Excited‐State Intramolecular Proton Transfer in a Blue Fluorescence Chromophore Induces Dual Emission

Compared with green fluorescence protein (GFP) chromophores, the recently synthesized blue fluorescence protein (BFP) chromophore variant presents intriguing photochemical properties, for example, dual fluorescence emission, enhanced fluorescence quantum yield, and ultra‐slow excited‐state intramolecular proton transfer (ESIPT; J. Phys. Chem. Lett., 2014 , 5, 92); however, its photochemical mechanism is still elusive. Herein we have employed the CASSCF and CASPT2 methods to study the mechanistic photochemistry of a truncated BFP chromophore variant in the S₀ and S₁ states. Based on the optimized minima, conical intersections, and minimum‐energy paths (ESIPT, photoisomerization, and deactivation), we have found that the system has two competitive S₁ relaxation pathways from the Franck–Condon point of the BFP chromophore variant. One is the ESIPT path to generate an S₁ tautomer that exhibits a large Stokes shift in experiments. The generated S₁ tautomer can further evolve toward the nearby S₁/S₀ conical intersection and then jumps down to the S₀ state. The other is the photoisomerization path along the rotation of the central double bond. Along this path, the S₁ system runs into an S₁/S₀ conical intersection region and eventually hops to the S₀ state. The two energetically allowed S₁ excited‐state deactivation pathways are responsible for the in‐part loss of fluorescence quantum yield. The considerable S₁ ESIPT barrier and the sizable barriers that separate the S₁ tautomers from the S₁/S₀ conical intersections make these two tautomers establish a kinetic equilibrium in the S₁ state, which thus results in dual fluorescence emission.     

Rational Approach Towards Designing Metallogels From a Urea‐Functionalized Pyridyl Dicarboxylate: Anti‐inflammatory,Anticancer, and Drug Delivery

A structural rationale was adopted to design a series of metallogels from a newly synthesized urea‐functionalized dicarboxylate ligand, namely, 5‐[3‐(pyridin‐3‐yl)ureido]isophthalic acid ( PUIA ), that produces metallogels upon reaction with various metal salts (Cu^II, Zn^II, Co^II, Cd^II, and Ni^II salts) at room temperature. The gels were characterized by dynamic rheology and transmission electron microscopy (TEM). The existence of a coordination bond in the gel state was probed by FTIR and ¹H NMR spectroscopy in a Zn^II metallogel (i.e., MG2 ). Single crystals isolated from the reaction mixture of PUIA and Co^II or Cd^II salts characterized by X‐ray diffraction revealed lattice inclusion of solvent molecules, which was in agreement with the hypothesis based on which the metallogels were designed. MG2 displayed anti‐inflammatory response (prostaglandin E₂ assay) in the macrophage cell line (RAW 264.7) and anticancer properties (cell migration assay) on a highly aggressive human breast cancer cell line (MDA‐MB‐231). The MG2 metallogel matrix could also be used to load and release (pH responsive) the anticancer drug doxorubicin. Fluorescence imaging of MDA‐MB‐231 cells treated with MG2 revealed that it was successfully internalized.     

Easy Access to Supramolecular Gels of the Nonsteroidal Anti‐inflammatory Drug Diflunisal: Synthesis,Characterization, and Plausible Biomedical Applications

By exploiting salt formation, a new series of primary ammonium monocarboxylate salts of a nonsteroidal anti‐inflammatory drug, namely, diflunisal, was synthesized. The majority of the salts thus synthesized turned out to be good gelators of various solvents, including the solvents (e.g., methyl salicylate and pure water) typically used for topical gel formulation. Single‐crystal X‐ray diffraction studies of a few gelator and nongelator salts in the series revealed details of the hydrogen‐bonding networks present in the salts. Furthermore, one such gelator salt, namely, the diflunisal salt of serinol, was found to be biocompatible (MTT assay), and its anti‐inflammatory (PGE₂ assay) response turned out to be as good as that of the parent drug, which is indicative of its potential in biomedical applications.     

