Dual-state Emission Research Articles

Achieving dual-state emission and high-contrast mechanochromic luminescence based on donor-acceptor type phenothiazine methanone derivatives

The development of fluorescent materials that can achieve dual-state emission (DSE) and high-contrast mechanochromic luminescence has attracted more and more attention. However, such materials are very rare, and there are no clear molecular design principles to follow. In current work, two novel DSE emitters are designed and synthesized by modifying phenothiazine and benzene or naphthalene rings on both sides of the methanone. Their photophysical properties in both dilute solution and the solid state are studied and compared. Optical researches have shown that D-A type phenothiazine methanone derivatives exhibit well-defined ICT (intramolecular charge transfer) characteristics. At the same time, they show intense luminescent efficiencies in both solution (ΦPTZ-CO-B = 0.645; ΦPTZ-CO-NA = 0.624) and solid states (ΦPTZ-CO-B = 0.386; ΦPTZ-CO-NA = 0.357), indicating their DSE properties. It is noteworthy that compounds PTZ-CO-B and PTZ-CO-NA exhibit significant fluorescence response to mechanical force, and the emitting wavelengths are red-shifted by 33 nm and 46 nm, respectively. XRD patterns and DSC curves confirm that the mechanochromism results from reversible changes in stacking pattern. The current work develops materials that can achieve DSE emission and high-contrast mechanochromism, laying the foundation for the development of such materials.

Influence of ethynyl extension on the dual-state emission properties of pyridinium-substituted ESIPT fluorophores

We describe the synthesis and photophysical properties in both solution and in the solid-state of dual-state emitters undergoing an Excited-State Intramolecular Proton Transfer (ESIPT) process. These fluorophores, built around the 2-(2′-hydroxyphenyl)benzoxazole (HBO) scaffold incorporate a N-methylpyridinium unit, at the 3 or 5 position of the phenol ring, prone to reduce the vibrational relaxation in the excited-state thanks to resonance effects and consequently allowing increasing the solution-state quantum yield (QY). In order to further optimize their optical properties, an additional ethynyl-extended triisopropylsilyl (TIPS) spacer was introduced with the aim to combine molecular rigidification and π-delocalization, as cooperative effects to enhance dual-state emission (DSE) properties. Molecular engineering studies and comparison with unsubstituted compounds reveal the significant impact of the position of the ethynyl-TIPS moiety on the photophysical properties, especially the fluorescence color. As several phenomena can compete with the ESIPT process, all dyes were modelled using ab initio calculations to rationalize the nature of the excited-states.

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.

Palladium-Catalyzed Cascade Reaction in Water to Imidazo[1,2-a]pyridazines as Switchable DSEgens, AIEgens, and ACQgens.

Dual-state emission (DSE) luminophores exhibit strong emissions in both solution and solid states, filling the gaps between aggregation-induced emissions (AIE) and aggregation-caused quenching (ACQ). However, limited design concepts and complicated synthetic strategies restrict the discovery of novel DSE molecules. Developing efficient and green methodologies to access novel DSE scaffolds via rational design remains highly desirable. In this work, we report a water-promoted Pd-catalyzed cascade reaction for the synthesis of multi-substituted imidazo[1,2-a]pyridazine derivatives with DSE properties. The intramolecular interactions of the neighboring benzene rings restrict molecular motion, leading to emissions in the solid state (quantum yield: 11 %), and the newly constructed core structure of imidazo[1,2-a]pyridazine ensures considerable planarity, allowing for emissions in solution. Further removal of the neighboring phenyl groups resulted in ACQgens, while additional methyl groups led to AIEgens. Subsequent live cell imaging investigations suggested that the novel DSEgens could serve as specific lipid droplet (LD) probes in a wide concentration range.

A novel fluorescent probe with ACQ-AIE conversion by alkyl chain engineering for simultaneous visualization of lipid droplets and lysosomes

The AIE bio-probes have attracted extensive attention because of their good brightness, long-term in situ retention ability, photostability and low cytotoxicity. Recently, the transformation of ACQ to AIE has become very popular, which is very important for the further development of AIE probes. Herein, a series of novel dyes (NR-Lyso-Ⅰ, NR-Lyso-Ⅱ, NR-Lyso-III, NR-Lyso-IV) were designed and synthesized. It was found that alkylation of 4-aminonaphthalimide could achieve the transformation of the dye from ACQ to AIE effect due to the growth of carbon chain. Moreover, the AIE probe NR-Lyso-IV exhibited dual-state emission (DSE) and large Stokes shift (＞100 nm), excellent selectivity, photostability, and low cytotoxicity, which was able to simultaneous visualize the lipid droplets (LDs) and lysosomes of HeLa cells and zebrafish.

