Facile synthesis and characterization of aza-bridged all-benzenoid quinoidal figure-eight and cage molecules.

Synthesis of conjugated compounds with unusual shape-persistent structures remains a challenge. Herein, utilizing thermodynamically reversible intermolecular Friedel-Crafts alkylation, a dynamic covalent chemistry (DCC) reaction, we facilely synthesized a figure-eight shaped macrocycle FEM and cage molecules CATPA/CACz. X-ray crystallographic analysis confirmed the chemical geometries of tetracation FEM4+(PF6 -)4 and hexacation CACz6+(SbF6 -)6. FEM and CATPA displayed higher photoluminescence quantum yield in solid states compared to that in solution, whereas CACz gave the reverse result. DFT calculations showed that fluorescence-related frontier molecular orbital profiles are mainly localized on their arms consisting of a p-quinodimethane (p-QDM) unit and two benzene rings of triphenylamine or carbazole. Owing to their space-confined structures, variable-temperature 1H NMR measurements showed that FEM, CATPA and FEM4+ have intramolecular restricted motion of phenyl rings on their chromophore arms. Accordingly, FEM and CATPA with flexible triphenylamine subunits displayed aggregation-induced emission behavior (AIE), whereas CACz with a rigid carbazole subunits structure showed no AIE behavior.

Solvent-Responsive Nonporous Adaptive Crystals Derived from Pyridinium Hydrochloride and the Application in Iodine Adsorption.

Nonporous adaptive crystals (NACs) are crystalline nonporous materials that can undergo a structural adaptive phase transformation to accommodate specific guest via porous cavity or lattice voids. Most of the NACs are based on pillararenes because of their flexible backbone and intrinsic porous structure. Here a readily prepared organic hydrochloride of 4-(4-(diphenylamino)phenyl)pyridin-1-ium chloride (TPAPyH), exhibiting the solvent dimension-dependent adaptive crystallinity is reported. Wherein it forms a nonporous α crystal in a solvent with larger dimensions, while forming two porous ß and γ crystals capable of accommodating solvent molecules in solvent with small size. Furthermore, the thermal-induced single-crystal-to-single-crystal (SCSC) transition from the ß to α phase can be initiated. Upon exposure to iodine vapor or immersion in aqueous solution, the nonporous α phase transforms to porous ß phase by adsorbing iodine molecules. Owing to the formation of trihalide anion I2Cl- within the crystal cavity, TPAPyH exhibits remarkable performance in iodine storage, with a high uptaking capacity of 1.27 g g-1 and elevated iodine desorption temperature of up to 110 and 82 °C following the first and second adsorption stage. The unexpected adaptivity of TPAPyH inspires the design of NACs for selective adsorption and separation of volatile compound from organic small molecules.

Controllable π-π coupling of intramolecular dimer models in aggregated states.

π-π coupling as a common interaction plays a key role in emissions, transport and mechanical properties of organic materials. However, the precise control of π-π coupling is still challenging owing to the possible interference from other intermolecular interactions in the aggregated state, usually resulting in uncontrollable emission properties. Herein, with the rational construction of intramolecular dimer models and crystal engineering, π-π coupling can be subtly modulated by conformation variation with balanced π-π and π-solvent interactions and visualized by green-to-blue emission switching. Moreover, it can rapidly respond to temperature, pressure and mechanical force, affording a facile way to modulate π-π coupling in situ. This work contributes to a deeper understanding of the internal mechanism of molecular motions in aggregated states.

Multi-stimulus Room Temperature Phosphorescent Polymers Sensitive to Light and Acid cyclically with Energy Transfer.

The stimulus-responsive room temperature phosphorescent (RTP) materials have endowed wide potential applications. In this work, by introducing naphthalene and spiropyran (SP) into polyacrylamide as the energy donor and acceptor respectively, a new kind of brilliant dynamic color-tunable amorphous copolymers were prepared with good stability and processibility, and afterglow emissions from green to orange in response to the stimulus of photo or acid, thanks to multi-responsibility of SP and the energy transfer between naphthalene and SP. In addition to the deeply exploring of the inherent mechanism, these copolymers have been successfully applied in dynamically controllable applications in information protection and delivery.

Highly Efficient Blue Organic Light Emitting Diodes Based on Cyclohexane-Fused Quinoxaline Acceptor.

