Chiral Dual Core Chromophores Based on Binaphthyl Acrylonitrile Motif with Tunable Dual Emission Bands and Anti-counterfeiting

Small organic molecules which can emit fluorescence with tunable dual emission bands are significant for fundamental research and broad applications. In this work, two binaphthyl based arylacrylonitrile derivatives with pyrene and triphenylamine unit (BiNp-Py and BiNp-TPA) were designed and synthesized, respectively, featuring chiral backbone and dual AIE-active cyanostyrene-linked chromophores. Excellent fluorescence emissions in a range of solution and solid states were observed with high quantum yields, indicative of the solvatochromism and mechanochromism. More interestingly, dual emission bands were found and tunable by the water fraction in THF, and speculatively attributed to the balancing of intramolecular charge transfer (ICT) and locally excited (LE) emission in solution and aggregate states. Furthermore, the potential application in anti-counterfeiting ink was also explored, indicating the very low concentration (5 ppm) for sufficient distinguishable vision and small colour migration (28 nm) for printing on the filter. The present work provides a new strategy to design organic luminescent structure having widely fluorescent emissions in dual states and a valuable reference for the study of chiral optical materials.

Extending Conjugation of Linear Cyanostilbene Derivatives via a Pyridine Moiety for Multi-Stimuli-Responsive Fluorescence Organogels

In the design of effective gelators with aggregation-induced emission behavior, amide and cholesterol moieties are generally used to provide multiple driving forces for gelation. In this work, a series of linear cyanostilbene gelators with a pyridine moiety and different lengths of the alkoxyl group, i.e., CSpy-Cn, have been synthesized through nucleophilic substitution and Knoevenagel reaction. The direct connection of pyridine extends the conjugation of the cyanostilbene moiety, while the alkoxyl group can regulate the solubility of the compounds so that the compounds can serve as gelators for common solvents such as acetonitrile, dimethyl sulfoxide, and ethanol at ultra-low concentrations. At the same time, the cyanostilbene group makes the compounds undergo photoisomerization and emit fluorescence under UV light, while the pyridine group can serve as an acid-base responsive group due to easy protonation. The gels can respond to temperature, light, and organic acid/base. The fluorescence intensity and color can reversibly change during the gel-sol transitions. Finally, a thin film based on the CSpy-C8 xerogel has been prepared and utilized as a multi-stimuli-responsive fluorescence display for information storage and anti-counterfeiting.

Resolving fluorescence signatures of a photoconvertible fluorophore by fluorescence spectroscopy and MCR-ALS-based combinatorial approach

Photoconvertible fluorophores are important for a myriad of applications in chemistry and biology. Here, we spectrally resolve and quantify individual photophysical information of a dual-emitting photoconvertible fluorophore by fluorescence spectroscopy and multivariate curve resolution-alternate least square techniques. We found that the reactant fluorophore, which shows a weak locally excited (LE) emission and a dominant intramolecular charge transfer (ICT) emission, also exhibits an intermolecular charge transfer emission. The ICT emission bands of both the reactant and product fluorophores are originated from their respective LE states. The reactant fluorophore is a mixture of its different ground state conformers. Higher yields of photoconversion of the yellow-emitting reactant fluorophore are achieved via a visible light photoreaction, leading to formation of pure white light at an intermediate photoreaction time. These findings together help us to glean new photophysical and photochemical insights into the photoreaction of a dual-emitting photoconvertible fluorophore.

A donor-π-acceptor aggregation-induced emission compound serving as a portable fluorescent sensor for detection and differentiation of methanol and ethanol in the gas phase

The design strategy of aggregation-induced emission (AIE) fluorophores with donor-π-acceptor (D-π-A) conjugation structure has greatly contributed to the development of luminescent materials and devices, including volatile organic compounds (VOCs) sensors. In this work, a D-π-A fluorophore DEBAB was synthesized, showing both AIE and intramolecular charge transfer (ICT) properties as confirmed by spectroscopic data and quantum chemical calculations. Furthermore, there is notable emission-enhancement when DEBAB is exposed to small-molecule alcohols, such as methanol and ethanol. Based on this phenomenon, a portable film sensor was fabricated, capable of detecting methanol and ethanol in gas phase, with detection limit (DL) as low as 8.02 ppm. Our systematic investigation suggests that hydrogen-bonding may be formed between DEBAB and alcohols, intensifying the AIE efficacy while influencing the ICT process. This working mechanism is supported by density functional theory (DFT) calculations including electrostatic potential mapping and molecular total energy. In addition, a sensor array was fabricated on a cellulose paper strip, showing different levels of emission changing in response to alcohols. Thus the detection and differentiation of methanol and ethanol are enabled.

