In this report, “fluorescent flippers” are introduced to create planarizable push–pull probes with the mechanosensitivity and fluorescence lifetime needed for practical use in biology. Twisted push–pull scaffolds with large and bright dithienothiophenes and their S,S-dioxides as the first “fluorescent flippers” are shown to report on the lateral organization of lipid bilayers with quantum yields above 80% and lifetimes above 4 ns. Their planarization in liquid-ordered (Lo) and solid-ordered (So) membranes results in red shifts in excitation of up to +80 nm that can be transcribed into red shifts in emission of up to +140 nm by Förster resonance energy transfer (FRET). These unique properties are compatible with multidomain imaging in giant unilamellar vesicles (GUVs) and cells by confocal laser scanning or fluorescence lifetime imaging microscopy. Controls indicate that strong push–pull macrodipoles are important, operational probes do not relocate in response to lateral membrane reorganization, and two flippers are indeed needed to “really swim,” i.e., achieve high mechanosensitivity.

In this report, we introduce synthetic anion transporters that operate with chalcogen bonds. Electron-deficient dithieno[3,2-b;2',3'-d]thiophenes (DTTs) are identified as ideal to bind anions in the focal point of the σ holes on the cofacial endocyclic sulfur atoms. Anion binding in solution and anion transport across lipid bilayers are found to increase with the depth of the σ holes of the DTT anionophores. These results introduce DTTs and related architectures as a privileged motif to engineer chalcogen bonds into functional systems, complementary in scope to classics such as 2,2'-bipyrroles or 2,2'-bipyridines that operate with hydrogen bonds and lone pairs, respectively.

Attached to electron-rich aromatic systems, sulfides are very weak acceptors; however, attached to electron-poor aromatics, they turn into quite strong donors. Here, we show that this underappreciated dual nature of sulfides deserves full consideration for the design of functional systems. Tested with newly designed and synthesized planarizable push-pull mechanophores, sulfide acceptors in the twisted ground state are shown to prevent oxidative degradation and promote blue-shifting deplanarization. Turned on in the planar excited state, sulfide donors promote red-shifting polarization. Impressive Stokes shifts are the result. Demonstrating the usefulness of time-resolved broadband emission spectra to address significant questions, direct experimental evidence for the ultrafast (3.5 ps), polarity-independent and viscosity-dependent planarization from the twisted Franck-Condon S1 state to the relaxed S1 state could be secured.

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New p-type, n-type, and ambipolar molecules were synthesized from commercially available 4,10-dibromoanthanthrone dye. Substitution at the 4,10- and 6,12-positions with different electron-rich and electron-poor units allowed the modulation of the optoelectronic properties of the molecules. A bis(dicyanovinylene)-functionalized compound was also prepared with a reduction potential as low as -50 mV versus Ag(+) with a crystalline two-dimensional lamellar packing arrangement. These characteristics are important prerequisites for air-stable n-type organic field-effect transistor applications.

Planarizable and polarizable dithieno[3,2‐b;2′,3′‐d]thiophene (DTT) dimers have been introduced recently as fluorescent probes that report on membrane fluidity with red shifts in excitation, i.e. planarization in the ground state. In this study, we elaborate on the hypothesis that twisted push‐pull probes could perform best in the presence of one unorthodox substituent that acts as a weak acceptor with electron‐rich and as a strong donor with electron‐poor aromatics. According to Hammett constants, we thought that sulfides could provide access to such a conceptually innovative donor‐acceptor switch. To elaborate on this hypothesis, we here describe the design, synthesis and evaluation of a comprehensive series of twisted push‐pull probes with turn‐on sulfide donors. Their planarization is explored in lipid bilayer membranes of different thickness and fluidity from liquid‐disordered to liquid‐ordered and solid‐ordered phases. Results from membranes are compared to the planarization of turn‐on mechanophores in crystals, proteins, and cyclodextrin macrocycles of varied diameter.

Thisa rticle assembles pertinenti nsights behind the concept of planarizable push-pullp robes. As ar esponse to the planarization of their polarized ground state, ar ed shift of their excitation maximum is expected to report on either the disorder, the tension,o rt he potential of biomembranes. The combination of chromophore planarization and polarization contributes to various, usually more complex processes in nature. Examples include the color change of crabs or lobsters during cooking or the chemistry of vision, particularly color vision. The summary of lessonsf rom nature is followed by an overview of mechanosensitive organic materials. Although often twisted and sometimes also polarized, their change of color under pressure usually originates from changes in their crystal packing. Intriguing exceptionsi nclude the planarization of severale legantly twisted phenylethynyl oligomers and polymers. Also mechanosensitive probes in plastics usuallyr espondt os tretching by disassembly.T rue ground-state planarization in response to molecular recognition is beste xemplified with the binding of thoughtfully twisted cationic polythiophenes to single-and double-stranded oligonucleotides. Molecular rotors, en vogue as viscosity sensors in cells, operate by deplanarization of the first excited state. Pertinent recent examples are described, focusing on l-ratiometry and intracellular targeting. Complementary to planarizationo ft he ground state with twisted push-pull probes, molecular rotors report on environmental changes with quenching or shifts in emission rather than absorption. The labeling of mechanosensitive channels is discussed as ab ioengineering approach to bypasst he challenge to create molecular mechanosensitivity andu se biological systems insteadt o sense membrane tension. With planarizable push-pull probes, this challengei sm et not with twistome screening, but with "fluorescent flippers," an ew concept to insert large and bright monomers into oligomeric probes to really feel the environmenta nd also shine when twisted out of conjugation.

A series of long-tail alkyl ethanolamine analogs containing amide-, urea-, and thiourea moieties was synthesized and the behavior of the corresponding monolayers was assessed on the Langmuir−Pockels trough combined with grazing incidence X-ray diffraction experiments and complemented by computer simulations. All compounds form stable monolayers at the soft air/water interface. The phase behavior is dominated by strong intermolecular headgroup hydrogen bond networks. While the amide analog forms well-defined monolayer structures, the stronger hydrogen bonds in the urea analogs lead to the formation of small three-dimensional crystallites already during spreading due to concentration fluctuations. The hydrogen bonds in the thiourea case form a two-dimensional network, which ruptures temporarily during compression and is recovered in a self-healing process, while in the urea clusters the hydrogen bonds form a more planar framework with gliding planes keeping the structure intact during compression. Because the thiourea analogs are able to self-heal after rupture, such compounds could have interesting properties as tight, ordered, and self-healing monolayers.