Tuning Emission Wavelength of Polymorphous Crystal via Controllable Alkyl Chain Stacking and Its Vapor- and Thermo-Responsive Fluorescence

Tuning fluorescence colour of solid-state materials has become a topic of increasing interest for both fundamental mechanism study and practical applications such as sensors, optical recording and security printing. In this work, a fluorescent colour tuneable molecule BA-C16 is rationally designed and facilely synthesized by attaching flexible long alkyl chains to 2-hydroxybenzophenone azine ( BA ), which shows both aggregation-induced emission (AIE) and excited-state intramolecular proton transfer (ESIPT) characteristics. Compared to BA , the simple introduction of long alkyl chains in BA-C16 leads to an emission wavelength redshift from 542 to 558 nm. This strategy of extending emission wavelength is rarely reported, and is ascribed to the enlarged through-space π-conjugation between interplanar molecules in the aggregate of BA-C16 . Three crystals of BA-C16 are obtained with green, yellowish green and yellow emission. According to characterization by X-ray crystallography, X-ray powder diffraction and differential scanning calorimetry, alkyl chains play an important role in inducing different stacking modes of the three crystals, which further leads to polymorph-dependent fluorescence colour. BA-C16 exhibits tuneable solid-state fluorescence upon vapor fumigation, or annealing based on a transition between a “near-monomer” crystalline state and a “dimer” crystalline state. BA-C16 is further applied for rewritable fluorescence printing tuned by vapor- and thermal-treatment.     

Synthesis and characterization of fluorescent N-CDs/ZnONPs nanocomposite for latent fingerprint detection by using powder brushing method

This study reports the first new approach of using nitrogen functionalized carbon dots coated on zinc oxide nanoparticles (N-CDs/ZnONPs) as a novel nanocomposite for latent fingerprint detection using the powder brushing method. N-CDs/ZnONPs nanocomposite was prepared using melamine, potato peel waste and zinc acetate dehydrate as precursors. This nanocomposite was characterized by Fourier-transform infrared spectroscopy, UV–Visible spectroscopy, Fluorescence spectroscopy, X-ray diffraction, Raman spectroscopy, Zeta nanosizer, Scanning Electron Microscope, Energy-Dispersive X-ray Spectrometry and Transmission Electron Microscopy. The size of N-CDs was around at 50–20 nm and ZnONPs was around at 40–50 nm. The quantum yield of N-CDs increased the fluorescence intensity of the fluorophore by 5.54%. The N-CDs were coated on surface of ZnONPs to increase the quantum yield and increase the blue emission after formation of N-CDs/ZnONPs by 5.12%.The N-CDs/ZnONPs nanocomposite demonstrated extraordinary sensitivity and selectivity for Latent Fingerprint (LPF) detection on the distinctive non-porous substrates which included aluminum foil, aluminum sheets, an aluminum rod, an iron disc, a compact disc, a black mat, white marble and magazine paper. This nanocomposite acts as a labeling agent and it helped to detect LFP with clear readability ridges and high contrast fingerprint images under UV light irradiation. N-CDs/ZnONPs nanocomposite additionally demonstrated superior ability to reveal readability ridges and clarity and high contrast LFP images with 415 nm and 450 nm light sources and a yellow filter by using a Living Image Microscope. This nanocomposite exhibited advantages such as improved efficiency, a non-toxic nature, good optical properties and good results in the LFP detection of the freshly applied fingerprints. N-CDs/ZnONPs nanocomposite is, therefore, a good alternative material for detection of latent fingerprints in crime investigations.     

High Excimer‐State Emission of Perylene Bisimides and Recognition of Latent Fingerprints

High excimer‐state emission in the H‐type aggregate of a novel asymmetric perylene bisimide derivative, 6 , with triethyleneglycol chains and lactose functionalization was achieved in water. Furthermore, its application for enhancing the visualization of transfer latent fingerprints from glass slides to the poly(vinylidene fluoride) (PVDF) membrane was explored, which showed clear images of the latent fingerprint in daylight and under 365 nm ultraviolet illumination.     