Peptide Conjugates of a Nonsteroidal Anti‐Inflammatory Drug as Supramolecular Gelators: Synthesis,Characterization, and Biological Studies

Indomethacin ( IND ), which is a well‐known nonsteroidal anti‐inflammatory drug (NSAID), was conjugated with various naturally occurring amino acids. Most of these bioconjugates were capable of gelling pure water, a solution of NaCl (0.9 wt %), and phosphate‐buffered saline (pH 7.4), as well as a few organic solvents. The gels were characterized by table‐top and dynamic rheology, and electron microscopy. Variable‐temperature ¹H NMR spectroscopy studies on a selected gel were performed to gain insights into the self‐assembly process during gel formation. Both 1D and 2D hydrogen‐bonded networks were observed in the single‐crystal structures of two of the gelators. Plausible biological applications of the hydrogelators were evaluated with the ultimate aim of drug delivery in a self‐delivery fashion. All hydrogelators were stable in phosphate‐buffered saline at pH 7.4 at 37 °C, and biocompatible in mouse macrophage RAW 264.7 cell line (3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide assay). Two of the most biocompatible hydrogelators displayed an anti‐inflammatory response comparable to that of the parent drug IND in prostaglandin E₂ assay. Release of the bioconjugates into the bulk solvent interfaced with the corresponding hydrogels indicated their plausible future application in drug delivery.     

Multidrug‐Containing,Salt‐Based,Injectable Supramolecular Gels for Self‐Delivery,Cell Imaging and Other Materials Applications

Both molecular and crystal‐engineering approaches were exploited to synthesize a new class of multidrug‐containing supramolecular gelators. A well‐known nonsteroidal anti‐inflammatory drug, namely, indomethacin, was conjugated with six different l ‐amino acids to generate the corresponding peptides having free carboxylic acid functionality, which reacted further with an antiviral drug, namely, amantadine, a primary amine, in 1:1 ratio to yield six primary ammonium monocarboxylate salts. Half of the synthesized salts showed gelation ability that included hydrogelation, organogelation and ambidextrous gelation. The gels were characterized by table‐top and dynamic rheology and different microscopic techniques. Further insights into the gelation mechanism were obtained by temperature‐dependent ¹H NMR spectroscopy, FTIR spectroscopy, photoluminescence and dynamic light scattering. Single‐crystal X‐ray diffraction studies on two gelator salts revealed the presence of 2D hydrogen‐bonded networks. One such ambidextrous gelator (capable of gelling both pure water and methyl salicylate, which are important solvents for biological applications) was promising in both mechanical (rheoreversible and injectable) and biological (self‐delivery) applications for future multidrug‐containing injectable delivery vehicles.     

Supramolecular Interactions of Nonsteroidal Anti‐inflammatory Drug in Nanochannels of Molecular Containers: A Spectroscopic,Thermogravimetric and Microscopic Investigation

Supramolecular host–guest complexation between the nonsteroidal anti‐inflammatory drug indomethacin (IMC) and molecular containers were investigated. The weakly fluorescent drug molecule becomes highly fluorescent on complexation with different molecular containers, and time‐resolved fluorescence emission spectroscopy reveals that the lifetime components of IMC significantly increase in the presence of molecular containers, compared with the lifetimes in neat water. The respective solid host–guest complexes were synthesised and characterised by Fourier transform infrared and ¹H nuclear magnetic resonance spectroscopic analysis. Microscopy techniques were used to analyse modifications of the surface morphology, owing to the formation of supramolecular complexes. The effect of the molecular container on the optical properties of IMC has also been investigated to determine the effect of nanochannels of different size and structure.     

Excited‐State Proton Transfer Can Tune the Color of Protein Fluorescent Markers

We show by quantum mechanical/molecular mechanical (QM/MM) simulations that phenylbenzothiazoles undergoing an excited‐state proton transfer (ESPT) can be used to probe protein binding sites. For 2‐(2′‐hydroxy‐4′‐aminophenyl)benzothiazole (HABT) bound to a tyrosine kinase, the absolute and relative intensities of the fluorescence bands arising from the enol and keto forms are found to be strongly dependent on the active‐site conformation. The emission properties are tuned by hydrogen‐bonding interactions of HABT with the neighboring amino acid T766 and with active‐site water. The use of ESPT tuners opens the possibility of creating two‐color fluorescent markers for protein binding sites, with potential applications in the detection of mutations in cancer cell lines.     

