Tethered Helical Ladder-Type Aromatic Lactams

Tethered nonplanar aromatics (TNAs) make up an important class of nonplanar aromatic compounds showing unique features. However, the knowledge on the synthesis, structures, and properties of TNAs remains insufficient. In this work, a new type of TNAs, the tethered aromatic lactams, is synthesized via Pd-catalyzed consecutive intramolecular direct arylations. These molecules possess a helical ladder-type conjugated system of up to 13 fused rings. The overall yields ranged from 3.4 to 4.3%. The largest of the tethered aromatic lactams, 6L-Bu-C14, demonstrates a guest-adaptive hosting capability of TNAs for the first time. When binding fullerene guests, the cavity of 6L-Bu-C14 became more circular to better accommodate spherical fullerene molecules. The host-guest interaction is thoroughly studied by X-ray crystallography, theoretical calculations, fluorescence titration, and nuclear magnetic resonance (NMR) titration experiments. 6L-Bu-C14 shows stronger binding with C70 than with C60 due to the better convex-concave π-π interaction. P and M enantiomers of all tethered aromatic lactams show distinct and persistent chiroptical properties and demonstrate the potential of chiral TNAs as circularly polarized luminescence (CPL) emitters.

Benzannulation Reactions: A Case for Perspective Change From Arene Decoration to Arene Construction

Benzannulation reactions involve construction of a benzene ring from acyclic precursors. This class of reactions offer a versatile and often superior alternative to aromatic substitution for construction of substituted arenes. Selected pioneering and recent reports of various benzannulation reactions are categorised and discussed in this review.

"Shadow" Synthesis, Structure, and Electronic Properties of [2.2](1,6)(1,8)Pyrenophane-1-monoene

An unexpected side product of a McMurry reaction was found to be a new [2.2]pyrenophane consisting of two pyrene units with different substitution patterns as well as different types and degrees of distortion from planarity. The new pyrenophane exhibits both monomer and intramolecular excimer fluorescence. Natural bond orbital (NBO) analysis revealed that there is an intramolecular charge-transfer interaction from the more distorted pyrene system to the less distorted one. The origin of the new pyrenophane was traced back to an impurity that was present a full five steps prior to the McMurry reaction from which it was isolated. The pathway to the pyrenophane shadowed that of the main synthetic route.

Gram-Scale Synthesis of the 1,1,n,n-Tetramethyl[n](2,11)teropyrenophanes

A gram-scale synthesis of a series of 1,1,n,n-tetramethyl[n](2,11)teropyrenophanes (n=7-9) has been accomplished as well as the first synthesis of the next higher homologue 1,1,10,10-tetramethyl[10](2,11)teropyrenophane. The scale-up of the original small-scale synthesis required the development of several heavily modified synthetic methods, including a chlorination/Friedel-Crafts alkylation protocol and an iodination/Wurtz coupling protocol, which were performed on 25-30 g and 30-60 g scales, respectively. Two separate sets of conditions for the key teropyrene-forming cyclodehydrogenation reaction at the end of the synthetic pathway were developed, an acid-promoted one for the two less strained congeners and an acid-free method for the two more strained homologues.

Enantioselective Synthesis of Polycyclic Aromatic Hydrocarbon (PAH)-Based Planar Chiral Bent Cyclophanes by Rhodium-Catalyzed [2+2+2] Cycloaddition

The enantioselective synthesis of polycyclic aromatic hydrocarbon (PAH)-based planar chiral cyclophanes was achieved for the first time by the rhodium-catalyzed intramolecular regio- and enantioselective [2+2+2] cycloaddition of tethered diyne-benzofulvenes followed by stepwise oxidative transformations. The thus synthesized planar chiral bent cyclophanes, that possess bent p-terphenyl- and 9-fluorenone-cores, were converted to 9-fluorenol-based ones with excellent ee values of >99 % by diastereoselective 1,2-reduction. These 9-fluorenol-based cyclophanes exhibited high fluorescence quantum yields, which were significantly higher than that of an acyclic reference molecule (78-82 % vs. 48 %). The bending effect on the chiroptical property was also examined, which revealed that the anisotropy factors (g_abs values) for electronic circular dichroism (ECD) of these 9-fluorenol-based planar chiral bent cyclophanes increase as the tether length becomes shorter.