Synthesis and characterization of fluorescent Europium (III) complex based on D-dextrose composite for latent fingerprint detection

A europium salt-Na[Eu(5,5′-DMBP)(phen)₃]·Cl₃ (Eu(III)-CPLx) was prepared by using various precursors such as 5,5′-Dimethyl-2,2′-bipyridyl (5,5′-DMBP), 1,10-phenanthroline (phen) and europium chloride hexahydrate (EuCl₃·6H₂O) by a complexation method. The red emission fluorescent Na[Eu(5,5′-DMBP)(phen)₃]·Cl₃/D-Dextrose (Eu(III)-CPLx/D-Dex) composite was synthesized by using an adsorption method with Eu(III)-CPLx and D-Dextrose (D-Dex). The Eu(III)-CPLx and fluorescent (Eu(III)-CPLx/D-Dex) composites were characterized by numerous techniques. The fluorescent (Eu(III)-CPLx/D-Dex) composite demonstrated a strong red emission and controlled fluorescence quenching in the solid state and was consequently used in latent fingerprint (LFP) detection. The LFPs were developed by using a powder dusting method (PDM) with Eu(III)-CPLx and fluorescent Eu(III)-CPLx/D-Dex composites on different substrates under daylight and UV-light irradiation at 365 nm. The fluorescent Eu(III)-CPLx/D-Dex composite was effectively explored for developing LFP images on various substrates and also acts as a better labeling agent for LFP detection in forensic science crime scene investigations.     

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.     

Bridged Stilbenes: AIEgens Designed via a Simple Strategy to Control the Non-radiative Decay Pathway

To broaden the application of aggregation-induced emission (AIE) luminogens (AIEgens), the design of novel small-molecular dyes that exhibit high fluorescence quantum yield (Φ_fl) in the solid state is required. Considering that the mechanism of AIE can be rationalized based on steric avoidance of non-radiative decay pathways, a series of bridged stilbenes was designed, and their non-radiative decay pathways were investigated theoretically. Bridged stilbenes with short alkyl chains exhibited a strong fluorescence emission in solution and in the solid state, while bridged stilbenes with long alkyl chains exhibited AIE. Based on this theoretical prediction, we developed the bridged stilbenes BPST[7] and DPB[7], which demonstrate excellent AIE behavior.     

Structural Engineering of Luminogens with High Emission Efficiency Both in Solution and in the Solid State

Developing molecules with high emission efficiency both in solution and the solid state is still a great challenge, since most organic luminogens are either aggregation‐caused quenching or aggregation‐induced emission molecules. This dilemma was overcome by integrating planar and distorted structures with long alkyl side chains to achieve DAπAD type emitters. A linear diphenyl–diacetylene core and the charge transfer effect ensure considerable planarity of these molecules in the excited state, allowing strong emission in dilute solution (quantum yield up to 98.2 %). On the other hand, intermolecular interactions of two distorted cyanostilbene units restrict molecular vibration and rotation, and long alkyl chains reduce the quenching effect of the π–π stacking to the excimer, eventually leading to strong emission in the solid state (quantum yield up to 60.7 %).     

Femtogram Detection of Explosive Nitroaromatics: Fluoranthene‐Based Fluorescent Chemosensors

Herein we report a novel fluoranthene‐based fluorescent fluorophore 7,10‐bis(4‐bromophenyl)‐8,9‐bis[4‐(hexyloxy)phenyl]fluoranthene ( S₃ ) and its remarkable properties in applications of explosive detection. The sensitivity towards the detection of nitroaromatics (NACs) was evaluated through fluorescence quenching in solution, vapor, and contact mode approaches. The contact mode approach using thin‐layer silica chromatograp‐ hic plates exhibited a femtogram (1.15 fg cm^?2) detection limit for trinitrotoluene (TNT) and picric acid (PA), whereas the solution‐phase quenching showed PA detection at the 2–20 ppb level. Fluorescence lifetime measurements revealed that the quenching is static in nature and the quenching process is fully reversible. Binding energies between model binding sites of the S₃ and analyte compounds reveal that analyte molecules enter into the cavity created by substituted phenyl rings of fluoranthene and are stabilized by strong intermolecular interactions with alkyl chains. It is anticipated that the sensor S₃ could be a promising material for the construction of portable optical devices for the detection of onsite explosive nitroaromatics.     

Bridged Stilbenes: AIEgens Designed via a Simple Strategy to Control the Non‐radiative Decay Pathway

To broaden the application of aggregation‐induced emission (AIE) luminogens (AIEgens), the design of novel small‐molecular dyes that exhibit high fluorescence quantum yield (Φ_fl) in the solid state is required. Considering that the mechanism of AIE can be rationalized based on steric avoidance of non‐radiative decay pathways, a series of bridged stilbenes was designed, and their non‐radiative decay pathways were investigated theoretically. Bridged stilbenes with short alkyl chains exhibited a strong fluorescence emission in solution and in the solid state, while bridged stilbenes with long alkyl chains exhibited AIE. Based on this theoretical prediction, we developed the bridged stilbenes BPST[7] and DPB[7], which demonstrate excellent AIE behavior.     

