From the Decks to the Bridges: Optoelectronics in [2.2]Paracyclophane Chemistry

Bent and Twisted: Synthesis of an Alkoxy-Substituted (1,5)Naphthalene-paracyclophanediene

This contribution describes the synthesis of [2.2](1,5)naphthalenoparacyclophane-1,13-diene in four steps from 1,5-bis(bromomethyl)naphthalene and 1,4-benzenedimethanethiol. Consisting of 2,6-dioctyloxynaphthalene and benzene moieties, the effects of differing arene size on the structure, strain energy, and chemical reactivity of the cyclophanediene are examined. Despite a strain energy of 24.3 kcal/mol, the naphthalenoparacyclophanediene was unreactive toward a library of olefin metathesis catalysts. This diminished reactivity can be explained by the steric hindrance of the twisted olefin. Incorporation of an electron donor (naphthalene) into the rigid paracyclophanediene structure can allow for applications in optoelectronics, chiral ligands, and planar chiral materials.

Nanosized Carbon Macrocycles Based on a Planar Chiral Pseudo Meta- [2.2]Paracyclophane

Structural designs combining cycloparaphenylenes (CPPs) backbone with planar chiral [2.2]Paracyclophane ([2.2]PCP) lead to optical-active chiral macrocycles with intriguing properties. X-ray crystal analysis revealed aesthetic necklace-shaped structures and size-dependent packages with long-range channels. The macrocycles exhibit unique photophysical properties with high fluorescence quantum yield of up to 82%, and the fluorescent color varies with ring size. In addition, size-dependent chiroptical properties with moderately large CPL dissymmetry factor of 10 -3 and CPL brightness in the range of 30 - 40 M -1 cm -1 were observed.

Planar Chirality: A mine for catalysis and structure discovery

Planar chirality is one of the most fascinating expressions of chirality, exploited by Nature to lock three-dimensional chiral conformations and, more recently, by chemists to create new chiral reagents, catalysts and functional organic materials. Nevertheless, the shortage of protocols able to induce and secure asymmetry during the generation of these unique chiral entities has dissuaded chemists to exploit their structural properties. This Minireview intends to illustrate the limited but remarkable catalytic methodologies reported for the production of planar chirality in strained molecules and serve as source of inspiration for the development of new unconventional protocols that are expected to come in the near future.

A Luminescent 1D Silver Polymer Containing [2.2]Paracyclophane Ligands

[2.2]Paracyclophane scaffolds have seen limited use as building blocks in supramolecular chemistry. Here, we report the synthesis and characterization of a 1D coordination polymer consisting of silver(I) ions bound to a [2.2]paracyclophane scaffold functionalized with two 4-pyridyl units. The structure of the polymer has been determined from single crystal X-ray diffraction analysis and reveals two different silver coordination motifs that alternate along the 1D coordination polymer. The coordination polymer exhibits strong blue and sky-blue fluorescence in solution and in the crystalline solid state, respectively.

New [2,2]Fluorenophanes Give Insights into Asymmetric Charge Transfer-Mediated Exciton Delocalization along the π-π Packing Direction

A series of new [2,2]fluorenophanes has been synthesized and characterized; among them, molecules of crystallographically asymmetric anti-[2.2](1,4)(4,1)fluorenophane (K2C-2) aggregate to form one-dimensional supramolecular chain structures through effective intermolecular π-π overlapping. This, in combination with the synergistic intramolecular π-π interaction, leads to prominent dual emission mediated by charge transfer (CT) exciton delocalization. Support of this new insight is given by mapping the transition density along the π-π packing direction where the intramolecular excitation and intermolecular CT coexist in K2C-2.

Chiral isothiourea-catalyzed kinetic resolution of 4-hydroxy[2.2]paracyclophane

We herein report a method for the kinetic resolution of racemic 4-hydroxy[2.2]paracyclophane by means of a chiral isothiourea-catalyzed acylation with isobutyric anhydride. This protocol allows for a reasonable synthetically useful s-factor of 20 and provides a novel entry to obtain this interesting planar chiral motive in an enantioenriched manner.

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.

A Pd3L6 supramolecular cage incorporating photoactive [2.2]paracyclophane units

In this work we report two photoactive [2.2]paracyclophane (pCp) ligands and a luminescent Pd₃L₆ cage incorporating these ligands, which exhibits blue emission both in solution and in the powder state.

