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

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.

Introduction

Macrocyclic compounds based on porphyrins, that is, porphyrinoid compounds, are often under the spotlight by virtue of their inspiring biological, photophysical, and photochemical properties, being frequently recognized due to their role in energy metabolism and light harvesting in photosynthesis.[1,2] Over the last few years, their potential in a great number of technological and medical applications[3] such as photocatalysis,[4] nonlinear optics,[5] molecular photovoltaics,[6,61] spintronics,[7] drug delivery,[8,9] and biological imaging and sensing[10,11] have undoubtedly expanded their use. Interestingly enough, the properties of porphyrinoid compounds can be modulated by structural modifications in the different positions of the peripheral substituents, as well as in the heterocyclic macrocycle, where the metal center directly affects their electronic and magnetic properties.[12,13]

In the context of surface science, investigations on intramolecular conformations, intermolecular coupling and supramolecular interactions of porphyrinoids have been performed in the last few years considering the interest of scientists from different fields in the study of such compounds adsorbed on well-defined interfaces.[14−16] While great efforts have been devoted to the systematic study of intramolecular conformations and noncovalent interactions containing different metals in the macrocycle and several peripheral-functionalized substituents; the intermolecular carbon–carbon (C–C) coupling of such compounds has been mainly limited to the fabrication of on-surface polymers and oligomers via the well-known Ullmann-like coupling[17−19] or dehydrogenative coupling,[20−23] among others.[24−29] However, further on-surface investigations of alternative chemical reactions with relevant implications in organic synthesis have remained unexplored. Among the novel on-surface reactions reported recently, the surface-catalyzed [3 + 3] cycloaromatization of isopropyl and isopropenyl substituents on arenes is of particular interest providing a novel pathway toward the fabrication of polymers that cannot be synthetized via conventional solution chemistry.[30]

Contemporarily, the field of carbon magnetism, with important advantages over classical semiconductors and metals, has gained increased attention in view of the recent progress made following a bottom-up synthesis approach.[31] Carbon-based nanomaterials may present exceptional electronic and magnetic features conceivable to be exploited in the fabrication of multifunctional spintronic devices. Magnetism in these compounds can arise from different aspects such as: (i) edge effects, where zigzag edge topologies allow the formation of low-energy localized edge states, (ii) non-Kekulé structures formed via Coulomb repulsion between valence electrons, or (iii) spin disparity caused due to a sublattice imbalance in the bipartite lattice. The research on the fundamental properties and applications of such magnetism makes the synthesis of magnetic carbon nanomaterials an appealing research area.[32−37]

Under this scenario, the comprehensive fabrication of magnetic (open-shell) porphyrinoid-based polymers emerges as a highly appealing field of research. However, the vast majority of the porphyrinoid compounds studied to date exhibit a nonmagnetic (closed-shell) character. Radicals stabilized by spin delocalization over the π-electron systems of porphyrinoids have been rarely found both in solution and on-surface chemistry.[38] Only recently, the on-surface formation of magnetic one-dimensional (1D) oligomers and polymers based on covalently linked porphyrinoid building blocks has been achieved.[18,23] Therefore, the fabrication of innovative magnetic porphyrinoid-based polymers would constitute the basis for further development toward molecular electronics expanding the current on-surface chemistry toolbox. In this article, we introduce an exemplary approach toward the bottom-up fabrication and characterization of unprecedented magnetic porphyrinoid-based polymers homocoupled via surface-catalyzed [3 + 3] cycloaromatization of isopropyl substituents studied on Au(111) under ultrahigh vacuum (UHV) conditions. To this end, we have synthesized a porphyrin precursor (1) in solution that can be sublimed intact on the gold surface. Porphyrin 1 was prepared according to previously reported methods developed by Lindsey for the synthesis of trans-A2B2 type porphyrins[39] involving BF3–ethanol cocatalysis.[40] The [2 + 2] condensation of 5-mesityldipyromethane[41−44] (1 equiv) and culminaldehyde (1 equiv) in CHCl3 containing 0.75% ethanol in the presence of BF3·Et2O at room temperature (RT) for 2 h, followed by irreversible oxidation of the porphyrinogene intermediate with DDQ for an additional 1 h, afforded porphyrin 1 in 20% yield (DDQ = 2,3-dichloro-5,6-dicyano-1,4-benzoquinone). Further synthesis and characterization details are provided in the Supporting Information. Our molecular-level investigations, accomplished by means of scanning probe techniques and complemented by multidisciplinary theoretical calculations, are shown in Scheme . A thermal activation step ensures the intramolecular cyclodehydrogenation and oxidative ring closure reactions, as well as the intermolecular oxidative coupling, that is, [3 + 3] cycloaromatization, of 1 through isopropyl substituents, which leads to the formation of polymer 2. Subsequent scanning tunneling microscopy (STM) tip-induced atom manipulation experiments allow the controllable cleavage of hydrogens from the two saturated carbon atoms of 2, giving rise to polymer 3. Interestingly, polymer 3 exhibits magnetic interactions between unpaired spins of adjacent porphyrinoid units, experimentally manifested as spin–flip excitations, revealing the crucial interplay between magnetic exchange and π-conjugation.