Exploring blue organic light emitting diodes (OLED) is an important but challenging issue. Herein, to achieve blue-shifted emission, cyclohexane is fused to quinoxaline to weaken the electron-withdrawing ability and conjugation degree of the acceptor. As a result, blue to cyan fluorescent emitters of Me-DPA-TTPZ, tBu-DPA-TTPZ, and TPA-TTPZ were designed and synthesized with donors of diphenylamine and triphenylamine, which exhibit high photoluminescence quantum yields and good thermal stability. In OLEDs with emitters of TPA-TTPZ, the sensitized and nonsensitized devices demonstrate deep-blue (449 nm) and blue (468 nm) emission with maximum external quantum efficiency and CIE coordinates of 6.1%, (0.15, 0.10) and 5.1%, (0.17, 0.22), respectively, validating their potential as blue emitters in OLEDs.

Multi-Stimuli-Responsive Amphiphilic Pyridinium Salt and Its Application in the Visualization of Level 3 Details in Latent Fingerprints.

Organic luminescent materials that can simultaneously achieve multimode mechanochromism and its water-vapor-induced recovery are desirable for practical applications but rarely reported. Herein, an amphiphilic compound, 4-(9H-carbazol-9-yl)-1-(2-hydroxyethyl)pyridin-1-ium bromide (CPAB), is designed by integrating a lipophilic aromatic unit and hydrophilic end in the molecular architecture. Self-recovered mechanochromism from brown to cyan is observed upon mechanical grinding in air. Comprehensive research by X-ray diffraction, infrared spectroscopy, and single-crystal analysis reveals that the photoluminescence switch originates from the variation in intermolecular hydrogen bonds and molecular packing mode. The amphiphilic nature of CPAB allows water molecules to enter the crystalline lattice, forming two polymorphs of the crystalline phase, namely CPAB-D and CPAB-W. The hydrosoluble CPAB exhibits excellent capability in probing the level 3 details of fingerprints because its lipophilic part can target the fatty acid residues of fingerprints, leading to strong aggregation-induced fluorescence. The research may inspire the design of latent fingerprint developers and application in forensics/anti-counterfeiting.

Direct demonstration of triplet excimer in purely organic room temperature phosphorescence through rational molecular design.

Organic luminogens with room temperature phosphorescence (RTP) have been paid great attention and developed rapidly for their wide application values. Until now, the internal mechanism and source of phosphorescence are still obscure, especially for the relationship between molecular dimer and RTP emission. Hence, we designed and synthesized eight phenothiazine 5,5-dioxide derivatives to directly reveal how the monomer and dimer in packing affect the RTP behavior. Dimers with strong π-π stacking (θ < 20.66°; d < 3.86 Å) lead to pure triplet excimer emission, while those with weak π-π stacking (27.02°< θ < 40.64°; 3.84 Å < d < 4.41 Å) contribute to dual RTP emissions of both monomer and triplet excimer. The valuable information of this work would promote the further development of this research field, as well as others in aggregate.

Surface Plasmon Resonance from Gallium-Doped Zinc Oxide Nanoparticles and Their Electromagnetic Enhancement Contribution to Surface-Enhanced Raman Scattering.

In recent years, semiconductor-based surface-enhanced Raman scattering (SERS) substrates have received considerable attention and led to a forefront in the SERS field. However, the lack of electromagnetic (EM) enhancement contributions highly precludes the development of semiconductive-substrate-based SERS. In this study, Ga-doped ZnO nanoparticles (NPs) were fabricated and employed as novel SERS substrates based on the EM enhancement contribution of surface plasmon resonance (SPR). The obtained Ga-doped ZnO NPs exhibited obvious SPR absorptions in the visible and near- and mid-infrared regions. SPR absorption can be readily tuned by changing the doping ratios of Ga3+ ions. The SERS spectra of Ga-doped ZnO/4-mercaptopyridine (MPy) were investigated at different excitation wavelengths of 488, 532, 633, and 785 nm. The spectral enhancement of Ga-doped ZnO substrates depended on the doping ratios, excitation wavelengths, and nearby SPR absorption. Ga-doped ZnO NPs with the highest free charge carrier density and the doping ratio of 5% showed the strongest SERS spectra. For the fixed doping ratio of 5%, the better is the match between excitation wavelengths and SPR absorption, the higher is the SERS spectral enhancement. This study showed the feasibility of EM contributions to SERS by using semiconductive substrates and can contribute to the development of the semiconductor-based EM mechanism.

Multicolored fluorescence variation of a new carbazole-based AIEE molecule by external stimuli.