(1,3)Pyrenophanes containing crown ether moieties as fluorescence sensors for metal and ammonium ions

Crown ether containing (1,3)pyrenophanes 1-6 were synthesized, and UV absorption and fluorescence spectroscopic studies were carried out to determine their abilities to form complexes with metal and ammonium ions. The fluorescence spectra of 1.0 × 10-5 M solutions of 1, 2, 4 and 6 in 1 : 1 v/v CH2Cl2 : CH3CN were comprised of both monomer and intramolecular excimer emission bands, while only monomer emission bands were present in the fluorescence spectra of 3 and 5. The intensities of the intramolecular excimer emission bands of 1, 2, 4 and 6 in 1 : 1 v/v CH2Cl2 : CH3CN decreased and those of the monomer emission increased in conjunction with the existence of isoemissive points upon the addition of increasing concentrations of various metal perchlorates. The fluorescence spectral changes were dependent on the sizes of crown ether rings and metal ions and, as such, they reflected equilibrium constants for the formation of metal-crown ether complexes. Addition of n-Bu2NH2+PF6- or (PhCH2)2NH2+PF6- to the solutions of the (1,3)pyrenophane linked crown ethers, which brought about similar fluorescence spectral changes, led to the formation of pseudo-rotaxanes as was evidenced by an analysis of 1H NMR spectra and Job's plots. The fluorescence changes of 1 occurred during 5 cycles of repetitive addition and removal of Ba2+. The ratio of intensities of the monomer to the intramolecular excimer emission bands of 1, 2, 4 and 6 increased as the temperature decreased. Based on the experimental observations and the results of DFT calculations, it is concluded that the (1,3)pyrenophanes exist in solution as equilibrium mixtures of anti monomer emitting and syn intramolecular excimer emitting conformers and the equilibrium favors the anti form when the crown ether moieties form complexes with metal or ammonium ions.

The Consequences of Twisting Nanocarbons: Lessons from Tethered Twisted Acenes

The properties of polycyclic aromatic hydrocarbons are determined by their size, shape, and functional groups. Equally important is their curvature, since deviation from planarity can affect their optical, electronic, and magnetic properties and also induce chirality. Acenes, which can be viewed as one-dimensional nanocarbons, are often twisted out of planarity. Although twisting is expected to affect the above-mentioned properties, it is often overlooked. This Account focuses on helically locked twistacenes (twisted acenes) having different twist angles and the effect of twisting on their electronic and optical properties. Various synthetic approaches to inducing backbone twist in acenes are discussed, with a focus on the introduction of a diagonal tether across the core, as this minimizes confounding substituent effects. Using such tethered acenes as our model, we then discuss the effects of twisting the aromatic core on twistacene properties. Electronic properties. Increasing the degree of twist only slightly affects the HOMO and LUMO energy levels. Twisting leads to a small increase in the HOMO level and a decrease in the LUMO level, which produces an overall decrease in the HOMO-LUMO gap. Optical properties. As the degree of twist increases, a slight bathochromic shift is observed in the absorption spectra, in accordance with the decrease in the HOMO-LUMO gap. The fluorescence quantum efficiency and the fluorescence lifetime also decrease. This is likely to be related to an increasing rate of intersystem crossing, which arises from increased spin-orbit coupling. In addition, computational studies indicate that the S⁰-T¹ energy gap decreases with increasing twist. Chiroptical properties. Increased twisting results in a larger Cotton effect and anisotropy factor, with the anisotropy factors of Ant-Cn being higher than those of longer helicenes. The parallel orientation of electric and magnetic transition dipole moments in twistacenes underlies this behavior and renders them as excellent chiroptical materials. The same trend is observed for the radical cations of twistacenes, which absorb in the NIR spectral region. Conjugation and delocalization. Twisting the anthracene radical cation up to 40° (13° per benzene ring) does not significantly affect spin delocalization, with the EPR spectra of twistacene radical cations showing that only slight localization occurs. This is in line with computational studies, which show only a small decrease in π-overlap for large acene twist. Overall, modifying the length of the tether in diagonally tethered acenes allows chemists to control core twist and to induce chirality. Twisting affects key optical, electronic, and chiroptical properties of acenes. Consequently, controlling the twist angle can improve the future design of nanocarbons with desired properties.

