Boosting the photocatalytic performance of the oligo (phenylene vinylene) moiety by copolymerization for the heterogeneous degradation of indigo carmine dye

The copolymerization with flexible and rigid spacers of 4,4’-((1E, 1′E)-(2,5-dimethoxy-1,4-phenylene)bis (ethene-2,1-diyl))dibenzoic acid, as a promising phenylene vinylene-based (PV) organic photocatalyst, was examined as a strategy to validate the effect of such chemical modifications in the modulation of the photocatalytic properties of this organic π-conjugated moiety. The polymers prepared here were fully characterized and evaluated as photocatalysts in the degradation of indigo carmine dye under two different irradiation scenarios (UVA and visible), consistently displaying a superior efficiency in contrast to their monomeric oligo (phenylene vinylene) (OPV) counterparts. Scavenging experiments confirmed that photocatalysis proceeds via the generation of superoxide radicals (O2 •–), singlet oxygen (1O2), and direct oxidation. The obtained results proved that the insertion of an aromatic rigid spacer not only prompts an enhancement in the photocatalytic activity of the phenylene vinylene-based polymers but also increases the stability of the OPV moiety by minimizing the reaction of the vinyl fragments with reactive oxygen species. This was demonstrated with the reuse experiments, where 96% of the photocatalytic activity was preserved throughout the first five reuse cycles.

Post-polymerisation approaches for the rapid modification of conjugated polymer properties

Post-polymerisation functionalisation provides a facile and efficient way for the introduction of functional groups on the backbone of conjugated polymers. Using post-polymerisation functionalisation approaches, the polymer chain length is usually not affected, meaning that the resulting polymers only differ in their attached functional groups or side chains, which makes them particularly interesting for investigating the influence of the different groups on the polymer properties. For such functionalisations, highly efficient and selective reactions are needed to avoid the formation of complex mixtures or permanent defects in the polymer backbone. A variety of suitable synthetic approaches and reactions that fulfil these criteria have been identified and reported. In this review, a thorough overview is given of the post-polymerisation functionalisations reported to date, with the methods grouped based on the type of reaction used: cycloaddition, oxidation/reduction, nucleophilic aromatic substitution, or halogenation and subsequent cross-coupling reaction. Instead of modifications on the aliphatic side chains of the conjugated polymers, we focus on modifications directly on the conjugated backbones, as these have the most pronounced effect on the optical and electronic properties. Some of the discussed materials have been used in applications, ranging from solar cells to bioelectronics. By providing an overview of this versatile and expanding field for the first time, we showcase post-polymerisation functionalisation as an exciting pathway for the creation of new conjugated materials for a range of applications.

[2.2.2.2]Paracyclophanetetraenes (PCTs): cyclic structural analogues of poly( p‑phenylene vinylene)s (PPVs)

Background: Poly( p-phenylene vinylene)s ( PPVs) and [2.2.2.2]paracyclophanetetraene ( PCT) are both composed of alternating π-conjugated para-phenylene and vinylene units. However, while the former constitute a class of π-conjugated polymers that has been used in organic electronics for decades, the latter is a macrocycle that only recently revealed its potential for applications such as organic battery electrodes. The cyclic structure endows PCT with unusual properties, and further tuning of these may be required for specific applications. Methods: In this article, we adopt an approach often used for tuning the properties of PPVs, the introduction of alkoxy (or alkylthio) substituents at the phenylene units, for tuning the optoelectronic properties of PCT. The resulting methoxy- and methylthio-substituted PCTs, obtained by Wittig cyclisation reactions, are studied by UV-vis absorption, photoluminescence, and cyclic voltammetry measurements, and investigated computationally using the visualisation of chemical shielding tensors (VIST) method. Results: The measurements show that substitution leads to slight changes in terms of absorption/emission energies and redox potentials while having a pronounced effect on the photoluminescence intensity. The computations show the effect of the substituents on the ring currents and chemical shielding and on the associated local and global (anti)aromaticity of the macrocycles, highlighting the interplay of local and global aromaticity in various electronic states. Conclusions: The study offers interesting insights into the tuneability of the properties of this versatile class of π-conjugated macrocycles.