In this article, we design and synthesize a new carbazole-based molecule, Cz2CN, with a twisted D-A structure, using the carbazole group as the donor and a dicyanoethylene fragment as the acceptor. Such a twisted D-A structure endows Cz2CN with two characteristic emission bands, LE emission and ICT emission. Cz2CN exhibits AIEE, solvatochromism, and different responses to anisotropic shearing force and isotropic hydrostatic pressure, due to the sensitivity of ICT emission to aggregation, solvent polarity and mechanical force. Aggregation benefits the ICT emission, leading to the AIEE of Cz2CN in the THF/water system. Polar solvents can help stabilize the ICT-excited states and make the ICT emission red shift. The original crystalline powder is strongly fluorescent with a high quantum yield of 40.4%. A single crystal of Cz2CN is obtained and dimers without π-π interactions among carbazole groups contribute to the strong emission. Anisotropic shearing alters the emission of Cz2CN powder from sky blue (474 nm) to green (520 nm). The single crystal of Cz2CN undergoes a distinct multicolored variation from sky blue (476 nm) to green (510 nm) and further to orange (590 nm) upon isotropic hydrostatic pressure. The emission of the original powder/crystal of Cz2CN is dominated by LE (locally excited state) emission in HLCT (intercrossed hybridized local and charge transfer excited state). To the best of our knowledge, Cz2CN is an interesting carbazole-based molecule that shows simultaneous AIEE, solvatochromism and force-induced multicolored variation.

Pressure-dependent distinct luminescent evolutions of pyrene and TPA-Py single crystals.

The effects of the high pressure on two single crystals, pyrene and N,N-diphenyl-4-(pyren-1-yl)aniline (TPA-Py), were studied by in situ fluorescent and Raman spectroscopies. During the compression, the pyrene with one structureless excimer emission band showed a continuous bathochromic-shift. In contrast, with the pressure increasing to 10.36 GPa, TPA-Py previously dominated with the hybridized local and charge transfer (HLCT) excited state gradually exhibited a new band at longer wavelengths, which is assigned to a new excited state species with the intramolecular charge transfer (ICT) state, caused by the pressure-induced changes on its molecular configuration. Accompanied by the spectral changes, a sequential color variation from blue to cyan was observed, giving a change to yellow and then red. The significant broadening of the full-width half-maximum (FWHM) of the TPA-Py is observed due to the enhanced dipole-dipole interaction and the existence of pressure gradient. Both pyrene and TPA-Py showed the delayed recovery of the luminescence in the compression-decompression cycle, which results from the poor reversibility of electronic structure caused by the compression-induced piezochromic effect. Furthermore, the evolutions of the Raman spectra of pyrene and TPA-Py indicated that the pressure-induced reversible transformation is caused by the molecular conformational change. This study is a deeper understanding of the structure-property relation of the HLCT species and will be a helpful reference for the regulation of photoluminescence in these intramolecular electron donor-acceptor crystal materials.

Reversible Luminescent Switching in an Organic Cocrystal: Multi-Stimuli-Induced Crystal-to-Crystal Phase Transformation.

A luminescent cocrystal system is reported to undergo crystal-to-crystal phase transformation from yellow-emitting polymorph I to green-emitting polymorph II, triggered by THF fuming or heating, and the green emission can recover to the initial yellow emission by grinding. The established spectroscopic and crystallographic analyses demonstrate that the phase transition occurred and benefits from the combined effect of similar molecular arrange sequence and unique alteration of intermolecular interactions from halogen/hydrogen bonds in I to π-π stacking in II. Furthermore, I and II exhibit red-shift emission under hydrostatic pressure. The emission of I and II shows a red-shift and recovers towards the initial emission upon acid-base fuming. This is a rare example of reversible luminescent switching of cocrystal based upon crystal-to-crystal phase transition, and provides an alternative strategy to develop multi-stimuli responsive materials.

Tetraphenylethene-based tetracationic dicyclophanes: synthesis, mechanochromic luminescence, and photochemical reactions.

Two tetraphenylethene-based tetracationic dicyclophanes 1 and 2 were synthesized via a one-step SN2 reaction. Based on the central TPE unit and the slight difference of the outer linkers, 1 and 2 exhibited a classic aggregation-induced emission but contrasting mechanochromic luminescence under grinding, vaporing, or hydrostatic pressure in the solid state, and photochemical reactions with various emitting colours induced by photoirradiation in the solution state.

Label-Free Detection of Multiplexed Metabolites at Single-Cell Level via a SERS-Microfluidic Droplet Platform.

