Fluorescent Flipper Probes: Comprehensive Twist Coverage

To image the membrane tension in living cells, planarizable push–pull probes have been introduced. The first operational probe is built around two dithieno[3,2-b:2′,3′-d]thiophenes (DTTs) that are twisted out of co-planarity and polarized with donors and acceptors at either end. In this report, the chemical space available for the twisting of “flipper probes” is assessed comprehensively. The result is, not surprisingly, that every atom matters: Removal of one methyl group in the twist region yields probes that planarize already in solution and are thus less sensitive to membrane tension. Addition of one or more carbons in the same region hinders non-interfering probe alignment along lipid tails and thus partitioning into lipid bilayer membranes as well as mechanosensitivity. However, substitution of one methyl by an isosteric trifluoromethyl group in the twist region, achieved by quite substantial multistep organic synthesis, yields excitation maxima that shift over +100 nm to the red in response to increasing order of the surrounding membrane. This record redshift comes with record changes in fluorescence intensity and lifetime, high push–pull transition dipoles and higher rotational barriers. Supported by distinct dependence on viscosity and twist of the push–pull probes, kinetic competition between dark, fully twisted and bright, fully planarized relaxed excited states emerges as unifying origin of fluorescence quantum yields.     

Twisted Push‐Pull Probes with Turn‐On Sulfide Donors

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.     

A Chalcogen‐Bonding Cascade Switch for Planarizable Push–Pull Probes

Planarizable push–pull probes have been introduced to demonstrate physical forces in biology. However, the donors and acceptors needed to polarize mechanically planarized probes are incompatible with their twisted resting state. The objective of this study was to overcome this “flipper dilemma” with chalcogen‐bonding cascade switches that turn on donors and acceptors only in response to mechanical planarization of the probe. This concept is explored by molecular dynamics simulations as well as chemical double‐mutant cycle analysis. Cascade switched flipper probes turn out to excel with chemical stability, red shifts adding up to high significance, and focused mechanosensitivity. Most important, however, is the introduction of a new, general and fundamental concept that operates with non‐trivial supramolecular chemistry, solves an important practical problem and opens a wide chemical space.     

Mechanosensitive Oligodithienothiophenes: Transmembrane Anion Transport Along Chalcogen‐Bonding Cascades

The design, synthesis, and evaluation of multifunctional dithieno[3,2‐b;2′,3′‐d]thiophene (DTT) trimers is described. Twisted push‐push‐pull or donor‐donor‐acceptor (DDA) trimers composed of one DTT acceptor and two DTT donors show strong mechanochromism in lipid bilayer membranes. Red shifts in excitation rather than emission and fluorescence recovery with increasing membrane order are consistent with planarization of the twisted, extra‐long mechanophores in the ground state. The complementary pull‐pull‐pull or AAA trimers with deep σ holes all along the scaffold are not mechanochromic in membranes but excel with submicromolar anion transport activity. Anion transport along membrane‐spanning strings of chalcogen‐bond donors is unprecedented and completes previous results on transmembrane cascades that operate with equally unorthodox interactions such as halogen bonds and anion‐π interactions.     

Ganglioside‐Selective Mechanosensitive Fluorescent Membrane Probes

The development of fluorescent probes to image forces in cells is an important challenge in chemistry and biology. Planarizable push‐pull probes have been introduced recently for this purpose. To provide most valuable information on forces in complex systems, these mechanosensitive ‘flipper’ probes will have to be localized by molecular recognition of targets of interest. Here we report fluorescent flippers that selectively recognize gangliosides on the surface of lipid bilayer membranes by formation of dynamic covalent boronate esters. The original flipper probes were equipped with 2‐fluorophenyl boronic acids and benzoboroxoles using consecutive triazole and oxime ligation. Evaluation was done in large unilamellar vesicles composed of EYPC/SM/CL/GM 40:40‐x:20:x to obtain mixed membranes with separate liquid‐disordered (L_d) and ganglioside (GM) containing liquid‐ordered (L_o) domains. With increasing GM concentration, fluorescence intensities increased and excitation maximum shifted to the red. Deconvolution of the spectra confirmed that these changes originate from a migration of the flipper probes from L_d to L_o domains upon binding to the gangliosides and thus the planarization in the more ordered environment. Control mechanophores without boronic acids failed to show the same response, and fructose partially inhibited the ganglioside sensitivity. These results demonstrate that it is possible to selectively accumulate mechanosensitive flipper probes in L_o domains and, more generally, that probe localization in complex membranes is possible and matters.     