Fluorescent probe as reporter on the local structure and dynamics in hydrolysis-condensation process of organotrialkoxysilanes

To get some information on the aggregation behaviors of the products derived from different organotrialkoxysilanes, the hydrolysis-condensation processes of some organotrialkoxysilanes have been examined by means of pyrene as fluorescent probe. The organotrialkoxysilanes used in the research were n-octadecyltri-methoxysilane (ODTMS), n-octyltrimethoxysilane (OTMS), 3-glycidoxypropyltrimethoxysilane (GTMS), 3-methacryloxypropyltrimethoxysilane (MAPTMS), and propyltrimethoxy-silane (PTMS). The results show that pyrene as fluorescence probe can respond sensitively not only to the organization state of the hydrolysates but also to the change in the organization state during the condensation process. The organization states during the hydrolysis and condensation can be explained in terms of structures of the products. In the initial stage, the silanols with long organic chains are amphiphilic molecules, and such nature of the silanols can be compared to that of a surfactant. Therefore, the excimer emission of pyrene is extremely obvious because of such silanols being prone to form aggregates. In the case of silanols having short alkyl groups or epoxy groups, these silanols homogenously disperse in solution, which results in the appearance of an only monomer emission of pyrene. In the late stage, the fluorescence behavior of pyrene is also sensitive to structural evolution of the silicates. The fluorescence spectra of pyrene during the condensation of the silanols with short alkyl groups or epoxy groups are almost in silence, indicating that the condensation products, with a low condensation degree, homogeneously disperse in solution. For the silanols with long hydrophobic substituents in different lengths, the changes in fluorescence spectra of pyrene during the condensation are varied. Commonly, the excimer emission is noticeable, implying that the condensation products with high condensation degree inhomogenously disperse in solution. However, the relative excimer/monomer fluorescence intensity is alkyl chain-length dependent. The longer alkyl chains in the condensation products result in the appearance of the obvious excimer emission. These phenomena imply that the condensation degree of the products increases with the length of the alkyl chains. Additionally, the distorted spectrum of pyrene appears in the case of the organotrialkoxysilanes with side chain substituent, illustrating that the steric hindrance between the substituents can be monitored by fluorescence of pyrene. All these results are verified by the fluorescence-quenching measurements. The approach in the present study gives new insights into the local structure and dynamics in hydrolysis-condensation process of organotrialkoxysilanes and emphasizes the influence of the self-assembling behavior.     

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.     

The photodimerization of a cinnamoyl moiety derivative in dilute solution based on the intramolecular chain interaction of gemini surfactant

A Gemini surfactant, sodium N, N'-di(4-n-butyloxy cinnamoly)-L-cystine, containing a cinnamoyl moiety in the alkyl chains and disulfide bond in the spacer was designed and synthesized. The incorporation of a cinnamoyl moiety into the alkyl chains of Gemini surfactant makes it easy to probe the conformational information of the amphiphile molecule. The UV/vis absorption spectra and steady-state fluorescence were investigated at a concentration far below the critical micelle concentration (cmc). Both blue-shift of absorption and red-shift of fluorescence emission spectra indicate the existence of intramolecular interaction between the two alkyl chains in Gemini surfactant in the singly dispersed state. Results based on the breakdown of the disulfide bond by dithiotheritol (DTT) further confirmed the conclusion. Moreover, the characteristic of intramolecular chain interaction in Gemini surfactant improves the topochemical geometrical requirements of cinnamoyl moiety and increases the local concentration of reactant in dilute solution. Utilizing the incorporation of cinnamoyl moiety into the alkyl chains of Gemini surfactant, the cinnamoyl moiety upon irradiation undergoes dimerization in dilute aqueous solution with high yield of 78%.     

Charge‐Transfer‐Induced Fluorescence Quenching of Anthracene Derivatives and Selective Detection of Picric Acid

2,6‐Divinylpyridine‐appended anthracene derivatives flanked by two alkyl chains at the 9,10‐position of the core have been designed, synthesized, and characterized by NMR, MALDI‐TOF, FTIR, and single‐crystal XRD. These anthracene derivatives are able to recognize picric acid (2,4,6‐trinitrophenol, PA) selectively down to parts per billion (ppb) level in aqueous as well as nonaqueous medium. Fluorescence emission of these derivatives in solution is significantly quenched by adding trace amounts of PA, even in the presence of other competing analogues, such as 2,4‐dinitrophenol (2,4‐DNP), 4‐nitrophenol (NP), nitrobenzene (NB), benzoic acid (BA), and phenol (PH). The high sensitivity of these derivatives toward PA is considered as a combined effect of the proton‐induced intramolecular charge transfer (ICT) as well as electron transfer from the electron‐rich anthracene to the electron‐deficient PA. Moreover, visual detection of PA has been successfully demonstrated in the solid state by using different substrates.     

