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.     

Azavinylidenephosphoranes: A Class of Cyclic Push–Pull Carbenes

The synthesis of a novel family of cyclic push–pull carbenes, namely, azavinylidene phosphoranes, is described. The methodology is based on a formal [3+2] cycloaddition between terminal alkynes and phosphine–imines followed by an oxidation/deprotonation step. Carbenes 6 , obtained by simple deprotonation, exhibit typical transient carbene reactivity like the intramolecular C?H insertion reaction and a pronounced ambiphilic character exemplified by [2+1] cycloaddition with electron‐poor methyl acrylate. Owing to the cyclic structure, carbenes 6 also exhibit an excellent coordination ability toward transition metals. Rh^I complex 10 was obtained in excellent yield and was fully characterized by multinuclear NMR spectroscopy and X‐ray crystallography. The corresponding Rh^I–carbonyl complex was also prepared; this indicates that carbenes 6 belong to the strongest σ‐donating ligands to date. DFT calculations confirmed the high σ‐donation ability of 6 and their classification as push–pull carbenes with a relatively small singlet–triplet energy gap of 23.2–24.3 kcal mol^?1.     

BODIPY‐Bridged Push–Pull Chromophores for Nonlinear Optical Applications

A set of linear and dissymmetric BODIPY‐bridged push–pull dyes are synthesized. The electron‐donating substituents are anisole and dialkylanilino groups. The strongly electron‐accepting moiety, a 1,1,4,4‐tetracyanobuta‐1,3‐diene (TCBD) group, is obtained by insertion of an electron‐rich ethyne into tetracyanoethylene. A nonlinear push–pull system is developed with a donor at the 5‐position of the BODIPY core and the acceptor at the 2‐position. All dyes are fully characterized and their electrochemical, linear and nonlinear optical properties are discussed. The linear optical properties of dialkylamino compounds show strong solvatochromic behavior and undergo drastic changes upon protonation. The strong push–pull systems are non‐fluorescent and the TCBD‐BODIPY dyes show diverse photochemistry and electrochemistry, with several reversible reduction waves for the tetracyanobutadiene moiety. The hyperpolarizability μβ of selected compounds is evaluated using the electric‐field‐induced second‐harmonic generation technique. Two of the TCBD‐BODIPY dyes show particularly high μβ (1.907 μm) values of 2050×10^?48 and 5900×10^?48 esu. In addition, one of these dyes shows a high NLO contrast upon protonation–deprotonation of the donor residue.     

Strong Enhancement of π‐Electron Donor/Acceptor Ability by Complementary DD/AA Hydrogen Bonding

π‐Conjugated organic materials possess a wide range of tunable optoelectronic properties which are dictated by their molecular structure and supramolecular arrangement. While many efforts have been put into tuning the molecular structure to achieve the desired properties, rational supramolecular control remains a challenge. Here, we report a novel series of supramolecular materials formed by the co‐assembly of weak π‐electron donor (indolo[2,3‐a]carbazole) and acceptor (aromatic o‐quinones) molecules via complementary hydrogen bonding. The resulting polarization creates a drastic perturbation of the molecular energy levels, causing strong charge transfer in the weak donor–acceptor pairs. This leads to a significant lowering (up to 1.5 eV) of the band gaps, intense absorption in the near‐IR region, very short π‐stacking distances (≥3.15 Å), and strong ESR signals in the co‐crystals. By varying the strength of the acceptor, the characteristics of the complexes can be tuned between intrinsic, gate‐, or light‐induced semiconductivity with a p‐type or ambipolar transport mechanism.     

Rational Design of Push–Pull Fluorene Dyes: Synthesis and Structure–Photophysics Relationship

Our work surveyed experimental and theoretical investigations to construct highly emissive D –π–A (D=donor, A=acceptor) fluorenes. The synthetic routes were optimised to be concise and gram‐scalable. The molecular design was first rationalised by varying the electron‐withdrawing group from an aldehyde, ketotriazole or succinyl to methylenemalonitrile or benzothiadiazole. The electron‐donating group was next varied from aliphatic or aromatic amines to saturated cyclic amines ranging from aziridine to azepane. Spectroscopic studies correlated with TD‐DFT calculations provided the optimised structures. The selected push–pull dyes exhibited visible absorptions, significant brightness, important solvatofluorochromism, mega‐Stokes shifts (>250 nm) and dramatic shifts in emission to the near‐infrared. The current library includes the comprehensive characterization of 16 prospective dyes for fluorescence applications. Among them, several fluorene derivatives bearing different conjugation anchors were tested for coupling and demonstrated to preserve the photophysical responses once further bound.     

