Hydrophobic tag-based protein degradation: Development, opportunity and challenge

Targeted protein degradation (TPD) has emerged as a promising approach for drug development, particularly for undruggable targets. TPD technology has also been instrumental in overcoming drug resistance. While some TPD molecules utilizing proteolysis-targeting chimera (PROTACs) or molecular glue strategies have been approved or evaluated in clinical trials, hydrophobic tag-based protein degradation (HyT-PD) has also gained significant attention as a tool for medicinal chemists. The increasing number of reported HyT-PD molecules possessing high efficiency in degrading protein and good pharmacokinetic (PK) properties, has further fueled interest in this approach. This review aims to present the design rationale, hydrophobic tags in use, and diverse mechanisms of action of HyT-PD. Additionally, the advantages and disadvantages of HyT-PD in protein degradation are discussed. This review may help inspire the development of more HyT-PDs with superior drug-like properties for clinical evaluation.

Compounds Derived from 9,9-Dialkylfluorenes: Syntheses, Crystal Structures and Initial Binding Studies (Part II)

New representatives of 2,4,7-trisubstituted 9,9-dialkyl-9H-fluorenes were prepared and used for crystallographic investigations as well as initial binding studies towards metal ions and carbohydrates. The binding studies, which included ¹ H NMR spectroscopic titrations and fluorescence measurements, demonstrated the ability of the tested fluorene-based compounds to act as complexing agents for ionic and neutral substrates. Depending on the nature of the subunits of the fluorene derivatives, "turn on" or "turn off" fluorescent chemosensors can be developed. Compounds composed of 4,6-dimethylpyridin-2-yl-aminomethyl moieties have the potential to be used as sensitive "turn-on" chemosensors for some metal ions.

Analytical Characterization of the Intercalation of Neutral Molecules into Saponite

Organo-modified layered materials characterization poses challenges due to their complexity and how other aspects such as contamination, preparation methods and degree of intercalation influence the properties of these materials. Consequently, a deep understanding of their interlayer organization is of utmost importance to optimize their applications. These materials can in fact improve the stability of photoactive molecules through intercalation, avoiding the quenching of their emission at the solid state, to facilitate their use in sensors or other devices. Two synthetic methods for the preparation of saponites with a cationic surfactant (CTABr) and a neutral chromophore (Fluorene) were tested and the obtained products were initially characterized with several complementary techniques (XRPD, SEM, TGA, IR, UV-Vis, Fluorescence and Raman spectroscopy), but a clear understanding of the organization of the guest molecules in the material could not be obtained by these techniques alone. This information was obtained only by thermogravimetry coupled with gas chromatography and mass spectroscopy (TGA-GC-MS) which allowed identifying the species present in the sample and the kind of interaction with the host by distinguishing between intercalated and adsorbed on the surface.

Covalent nanosynthesis of fluorene-based macrocycles and organic nanogrids

Gridization is an alternative way to create macromolecules of various sizes in addition to linear and dendritic polymerization as well as cyclization. Organic nanogrids are an expanding family of macrocycle-like closed structures at the nanoscale, but with a series of well-defined extension edges and vertices. Cyclic nanogrids can be used as nanoscale building blocks for the fabrication of not only rotaxanes, catenanes, knots, 3D cages, but also nanopolymers, covalent organic frameworks (COFs), metal-organic frameworks (MOFs), and complex molecular cross-scale architectures. In this review, the history of fluorene-based macrocycles has first been explored, followed by the development of the synthetic methodologies; in particular, fluorene-based nanogrids are highlighted owing to their features and applications. Typically, fluorenes are fused arenes with a hybrid entity between tetrahedral Csp³ and Csp². Four ingenious connection modes of fluorene-based macrocycles, including 2,7-, 3,6-, 9,9-, and 2,9-linkages, fully demonstrate the geometric possibilities of the macrocycles and nanogrids. Such fluorene-based nanogrids will give birth to organic intelligence.

Synthesis, Photophysics, and Langmuir Films of Polyfluorene/Permodified Cyclodextrin Polyrotaxanes

In the present study, we investigated the effect of permodified 2,3,6-tri-O-trimethylsilyl β- and γ-cyclodextrin (TMS·β-CD, TMS·γ-CD) encapsulation on the optical, electrochemical, morphological, and supramolecular arrangements of a poly[2,7'-(9,9-dioctylfluorene-alt-2',7-fluorene)] PF copolymer. For this purpose, the photophysical properties and Langmuir monolayer formation of PF·TMS·β-CD and PF·TMS·γ-CD polyrotaxanes were investigated and compared with those of the reference PF. Surface pressure-area isotherms and Brewster angle microscopy studies indicated the capability of both polyrotaxanes to organize into larger and homogeneous 2D supramolecular assemblies at the air-water interface. The obtained results suggest that the presence of the surrounding TMS·β-CD and TMS·γ-CD macrocycles on the PF backbones leads to changes in the conformation and hydrophobicity of the film surfaces. Our investigation offers a method to assess the impact of TMS-CD encapsulation on the control of 2D monolayer formation, with particular attention on the generation of stable PF monolayers for organic electronic devices.

