New insights in the study of pyrene excimer fluorescence to characterize macromolecules and their supramolecular assemblies in solution

Cyclotriphosphazene: A Versatile Building Block for Diverse Functional Materials

Cyclotriphosphazene (CP) is a cyclic inorganic compound with the chemical formula N3P3. This unique molecule consists of a six-membered ring composed of alternating nitrogen and phosphorus atoms, each bonded to two chlorine atoms. CP exhibits remarkable versatility and significance in the realm of materials chemistry due to its easy functionalization via facile nucleophilic substitution reactions in mild conditions as well as intriguing properties of resultant final CP-based molecules or polymers. CP has been served as an important building block for numerous functional materials. This review provides a general and broad overview of the synthesis of CP-based small molecules through nucleophilic substitution of hexachlorocyclotriphosphazene (HCCP), and their applications, including flame retardants, liquid crystals (LC), chemosensors, electronics, biomedical materials, and lubricants, have been summarized and discussed. It would be expected that this review would offer a timely summary of various CP-based materials and hence give an insight into further exploration of CP-based molecules in the future.

Glycogen β-particles surface characterized by a combination of size exclusion chromatography and pyrene excimer fluorescence before and after β-amylolysis

Size exclusion chromatography (SEC) and pyrene excimer formation (PEF) experiments were conducted to characterize the local density profile inside a glycogen sample before (Glycogen) and after (Gly-β-LD) treatment with β-amylase. These experiments were conducted to assess whether the density at the periphery of the glycogen particles was very high to limit access to proteins involved in the metabolism of glycogen as predicted by the Tier model or low as suggested by the Gilbert model. SEC analysis indicated that the density inside the Glycogen and Gly-β-LD samples remained constant with particle size and was not affected by β-amylolysis. Analysis of the PEF experiments conducted on the Glycogen and Gly-β-LD samples labeled with 1-pyrenebutyric acid showed that the particles have a dense interior and loose corona. The conclusions reached by the SEC and PEF experiments agree with the Gilbert model and have implications for the association of glycogen β-particles into larger α-particles.

Synthesis, Photophysical and Electronic Properties of a D-π-A Julolidine-Like Pyrenyl-o-Carborane

We synthesized 2-(1-1,2-dicarbadodecaboranyl(12))-6,6,12,12-tetramethyl-7,8,11,12-tetrahydro-6H,10H-phenaleno[1,9-fg]pyrido[3,2,1-ij]quinoline (4), a julolidine-like pyrenyl-o-carborane, with pyrene substituted at the 2,7-positions on the HOMO/LUMO nodal plane. Using solid state molecular structures, photophysical data, cyclic voltammetry, DFT and LR-TDDFT calculations, we compare o-carborane and B(Mes)2 (Mes=2,4,6-Me3C6H2) as acceptor groups. Whereas the π-acceptor strength of B(Mes)2 is sufficient to drop the pyrene LUMO+1 below the LUMO, the carborane does not do this. We confirm the π-donor strength of the julolidine-like moiety, however, which raises the pyrene HOMO-1 above the HOMO. In contrast to the analogous pyrene-2-yl-o-carborane, 2-(1-1,2-dicarbadodecaboranyl(12))-pyrene VI, which exhibits dual fluorescence, because the rate of internal conversion between locally-excited (LE) and charge transfer (CT) (from the pyrene to the carborane) states is faster than the radiative decay rate, leading to a thermodynamic equilibrium between the 2 states, 4 shows only single fluorescence, as the CT state involving the carborane as the acceptor moiety in not kinetically accessible, so a more localized CT emission involving the julolidine-like pyrene moiety is observed.

Location of a Hydrophobic Load in Poly(oligo(ethylene glycol) methyl ether methacrylate)s (PEGMAs) Dissolved in Water and Probed by Fluorescence

Two series of pyrene-labeled poly(oligo(ethylene glycol) methyl ether methacrylate)s referred to as PyEG5-PEGnMA and PyC4-PEGnMA were prepared to probe the region surrounding the polymethacrylate backbone by using the fluorescence of the dye pyrene. PyEG5-PEGnMA and PyC4-PEGnMA were prepared by copolymerizing the EGnMA methacrylate monomers with penta(ethylene glycol) 1-pyrenemethyl ether methacrylate or 1-pyrenebutyl methacrylate, respectively. In organic solvents, the much longer 18 non-hydrogen atom linker connecting the pyrene moieties to the polymethacrylate backbone in the PyEG5-PEGnMA samples enabled the deployment of the pyrenyl labels into the solution. In water, however, an excited pyrene for PyEG5-PEGnMA was found to probe a same volume as for the PyC4-PEGnMA samples where a much shorter 6 non-hydrogen atom spacer connected pyrene to the backbone. Another surprising observation, considering that the hydrophobicity of pyrene induces strong pyrene aggregation for many pyrene-labeled water-soluble polymers (Py-WSPs) in water, was the little pyrene aggregation found for the PyEG5-PEGnMA and PyC4-PEGnMA samples in water. These effects could be related to the organic-like domain (OLD) generated by the oligo(ethylene glycol) side chains densely arranged around the polymethacrylate backbone of the polymeric bottlebrush (PBB). Additional fluorescence experiments conducted with the penta(ethylene glycol) 1-pyrenemethyl ether derivative indicated that the cylindrical OLD surrounding the polymethacrylate backbone had a chemical composition similar to that of ethylene glycol. Binding of hydrophobic pyrene molecules to unlabeled PEGnMA bottlebrushes in water further supported the existence of the OLD. The demonstration, that PEGnMA samples form an OLD in water, which can host and protect hydrophobic cargoes like pyrene, should lead to the development of improved PEGnMA-based drug delivery systems.

