Amelia
Domínguez-Celorrio
ab,
Carlos
Garcia-Fernandez
cd,
Sabela
Quiroga
e,
Peter
Koval
f,
Veronique
Langlais
b,
Diego
Peña
e,
Daniel
Sánchez-Portal
cd,
David
Serrate
*agh and
Jorge
Lobo-Checa
*ag
aInstituto de Nanociencia y Materiales de Aragón (INMA), CSIC-Universidad de Zaragoza, Zaragoza 50009, Spain. E-mail: serrate@unizar.es; jorge.lobo@csic.es
bCentre d'Elaboration de Materiaux et d'Etudes Structurales – Centre National de la Recherche Scientifique, Toulouse, France
cCentro de Física de Materiales CSIC/UPV-EHU-Materials Physics Center, Manuel Lardizabal 5, E-20018 San Sebastián, Spain
dDonostia International Physics Center, Paseo Manuel de Lardizabal 4, E-20018 San Sebastian, Spain
eCentro de Investigación en Química Biolóxica e Materiais Moleculares (CIQUS), Departamento de Química Orgánica, Universidade de Santiago de Compostela, Santiago de Compostela 15782, Spain
fSimune Atomistics S.L., Avenida Tolosa 76, 20018, San Sebastian, Spain
gDepartamento de Física de la Materia Condensada, Universidad de Zaragoza, E-50009 Zaragoza, Spain
hLaboratorio de Microscopías Avanzadas, Universidad de Zaragoza, E-50018, Zaragoza, Spain
First published on 27th April 2022
The synthesis of novel organic prototypes combining different functionalities is key to achieve operational elements for applications in organic electronics. Here we set the stage towards individually addressable magneto-optical transducers by the on-surface synthesis of optically active manganese-phthalocyanine derivatives (MnPc) obtained directly on a metallic substrate. We created these 2D nanostructures under ultra-high vacuum conditions with atomic precision starting from a simple phthalonitrile precursor with reversible photo-induced reactivity in solution. These precursors maintain their integrity after powder sublimation and coordinate with the Mn ions into tetrameric complexes and then transform into MnPcs on Ag(111) after a cyclotetramerization reaction. Using scanning tunnelling microscopy and spectroscopy together with DFT calculations, we identify the isomeric configuration of two bi-stable structures and show that it is possible to switch them reversibly by mechanical manipulation. Moreover, the robust magnetic moment brought by the central Mn ion provides a feasible pathway towards magneto-optical transducer fabrication. This work should trigger further research confirming such magneto-optical effects in MnPcs both on surfaces and in liquid environments.
When organic complexes are adsorbed on surfaces, important interfacial interactions emerge that influence their electronic and magnetic properties.5–7 In the case of metallated Pcs and porphyrins with first-row transition-metal (TM) ions, significant conformational and spin state modifications have been reported.7–10 These are strongly dependent on the local adsorption sites of metal centers and the arrangement of their external ligands, which alter the relative position of the metal center with respect to the π-conjugated macrocyclic plane. Similarly, selective adsorption of external and axial ligands on the metal centers modify these metal–organic complex properties.11–14 Such local variability limits the scalability of single-molecule device production since building blocks require dynamical control and operational reversibility while maintaining their overall chemical and physical environments.
A way to overcome these local conformation limitations could be inserting active molecular switches into molecular complexes so that morphological control can be exerted using external stimuli, in the form of photons,15,16 focused electric fields17,18 or electric currents.19,20 Light illumination is particularly simple to technically control, but demands that the spin centers of molecular complexes are strongly coupled to switchable ligands whose conformations can be changed by photon adsorption. The synthesis and deposition on the surfaces of such magnetic and optically active molecules by direct thermal evaporation is still pending, as it is compromised by the structural fragility of these metal–organic complexes. Such molecular integrity limitation has prevented its fabrication and implementation into magneto-optical transducers.
