Functionalisation of conjugated macrocycles with type I and II concealed antiaromaticity via cross-coupling reactions

Abstract

Conjugated macrocycles can exhibit concealed antiaromaticity; that is, despite not being antiaromatic, under specific circumstances, they can display properties typically observed in antiaromatic molecules due to their formal macrocyclic 4n π-electron system. Paracyclophanetetraene (PCT) and its derivatives are prime examples of macrocycles exhibiting this behaviour. In redox reactions and upon photoexcitation, they have been shown to behave like antiaromatic molecules (requiring type I and II concealed antiaromaticity, respectively), with such phenomena showing potential for use in battery electrode materials and other electronic applications. However, further exploration of PCTs has been hindered by the lack of halogenated molecular building blocks that would permit their integration into larger conjugated molecules by cross-coupling reactions. Here, we present two dibrominated PCTs, obtained as a mixture of regioisomers from a three-step synthesis, and demonstrate their functionalisation via Suzuki cross-coupling reactions. Optical, electrochemical, and theoretical studies reveal that aryl substituents can subtly tune the properties and behaviour of PCT, showing that this is a viable strategy in further exploring this promising class of materials.

Scheme 1. Redox reaction and aromaticity switching of paracyclophanetetraene (PCT; grey shadings indicate aromaticity). In the neutral state, the antiaromaticity of the formal macrocyclic 24 (4n) π-electron system (bold bonds) is concealed by the locally aromatic subunits, but the dianion can still become globally aromatic.

Scheme 2. Previous work: squarephaneic tetraanhydride as a building block for the synthesis of molecules with cross-conjugated imide groups between the PCT core and the aryl groups. This work: dibrominated PCTs as building blocks for the synthesis of molecules with linear conjugation between the PCT core and the aryl groups.

Scheme 3. Synthesis of dibrominated PCTs 3-cis and 3-trans, which were isolated as mixture 3. Reactants: N-bromosuccinimide (NBS), dibenzoyl peroxide (DBPO), diethylphosphite (DEP), N,N-diisopropylethylamine (DIPEA), triphenylphosphine (PPh₃), lithium methoxide (LiOMe).

Scheme 4. Suzuki cross-coupling reactions of the mixture of dibrominated PCT isomers 3 and the respective boronic acid pinacol esters (Aryl-BPin), yielding dithienyl-substituted PCT 4 and bis(bithienyl)-substituted PCT 5 as mixtures of isomers and bis(3-hexylthienyl)-substituted PCT 6 as separated isomers 6-cis and 6-trans following purification by recycling preparative GPC.

Fig. 1. ¹H NMR (CDCl₃, 400 MHz) spectra and molecular structures of 6-cis (top) and 6-trans (bottom).

Fig. 2. UV-vis absorption (solid lines) and photoluminescence spectra (dashed lines) of the macrocycles in CHCl₃ solution (approx. 5 and 10 μM). For recording the PL spectra, the macrocycles were excited at their absorption maximum wavelengths.

Fig. 3. Cyclic voltammograms of the macrocycles (a) in dimethylformamide (DMF) solution, and (b) in thin films dropcast onto the working electrode. The voltammograms were recorded using a glassy carbon working electrode, a platinum counter electrode, and a silver wire quasi-reference electrode (QRE) at a scan rate of 0.1 V s⁻¹. 0.1 M [n-Bu₄N]PF₆ solutions in either DMF (solution) or acetonitrile (thin film) were used as the supporting electrolyte.

Fig. 4. Analysis of the neutral and doubly charged states for 5-cis and 5-trans. Centre: VIST plots. Left and right: Dominant natural difference orbitals (NDOs) between the neutral states and the dications (blue/red NDOs for electron detachment) and the dianions (green/orange NDOs for attachment). See ESI Fig. S25 for the analogous plots of 4-cis and 4-trans.

Fig. 5. Analysis of the S₀, S₁, and T₁ states for 4-cis. Centre: VIST plots (using T₁ also at the S₁ geometry). Left and right: Dominant natural transition/difference orbitals (NTOs/NDOs) for the S₁ and T₁ states (blue/red for electron detachment; green/orange for attachment). See ESI Fig. S27–S29 for the analogous plots of 4-trans, 5-cis, and 5-trans.

Florian Glöcklhofer