OMARG · Photoactive napthalenediimide pi-stacking architectures - Versatile building blocks for zipper assembly of cascade redox gradients

Photoactive napthalenediimide pi-stacking architectures - versatile building blocks for zipper assembly of cascade redox gradients

The general objective of this project was to find general methods for facile access to multicomponent architectures on solid surfaces (e.g. OMARG supramolecular heterojunctions (hSHJs)) that contain, for the first time in the host group, oligothiophenes. OMARG-SHJs refer to architectures that contain co-axial channels for the transport of electrons and holes in opposite directions along antiparallel redox gradients. Applying lessons from biological photosystems, this innovative concept aims to minimise the recombination of charges generated with light; that is losses in photonic energy. To preserve the directionality needed to place antiparallel gradients, it was clear from the beginning that we have to learn how to grow multicomponent architectures directly on oxide surfaces. With the introduction of methods such as zipper assembly or self-organising surface-initiated polymerisation (SOSIP), the host group has developed appreciable expertise to achieve this challenging task. These methods have been developed mainly with the most convenient naphthalenediimide (NDI) stacks for charge transport. More common but less well-behaved components, e.g. oligothiophenes, have not been explored so far in the host group. However, oligothiophenes are particularly interesting in this context because their structures are exceptionally variable, ideal for the fine-tuning of multcomponent charge cascades. With a Doctor of Philosophy (PhD) degree on the topic, the researcher was perfectly prepared to develop routes to oriented double-channel architectures with oligothiophenes. Upon arrival in the host group, the researcher initially had to familiarise herself with the methods being developed. Zipper assembly was explored first with a study on covalent capture of supramolecular architectures. This study, published in Energy Environ. Sci. (IF 9.6), suggested that zipper assembly, although fascinating as high precision approach, will be synthetically too demanding to achieve important objectives within reasonable time. SOSIP introduced as user-friendly low-cost alternative to zipper assembly, was tested next. Contributing to an ongoing study on self-sorting during co-SOSIP, a rational approach to alternate donor-acceptor architectures was found (published in JACS, IF 9.9). This fruitful experience with SOSIP suggested that this method would be ideal to build double-channel architectures with oligothiophenes. To create SOSIP architectures with oligothiophenes, initiator 1 and propagator 2 were synthesised. They both contain a quaterthiophene that is embedded in a peptide-like hydrogen-bonded network to assure self-organisation of the system. The initiator further contains two NDI templates and four diphosphonate feet to bind to an indium tin oxide surface. Thiolates are produced on the surface by disulfide reduction with DTT. These thiolates then react with the strained disulfides in the propagator to initiate ring-opening disulfide exchange polymerisation. The resulting photosystem 3 with oriented, hole-transporting oligothiophene stacks is almost inactive. To add a co-axial stack for the transport of electrons, the benzaldehyde in 3 was removed with hydroxylamine. The hydrazide-rich pores drilled into photosystem 4 were filled with aldehydes of free choice. According to the respective absorption maxima of the resulting double-channel photosystems 5, this post-SOSIP stack exchange worked quantitatively for NDIs 6 and 7. Broadened stacks with core-expanded NDI 8 (a very promising new electron transporter from China) were not well tolerated, whereas the elongated PDI 9 (a very popular component) was very well accepted. Compared to photosystem 3, activities increased up to 64-times (with red NDI stacks from 7). These results are very important. They report the first oriented double-channel SOSIP architectures with oligothiophenes. Compatibility of this approach with OMARG-SHJs has already been demonstrated with NDIs. The variability (and very powerful properties) of oligothiophenes now offer many, exceptionally promising perspectives to refine the architectures (terminal substituents, lateral substituents (EDOTs, etc.), sulphur replacements, oligomer elongation, mixed oligomers, and so on). A first manuscript on double-channel photosystems with oligothiophenes, i.e. the general objective of this project, has been submitted to Angew. Chem. (IF 13.5).

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