In this work, we achieved an adequate strategy to create metal–organic complexes by on-surface synthesis (OSS) featuring optically active moieties. In particular, we generated active Pcs with a central magnetic atom by means of cyclotetramerization of a functionalized phthalonitrile precursor on the Ag(111) surface. We chose Mn atoms as metallic centers based on their expected magnetic response that is evidenced in their reported Kondo signals on Ag substrates.21,22 We find that the produced manganese phthalocyanines (MnPcs) have the central magnetic ions coupled to four diarylethylene (DAE) moieties. These MnPcs on Ag(111) show at each branch two possible conformations that we image and study electronically by scanning tunneling microscopy (STM) and spectroscopy (STS) techniques. Density functional theory (DFT) calculations allow us to identify the relevant configurations of the open and closed DAE isomers on the Ag(111) surfaces, and confirm the robust magnetic moment localized at the Mn ion. Notably, we induce reversible changes between the open and closed conformations by lateral tip-molecule manipulation. Although magneto-optical experiments were not attempted here, this OSS process ensures that each MnPc we formed contains four switchable branches.
When a UV light of 365 nm wavelength irradiates a solution of compound 1a in CH2Cl2, the optical activity is photochromatically evident: the initial colorless solution corresponding to the open isomer 1a changes into blue as these molecules transform into the closed isomer 1b (Fig. 1a). The photochemically induced electrocyclic reaction involves six π-electrons, and therefore, according to the Woodward–Hoffmann rules the cyclization should be conrotatory, giving product trans-1b. The isomerization is reversible since the solution changes back to colorless once the UV irradiation is stopped (see ESI Fig. S1†).
Dehydrogenation of the closed isomer 1b leads to the formation of conjugated dicyanonaphthalene 2 (Fig. 1b), which does not vary its color in solution when irradiated with the same UV light. Its unique isomeric configuration and planar geometry will be used as a control to ensure the presence of the optically active ligands of 1.
The pathway that could lead to the generation of magneto-optical transducers requires to fulfill two conditions: first, deposition of optically active precursors that preserve their moiety integrity and, second, their combination with magnetic atoms. To achieve this, we evaporated small, optically active precursor molecules under UHV conditions so they self-assemble with Mn atoms on Ag(111). This surface is then used to catalyze a more complex structure by OSS. In order to prevent the Mn from clustering, we simultaneously deposited Mn atoms and precursor 1a on a Ag(111) surface kept at 50 °C (see the Methods section in the ESI†). As shown in Fig. 2a, squared-lattice islands are formed by the self-assembly of tetramer units that consist of Mn atoms (imaged as dark depressions) coordinated to four molecules by their cyano groups.24,25 These metal–organic tetramers are quite stable since they conserve their integrity after lateral atomic manipulation with an STM tip (see the inset of Fig. 2a). The islands of Mn + four 1a ligands are stabilized by the formation of hydrogen bonds with adjacent molecules (see Fig. S2†). In the coordinated islands, STM topography reveals two types of asymmetric ligands with different external lobes, also found for other diaryl derivatives on Ag(111).26 For convenience, we will refer to the brighter lobes as A-configuration and the darker lobes as B-configuration (cf. red and green arrows in the inset of Fig. 2a). These lobes are randomly distributed and for an island with 452 molecules of 1 we find an A/B ratio of 0.72, as if the interactions between neighboring tetramer units were independent of their external conformations. This randomness suggests that both configurations are structurally very similar, as expected from the two isomers 1a and 1b sketched in Fig. 1a.
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Fig. 2 Representative STM images of the metal–organic structures. (a) Survey and closeups of precursors 1a (a) and 2 (b) co-evaporated with Mn atoms on Ag(111). Each Mn atom is surrounded by four ligands forming stable metal–organic coordination units. In contrast to (b), the Mn + 1 units cluster into islands (see the molecular model in Fig. S2†) where each complex with four ligands can be manipulated and extracted without damage (cf. inset). The DAE moieties present two types of lobes (named A and B) which are marked by red and green arrows in the inset. Post-annealing the above structures to ∼300 °C and ∼350 °C induces cyclotetramerization of the ligands forming the MnPcs 3 (c) and 4 (d), respectively. As evidenced in the insets, the Pc core in 3 displays branches with two distinct appearances, whereas 4 is fully conjugated and structurally flat in comparison (see bond-resolved STM images in Fig. S3†). STM images: (a) 40 × 40 nm2, 0.6 V, 60 pA, inset (a) 3.5 × 3.5 nm2, 0.6 V, 60 pA; (b) 40 × 40 nm2, −1 V, 100 pA, inset (b) 3.5 × 3.5 nm2, −1 V, 100 pA; (c) 50 × 50 nm2, 10 mV, 50 pA, inset (c) 2.5 × 2.5 nm2, 0.4 V, 100 pA; and (d) 50 × 50 nm2, 10 mV, 50 pA, inset (d) 2.5 × 2.5 nm2, −0.4 V, 20 pA. |
An identical process of co-depositing Mn + compound 2 on Ag(111) leads to very different structures. The similarity with the previous metal–organic complex is restricted to the formation of coordinated units of Mn atoms and four molecules by their cyano groups24,25 (cf.Fig. 2b). However, these complexes are monodispersed and do not self-assemble into islands. Moreover, the ligands do not present visible differences in the external groups. We assign the absence of island aggregation to the lack of the two additional hydrogen atoms at their external DAE moieties compared to 1a and to the enhanced structural conjugation of the 2 precursors. All these findings confirm the molecular integrity of both 1 and 2 on the Ag(111) surface and the coordination with Mn into tetramer units that sets the stage for the MnPc formation by OSS.