Cover Feature: Asymmetric Dual‐State Emitters Featuring Thiazole Acceptors (Eur. J. Org. Chem. 35/2022)

The Cover Feature shows 2,5-disubstituted thiazole fluorescent dyes with an emergent phenomenon called dual-state emission (DSE). The cover shows the sunset in the courtyard of the famous UNAM library building in harmony with the glowing compounds. The cover image was designed by Armando Navarro Huerta (Institute of Chemistry of UNAM). More information can be found in the Research Article by J. L. Belmonte-Vázquez, B. Rodríguez-Molina et al.

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.

A Mild Silica Gel Promoted Synthesis and Initial Functional Study of Tetrapyridyl Tetrahydropyrrolopyrrolones.

A one-pot strategy with yields up to 82% was reported to generate 2-(pyridin-2-yl)-2-(3,3a,6-tris(5-pyridin-2-yl)-5-oxohexahydropyrrolo[3,2-b] pyrrol-2(1H)-ylidene)acetonitrile 1a and its derivatives 1b-d. Silica gel promoted quantitative conversion from stable intermediate to 1a within 30 min at room temperature. Finally, four chemical σ bonds and two chiral carbons with high diastereoselectivity were achieved. Compound 1a can act as a novel high selective UV-vis and fluorescence "turn-on" probe for Zn2+ and Cd2+, respond to proton, and show dual-state emission (DSE) characteristics.

α-Cyanostilbene functionalized carbazole derivatives exhibiting dual-state emission and multi-stimuli responsive fluorescent switching

Interest in organic luminescent materials that show strong dual state emission (DSE) in both solution and the solid state is rapidly growing due to their widespread applications. The development of DSE luminogens (DSEgens) with multi-stimuli responsive properties capable of achieving multi-parameter detection in complicated environments remains challenging due to the lack of an explicit molecular design strategy. In this work, we devised and prepared two D-π-A-π-D-structured carbazole derivatives, PY-CZ-Pa and PY-CZ-Ta, with intramolecular charge transfer (ICT) characteristics and excellent DSE properties. Both the compounds as fluorescent probes are highly sensitive to trace amount of water and picric acid (PA) in organic solvents. The limits of detection are determined to be 0.016% (PY-CZ-Pa) or 0.040% (PY-CZ-Ta) for water and 320 nM (PY-CZ-Pa) or 538 nM (PY-CZ-Ta) for PA. Moreover, the two compounds exhibit reversible mechanofluorochromism, with spectral shifts of over 24 nm. The results presented in this work suggest that bright DSEgens with multi-stimuli responsive properties are promising materials for applications at both the molecular and aggregated levels.

Strong dual-state emission of unsymmetrical and symmetrical thiazolothiazole-bridged imidazolium salts

A novel thiazolothiazole-bridged imidazole derivative (1) was found to exhibit blue fluorescence in gaseous state or in methanol and yellow fluorescence in solid state. The N-alkylation of imidazole subunit(s) in 1 using n-propyl iodide generated unsymmetrically or symmetrically alkylated thiazolothiazole-bridged imidazolium salts with good water solubility and remarkably strong emission in solution. Furthermore, the replacement of iodide counter-anion by triflate or bis(trifluoromethane sulfonyl)imide achieved remarkably strong emission in solid state and in solution as well as good water solubility. The strong fluorescence of dicationic salts with triflate and NTf2– counter-anions in solid state can be ascribed to their twisted and rigid structures induced by interionic C−H···F hydrogen bonding.

Dual-state emission of a coumarin acylhydrazone pH dye as colorimetric and fluorometric dual-channel indicator for seafood spoilage taggant

A coumarin hydrazone ligand 1 not only displays a dual-state emission but also has an ability to behave as a solid-state fluorescent sensor for vapor detection of volatile acid and ammonia (1 = 7-(diethylamino)-2-oxo-N′-(pyridin-2-ylmethylene)-2H-chromene-3-carbohydrazide). The mechanism of “OFF” fluorescent signal of conjugate acid of 1 (H1+) is due to the formation of H-aggregate with aggregation-caused quenching (ACQ) effect, however, molecule stacking of protonated counterpart H1+·ClO4·DCE with staircase arrangement of J-aggregates exhibited an orange emission in the solid state, acting as crystallization-induced emission (CIE) active nature. Moreover, the color and fluorescence emission of CIE-active molecule can be switched by external stimuli including grinding and solvent vapor. Utilizing the excellent OFF–ON response to HCl-NH3 vapor in the solid state, H1+ acts as a colorimetric and fluorometric dual-channel indicator upon exposure to total basic volatile nitrogen (TVB-N) compounds from fish as it spoils. The freshness of different batches, species and temperatures of fillets were readily monitored using standard chemometric and fluorometric approaches, providing in situ quality assurance (QA) information for fisheries industries and consumers.

