Circularly Polarized Luminescence from Cyclic (Alkyl)(Amino)Carbene Derived Propellers

Crassous J., Fuchter M.J., Freedman D.E., Kotov N.A., Moon J., Beard M.C., Feldmann S.

Solution-processable semiconductors based on small molecules, polymers or halide perovskites combine sustainable manufacturing with exceptional optoelectronic properties that can be chemically tailored to achieve flexible and highly efficient optoelectronic and photonic devices. A new exciting research direction is the study of the influence of chirality on light–matter interactions in these soft materials and its exploitation for the simultaneous control of charge, spin and light. In this Viewpoint, researchers working on different types of chiral semiconductors discuss the most interesting directions in this rapidly expanding field.

Felder S., Delcourt M., Contant D., Rodríguez R., Favereau L., Crassous J., Micouin L., Benedetti E.

Due to their unique three-dimensional framework and intriguing electronic properties, [2.2]paracyclophanes have been employed over the years as building blocks in materials science for the development of organic light-emitting diodes...

Chu W., Zhou T., Bisz E., Dziuk B., Lalancette R., Szostak R., Szostak M.

We report a merger of CAAC ligands with highly-hindered IPr*. The ligands are strongly electron-rich, bulky and flexible. The availability of various IPr* and CAAC templates offers a significant potential to expand the existing arsenal of NHCs.

Kemper M., Reese S., Engelage E., Merten C.

Stabilization of chiral propeller conformations in triaryl compounds is challenging due to generally low racemization barriers. Nonetheless, it was recently found that chiral conformational preferences can be induced to triaryl boranes by incorporating point-chiral alkylether chains to the aryl blades and subsequently locking the structure with ammonia. A four-point interaction, meaning that the cooperative effects of Lewis-adduct formation and three hydrogen bonds, was proposed as stabilizing mechanism. Herein, it was shown that three such strong interactions suffice to introduce a preferential propeller handedness. Although DFT calculations predict no noteworthy preferences for either P- or M-chiral propellers for some of the investigated triarylborane-amine adducts that were prepared with chiral primary amines, vibrational circular dichroism (VCD) spectroscopic characterizations revealed that there is indeed a measurable excess of one propeller handedness. Furthermore, the steric demand of the amine was found to play a key role in the induction process and especially in preventing blade rotations.

Madron du Vigné A., Cramer N.

Breitwieser K., Munz D.

The chemistry of cyclic (alkyl)(amino) carbene (CAAC) ligands is presented. Their electronic structure is discussed and put into the context of conventional N-heterocyclic carbene (NHC) ligands. Then, we present their ligand-centered reactivity as well as redox- and photochemistry in detail. This includes the coordination chemistry of low-valent transition metal- and main group complexes. Accordingly, an overview regarding the photochemistry with CAAC complexes as well as organic CAAC-scaffolds is given. Further, their chemical non-innocence or rather “ligand cooperativity” is summarized focusing on their capability to reversibly bind hydrides. For selected compounds, specific applications in catalysis and as chromophores (luminophores, respectively) are highlighted.

Kumar Kushvaha S., Mishra A., Roesky H.W., Chandra Mondal K.

Isolation of cyclic (alkyl) amino carbenes (cAACs) in 2005 has been a major achievement in the field of stable carbenes due to their better electronic properties. cAACs and bicyclic(alkyl)(amino)carbene (BicAAC) in essence are the most electrophilic as well as nucleophilic carbenes are known till date. Due to their excellent electronic properties in terms of nucleophilic and electrophilic character, cAACs have been utilized in different areas of chemistry, including stabilization of low valent main group and transition metal species, activation of small molecules, and catalysis. The applications of cAACs in catalysis have opened up new avenues of research in the field of cAAC chemistry. This review summarizes the major results of cAAC chemistry published until August 2021.

Vermersch F., Oliveira L., Hunter J., Soleilhavoup M., Jazzar R., Bertrand G.

Using readily available preallylated aldehydes, we report a simple and divergent synthesis of cyclic (alkyl)(amino)carbene (CAAC) iminium precursors. Using a combination of crystallographic data and steric maps, we further elaborate on the specific steric properties of CAAC ligands with respect to state-of-the-art phosphine and carbene ligands.

Singh R.K., Khan T.K., Misra S., Singh A.K.