Focusing on 3, from the high resolution images of individual molecules extracted from the island by atomic manipulation (Fig. 3a–d), we still identify two apparent configurations of the peripheral DAE groups. We keep naming A for the brighter lobes (e.g. top branch in Fig. 2c inset) and B for the darker ones. These lobes must correspond to different benzothiophene arrangements since these two configurations are randomly present in the MnPc branches. These MnPcs (3) are far from being planar, with A lobes being further corrugated than B lobes. Indeed, recording constant-height and bond-resolved images with a functionalised CO-tip (see Fig. S3†) is only feasible by vertically adjusting the tip height for each kind of lobe. Interestingly, the external branches in 3 do not show mirror symmetry with respect to their adjacent pyrrole ring (see Fig. 3). The superimposed chemical structure on a switchable MnPc with two A and two B branch configurations duplicated in Fig. 3c and d includes the branch configurations with the lowest energies (see DFT calculations in Table 1). The open-rot configuration fails when fitting the underlying topographic STM footprints both for A and B lobes at the red arrows (cf. Fig. S4†). In contrast, the molecular models for open and closed configurations in Fig. 3d satisfactorily reproduce the observed topography.
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Fig. 3 High resolution imaging and DFT structural models of 3. (a)–(c) STM images with a functionalized tip (2.5 × 2.5 nm2, −50 mV, 20 pA) of three different switchable MnPcs (3) isolated by lateral atomic manipulation (manipulation set point 5 mV and 15–40 nA). (d) is duplicated from (c) to compare two different superimposed molecular models from the three DFT branch configurations (see Table 1 and Fig. S4†). The positions where the model does not fit their underlying topographic STM footprints are indicated by red arrows in (c). This corresponds to the open-rot, which does neither fit A nor B lobes (cf. Fig. S4†). The correct molecular structure is overlaid in (d). The perpendicular dotted lines in (a) and (b) indicate the symmetrical axis expected for square-planar molecules. |
By contrast, the cyclotetramerized molecules 4 in Fig. 2d† present four similar and symmetric branches with a rather homogenous intensity distribution. The formation of a cyclodehydrogenated MnPc with its expected planar structure is confirmed by bond resolved STM images (see Fig. S3†), corroborating the cyclotetramerization reaction process of these MnPcs on the surface. Moreover, the differences in the STM images of 3 confirms the integrity of DAE moieties in the latter, based on the A/B branch lobe detection that maintains a bimodal distribution.
These DFT atomic arrangements must agree with the STM/STS datasets. We can start by discarding the symmetric cis-closed arrangement shown in Fig. S8† since none of the branches of 3 exhibits mirror symmetry with respect to the pyrrole ring (cf.Fig. 3a–d). In a similar way, the more compressed structural model of the open-rot isomer (superimposed to the MnPc in Fig. 3c) does not match the topographic STM footprint neither for A nor B lobes. Not only is energetically less stable than the open configuration, but it also shows almost identical LDOS for the molecular orbitals near the Fermi level (see ESI Fig. S9†). Thus, it is justified to rule out the presence of the open-rot isomer and restrict our external atomic arrangements to the trans-closed and open configurations. Their sideview comparison in Table 1 shows a larger apparent height and a more pronounced asymmetry for the latter, which agrees with the corrugation found in the constant-current images in Fig. 3. Consequently, we associate the open arrangement with the A configuration and the trans-closed with the B lobes.