Scheme 1

On-Surface Reaction Scheme toward the Synthesis of 1D Porphyrinoid Polymers

Results and Discussion

To achieve the formation of the desired porphyrinoid polymer, precursor 1 was sublimed onto a smooth Au(111) surface held at RT under UHV conditions. Figure a shows a representative STM image after the sublimation of 1 on Au(111), revealing the predominant presence of several self-assembled domains stabilized by CH···π interactions between adjacent precursors coexisting with some disordered regions (see Figure S1). Annealing of the sample at 200 °C (T1 in Scheme ) gives rise to the efficient formation of relatively long 1D chains, where porphyrinoid units, linked to each other, can be discerned as shown in Figure b. To access the chemical structure of such polymers, frequency-shift noncontact atomic force microscopy (nc-AFM) images acquired with a CO-functionalized tip were performed.[45] The resulting image (Figure c) suggests that the thermal energy provided by annealing the sample at 200 °C is sufficient to afford a one-step reaction process toward the formation of polymer 2. On the one hand, nc-AFM images reveal the formation of a new phenylene ring between porphyrinoid units, which arises from the intermolecular oxidative coupling of isopropyl substituents (highlighted by the dashed blue circle in Figure c). The mechanism of such a chemical reaction, unexpected to occur in conventional solution chemistry, has only recently been described in on-surface synthesis by the seminal publication of Kinikar et al.[30] On the other hand, two intramolecular reactions took place: (i) oxidative ring closure of the four methyl substituents per porphyrinoid unit, providing four hexagonal rings (highlighted by the dashed green circle in Figure c) and (ii) cyclodehydrogenation of the phenyl groups adjacent to the isopropyl substituents, which admit two equal rotation possibilities upon activation, affording the formation of two five-membered rings (highlighted by the dashed orange circle in Figure c). In addition, four features of increased frequency shift per porphyrinoid unit are identified (white arrows in Figure c). Such features are typically attributed to hydrogens linked to unsaturated carbon atoms adopting a nonplanar configuration. Notably, the extra-hydrogenation of the two carbon atoms at the apex of the five-membered rings was unexpected and is attributed to the presence of hydrogen on the surface, being available to passivate the most reactive sites of the studied compounds. Such experimental features are well reproduced by the corresponding nc-AFM simulations[46] of polymer 2 (Figure d), which further support the highly nonplanar configuration of several hydrogens with respect to the underlying surface.