Solid‐State Hierarchical Cyclodextrin‐Based Supramolecular Polymer Constructed by Primary,Secondary, and Tertiary Azido Interactions

The crystallization of a di‐azido‐α‐cyclodextrin revealed a polymeric self‐assembly involving a variety of azido‐type interactions. The crystal arrangement relies on the cooperativity of a primary azido inclusion, a secondary azido–azido interaction involving an unprecedented distribution of canonical forms, and a tertiary azido–groove interaction. The second azido group brings in a major contribution to the supramolecular structure illustrating the benefit of a difunctionalization for the generation of hierarchy.     

Excited‐State Proton Transfer and Intramolecular Charge Transfer in 1,3‐Diketone Molecules

The photophysical signature of the tautomeric species of the asymmetric (N,N‐dimethylanilino)‐1,3‐diketone molecule are investigated using approaches rooted in density functional theory (DFT) and time‐dependent DFT (TD‐DFT). In particular, since this molecule, in the excited state, can undergo proton transfer reactions coupled to intramolecular charge transfer events, the different radiative and nonradiative channels are investigated by making use of different density‐based indexes. The use of these tools, together with the analysis of both singlet and triplet potential energy surfaces, provide new insights into excited‐state reactivity allowing one to rationalize the experimental findings including different behavior of the molecule as a function of solvent polarity.     

The Host/Guest Type of Excited‐State Proton Transfer; a General Review

Contemporary progress regarding guest/host types of excited‐state double proton transfer has been reviewed, among which are the biprotonic transfer within doubly H‐bonded host/guest complexes, the transfer through a solvent bridge relay, the intramolecular double proton transfer and solvation dynamics coupled proton transfer. Of particular emphases are the photophysical and photochemical properties of excited‐state double proton transfer (ESDPT) in 7‐azaindole and its corresponding analogues. From the chemical aspect, two types of ESDPT reaction, namely the catalytic and non‐catalytic types of ESDPT, have been classified and reviewed separately. For the case of static host/guest hydrogen‐bonded complexes both hydrogen‐bonding strength and configuration (i.e. geometry) play key roles in accounting for the reaction dynamics. In addition to the dynamical concern, excited‐state thermodynamics are of importance to fine‐tune the proton transfer reaction in the non‐catalytic host/guest type of ESDPT. The mechanisms of protic solvent assisted ESDPT, depending on host molecules and proton‐transfer models, have been reviewed where the plausible resolution is deduced. Particular attention has been given to the excited‐state proton transfer dynamics in pure water, aiming at its future perspective in biological applications. Finally, the differentiation in mechanism between solvent diffusive reorganization and solvent relaxation to affect the host/guest ESPT dynamics is made and discussed in de tail.     

One‐Pot Mechanosynthesis with Three Levels of Molecular Self‐Assembly: Coordination Bonds,Hydrogen Bonds and Host–Guest Inclusion

Liquid‐assisted grinding (LAG) was used to combine three levels of molecular self‐assembly into a one‐pot mechanochemical approach for the construction of metal–organic materials. The approach was applied for the construction of three adducts of cobalt(II) dibenzoylmethanate with isonicotinamide, nicotinamide and imidazole, to screen for their inclusion compounds. The one‐pot process consists of: i) The coordination‐driven binding of addends to the equatorially‐protected metal ion, resulting in “wheel‐and‐axle”‐shaped complexes; ii) self‐assembly of resulting complexes by way of hydrogen‐bonded synthons to form metal–organic inclusion hosts; iii) in situ inclusion of the grinding liquid in the resulting host. This approach provided quantitatively and within 20 min the known inclusion compounds of the bis(isonicotinamide) adduct in a single synthetic step. Changing the liquid phase in LAG was used to explore the inclusion behaviour of new wheel‐and‐axle adducts with nicotinamide and imidazole, revealing several inclusion compounds, as well as two polymorphs, of the bis(nicotinamide) host. Preliminary results suggest that one‐pot LAG is superior to solution synthesis in screening for metal–organic inclusion compounds. The difference between the methods is rationalised in terms of reactant solubility and solvent competition. In contrast to the nicotinamide adduct, the bis(imidazole) adduct did not form inclusion compounds. The difference in the inclusion properties of the two adducts is rationalised by structural information gathered by single crystal and powder X‐ray diffraction.     

