On‐Surface Driven Formal Michael Addition Produces m‐Polyaniline Oligomers on Pt(111)

Abstract On‐surface synthesis is emerging as a highly rational bottom‐up methodology for the synthesis of molecular structures that are unattainable or complex to obtain by wet chemistry. Here, oligomers of meta‐polyaniline, a known ferromagnetic polymer, were synthesized from para‐aminophenol building‐blocks via an unexpected and highly specific on‐surface formal 1,4 Michael‐type addition at the meta position, driven by the reduction of the aminophenol molecule. We rationalize this dehydrogenation and coupling reaction mechanism with a combination of in situ scanning tunneling and non‐contact atomic force microscopies, high‐resolution synchrotron‐based X‐ray photoemission spectroscopy and first‐principles calculations. This study demonstrates the capability of surfaces to selectively modify local molecular conditions to redirect well‐established synthetic routes, such as Michael coupling, towards the rational synthesis of new covalent nanostructures.


Introduction
Quinone and quinone imines in general, and 1,4-benzoquinone monoimine in particular, are very useful building blocks for the synthesis of aw ide variety of compounds, [1] from natural products to polymers,w ith ab road range of applications.Despite the apriori structural simplicity of such molecules,t hey are characterized by al ack of control of the reactive sites due to the electrophilic character of the different positions on the ring (see positions 2, 3, 4, and 5i n equation a, Scheme 1). [2] Therefore,i no rder to target the synthesis of aspecific molecular structure,itiscrucial to find areaction that induces selectivity.Inthis work, we show that ar oute based on the well-known Michael addition, [3,4] complemented with the unique properties of metal surfaces, generates high selectivity towards nucleophilic attack at site 4 (see Scheme 1,panel b), such that starting from simple paraaminophenol (p-AP) precursors we obtain meta-coupled oligomers of polyaniline (PANI). Conjugated polymers such as PA NI are af amily of polymers that have attracted huge scientific and technological attention during the last decade. [5][6][7] Theemeraldine base form is generally regarded as the most useful, since it becomes electrically conducting upon simple protonation of the imine nitrogen atoms with an acid. Indeed, one of their most important properties is the reversibility of electronic character and the modification of electrical conductivity through doping (protonation) and undoping (deprotonation) treatments that, combined with their relatively facile processability and good environmental stability,m ake them interesting materials for aw ide range of applications. [8] Additionally, there is growing recent interest in PA NI chains polymerized in the meta-position (m-PANI), since its well-known roomtemperature ferromagnetic behaviour [9,10] has become of interest for tuning electronic and magnetic states,s pin transport and spin interactions that may have important implications for applications in organic conductors and spintronics [11] and as possible precursor materials for the development of carbon magnets. [12] Albeit there are several synthetic routes to m-PANI homopolymers [13][14][15][16] or m-PANI/ p-PANI copolymer-like structures, [17,18] these reactions can be challenging,r equiring multistep processes to obtain adequately pure materials.
On-surface synthesis has consolidated as aunique strategy to obtain unprecedented low-dimensional carbon-based nanomaterials with atomic precision showing intriguing properties. [19] Among the variety of reactions successfully tested on surfaces,U llmann coupling remains the preferred methodology for inducing on-surface homo-and heterocoupling of individual molecular precursors.W ith such strategies,d iverse one-dimensional nanostructures including conducting polymers and atomically precise graphene nanoribbons (GNRs) have been recently synthesized. [20][21][22][23][24][25][26][27] However,t his methodology presents several drawbacks,s uch as surface poisoning by the detached halogen, or the need to dispose of halogen modified precursors.Although methods to reduce these limitations have been described, [28] alternative strategies,b ased on innovative reaction mechanisms,a re needed. Moreover,o ne of the most important merits of onsurface synthesis is the ability to open new reaction pathways that are inaccessible via solution-based organic chemistry, thanks to the particular catalytic role played by surfaces.Thus, in this work we describe am echanism for surface-induced selectivity in af ormal 1,4 Michael-type addition, without the need for ahalogen substituent. To illustrate this,wehave used platinum, ametal well-known for its strong dehydrogenation capability. [30,31] Specifically,weshow that the unique reaction pathway induced by the Pt(111) surface leads to the formation of linear meta-polyaniline (m-PANI) oligomers from ap recursor monomer functionalized at para positions (p-AP). We demonstrate that the interaction with the platinum surface not only drives the reaction mechanism at as pecific site but also aligns the growing oligomers along the crystallographic directions of Pt, self-limiting the length of the oligomers due to stress accumulation. These type of reactions,w here the position of attack differs from the activated site,are very rare, thus very interesting as they open new reaction pathways.