• CAACs form strong bonds with transition metals as well as main group elements. • CAAC ligand families (CAAC-5, CAAC-6, BiCAACs, FunCAACs, CAArCs, CAAmCs, ChiCAACs) are described. • CAAC-based transition metal complexes are shown to be robust catalysts. • Complexes with CAAC ligands find applications in coordination chemistry, catalysis, bio-inorganic chemistry, medicinal, and material science. CAACs have emerged as an attractive class of ancillary ligands, forming strong bonds with transition metals as well as main-group elements and stabilise both low and high oxidation states. These strongly nucleophilic and electrophilic carbenes have a small HOMO−LUMO energy gap compared to N -heterocyclic carbenes (NHCs). The robust metal complexes formed with CAAC ligands have been utilised in different areas such as catalysis, small-molecule activation, medicinal chemistry, and bioinorganic chemistry. This review describes recent developments in the synthesis of various types of CAAC ligands, their binding ability to transition metals and applications of CAAC−based metal complexes in homogeneous catalysis via hydrogenation, hydroboration, coupling reactions, olefin metathesis and asymmetric transformations.

Yang S., Qu Y., Liao L., Jiang Z., Lee S.

Organic semiconductors can be designed and constructed in π-stacked structures instead of the conventional π-conjugated structures. Through-space interaction (TSI) occurs in π-stacked optoelectronic materials. Thus, unlike electronic coupling along the conjugated chain, the functional groups can stack closely to facilitate spatial electron communication. Using π-stacked motifs, chemists and materials scientists can find new ways for constructing materials with aggregation-induced emission (AIE), thermally activated delayed fluorescence (TADF), circularly polarized luminescence (CPL), and room-temperature phosphorescence (RTP), as well as enhanced molecular conductance. Organic optoelectronic devices based on π-stacked molecules have exhibited very promising performance, with some of them exceeding π-conjugated analogues. Recently, reports on various organic π-stacked structures have grown rapidly, prompting this review. Representative molecular scaffolds and newly developed π-stacked systems could stimulate more attention on through-space charge transfer the well-known through-bond charge transfer. Finally, the opportunities and challenges for utilizing and improving particular materials are discussed. The previous achievements and upcoming prospects may provide new insights into the theory, materials, and devices in the field of organic semiconductors.

Bellotti P., Koy M., Hopkinson M.N., Glorius F.

N-Heterocyclic carbenes, despite being isolated and characterized three decades ago, still capture scientists’ interest as versatile, modular and strongly coordinating moieties. In the last decade, driven by the increasingly refined fundamental understanding of their behaviour, the emergence of new carbene frameworks and cogent sustainability issues, N-heterocyclic carbenes have experienced a tremendous increase in utilization across several disparate fields. In this Review, a concise overview of N-heterocyclic carbenes encompassing their history, properties and applications in transition metal catalysis, on-surface chemistry, main group chemistry and organocatalysis is provided. Emphasis is placed on developments emerging in the last seven years and on envisaging future directions. N-Heterocyclic carbenes continue to mesmerize scientists and open up new research avenues for academia and industry. Here, we provide a concise and up-to-date overview of N-heterocyclic carbenes, encompassing their history, properties and applications in transition metal catalysis, main group chemistry, on-surface chemistry and organocatalysis.

Zhan X., Xu F., Zhou Z., Yan Y., Yao J., Zhao Y.S.

3D laser displays play an important role in next-generation display technologies owing to the ultimate visual experience they provide. Circularly polarized (CP) laser emissions, featuring optical rotatory power and invariability under rotations, are attractive for 3D displays due to potential in enhancing contrast ratio and comfortability. However, the lack of pixelated self-emissive CP microlaser arrays as display panels hinders the implementation of 3D laser displays. Here, full-color 3D laser displays are demonstrated based on CP lasing with inkjet-printed cholesteric liquid crystal (CLC) arrays as display panels. Individual CP lasers are realized by embedding fluorescent dyes into CLCs with their left-/right-handed helical superstructures serving as distributed feedback microcavities, bringing in ultrahigh circular polarization degree values (gem = 1.6). These CP microlaser pixels exhibit excellent far-field color-rendering features and a relatively large color gamut for high-fidelity displays. With these printed CLC red–green–blue (RGB) microlaser arrays serving as display panels, proof-of-concept full-color 3D laser displays are demonstrated via delivering images with orthogonal CP laser emissions into one's left and right eyes. These results provide valuable enlightenment for the development of 3D laser displays.

Prusinowska N., Czapik A., Kwit M.

