From molecular shape to molecular function
On-surface 2-D chemistry is a means to synthesize defined molecular nanosystems with unique properties. We combined photo- and redox-active porphyrins with rigid 3-D hydrocarbon scaffolds to construct organic electronic elements, such as a molecular resistor. Using the molecular shape of isosteres to control the chemical function in organic superstructures opens new avenues in materials research.
Focus Group: Molecular and Interfacial Engineering of Organic Nanosystems
Prof. Mathias O. Senge (Trinity College Dublin), Alumnus Hans Fischer Senior Fellow
Biao Yang, Miguel Gavara-Edo (TUM), Postdoctoral Researchers
Hosts: Prof. Johannes V. Barth, Prof. Willi Auwärter (TUM)
Porphyrins
Porphyrins, known as the “pigments of life” (e.g., heme and chlorophyll) are a class of ubiquitous natural compounds and are targets of strategic importance in biology, industry (pigments, catalysts, sensors, nanomaterials), and medicine (theranostics) (Fig. 1). Their chemical and functional versatility, tunable photophysical and electrochemical properties, and ability to act as sensors and catalysts make them molecular engineering test cases and building blocks of choice. On a fundamental basis, these effects are related to their chemical properties, namely their photochemical (energy and exciton transfer), redox (electron transfer, catalysis), and coordination properties (metal and axial ligand binding) and their conformational flexibility (functional control). Their chemical reactivity can be modulated by introduction of different metals or functionalization of the macrocycle. Advances in their synthesis, molecular engineering, and physical characterization on surfaces have opened application opportunities for functional nanodevices. We aimed to use a combination of molecular and interfacial engineering of porphyrins as innovative “molecular systems” for an interdisciplinary approach toward organic nanosystems.
Figure 1 and 2
Fig. 2 (left), The isostere concept and bicyclo[1.1.1]pentane and cubane are rigid linear drop-in replacements for alkinyl and benzene units.
Molecular shape and function
Imprinting molecular function, be it in terms of chemical reactivity, biological activity, or physical properties, is a fundamental problem in chemistry. A logical effector design to develop functional and diagnostic molecules and nanoarrays requires construction principles that impart the necessary functions (substrate binding/activation, photophysics, electron transport) and action modes (e.g., light absorption, (photo)catalytic activity) at the molecular level. Molecules can be synthesized for many applications, yet it is more problematic to develop general concepts and sets of design criteria for polychromophoric nanomaterials where additional functionality is derived from an ordered structure of multiple photo- and/or electroactive components such as porphyrins. One approach is altering the shape of a given molecule to change its chemical reactivity using conformational control. For this we established an analytical framework to identify distortion modes based on normal-structural decomposition of porphyrins. [1] In practical terms, nonplanar atropisomeric porphyrins were developed as organocatalysts and sensors. Similarly, molecules with a unique shape can be used to construct larger arrays in a spatially defined manner. To some extent both aspects are exemplified in the isostere concept (Fig. 2). Replacing a planar unit such as benzene with a three-dimensional one like cubane or bicyclo[1.1.1]pentane (BCP) changes potential interactions from 2-D to 3-D. This “escape from flatland” not only gives rise to novel biological, medical, and materials properties, but also delivers novel tectons for use in on-surface chemistry as it replaces linear electronically conjugating linkers with rigid isolating ones.
On-surface chemistry
On-surface chemistry allows the construction and characterization of functional molecular nanostructures and architectures at interfaces with atomic resolution. Surface studies on porphyrins have established their structural, electronic, and chemical characteristics at interfaces and yielded methods to manipulate their intramolecular and organizational features based on unique properties resulting from interfacial confinement. Surface-assisted C–C/X fusing reactions [2] in conjunction with supramolecular chemistry also offer a means to chemo- and regioselectively [3] construct devices comprised of functionally different components. Further studies aimed at functional group interconversions of porphyrins and organometallic derivatives thereof (Fig. 3), dye-nanographene structures, nonplanar porphyrin on surfaces, reactivity studies of cubane and BCP, and the reaction of porphyrins with rigid hydrocarbon linkers.