Ther eaction mechanism is unveiled by ac ombination of in situ advanced microscopy and spectroscopy techniques, including non-contact atomic force microscopy (nc-AFM), scanning tunneling microscopy (STM) and spectroscopy (STS), synchrotron-based X-ray photoemission spectroscopy (XPS), and theoretical calculations.Our results unequivocally demonstrate the chemical transformation of the precursors upon annealing and the chemical structure of the synthetized oligomers.Moreover,these techniques allow us to determine the electronic structure of the oligomers,w hich is distinguished by the presence of unoccupied localized electronic states in the chains.This work is aclear example demonstrating that chemical routes that are complex or inefficient in solution-based chemistry can be accomplished via on-surface chemistry,o pening the door to new approaches for the synthesis of defined supramolecular structures.

Results and Discussion
Theinteraction of p-AP molecules on aPt(111) surface at room temperature has been previously described whereby the formation of partially ordered molecular layers was found. [34] Figure 1a shows atypical STM/nc-AFM image obtained after depositing p-AP molecules on aPt(111) surface at atemperature of around 525 K. This image shows some individual isolated molecules that have not reacted together along with two types of elongated structures.O nt he one hand, curved chains with an inhomogeneous appearance are observed. On the other, homogenous linear chains can be seen, made of individual links ranging from 3t o1 2u nits.T he latter chains are oriented at 308 8 with respect to the main crystallographic directions of Pt(111) and show an average internal periodicity   (Figure 1b), the chain appears as ap eriodic repetition of bright elliptical lobes,w hilst in the constant height image (Figure 1c)t he lobes have ar ounded shape.I mportantly, these maxima present an identical appearance in all the oligomers observed, suggesting that each has the same local structure.I no rder to improve the intramolecular resolution, af requency-shift (Df)i mage of the tip-oligomer force interaction was performed on the same chain using aC O functionalized tip (Figure 1d). In this image,t he lobes are resolved as tilted hexagonal shapes,which we attribute to the phenyl ring of each building block. In addition, some rounded protrusions appear when the tip is close to the sample.A t present, we do not have aclear interpretation of the nature of these protrusions.Itcan also be seen that the top and bottom edges of the frequencys hift image show protrusions that differ in appearance,w hich is in good agreement with the expected differences between the chain terminations,a s discussed below.
To unveil the chemical nature of these oligomers,wehave studied the thermal behaviour of the core levels associated to the molecules using synchrotron radiation-based XPS following two different approaches.O nt he one hand, we have performed a"fast" (real-time) XPS of the C1s, N1s, and O1s core level peaks while ramping the sample temperature from 120 Kt o1 025 K. Ther esults are plotted in the 2D representation shown in the top panel of Figure 2. [35] This type of experiments allow for aone-shot qualitative understanding of the chemical transformations experienced by the molecule during annealing. Although the absolute temperatures are usually overestimated in this type of measurements due to kinetic considerations (the system has no time to thermalize), these pictures are very useful to determine at ag lance the temperature regions where chemical transformations take place.T hree different phases can be distinguished in the "fast" XPS,a nd detailed high-resolution XPS spectra have been recorded at each of the selected regions (lower panel in Figure 2). Theb ottom spectra in Figure 2( black line) correspond to am ultilayer of p-AP molecules recorded at 80 K, used as ar eference for the core level binding energies (BE). TheB Eo btained for the amino group,t he alcohol group,a nd the carbon ring are 399.5 eV,5 32.8 eV,a nd 284.7 eV,r espectively.T hese values appear shifted by about 0.4 eV to higher BE than the values reported in literature for adsorbed molecules [36][37][38] due to the presence of am ultilayer thick enough to prevent acontribution from the contact layer, which experiences afinal state screening by the Pt substrate.