Accurate Determination of the Average Rate Constant of Pyrene Excimer Formation for Pyrene-Labeled Macromolecules from the Analysis of Individual Fluorescence Decays with Sums of Exponentials

The average rate constant (⟨k⟩) for pyrene excimer formation (PEF) between an excited and a ground-state pyrenyl label covalently attached to a pyrene-labeled macromolecule (PyLM) has been found to be an ideal parameter to probe macromolecular conformations due to its proportionality to the local concentration ([Py]loc) of pyrenyl labels in PyLM. To date, ⟨k⟩ has only been determined with the model-free analysis (MFA) involving the global analysis of the pyrene monomer and excimer fluorescence decays of PyLM. Unfortunately, the MFA is computationally demanding which prevents its widespread use. To circumvent this complication, a methodology is introduced that involves the analysis of individual fluorescence decays with sums of exponentials (SoE), which are commonly used in the analysis packages of commercial time-resolved fluorometers. The individual fluorescence decays of the pyrene monomer acquired with 286 PyLM were analyzed with a SoE to yield ⟨kSoE-M⟩. The strong correlation between ⟨kSoE-M⟩ and ⟨kMF⟩ obtained from the global MFA indicated that ⟨kSoE-M⟩ was a good representation of ⟨k⟩. Furthermore, the AE-/AE+ ratio, equal to the ratio of the sum of the negative pre-exponential factors over the sum of the positive pre-exponential factors, was determined by fitting the individual pyrene excimer fluorescence decays of the 286 PyLM with a SoE. AE-/AE+ was found to take a value between -1.0 and -0.8, indicating that the pyrenyl labels were not aggregated. This result indicated that [Py]loc was well described by ⟨kSoE-M⟩, so that ⟨kSoE-M⟩ could be used to describe the conformation of macromolecules in the same manner as ⟨kMF⟩. Consequently, the methodology based on the analysis of individual fluorescence decays with sums of exponentials to determine ⟨kSoE-M⟩ and AE-/AE+ provides a robust alternative to the use of the MFA for the study of PyLM to many scientists interested in the characterization of macromolecular conformations.

4,9- and 4,10-Substituted pyrenes: synthesis, successful isolation, and optoelectronic properties

We report herein a way to prepare and purify optoelectronic functional 4,9- and 4,10-substituted pyrene isomers. By tuning the size of substituents, the designed 4,9- and 4,10-isomers can be successfully isolated by recycling preparative size-exclusion chromatography (SEC) and/or repeated recrystallization. The structure and purity of the isolated compounds 1-5 have been confirmed by 1H NMR, 13C NMR, and HRMS. The photophysical and electrochemical properties of compounds 1-5 have been studied in detail both experimentally and theoretically. The lowest transitions of these pyrenes, 1-5, are allowed, with moderate to high fluorescence quantum yields and radiative decay rates around 108 s-1. The differences between the electrochemical and photophysical properties of 4,9-, 4,10-, 1,6-, and 2,7-substituted isomers are compared and concluded.

Probing the inner local density of complex macromolecules by pyrene excimer formation

The direct relationship existing between the average rate constant 〈k〉 for pyrene excimer formation and the local concentration [Py]loc of ground-state pyrenyl labels covalently attached to a macromolecule was established for 55 pyrene-labeled macromolecules (PyLM). These PyLM belonged to three different families of macromolecules with the first representing short monodisperse linear chains end-labeled with pyrene (polystyrene, poly(ethylene oxide), and poly(N-isopropyl acrylamide)), the second representing long polydisperse linear chains randomly labeled with pyrene (poly(methyl acrylate), poly(methyl methacrylate), polystyrene, poly(butyl methacrylate), poly(methoxyethyl methacrylate), and poly(N-isopropyl acrylamide)), and the third being comprised of two series of pyrene end-labeled low generation dendrimers with a bis(hydroxymethyl)propionic acid or a polyamidoamine backbone. The assumption, that the polymeric segments probed by an excited pyrenyl label covalently attached to one of these macromolecules obeyed Gaussian statistics, enabled the calculation of their square root average squared end-to-end distance (LPy), which was applied to calculate [Py]loc. The log-log plots of 〈k〉 as a function of [Py]loc yielded straight lines with a slope of unity for all families of macromolecules studied in four different organic solvents demonstrating the validity and generality of the 〈k〉-vs.-[Py]loc relationship. Since an experimentalist knows how the the pyrenyl labels are covalently attached onto a macromolecule, [Py]loc offers a means to probe the local density of a macromolecule, which can be employed to characterize its conformation in solution. Consequently, the 〈k〉-vs.-[Py]loc relationship provides a novel experimental means to probe the conformation of macromolecules which should establish pyrene excimer formation as an appealing method for conformational studies of macromolecules in solution, which should nicely complement scattering techniques.

Persistence Length of PEGMA Bottle Brushes Determined by Pyrene Excimer Fluorescence

Seven pyrene-labeled poly(oligo(ethylene glycol) methyl ether methacrylate)s (PyEG5-PEGnMAs) were prepared with n = 0, 3, 4, 5, 7, 9, and 19 ethylene glycol units by copolymerizing a small amount of penta(ethylene glycol) 1-pyrenemethyl ether methacrylate with an EGnMA monomer. The conformation of the PyEG5-PEGnMA polymers evolved from a random coil for PyEG5-PEG0MA or poly(methyl methacrylate) to a polymeric bottle brush (PBB) architecture with increasing side chain length. The fluorescence decays of the PyEG5-PEGnMA samples were fitted according to the fluorescence blob model (FBM) whose parameters were used, in combination with the Kratky-Porod equation, to calculate the persistence length of these polymers. The persistence lengths obtained from the PEF experiments were found to increase with the square of the number (NS) of non-hydrogen atoms in the side chain as expected theoretically. The persistence lengths found with the PyEG5-PEGnMA samples in DMF also matched those found earlier for another series of PEGnMA samples labeled with 1-pyrenebutanol. The good agreement found between the persistence lengths obtained with the PEGnMA samples labeled with two different pyrene derivatives illustrates the robustness of the method and its applicability for measuring the unknown persistence length of polydisperse polymer samples.

