Nonconventional Luminescent Piperazine-Containing Hyperbranched Polysiloxanes with Pure n-electron

Nonconventional luminogens with high quantum yield (QY) possess very potential applications in various fields. However, the preparation of such luminogens remains a great challenge. Herein, the first example of piperazine-containing hyperbranched polysiloxane exhibiting blue and green fluorescence is reported under the irradiation of different excitation wavelength and a high QY of 20.9%. The density functional theory (DFT) calculations and experimental results revealed that the through-space conjugation (TSC) within the clusters of N and O atoms is produced via the induction of multiple intermolecular hydrogen bonds and flexible SiO units, which is accountable for the fluorescence. Meanwhile, the introduction of the rigid piperazine units not only rigidifies the conformation, but also enhances the TSC. In addition, the fluorescence of both P1 and P2 shows concentration-, excitation-, and solvent-dependent emission, especially exhibits significant pH-dependent emission and obtains an ultrahigh QY of 82.6% at pH 5. The synthetic luminogens show excellent applications in fluorescence detection for Fe3+ and Co2+ , information encryption, and fluorescent film. This study provides a novel strategy to rationally design high-efficiency nonconventional luminogens.

Structure-Dependent Nontraditional Intrinsic Fluorescence of Aliphatic Hyperbranched Polyureas

The nontraditional intrinsic fluorescence (NTIF) of polymers containing heteroatoms has gained considerable attention due to its promising applications in label-free bioimaging. Aliphatic hyperbranched polyureas (aBPUs), which have recently shown great promise in the field of nanomedicine, bear controllable urea groups distributed on the branch points and thus are potential candidate luminogens. However, their NTIF properties and how their structures influence the NTIF properties have not been illustrated yet. Here, we addressed these issues by synthesizing a series of aBPUs with different degrees of branching (DBs) or different modifications. aBPUs exhibited an obvious NTIF phenomenon and with the increase of DBs, the NTIF enhanced as well. Chemical modifications either at the branching ends or in the interior of aBPUs could affect the NTIF performances, which were highly dependent on the types of modification. Disruption of the intra-/intermolecular hydrogen-bonding interactions decreased the NTIF. In addition, poly(ethylene glycol) (PEG)-modified aBPUs could self-assemble into nanospheres, and the formation of nanoassembly led to 89% enhancement on NTIF compared with the homogeneous solution of aBPUs-PEG in dimethylformamide (DMF). Finally, aBPUs-PEG nanoassembly demonstrated a capability in realizing label-free material imaging in vitro. These results shed light on the rational design of the polymer structures to achieve desired fluorescence with unconventional luminophores.

Thiolactone chemistry, a versatile platform for macromolecular engineering

This review covers the extensive use of γ-thiolactone chemistry as a versatile and powerful tool for macromolecular engineering and the preparation of various polymer architectures, such as functional, alternating, or sequence-controlled (co)polymers.

Synthesis of thermoresponsive nonconjugated fluorescent branched poly(ether amide)s via oxa-Michael addition polymerization

Novel thermoresponsive nonconjugated fluorescent branched poly(ether amide)s with tunable LCST via t-BuP2-catalyzed oxa-Michael addition polymerization of N,N′-methylenebis(acrylamide) with triols.

Nonconventional luminophores: characteristics, advancements and perspectives

Nonconventional luminophores devoid of remarkable conjugates have attracted considerable attention due to their unique luminescence behaviors, updated luminescence mechanism of organics and promising applications in optoelectronic, biological and medical fields. Unlike classic luminogens consisting of molecular segments with greatly extended electron delocalization, these unorthodox luminophores generally possess nonconjugated structures based on subgroups such as ether (-O-), hydroxyl (-OH), halogens, carbonyl (CO), carboxyl (-COOH), cyano (CN), thioether (-S-), sulfoxide (SO), sulfone (OSO), phosphate, and aliphatic amine, as well as their grouped functionalities like amide, imide, anhydride and ureido. They can exhibit intriguing intrinsic luminescence, generally featuring concentration-enhanced emission, aggregation-induced emission, excitation-dependent luminescence and prevailing phosphorescence. Herein, we review the recent progress in exploring these nonconventional luminophores and discuss the current challenges and future perspectives. Notably, different mechanisms are reviewed and the clustering-triggered emission (CTE) mechanism is highlighted, which emphasizes the clustering of the above mentioned electron rich moieties and consequent electron delocalization along with conformation rigidification. The CTE mechanism seems widely applicable for diversified natural, synthetic and supramolecular systems.

