Publication date: 11th July 2022
We attained the synthesis of mesoscale ordered 2D polymers by the topochemical on-surface photopolymerization of fluorinated anthracene-triptycene (fantrip) monomers. The underlying protocol is two-staged: (1) Self-assembly of the monomers into a photopolymerizable monolayer structure, where the photoactive anthracene moieties are face-to-face stacked; (2) cross-linking of the self-assembled monolayer into a covalent 2D polymer by photochemically excited [4+4] cycloadditions between the antiparallel aligned anthracene blades. Thereby, the long-range order attained in (1) is transferred into the covalent state. Yet, the topochemical approach crucially depends on achieving the reactive packing with the appropriate mutual alignment of the anthracene blades. For the self-assembly the underlying surface plays a decisive role. We used graphite substrates, but additional passivation with an alkane monolayer was necessary to weaken molecule-surface interactions. As a result, the desired monolayer that is determined by molecule-molecule interactions became thermodynamically favored. STM has proven as ideal analytical tool for monitoring intermediate and final structures with the ultimate single-linkage resolution. The [4+4] cycloadditions induce a sizable increase of the HOMO-LUMO gap, which translates into an unambiguous change of STM contrast. This possibility to identify individual newly formed covalent linkages facilitated studies of the polymerization progression and allowed to assess the temperature dependence of polymerization rates, where an increase with temperature indicated a small energy barrier in the photoexcited state. In addition, successful photopolymerization could be corroborated by complementary local IR spectroscopy.
The Nanosystems-Initiative-Munich (M.L. and W.M.H.) Cluster of Excellence and the Deutsche Forschungsgemeinschaft (grant no. LA 1842/9-1, M.L.) are gratefully acknowledged for funding.
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