Bright and Stable Nanomaterials for Imaging and Sensing

This review covers strategies to prepare high-performance emissive polymer nanomaterials, combining very high brightness and photostability, to respond to the drive for better imaging quality and lower detection limits in fluorescence imaging and sensing applications. The more common approaches to obtaining high-brightness nanomaterials consist of designing polymer nanomaterials carrying a large number of fluorescent dyes, either by attaching the dyes to individual polymer chains or by encapsulating the dyes in nanoparticles. In both cases, the dyes can be covalently linked to the polymer during polymerization (by using monomers functionalized with fluorescent groups), or they can be incorporated post-synthesis, using polymers with reactive groups, or encapsulating the unmodified dyes. Silica nanoparticles in particular, obtained by the condensation polymerization of silicon alcoxides, provide highly crosslinked environments that protect the dyes from photodegradation and offer excellent chemical modification flexibility. An alternative and less explored strategy is to increase the brightness of each individual dye. This can be achieved by using nanostructures that couple dyes to plasmonic nanoparticles so that the plasmon resonance can act as an electromagnetic field concentrator to increase the dye excitation efficiency and/or interact with the dye to increase its emission quantum yield.

Intramacromolecular Conformational Changes in Low Generation PAMAM Dendrimers Probed by Pyrene Excimer Formation

Pyrene excimer formation (PEF) was used to probe the intramacromolecular conformational change experienced by low generation pyrene-labeled PAMAM dendrimers referred to as PyCX-PAMAM-GY, where X (=4, 8, or 12) and Y (=0, 1, or 2) represent the number of atoms in the pyrenyl linker and the dendrimer generation, respectively. Each sample was studied in N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) with and without 5 mM HCl. Global analysis of the monomer and excimer time-resolved fluorescence decays using the model free analysis (MFA) yielded the average rate constant of excimer formation, ⟨k⟩, which was compared with the local pyrene concentration ([Py]loc) of the PyCX-PAMAM-GY samples calculated by assuming that the oligomeric segments constituting the dendrimer's interior obeyed Gaussian statistics. A notable decrease in ⟨k⟩ was observed upon the addition of 5 mM HCl to the PyCX-PAMAM-GY solutions and was attributed to swelling of the dendrimers resulting from the protonation of the internal tertiary amines. The reversibility of this conformational change could also be monitored via PEF. Solvent differences between DMF and DMSO were accounted for by dividing ⟨k⟩ by kdiff, the bimolecular rate constant for diffusive PEF of a n-hexyl-1-pyrenebutyramide model compound, to yield the ⟨k⟩/kdiff ratio. Comparison between the ⟨k⟩/kdiff ratios obtained for all the PyCX-PAMAM-GY samples with and without 5 mM HCl revealed a 13% increase in the radius of the PAMAM-GY dendrimers upon protonation of their internal tertiary amines in agreement with earlier reports. These experiments illustrate that PEF represents a powerful experimental means to quantitatively probe the intramacromolecular conformational changes of complex macromolecules in solution, in a manner that complements scattering techniques.

Novel Tripodal Polyamine Tris-Pyrene: DNA/RNA Binding and Photodynamic Antiproliferative Activity

A novel tri-pyrene polyamine (TAL3PYR) bearing net five positive charges at biorelevant conditions revealed strong intramolecular interactions in aqueous medium between pyrenes, characterised by pronounced excimer fluorescence. A novel compound revealed strong binding to ds-DNA and ds-RNA, along with pronounced thermal stabilisation of DNA/RNA and extensive changes in DNA/RNA structure, as evidenced by circular dichroism. New dye caused pronounced ds-DNA or ds-RNA condensation, which was attributed to a combination of electrostatic interactions between 5+ charge of dye and negatively charged polynucleotide backbone, accompanied by aromatic and hydrophobic interactions of pyrenes within polynucleotide grooves. New dye also showed intriguing antiproliferative activity, strongly enhanced upon photo-induced activation of pyrenes, and is thus a promising lead compound for theranostic applications on ds-RNA or ds-DNA targets, applicable as a new strategy in cancer and gene therapy.

Association between N-Terminal Pyrenes Stabilizes the Collagen Triple Helix

Collagen model peptides featuring the fluorophore pyrene at their N-termini have been synthesized, and their thermal denaturation has been examined using circular dichroism (CD) and fluorescence spectroscopies. Flanking the (Pro-Hyp-Gly)7 core of the peptide monomers at positions 1 and/or 23 in the primary sequence, Lys residues were introduced to ensure water solubility. Triple helices derived from such peptides show a broad excimer emission at ∼480 nm, indicative of interaction between the pyrene units. CD experiments show that the fluorophores enhance helix stability primarily through entropic effects. Unfolding temperatures (Tm) increase by up to 7 °C for systems with N-terminal lysine residues and by up to 21 °C for systems in which the first-position Lys is replaced by Ala. Tm values derived from fluorescence measurements (at 50 μM) typically lie within ∼1 °C of those obtained using CD (at 200 μM). Computational modeling in a water continuum using B3LYP-GD3 and M06-2X functionals predicts that face-to-face association of fluorophores can occur while H-bonding within the [(POG)n]3 assembly is retained. Such parallel stacking is consistent with hydrophobically driven stabilization. Labeling collagen peptides with pyrene is a synthetically simple way to promote triple helicity while providing a means to obtain Tm data on relatively dilute samples.

