Ultrahigh-Temperature Long-Persistent Luminescence from B2O3-Confined Polycyclic Aromatic Compounds.

Organic molecules and polymers have recently been intensively explored for afterglow materials owing to their low cost and flexible design. However, they normally fail to generate long-persistent luminescence at elevated temperatures, mostly due to the fast deactivation of triplet excited states. Here, we report that polycyclic aromatic compounds (PACs) individually confined in a B2O3 crystalloid emit long-persistent luminescence at high temperatures up to 400 °C. This is facilely accomplished by dispersing a series of aromatic derivatives in an aqueous solution of boric acid, followed by drying, melting, and dehydrating. The resulting highly rigid and thermostable B2O3 crystalloid network provides a matched ultrastrong confinement effect and completely restricts the vibration and rotation of the molecularly distributed PACs even at ultrahigh temperatures and thereby prevents the nonradiative dissipation of triplet excitons and promotes the generation of ultrahigh-temperature long-persistent luminescence. The afterglow colors are responsive to both temperature and time, spanning from ultraviolet to near-infrared regions over a wide temperature range, which is substantially modulated by the subtle balance of phosphorescence and thermally activated delayed fluorescence. These features favor the creation of advanced afterglow materials for visual 3D temperature probing, anticounterfeiting, and data encryption in extreme environments.

Enantiomerization of five-membered-heterocycle-embedded helicenes: A DFT study.

In this work, DFT theoretical calculations were employed to investigate the enantiomerization of helicenes embedded with five-membered heterocycles. The original benzene rings in the helicene backbone were replaced by heterocycles such as furan, thiophene, pyrrole, or phosphole to create [n]helicenes with n ranging from 4 to 7. The impact of the type, position, and number of heterocycles on the enantiomerization barrier was systematically evaluated. Notably, the enantiomerization barrier was found to be significantly dependent on the rotatory angle and the position of the heterocycles, particularly for [4, 5]helicenes. With less rotatory angle of heterocycle, the enantiomerization barrier of helicenes was revealed to be lower, while when the heterocycle was close to the central part of the helicene chain, the barrier was also lower. Furthermore, the number of thiophene rings also had a marked effect on enantiomerization, showing a decrease of the barrier with more thiophene rings placed on the helicenes backbone. We expect this work would deliver new perspective on the relative studies for the helicene conformational conversion.

Theoretical Study on Vibrationally Resolved Electronic Spectra of Chiral Nanographenes.

Nanographenes are of increasing importance owing to their potential applications in the photoelectronic field. Meanwhile, recent studies have primarily focused on the pure electronic spectra of nanographenes, which have been found to be inadequate for describing the experimental spectra that contain vibronic progressions. In this study, we focused on the vibronic effect on the electronic transition of a range of chiral nanographenes, especially in the low-energy regions with distinct vibronic progressions, using theoretical calculations. All the calculations were performed at the PBE0-D3(BJ)/def2-TZVP level of theory, adopting both time-dependent and time-independent approaches with Franck-Condon approximation. The resulting calculated curves exhibited good alignment with the experimental data. Notably, for the nanographenes incorporating helicene units, owing to the increasing π-extension, the major vibronic modes in the vibrationally resolved spectra differed significantly from those of the primitive helicenes. This investigation suggests that calculations that account for the vibronic effect could have better reproducibility compared with calculations based solely on pure electronic transitions. We anticipate that this study could pave the way for further investigations into optical and chiroptical properties, with a deeper understanding of the vibronic effect, thereby providing theoretical explanations with higher precision on more sophisticated nanographenes.

π-Extended Diaza[7]helicenes with Dual Negatively Curved Heptagons: Extensive Synthesis and Spontaneous Resolution into Strippable Homochiral Lamellae with Helical Symmetry.

