The work performed from the beginning of the project quickly demonstrated that N-unsubstituted DPPs are versatile functional monomers that can be polymerised/copolymerised (oxidative coupling) directly into thin film form via oxidative chemical vapour deposition (oCVD). Initial experiments were conducted using commercially available 3,6-di(2-thienyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (TDPP). TDPP was selected as "model compound" due to the presence of thienyl moieties, which are commonly used in oCVD. Interestingly, grafting of the thienyl group directly to the electron-deficient DPP core did not significantly decrease its electron-rich character, which is crucial for conducting the intermolecular dehydrogenative coupling reaction, and TDPP readily polymerised in oCVD using FeCl3 as oxidant. The influence of deposition conditions (substrate, temperature, pressure, etc.) on the properties of the resulting thin films were studied.
To further investigate the formation of higher oligomeric species, experiments were conducted with alkylated TDPP, i.e. 2,5-bis(2-ethylhexyl)-3,6-di(2-thienyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (BEHTDPP). Following previous solution-based studies, grafting long alkyl chains to the axial positions of the TDPP motif enhanced the solubility of the formed polymeric chains by reducing π-π intermolecular interactions between aromatic systems and preventing hydrogen bonding between amide functional groups. This modification enabled the analysis of formed polymers using GPC (solution-based analysis). The GPC spectrum only displayed a broad peak indicating masses up to 32,000 g·mol-1. Given the similarities in the UV/Vis/NIR and LDI-HRMS spectra of polymerized TDPP and BEHTDPP, it can be assumed that the product of oCVD on TDPP materials also consists of polymeric chains with molecular weights in the range of tens of thousands g·mol-1.
To gain a deeper understanding of the potential of the selected oCVD technique, further studies were conducted on other organic dyes. The strong electron-deficient character of the DPP core hindered polymerization of most DPP derivatives under oCVD conditions. Nevertheless, DPP derivatives substituted with strong electro-donating groups, such as thienyl groups (TDPP and BEHTDPP), were readily polymerised via oCVD and their photo-electronic and photo-electro-catalytic were thoroughly studied. Noteworthy, two-photon photoelectron spectroscopy evidenced that the DPP-based conjugated polymers bearing no N-alkyl substituent exhibits a systematic increase of all lifetimes compared to N-alkylated DPP-based conjugated polymers, notably an increase in the fraction of the very long-lived excited species. The ability to synthesise N-alkylated DPP-based conjugated polymer thin films (only possible using the oCVD approach) and the demonstration of their superior photo-electronic properties is of high interest for photovoltaic applications. Besides, the photo-electronic and photo-electro-catalytic of the conjugated copolymers derived other dyes show remarkable photoactivity.
The results of the TODAM project were already partly disclosed as an open access peer-reviewed research article (M. K. Charyton, T. Reiker, K. Kotwica, M. Góra, H. Zacharias N. D. Boscher Mater. Adv., 2023,
https://doi.org/10.1039/D3MA00197K(öffnet in neuem Fenster)) and via an oral contribution at an international conference (AVS 68th International Symposium & Exhibition 6-11/11/2022 Pittsburgh, USA). Additional open access peer-reviewed publications are currently under preparation. Besides, in the framework of TODAM, the ER and the supervisor established collaboration with multiple universities from Poland, Germany, Austria and Luxembourg and prepared follow-up research proposal applications.