Single-cell metabolomics could be used to discover the chemical strategies of cells for coping with chemical or environmental stress because metabolomics provides a more immediate and dynamic picture of cell functionality. However, these small-molecule metabolites are the most difficult to measure because they characterize rapid metabolic dynamics, structural diversity, and incapacity to achieve signal amplification or labeling. In order to solve above problems, we presented a surface-enhanced Raman scattering (SERS)-microfluidic droplet platform to realize the label-free simultaneous analysis of multiplexed metabolites at the single-cell level via a versatile magnetic SERS substrate composed by silver nanoparticles (AgNPs, 30 nm)-decorated 400 nm Fe3O4 magnetic microspheres. This metal-magnetic composite substrate is beneficial to efficient adsorption of single cell metabolites, fast separation from complex matrixes, and high SERS sensitivity. Also, the endocytosis effect can be fully prohibited due to its relatively large size. This work achieves label-free, nondestructive, simultaneous determination of three single-cell metabolites, pyruvate, adenosine triphosphate (ATP), and lactate, owing to their intrinsic SERS fingerprints. The "hotspots" effect induced by the magnetic aggregation of Fe3O4@AgNPs allows highly sensitive SERS detection. Encapsulating metabolites in such a limited and isolated droplet accelerates the process of diffusion and adsorption equilibriums. Activation with saponin for metabolites was assessed on different cell lines. The SERS-microdroplet platform is a powerful tool for exploring single-cell heterogeneity at the metabolic level.

Solvation-Enhanced Intermolecular Charge Transfer Interaction in Organic Cocrystals: Enlarged C-C Surface Close Contact in Mixed Packing between PTZ and TCNB.

The mixed π-π packing of the donor (D) and acceptor (A) molecules is the highlighting feature of the intermolecular interactions following charge transfer (CT) issues in organic cocrystal systems. There is an inverse relationship between the D-A interplanar distance and the intermolecular CT interaction. However, the D-A C-C surface close contact (relative areas) on the intermolecular CT interactions in organic cocrystal systems is rarely investigated. Herein, we designed and constructed a novel cocrystal and its solvate cocrystal. The structural and electrostatic potential analyses suggest that the solvation destroys the N-H···N hydrogen bond interaction between phenothiazine (PTZ) and 1,2,4,5-tetracyanobenzene (TCNB), which causes the TCNB molecules to have a 90° rotation along the normal axis of the PTZ plane. Thus, the D-A C-C surface close contact is enlarged, strengthening the intermolecular π-π stacking interactions and intermolecular CT interaction between PTZ and TCNB, which are further evidenced by the absorption and Raman spectroscopic analyses. This study provides rare evidence of the enlarged C-C surface close contact in the mixed packing between D and A that greatly contributes to the intermolecular CT interaction in a D-A cocrystal system. It also provides a deeper understanding of the role of solvation in the structure-property relationship of organic cocrystal materials.

Remarkable pressure-induced emission enhancement based on intermolecular charge transfer in halogen bond-driven dual-component co-crystals.

A series of two-component co-crystals driven by IN interactions based on the bipyridine (BIPY) chromophore with one among three different co-former building blocks, iodopentafluorobenzene (IPFB), 1,4-diiodotetrafluorobenzene (DITFB) and 1,3,5-trifluoro-2,4,6-triiodobenzene (IFB), were prepared and analysed via infared spectroscopy and single-crystal X-ray diffraction. By comparing the IN distances in the co-crystal structures, we found that the higher the -F ratio in the building blocks the closer the contact of the IN bond, enhancing the intermolecular interactions in these co-crystals as well. That is, the positive electrostatic potential on the iodine atom(s) in the co-formers was enhanced by the presence of strong electron-withdrawing groups. The distinct spectroscopic behaviours (fluorescence and Raman spectra) among the two-component BIPY co-crystal systems in response to hydrostatic pressure were also investigated. Interestingly, the fluorescence of BIPY-DITFB presented intriguing abnormal evolution from dark to bright, suggesting a new charge transfer state due to the decreased intermolecular distance and the enhanced IN interactions. Theoretical simulations by Materials Studio also showed the shortened IN distance and the increased angle of C-IN, evidencing the enhanced IN interactions. In contrast, BIPY-IFB showed only slightly enhanced fluorescence intensity at 550 nm consistent with BIPY-DITFB. Once the pressure was relieved, both the Raman and fluorescence spectra for BIPY co-crystal systems entirely self-recovered. Remarkable emission enhancement in a solid-state co-crystal has been rarely reported in previous publications and in fact, this study paves a unique way for designing and developing novel stimuli-responsive photo-functional materials.

Direct observation of the wrapping/unwrapping of ssDNA around/from a SWCNT at the single-molecule level: towards tuning the binding mode and strength.