Dual emission in purely organic materials for optoelectronic applications

Purely organic molecules, which emit light by dual emissive (DE) pathways, have received increased attention in the last decade. These materials are now being utilized in practical optoelectronic, sensing and biomedical applications. In order to further extend the application of the DE emitters, it is crucial to gain a fundamental understanding of the links between the molecular structure and the underlying photophysical processes. This review categorizes the types of DE according to the spin multiplicity and time range of the emission, with emphasis on recent experimental advances. The design rules towards novel DE molecular candidates, the most perspective types of DE and possible future applications are outlined. These exciting developments highlight the opportunities for new materials synthesis and pave the way for accelerated future innovation and developments in this area.

D-A type sensor array for differentiation and identification of white wine varieties based on specific solvent effect activated by CT-LE transition

In this work, we synthesize a series of compounds with electron donor (D) and acceptor (A) units. They show general solvent effect in aprotic solvents, suggesting a charge transfer (CT) process. While in protic solvents including water, ethanol and methanol, the spectra exert no polarity-dependence but a remarkable hypochromatic shift together with the fading of CT band. Dynamic analysis implies that intermolecular hydrogen bond will be formed between carboxylic acid and protic solvent, boosting another deactivation pathway that jumps off a bigger energy gap, in other words, favoring the locally excited (LE) state emission. The CT-LE transition involves variations in both absorption and emission spectra, and further poses competition with other mechanisms including activated/restricted intramolecular rotation (IR/RIR). Inspired by the cross-reactivity, we turn our attention to the development of sensor array, in order to identify white wine varieties. The differential spectral responses are recorded, generating multiple factors including absorption wavelength (λ_ab), emission wavelength (λ_em), absorbance (Abs.) and emission intensity (Int.). These factors are processed with principal component analysis (PCA), creating a three-dimensional fingerprint data base for white wines. The data points in the coordinate system are clustered into 10 different groups, demonstrating a clear differentiation of all the white wines. More importantly, as our final test for whether the sensor array can identify the counterfeits, an adulterated liquor sample, which is provided by police officers, is fingerprinted on the three-dimensional diagram. Its canonical factors fall into an area distinct from the adulterated wine, indicating a clear identification.

A turn-on type stimuli-responsive fluorescent dye with specific solvent effect: Implication for a new prototype of paper using water as the ink

In this study, we reported the photoluminescence (PL) behaviour of a new intramolecular charge transfer (ICT) compound, ((E)-2-(((2-hydroxynaphthalen-1-yl)methylene)amino)benzoic acid, (HABA), which shows ICT solvent effect in aprotic solvents as confirmed by absorption and emission spectra. While in protic solvents including water and ethanol, the charge transfer (CT) band significantly reduces. Remarkable fluorescence enhancement in the blue region was also observed for HABA in polar protic solvents. We described such phenomena as "specific solvent effect". It can be ascribed to the hydrogen bonding formation between HABA and protic solvents, which not only causes significant reduction in the rate of internal conversion but also elevates the energy gap. Density functional theory (DFT) calculations as well as the dynamics analysis were performed to further verify the existence of hydrogen bonding complexes. Stronger emission turn-on effect was observed on HABA solid film when it is treated with water and base solution. The stimuli-responsive fluorescence of HABA enables a new green printing technique that uses water/base as the ink, affording fluorescent handwritings highly distinct from the background. Thermoanalysis of the dye suggests the nice thermostability, which is highly desired for real-world printing in a wide temperature range.

Axially chiral amino acid scaffolds as efficient fluorescent discriminators of methanol–ethanol

Differential solvation guided H-bonding interaction allows novel axially chiral amino acid scaffolds for efficient discrimination of ethanol–methanolviaa switch-on fluorescence response.