To further increase the confidence in the assignment of the atomic arrangements of A/B lobes we calculated the local density of states (LDOS) in the energy window (−1, −0.5) eV corresponding to the highest fully occupied molecular orbital of 3 with four identical branches. The resulting LDOS for the open and trans-closed MnPcs is shown in Fig. 4c and d. The open branched MnPc exhibits a higher electronic density mainly localized at the pyrrole rings, whereas the trans-closed shows more weight towards the external branches. Moreover, our calculated PDOS (see ESI Fig. S9†) support a shift to lower energies in the HOMO position for the trans-closed structure due to a higher conjugation of its branches with the pyrrole ring center. Then, based on this feature of the HOMO, we correlated the calculated molecular orbital of Fig. 4d for the trans-closed geometry with the experimental dI/dV signal observed at −1.3 V of Fig. 4e. In contrast to constant current images, the darker B branches (number 4 in Fig. 4b) display a dominant character in the charge density distribution in the dI/dV maps of occupied states. Indeed, the trans-closed intensity maximizes closer to the sulfur atoms for the HOMO (cf.Fig. 4e). In contrast, the A branches (numbers 1, 2 and 3 in Fig. 4b) featuring brighter signals in the constant current, become practically irrelevant and only reflect the higher corrugation inherent to the open configuration in the dI/dV map. The LDOS of this occupied molecular orbital qualitatively agree with the features described for the calculated HOMO shown in Fig. 4c.
We also tested the use of bias pulses and slow ramps in the current or electric field to drive the molecular switches.17,18,28,29 However, we discarded these methods since no changes were observed below a 3V threshold, whereas biasing the molecules above this value systematically led to non-reversible structural changes of the branches, in the form of dehydrogenation with the appearance of 4-type branches (see ESI Fig. S6†).
DFT theory sheds light on the origin of this Kondo resonance that we locate at the pyrrole ring center, in agreement with previous work.21,22 The calculated density of states projected onto the Mn d orbitals indicate that the dx2−y2 orbital is emptied, whereas the other four d orbitals are partially filled (see ESI Fig. S10†). This electronic arrangement results in a total magnetic moment slightly less than 2μB where the main contribution is associated with the Mn center but with an opposite magnetic contribution from the molecular framework. Previously, the Kondo resonance was associated with the many-body screening of the dz2 orbital by the conduction electrons of the substrate,21 which in our case is substantiated by the nearly half-filling observed for this orbital. Also, the degenerated dxz/yz orbitals exhibit a strong contribution just below the Fermi level, accounting for the tail to the left of the Kondo resonance.
A critical parameter for the Kondo state is the screening strength, which affects the Kondo temperature and resonance line shape when, for instance, altering the distance of the central Mn to the metallic substrate. Despite the possibility of switching the branch configurations between open and trans-closed, we experimentally find for our switchable MnPcs that the low energy Mn spectra do not change (cf. ESI Fig. S7†). This can be understood if the Mn vertical distance turns out to be insensitive to the branch configuration, as deduced by theory (cf. ΔzMn in Table 1). Furthermore, the identical planar environment of the Mn atoms in both configurations leads to nearly the same d orbital decomposition (ESI Fig. S10†). The use of bulkier groups at the hydrogen positions that are still optically active could lead to significant Mn height changes when switching between different open/closed isomers, thereby bearing the prospect of fulfilling the magnetic transducer functionality.
The flexibility of the OSS process to produce complex structures, where the optical activity is brought by diarylethylene (DAE) moieties and the magnetism by metal centers exchangeable by metallation, opens vast opportunities. For instance, we foresee the implementation of different readout signals changing the magnetic state of the ion, the magneto crystalline anisotropy,30,31 the energy resolved spin polarization of the local DOS,32,33 or the Kondo screening.21,22 Also, we envision the use of bulkier ligands to alter the adsorption geometry and height of the pyrrole ring around the metal center, thereby correlating the Kondo resonance to the different combinations of isomer ligands. All these possibilities expand the well-stablished spin-crossover photo-magnetism approach at the individual molecular level.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2nr00721e |
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