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On-surface photopolymerization of two-dimensional polymers ordered on the mesoscale

Abstract

The use of solid supports and ultra-high vacuum conditions for the synthesis of two-dimensional polymers is attractive, as it can enable thorough characterization, often with submolecular resolution, and prevent contamination. However, most on-surface polymerizations are thermally activated, which often leads to high defect densities and relatively small domain sizes. Here, we have obtained a porous two-dimensional polymer that is ordered on the mesoscale by the two-staged topochemical photopolymerization of fluorinated anthracene triptycene (fantrip) monomers on alkane-passivated graphite surfaces under ultra-high vacuum. First, the fantrip monomers self-assemble into highly ordered monolayer structures, where all anthracene moieties adopt a suitable arrangement for photopolymerization. Irradiation with violet light then induces complete covalent crosslinking by [4+4] photocycloaddition to form a two-dimensional polymer, while fully preserving the long-range order of the self-assembled structure. The extent of the polymerization is confirmed by local infrared spectroscopy and scanning tunnelling microscopy characterization, in agreement with density functional theory calculations, which also gives mechanistic insights.

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Fig. 1: Fantrip structures.
Fig. 2: Characterization of the fantrip self-assembly by STM imaging.
Fig. 3: Polymerization propagation.
Fig. 4: Polymerization-induced changes.
Fig. 5: Fully polymerized 2DP.
Fig. 6: Nano-FTIR of supramolecular networks versus 2DP.

Data availability

Data supporting the findings of this study are available within the paper, Supplementary Information and data files. Data for Supplementary Figs. 18 and 19 and DFT-calculated structures are provided as Supplementary Data files. Source data are provided with this paper.

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Acknowledgements

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. J.B. and J.R. acknowledge funding from the Swedish Research Council and the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU no. 2009 00971, received by Linköping University). J.R. also acknowledges funding from the Knut and Alice Wallenberg Foundation. Computational resources were allocated by the Swedish National Infrastructure for Computing and carried out at the National Supercomputer Centre, Sweden.

Author information

Authors and Affiliations

Authors

Contributions

M.L., L.G. and W.M.H. conceived and designed the experiments. B.T.K. designed, synthesized and purified the fantrip monomer. L.G. developed the sample preparation, carried out all STM experiments and analysed the data. S.R. performed the ToF-SIMS experiments and analysed the data. N.H. performed the nano-FTIR experiments and analysed the data. J.B. and J.R. conducted and analysed the DFT calculations. M.L. co-wrote the manuscript, with contributions from all authors.

Corresponding author

Correspondence to Markus Lackinger.

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The authors declare no competing interests.

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Peer review information Nature Chemistry thanks the anonymous reviewers for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Fantrip adsorption.

DFT-optimized structures of single fantrip molecules with their anthracene blades adsorbed a, on edge (−1.11 eV) vs. b, parallel (−1.89 eV) on pristine bilayer graphene; as well as c, on edge (−0.93 eV) vs. d, parallel (−1.48 eV) on alkane-passivated bilayer graphene. Top and side views are shown in the upper and lower row, respectively. Adsorption energies with respect to single geometry-optimized fantrip molecules in vacuum are stated in parenthesis. The hexacosane monolayer was modelled as densely packed monolayer of infinite polyethylene strands aligned along a graphite high symmetry direction as experimentally observed for hexacosane (Supplementary Fig. 2). Atom colour code: F, green; C, grey; H, light grey.

Extended Data Fig. 2 Thermal stability illuminated versus non-illuminated.

First, fantrip was deposited onto hexacosane-passivated graphite and heated at 80 °C to afford self-assembly of exclusively the hexagonal structure. a, STM image of the as prepared sample, that is without further illumination. b, STM image of a sample prepared as in a, but illuminated with laser light to afford a fully cross-linked 2DP. Then both samples were heated to 150 °C for 20 min under identical conditions. c, Line-profile as indicated in b. d, STM image of the non-illuminated sample acquired after heating. The original hexagonal self-assembly was converted into the densely packed fantrip structure as observed for direct deposition onto graphite (Fig. 2a of main manuscript and Supplementary Fig. 1). Even though the ordering has improved, a unit cell can still not be assigned. The conversion is explained by reorganization of fantrip as a consequence of desorption of the hexacosane buffer layer. e, STM image of the illuminated sample acquired after heating, confirming the continued existence of the 2DP. f, Line-profile as indicated in e. The hexacosane monolayer was mostly desorbed, but remnants are still visible at the 2DP boundary. This allows measuring apparent heights of the 2DP of 0.52 nm with regards to graphite and of 0.20 nm with regards to hexacosane, suggesting that the hexacosane monolayer may still be present underneath the 2DP. (Tunneling parameters and scale bars: a −3.0 V, 3 pA, 10 nm; b −3.0 V, 4 pA, 30 nm; d −3.3 V, 3 pA, 10 nm; e -3.0 V, 4 pA, 40 nm).