Role of Sterically Bulky Azobenzenes in the Molecular Assembly of Pyrene Derivatives: Rectangular Sheet-like Structures and Their Emission Characteristics

The development of pyrene-based fluorescent assembled systems with desirable emission characteristics by reducing conventional concentration quenching and/or aggregation-induced quenching (ACQ) is highly desirable. In this investigation, we designed a new azobenzene-functionalized pyrene derivative (AzPy) in which sterically bulky azobenzene is linked to pyrene. Absorption and fluorescence spectroscopic results before and after molecular assembly indicate that even in a dilute N,N-dimethylformamide (DMF) solution (~10 μM), AzPy molecules experienced significant concentration quenching, whereas the emission intensities of AzPy DMF-H₂O turbid suspensions containing self-assembled aggregates were slightly enhanced and showed similar values regardless of the concentration. The shape and size of sheet-like structures, from incomplete flakes less than one micrometer in size to well-completed rectangular microstructures, could be adjusted by changing the concentration. Importantly, such sheet-like structures exhibit concentration dependence of their emission wavelength from blue to yellow-orange. Comparison with the precursor (PyOH) demonstrates that the introduction of a sterically twisted azobenzene moiety plays an important role in converting the spatial molecular arrangements from H- to J-type aggregation mode. Thus, AzPy chromophores grow into anisotropic microstructures through inclined J-type aggregation and high crystallinity, which are responsible for their unexpected emission characteristics. Our findings provide useful insight into the rational design of fluorescent assembled systems.

Assembly of Biologically Functional Structures by Nucleic Acid Templating: Implementation of a Strategy to Overcome Inhibition by Template Excess

Delivery of therapeutic molecules to pathogenic cells is often hampered by unintended toxicity to normal cells. In principle, this problem can be circumvented if the therapeutic effector molecule is split into two inactive components, and only assembled on or within the target cell itself. Such an in situ process can be realized by exploiting target-specific molecules as templates to direct proximity-enhanced assembly. Modified nucleic acids carrying inert precursor fragments can be designed to co-hybridize on a target-specific template nucleic acid, such that the enforced proximity accelerates assembly of a functional molecule for antibody recognition. We demonstrate the in vitro feasibility of this adaptation of nucleic acid-templated synthesis (NATS) using oligonucleotides bearing modified peptides (“haplomers”), for templated assembly of a mimotope recognized by the therapeutic antibody trastuzumab. Enforced proximity promotes mimotope assembly via traceless native chemical ligation. Nevertheless, titration of participating haplomers through template excess is a potential limitation of trimolecular NATS. In order to overcome this problem, we devised a strategy where haplomer hybridization can only occur in the presence of target, without being subject to titration effects. This generalizable NATS modification may find future applications in enabling directed targeting of pathological cells.

Cluster Size of Amylopectin and Nanosized Amylopectin Fragments Characterized by Pyrene Excimer Formation

Amylopectin from waxy corn and the three nanosized amylopectin fragments (NAFs)—NAF(56), NAF(20), and NAF(8)—from waxy corn starch with a hydrodynamic diameter of 227, 56, 20, and 8 nm, respectively, were randomly labeled with 1-pyrenebutyric acid. The efficiency of these pyrene-labeled amylopectin-based polysaccharides (Py-AbPS) for pyrene excimer formation (PEF) upon diffusive encounter between an excited and a ground-state pyrene increased with increasing concentration of unlabeled NAF(56) in Py-AbPS dispersions in DMSO. Fluorescence decay analysis of the Py-AbPS dispersions in DMSO prepared with increasing [NAF(56)] yielded the maximum number (N_blob^exp) of anhydroglucose units (AGUs) separating two pyrene-labeled AGUs while still allowing PEF. Comparison of N_blob^exp with N_blob^theo, obtained by conducting molecular mechanics optimizations on helical oligosaccharide constructs with HyperChem, led to a relationship between the interhelical distance (d_h-h) in a cluster of oligosaccharide helices, [NAF(56)], and the number of helices in a cluster. It was found that the AbPSs were composed of building blocks made of 3.5 (±0.9) helices that self-assembled into increasingly larger clusters with increasing [NAF(56)]. The ability of PEF-based experiments to yield the cluster size of AbPSs provides a new experimental means to probe the interior of AbPSs at the molecular level.

Characterization of the Interactions between an Unassociated Cationic Pyrene-Labeled Gemini Surfactant and Anionic Sodium Dodecyl Sulfate

The gemini surfactant PyO-3-12, made of two dimethylammonium bromides joined by a propyl linker and bearing a dodecyl pendant on one side and a 1-pyrenemethoxyhexyl group on the other side, was employed to probe the interactions between positively charged PyO-3-12 and negatively charged sodium dodecyl sulfate (SDS). PyO-3-12 was selected for its ability to respond to the polarity of its local environment through the fluorescence intensity ratio I₁/I₃ of the first-to-third fluorescence peaks of the pyrene monomer and the local pyrene concentration [Py]_loc through the I_E/I_M ratio of the pyrene excimer-to-pyrene monomer fluorescence intensity. Furthermore, analysis of the fluorescence decays of aqueous solutions of PyO-3-12 and SDS yielded a measure of the internal dynamics, local concentration, and state (associated vs unassociated) of PyO-3-12 in solution. By following these parameters for aqueous solutions prepared with a constant PyO-3-12 concentration of either 1, 4, or 16 μM and SDS concentrations ranging from 0 to 200 mM, six SDS concentration regimes were identified to describe the interactions between PyO-3-12 and SDS in pure water. Sharp transitions of the parameters describing the fluorescence of pyrene marked the boundaries between the different regimes. Perhaps the most important transition was the one defining the formation of the PyO-3-12/SDS aggregates, which was completed at the equicharge point, implying that they were constituted of 1 meq of PyO-3-12 and 2 meq of SDS. The low I₁/I₃ ratio obtained for the PyO-3-12/SDS aggregates suggested that they were multilamellar aggregates, which would shield the pyrenyl labels from polar water. The formation of these multilamellar aggregates was confirmed by transmission electron microscopy (TEM), which demonstrated the existence of multilamellar vesicles, whose presence increased with decreasing PyO-3-12 concentration. This study suggests that the combination of pyrene excimer formation and TEM provides an interesting experimental means to probe the assemblies generated from oppositely charged surfactants at surfactant concentrations, which are much lower than their critical micelle concentration.