Excited‐State Proton Transfer and Conformational Relaxation of 2‐(4′‐Pyridyl)benzimidazole in Nafion Films

The effect of annealing on the acidity and water uptake of Nafion films has been studied by using the acidity sensing fluorophore 2‐(4′‐pyridyl)benzimidazole (4PBI). The difference in acidity and the microenvironment of the fluorophore in annealed and nonannealed films is brought out in this study. The annealed film is found to have less water uptake than nonannealed films. The amount of water uptake increases upon acid treatment of the films, as all the steady‐state and time‐resolved behaviour of the molecule in nonannealed films is restored. These observations are rationalised by the formation of anhydrides upon annealing and their hydrolysis to sulfonic acid groups upon acid treatment. Interestingly, the acidity of annealed films is found to be even less than that of Na⁺exchanged films, indicating that annealing removes more protons from the Nafion films than cation exchange can.     

Supramolecular Proteoglycan Aggregate Mimics: Cyclodextrin‐Assisted Biodegradable Polymer Assemblies for Electrostatic‐Driven Drug Delivery

Self‐assembled, noncovalent polymeric biodegradable materials mimicking proteoglycan aggregates were synthesized from inclusion complexes of cationic surfactants with γ‐cyclodextrin and the natural anionic polymer hyaluronan. The amorphous structure of this ternary system was proven by X‐ray diffraction and thermal analysis. Light‐scattering measurements showed that there was a competition between hyaluronic acid and the surfactant for the cyclodextrin cavity. These self‐assembled supramolecular matrices were loaded with both hydrophilic and lipophilic drug substances for dissolution studies. The release of the entrapped drugs was found to be controlled by cations in the surrounding media and by biodegradation. Slow drug release in an ion‐free medium became faster in physiological salt solution in which the macroscopic polymer matrix was disassembled. In contrast, the enzymatic degradation of hyaluronan was hindered in the polymeric matrix. The supramolecular systems consisting of γ‐cyclodextrin as a macrocyclic host, a cationic surfactant guest, and hyaluronic acid as the anionic polymer electrostatically cross‐linked by the inclusion complex of the first two was found to be a novel drug‐delivery system for the controlled release of traditional drugs such as curcumin and ketotifen and proteins such as bovine serum albumin.     

Nano‐Supramolecular Assemblies Constructed from Water‐Soluble Bis(calix[5]arenes) with Porphyrins and Their Photoinduced Electron Transfer Properties

What a PET! Two well‐defined nanoarchitectures with 2D netlike and 1D linear topological structures are constructed by bis(p‐sulfonatocalix[5]arenes) and cationic porphyrins, respectively, in which an unambiguous PET process is observed. As a result, the supramolecular aggregates possess, in principle, benign photoelectric properties with the transport of an electron between the building blocks in the nanoscale region.

     

Dual Photo‐ and pH‐Responsive Supramolecular Nanocarriers Based on Water‐Soluble Pillar[6]arene and Different Azobenzene Derivatives for Intracellular Anticancer Drug Delivery

Two novel types of supramolecular nanocarriers fabricated by the amphiphilic host–guest inclusion complex formed from water‐soluble pillar[6]arene ( WP6 ) and azobenzene derivatives G1 or G2 have been developed, in which G1 is structurally similar to G2 but has an extra phenoxy group in its hydrophobic region. Supramolecular micelles can be initially formed by WP6 with G1 , which gradually transform into layered structures with liquid‐crystalline properties, whereas stable supramolecular vesicles are obtained from WP6 and G2 , which exhibit dual photo‐ and pH‐responsiveness. Notably, the resulting WP6 ? G2 vesicles can efficiently encapsulate anticancer drug mitoxantrone (MTZ) to achieve MTZ‐loaded vesicles, which maintain good stability in a simulated normal physiological environment, whereas in an acid environment similar to that of tumor cells or with external UV irradiation, the encapsulated drug is promptly released. More importantly, cytotoxicity assay indicates that such vesicles have good biocompatibility and the MTZ‐loaded vesicles exhibit comparable anticancer activity to free MTZ, especially with additional UV stimulus, whereas its cytotoxicity for normal cells was remarkably reduced. Flow cytometric analysis further confirms that the cancer cell death caused by MTZ‐loaded vesicles is associated with apoptosis. Therefore, the dual pH‐ and UV‐responsive supramolecular vesicles are a potential platform for controlled release and targeted anticancer drug delivery.     

