Triangulene, the smallest triplet ground state polybenzenoid (also known as Clar’s hydrocarbon), has been an enigmatic molecule ever since its existence was first hypothesized [1] . Despite containing an even number of carbons (22, in 6 fused benzene rings), it is not possible to draw Kekulé-style resonant structures for the whole molecule: any attempt results in two unpaired valence electrons [2]. Synthesis and characterization of unsubstituted triangulene have not been achieved because of its extreme reactivity [1], although the addition of substituents has allowed the stabilization and synthesis of the triangulene core [3, 4] and verification of the triplet ground state via electron paramagnetic resonance measurements [5]. Here we show the on-surface generation of unsubstituted triangulene consisting of six fused benzene rings. The tip of a combined scanning tunnelling and atomic force microscope (STM/AFM) was used to dehydrogenate precursor molecules. STM measurements in combination with density functional theory (DFT) calculations confirmed that triangulene keeps its free molecule properties on the surface, whereas AFM measurements resolved its planar, threefold symmetric, molecular structure. The unique topology of such non-Kekulé hydrocarbons results in open-shell pi-conjugated graphene fragments [6] which give rise to high-spin ground states potentially useful in organic spintronic devices [7, 8]. Our generation method renders manifold experiments possible to investigate triangulene and related open-shell fragments at the single-molecule level.
REFERENCES:
1. Clar, E. & Stewart, D. G., J. Am. Chem. Soc. 75, 2667 (1953).
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By: Niko Pavlicek, Anish Mistry, Zsolt Majzik, Nikolaj Moll, Gerhard Meyer, David J. Fox, Leo Gross
Published in: RZ3915 in 2016
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