Nontraditional Luminescent Molecular Aggregates Encapsulated by Wormlike Silica Nanoparticles for Latent Fingerprint Detection

The phenomenon of nontraditional luminescence has attracted wide attention and curiosity of researchers due to its inexplicable photoluminescence paradigm without aromatic or extended π-systems. The present work puts forward a neotype of a light-emitting nitrogenous small molecule, namely, N-stearoyl-hydroxyproline (L-C₁₆-Hyp), which could emit weak light in aggregation states through the restriction mechanism of intramolecular motion, exhibiting properties comparable to those of AIEgens. Using these molecular aggregates as anionic surfactant micelles to incorporate within the silica matrix, we prepared fluorescent nanoparticles (FL-NPs) by a one-pot method for expedient visualization of latent fingerprints (LFPs). The FL-NPs exhibit an excitation range from 335 to 365 nm, resulting in nontraditional luminescence observed between 410 and 440 nm. The enhanced luminescent FL-NPs may derive from the collective entities or assemblies of restricted L-C₁₆-Hyp, which can be reasonably explicated by an effect termed as cluster-triggered emission (CTE). Theoretical calculations demonstrated that this luminescence pattern belongs to partial charge transfer, which is mainly attributed to the close interaction between the tertiary amino and adjacent carboxyl in the L-C₁₆-Hyp structure. Moreover, some merits of FL-NPs, such as wormlike nanomorphology, stable photophysical properties, low toxicity, great adhesion to multiple substrates, easy to get raw material, an inexpensive, simple process, and rapid detection without any further modification or assistance, provide the feasibility of efficacious LFP detection. Overall, this study will provide insights into the design and application of luminescent materials with unconventional groups.

Metal Cation-Responsive and Excitation-Dependent Nontraditional Multicolor Fluorescent Hydrogels for Multidimensional Information Encryption

Fluorescent polymeric hydrogels especially multicolor fluorescent polymeric hydrogels (MFPHs) have important applications in information storage, encryption, and encoding. MFPHs are generally prepared by incorporating multiple traditional fluorescent materials into hydrogels. In recent years, nontraditional luminescent polymers without any traditional π-conjugated chromophores have received increasing attention. Here, we report a novel type of nontraditional MFPHs prepared by in situ polymerization of acrylamide (AAm) in the presence of poly(itaconic acid) (PITAc). The hydrogen-bonded mechanically strong PAAm/PITAc hydrogels show strong intrinsic fluorescence, and the fluorescence emission is excitation-dependent and metal cation-responsive. More impressively, the hydrogels treated with metal cations also possess excitation-dependent fluorescence. We developed a multi-ion inkjet printing (MIIP) technique to print texts or designed patterns onto the hydrogel surface using different metal cation solutions as inks, and then variable texts or patterns appear under the irradiation of UV, violet, and blue lights. Patterns can be further changed by selective printing, erasing, or reprinting on some regions. Therefore, multidimensional information encryption is achieved. This work provides a new strategy for preparing MFPHs for wide applications.

Dramatically Enhanced and Red-shifted Photoluminescence Achieved by Introducing an Electron-withdrawing Group into a Non-traditional Luminescent Small Organic Compound

Small organic compounds without any traditional fluorescent chromophores are generally non-emissive, and only very few are reported to emit weak blue fluorescence. Here we synthesized a non-traditional luminescent small organic compound N-(2,2,2-trifluoroethyl)acrylamide (TFAM) with dramatically enhanced and red-shifted photoluminescence by introducing a strong electron-withdrawing group into acrylamide (AM). Very impressively, TFAM emits cyan (472 nm) and yellow-green (560 nm) fluorescence in solutions and solid state, respectively. TFAM also shows aggregation-induced emission enhancement (AIEE) and excitation-dependent fluorescence (EDF) characteristics, as well as temperature and metal cations-responsive fluorescence. Theoretical calculations show that the introduction of electron-withdrawing group leads to a lower energy gap between the HOMO-LUMO energy levels in TFAM than in AM. And strong cooperative hydrogen bonds are formed in TFAM molecules, resulting in rigidification of molecular conformations. The study provides a strategy for preparing non-traditional luminescent compounds with enhanced and red-shifted photoluminescence.