We report a unique category of π-extended diaza[7]helicenes with double negative curvatures. This is achieved by two-fold regioselective heptagonal cyclization of the oligoarylene-carbazole precursors through either intramolecular C-H arylation or Scholl reaction. The fusion of two heptagonal rings in the helical skeleton dramatically increases the intramolecular strain and forces the two terminal carbazole moieties to stack in a compressed fashion. The presence of the deformable negatively curved heptagonal rings endows the resulting diaza[7]helicenes with dynamic chiral skeletons, aggregation-induced emission feature and relatively low racemization barrier of ca. 25.6 kcal mol-1 . Further π-extension on the carbazole moieties subsequently leads to a more sophisticated C2 -symmetric homochiral triple helicene. Notably, these π-extended diaza[7]helicenes show structure-dependent stacking upon crystallization, switching from heterochiral packing to intra-layer homochiral stacking. Interestingly, the C2 -symmetric triple helicene molecules spontaneously resolve into a homochiral lamellar structure with 31 helix symmetry. Upon ultrasonication in a nonsolvent, the crystals can be readily exfoliated into large-area ultrathin nanosheets with height of ca. 4.4 nm corresponding to two layers of stacked triple helicene molecules and relatively thicker nanosheets constituted by even-numbered molecular lamellae. Moreover, regular hexagonal thin platelets with size larger than 30 µm can be readily fabricated by flash aggregation.

A Helicene-Based Single-Molecule Inductor and Capacitor with Frequency-Dependent Charge-Transport Pathways.

Despite extensive studies has been explored on single-molecule switches and rectifiers, the design of single-molecule inductors has not been explored due to the experimental challenges in the investigation of frequency-dependent charge transport at the single-molecule scale. In this study, we synthesized a helicene-based helical molecular wire and carried out meticulous single-molecule conductance measurements, combined with current-voltage (IV) studies with varying frequencies using the scanning tunneling microscope break junction (STM-BJ) technique. Our results reveal the formation of a single-molecule junction and highlight the unique behavior of the molecular wire in response to different alternating current (AC) varying frequencies. The transport of charges occurs selectively either through the coiled backbone of the conjugated helical structure or vertically via π-π stacking, depending on the frequency of the applied AC. Notably, our investigation demonstrates the functionality of the wire as an inductor at low frequencies, and a capacitor at high frequencies. This work lays the foundation for a systematic approach to designing, fabricating, and implementing single-molecule logic devices such as inductors and wave filters.

[1,4]Diazocine-Embedded Electron-Rich Nanographenes with Cooperatively Dynamic Skeletons.

Curved nanographenes (NGs) are emerging as promising candidates for organic optoelectronics, supramolecular materials, and biological applications. Here we report a distinctive type of curved NGs bearing a [1,4]diazocine core that is fused with four pentagonal rings. This is formed by Scholl-type cyclization of two adjacent carbazole moieties through an unusual diradical cation mechanism followed by C-H arylation. Owing to the strain in the unique 5-5-8-5-5-membered ring skeleton, the resulting NG adopts an interesting concave-convex cooperatively dynamic structure. By peripheral π-extension, a helicene moiety with fixed helical chirality can be further mounted to modulate the vibration of the concave-convex structure, through which the distant bay region of the curved NG inherits the chirality of the helicene moiety in a reversed fashion. The [1,4]diazocine-embedded NGs show typical electron-rich characteristics and form charge transfer complexes with tunable emissions with a series of electron acceptors. The relatively protruding armchair edge also allows the fusion of three NGs into a C2 symmetric triple diaza[7]helicene which reveals a subtle balance of fixed and dynamic chirality.

DNA-induced circularly polarized luminescence of helicene racemates.

Enantiopure helicenes have been extensively investigated due to their outstanding chiroptical properties, while helicene racemates are considered as chiroptically silent. Here, we describe a facile method to produce circularly polarized luminescence (CPL) from helicene racemates via supramolecular association with DNA in aqueous solution. Racemic cationic helicene derivatives are immobilized in the grooves of commercially available double-stranded right-handed DNA, and the discrimination of left- and right-handed helicenes by chiral DNA is monitored by single molecule force spectroscopy. This subsequently leads to the generation of prominent CPL with dissymmetric factor |glum | of close to 0.01, which is approximate to enantiopure helicenes. The strategy developed in this work avoids the tedious and expensive chiral resolution process and provides a distinctive insight into the fabrication of CPL-emitting systems.