In the temperature range from 120 Kto275 K(region A), most of the molecules preserve their canonical structure,with am inority of them undergoing the initial stages of the dehydrogenation of the alcohol groups.T he second phase occurs between 275 Ka nd 475 K( range B) and corresponds to STM images showing individual molecules.The intensity of all peaks slightly decreases,b ut the total stoichiometry derived from XPS quantitative analysis (see Supporting Information) indicates that neither Nn or Oa toms are removed from the molecules.However,full dehydrogenation of the hydroxyl moieties leads to the appearance of new components in the O1 sc ore level peak. Most of the species present as trong interaction with the Pt surface (new component at 530.3 eV for O1 s) through activated site 6 (see Scheme 1), while the rest adopt aq uinone form as suggested by the binding energy of the related peak (532.3 eV). [38][39][40] On the other hand, most of the amine groups remain intact (BE of 399.35 eV for N1s) while aminority of the molecules present partially and fully dehydrogenated nitrogen species (peaks with BE of 399.9 eV and 397.6 eV, respectively). [41] Finally,a tt emperatures around 475 K( range C), linear chains are formed through covalent coupling between the activated molecules,ascan be observed in the STM images of Figure 1. XPS reveals that oxygen is fully removed from the surface,leaving an insignificant contribution at 533 eV,due to adventitious oxygen (note that XPS measurements are carried out at 120 K). On the other hand, two contributions for N1 sa re observed, located at 399.8 and 397.6 eV,w hich suggest that the linear chains are comprised of aryl units linked by nitrogen atoms.I nd ifferent studies related with polyanilines,t he core level associated with benzenoid amine varies in the range of 399.9-399.3 eV and for quinone imine between 398-398.9 eV. [42][43][44] Thev alue of 397.6 eV corresponding to the quinone imine is lower than expected, but this O1sXPS core levels as afunction of the annealing temperature after evaporation of p-AP on Pt(111). The N1s"fast" XPS scale ranges from the less intense signal in black to the more intense in yellow,while for O1s ranges from the less intense in blue to the more intense in red colour.Three different regions (A, B, C) are observed. At right side, characteristic STM images for these regions. Lower part:C haracteristic high-resolution C1s, N1sand O 1s XPS core levels spectra recorded after depositing at high temperature and cooling at 120 Katt he three different regions (A, B, C) observed in the top panel. The blue spectra correspond to amolecular multilayer,while the purple and green ones are associated to monolayer or submonolayer coverage due to moleculardesorption during sample annealing. difference can be attributed to charge redistribution due to the interaction with the metal surface.T he C1 sc ore-level peak presents three components at 283.4 eV,2 83.9 eV,a nd 284.8 eV,t hat can be attributed to the C ring -Pt bonds,Cring and azo-linkages,respectively.Therefore,the XPS analysis of N1sand C1score levels indicates that the oligomers depicted in Figure 1p resent aP ANI structure,w ith ac o-existence of imine and secondary amine linkages.
Region Ci nt he "Fast" XPS data in Figure 2s hows an important decrease in the N1 si ntensity (much less pronounced in the C1speak) and an increasing peak shift upon increasing temperature.T hese effects can be rationalized in terms of agradual temperature-induced decomposition of the oligomers and aconcomitant loss of Natoms as aconsequence of the interaction with the metal surface (see corresponding shift of the C1 sp eak towards lower BE characteristic of Cmetal interactions). Interestingly,atthe extreme temperature of 1025 K, the N1 sp resents two components at approximately 400.5 eV and 398.3 eV.T he former is attributed in literature to Na toms embedded in ag raphene lattice, [45,46] whilst we assign the latter to the remaining oligomers that partially preserve their structural integrity.Decomposition of azo-derived molecules at high temperature on platinum surfaces and formation of small graphene areas has already been reported (see Supporting Information). [30,47] Figure 3a shows the thermal evolution of the normalized intensity of the three components of the N1 sphotoemission peak, as extracted from adetailed deconvolution of the XPS peaks.I tc an be observed that at RT the NH 2 moiety predominates (60 %), as is expected for the adsorbed molecule,t ogether with lower contributions of -NH (30 %) and -N= (10 %). As previously mentioned, from the STM images two types of elongated molecular structures can be distinguished on the surface:l inear (discussed above) and curved structures.F igure 3b shows aS TM image of az one where both structures coexist. Theformer (highlighted by red ellipses) are assigned to fully oxidized PA NI oligomers based on imine links,while the latter structure (highlighted by blue ellipses) would correspond to the reduced form of PA NI formed by -NH-(amine) links.T his assignment is based on the experimental observation from STM images of an increasing proportion of linear vs.c urved oligomers with temperature,t he former being prevalent at 625 K( see Supporting Information). We rationalize this by considering as urface oxidation of the amine links into imine links. Moreover,t hese findings are corroborated by the structural models inferred from theoretical calculations,aswewill argue below.