Merging antimicrobial and visible emission properties within 1,3,4-trisubstituted-1,2,3-triazolium salts

Bioactive molecules displaying visible wavelength emission can be useful for bioimaging, chemosensing and photodynamic therapy applications. Reported herein are 1,3,4-trisubsituted-1,2,3-triazolium salts displaying both antimicrobial and visible emission properties. Using a click chemistry approach, 2-fluorenyl, 1-naphthyl, 2-naphthyl, 2-anthracenyl and 1-pyrenyl units were incorporated at the N1 position, imparting visible emission properties to their triazolium bromide salts with Stokes shifts greater than 100 nm relative to the emission of their triazole precursors. The increasing size of such hydrophobic aryl units impacts minimum inhibitory concentration (MIC) values against Gram-positive bacteria, Gram-negative bacteria and yeast, and can be counterbalanced by hydrophobic substituent variation at other positions of the molecule in order to preserve bioactivity. Among the series of compounds studied are analogs displaying blue, green and yellow colored emission and MIC values as low as 0.4 μM (Gram-positive bacteria), 8 μM (Gram-negative bacteria) and 2 μM (yeast). XRD analysis validates the regioselective benzylation at the N3 position of the 1,2,3-triazole ring and the ability of such compounds to associate through dimeric intermolecular π-stacking interactions.

Highly Fluorescent Dyes Containing Conformationally Restrained Pyrazolylpyrene (Pyrazoolympicene) Chromophore

The triflic-acid-promoted cyclization of 1-phenyl-3-(pyren-1-yl)-1H-pyrazole-4-carbaldehyde afforded a mixture of 9-phenyl-7,9-dihydropyreno (10,1-fg)indazole and 9-phenylpyreno(10,1-fg)indazole-7(9H)-one, readily separable by column chromatography. Both products contained a rigid six-ringed pyrazoolympicene backbone and exhibited bright fluorescence in chloroform solution and a weak fluorescence in the solid state. DFT and TD DFT calculations revealed that the lowest excited state (S1) of these compounds is populated via HOMO →LUMO π-π * transition. Furthermore, the synthesized compounds behaved as weak bases and their emission spectra showed substantial changes upon protonation. Therefore, they may be of interest for sensing of strongly acidic fluorophore environments.

Pyrene label used as a scale for sequence-controlled functionalized polymers

Pyrene-labeled sequence-controlled polymers (SCPs) was synthesized, and the differences among sequences were reflected directly by pyrene luminescence. The pyrene label can be used as a quantitative scale for characterizing SCPs.

Luminescent Covalent Organic Frameworks for Biosensing and Bioimaging Applications

Luminescent covalent organic frameworks (LCOFs) have attracted significant attention due to their tunability of structures and photophysical properties at molecular level. LCOFs are built to highly ordered and periodic 2D or 3D framework structures through covalently assembling with various luminophore building blocks. Recently, the advantages of LCOFs including predesigned properties of structure, unique photoluminescence, hypotoxicity and good biocompatibility and tumor penetration, broaden their applications in biorelated fields, such as biosensing, bioimaging, and drug delivery. A specific review that analyses the advances of LCOFs in the field of biosensing and bioimaging is thus urged to emerge. Here the construction of LCOFs is reviewed first. The synthetic chemistry of LCOFs highlights the key role of chemical linkages, which not only concrete the building blocks but also affect the optical properties and even can act as the responsive sites for potential sensing applications. How to brighten LCOFs are clarified through description of structure managements. The ability to utilize the luminescence of LCOFs for applications in biosensing and bioimaging is discussed using state-of-the-art examples of varied practical goals. A prospect finally addresses opportunities and challenges the development of LCOFs facing from chemistry, physics to the applications, according to their current progress.

Visible-Light-Induced Photoannulation of α-Naphthyl Cyclopropane Carboxylic Esters to Functionalized Dihydrophenalenes

A general synthetic entry to functionalized dihydrophenalenes has been found using naphthyl-cyclopropane esters as starting materials. The desired annulation was possible with visible light, Ir(Fppy)₃ as photocatalyst, BnNMe₂ or DABCO as electron donor, HAT-catalyst, and proton source. A broad scope of substituted naphthyl and azanaphthyl derivatives provided the photoannulation products in high yield. Deuteration studies support a photoredox mechanism involving the photoreductive cyclopropane opening to an enolate radical followed by an aryl radical trapping.