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.

Accessing conjugated and twisted structures for efficient dual-state emission fluorophore and its sensitive lysosomal imaging

Herein, based on 1,8-naphtholactam, we have developed twisted and conjugated fluorophores, NPM-1 and NPM-2, which featured efficient emissions in both dispersion and aggregation states with quantum yields up to 0.76 and 0.84, respectively. From spectral and density functional theory (DFT) calculation, the luminescence mechanism of both compounds was intramolecular charge transfer (ICT) in this D-A type molecules, and the structural differences lead to differences in the intensity of ICT effects. Moreover, NPM-1 exhibited efficient and stable emissions in a variety of environments due to its rational structural design. It was a sensitive lysosome-targeting dye and retains its fluorescence intensity under continuous laser irradiation. All these features made it an unprecedented chemical tool for real-time monitoring of lysosomal status and changes, leading to a better understanding of physiological and pathological processes.

Ionic Rigid Organic Dual-State Emission Compound With Rod-Shaped and Conjugated Structure for Sensitive Al3+ Detection.

Dual-state emission (DSE) luminogens, a type of luminescent material which can effectively emit light in both dilute solution and solid states, have attracted tremendous attention, due to their widespread applications in chemical sensing, biological imaging, organic electronic devices, and so on. They overcome the shortcomings of aggregation-induced emission (AIE)-type compounds that do not emit light in dilute solutions and aggregation-caused quenching (ACQ)-type compounds that do not emit light in a concentrated or aggregated state. This work reports a novel ionic DSE material based on rigid rod-shaped organic conjugated structure using 4,4′-bis(2-sulfonatostyryl) biphenyl disodium salt (BSBDS); the ion repulsion effect can reduce the strong π–π interaction in aggregation and achieve high-efficiency luminescence in solution and solid states. In addition to excellent DSE characteristics, BSBDS also exhibits a mechanochromic nature and sensitive detection performance for aluminum ion (Al3+).

Dual-state emissionversusno emission by manipulating the molecular structures of cyanovinyl–benzofuran derivatives

A very small change in the molecular structure of D–A cyanostilbene derivatives enables non-emissive derivatives to become ones presenting high luminescence both in solution and in the solid state.

Highly efficient dual-state emission and two-photon absorption of novel naphthalimide functionalized cyanostilbene derivatives with finely tuned terminal alkoxyl groups

Cyanostilbene and naphthalimide are incorporated to construct novel dual-state emission luminogens (DSEgens), which show highly efficient photoluminescence and two-photon absorption properties in both dilute solution and the aggregated state.

D–A–D structured triphenylamine fluorophore with bright dual-state emission for reversible mechanofluorochromism and trace water detection

A donor–acceptor–donor-structured triphenylamine chromophore showing bright dual-state emission (DSE) has been synthesized and demonstrated multi-stimuli responsive properties, including water response, mechanochromism, and solvatochromism.

Pyrene and triphenylamine substituted cyanostyrene and cyanostilbene derivatives with dual-state emission for high-contrast mechanofluorochromism and cell imaging

Two α-cyanostilbene derivatives exhibiting bright emission in both well-dissolved solutions (Φ > 49%) and the solid state (Φ > 95%) have been synthesized as novel DSEgens for high-contrast mechanofluorochromism (>50 nm) and live cell imaging.

Synthesis, structures and fluorescence properties ofgem-linked cyclic tetraphenylethylenes and cyclic hexaphenylethylenes

Five rigid macrocyclic molecules ([n]CTPEs) and [n]CHPEs) have been synthesized and their structural features were analysed by DFT calculations. [2]CTPE exhibits dual-state emission property and a long solid-state fluorescence lifetime.

D–A-Type fluorophores with efficient dual-state emission for imaging at ultralow concentration

We demonstrate a D–A-type organic fluorophore with efficient dual-state emission and highlight its advantages in bioimaging applications.