Poly(Pyridinium Salt)s Containing 2,7-Diamino-9,9'-Dioctylfluorene Moieties with Various Organic Counterions Exhibiting Both Lyotropic Liquid-Crystalline and Light-Emitting Properties

A series of poly(pyridinium salt)s-fluorene main-chain ionic polymers with various organic counterions were synthesized by using ring-transmutation polymerization and metathesis reactions. Their chemical structures were characterized by Fourier Transform Infrared (FTIR), proton (¹H), and fluorine 19 (¹⁹F) nuclear magnetic resonance (NMR) spectrometers. These polymers showed a number-average molecular weight (M_ns) between 96.5 and 107.8 kg/mol and polydispersity index (PDI) in the range of 1.12–1.88. They exhibited fully-grown lyotropic phases in polar protic and aprotic solvents at different critical concentrations. Small-angle X-ray scattering for one polymer example indicates lyotropic structure formation for 60–80% solvent fraction. A lyotropic smectic phase contains 10 nm polymer platelets connected by tie molecules. The structure also incorporates a square packing motif within platelets. Thermal properties of polymers were affected by the size of counterions as determined by differential scanning calorimetry and thermogravimetric analysis measurements. Their ultraviolet-visible (UV-Vis) absorption spectra in different organic solvents were essentially identical, indicating that the closely spaced π-π* transitions occurred in their conjugated polymer structures. In contrast, the emission spectra of polymers exhibited a positive solvatochromism on changing the polarity of solvents. They emitted green lights in both polar and nonpolar organic solvents and showed blue light in the film-states, but their λ_em peaks were dependent on the size of the counterions. They formed aggregates in polar aprotic and protic solvents with the addition of water (v/v, 0–90%), and their λ_em peaks were blue shifted.

Syntheses of Acyclic and Macrocyclic Compounds Derived from 9,9-Diethylfluorene (Part I)

A series of new 9,9-diethylfluorenes consisting of three side-arms each bearing a heterocyclic, bis(carboxymethyl)amino, bis(carbamoylmethyl)amino, bis(ethoxycarbonylmethyl)amino or an amino group were prepared on the basis of 2,4,7-tris(bromomethyl)-9,9-diethylfluorene. Imidazolyl, benzimidazolyl, pyrazolyl, pyrrolyl, 1,3-dioxoisoindolyl and pyridinium groups were taken into account as heterocyclic units, attached to the aromatic skeleton via -CH2-, -CH2NHCH2- or -CH2N=CH- linkers. In addition to the seventeen 2,4,7-trisubstituted 9,9-diethylfluorenes, two macrocyclic compounds were prepared on the basis of 2,7-bis(aminomethyl)-9,9-diethylfluorene. The excellent yield of the macrocyclization reaction is worth a special mention. Both the acyclic and the macrocyclic fluorene-based compounds have, among other things, the potential to act as artificial receptors for different substrates in analogy to the known receptors consisting of a benzene or biphenyl core.

Intermolecular Three-Component Synthesis of Fluorene Derivatives by a Rhodium-Catalyzed Stitching Reaction/Remote Nucleophilic Substitution Sequence

A three-component synthesis of multisubstituted fluorene derivatives has been developed by devising a rhodium-catalyzed stitching reaction/remote nucleophilic substitution sequence. A variety of nucleophiles can be installed in the second step including both heteroatom and carbon nucleophiles. An efficient synthesis of 5H-benzo[a]fluoren-5-ones has also been realized using N-(2-alkynyl)benzoylpyrrole as the reaction partner through a new reaction pathway.

Fluorene benzothiadiazole co-oligomer based aqueous self-assembled nanoparticles

Self-assembled π-conjugated nanoparticles with tunable optical characteristics are appealing for sensing and imaging applications due to their intrinsic fluorescence, supramolecular organization and dynamics.

A denitrogenative palladium-catalyzed cascade for regioselective synthesis of fluorenes

We herein report a denitrogenative palladium-catalyzed cascade for the modular and regioselective synthesis of polysubstituted fluorenes. Hydrazone facilitates the Pd(ii) to Pd(iv) oxidative addition in a Catellani pathway and is also the methylene synthon in the proposed reaction. Aryl iodides and 2-bromoarylaldehyde hydrazones undergo a norbornene-controlled tandem reaction sequence to give a broad scope of fluorenes in the presence of a palladium catalyst. The method described is scalable and adaptable to a three-component reaction with in situ generation of the hydrazone group. Preliminary mechanistic investigations have been conducted.