Synthesis and Characterization of a Pyrene-Labeled Gemini Surfactant Sensitive to the Polarity of Its Environment

The cationic gemini surfactant PyO-3-12 was designed to include two dimethyl ammonium groups, one dodecyl tail, and 1-pyrenemethyl hexyl ether tail into the structure of the surfactant. The pyrenyl label ensured that the fluorescence of pyrene could be employed to probe the behavior of PyO-3-12 at the molecular level. The introduction of the oxygen atom in the β-position to pyrene was found to be critical for restoring the sensitivity of the pyrenyl label to the polarity of its environment. The properties of PyO-3-12 were characterized in water by surface tension and a fluorescence methodology that involved the global model-free analysis (MFA) of the pyrene monomer and excimer fluorescence decays to provide quantitative information about the state (unassociated-vs-aggregated) of PyO-3-12. The MFA was combined with a fluorescence quenching study with 2,6-dinitrotoluene to determine the size of the PyO-3-12 micelles. PyO-3-12 was found to behave like a typical gemini surfactant, exhibiting a critical micelle concentration (CMC) of 0.38 (±0.05) mM and an aggregation number (N_agg) equal to 23 (±2). Besides allowing PyO-3-12 to probe the polarity of its environment, the oxygen atom in the β-position next to pyrene brought some pyrenyl labels closer to the interface between the micellar interior and the aqueous phase, in a process that increased the effective volume of the hydrophobic part of PyO-3-12. This led to an increase in the packing parameter of PyO-3-12 and, consequently, an increase in N_agg compared to the N_agg value of 14 (±0.2) obtained for Py-3-12, a gemini surfactant, whose chemical structure was similar to that of PyO-3-12 but without the oxygen in the β-position to pyrene. The methodology described in this study to prepare and characterize pyrene-labeled surfactants is general and can be applied to study any pyrene-labeled surfactant and its interactions with oppositely charged macromolecules.

Cyclodextrins with Multiple Pyrenyl Groups: An Approach to Organic Molecules Exhibiting Bright Excimer Circularly Polarized Luminescence

Circularly polarized luminescence (CPL) has attracted much attention because of its potential for electronic and biological applications. Herein, we report a simple and effective approach to organic molecules exhibiting bright CPL by combining a chiral cyclic molecular scaffold and multiple excimer-enabling moieties. An a-cyclodextrin ( CyD ) scaffold was modified with six pyrenyl groups to obtain pyrene-cyclodextrins ( PCD s) in a one-step synthesis from commercially available compounds. The PCDs exhibited high molar extinction coefficients (ε ~ 10 5 M -1 cm -1 ), polarized emission with a good dissymmetry factor (| g lum | ~ 10 -2 ), and quantum yield ( Φ f ~ 0.5). Owing to the excellent photophysical properties of the PCDs, the circularly polarized luminescence brightness ( B CPL ) reached 340 M -1 cm -1 . Comprehensive photophysical and chiroptical studies of the PCD s with only five pyrene units and with linkers of various lengths connecting the CyD with the pyrene units revealed that the formation of a pyrene excimer in a spatially crowded environment is crucial for CPL anisotropy. This study paves the way for the development of bright CPL organic molecules.

Triphasic Polymer Particles Assembled via Microphase Separation with Multiple Functions

This work investigated a unique type of triphasic colloidal particles composed of an azo polymer (PCNAZO), a fluorescent pyrene-containing polymer [P(MMA-co-PyMA)], and a poly(dimethylsiloxane)-based polymer (H₂pdca-PDMS), focusing on the synthesis, forming mechanism, morphology control, and functions. The triphasic particles with well-defined morphologies were assembled through the microphase separation of the components in dichloromethane (DCM) droplets in an aqueous medium, induced by the gradual evaporation of the organic solvent. The real-time fluorescence emission spectra of the pyrenyl moieties and in situ microscopic observations show that the formation of the triphasic particles undergoes the segregation of the PCNAZO-rich phase, separation between P(MMA-co-PyMA)-rich and H₂pdca-PDMS-rich phases, coalescence, and solidification in the dispersed droplets. The structure formation is due to the strong phase separation of the polymers as revealed by the calculations based on the Flory-Huggins theory. The morphologies and phase boundaries of the particles are found to be controlled by the interfacial energy between the phases and processing conditions. The triphasic particles thus obtained possess a series of interesting functions stemming from the polymers and the triple-compartmentalized structures. After being deposited on a substrate, the H₂pdca-PDMS parts can tightly adhere on the surface, caused by the spreading nature of the polymer when slightly swelled by DCM. Upon irradiation with a linearly polarized laser beam at 488 nm, the azo polymer compartments show a significant elongation along the electric vibration direction of the polarized light, accompanied by the cooperative deformation of the H₂pdca-PDMS pads. When dispersed in water and adhered on the substrate surface, the triphasic particles exhibit tunable colors originating from the fluorescence of the pyrenyl fluorophores and light absorption of the azo chromophores. The real-time investigation methods developed here could lead to the deep understanding of the structure formation process in the confined volume and be applied in phase-separation study of other polymers as well.

Unfolding of Helical Poly(L-Glutamic Acid) in N,N-Dimethylformamide Probed by Pyrene Excimer Fluorescence (PEF)

The denaturation undergone by α–helical poly(L-glutamic acid) (PLGA) in N,N-dimethylformamide upon addition of guanidine hydrochloride (GdHCl) was characterized by comparing the fluorescence of a series of PLGA constructs randomly labeled with the dye pyrene (Py-PLGA) to that of a series of Py-PDLGA samples prepared from a racemic mixture of D,L-glutamic acid. The process of pyrene excimer formation (PEF) was taken advantage of to probe changes in the conformation of α–helical Py-PLGA. Fluorescence Blob Model (FBM) analysis of the fluorescence decays of the Py-PLGA and Py-PDLGA constructs yielded the average number (<N_blob>) of glutamic acids located inside a blob, which represented the volume probed by an excited pyrenyl label. <N_blob> remained constant for randomly coiled Py-PDLGA but decreased from ~20 to ~10 glutamic acids for the Py-PLGA samples as GdHCl was added to the solution. The decrease in <N_blob> reflected the decrease in the local density of PLGA as the α–helix unraveled in solution. The changes in <N_blob> with GdHCl concentration was used to determine the change in Gibbs energy required to denature the PLGA α–helix in DMF. The relationship between <N_blob> and the local density of macromolecules can now be applied to characterize the conformation of macromolecules in solution.