Through-space π-delocalization in a conjugated macrocycle consisting of [2.2]paracyclophane

Herein, we report the synthesis and characterization of a [2.2]paracyclophane-containing macrocycle (PCMC) as a new through-space conjugated macrocycle using only benzene groups as the skeleton. For comparison, a diphenylmethane-containing nanohoop macrocycle (DCMC) with a non-conjugated linker was also synthesized. Their structures were confirmed by NMR and HR-MS, and their photophysical properties were studied by UV-vis and fluorescence spectroscopies combined with theoretical calculations. The strain energy of PCMC was estimated to be as high as 72.58 kcal mol-1.

Indeno[1,2-b]fluorene-Based [2,2]Cyclophanes with 4n/4n and 4n/[4n+2] π Electrons: Syntheses, Structural Analyses, and Excitonic Coupling Properties

Indeno[1,2-b]fluorene-based [2,2]cyclophanes with 4n/4n and 4n/[4n+2] π-electron systems were prepared, and their structures were identified by X-ray crystallography. With short π-π distances around 3.0 Å, [2.2](5,11)indeno[1,2-b]fluorenophane and its precursor [2.2](5,11)indeno[1,2-b]fluorene-6,12-dionophane exhibit remarkable transannular interactions, leading to their unusual electrochemical and photophysical properties. With the aid of femtosecond transient absorption spectroscopy, the transition from the monomeric excited state to the redshifted H-type dimeric state was first observed, correlating to the calculated excitonic energy splitting and the steady-state absorption spectra induced by charge-transfer-mediated superexchange interaction.

Synthesis, aggregation induced emission and through space conjugation of triphenylvinylphenyl substituted [2.2]paracyclophane-1,9-diene

4-Bromo substituted [2.2]paracyclophane-1,9-diene was synthesized from the corresponding dithia[3.3]paracyclophane in three steps through benzyne Steven rearrangement, oxidation, and a thermal elimination reaction. 4-Triphenylvinylphenyl substituted [2.2]paracyclophane-1,9-diene was successfully prepared by the Suzuki-Miyaura cross-coupling reaction of 4-bromo substituted [2.2]paracyclophane-1,9-diene and 4,4,5,5-tetramethyl-2-(4-(1,2,2-triphenylvinyl)phenyl)-1,3,2-dioxaborolane using Pd(OAc)2 as a catalyst, S-Phos as a ligand and K3PO4 as a base. The structures of bromo substituted [2.2] paracyclophane-1,9-diene and triphenylvinylphenyl substituted [2.2]paracyclophane-1,9-diene were fully characterized by 1H NMR spectroscopy and X-ray crystallography. 4-Triphenylvinylphenyl substituted [2.2]paracyclophane-1,9-diene exhibited aggregation-induced emission characteristics when the water fraction was higher than 80% in the THF/water mixtures. 4-Triphenylvinylphenyl substituted [2.2]paracyclophane-1,9-diene displays much higher fluorescence when the water fraction is 90% compared to that of model compounds due to both through bond and through space conjugation. To the best for our knowledge, we are the first to synthesize triphenylvinylphenyl substituted [2.2]paracyclophane-1,9-diene with aggregation-induced emission characteristics.

Remarkable influence of 'phane effect' on the excited-state properties of cofacially oriented coumarins

A comprehensive investigation of the photophysics of a cofacially oriented bis-coumarin based on naphthalene, i.e., Cou-Nap, designed and synthesized to examine the influence of π-electronic communication between the two fluorophores, reveals exceptional excited-state properties. While the anticipated [2 + 2] photocycloaddition is not observed despite the fact that the two reactive coumarin units are at a distance of 3.8 Å, the fluorescence quantum yields and singlet lifetimes in different solvents are found to be remarkably higher when compared to those of the parent coumarin and a mono-coumarin model system, i.e., Cou-Dur. In addition to large solvent-induced Stokes shifts, Cou-Nap displays intriguing temperature-dependent emission in a nonpolar solvent such as cyclohexane. The observed photophysical properties are reconciled based on the so-called 'phane effect' that is operative in cyclophanes. In the latter, an effective π-π interaction between the aromatic rings modifies the attributes of the chromophore in such a manner that the observed properties cannot be associated with the individual aromatic rings. The temperature-dependent emission is proposed to arise as a consequence of thermally activated ISC from the singlet-excited state to one of the higher energy triplet states. The results constitute, for the first time, the demonstration of modification of the excited-state properties of a fluorophore in a non-cyclophane system by 'phane effect'.

