abstract
Graphene nanoribbons (NRs) constitute a versatile platform for developing novel materials, where their structure governs their optical, electronic, and magnetic properties while also shaping their excited-state dynamics. Here, we investigate a set of three twisted N-doped molecular NRs of increasing length, obtained by linearly fusing perylene diimide to pyrene and pyrazino- or thiadiazolo-quinoxaline residues. By employing various temperature-dependent time-resolved spectroscopy techniques, we reveal how the flexible twisted NR geometry promotes the formation of a mixed electronic state with varying contributions from locally excited and charge-transfer (CT) states. The fate of this mixed state is highly sensitive to the molecular geometry, length, and solvent polarity. For the shortest NR, intersystem crossing dominates the deactivation pathway, efficiently generating triplets in low-polarity solvents. In contrast, for the extended NRs, intramolecular singlet fission (SF) takes place within a single nanoribbon. This is enabled by enhanced superexchange coupling due to a pronounced push-pull nature and the existence of multiple localized pi-electron states caused by heteroatom doping, thereby circumventing the need for dimeric interactions typically associated with conventional SF systems. In higher-polarity environments, evidence of a (diabatic) CT state emerges. These findings underscore the intricate relationship between geometry, energy levels, and excited-state dynamics in twisted N-doped NRs.
keywords
TRIPLET EXCITED-STATE; ELECTRON-TRANSFER; BAND-GAP; FLUORESCENCE; SEPARATION; DESIGN; MODEL
subject category
Chemistry
authors
Greissel, PM; Beneventi, GM; Weiss, R; Wollny, AS; Dubey, RK; Melle-Franco, M; Clark, T; Mateo-Alonso, A; Guldi, DM
our authors
Manuel Melle-Franco
Principal Researcheracknowledgements
This work was carried out with support from the Basque Science Foundation for Science (Ikerbasque), POLYMAT, the University of the Basque Country, Fomento San Sebastian, Diputacion de Guipuzcoa, Gobierno Vasco (PIBA_2024_1_0030 and BERC programme) and Agencia Estatal de Investigacion (Projects PID2021-124484OB-I00, PCI2022-132921, CEX2020-001067-M and Mar & imath;a de Maeztu Excellence Unit CEX2023-001303-M funded by MCIN/AEI/10.13039/501100011033 and European Union NextGenerationEU/PRTR). This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 Research and Innovation Programme (Grant Agreement No. 722951). This Project has received funding from the European Union under the Horizon 2020 Research and Innovation Programme M-ERA.NET 2021 (SuperSuper). This work was funded by the European Union under the Horizon Europe grant 101046231 (FantastiCOF). Technical and human support provided by SGIker of UPV/EHU is acknowledged. Furthermore, D.M.G. acknowledges financial support from the Deutsche Forschungsgemeinschaft (DFG) under grant numbers GU 517/27-1 and GU 517/32-1.