How to Manipulate Through-Space Conjugation and Clusteroluminescence of Simple AIEgens with Isolated Phenyl Rings

Apart from the traditional through-bond conjugation (TBC), through-space conjugation (TSC) is gradually proved as another important interaction in photophysical processes, especially for the recent observation of clusteroluminescence from nonconjugated molecules. However, unlike TBC in conjugated chromophores, it is still challenging to manipulate TSC and clusteroluminescence. Herein, simple and nonconjugated triphenylmethane (TPM) and its derivatives with electron-donating and electron-withdrawing groups were synthesized, and their photophysical properties were systematically studied. TPM was characterized with visible clusteroluminescence due to the intramolecular TSC. Experimental and theoretical results showed that the introduction of electron-donating groups into TPM could red-shift the wavelength and increase the efficiency of clusteroluminescence simultaneously, due to the increased electronic density and stabilization of TSC. However, TPM derivatives with electron-withdrawing groups showed inefficient or even quenched clusteroluminescence caused by the vigorous excited-state intramolecular motion and intermolecular photoinduced electron transfer process. This work provides a reliable strategy to manipulate TSC and clusteroluminescence.

Monitoring cysteine level changes under LPS or H2O2 induced oxidative stress using a polymer-based ratiometric fluorescent probe

Cysteine (Cys) is a critical amino acid that involves in many physiological and pathological processes in the human body, and it plays an important role in maintaining redox homeostasis in living systems. The concentration of intracellular Cys is abnormal under oxidative stress thus leading to many diseases. Therefore, it is significant to develop an effective method for detection of Cys under oxidative stress. In this work, we propose a new polymer-based ratiometric fluorescent probe with good selectivity and sensitivity for detecting Cys. The bioimaging experiments results show that the novel probe has a rapid ratiometric response to Cys, which can be used to monitor Cys level changes during LPS or H₂O₂ induced oxidative stress in living cells and zebrafish.

Disulfide-Containing Polysiloxane with Multicolor Fluorescence and Visible-Light Excitation: Design, Synthesis, and Mechanism Exploration

Aggregation-induced emission (AIE) polysiloxane has attracted growing attention in recent years due to its outstanding biocompatibility. However, polysiloxane usually requires high-energy UV light for excitation and exhibits monochromatic blue emission. Moreover, the experimental selection process of polysiloxane with designed features is time-consuming and laborious. So, in this paper, we developed a new molecular structure selection strategy using theoretical calculations instead of experiments, and a linear disulfide-containing polysiloxane (L1) is selected and synthesized. To our surprise, L1 can be excited by low-energy visible light (Ex = 508 nm and Em = 588 nm) and emit multicolor fluorescence under different excitation wavelengths. A further study of the luminescence mechanism was carried out through calculations about the quantum states of L1. Moreover, L1 shows multiple stimuli-responsiveness, such as redox, pH, metal ions, and solvent. This work provides an integrated route for the molecular design of macromolecular AIE luminogens with attractive fluorescence properties.

Michael Polyaddition Approach Towards Sulfur Enriched Nonaromatic Polymers with Fluorescence-Phosphorescence Dual Emission

Nonaromatic photoluminescent polymers have attracted great attention due to their intriguing photophysical properties and promising implications in optoelectronic and biological areas. The luminescence from these nonconventional luminophores can be well rationalized by the clustering-triggered emission mechanism. Sulfur, although as an n-electron-rich element with big radius, is not been widely utilized in construction of nonconventional luminophores despite of its potential competitiveness in nonaromatic photoluminescent polymers. Herein, the "click" type Michael polyaddition is utilized to construct sulfur-bearing nonconventional luminophores, and two sulfur enriched nonaromatic poly(thioether sulfone)s (PES) are obtained, which demonstrate fluorescence-phosphorescence dual emission. More investigations concerning the monomer of bis(vinylsulfonyl)methane are further proceeded to support acquired results. Finally, the application of explosive detection by the prepared PES is also conducted.