Figure 1

Synthesis and structural characterization of polymer 2 on Au(111). (a) Overview STM topography image of the Au(111) surface after RT deposition of 1, showing the formation of self-assembled islands coexisting with some disordered regions. Vb = −1.0  V, It = 100  pA, scale bar = 5 nm. The inset shows a high-resolution STM image of an individual molecule where four bright protrusions of identical apparent height are attributed to the tilt of the four peripheral benzene rings due to steric hindrance with the porphyne backbone. Image size 3.5 nm × 3.5 nm, Vb = −0.5  V, It = 50  pA. (b) Overview STM topography image showing the formation of 1D chains (polymer 2) after deposition of 1 and subsequent annealing at 200 °C. Vb = 0.5  V, It = 110  pA, scale bar = 5 nm. (c) Constant-height frequency-shift nc-AFM image acquired with a CO-functionalized tip (z offset −43 pm below STM set point = 5 mV, 50 pA), scale bar = 1 nm. The dashed circles and white arrows highlight the chemical reactions resulted from the annealing of 1 at 200 °C and the out-of-plane hydrogens attached to unsaturated carbon atoms, respectively (see Scheme ). (d) Simulated nc-AFM image of (c). Scale bar = 1 nm.

Further annealing of the sample at 280 °C affords the partial cleavage of the aforementioned hydrogen atoms, though the applied thermal energy is also compatible with the unselective C–H bond dissociation of other segments of polymer 2 (see Figure S2). Thus, to achieve the fabrication of polymer 3, STM tip-induced atom manipulation experiments were performed, in agreement with previous works.[32,47−52]Figure a,b shows a profound difference between constant-height STM images of a polymer segment before and after the tip-induced manipulation event, which anticipates a modification of its electronic properties. The successful removal of one of the two hydrogens in each sp3 carbon site is further confirmed by the corresponding nc-AFM images (Figure c,d), where the absence of the bright features located at the apex of the six five-membered rings after the tip manipulation is clearly discerned (highlighted with white arrows). Therefore, as anticipated in Scheme , every hydrogen dissociation step introduces one delocalized π radical into the system. Furthermore, the planar appearance shown in the scanning probe experiments together with density functional theory (DFT)[53] calculations evidence that polymer 3 is physisorbed on the gold surface (see Figure S3).

Figure 2

Transformation of polymer 2 into polymer 3 via STM tip-induced hydrogen dissociation. (a,b) High-resolution constant-height STM images acquired with a CO-functionalized tip that show the generation of polymer 3 through voltage-pulse-induced dissociation of individual hydrogen atoms from polymer 2. The blue crosses specified the positions of the tip where the voltage pulse was performed. Vb = 5  mV, It = 30  pA. (c,d) Laplace-filtered constant-height frequency-shift nc-AFM images of (a,b) that evidences the absence of the bright protrusion (two per porphyrinoid unit) after the tip manipulation event, which is assigned to the controlled dissociation of one hydrogen atom per carbon site. The white arrows highlight the presence (c) and absence (d) of such bright protrusion. z offset −50 pm below STM set point = 5 mV, 40 pA. All scale bars = 1 nm.

In addition to the detailed structural investigation of polymers 2 and 3, we have performed scanning tunneling spectroscopy (STS) measurements to probe the electronic structure of the resulting polymers on Au(111). Long-range differential conductance dI/dV spectra acquired on polymers 2 and 3 display strong features in the density of states at −0.63 and +0.85, and −0.85 and +0.90 V respectively, which are assigned to the valence band (VB) maximum and the conduction band (CB) minimum, corresponding to STS band gaps of 1.48 and 1.75 eV (see Figure S4). The experimental dI/dV maps acquired at the energy positions specified in Figure S4a,c allow resolving the spatial distribution of the VB and the CB, which match well with the simulated dI/dV maps of the frontier bands[54] calculated for a free-standing infinite segment of polymers 2 and 3 using DFT (Figure S4b,d).