Charge‐Recombination Fluorescence from Push–Pull Electronic Systems Constructed around Amino‐Substituted Styryl–BODIPY Dyes

A small series of donor–acceptor molecular dyads has been synthesized and fully characterized. In each case, the acceptor is a dicyanovinyl unit and the donor is a boron dipyrromethene (BODIPY) dye equipped with a single styryl arm bearing a terminal amino group. In the absence of the acceptor, the BODIPY‐based dyes are strongly fluorescent in the far‐red region and the relaxed excited‐singlet states possess significant charge‐transfer character. As such, the emission maxima depend on both the solvent polarity and temperature. With the corresponding push–pull molecules, there is a low‐energy charge‐transfer state that can be observed by both absorption and emission spectroscopy. Here, charge‐recombination fluorescence is weak and decays over a few hundred picoseconds or so to recover the ground state. Overall, these results permit evaluation of the factors affecting the probability of charge‐recombination fluorescence in push–pull dyes. The photophysical studies are supported by cyclic voltammetry and DFT calculations.     

Is Our Way of Thinking about Excited States Correct? Time‐Resolved Dispersive IR Study on p‐Nitroaniline

Low‐lying excited electronic states of an important class of molecules known as push–pull chromophores are central to understanding their potential nonlinear optical properties. Here we report that a combination of high‐sensitivity nanosecond time‐resolved dispersive IR spectroscopy and DFT calculations on p‐nitroaniline (PNA), a prototypical push–pull molecule, reveals that PNA in the lowest excited triplet state has a partial quinoid structure. In this structure, the quinoid configuration is restricted to a part of the phenyl ring adjacent to the NO₂ group. The partial quinoid structure of PNA cannot be explained by a commonly used hybrid of a neutral form and a zwitterionic charge‐transfer form. Our findings not only cast doubt on the general applicability of the classical way of looking at excited states, based exclusively on characteristic resonance structures, but also provide deeper insights into excited‐state structure of highly polarizable molecular systems.     

Dipolar Photosystems: Engineering Oriented Push–Pull Components into Double‐ and Triple‐Channel Surface Architectures

Push–pull aromatics are not popular as optoelectronic materials because their supramolecular organization is difficult to control. However, recent progress with synthetic methods has suggested that the directional integration of push–pull components into multicomponent photosystems should become possible. In this study, we report the design, synthesis, and evaluation of double‐ or triple‐channel architectures that contain π stacks with push–pull components in parallel or mixed orientation. Moreover, the parallel push–pull stacks were uniformly oriented with regard to co‐axial stacks, either with inward or outward oriented push–pull dipoles. Hole‐transporting (p) aminoperylenemonoimides (APIs) and aminonaphthalimides (ANIs) are explored for ordered push–pull stacks. For the co‐axial electron‐transporting (n) stacks, naphthalenediimides (NDIs) are used. In double‐channel photosystems, mixed push–pull stacks are overall less active than parallel push–pull stacks. The orientation of the parallel push–pull stacks with regard to the co‐axial NDI stacks has little influence on activity. In triple‐channel photosystems, outward‐directed dipoles in bridging stacks between peripheral p and central n channels show higher activity than inward‐directed dipolar stacks. Higher activities in response to direct irradiation of outward‐directed parallel stacks reveal the occurrence of quite remarkable optical gating.     