We have proven the usability and versatility of chiral triphenylacetic acid esters, compounds of high structural diversity, as chirality-sensing stereodynamic probes and as molecular tectons in crystal engineering. The low energy barrier to stereoisomer interconversion has been exploited to sense the chirality of an alkyl substituent in the esters. The structural information are cascaded from the permanently chiral alcohol (inducer) to the stereodynamic chromophoric probe through cooperative interactions. The ECD spectra of triphenylacetic acid esters are highly sensitive to very small structural differences in the inducer core. The tendencies to maximize the C–H···O hydrogen bonds, van der Waals interactions, and London dispersion forces determine the way of packing molecules in the crystal lattice. The phenyl embraces of trityl groups allowed, to some extent, the control of molecular organization in the crystal. However, the spectrum of possible molecular arrangements is very broad and depends on the type of substituent, the optical purity of the sample, and the presence of a second trityl group in the proximity. Racemates crystallize as the solid solution of enantiomers, where the trityl group acts as a protecting group for the stereogenic center. Therefore, the absolute configuration of the inducer is irrelevant to the packing mode of molecules in the crystal.

Deng Y., Wang M., Zhuang Y., Liu S., Huang W., Zhao Q.

Circularly polarized light exhibits promising applications in future displays and photonic technologies. Circularly polarized luminescence (CPL) from chiral luminophores is an ideal approach to directly generating circularly polarized light, in which the energy loss induced by the circularly polarized filters can be reduced. Among various chiral luminophores, organic micro-/nano-structures have attracted increasing attention owing to the high quantum efficiency and luminescence dissymmetry factor. Herein, the recent progress of CPL from organic micro-/nano-structures is summarized. Firstly, the design principles of CPL-active organic micro-/nano-structures are expounded from the construction of micro-/nano-structure and the introduction of chirality. Based on these design principles, several typical organic micro-/nano-structures with CPL activity are introduced in detail, including self-assembly of small molecules, self-assembly of π-conjugated polymers, and self-assembly on micro-/nanoscale architectures. Subsequently, we discuss the external stimuli that can regulate CPL performance, including solvents, pH value, metal ions, mechanical force, and temperature. We also summarize the applications of CPL-active materials in organic light-emitting diodes, optical information processing, and chemical and biological sensing. Finally, the current challenges and prospects in this emerging field are presented. It is expected that this review will provide a guide for the design of excellent CPL-active materials.

Qiu Z., Ju C., Frédéric L., Hu Y., Schollmeyer D., Pieters G., Müllen K., Narita A.

π-Extended helicenes constitute an important class of polycyclic aromatic hydrocarbons with intrinsic chirality. Herein, we report the syntheses of π-extended [7]helicene 4 and π-extended [9]helicene 6 through regioselective cyclodehydrogenation in high yields, where a prefusion strategy plays a key role in preventing undesirable aryl rearrangements. The unique helical structures are unambiguously confirmed by X-ray crystal structure analysis. Compared to the parent pristine [7]helicene and [9]helicene, these novel π-extended helicenes display significantly improved photophysical properties, with a quantum yield of 0.41 for 6. After optical resolution by chiral high-performance liquid chromatography, the chiroptical properties of enantiomers 4-P/M and 6-P/M are investigated, revealing that the small variation in helical length from [7] to [9] can cause an approximately 10-fold increase in the dissymmetry factors. The circularly polarized luminescence brightness of 6 reaches 12.6 M-1 cm-1 as one of the highest among carbohelicenes.

Ye F., Hu M., Luo J., Yu W., Xu Z., Fu J., Zheng Y.

Hattori Y., Matsuoka R., Baba A., Yoshida S., Yamada M., Fujita R., Suzuki D., Imai Y., Uchida K., Kusamoto T., Rapenne G., Kawai T.

AbstractTwo units of a highly stable luminescent triarylmethyl radical (PyBTM) were bridged using a chiral octahydrobinaphthyl moiety, resulting in a diradical with sufficient stability to enable the measurement of its chiroptical properties. To synthesize this diradical, a novel boronic ester radical precursor, αH‐PyBTM‐B(Epin), was designed. The use of this precursor significantly improved the yield and streamlined the preparation of stable luminescent radical‐substituted molecules. The photoluminescence quantum yield (PLQY) of the diradical was measured to be 10 % in chloroform. Furthermore, both circular dichroism (CD) and circularly polarized luminescence (CPL) were successfully observed.

Arumugam S., Kushvaha S.K., Ravichandran P., Kumar J., Gorantla S.M., Mondal T., Roesky H.W., Mondal K.C.