Next, in order to obtain information about the transition from nonmagnetic to magnetic nature of polymers 2 and 3, respectively, we have performed a detailed analysis of the low-bias dI/dV spectra (in a range of ±50 mV), where the appearance of low-energy spectral features can be associated with a magnetic fingerprint.[32−37] First, we have inspected a segment of polymer 2. Herein, no spectral features were observed, as shown in Figure a. However, this scenario is drastically modified after tip-induced dehydrogenation at the positions specified in the STM images shown in Figure a–c.

Figure 3

Intra- and intermolecular characterization of the magnetic coupling of 1D porphyrinoid polymers on Au(111). (a–d) Chemical sketches, constant-current STM images, and low-bias dI/dV spectra acquired at positions α–δ (depicted in the chemical sketches) upon the sequential dissociation of hydrogens from the unsaturated carbon atoms at the five-membered rings of each porphyrinoid unit. The blue crosses shown in the STM images indicate the position, where the tip-induced manipulation was performed. The orange curve corresponds to the reference dI/dV spectrum acquired on Au(111). The two black rectangles in the green spectrum displayed in (c) highlight the inelastic excitation threshold obtained at the half maximum width, i.e., ±4.6 mV, of the observed resonances. STM images: Vb = 5  mV, It = 30  pA, scale bars = 1 nm. dI/dV spectra: (Vb = 50  mV, It = 0.45  nA, and Vrms = 0.8  mV) (e) dI/dV spectrum shown in position α of (b), fitted using the Frota function including the vibration plus spin-flip fits (red line). Clearly, the fit reproduces the observed spectral features, which further support the existence of a ferromagnetic (triplet) ground state of a porphyrinoid unit predicted by DFT calculations. (f) dI/dV spectrum shown in position α of (c) showing the nice fitting (red dashed line) to a single Frota function. The fit is now very satisfactory without the spin–flip component. The black arrows shown in (e,f) highlight the vibrations associated with the CO molecule used to functionalize the tip observed at ∼±32 mV (g) Chemical sketch of polymer 3. The red line highlights the π-conjugation pathway between porphyrinoid units.

After a first tip-induced hydrogen dissociation event at positions α and β shown in the chemical sketch in Figure a (realized with the tip located at the center of the porphyrinoid unit), two zero-energy excitation features mainly located at the five-membered rings appear. Such features are typically characteristic of a Kondo resonance (total spin S = 1/2)[55] or attributed to ferromagnetic correlations (triplet ground state, S = 1).[36,56−58] To unravel their nature, we have performed DFT as well as many-body calculations of a porphyrine unit revealing the triplet ground state of the porphyrinoid unit once the two extra hydrogens located at positions α and β are removed (see Figures S5–S8, Table S1 and underlying discussion in the Supporting Information for further details). Furthermore, the fit of the experimental data corroborates the theoretical predictions about the triplet ground state in the porphyrinoid unit, resulting in a ferromagnetic intraporphyrin coupling (Figures e and S9a, where the fittings of the spectra from Figure b acquired on top of α and β are shown). Notably, a single Frota line shape, that is well-known to fit Kondo resonances, cannot fit our experimental spectra over the whole voltage range shown. Instead, an underscreened S = 1 Kondo with spin-flip signatures does lead to a good fit (exchange coupling energy of Jeff = 11.9 meV derived from the fit, Table S2), in agreement with the theoretically predicted S = 1 ground state. In addition, we have investigated the anisotropy of the current-induced density (ACID),[59] which has proven to be an extremely versatile and descriptive method used to analyze the density of delocalized electrons in ground state molecules and polymers.[60] ACID calculations performed on a single basic unit of polymer 3 suggest that the two formed five-membered rings (colored in orange in Scheme ) show a clear paratropic contribution in the delocalized π-system, while the remaining parts reveal a diatropic current around their peripheries. The picture depicted by the ACID calculations in Figure S7a can be intuitively explained by the fact that the phenalene subunits formed upon the intramolecular ring closure impose the break of the π-conjugation pathway, largely influencing the chemical reactivity as well as the electronic and magnetic properties of the polymer (see the Supporting Information for detailed discussion).