Push–Pull Porphyrins via β-Pyrrole Functionalization: Evidence of Excited State Events Leading to High-Potential Charge-Separated States

A new set of free-base and zinc(II)-metallated, β-pyrrole-functionalized unsymmetrical push–pull porphyrins were designed and synthesized via β-mono- and dibrominated tetraphenylporphyrins using Sonogashira cross-coupling reactions. The ability of donors and acceptors on the push–pull porphyrins to produce high-potential charge separated states was investigated. The porphyrins were functionalized at the opposite β,β′-pyrrole positions of porphyrin ring bearing triphenylamine push groups and naphthalimide pull groups. Systematic studies involving optical absorption, steady-state and time-resolved emission revealed existence of intramolecular type interactions both in the ground and excited states. The push–pull nature of the molecular systems was supported by frontier orbitals generated on optimized structures, wherein delocalization of HOMO over the push group and LUMO over the pull group connecting the porphyrin π-system was witnessed. Electrochemical studies were performed to visualize the effect of push and pull groups on the overall redox potentials of the porphyrins. Spectroelectrochemical studies combined with frontier orbitals helped in characterizing the one-electron oxidized and reduced porphyrins. Finally, by performing transient absorption studies in polar benzonitrile, the ability of push–pull porphyrins to produce charge-separated states upon photoexcitation was confirmed and the measured rates were in the range of 10⁹ s⁻¹. The lifetime of the final charge separated state was around 5 ns. This study ascertains the importance of push–pull porphyrins in solar energy conversion and diverse optoelectronic applications, for which high-potential charge-separated states are warranted.     

Mechanism and Nature of the Different Viscosity Sensitivities of Hemicyanine Dyes with Various Heterocycles

A series of hemicyanine derivatives are excellent fluorescent viscosity sensors in live cells and in imaging of living tissues due to their low quantum yields in solution but large fluorescence enhancements in viscous environments. Herein, three carbazole‐based hemicyanine dyes with different heterocycles are studied. They have different background quantum yields, and hence different sensitivities to viscosity detection, large Stokes shifts, and high sensitivity. Better understanding of the structure–property relationships for viscosity sensitivity could benefit the design of improved dyes. Computational studies on these dyes reveal the mechanism of viscosity sensitivity of fluorescent molecular rotors and the nature of the difference in viscosity sensitivity of the three dyes. The results show that the greatly raised HOMO and greatly lowered LUMO in the S₁ state compared with the S₀ state are responsible for the large Stokes shift of the three dyes. The heterocyclic moieties have the primary influence on the LUMO levels of the three hemicyanine dyes. Rotation about the C? C bond adjacent to the carbazole moiety of the three dyes drives the molecule toward a small energy gap between the ground state and the first excited state, which causes mainly nonradiative deactivation. The oscillator strengths in the lowest singlet excited state drop rapidly with increasing rotation between 0 and 95°, which leads to a dark state for these dyes when fully twisted at 95°. We draw a mechanistic picture at the molecular level to illustrate how these dyes work as viscosity‐sensitive fluorescent probes. The activation barriers and energy gaps of C? C bond rotation strongly depend on the choice of heterocycle, which plays a major role in reducing fluorescence quantum yield in the free state and provides high sensitivity to viscosity detection in viscous environments for the carbazole‐based hemicyanine dyes.     

Molecular rotors: synthesis and evaluation as viscosity sensors

It has been shown that compounds containing the p-N,N-dialkylaminobenzylidene cyanoacetate motif can serve as fluorescent non-mechanical viscosity sensors. These compounds, referred to as molecular rotors, belong to a class of fluorescent probes that are known to form twisted intramolecular charge-transfer complexes in the excited state. In this study we present the synthesis and spectroscopic characterization of these compounds as viscosity sensors. The effects of the molecular structure and electronic density of these rotors to the emission wavelength, fluorescence intensity, and viscosity sensitivity are discussed.     