Ultrafast Broadband Transient Absorption and Circular Dichroism Reveal Relaxation of a Chiral Copolymer

We present a study of the photoinduced dynamics of the chiral polyfluorene-phenylene copolymer PFPh in THF and in cholesteric thin films. After photoexcitation at 370 nm in THF, ultraviolet-visible-near-infrared (UV-vis-NIR) transient absorption spectra show fast subpicosecond to picosecond intrachain migration of singlet excitons, solvation dynamics, and an exciton lifetime of 410 ps. The PFPh thin film features also interchain singlet exciton migration and exhibits shorter (2.1 and 240 ps) and longer lifetime components (2800 ps, interchain recombination). Furthermore, a setup for ultrafast UV-vis broadband transient circular dichroism (TrCD) spectroscopy has been developed. Fast supramolecular relaxation processes are observed, which are linked to changes in the anisotropic polarizability and pitch length of the cholesteric film. Such combined ultrafast transient CD and absorption experiments hold promise to reveal not only details of relaxation processes in supramolecular arrangements but also structural rearrangements of chiral molecular systems featuring CD signals in the UV-vis region.

Improving the Electropolymerization Properties of Fluorene-Bridged Dicarbazole Monomers through Polyfluoroalkyl Side Chains

The facile functionalization of the fluorene scaffold at the 2,7-positions was utilized to provide access to two soluble carbazole-π-carbazole derivatives CFC-H1 and CFC-F1 featuring fully hydrogenated and polyfluorinated alkyl chains at the 9-position of the fluorene π-bridging unit, respectively. The optical and electrochemical properties of the new dicarbazoles were investigated. Their electrochemical polymerization over Pt and indium tin oxide electrodes allowed the generation of electroactive polymeric films, whose physicochemical characteristics were strongly dependent on the kind of alkyl chain present on the fluorene bridge. In particular, the electropolymerization of the polyfluorinated monomer allowed the fabrication of thin films with good electrical conductivity, reversible electrochemical processes, good electrochromic properties, and enhanced water repellency with respect to its nonfluorinated analogue.

The design, synthesis and order-enhanced emission of luminescent liquid crystalline polymers based on a “jacketing” effect

Two resultant luminescent liquid crystalline polymers (LLCPs) show higher emissive efficiency and the polymers form more ordered structures after thermal annealing, which means that an improvement in ordering can enhance emission.

Conjugated Nanowire Sensors via High-Energy Single-Particle-Induced Linear Polymerization of 9,9'-Spirobi[9 H-fluorene] Derivatives

Nanostructures composed of conjugated polymers or π-conjugated molecules provide sensing platforms with large specific surface areas. One of the feasible approaches to accessing such nanostructured miniaturized sensors with ultrahigh sensitivity is to develop a network of organic nanowires with optical/electronic properties that can measure signals upon interacting with the analytes at their surfaces. In this work, organic nanowires with controlled number density and uniform length were fabricated by one-dimensional solid-state polymerization of 9,9'-spirobi[9 H-fluorene] (SBF) derivatives triggered by high-energy single particles. SBF was chosen as a conjugated molecular motif with the interplay of high density of π-electrons, high solubility, and uniform solid-state structures, allowing us to fabricate sensing platforms via solution processing. The as-deposited energy density in linear polymerization nanospace was theoretically analyzed by a collision model, interpreting nanowire sizes at subnanometer levels. The substitution of bromine atoms was confirmed to be effective not only for the higher collision probability of the incident particles but also for the remarkable increase in radiolytic neutral radical yield via C-Br cleavages or electron-dissociative attachments onto the bromine atoms. The fluorescence spectra of SBF-based nanowires were different from those of SBF derivatives due to extended bond formation as a result of polymerization reactions. Fluorescence was quenched by the addition of nitrobenzene, indicating the potential use of our nanowires for fluorometric sensing applications. Microwave-based conductivity measurements revealed that the SBF-based nanowires exhibited charge carrier transport property upon photoexcitation, and that the conductivity was changed upon treatment with nitrobenzene vapors. The presented strategy of bromination of aromatic rings for efficient fabrication of controlled nanowire networks with favorable fluorescent and charge transport properties of nanowires advances the development of nanostructured sensing systems.

Synthesis of spiro quasi[1]catenanes and quasi[1]rotaxanes via a templated backfolding strategy

Due to their well-defined three-dimensional geometry, spiro compounds are widely utilized in drug research. From the central tetrahedral carbon atom, besides the regular structure, an inverted spiro connectivity may be envisioned. Here we disclose the synthesis of this molecule class that we have coined quasi[1]catenanes. Next to their fascinating and aesthetic shape, the higher compactness as compared to regular spiro bicycles is noteworthy. To enable synthetic access to compact entangled multimacrocyclic molecules, we have developed a new strategy. The key element is a template, which is covalently connected to the linear precursors, and spatially directs the sterically congested backfolding macrocyclizations that are required to give quasi[1]catenanes. Similarly, quasi[1]rotaxanes are made.