Further hydrogen dissociation realized on an adjacent porphyrinoid unit (position γ in Figure a–d) unveils a new scenario. Low-bias dI/dV spectra acquired at positions β and γ reveal a conductance step at ±4.6 mV, symmetric with respect to the Fermi energy, indicative of inelastic excitations (Figure c,d). Such inelastic excitations are ascribed to spin excitations (singlet–triplet magnetic excitation due to the tunneling electrons), where the two closer spins from adjacent porphyrinoid units are antiferromagnetically coupled, with an effective exchange parameter Jeff = 4.6 mV, in good agreement with the calculated Jeff = 7 mV (see Figure S10 and Table S3 for a dimer model adopted to estimate the effective magnetic exchange interactions between the individual radical states). Remarkably, the low-bias dI/dV spectrum repeated at position α in Figure c presents a single peak at the Fermi energy that, in contrast to the previous spectrum acquired on α prior to the removal of the extra hydrogen at position γ (Figure b), now it is well fitted to a single Frota peak centered at the Fermi level (Figure f). We assign this feature to a Kondo resonance (S = 1/2) of an unpaired electron spin located at position α. The disappearance here of the spin–flip signal (Figure S9c) is attributed to the prevalence of the antiferromagnetic coupling between adjacent porphyrinoid units (between positions β and γ from Figure ), over the previous ferromagnetic coupling between α and β. Such prevalence is tentatively related to the formation of a π-conjugated path between β and γ.

Finally, we have performed tip-induced hydrogen dissociation experiments in order to cleavage one of the two hydrogens located at position δ (Figure a–d). Herein, as expected, the low-bias dI/dV spectrum obtained at position δ reveals a zero-energy excitation with an identical shape to the one observed for position α in Figure c,d, while maintaining the antiferromagnetic coupling between β and γ. Therefore, we can conclude that the oxidative coupling of isopropyl substituents, that is, formation of a benzene ring and subsequent tip-induced dehydrogenation, provides a π-conjugated pathway between unpair spins of adjacent porphyrinoid units, which allows their antiferromagnetic coupling (see the sketch in Figure g).

Conclusions

We have demonstrated an illustrative approach toward the fabrication of magnetic porphyrinoid-based 1D polymers on the Au(111) surface. Polymer 2 is achieved by thermal activation of precursor 1 at 200 °C, which induces the expected intramolecular oxidative ring closure and cyclodehydrogenation reactions; while an intermolecular oxidative coupling, that is, [3 + 3] cycloaromatization via isopropyl substituents provides the linkage between porphyrinoid units. Successive STM single-atom manipulation experiments ensure the controllable dissociation of hydrogen atoms from the saturated carbon atoms of polymer 2, promoting the formation of polymer 3. The chemical structures of both polymers have been clearly elucidated by STM and nc-AFM. In addition, a detailed electronic characterization, performed by STS measurements and DFT calculations, reveals a nonmagnetic ground state for polymer 2 as no spectral features in the low-bias regime were detected. Interestingly, the removal of hydrogens from the two unsaturated carbon atoms at the five-membered rings of a porphyrinoid unit reveals zero-energy excitation features ascribed to its triplet ground state (S = 1). However, further tip-induced hydrogen dissociation experiments performed at adjacent units unveils a scenario, where an antiferromagnetic coupling, distinguished by an experimental exchange-coupling strength of 4.6 meV, is observed between the two closer spins from adjacent porphyrinoid units. Our work highlights the relevance of the π-conjugation in the correlations between spins, representing a profitable step toward the on-surface synthesis of covalently linked 1D magnetic polymers with application prospects in nanoscale spintronic devices. In more general terms, we envision that our on-surface synthetic approach can be expanded to the controlled fabrication of two-dimensional magnetic polymers by further rationalization of the precursor chemical design.