Extended Data Fig. 3 Photopolymerization yield versus temperature.

To evaluate the temperature dependence of the photopolymerization yield, supramolecular fantrip structures on hexacosane-passivated graphite were illuminated with laser light for a similar duration of 4 h while maintaining constant temperatures of the sample environment in a range between 280 K and 310 K. To this end, samples were loaded into the STM for illumination. Constant temperatures were maintained for the cryostat to which the instrument is mounted by simultaneously cooling with liquid nitrogen and feedback controlled counter heating. Accordingly, stated temperatures do not directly correspond to surface temperatures, which lie well above. Representative STM images acquired for cryostat temperatures during illumination of 280 K (a), 290 K (b), 300 K (c) and 310 K (d). e, Plot of the polymerized fraction as obtained from statistical analysis of STM data vs. cryostat temperature. Red dots represent data points corresponding to the mean value obtained from four 25 × 25 nm² STM images; error bars denote standard deviations of the experimental means; the blue line serves as guide to the eye. The graph corroborates increasing degrees of polymerization for increasing surface temperatures around room temperature. (Tunneling parameters: a −3.0 V, 3 pA; b -2.8 V, 3 pA; c -3.0 V, 3 pA; d -2.8 V, 3 pA; all scale bars: 20 nm).

Source data

Extended Data Fig. 4 Unpolymerized linkages within 2DP.

a, Overview STM image of a nearly completely polymerized sample, showing two distinct types of bright features on top; Features with lower apparent height (red circle) are more abundant. These appear with fairly uniform contrast, whereas the higher appearing features (green circle) show a broader size and shape variation. We tentatively attribute the latter to fantrip molecules that became relocated onto the 2DP during the polymerization. The size and shape variability suggests that these features could be comprised of more than one fantrip monomer. b Close-up STM image of a 2DP, where covalent linkages appear as round protrusions. Several brighter appearing features are accumulated in the lower right corner that correspond to the feature highlighted by the red circle in (a). Both their positioning at the network vertices and their uniform internal contrast suggest that these features correspond to non-polymerized fantrip monomers or individual anthracene blades of monomers within the network. (Tunneling parameters and scale bars: a -2.8 V, 3 pA, 10 nm; b -2.8 V, 3 pA, 3 nm).

Extended Data Fig. 5 AFM images for nano-FTIR.

The regions of interest for locally acquiring IR spectra by nano-FTIR were identified in previously recorded AFM topographies of supramolecular networks (a) and 2DP (b), both on hexacosane-passivated graphite. The AFM images were acquired in tapping mode. Crosses indicate the positions, where IR spectra were acquired (colours correspond to Fig. 6 of main manuscript) from two independent domains for each. These AFM images also indicate lateral domain extensions in the order of 500 nm. (a 150 px × 150 px; b 100 px × 100 px, 10 ms per pixel, scale bars: 500 nm).

Extended Data Fig. 6 Mass spectrometry illuminated versus non-illuminated.

ToF-SIMS data (counts vs. ion mass in AMU, positive ions detected) acquired from self-assembled fantrip monolayer on hexacosane-passivated graphite (as in Fig. 2b of main manuscript) (a) and similarly prepared sample after illumination under conditions that afford complete photopolymerization (as in Fig. 5 of main manuscript) (b); Note the logarithmic scale on the vertical axis.

Extended Data Fig. 7 Photopolymerization at elevated temperatures.

The temperature dependent photopolymerization experiments (Extended Data Fig. 3) also indicated a lower density of Stone-Wales defects. This overview STM image was acquired from a fully polymerized 2DP that was prepared by 14 h illumination with the sample loaded into the STM and the cryostat temperature held constant at 330 K. While for room temperature polymerization ~10 Stone-Wales defects were observed per 100 × 100 nm² of the 2DP (Fig. 5 of main manuscript), this image indicates a lower density with only 2-3 Stone-Wales defects per 100 × 100 nm² of the 2DP. The inset shows the corresponding FFT, corroborating a hexagonal symmetry and the absence of orientational disorder. (Tunneling parameters and scale bars: a -2.8 V, 3 pA, 50 nm; inset 1 nm-1).

Supplementary information

Supplementary Information

Supplementary Figs. 1–26 and synthesis protocol for fantrip monomers.

Supplementary Data 1

Source data for Supplementary Figs. 19 and 20. Calculated energies per unit cell versus lattice parameters.

Supplementary Data 2

Zip file summarizing all coordinates of DFT-calculated structures as *.xyz files.

Source data

Source Data Fig. 3f

Polymerized fraction versus illumination time: mean values and standard deviations.

Source Data Extended Data Fig. 3e

Polymerized fraction versus sample temperature: mean values and standard deviations.

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Grossmann, L., King, B.T., Reichlmaier, S. et al. On-surface photopolymerization of two-dimensional polymers ordered on the mesoscale. Nat. Chem. 13, 730–736 (2021). https://doi.org/10.1038/s41557-021-00709-y

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