Determination of the Aggregation Number of Pyrene-Labeled Gemini Surfactant Micelles by Pyrene Fluorescence Quenching Measurements

A cationic gemini surfactant referred to as Py-3-12 and composed of two alkylated diammonium bromide head groups, a propyl spacer, and dodecyl and 1-pyrenehexyl hydrophobic tails was synthesized. Its critical micellar concentration (CMC) was determined to equal 0.15 (±0.02) mM by surface tension and time-resolved fluorescence measurements. The state of the pyrene molecules, whether they were incorporated inside the Py-3-12 micelles or unassociated in the aqueous solution, was determined by applying the global model-free analysis (MFA) to the fluorescence decays acquired with Py-3-12 aqueous solutions. The unassociated Py-3-12 surfactants emitted as pyrene monomers and showed a long fluorescence lifetime. The excited pyrenyl groups located inside Py-3-12 micelles formed an excimer by a rapid encounter with a ground-state pyrene with an average rate constant equal to 0.69 (±0.06) ns^-1. After having the photophysical properties of Py-3-12 in aqueous solution characterized, the number (N_agg) of surfactants per micelle was determined by conducting quenching experiments with dinitrotoluene (DNT). Although DNT is fairly hydrophobic, it was found to partition itself between the Py-3-12 micelles and the aqueous phase. Fluorescence quenching experiments performed on the pyrene monomer and excimer generated by the Py-3-12 aqueous solutions yielded the concentration ([Q]_b) of DNT bound to the Py-3-12 micelles and the average number ⟨n⟩_d of DNT quenching an excimer by diffusive encounters. A combination of steady-state and time-resolved fluorescence quenching experiments on the excimer yielded the number (⟨n⟩_s) of DNT molecules that were bound to the micelles and quenched the excimer in a static manner. A plot of the sum ⟨n⟩_d + ⟨n⟩_s as a function of [Q]_b yielded an N_agg value of 14.0 (±0.2) Py-3-12 units per micelle. This study represents the first example in the literature where N_agg is determined for a micelle, where each surfactant molecule is labeled with pyrene.

Pyrene-Terminated Tin Sulfide Clusters: Optical Properties and Deposition on a Metal Surface

Herein, we present the synthesis of two pyrene-functionalized clusters, [(Rpyr Sn)4 S6 ]⋅2 CH2 Cl2 (4) and [(Rpyr Sn)4 Sn2 S10 ]⋅n CH2 Cl2 (n=4, 5 a; n=2, 5 b; Rpyr =CMe2 CH2 C(Me)N-NC(H)C16 H9 ), both of which form in reactions of the organotin sulfide cluster [(RN Sn)4 S6 ] (C; RN =CMe2 CH2 C(Me)N-NH2 ) with the well-known fluorescent dye 1-pyrenecarboxaldehyde (B). In contrast, reactions using an organotin sulfide cluster with another core structure, [(RN Sn)3 S4 Cl] (A), leads to formation of small molecular fragments, [(Rpyr Cl2 Sn)2 S] (1), (pyren-1-ylmethylene)hydrazine (2), and 1,2-bis(pyren-1-ylmethylene)hydrazine (3). Besides synthesis and structures of the new compounds, we report the influence of the inorganic core on the optical properties of the dye, which was analyzed exemplarily for compound 5 a via absorption and fluorescence spectroscopy. This cluster was also used for exploring the potential of such non-volatile clusters for deposition on a metal surface under vacuum conditions.

2- and 2,7-Substituted para-N-Methylpyridinium Pyrenes: Syntheses, Molecular and Electronic Structures, Photophysical, Electrochemical, and Spectroelectrochemical Properties and Binding to Double-Stranded (ds) DNA

Two N-methylpyridinium compounds and analogous N-protonated salts of 2- and 2,7-substituted 4-pyridyl-pyrene compounds were synthesised and their crystal structures, photophysical properties both in solution and in the solid state, electrochemical and spectroelectrochemical properties were studied. Upon methylation or protonation, the emission maxima are significantly bathochromically shifted compared to the neutral compounds, although the absorption maxima remain almost unchanged. As a result, the cationic compounds show very large apparent Stokes shifts of up to 7200 cm-1 . The N-methylpyridinium compounds have a single reduction at ca. -1.5 V vs. Fc/Fc+ in MeCN. While the reduction process was reversible for the 2,7-disubstituted compound, it was irreversible for the mono-substituted one. Experimental findings are complemented by DFT and TD-DFT calculations. Furthermore, the N-methylpyridinium compounds show strong interactions with calf thymus (ct)-DNA, presumably by intercalation, which paves the way for further applications of these multi-functional compounds as potential DNA-bioactive agents.

Energy transduction through FRET in self-assembled soft nanostructures based on surfactants/polymers: current scenario and prospects

The self-assembled systems of surfactants/polymers, which are capable of supporting energy funneling between fluorophores, have recently gained significant attraction. Surfactant and polymeric micelles form nanoscale structures spanning a radius of 2-10 nm are generally suitable for the transduction of energy among fluorophores. These systems have shown great potential in Förster resonance energy transfer (FRET) due to their unique characteristics of being aqueous based, tendency to remain self-assembled, spontaneous formation, tunable nature, and responsiveness to different external stimuli. This review presents current developments in the field of energy transfer, particularly the multi-step FRET processes in the self-assembled nanostructures of surfactants/polymers. The part one of this review presents a background and brief overview of soft systems and discusses certain aspects of the self-assemblies of surfactants/polymers and their co-solubilization property to bring fluorophores to close proximity to transduce energy. The second part of this review deals with single-step and multi-step FRET in the self-assemblies of surfactants/polymers and links FRET systems with advanced smart technologies including multicolor formation, data encryption, and artificial antenna systems. This review also discusses the diverse examples in the literature to present the emerging applications of FRET. Finally, the prospects regarding further improvement of FRET in self-assembled soft systems are outlined.