Complexation of single-stranded DNA (ssDNA) with a chiral single-walled carbon nanotube (SWCNT) exhibits surprising efficacy in CNT dispersion and sorting, optical sensing, and nanoelectronic device design. Studying the wrapping/unwrapping mechanism is challenging because an in situ method at the single-molecule level is required. Here, we developed a method based on single-molecule force spectroscopy to monitor the unwrapping/wrapping of ssDNA from/around a SWCNT. Our results reveal that the wrapping/unwrapping processes are reversible in water, and these processes occur in an equilibrium manner driven mainly by π-π interactions between DNA bases and CNTs. In phosphate buffered saline, the unwrapping process is loading rate-dependent, and ssDNA wrapping around a CNT undergoes two distinct stages dominated by both π-π interactions and hydrogen bonding. In addition, our results show that salts could further stabilize ssDNA/CNT complexes by blocking the electrostatic interactions between adjacent DNA segments and by catalyzing the formation of hydrogen bonds between DNA bases. The stability of ssDNA/CNT is dependent on the DNA sequence and CNT chirality. These results deepen our understanding of ssDNA-CNT interactions and provide effective means to tune the binding mode and strength.

Pressure-induced emission band separation of the hybridized local and charge transfer excited state in a TPE-based crystal.

We herein report a newly synthesized simple molecule, named TPE[double bond, length as m-dash]C4, with twisted D-A structure. TPE[double bond, length as m-dash]C4 showed two intrinsic emission bands ascribed to the locally excited (LE) state and the intramolecular charge transfer (ICT) state, respectively. In the crystal state, the LE emission band is usually observed. However, by applying hydrostatic pressure to the powder sample and the single crystal sample of TPE[double bond, length as m-dash]C4, dual-fluorescence (445 nm and 532 nm) was emerged under high pressure, owing to the pressure-induced emission band separation of the hybridized local and charge transfer excited state (HLCT). It is found that the emission of TPE[double bond, length as m-dash]C4 is generally determined by the ratio of the LE state to the ICT state. The ICT emission band is much more sensitive to the external pressure than the LE emission band. The HLCT state leads to a sample with different responsiveness to grinding and hydrostatic pressure. This study is of significance in the molecular design of such D-A type molecules and in the control of photoluminescence features by molecular structure. Such results are expected to pave a new way to further understand the relationship between the D-A molecular structure and stimuli-responsive properties.

Investigation of supramolecular interaction in 4, 4'-bipyridine crystal by hydrostatic pressure spectroscopies.

The luminescence and structural changes of 4, 4'-bipyridine in the crystal and powder forms under the effect of high pressure applied by a diamond anvil cell has been investigated through the fluorescence and Raman spectroscopies. In its single crystal structure, the 4, 4'-bipyridine molecules are paralleled arranged with the identifiable CH⋯N and π⋯π interactions among molecules. However, in the powder form, these intermolecular interactions nearly diminish. The 4, 4'-bipyridine crystal shows the obvious bathochromic-shifting of the emission band, which is different from the powder sample that displays a fixed luminescent band during compression. Additionally, the Raman bands of them both show shifts to higher wavenumbers as different degrees. The detailed peak assignments are performed based on the theoretical calculation through B3LYP method. Comparisons in spectral behaviors between the crystal and powder under compression show the crystal form exhibits a superior mechanochromic performance relative to the powder one, because the intermolecular interactions in the crystal form play dominating roles and they can be easily tuned along with pressure in such a highly ordered structure compared to the powder form. The relation investigation between property and supramolecular interactions not only makes deeper understanding in the mechanochromic mechanisms of 4, 4'-bipyridine, but also gives a helpful reference for the molecular designs of coordination polymers and co-crystals with 4, 4'-bipyridine involved.

A Single Crystal with Multiple Functions of Optical Waveguide, Aggregation-Induced Emission, and Mechanochromism.

A novel single crystal, PyB, is produced in a high yield by the simple method of connecting a pyrene unit and a rhodamine B moiety together. PyB shows multiple functions of aggregation-induced emission, low-loss optical waveguiding, and tricolored mechanochromism. The crucial point for fabricating such a multifunctional single crystal is selecting the C═N group as a spacer, which simplifies the synthetic procedure, confines the molecular conformation to develop single crystals, and allows one to dynamically observe the color variation in situ and quantitatively analyze the effect of applied pressures. Such a simple approach may be extended to other fluorophores, thus providing a new opportunity for the real world application of mechanochromic materials for mechanical sensors, optical encoding, and optoelectronic devices, etc.