unique optical properties are expected to be utilized for detecting polarity of system and deformation. Moreover, according to the results of cell imaging and flow cytometry, TBO molecular were easily internalized into Hela cells and distributed in the cytoplasm with strong red fluorescence. Therefore, this research inspires more insight into development of NIR luminogens for biomedical imaging.

Dual-state emission, mechanofluorochromism, and lipid droplet imaging of asymmetric D-π-A-D'-type triads.

Dual-state emission (DSE) is an emerging phenomenon wherein organic luminescent molecules display bright emissions in both molecularly isolated and packed states, addressing the challenge associated with the traditional paradigm of dyes with mono-state emission. This study presents the design and synthesis of two unsymmetrical triads, TPCA and TPCT, featuring a D-π-A-D' electronic structure by integrating phenothiazines, triphenylamines, and cyanostilbene. Photophysical assessments reveal that both molecules serve as robust DSEgens, exhibiting strong emissions in both solution and solid phases. TPCA displays ΦTHF 53.2% and Φsolids 43.2%, while TPCT exhibits ΦTHF 49.6% and Φsolids 37.5%. However, due to differences in molecular conformation and packing, they diverge in solid-state emission wavelengths and mechanofluorochromic behavior. In the solid state, TPCA emits strong red fluorescence, contrasting with TPCT, which emits orange fluorescence. Furthermore, TPCA demonstrates significant mechanofluorochromism (MFC), shifting from yellow to yellow-red upon mechanical grinding, while TPCT exhibits negligible MFC owing to conformational distinctions. As robust and low-toxic bioimaging agents, both TPCA and TPCT prove highly effective for lipid-droplet imaging studies. This research contributes valuable insights to the evolving field of DSE materials, elucidating the promising applications and mechanisms governing their versatile emission behaviors.