In situ monitoring of PISA morphologies

Fluorescent spectroscopy is a convenient method for monitoring the progression of polymerization-induced self-assembly (PISA).

Direct Measure of the Local Concentration of Pyrenyl Groups in Pyrene-Labeled Dendrons Derived from the Rate of Fluorescence Collisional Quenching

The model-free analysis (MFA) was applied to measure the average rate constant (<k>) for pyrene excimer formation (PEF) in a series of pyrene-labeled dendrons referred to as Py_x-G(N), where x (= 2^N) is the number of pyrenyl labels born by a dendron of generation N ranging from 1 to 6. <k> was measured in four different solvents, namely tetrahydrofuran (THF), toluene, N,N-dimethylformamide (DMF), and dimethylsulfoxide (DMSO). <k> was found to increase linearly with increasing local pyrene concentration ([Py]_loc), where [Py]_loc had been determined mathematically for the Py_x-G(N) dendrons. The slope of each straight line changed with the nature of the solvent and represented k_diff, the bimolecular rate constant for PEF. k_diff depended on the solvent viscosity (η) and the probability (p) for PEF upon encounter between an excited and a ground-state pyrene. In a same solvent, k_diff for the Py_x-G(N) dendrons was about 360 ± 30 times smaller than k_diff obtained for ethyl 4-(1-pyrene)butyrate (PyBE), a pyrene model compound similar to the pyrene derivative used to label the dendrons. The massive decrease in k_diff observed for the Py_x-G(N) samples reflected the massive loss in mobility experienced by the pyrenyl labels after being covalently attached onto a macromolecule compared to freely diffusing PyBE. Interestingly, the k_diff values obtained for the Py_x-G(N) dendrons and the PyBE model compound followed similar trends as a function of solvent, indicating that the difference in behavior between the k_diff values obtained in different solvents were merely due to the changes in the η and p values between the solvents. Normalizing the <k> values obtained with the Py_x-G(N) dendrons by the k_diff values obtained for PyBE in the same solvents accounted for changes in η and p, resulting in a master curve upon plotting <k>/(f_diff × k_diff) as a function of [Py]_loc, where f_diff was introduced to account for some pyrene aggregation in the higher generation dendron (Py₆₄-G(6)). This result demonstrates that <k> represents a direct measure of [Py]_loc in pyrene-labeled macromolecules.

Synthesis of 2-Azapyrenes and Their Photophysical and Electrochemical Properties

A series of 5,7,9-substituted 2-azapyrenes were synthesized for the first time. The synthesis relies on Brønsted acid promoted benzannulation of alkyne precursors prepared by palladium-catalyzed cross-coupling reactions. The synthetic strategy is efficient and the scope covers a variety of functional groups. The electrochemical behavior and photophysical properties of the products were investigated by UV-vis and fluorescence spectroscopy, cyclic voltammetry, and DFT calculations.

Hybrid Fluorescent Poly(silsesquioxanes) with Amide- and Triazole-Containing Side Groups for Light Harvesting and Cation Sensing

Hybrid polymers containing pyrene (Py) units bound to linear poly(silsesquioxane) (LPSQ) chains through flexible linkers containing heteroatoms (S, N, O) (LPSQ-triazole-Py and LPSQ-amide-Py) exhibit intense fluorescence emission, both in very diluted solutions (c = 10-8 mol/L) and in the solid state. The materials are thermally stable and exhibit good thin film forming abilities. Their optical and physicochemical properties were found to be strongly dependent on the structure of the side chains. Comparative studies with octahedral silsesquioxane (POSS) analogues (POSS-triazole-Py and POSS-amide-Py) emphasized the role of the specific double-strand architecture of the LPSQ backbone and distribution of side Py groups for their photo-luminescent properties. The new hybrid materials were tested as fluorescence energy donors to red-emitting dyes (Nile Red and Coumarine 6). All the silsesquioxanes studied were found to be able to transfer FL emission energy to Coumarin 6, irrespectively of their spatial structure. However, due to the differences in the wavelength range of FL emission, only LPSQ-triazole-Py were able to act as energy donors to Nile Red. The Py-grafted LPSQ may be also applied for development of soluble and highly emissive chemosensors. Their fluorescent nature was explored for the detection of Cu(II), Fe(III), Co(II), Ag(I), Hg(II), Mg(II), Ca(II), Pb(II) and Zn(II). The morphology of the side chains and hydrogen-bonding interactions influenced the sensing capacity of all the studied materials.

Development of pyrene-based fluorescent ether lipid as inhibitor of SK3 ion channels

We report the synthesis of three bioactive pyrene-based fluorescent analogues of Ohmline which is the most efficient and selective inhibitor of SK3 ion channel. The interaction of these Ohmline-pyrene (OP1-3) with liposomes of different composition reveals that only OP2 and OP3 are readily integrated into liposomes. Fluorescence measurements indicate that, depending on their concentration, OP2 and OP3 exist either as monomer or as a mixture of monomer and excimers within the liposome bilayer. Among the three Ohmline Pyrene compounds (OP1-3) only OP2 is able to reduce SK3 currents and is the first efficient fluorescent modulator of SK3 channel as revealed by patch clamp measurements (- 71.3 ± 13.3% at 10 μM) and by its inhibition of SK3-dependent cancer cell migration at (-32.5% ± 4.8% at 1 μM). We also report the first fluorescence study on living breast cancer cells (MDA-MB-231) showing that OP2 is rapidly integrated in bio-membranes followed by cell internalization.