A Host–Guest Supramolecular Complex with Photoregulated Delivery of Nitric Oxide and Fluorescence Imaging Capacity in Cancer Cells

Herein we report the design, preparation, and properties of a supramolecular system based on a tailored nitric oxide (NO) photodonor and a rhodamine‐labeled β‐cyclodextrin conjugate. The combination of spectroscopic and photochemical experiments shows the absence of significant interchromophoric interactions between the host and the guest in the excited states. As a result, the complex is able to release NO under the exclusive control of visible light, as unambiguously demonstrated by direct detection of this transient species through an amperometric technique, and exhibits the typical red fluorescence of the rhodamine appendage. The supramolecular complex effectively internalizes in HeLa cancer cells as proven by fluorescence microscopy, shows a satisfactory biocompatibility in the dark, and induces about 50 % of cell mortality upon irradiation with visible light. The convergence of all these properties in one single complex makes the present host–guest ensemble an appealing candidate for further delevopment of photoactivatable nanoscaled systems addressed to photostimulated NO‐based therapy.     

Gelation‐Induced Enhanced Fluorescence Emission from Organogels of Salicylanilide‐Containing Compounds Exhibiting Excited‐State Intramolecular Proton Transfer: Synthesis and Self‐Assembly

Self‐assembly structure, stability, hydrogen‐bonding interaction, and optical properties of a new class of low molecular weight organogelators (LMOGs) formed by salicylanilides 3 and 4 have been investigated by field‐emission scanning electron microscopy (FESEM), X‐ray diffraction (XRD), UV/Vis absorption and photoluminescence, as well as theoretical studies by DFT and semiempirical calculations with CI (AM1/PECI=8) methods. It was found that salicylanilides form gels in nonpolar solvents due to π‐stacking interaction complemented by the presence of both inter‐ and intramolecular hydrogen bonding. The supramolecular arrangement in these organogels predicted by XRD shows lamellar and hexagonal columnar structures for gelators 3 and 4 , respectively. Of particular interest is the observation of significant fluorescence enhancement accompanying gelation, which was ascribed to the formation of J‐aggregates and inhibition of intramolecular rotation in the gel state.     

Enhanced In Vitro Antitumor Efficacy and Strong Anti‐Cell‐Migration Activity of a Hydroxycamptothecin‐Encapsulated Magnetic Nanovehicle

A 10‐hydroxycamptothecin‐encapsulated magnetic nanovehicle (HEMN) was fabricated by coencapsulating Fe₃O₄ nanoparticles and 10‐hydroxycamptothecin (HCPT) into a micelle core self‐assembled from the amphiphilic copolymer methoxy‐poly(ethylene glycol)–poly(d,l ‐lactide‐co‐glycolide) through a facile dialysis method. A satisfactory drug‐loading content of (9.03±0.67) % and a relatively high encapsulation efficiency of (53.52±6.46) % were achieved. In vitro drug release was performed by membrane dialysis and a pH‐dependent release behavior was observed. In comparison with free HCPT dissolved in dimethylsulfoxide, HEMNs showed a greatly improved in vitro antitumor efficacy against three different human cancer cell lines—HeLa, A549, and HepG2—and lower IC₅₀ values were measured. The mechanism of cell death was investigated, and it was clearly demonstrated that the apoptosis process was triggered. An in vitro wound‐healing assay and a transwell assay indicated that HEMNs exerted much stronger activity in inhibiting HeLa cell migration. The cellular uptake of HEMNs in a desired area can be significantly enhanced by an external magnetic field. These results demonstrate HCPT‐encapsulated magnetic nanovehicles might have important potential in clinical applications for inhibiting tumor metastasis and for targeted drug delivery.