Highly Asymmetrical Porphyrins with Enhanced Push–Pull Character for Dye‐Sensitized Solar Cells

A porphyrin π‐system has been modulated by enhancing the push–pull character with highly asymmetrical substitution for dye‐sensitized solar cells for the first time. Namely, both two diarylamino moieties as a strong electron‐donating group and one carboxyphenylethynyl moiety as a strong electron‐withdrawing, anchoring group were introduced into the meso‐positions of the porphyrin core in a lower symmetrical manner. As a result of the improved light‐harvesting property as well as high electron distribution in the anchoring group of LUMO, a push–pull‐enhanced, porphyrin‐sensitized solar cell exhibited more than 10 % power conversion efficiency, which exceeded that of a representative highly efficient porphyrin (i.e., YD2)‐sensitized solar cell under optimized conditions. The rational molecular design concept based on highly asymmetric, push–pull substitution will open the possibilities of further improving cell performance in organic solar cells.     

The Photoisomerization Pathway(s) of Push–Pull Phenylazoheteroarenes**

Azoheteroarenes are the most recent derivatives targeted to further improve the properties of azo-based photoswitches. Their light-induced mechanism for trans–cis isomerization is assumed to be very similar to that of the parent azobenzene. As such, they inherited the controversy about the dominant isomerization pathway (rotation vs. inversion) depending on the excited state (nπ* vs. ππ*). Although the controversy seems settled in azobenzene, the extent to which the same conclusions apply to the more structurally diverse family of azoheteroarenes is unclear. Here, by means of non-adiabatic molecular dynamics, the photoisomerization mechanism of three prototypical phenyl-azoheteroarenes with increasing push–pull character is unraveled. The evolution of the rotational and inversion conical intersection energies, the preferred pathway, and the associated kinetics upon both nπ* and ππ* excitations can be linked directly with the push–pull substitution effects. Overall, the working conditions of this family of azo-dyes is clarified and a possibility to exploit push–pull substituents to tune their photoisomerization mechanism is identified, with potential impact on their quantum yield.     

Tuning the Photophysical Properties of Push‐Pull Azaheterocyclic Chromophores by Protonation: A Brief Overview of a French‐Spanish‐Czech Project

Conjugated push‐pull molecules that incorporate nitrogen heterocycles as electron‐withdrawing units are interesting materials because of their luminescence properties. These chromophores can be easily and reversibly protonated at the nitrogen atom of the heterocyclic ring and this can cause dramatic color changes. White and multi‐color photoluminescence both in solution and in the solid state can be obtained by an accurate control of the amount of acid. Thus, with a suitable design these compounds have potential applications in the development of colorimetric pH sensors and the fabrication of OLEDs based on only one material. We provide here a brief overview of our collaborative efforts made in this area.     

Anion–π and Cation–π Interactions on the Same Surface

Herein, we address the question whether anion–π and cation–π interactions can take place simultaneously on the same aromatic surface. Covalently positioned carboxylate–guanidinium pairs on the surface of 4‐amino‐1,8‐naphthalimides are used as an example to explore push–pull chromophores as privileged platforms for such “ion pair–π” interactions. In antiparallel orientation with respect to the push–pull dipole, a bathochromic effect is observed. A red shift of 41 nm found in the least polar solvent is in good agreement with the 70 nm expected from theoretical calculations of ground and excited states. Decreasing shifts with solvent polarity, protonation, aggregation, and parallel carboxylate–guanidinium pairs imply that the intramolecular Stark effect from antiparallel ion pair–π interactions exceeds solvatochromic effects by far. Theoretical studies indicate that carboxylate–guanidinium pairs can also interact with the surfaces of π‐acidic naphthalenediimides and π‐basic pyrenes.     

Uranium Metalla‐Allenes with Carbene Imido R2C=UIV=NR′ Units (R=Ph2PNSiMe3; R′=CPh3): Alkali‐Metal‐Mediated Push–Pull Effects with an Amido Auxiliary

We report uranium(IV)‐carbene‐imido‐amide metalla‐allene complexes [U(BIPM^TMS)(NCPh₃)(NHCPh₃)(M)] (BIPM^TMS=C(PPh₂NSiMe₃)₂; M=Li or K) that can be described as R₂C=U=NR′ push–pull metalla‐allene units, as organometallic counterparts of the well‐known push–pull organic allenes. The solid‐state structures reveal that the R₂C=U=NR′ units adopt highly unusual cis‐arrangements, which are also reproduced by gas‐phase theoretical studies conducted without the alkali metals to remove their potential structure‐directing roles. Computational studies confirm the double‐bond nature of the U=NR′ and U=CR₂ interactions, the latter increasingly attenuated by potassium then lithium when compared to the hypothetical alkali‐metal‐free anion. Combined experimental and theoretical data show that the push–pull effect induced by the alkali metal cations and amide auxiliary gives a fundamental and tunable structural influence over the C=U^IV=N units.     