AbstractA series of significantly bulky mono‐ and di‐substituted cyclic alkyl‐amino carbene (cAAC)‐ functionalized cyclopentadiene ring (Cp) compounds were synthesized. The functionalization of the Cp ring with cAAC ligands makes them significantly bulkier, while retaining their ligation properties. These compounds display interesting fluorescence properties. In these compounds, intra‐molecular charge transfer is observed from electron‐rich carbene to electron‐deficient cyclopentadiene unit. These high yielding compounds have been characterized by X‐ray single‐crystal diffraction and their emission properties have been studied. Rotational conformers (via C−C bond rotation) play a pivotal role with possible different extent of intramolecular charge‐transfer (ICT) from carbene to cyclopentadiene ring. Variable temperature‐dependent NMR studies were performed along with NOSY, COSY and different 2D NMR techniques to estimate the energy barriers and 1,5‐Hydrogen shift.

Yu W., Hu M., Xu Z., Fu J., Zheng Y.

Coehlo M., Frédéric L., Poulard L., Ferdi N., Estaque L., Desmarchelier A., Clavier G., Dognon J., Favereau L., Giorgi M., Naubron J., Pieters G.

Very recently, the control of dynamic chirality has emerged as a powerful strategy to design chiral functional materials. In this context, we describe herein a molecular design in which a tethered configurationally stable binaphthyl chiral unit efficiently controls the dynamic chirality of donor‐acceptor fluorophores, involving diverse indolocarbazoles as electron donors and terephthalonitrile as an electron acceptor. The high conformational discrimination in such a molecular system suggested by density functional theory calculations is experimentally probed using electronic and vibrational circular dichroism and confirmed by the crystallization of these chiral molecules in gel and their single crystal X‐ray diffraction analysis. In addition to extending the scope of dynamic chirality control to donor‐acceptor fluorophores, this work also highlights the positive effect of the configurationally stable chiral inductor on the magnitude of the dissymmetry factors of the active dynamically chiral fluorophores, both in ground and excited states, through chiral perturbation.

Coehlo M., Frédéric L., Poulard L., Ferdi N., Estaque L., Desmarchelier A., Clavier G., Dognon J., Favereau L., Giorgi M., Naubron J., Pieters G.

Very recently, the control of dynamic chirality has emerged as a powerful strategy to design chiral functional materials. In this context, we describe herein a molecular design in which a tethered configurationally stable binaphthyl chiral unit efficiently controls the dynamic chirality of donor‐acceptor fluorophores, involving diverse indolocarbazoles as electron donors and terephthalonitrile as an electron acceptor. The high conformational discrimination in such a molecular system suggested by density functional theory calculations is experimentally probed using electronic and vibrational circular dichroism and confirmed by the crystallization of these chiral molecules in gel and their single crystal X‐ray diffraction analysis. In addition to extending the scope of dynamic chirality control to donor‐acceptor fluorophores, this work also highlights the positive effect of the configurationally stable chiral inductor on the magnitude of the dissymmetry factors of the active dynamically chiral fluorophores, both in ground and excited states, through chiral perturbation.

Volk J., Heinz M., Guthardt R., Yadav S., Bruhn C., Holthausen M.C., Siemeling U.

AbstractThe recently described crystalline cyclic (alkyl)(amino)carbene with a 1,1’‐ferrocenylene (fc) backbone fc(CPh2−C−NMes) (A, Mes=mesityl) is highly reactive due to its particularly pronounced ambiphilicity and is thermally not stable in solution due to an intramolecular insertion of the divalent carbon atom into a methyl C−H bond of the Mes substituent. The closely related congener fc(CPh2−C−N‐p‐C6H4‐tBu) (1) cannot undergo such an insertion reaction. Nevertheless, 1 is too short‐lived for isolation due to a rapid 1,2‐shift of a phenyl group, furnishing the isomeric cyclic enamine fc[C(Ph)=C(Ph)−N‐p‐C6H4‐tBu] (1’) in a specific decomposition process unprecedented for CAACs. Trapping of 1 was possible with carbon monoxide, elemental selenium and with [CuBr(SMe2)], respectively affording the aminoketene 1=C=O, the selenoamide 1=Se and the homoleptic CuI complex [Cu(1)2][CuBr2]. 1 is an even stronger ambiphile than A according to NMR spectroscopic data. Similar to A, 1 does not react with H2, because the experimentally observed intramolecular process is kinetically more favourable according to DFT results.