Interplay between π-Conjugation and Exchange Magnetism in One-Dimensional Porphyrinoid Polymers

Abstract

The synthesis of novel polymeric materials with porphyrinoid compounds as key components of the repeating units attracts widespread interest from several scientific fields in view of their extraordinary variety of functional properties with potential applications in a wide range of highly significant technologies. The vast majority of such polymers present a closed-shell ground state, and, only recently, as the result of improved synthetic strategies, the engineering of open-shell porphyrinoid polymers with spin delocalization along the conjugation length has been achieved. Here, we present a combined strategy toward the fabrication of one-dimensional porphyrinoid-based polymers homocoupled via surface-catalyzed [3 + 3] cycloaromatization of isopropyl substituents on Au(111). Scanning tunneling microscopy and noncontact atomic force microscopy describe the thermal-activated intra- and intermolecular oxidative ring closure reactions as well as the controlled tip-induced hydrogen dissociation from the porphyrinoid units. In addition, scanning tunneling spectroscopy measurements, complemented by computational investigations, reveal the open-shell character, that is, the antiferromagnetic singlet ground state (S = 0) of the formed polymers, characterized by singlet–triplet inelastic excitations observed between spins of adjacent porphyrinoid units. Our approach sheds light on the crucial relevance of the π-conjugation in the correlations between spins, while expanding the on-surface synthesis toolbox and opening avenues toward the synthesis of innovative functional nanomaterials with prospects in carbon-based spintronics.

Structural Expansion of Cyclohepta[def]fluorene towards Azulene‐Embedded Non‐Benzenoid Nanographenes

“…[30] A zoomed-in nc-AFM image of the majority reaction product 1 (~80 % of the studied species) is shown in Figure 2b, revealing the chemical structure of the molecule with a nonplanar conformation highlighted by the increased frequency shift (yellow and pink arrows in Figure 2b). We propose, as shown in the chemical sketch of Figure 2c, that the feature highlighted with the yellow arrow is related to the saturation with two H atoms on the apical carbon atom [21,[31][32][33][34] of the heptagonal ring, while the pink arrow points to steric hindrance between two adjacent H atoms, where the expected cyclodehydrogenation reaction did not occur, lifting up a part of the DMPA segment. Sporadically, we observed the cyclodehydrogenation at both CÀ C sites, as shown in the highresolution STM image, along with the corresponding chemical sketch of a similar molecule 1' (Figure S2).…”

Sterically Selective [3 + 3] Cycloaromatization in the On-Surface Synthesis of Nanographenes

“…This safeguard is attributed to the steric protection from the large benzotetracene core, which is not present when the isopropyl group is affixed to smaller arenes. 4,16 Supporting this, the key stages of the cross-linking reaction pathway are outlined in Scheme 3a. 6a, a cycloaromatization intermediate, is highly reactive.…”

Steering Electron-Induced Surface Reaction via a Molecular Assembly Approach

“…In surface chemistry, the latter process usually takes place for a molecule positioned between the substrate surface and the tip of a scanning tunneling microscope (STM). As such, the conversion of the molecule can be triggered by the application of STM-tip voltage pulses. ,− As a surface characterization technique with an angstrom-scale spatial resolution, STM can possibly provide a mechanistic understanding of the electron-induced surface reaction on the molecular scale. ,− Moreover, the STM-based electron-induced chemical conversion is highly localized and controllable, which enables selective activation of a single chemical bond and accordingly offers a way to synthesizing functional molecular nanostructures with atomic precision on the surface. − …”