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.     

Three-Dimensional Scaffolds for Light Emission

Organic fluorophores with highly efficient luminescence in both solution and solid states have attracted significant attention due to their ability to circumvent the limitations of aggregation-caused quenching and aggregation-induced emission type molecules. However, their development and wide-range applications are hampered by extremely complex synthetic methodologies and limited frameworks with dual-state emission (DSE) structural characteristics. In sharp contrast to the reported luminogens with big and planar π systems or highly conjugated and twisted structures, we discovered novel three-dimensional scaffolds in one molecule to achieve DSE. These molecules allowing for rapid access showed completely different molecular packing manners from those of planar conjugated molecules and exhibited excellent optoelectronic properties with diminished intermolecular π−π stacking interactions due to steric hindrance. Our findings should open new avenues for designing DSE molecules with new frameworks, which will enable more successful development of dual-state emitters for their broad applications in the future.     

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.     

Enhanced emission efficiency and excited state lifetime due to restricted intramolecular motion in silole aggregates

The aggregation-induced emission (AIE) properties of 1,1,2,3,4,5-hexaphenylsilole (HPS) and poly{11-[(1,2,3,4,5-pentaphenylsilolyl)oxy]-1-phenyl-1-undecyne} (PS9PA) were studied by time-resolved fluorescence technique. The enhanced fluorescence and long fluorescent lifetime were obtained for the sample in an aggregate state as compared to the sample in solution. The time-decay of fluorescence of HPS and PS9PA in high viscosity solvents and low-temperature glasses has also been measured in detail to further investigate the possible mechanism for AIE. Enhanced light emission and long fluorescence lifetime were detected for both HPS and PS9PA in the solution-thickening and -cooling experiments. These results provided direct evidence that the enhanced photoluminescence (PL) efficiency is due to restricted intramolecular motion, which ascribes AIE to the deactivation of nonradiative decay caused by restricted torsional motions of the molecules in the solid state or aggregate form.     

Alkyl chains length dependent fluorescence emission and reversible mechanofluorochromism of AIEE-based quinoline derivatives

Five quinoline derivatives containing different alkyl chains (QLACn, n?=?2, 4, 6, 8, 12) were synthesized to investigate the effect of alkyl chains on their photophysical properties. The fluorescent properties of the QLACn were affected by the alkyl chains, which indicated obvious blue-shifting trend and fluorescence emission are alkyl chains length-dependent by grinding-induced spectral shifts (Δλ_MFC). Longer alkyl-length revealed larger Δλ_MFC. Powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) measurements indicated that the transformation between microcrystalline and amorphous states upon external stimuli should be responsible for the observed reversible MFC behavior.     

Nanoparticle‐Enhanced Fluorescence Imaging of Latent Fingerprints Reveals Drug Abuse

No sweat! The sweat in a latent fingerprint (LFP) can contain orally ingested drugs and their metabolites. In a new method for drug detection, primary antibodies(Abs) against drug metabolites are conjugated to magnetic nanoparticles (NPs). The LFP is incubated with the NPs, excess particles removed, and the LFP treated with a fluorescently labeled secondary antibody. Fluorescence imaging then allows characterization.

     

Synthesis, two-photon absorption and AIE properties of multibranched thiophene-based triphenylamine derivatives with triazine core

Two new multibranched thiophene-based triarylamine derivatives with 1,3,5-triazine core are synthesized and characterized. Their one-and two-photon absorption properties and aggregation-induced emission effect have been investigated. Both the STAPA-based compounds are AIE active. The two-photon absorption (2PA) cross sections measured by the open aperture Z-scan technique are determined to be 620 and 1610 GM for STAPA-a and STAPA-b in chloroform,respectively, which dramatically increase with the introduction of alkyl chains. The relationship between their structures and properties on one-and two-photon absorption and aggregation-induced emission is discussed, which allows us to examine the effect of introducing alkyl chains. In addition, solvent effects also show a significant influence on the 2PA cross section. The two compounds with excellent AIE and 2PA properties provide attractive alternatives for the biophotonic materials.