Transformation of Crowded Oligoarylene into Perylene-Cored Chiral Nanographene by Sequential Oxidative Cyclization and 1,2-Phenyl Migration.

Synthetic innovation for constructing sophisticated nanographenes is of fundamental significance for a variety of advanced applications. Herein, we report a distinctive method to prepare π-extended chiral nanographenes with 29 benzenoid rings and two helical breaches from a highly crowded perylene-cored oligoarylene precursor. Under Scholl's conditions, the reaction predominantly involves the regioselective and sequential cyclization in the peri- and bay regions of the perylene core, and the complanation of the 1-phenyl[5]helicene intermediate module via 1,2-phenyl migration. The resulting chiral nanographenes are configurationally stable at 180 °C due to the high diastereomerization barriers of ca. 45 kcal mol-1 . These molecules also possess globally delocalized π-systems with low HOMO/LUMO gaps, leading to nearly panchromatic absorption, intensive electronic circular dichroism signals and deep-red circularly polarized luminescence.

Amplifiable Symmetry Breaking in Aggregates of Vibrating Helical Molecules.

Symmetry breaking in the self-assembly of achiral constituents is of vital importance for the origin of molecular homochirality and developing advanced chiral materials. Here, we report a unique mode of spontaneous symmetry breaking in the aggregates of aza[4]helicenes with an achiral vibrating helical conjugated structure. The achiral molecules initially form clustered aggregates with a slight chiral bias of the P and M isomers, and subsequently the chiral imbalance is amplified by the conversion of the P and M conformations to favor a more thermodynamic stable π-π stacking (from PM to PP or MM stacking). The dynamical P/M transformation not only promotes the evolution of optical activity following the initial spontaneous symmetry breaking but also favors the healing of chirality after the majority is eliminated by heating. Notably, the aggregates reveal prominent circularly polarized luminescence with the absolute dissymmetry factor approaching 0.01. This work provides additional insights into the pathway of chiral symmetry breaking and illustrates a unique route to develop optically active materials from achiral helical molecules.

Chiral Recognition of Hexahelicene on a Surface via the Forming of Asymmetric Heterochiral Trimers.

Chiral recognition among helical molecules is of essential importance in many chemical and biochemical processes. The complexity necessitates investigating manageable model systems for unveiling the fundamental principles of chiral recognition at the molecular level. Here, we reported chiral recognition in the self-assembly of enantiopure and racemic hexahelicene on a Au(111) surface. Combing scanning tunneling microscopy (STM) and atomic force microscopy (AFM) measurements, the asymmetric heterochiral trimers were observed as a new type of building block in racemic helicene self-assembly on Au(111). The intermolecular recognition of the heterochiral trimer was investigated upon manual separation so that the absolute configuration of each helicene molecule was unambiguously determined one by one, thus confirming that the trimer was "2+1" in handedness. These heterochiral trimers showed strong stability upon different coverages, which was also supported by theoretical calculations. Our results provide valuable insights for understanding the intermolecular recognition of helical molecules.

Chiral Organic Cages with a Triple-Stranded Helical Structure Derived from Helicene.

We report the use of helicene with an intrinsic helical molecular structure to prepare covalent organic cages via imine condensation. The organic cages revealed a [3+2]-type architecture containing a triple-stranded helical structure with three helicene units arranged in a propeller-like fashion with the framework integrally twisted. Such structural chirality was retained upon dissolution in organic solvents, as indicated by a strong diastereotopy effect in proton NMR and unique Cotton effects in circular dichroism spectra. Further study on chiral adsorption showed that the chiral organic cages possess considerable enantioselectivity toward a series of aromatic racemates.

Chiral Transmission to Cationic Polycobaltocenes over Multiple Length Scales Using Anionic Surfactants.