Self‐Assembly of Highly Stable Zirconium(IV) Coordination Cages with Aggregation Induced Emission Molecular Rotors for Live‐Cell Imaging

The self‐assembly of highly stable zirconium(IV)‐based coordination cages with aggregation induced emission (AIE) molecular rotors for in vitro bio‐imaging is reported. The two coordination cages, NUS‐100 and NUS‐101, are assembled from the highly stable trinuclear zirconium vertices and two flexible carboxyl‐decorated tetraphenylethylene (TPE) spacers. Extensive experimental and theoretical results show that the emissive intensity of the coordination cages can be controlled by restricting the dynamics of AIE‐active molecular rotors though multiple external stimuli. Because the two coordination cages have excellent chemical stability in aqueous solutions (pH stability: 2–10) and impressive AIE characteristics contributed by the molecular rotors, they can be employed as novel biological fluorescent probes for in vitro live‐cell imaging.     

Addressing Charge-Transfer and Locally-Excited States in a Twisted Biphenyl Push-Pull Chromophore

We present the synthesis and spectroscopic characterization of a twisted push–pull biphenyl molecule undergoing photoinduced electron transfer. Steady-state and transient absorption spectra suggest, in this rigid molecular structure, a subtle interplay between locally-excited and charge-transfer states, whose equilibrium and dynamics is only driven by solvation. A theoretical model is presented for the solvation dynamics and, with the support of quantum chemical calculations, we demonstrate the existence of two sets of states, having either local or charge-transfer character, that only “communicate” thanks to solvation, which is the sole driving force for the charge-separation process.     

N‐Méthyl succinimide

The molecular structure of 1‐methylpyrrolidine‐2,5‐dione, C₅H₇NO₂, corresponds to the dicarbonyl tautomer with an envelope ring conformation. The packing is stabilized by weak intermolecular hydrogen bonds and presents push–pull nucleophile–electrophile interactions of the carbonyl groups.     

Synthetic Control of the Excited‐State Dynamics and Circularly Polarized Luminescence of Fluorescent “Push–Pull” Tetrathia[9]helicenes

A series of fluorescent “push‐pull” tetrathia[9]helicenes based on quinoxaline (acceptor) fused with tetrathia[9]helicene (donor) derivatives was synthesized for control of the excited‐state dynamics and circularly polarized luminescence (CPL) properties. In this work, introduction of a quinoxaline onto the tetrathia[9]helicene skeleton induced the “push–pull” character, which was enhanced by further introduction of an electron‐releasing Me₂N group or an electron‐withdrawing NC group onto the quinoxaline unit (denoted as Me₂N‐QTTH and NC‐QTTH, respectively). These trends were successfully discussed in terms of by electrochemical measurements and density functional theory (DFT) calculations. As a consequence, significant enhancements in the fluorescence quantum yields (Φ_FL) were achieved. In particular, the maximum Φ_FL of Me₂N‐QTTH was 0.43 in benzene (NC‐QTTH: Φ_FL=0.30), which is more than 20 times larger than that of a pristine tetrathia[9]helicene (denoted as TTH; Φ_FL=0.02). These enhancements were also explained by kinetic discussion of the excited‐state dynamics such as fluorescence and intersystem crossing (ISC) pathways. Such significant enhancements of the Φ_FL values thus enabled us to show the excellent CPL properties. The value of anisotropy factor g_CPL (normalized difference in emission of right‐handed and left‐handed circularly polarized light) was estimated to be 3.0×10^?3 for NC‐QTTH.