Control of Axial Chirality by Planar Chirality Based on Optically Active [2.2]Paracyclophane

Optically active X-shaped molecules based on the planar chiral [2.2]paracyclophane building block were prepared, in which di(methoxy)terphenyl units were stacked on the central benzene rings. At 25 °C, anisolyl rings freely rotate in solution, while in the crystal form, they are fixed by intramolecular CH–π interactions, thereby leading to the expression of the axial chirality, i.e., propeller chirality was exhibited by the planar chiral [2.2]paracyclophane moiety. The X-shaped molecule exhibited good circularly polarized luminescence (CPL) profiles with moderate Φ_PL and a large g_lum value in the order of 10⁻³ at 25 °C, in solution. In contrast, at −120 °C, dual CPL emission with opposite signs was observed. According to the theoretical studies, the rotary motion of the anisolyl units is suppressed in the excited states, and so emission from two isomers could be observed. These results demonstrate that the axial chirality was controlled by the planar chirality, leading ultimately to propeller chirality.     

Heterocyclic effect for optical properties of naphthoquinone-based pigment: 2-methyl-3-heteroarylthio-1,4-naphthalenedione

Three novel naphthoquinone-based heterocyclic pigments, 2-methyl-3-[(1-methyl-1H-imidazol-2-yl)thio-1,4-naphthalenedione, (4-methyl-4H-1,2,4-triazol-3-yl)thio-1,4-naphthalenedione, and (1-methyl-1H-tetrazol-5-yl)thio]-1,4-naphthalenedione, are synthesized, and their optical properties in both solution and solid states are investigated. Depending on the heteroarylthio ring in the pigment, variation in optical properties is observed, e.g. characteristic colours for each pigment in the solution and solid states. The achiral pigment containing the 1-methyl-1H-tetrazol-5-yl ring exhibits a chiral space group and a CD signal in the solid state.     

Multiple Fused Anthracenes as Helical Polycyclic Aromatic Hydrocarbon Motif for Chiroptical Performance Enhancement

Polycyclic aromatic hydrocarbons consisting of three fused anthracene units were designed as new π‐conjugated compounds having helical structures. These expanded helicenes were synthesized by Pt‐catalyzed cycloisomerization of the corresponding ethynyl‐substituted precursors. The nonplanar and helical structure was confirmed by X‐ray analysis and DFT calculations, and the barrier to helical inversion was estimated to be 34 kJ mol^?1. The enantiomers of the diphenyl derivative were successfully resolved by chiral HPLC. Enantiopure samples showed good chiroptical performance in the CD (|Δ?| 1380 L mol^?1 cm^?1) and CPL (|g_lum| 0.013) spectra, and these values were considerably large for simple organic molecules. The unique chiroptical properties are discussed on the basis of the molecular structure and the electronic state with the aid of time‐dependent DFT calculations.     

Control of the Solid‐State Chiral Optical Properties of a Supramolecular Organic Fluorophore Containing 4‐(2‐Arylethynyl)‐Benzoic Acid

The solid‐state chiral optical properties of a 4‐(2‐arylethynyl)‐benzoic acid/amine supramolecular organic fluorophore can be controlled by changing the arylethynyl group of the achiral 4‐(2‐arylethynyl)‐benzoic acid component molecule rather than the chirality of the amine component molecule.     

A charge-transfer host system composed of a chiral 1,1′-bi-2-naphthol cluster and benzylviologen

Chiral charge-transfer (CT) complexes composed of a chiral 1,1′-bi-2-naphthol cluster as the electron donor and 1,1′-dibenzyl-4,4′-bipyridinium dichloride as the electron acceptor serve as a host system for molecular recognition. CT complexes that include guest alcohols show different diffuse reflectance spectra (DRS) depending on the included guest.     

A Solid‐State Fluorescent Host System with a 21‐Helical Column Consisting of Chiral (1R,2S)‐2‐Amino‐1,2‐diphenylethanol and Fluorescent 1‐Pyrenecarboxylic Acid

A solid‐state fluorescent host system was created by self‐assembly of a 2₁‐helical columnar organic fluorophore composed of (1R,2S)‐2‐amino‐1,2‐diphenylethanol and fluorescent 1‐pyrenecarboxylic acid. This host system has a characteristic 2₁‐helical columnar hydrogen‐ and ionic‐bonded network. Channel‐like cavities are formed by self‐assembly of this column, and various guest molecules can be included by tuning the packing of this column. Moreover, the solid‐state fluorescence of this host system can change according to the included guest molecules. This occurs because of the change in the relative arrangement of the pyrene rings as they adjust to the tuning of the packing of the shared 2₁‐helical column, according to the size of the included guest molecules. Therefore, this host system can recognize slight differences in molecular size and shape.     

Circularly polarized luminescence (CPL) characteristics of hydrophobic pyrene derivatives/γ-cyclodextrin (γ-CD) complexes in aqueous solution dissolved by grinding

Circularly polarized luminescence (CPL) organic dyes are currently receiving a great interest, but there are still not many reported observations of CPL spectra of hydrophobic dyes from aqueous solution. We have prepared hydrophobic pyrene derivatives and dissolved them into aqueous solutions with γ-cyclodextrin (γ-CD) by using grinding technique. Among these derivatives, (pyrene-1-carbonyl)serine (PySer) forms a spatially restricted dimer in the hydrophobic chiral cavity of γ-CD and exhibits excimer emission with a high quantum yield of Φ_f?=?0.68. In addition, circular dichroism and CPL signals were induced for the complex. The strong g_CPL value of g_CPL?=?+?2.2?×?10^?3 was obtained, which may be attributed to the interaction between the hydroxyl groups in the side chain of PySer with those of γ-CD and it strengthens the chiral dimeric structure.

Graphic abstract

     

Solid-state optical properties of a chiral supramolecular fluorophore consisting of chiral (1R,2R)-1,2-diphenylethylenediamine and fluorescent carboxylic acid derivatives

By using (1R,2R)-1,2-diphenylethylenediamine as a chiral molecule and 2-anthracenecarboxylic acid as a fluorescent molecule, we created a chiral supramolecular organic fluorophore having circularly polarized luminescence properties in the solid-state.     

Non-classical control of solid-state aggregation-induced enhanced circularly polarized luminescence in chiral perylene diimides

Organic fluorescent molecules are gaining importance because of their potential applications in many devices. Optically active N,N′-bis((1R)-1-naphthylethyl)perylene-3,4,9,10-tetracarboxylic diimide [(R,R)-1-BNP] and N,N′-bis((1R)-2-naphthylethyl)perylene-3,4,9,10-tetracarboxylic diimide [(R,R)-2-BNP] and their antipode, [(S,S)-1-BNP and (S,S)-2-BNP], emit aggregation-induced enhanced (AIEnh) circularly polarized luminescence (CPL) on both a solid organic polymer film (poly(methyl methacrylate)) and solid inorganic KBr pellet. An opposite chirality is essential for generating CPL of inverted sign. However, a pair of enantiomeric organic molecules may not always be easy to prepare. Interestingly, the chiral perylene fluorophores synthesized in this study can emit both positive and negative AIEnh-CPL in the solid state, depending on their position on the naphthylene groups. In addition, no CPL was observed in these compounds from their dilute solutions.