Chiral polymers are ubiquitous in nature, and the self-assembly of chiral materials is a field of widespread interest. In this paper, we describe the formation of chiral metallopolymers based on poly(cobaltoceniumethylene) ([PCE] n+), which have been prepared through oxidation of poly(cobaltocenylethylene) (PCE) in the presence of enantiopure N-acyl-amino-acid-derived anionic surfactants, such as N-palmitoyl-l-alanine (C16-l-Ala) and N-palmitoyl-d-alanine (C16-d-Ala). It is postulated that the resulting metallopolymer complexes [PCE][C16-l/d-Ala] n contain close ionic contacts, and exhibit chirality through the axially chiral ethylenic CH2-CH2 bridges, leading to interaction of the chromophoric [CoCp2]+ units through chiral space. The steric influence of the long palmitoyl (C16) surfactant tail is key for the transmission of chirality to the polymer, and results in a brushlike amphiphilic macromolecular structure that also affords solubility in polar organic solvents (e.g., EtOH, THF). Upon dialysis of these solutions into water, the hydrophobic palmitoyl surfactant substituents aggregate and the complex assembles into superhelical ribbons with identifiable "handedness", indicating the transmission of chirality from the molecular surfactant to the micrometer length scale, via the macromolecular complex.

Redox-Active Chiroptical Switching in Mono- and Bis-Iron Ethynylcarbo[6]helicenes Studied by Electronic and Vibrational Circular Dichroism and Resonance Raman Optical Activity.

Introducing one or two alkynyl-iron moieties onto a carbo[6]helicene results in organometallic helicenes (2 a,b) that display strong chiroptical activity combined with efficient redox-triggered switching. The neutral and oxidized forms have been studied in detail by electronic and vibrational circular dichroism, as well as by Raman optical activity (ROA) spectroscopy. The experimental results were analyzed and spectra were assigned with the help of first-principles calculations. In particular, a recently developed method for ROA calculations under resonance conditions has been used to study the intricate resonance effects on the ROA spectrum of mono-iron ethynylhelicene 2 a.

Enantiopure versus Racemic Naphthalimide End-Capped Helicenic Non-fullerene Electron Acceptors: Impact on Organic Photovoltaics Performance.

Impact of the enantiopurity on organic photovoltaics (OPV) performance was investigated through the synthesis of racemic and enantiomerically pure naphthalimide end-capped helicenes and their application as non-fullerene molecular electron acceptors in OPV devices. A very strong increase of the device performance was observed by simply switching from the racemic to the enantiopure forms of these π-helical non-fullerene acceptors with power conversion efficiencies jumping from 0.4 to about 2.0 % in air-processed poly(3-hexylthiophene)-based devices, thus highlighting the key role of enantiopurity in the photovoltaic properties.

Synthesis and Chiroptical Properties of Hexa-, Octa-, and Deca-azaborahelicenes: Influence of Helicene Size and of the Number of Boron Atoms.

Four members of a new class of cycloborylated hexa-, octa-, and deca-helicenes (1 a-d) have been prepared in enantiopure form, along with two cycloplatinated deca-helicenes (1 d', 1 d1 ), further extending the family of cycloplatinated hexa- and octa-helicenes reported previously. The azabora[n]helicenes display intense electronic circular dichroism and large optical rotations; the dependence of the optical activity on the size of the helix (n=6, 8, 10) and the number of boron atoms (1 or 2) has been examined in detail both experimentally and theoretically. The photophysical properties (nonpolarized and circularly polarized luminescence) of these new fluorescent organic helicenes have been measured and compared with the corresponding organometallic phosphorescent cycloplatinated derivatives (1 a1 -d1 ).

Iron Alkynyl Helicenes: Redox-Triggered Chiroptical Tuning in the IR and Near-IR Spectral Regions and Suitable for Telecommunications Applications.

The combination of a bis-alkynyl-helicene moiety with two iron centers leads to novel electroactive species displaying unprecedented redox-triggered chiroptical switching. Upon oxidation, strong changes of vibrational modes (either local or extended coupled modes) are detected by vibrational circular dichroism and Raman optical activity. Remarkably, the sign of the optical rotation at 1.54 µm (that is, at wavelengths typically used for telecommunications) changes upon oxidation while the topology and stereochemistry of the helicene remain unchanged.

Enantioseparation on Riboflavin Derivatives Chemically Bonded to Silica Gel as Chiral Stationary Phases for HPLC.