Clusteroluminescence from Cluster Excitons in Small Heterocyclics Free of Aromatic Rings

The study of nonconventional luminescence is important for revealing the luminescence of natural systems and has gradually drawn the attention of researchers in recent years. However, the underlying mechanism is still inexplicable. Herein, the luminescence behavior of two series of simple, heteroatom-containing small molecules without aromatic rings, i.e., maleimide and succinimide derivatives, are studied to gain further mechanistic insight into the nonconventional luminescence process. It has been unveiled that all the molecules exhibit bright and visible luminescence in concentrated solution and solid state and the formation of clusters is the root cause for such behaviors, which can effectively increase the possibility of both the nonradiative n-π* and favorable π-π* transitions and stabilize the excitons formed in the excited state. The distinctive luminescent phenomena and intriguing mechanism presented in this work will be significant for understanding the mechanism of clusteroluminescence and provide new strategies for the rational design of novel luminescent materials.

Recent Advances in Clusteroluminescence

Clusteroluminescence is a phenomenon whereby the aggregation or clustering of non-conjugated electron-rich units leads to the emission of light at long wavelengths. This phenomenon was first discovered in poly(amido amine) (PAMAM) dendrimers. In recent years, clusteroluminescence has attracted growing research interest and its photophysical properties and mechanism have been thoroughly studied. In this review, we first briefly introduce the development of different types of clusteroluminogens. Then we highlight recent developments in clusteroluminescence, including mechanistic studies, the disclosure of room-temperature phosphorescence, and the extension of emission to the longer-wavelength region. Lastly, we demonstrate a few applications in various fields. With advantages such as being earth-abundant, biocompatible and biodegradable, clusteroluminogens are envisioned to be commonplace in the future.

A novel polythioether-based rhodamine B fluorescent probe via successive click reaction and its application in iron ion detection and cell imaging

Polythioether has good chemical stability and biocompatibility and is a kind of promising polymers for the application of optical materials, medical materials and energy conversion materials. However, the fluorescent probe based on polythioether is still rare. Herein, a series of polythioether based polymer fluorescent probes were synthesized by successive thiol click reaction under ultraviolet light at room temperature. The poly(thioether)s have good selectivity and responsiveness to iron ions and can be applied in cell imaging, which indicate that the broad application prospects of polythioether-based fluorescent probes in ion detection and bioimaging.

A New Kind of Nonconventional Luminogen Based on Aliphatic Polyhydroxyurethane and Its Potential Application in Ink-Free Anticounterfeiting Printing

Organic luminogens have extensive applications due to their unique photophysical properties. In recent years, nonconjugated organic luminogens, in contrast to traditional conjugated luminogens, have gained much attention because of their facile preparation, environmental friendliness, and biocompatibility. In this study, a new kind of nonconventional luminogen based on dynamic covalent cross-linked polyhydroxyurethane is reported for the first time. The new luminogen not only exhibits intrinsic strong fluorescent emission in the solid state but also possesses high mechanical properties along with good shape memory and self-healing properties. In addition, the new luminogens are synthesized from aliphatic polyfunctional cyclic carbonate and amines via a much more straightforward method, avoiding the use of toxic isocyanates. Investigations indicated that the intrinsic luminescence of the resultant luminogens was induced by the cross-linking of polymer chains and could be well tuned by controlling the degree of cross-linking. By taking advantage of the unique characteristics of the resultant polymer luminogens, we further developed a facile method, named "light-mediated ink-free screen printing", for anticounterfeiting paper fabrication. Different from traditional ink-based printing technology, the new method used UV-light instead of expensive security ink to encode anticounterfeiting information on natural cellulose paper. The anticounterfeiting information is stable under various wet conditions, showing promising applications in the fast-growing counterfeiting of pharmaceuticals, packaging, and the food industry.

Recent advances in the development and applications of nonconventional luminescent polymers

Recently, nonconventional luminescent polymers (NLPs) have emerged as the most sought-after alternative luminescent materials. This review provides a thorough description of the importance and applications of each class of state-of-the-art NLPs.

Barbier Hyperbranching Polymerization-Induced Emission toward Facile Fabrication of White Light-Emitting Diode and Light-Harvesting Film

Luminescent polymers are generally constructed through polymerization of luminescent moieties. Polymerization itself, however, is mainly used for constructing polymer main chain, and the importance of polymerization on luminescence has yet to be explored. Here, we demonstrate a polymerization-induced emission strategy producing luminescent polymers by introducing Barbier reaction to hyperbranching polymerization, which allows luminescent properties to be easily tuned from the traditional type to an aggregation-induced emission type by simply adjusting the monomer structure and the polymerization time. When rotation about the phenyl groups in hyperbranched polytriphenylmethanols (HPTPMs) is hindered, HPTPMs exhibit traditional emission property. When all phenyl groups of HPTPM are rotatable, i.e., p,p',p″-HPTPM, it exhibits interesting aggregation-induced emission property with tunable emission colors from blue to yellow, by just adjusting polymerization time. Further applications of aggregation-induced emission type luminescent polymers are illustrated by the facile fabrication of white light-emitting diode (LED) and light-harvesting film with an antenna effect >14. This Barbier hyperbranching polymerization-induced emission provides a new strategy for the design of luminescent polymers and expands the methodology and functionality library of both hyperbranching polymerization and luminescent polymers.