Taking into account the aforementioned information provided by STM/AFM and XPS,o ne can conclude that p-AP molecules on Pt(111) lead to the formation of PA NI oligomers above 475 K. Af irst approximation would be to assume that the polymerization takes place in a para configuration (positions 1-5 in Scheme 1), as oxygen has been cleaved from the carbon ring.However,adetailed analysis of the experimental results indicates that this cannot be the case. Thep eriodicity within the oligomers,r epresented by the distance between adjacent phenyl rings for free p-PANI, is expected to be 5.2 ,a ss hown by accurate gas phase DFT calculations and elemental arguments based on the wellaccepted dimensions for these molecules.H owever,w eh ave found an experimental periodicity of 4.6 AE 0.1 ,l eaving adiscrepancy of 13 %, which is too large to be accommodated by surface-induced molecular distortions.Moreover,F igure 1 shows that all repeat units within each oligomer are equivalent, whereas the p-PANI structure would present az ig-zag configuration resulting from out-of-plane alternate tilting of the phenyl rings.Therefore,todetermine the atomic structure of the oligomer,w eh ave performed an extensive number of DFT calculations and the optimization of different plausible structures.Abest candidate that fits well with both XPS and STM/AFM data has been found and is presented in Figure 4 (see Supporting Information for the rest of the structures). This model suggests that the oligomers adopt an m-PANI configuration that is stabilized along the [21 1 ]s urface direction (and equivalent symmetry-related directions), in agreement with the experimental observation of Figure 1a, whilst maintaining as ignificant interaction with the surface through position 5 of the phenyl ring (see Scheme 1). In fact, this interaction is reflected in the short adsorption distance between C5 and Pt surface,2.1 ,indicating covalent bonding of Ca tom with the surface,a nd in the C1 sc ore level signal where as mall peak at 283.4 eV appears (one sixth of the C intensity,c orresponding to the expected ratio between C species). As aconsequence,the different phenyl rings present an in-phase torsion angle ranging between 208 8 and 308 8 that is responsible for the shape of the aryl repeat units observed in Figure 1d.F inally,the Pt atoms connecting with these lowest Ca toms exhibit ap erpendicular out-of-plane buckling of around 0.15 ,c onfirming as ignificant build up interaction, whereas the rest of the Pt atoms maintain the in-plane configuration.
Another important result derived from the calculations is that the oligomers are not commensurate with the Pt surface. Thep referred model (Figure 4) exhibits ac omputed periodicity of 4.62-4.68 (to be compared with the experimentally measured average periodicity of 4.6 AE 0.1 ), which does not fit with the Pt-Pt distance of 4.81 along the [21 1 ]h ighsymmetry direction. This fact implies that each arylamine unit added to the chain accumulates stress corresponding to Panel c) of Figure 4s hows the computed STM image in constant-current mode.T he correlation between simulated and experimental images is excellent, both showing al inear sequence of rounded protrusions,w ith one of the extremes presenting adifferent appearance due to the presence of the NH 2 group.T he superposition of the geometrical model over both the nc-AFM image (Figure 4b)and the theoretical STM image (Figure 4c)a llows the assignment of each rounded protrusion to the aryl units in the chain, except the one closest to the NH 2 extreme,w here the intensity of the ring is attenuated and as maller protrusion associated to the C connected to the NH 2 group appears,a si ti so bserved in the experiment. The Df image in Figure 4b provides aclear view of the asymmetry in the terminal groups of the oligomer.