Acetylated and/or 3,5-dimethylphenylcarbamated riboflavins were prepared and the resulting riboflavin derivatives as well as natural riboflavin were regioselectively immobilized on silica gel through chemical bonding at the 5'-O- or 3-N-position of the riboflavin to develop novel chiral stationary phases (CSPs) for enantioseparation by high-performance liquid chromatography (HPLC). The chiral recognition abilities of the obtained CSPs were significantly dependent on the structures of the riboflavin derivatives, the position of the chemical bonding on the silica gel, and the structures of the racemic compounds. The CSPs bonded at the 5'-O-position on the silica gel tended to well separate helicene derivatives, while the CSPs bonded at the 3-N-position composed of acetylated and 3,5-dimethylphenylcarbamated riboflavins showed a better resolving ability toward helicene derivatives and bulky aromatic racemic alcohols, respectively, and some of them were completely separated into the enantiomers. The observed difference in the chiral recognition abilities of these riboflavin-based CSPs is discussed based on the difference in their structures, including the substituents of riboflavin and the positions immobilized on the silica gel.

Helicene quinones: redox-triggered chiroptical switching and chiral recognition of the semiquinone radical anion lithium salt by electron nuclear double resonance spectroscopy.

We present the synthesis and characterization of enantiomerically pure [6]helicene o-quinones (P)-(+)-1 and (M)-(-)-1 and their application to chiroptical switching and chiral recognition. (P)-(+)-1 and (M)-(-)-1 each show a reversible one-electron reduction process in their cyclic voltammogram, which leads to the formation of the semiquinone radical anions (P)-(+)-1(•-) and (M)-(-)-1(•-), respectively. Spectroelectrochemical ECD measurements give evidence of the reversible switching between the two redox states, which is associated with large differences of the Cotton effects [Δ(Δε)] in the UV and visible regions. The reduction of (±)-1 by lithium metal provides [Li(+){(±)-1(•-)}], which was studied by EPR and ENDOR spectroscopy to reveal substantial delocalization of the spin density over the helicene backbone. DFT calculations demonstrate that the lithium hyperfine coupling A((7)Li) in [Li(+){(±)-1(•-)}] is very sensitive to the position of the lithium cation. On the basis of this observation, chiral recognition by ENDOR spectroscopy was achieved by complexation of [Li(+){(P)-(+)-1(•-)}] and [Li(+){(M)-(-)-1(•-)}] with an enantiomerically pure phosphine oxide ligand.

Site-selective α-C(sp3)-H arylation of dialkylamines via hydrogen atom transfer catalysis-enabled radical aryl migration.

Site-selective C(sp3)-H arylation is an appealing strategy to synthesize complex arene structures but remains a challenge facing synthetic chemists. Here we report the use of photoredox-mediated hydrogen atom transfer (HAT) catalysis to accomplish the site-selective α-C(sp3)-H arylation of dialkylamine-derived ureas through 1,4-radical aryl migration, by which a wide array of benzylamine motifs can be incorporated to the medicinally relevant systems in the late-stage installation steps. In contrast to previous efforts, this C-H arylation protocol exhibits specific site-selectivity, proforming predominantly on sterically more-hindered secondary and tertiary α-amino carbon centers, while the C-H functionalization of sterically less-hindered N-methyl group can be effectively circumvented in most cases. Moreover, a diverse range of multi-substituted piperidine derivatives can be obtained with excellent diastereoselectivity. Mechanistic and computational studies demonstrate that the rate-determining step for methylene C-H arylation is the initial H atom abstraction, whereas the radical ipso cyclization step bears the highest energy barrier for N-methyl functionalization. The relatively lower activation free energies for secondary and tertiary α-amino C-H arylation compared with the functionalization of methylic C-H bond lead to the exceptional site-selectivity.

Chemoselective Synthesis of α-Halo-α,α-difluoromethyl Ketones from Trimethyl(phenylethynyl)silane.

Here, we describe a novel strategy for chemoselective synthesis of α-halo-α,α-difluoromethyl ketones (-COCF3 and -COClCF2 motifs) from trimethyl(phenylethynyl)silane under catalyst-free and mild conditions. Commercially available Selectfluor or additional NaCl as halogen reagent was employed to complete this transformation, thereby demonstrating the potential synthetic value of this new reaction in organic synthesis.