Case study – A molecular isolator
The propeller-shaped BCP motif is of growing importance to the pharmaceutical industry as a sp3-rich bioisostere of benzene rings and as molecular building blocks in materials science. We explored the behavior of 1,3-disubstituted BCP moieties on metal surfaces by combining low-temperature scanning tunneling microscopy / non-contact atomic force microscopy studies with density functional theory modeling. We examined the configuration of individual BCP-containing precursors on Au(111), their supramolecular assembly, and thermally activated dehalogenative coupling reactions, affording polymeric chains with incorporated electronically isolating units (Fig. 4). This study not only provided the first sub-molecular insights of the BCP scaffold on surfaces but also extended the scope of on-surface synthesis to 3-D isosteres. Bicyclo[1.1.1]pentane scaffolds were identified as versatile resistor/isolator units in molecular nanostructures at surfaces, opening novel avenues for the exploration of further aspects and tunability in organic electronics, e.g., via incorporation of tetrapyrroles or other functional units in custom-designed surface architectures. [4]
Figure 4
Beyond the Focus Group
The on-surface projects outlined above will be continued by the members of the Focus Group aiming at heteromolecular functional devices. Notably, the Fellowship facilitated the establishment of several additional collaborations at TUM. Among others, these include Angela Casini (gold porphyrins and metallacages), Nicole Strittmatter (mass spectrometry, CO2) activation), Romy Ettlinger (metal-organic frameworks with rigid hydrocarbon linkers for water purification), and Roland A. Fischer (nonplanar porphyrins as MOF components [5]) and many of their group members. In the spirit of the Fellowship’s namesake – Hans Fischer’s lifework was focused on pyrrole compounds – much of this work is centered on porphyrins and will occupy us for years to come. We will continue using their unique optical, photophysical, electrochemical, and structural properties in conjunction with novel linker structures to affect a transformational advance in the molecular design and control of 1-D, 2-D, and 3-D nanoconstructs with tunable electronic, photonic and chemical functionalities. Synthesis, characterization, and materials aside, the Fellowship, which has delivered 38 publications thus far, also allowed us to venture farther afield to the art-science interface. [6] Furthermore, a deeper analysis of the rich legacy (e.g., Willstätter, Fischer, Treibs) and contemporary research on porphyrins in Munich could serve to highlight and illustrate the development of the field in its societal, historical, and political context.
[1]
Kingsbury (2024).
[2]
Cao (2021)
[3]
Cao (2023)
[4]
Yang (2023)
[5]
Hemmer (2024)
[6]
Kingsbury (2024)
Selected publications
- N. Cao, A. Riss, E. Corral-Rascon, A. Meindl, W. Auwärter, M. O. Senge, M. Ebrahimi and J. V. Barth, “Surface-confined formation of conjugated porphyrin-based nanostructures on Ag(111),” Nanoscale, vol. 13, no. 47, Dec., pp.19884-19889 (2021).
- N. Cao, J. Björk, E. Corral-Rascon, Z. Chen, M. Ruben, M. O. Senge, J. V. Barth and A. Riss, “The role of aromaticity in the cyclization and polymerization of alkyne-substituted porphyrins on Au(111),” Nature Chemistry, vol. 15, Dec.,
pp.1765-1772 (2023).
- B. Yang, K. Niu, N. Cao, N. Grover, W. Zhao, A. Riss, J. Björk, W. Auwärter, J. V. Barth and M. O. Senge, “On-Surface Synthesis of Polyphenylene Wires Comprising Rigid Aliphatic Bicyclo[1.1.1]Pentane Isolator Units,” Angewandte Chemie International Edition, vol. 62, no. 19, May, e202218211 (2023).
- K. Hemmer, S. M. Kronawitter, N. Grover, B. Twamley, M. Cokoja, R. A. Fischer, G. Kieslich and M. O. Senge, “Understanding and Controlling Molecular Compositions and Properties in Mixed-Linker Porphyrin Metal−Organic Frameworks,” Inorganic Chemistry, vol. 63, no. 4, Jan.,
pp.2122-2130 (2024).
- C. J. Kingsbury and M. O. Senge, “Molecular Symmetry and Art: Visualizing the Near-Symmetry of Molecules in Piet Mondrian’s De Stijl,” Angewandte Chemie International Edition, vol. 63, no. 25, June, e202403754 (2024).