As we have mentioned, m-PANI chains are not commensurate with the Pt substrate,r egardless of their length. To understand the reasons,w ee xplore the energetics and electronic properties of the oligomer adsorbed on the surface. In the proposed model of Figure 4, monomers are linked by C À N = Cu nits,w hich have bond lengths between Ca nd No f 1.33 and Pauling bond orders of around 1.5, reflecting the resonance between as ingle and ad ouble bond. Such ac onfiguration stores À5.3 eV per CÀNb ond (gas phase) and results in ar igid configuration, which does not accommodate rotations easily.F urther, it dictates al ength for the repeating unit of the polymer that is 0.2 shorter,w hich prevents commensuration with the underlying Pt lattice. Therefore,t he interplay between its internal energy and the interaction with the substrate results in stress for the growing polymer chain. To complete our understanding on the formation of the oligomer,w eh ave performed additional simulations;i tis interesting to briefly discuss the case where monomers are joined via C-NH-C units (e.g. configuration II in Figure S1). Here,C À Nbond lengths extend to 1.39 ,and it stores À3.9 eV (gas phase), with ab ond order close to 1. Such ac onfiguration is more flexible than the previous one and it generates ar epeat unit in the polymer that can commensurate better with the substrate.H owever,t he commensurability only increases the interaction energy per monomer with the substrate from 1.3 to 1.95 eV (D = À0.65 eV), which explains the tendency of monomers to link via C À N = Cg roups,d espite the induced stress.F inally,w e comment on passing that since C-NH-C groups accommodate rotations better, and the XPS signal proves that these groups persist especially at lower temperatures,itislikely that these are related to the images where ac urved configuration has been observed (e.g. Figure 3).
We have also investigated, from atheoretical perspective, the most viable coupling mechanism that could lead to the onsurface formation of m-PANI oligomers.W eh ave employed the Climbing-Image Nudged-Elastic Band methodology to explore different scenarios for the dimerization of p-AP on the Pt surface,i ncluding distinct initial molecular configurations and attacking sites. Figure 5p resents ap ictorial view of the main results obtained for the two most favourable dimerization couplings taking place at positions 1 and 4 (other less favourable coupling reactions are reported in the Supporting Information). It is worthy to note that, in both cases,t he initial molecular composition employed was that corresponding to the results obtained from XPS at the temperature of interest, that is,t he precursor molecules are dehydroxylated and the amino group is either partially or fully oxidized. Coupling I corresponds to the dimerization between two C 6 H 4 Np recursors anchored to the surface through ring position 5.Inthis pathway,the C À Hbond (site 4)reacts with the C À Nbond of the adjacent molecule to form aC -NH-C linkage with an enthalpy gain of À3.53 eV and abarrier for the transition state of DE TS = 0.52 eV.C oupling II shows the dimerization between two C 6 H 4 NH precursors.I nt his pathway,aC ÀH bond reacts with aC-NH to form C-N-C bond and agas-phase H 2 molecule,with an enthalpy gain and abarrier of À2.37 eV and 0.44 eV,respectively.Notice that these values correspond to species adsorbed on the surface and include the influence of the substrate on the polymerization energy.T hus,a ccording to these results,t he most energetically favourable dimer coupling would correspond to Coupling I. However,the very small difference in the energy barrier (D =+0.08 eV) implies that both coupling mechanisms could be kinetically viable on the surface;C oupling II leads to the growth of the proposed m-PANI linear oligomers,w hereas Coupling Ic ould explain curved chains due to its enhanced flexibility towards small rotations.