Molecular Ansa-Basket: Synthesis of Inherently Chiral All-Carbon [12](1,6)Pyrenophane

The synthesis of inherently chiral all-carbon C2-symmetric [12](1,6)pyrenophane 1 is reported. The cyclophane 1 was obtained via a ring-closing alkyne metathesis reaction using Mortreux's catalyst molybdenum hexacarbonyl and 2-fluorophenol as a phenol additive. The M and P enantiomers of the all-carbon pyrenophane 1 were demonstrated to be very stable in their enantiopure form even upon prolonged heating at 200 °C. [12](1,6)Pyrenophane-6-yne 1 was fully characterized by high-resolution mass spectrometry, nuclear magnetic resonance, UV-vis, and measured and calculated electronic circular dichroism spectroscopy.

Chiroptical Properties of Twisted Acenes: Experimental and Computational Study

Acenes that are twisted out of planarity are expected to display chiroptical properties. However, the effect of twisting on the chiroptical properties of acenes has not been investigated computationally or experimentally. Herein, we present a computational investigation of the chiroptical properties of anthracenes to pentacenes, combined with an experimental study using a series of helically locked acenes, twisted to different torsional angles in their enantiopure form. The lowest energy transition, which is relatively weak in acenes, becomes dominant in their circular dichroism spectra upon twisting. We find that the rotational strength of acenes consistently increases with increasing twist. The experimental data obtained from enantiopure tethered twistacenes show the same trend as the calculated result, with a strong Cotton effect and anisotropy factor, rendering twisted acenes as excellent chiroptical materials.

Helically Locked Tethered Twistacenes

Twisting linear acenes out of planarity affects their electronic and optical properties, and induces chirality. However, it is difficult to isolate the effect of twisting from the substituent effect. Moreover, many twistacenes (twisted acenes) readily racemize in solution. Here, we introduce a series of twistacenes having an anthracene backbone diagonally tethered by an n-alkyl bridge, which induces a twist of various angles. This allows us to systematically monitor the effect of twisting on electronic and optical properties. We find that absorption is bathochromically shifted with increasing twist, while fluorescence quantum efficiency drops dramatically. The tethered twistacenes were isolated to their enantiomerically pure form, displaying strong chiroptical properties and anisotropy factor ( g-value). No racemization was observed even upon prolonged heating, rendering these tethered twistacenes suitable as enantiopure helical building units for π-conjugated backbones.

Cyclophanes containing large polycyclic aromatic hydrocarbons

Cyclophanes have been firmly entrenched as a distinct class of compounds for well over half a century. The two main factors that have kept this field of chemistry going so strongly for such a long time are tremendous structural diversity and the interesting behaviour that is often observed. Although a very large number cyclophanes has been reported, only a very small proportion of them contain polycyclic aromatic systems that can be thought of as "large", i.e. with ≥4 rings. This Review puts the spotlight on such cyclophanes, illuminating both the chemistry that was used to synthesize them and what was learned from studying them. Context for the main body is provided by the careful consideration of the anatomy of a cyclophane and the classification of general synthetic approaches. The subsequent sections cover eleven different PAHs and are organized primarily according to increasing size of the aromatic system, starting with pyrene (C16, the only large polycyclic aromatic system to have been incorporated into numerous cyclophanes) and ending with hexabenzo[bc,ef,hi,kl,no,qr]coronene (C42).

Synthesis of macrocyclic receptors with intrinsic fluorescence featuring quinizarin moieties

An unprecedented class of macrocycles with intrinsic fluorescence consisting of phenolic trimers and quinizarin is developed. Though they are lacking strong hydrogen bonds as observed in calixarenes, the two examples introduced here each adopt a vase-like conformation with all four aromatic units pointing in one direction (syn orientation). This "cone" conformation has been confirmed by NMR spectroscopy, molecular modeling, and X-ray crystallography. The laminar, electron-rich fluorophore as part of the macrocycle allows additional contacts to enclosed guest molecules.

Synthesis of zirconium oxycarbide powders using metal–organic framework (MOF) compounds as precursors

ZrC_xO_y oxycarbides were synthesized for the first time by using metal–organic framework (MOF) compounds as precursors.