Self-Assembling Nonconjugated Poly(amide-imide) into Thermoresponsive Nanovesicles with Unexpected Red Fluorescence for Bioimaging

Nonconjugated red fluorescent polymers have been increasingly studied to improve the biocompatibility and penetration depth over conventional fluorescent materials. However, the accessibility of such polymers remains challenging due to the scarcity of nonconjugated fluorophores and lacking relevant mechanism of red-shifted fluorescence. Herein, we discovered that the combination of hydrogen bonding and π-π stacking interactions provides nonconjugated poly(amide-imide) with a large bathochromic shift (>100 nm) from blue-green fluorescence to red emission. The amphiphilic PEGylated poly(amide-imide) derived from in situ PEGylation self-assembled into nanovesicles in water, which isolated the aminosuccinimide fluorophore from the solvents and suppressed the hydrogen bonds formation between aminosuccinimide fluorophores and water. Therefore, the fluorescence of PEGylated poly(amide-imide) in water was soundly retained. Furthermore, the strong hydrogen bonding and hydrophobic interactions with water provided PEGylated poly(amide-imide) with a reversible thermoresponsiveness and presented a concentration-dependent behavior. Finally, accompanied by the excellent biostability and photostability, PEGylated poly(amide-imide) exhibited as a good candidate for cell imaging.

Sulphur-containing nonaromatic polymers: clustering-triggered emission and luminescence regulation by oxidation

Sulphur-containing nonconventional luminophores with clustering-triggered emission characteristics are developed, whose emission is readily modulated through oxidation.

A non-conjugated polyethylenimine copolymer-based unorthodox nanoprobe for bioimaging and related mechanism exploration

A non-conjugated polyethylenimine copolymer-based nanoprobe for lysosome-specific staining and tumor-targeted bioimaging and related mechanism exploration.

Unusual Emission of Polystyrene-Based Alternating Copolymers Incorporating Aminobutyl Maleimide Fluorophore-Containing Polyhedral Oligomeric Silsesquioxane Nanoparticles

In this study, we synthesized an unusual 2-aminobutyl maleimide isobutyl polyhedral oligomeric silsesquioxane (MIPOSS-NHBu) monomer lacking conventional fluorescent groups. We then prepared poly(styrene-alt-2-aminobutyl maleimide isobutyl POSS) [poly(S-alt-MIPOSS-NHBu)] and poly(4-acetoxystyrene-alt-2-aminobutyl maleimide isobutyl POSS) [poly(AS-alt-MIPOSS-NHBu)] copolymers through facile free radical copolymerizations using azobisisobutyronitrile as the initiator and tetrahydrofuran as the solvent. A poly(4-hydroxystyrene-alt-2-aminobutyl maleimide isobutyl POSS) [poly(HS-alt-MIPOSS-NHBu)] copolymer was prepared through acetoxyl hydrazinolysis of poly(AS-alt-MIPOSS-NHBu). We employed ¹H, ¹³C, and ²⁹Si nuclear magnetic resonance spectroscopy; Fourier transform infrared spectroscopy; differential scanning calorimetry; and photoluminescence spectroscopy to investigate the structures and the thermal and optical properties of the monomers and novel POSS-containing alternating copolymers. Intramolecular hydrogen bonding between the amino and dihydrofuran-2,5-dione group and clustering of the locked C=O groups from the POSS nanoparticles in the MIPOSS-NHBu units restricted the intramolecular motion of the polymer chain, causing it to exhibit strong light emission. As a result, the MIPOSS-NHBu monomer and the poly(AS-alt-MIPOSS-NHBu) copolymer both have potential applicability in the detection of metal ions with good selectivity.

C–H imidation: a distinct perspective of C–N bond formation

The direct imidation strategy proficiently constructs C–N bonds and creates the useful amine functional group in the molecular template.