Controllable Fluorescence Switching of a Coordination Chain Based on the Photoinduced Single-Crystal-to-Single-Crystal Reversible Transformation of a syn-[2.2]Metacyclophane

The observation of a reversible chemical transformation corresponding to an external stimulus in the solid state is intriguing in the exploration of smart materials, which can potentially be applied in molecular machines, molecular switches, sensors, and data storage devices. The solid-state photodimerization reaction of 1,3-bis[2-(4-pyridyl)ethenyl]benzene (1,3-bpeb) in a one-dimensional coordination polymer {[Cd₂(1,3-bpeb)₂(4-FBA)₄]·H₂O}_n (4-FBA = 4-fluorobenzoate) with 365 nm UV light afforded syn-tetrakis(4-pyridyl)-1,2,9,10-diethano[2.2]metacyclophane (syn-tpmcp) in quantitative yield via a single-crystal-to-single-crystal (SCSC) transformation. Upon irradiation with 254 nm UV light, an SCSC conversion from syn-tpmcp to 1,3-bpeb was also achieved in quantitative yield within the syn-tpmcp-supported coordination polymer {[Cd₂(syn-tpmcp)(4-FBA)₄]·H₂O}_n. In particular, accompanied by the reversible transformation between 1,3-bpeb and syn-tpmcp, the coordination chain exhibits photocontrollable fluorescence-switching behavior, which makes this intelligent material an appealing candidate for practical applications.

(Deep) blue through-space conjugated TADF emitters based on [2.2]paracyclophanes

The first examples of through-space conjugated thermally activated delayed fluorescence (TADF) emitters based on a [2.2]paracyclophane (PCP) skeleton with stacked (coplanar) donor–acceptor groups have been synthesized.

Metal-Organic Coordination versus Hydrogen Bonding: Highly Efficient Templated Photocycloadditions of Trisubstituted Isomeric Olefins in the Solid State

Trisubstituted olefins are reactants in template-directed, solid-state [2+2] photocycloaddition reactions. Whereas hydrogen-bond-based templates afford crystalline assemblies with olefins that are photostable, Ag^I coordination results in carbon-carbon double bonds that react stereoselectively and result in quantitative yield.

Redox-Responsive Viologen-Mediated Self-Assembly of CB[7]-Modified Patchy Particles

Sulfonated surface patches of poly(styrene)-based colloidal particles (CPs) were functionalized with cucurbit[7]uril (CB[7]). The macrocycles served as recognition units for diphenyl viologen (DPV(2+)), a rigid bridging ligand. The addition of DPV(2+) to aqueous suspensions of the particles triggered the self-assembly of short linear and branched chainlike structures. The self-assembly mechanism is based on hydrophobic/ion-charge interactions that are established between DPV(2+) and surface-adsorbed CB[7]. DPV(2+) guides the self-assembly of the CPs by forming a ternary DPV(2+)⊂(CB[7])2 complex in which the two CB[7] macrocycles are attached to two different particles. Viologen-driven particle assembly was found to be both directional and reversible. Whereas sodium chloride triggers irreversible particle disassembly, the one-electron reduction of DPV(2+) with sodium dithionite causes disassembly that can be reversed via air oxidation. Thus, this bottom-up synthetic supramolecular approach allowed for the reversible formation and directional alignment of a 2D colloidal material.

Charge density investigations on [2,2]-paracyclophane – in data we trust

Four datasets on [2,2]-paracyclophane were collected in-house and at the Advanced Photon Source at two different temperatures for charge density investigation. Global data quality indicators such as high resolution, high I/σ(I) values, low merging R values and high multiplicity were matched for all four datasets. The structural parameters did not show significant differences, but the synchrotron data depicted deficiencies in the topological analysis. In retrospect these deficiencies could be assigned to the low quality of the innermost data, which could have been identified by e.g. merging R values for only these reflections. In the multipole refinement these deficiencies could be monitored using DRK-plot and residual density analysis. In this particular example the differences in the topological parameters were relatively small but significant.

13C NMR spectroscopy for the quantitative determination of compound ratios and polymer end groups

(13)C NMR spectroscopic integration employing short relaxation delays was evaluated as a quantitative tool to obtain ratios of diastereomers, regioisomers, constitutional isomers, mixtures of unrelated compounds, peptoids, and sugars. The results were compared to established quantitative methods such as (1)H NMR spectroscopic integration, gas chromatography, and high-performance liquid chromatography and were found to be within <3.4% of (1)H NMR spectroscopic values (most examples give results within <2%). Acquisition of the spectra took 2-30 min on as little as 10 mg of sample, proving the general utility of the technique. The simple protocol was extended to include end group analysis of low molecular weight polymers, which afforded results in accordance with (1)H NMR spectroscopy and matrix-assisted laser desorption-ionization time-of-flight spectrometry.

Influence of the anion nature on styryl dye crystal packing and feasibility of the direct and back [2 + 2] photocycloaddition reactions without single crystal degradation

By X-ray diffractometry, [2 + 2]-photocycloaddition reaction and its back reaction were investigated.