Alphabet‐Inspired Design of (Hetero)Aromatic Push–Pull Chromophores

Push–pull molecules represent a unique and fascinating class of organic π‐conjugated materials. Herein, we provide a summary of their recent extraordinary design inspired by letters of the alphabet, especially focusing on H‐, L‐, T‐, V‐, X‐, and Y‐shaped molecules. Representative structures from each class were presented and their fundamental properties and prospective applications were discussed. In particular, emphasis is given to molecules recently prepared in our laboratory with T‐, X‐, and Y‐shaped arrangements based on indan‐1,3‐dione, benzene, pyridine, pyrazine, imidazole, and triphenylamine. These push–pull molecules turned out to be very efficient charge‐transfer chromophores with tunable properties suitable for second‐order nonlinear optics, two‐photon absorption, reversible pH‐induced and photochromic switching, photocatalysis, and intercalation.

     

From Homoconjugated Push–Pull Chromophores to Donor–Acceptor‐Substituted Spiro Systems by Thermal Rearrangement

Series of homoconjugated push–pull chromophores and donor–acceptor (D–A)‐functionalized spiro compounds were synthesized, in which the electron‐donating strength of the anilino donor groups was systematically varied. The structural and optoelectronic properties of the compounds were investigated by X‐ray analysis, UV/Vis spectroscopy, electrochemistry, and computational analysis. The homoconjugated push–pull chromophores with a central bicyclo[4.2.0]octane scaffold were obtained in high yield by [2+2] cycloaddition of 2,3‐dichloro‐5,6‐dicyano‐p‐benzoquinone (DDQ) to N,N‐dialkylanilino‐ or N,N‐diarylanilino‐substituted activated alkynes. The spirocyclic compounds were formed by thermal rearrangement of the homoconjugated adducts. They also can be prepared in a one‐pot reaction starting from DDQ and anilino‐substituted alkynes. Spiro products with N,N‐diphenylanilino and N,N‐diisopropylanilino groups were isolated in high yields whereas compounds with pyrrolidino, didodecylamino, and dimethylamino substituents gave poor yields, with formation of insoluble side products. It was shown by in situ trapping experiments with TCNE that cycloreversion is possible during the thermal rearrangement, thereby liberating DDQ. In the low‐yielding transformations, DDQ oxidizes the anilino species present, presumably via an intermediate iminium ion pathway. Such a pathway is not available for the N,N‐diphenylanilino derivative and, in the case of the N,N‐diisopropylanilino derivative, would generate a strained iminium ion (A1,3 strain). The mechanism of the thermal rearrangement was investigated by EPR spectroscopy, which provides good evidence for a proposed biradical pathway starting with the homolytic cleavage of the most strained (CN)C?C(CN) bond between the fused four‐ and six‐membered rings in the homoconjugated adducts.     

Functionalization of a Ruthenium–Diacetylide Organometallic Complex as a Next‐Generation Push–Pull Chromophore

The design and preparation of an asymmetric ruthenium–diacetylide organometallic complex was successfully achieved to provide an original donor–π–[M]–π–acceptor architecture, in which [M] corresponds to the [Ru(dppe)₂] (dppe: bisdiphenylphosphinoethane) metal fragment. The charge‐transfer processes occurring upon photoexcitation of the push–pull metal–dialkynyl σ complex were investigated by combining experimental and theoretical data. The novel push–pull complex, appropriately end capped with an anchoring carboxylic acid function, was further adsorbed onto a semiconducting metal oxide porous thin film to serve as a photosensitizer in hybrid solar cells. The resulting photoactive material, when embedded in dye‐sensitized solar cell devices, showed a good spectral response with a broad incident photon‐to‐current conversion efficiency profile and a power conversion efficiency that reached 7.3 %. Thus, this material paves the way to a new generation of organometallic chromophores for photovoltaic applications.