As we have commented, even though the apriori natural linking positions between the p-AP precursors would be the para positions (1-5,Scheme 1), this situation can be ruled out since the 1-5 coupling would result in am uch larger repeat distance,a bove 5.2 .T og row on the Pt(111) surface,t his large difference implies strong deformations in the oligomers that are not observed in the straight chains. Now that we have determined the nature of the so-formed oligomers,w eh ave turned our attention to their electronic local density of states (LDOS) as probed by dI/dV maps (Figure 6a-c) and single-point dI/dV spectra (Figure 6b). Figure 6b apresents aset of single-point spectra recorded at different positions along the oligomer chains,t ogether with the curve corresponding to the surface,w hich is used as reference.I tc an be clearly observed that two unoccupied states are present at around 200 and 650 mV.I no rder to determine the spatial distribution of these states,c onstant height topography and differential-conductance map images have been acquired at the maximum energy position of each and are presented in Figures 6a and c. Interestingly,t here is an evident change in the appearance of the oligomers.While at 200 mV they can be visualized as ad ouble maximum, at 650 mV each unit is made of asingle rounded feature located at the centre of the two protrusions present at 200 mV,i n marked contrast to the elliptical appearance of the constant current image of Figure 1b.E ven more interesting,t he simulations indicate that these maxima correspond to the tilted phenyl rings and not to the linking Na toms,w hich are closer to the surface.Furthermore,the state at 200 mV is not symmetric,b ut one lobe is higher than the other (Figure 6a, lower links of the chain:left lobe stronger than right lobe) and as we move along the chain the situation is inverted (Figure 6a,upper links of the chain:right lobe stronger than left lobe).
Forc omparison, we have computed differential-conductance STM images (constant-current mode) at 0.1 nA and 200 AE 25 and 700 AE 25 mV on an 11-link oligomer. Figure 6a and cs how the good agreement of both images with the experimental maps.A dditionally,w eh ave computed the Projected Density of States (PDOS) on a5 -unit m-PANI oligomer on Pt(111) (see details in supporting information). Albeit difficult because of the hybridization, the single-lobe state at 650 mV seems to originate from apronounced feature in the PDOS profile arising between 650 and 800 mV. Similarly,t he two-lobe state at 200 mV,w hich also has ar eflection in the PDOS profile,c an be associated with the LUMO of the oligomer.
Our theoretical calculations indicate that these oligomers, albeit lacking one hydrogen atom in the repeat units,present ferromagnetic properties. [9,10] However,e lectronic hybridization with the substrate quenches the ferromagnetism when the oligomer is on the surface (see supplementary information).  . Electronic local density of states (LDOS) on am-PANI chain composed of 8u nits, as extracted from STS measurements. a) from left to right:T unneling current in constant height (regulation set point in the middle of the chain at 50 pA and À200 mV) recorded at 200 mV, Differential-conductance(dI/dV)maps of unoccupied states (modulation 8.7 mV,) and theoretical dI/dV map on a11units oligomer.b)dI/ dV spectra for the specific positions of the chain indicated as bolddots in Figure 6a.The brown one corresponds to the typical LDOS of clean Pt(111). The green and the blue spectra are recorded at the edges of the chain and the red one represents the electronic states at the center of the chain. c) same as panel (a), but recorded at 650 mV. Bottom panel shows adetail of the dI/dV map to emphasize the double lobe structure for low voltages (left side) and the single lobe for higher values (right side).
Theabove-described mechanism is based on asequence of different dehydrogenation reactions in amine moieties and carbon rings,w hich have been previously described to take place stepwise at different temperatures on different surfaces. [31,48,49] Moreover,the proposed mechanism, confirmed by DFT calculations,explains why coupling occurs at adifferent site than the radical formation, as one could expect ap riori. We show that although para-precursors are used the bonding with the surface changes the coupling position resulting in a meta-coupling of the chains. [29,32,33]

Conclusion
We report an unprecedented on-surface driven formal Michael-coupling leading to the formation of meta-polyaniline oligomers on Pt(111) surfaces directly from p-aminophenol precursors,w hich we investigate using advanced scanning probe microscopies and electron spectroscopies, combined with theoretical methods.W es how that the paraaminophenol molecules adsorbed on Pt(111) at room temperature are reduced to p-quinone imines and adopt an appropriate spatial position on the surface that promotes atemperature activated coupling leading to the formation of m-PANI metal-oligomers.T he mechanism has been rationalized by first principles calculations that suggest only as mall energy barrier for the process.T he m-PANI oligomers are incommensurate with respect to the Pt surface.A st he molecule grows adding more repeat units,t he accumulated mismatch leads to an important stress increase that limits the number of repeat units in the oligomer chains.The oligomers present well-defined unoccupied electronic states at 200 and 650 mV that are spatially localized on the carbon rings.T he reaction pathway proposed here is unviable in solution chemistry and it presents ah igh directionality that may be used for ar ational synthesis of novel nanostructures of interest in the emerging fields of nanomagnetic materials and spintronics.