Persistent peri-Heptacene: Synthesis and In-Situ Characteriza­tion

The synthesis of the longest peri-acene, i.e., peri-Heptacene (7-PA) is accomplished by the researchers from the group of Prof. Xinliang Feng in collaboration with Politecnico di Milano, Leibniz Institute for Solid State and Materials Research, and Leibniz-Institut for Polymerforschung Dresden e. V. The work has recently reported as a communication “Persistent peri-Heptacene: Synthesis and In-Situ Characterization” in Angewandte Chemie International Edition. n-peri-Acenes (n-PAs) have gained enormous interest as fundamental model systems of zigzag-edged graphene nanoribbons for the potential applications in synthetic carbon nanoelectronics and spintronics. However, the synthesis of n-PAs larger than peri-tetracene (4-PA) remains challenging because of their intrinsic open-shell character and high reactivity. In this communication, the authors reported the synthesis of the hitherto unknown 7-PA, by a rational molecular design strategy, in which the reactive zigzag edges are kinetically protected with eight 4-tert-butylphenyl groups. The successful formation of 7-PA from its tetrahydro-precursor 3 was clearly validated by high-resolution mass spectrometry and in-situ FT-Raman spectroscopy. The resultant 7-PA displays a narrow optical energy gap of 1.01 eV and exhibits a persistent stability (half-life, t_1/2 ~ 25 min) under inert conditions. Moreover, electron spin resonance (ESR) measurements and theoretical studies reveal that 7-PA exhibits an open-shell feature and a significant tetraradical character (y₀ = 1.0 and y₁ = 0.2). This synthetic strategy could be considered as a modular approach for the construction of next generation (3N+1)-PAs (where N = 3, 4, etc.).

Reference: M. R. Ajayakumar, J. Ma, A. Lucotti, K. S. Schellhammer, G. Serra, E. Dmitrieva, M. Rosenkranz, H. Komber, J. Liu, F. Ortmann, M. Tommasini, and Xinliang Feng, Angew. Chem. Int. Ed. 2021, https://doi.org/10.1002/anie.202102757

Acknowledgments:  This research was financially supported by the EU Graphene Flagship (Graphene Core 3, 881603), ERC Consolidator Grant (T2DCP, 819698), the Center for Advancing Electronics Dresden (cfaed) and DFG-NSFC Joint Sino-German Research Project (EnhanceNano, No. 391979941), as well as the DFG-SNSF Joint Switzerland-German Research Project (EnhanTopo, No. 429265950). We thank Mr. Federico Lombardi and Prof. Dr. Lapo Bogani for helpful discussions. The authors acknowledge the use of computational facilities at the Center for information services and high performance computing (ZIH) at TU Dresden. J. Liu is grateful for the startup funding from The University of Hong Kong. G.S. and M.T acknowledge funding by the Italian Ministry of Education, Universities and Research (MIUR) through the PRIN 2017 program (Project No. 2017PJ5XXX “MAGIC DUST”).