Spiro compounds contain two or more rings linked together through one common atom. Here the authors provide a method to backfold both rings, producing spiro quasi[1]catenanes, via a strategy of temporarily linking the linear intermediates with covalent bonds.

Development of emissive aminopentaazaphenalene derivatives employing a design strategy for obtaining luminescent conjugated molecules by modulating the symmetry of molecular orbitals with substituent effects

This communication describes the transformation of a non-emissive heterocycle into a luminophore via modulation of molecular orbitals by the introduction of dialkylamine group.

Experimental and theoretical investigations on spectroscopic properties of the imidazole-fused phenanthroline and its derivatives

Two phenanthroline derivatives, 1H-imidazo[4,5-f][1,10]phenanthroline (imPhen) and 2-(9H-fluoren-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline (Flu-imPhen), have been synthesized and characterized and the corresponding absorption and emission spectroscopic properties have been studied in CH2Cl2 solution. The imPhen exhibits the main two absorption bands at 282 nm and 229 nm and these bands are assigned as the typical π→π*(Phen) state. In addition, the weak absorption bands at 313 nm associated with a shoulder near 302 nm were assigned to the π→π*(Phen) state with partial charge transfer (CT) character. A similar absorption spectra are observed in the case of the Flu-imPhen in the region of 200-300 nm, while the region of 300-400 nm of the spectra are dominated by the characteristic π→π* transition of the fluorene moiety. imPhen shows the typical ligand-centered (1)π→π* emission, while Flu-imPhen emits from the mixed (1)π→π*/CT states. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) were employed to rationalize the photophysical properties of these ligands studied. The theoretical data confirm the assignment of the experimental absorption spectra and the nature of the emitting states.

Supramolecular block copolymer photovoltaics through ureido-pyrimidinone hydrogen bonding interactions

Hydrogen bonding interactions are incorporated into a model polymer-blend OPV system through self-associative endgroups. Supramolecular interactions are shown to increase the resistance for both charge recombination and bulk charge transport.

Transition-metal-free intramolecular carbene aromatic substitution/Büchner reaction: synthesis of fluorenes and [6,5,7]benzo-fused rings

Intramolecular aromatic substitution and Büchner reaction have been established as powerful methods for the construction of polycyclic compounds. These reactions are traditionally catalyzed by Rh(II) catalysts with α-diazocarbonyl compounds as the substrates. Herein a transition-metal-free intramolecular aromatic substitution/Büchner reaction is presented. These reactions use readily available N-tosylhydrazones as the diazo compound precursors and show wide substrate scope.

Palladium-catalysed direct cross-coupling of organolithium reagents with aryl and vinyl triflates

A palladium-catalysed cross-coupling of organolithium reagents with aryl and vinyl triflates is presented. The reaction proceeds at 50 or 70 °C with short reaction times, and the corresponding products are obtained with moderate to high yields, with a variety of alkyl and (hetero)aryl lithium reagents.

Supramolecular luminescence from oligofluorenol-based supramolecular polymer semiconductors

Supramolecular luminescence stems from non-covalent exciton behaviors of active π-segments in supramolecular entities or aggregates via intermolecular forces. Herein, a π-conjugated oligofluorenol, containing self-complementary double hydrogen bonds, was synthesized using Suzuki coupling as a supramolecular semiconductor. Terfluorenol-based random supramolecular polymers were confirmed via concentration-dependent nuclear magnetic resonance (NMR) and dynamic light scattering (DLS). The photoluminescent spectra of the TFOH-1 solution exhibit a green emission band (g-band) at approximately ~520 nm with reversible features, as confirmed through titration experiments. Supramolecular luminescence of TFOH-1 thin films serves as robust evidence for the aggregates of g-band. Our results suggest that the presence of polyfluorene ketone defects is a sufficient condition, rather than a sufficient-necessary condition for the g-band. Supramolecular electroluminescence will push organic devices into the fields of supramolecular optoelectronics, spintronics, and mechatronics.

Facile doping of anionic narrow-band-gap conjugated polyelectrolytes during dialysis

PCPDTBTSO3 K, an anionic, narrow-band-gap conjugated polyelectrolyte, was found to be doped after dialysis. The proposed doping mechanism involves protonation of the polymer backbone, followed by electron transfer from a neutral chain, to generate radical cations, which are stabilized by the pendant sulfonate anions. Formation of polarons is supported by spectroscopy and electrical-conductivity measurements.