Surface‐Assisted Synthesis of N‐Containing π‐Conjugated Polymers

Abstract On‐surface synthesis has recently emerged as a powerful strategy to design conjugated polymers previously precluded in conventional solution chemistry. Here, an N‐containing pentacene‐based precursor (tetraazapentacene) is ex‐professo synthesized endowed with terminal dibromomethylene (:CBr2) groups to steer homocoupling via dehalogenation on metallic supports. Combined scanning probe microscopy investigations complemented by theoretical calculations reveal how the substrate selection drives different reaction mechanisms. On Ag(111) the dissociation of bromine atoms at room temperature triggers the homocoupling of tetraazapentacene units together with the binding of silver adatoms to the nitrogen atoms of the monomers giving rise to a N‐containing conjugated coordination polymer (P1). Subsequently, P1 undergoes ladderization at 200 °C, affording a pyrrolopyrrole‐bridged conjugated polymer (P2). On Au(111) the formation of the intermediate polymer P1 is not observed and, instead, after annealing at 100 °C, the conjugated ladder polymer P2 is obtained, revealing the crucial role of metal adatoms on Ag(111) as compared to Au(111). Finally, on Ag(100) the loss of :CBr2 groups affords the formation of tetraazapentacene monomers, which coexist with polymer P1. Our results contribute to introduce protocols for the synthesis of N‐containing conjugated polymers, illustrating the selective role of the metallic support in the underlying reaction mechanisms.


General methods.
Chemicals and reagents were purchased from commercial suppliers and used as received. Analytical thin-layer chromatography (TLC) was performed using aluminum-coated Merck Kieselgel 60 F254 plates. Purification by column chromatography was performed using silica gel (Merck, Kieselgel 60, 230-240 mesh or Scharlab 60, 230-240 mesh). NMR spectra were recorded on a Bruker Advance 300 ( 1 H: 400 MHz; 13 C: 101 MHz) spectrometer at 298 K using partially deuterated solvents as internal standards. Coupling constants (J) are denoted in Hz and chemical shifts (δ) in ppm. Multiplicities are denoted as follows: s = singlet, d = doublet, t = triplet, m = multiplet.

DFT calculations of P3 on Ag(100)
To model the structure of P3 species, we first calculated different adsorption configurations of a single molecule with the =CBr 2 groups removed, both with and without hydrogen atoms bonded to the central carbon atoms of the molecule. The adsorption configurations, with their relative energies, are shown in Figure SI8 and SI9 without and with hydrogen terminations, respectively.  Based on the most stable adsorption configurations both with and without H on the former =CBr 2 sites, periodic self-assembled structures were constructed and geometrically optimized.
For the optimized geometries we performed STM simulations. The optimized structures are shown in Figure SI10 and the corresponding STM simulations in Figure SI11.

Coupling mechanisms
For the calculations of coupling mechanisms on Ag(111) and Au(111) we started from dehalogenated two molecules in the most stable adsorption configurations on the two surfaces.
In other words, we first investigated the most stable adsorption configurations on the two surfaces. In Figure SI12 and SI13 the most stable adsorption configurations of the dehalogenated molecules are shown for Ag(111) and Au(111), respectively, and the coupling mechanisms are shown in Figure SI14 and SI15.      Computed reaction mechanism for the step-wise transformation of a four-unit oligomer connected by ethynylene bridges (S0) to an oligomer with pyrrole-pyrrole connects (S3) in gas-phase, depicting local minima (S0, S1, …) and transition states (TS1, TS2, … ) and corresponding energy profile. Units in eV.

Adsorption configurations of oligomers on Au(111) and Ag(111)
For the calculations of the oligomer transformation reactions different adsorption configurations were considered for both reactants and products, shown in Figure SI18-SI21 for the atomically flat surfaces. Furthermore, on Ag(111) we considered the reaction with Ag adatoms, for which different possible initial configurations are shown in Figure SI22 (the most stable one was used as starting point for the reaction shown in Figure 4b of the main manuscript.) Finally, on Ag(111) we also did calculations of an oligomer consisting of six units, both without and with adatoms ( Figure SI23-SI24). The most stable configuration in each case was used for the STM simulations.

Adatom entropy considerations
To estimate the entropy of Ag adatoms on the Ag(111) surface we employed the complete potential energy sampling (CPES) 3 . In this approach, the translational partition function at a temperature is given by where is the mass (of the Ag adatom in this case), is Boltzmann's constant and is Planck's constant.
is the potential energy of the adatom at different lateral positions and . The integration is performed numerically over a surface cell of area while the area is the available area per adatom (i.e., the inverse of the adatom surface concentration). The fraction is needed to include the concentration of adatoms into the partition function. If the adatoms are assumed to move freely on the surface, in other words , the partition function reduces to the free translator (FT) partition function , i.e., the translational partition function for a gas of one particle confined to an area .
Using either of the partition functions, the entropy in the canonical ensemble is given according to ( ) .
For the free-translator partition function, this gives the 2D Sackur-Tetrode equation.
To calculate the potential energy landscape used to calculate we mapped the potential energy surface of an Ag adatom on the Ag(111) surface by freezing the lateral position of the Ag adatom at positions on the surface while allowing optimization of remaining degrees of freedom (including the uppermost layers of the Ag slab). The resulting potential energy surface is shown in Figure SI25. This potential energy surface was then used to construct the translational entropy within the CPES picture, and compared to the freetranslator model, for different surface coverages and temperatures in Figure SI26. If assuming a surface concentration of 0.1 nm -2 (close to the value used in the article mentioned by the reviewer) the entropy is 0.99 meV/K at 200 according to the CPES calculations, which corresponds to a free energy contribution of -0.47 eV per adatom. Figure SI27 compare the energy profiles with and without the adatom entropy for the ladderization on Ag(111), demonstrating that including adatom entropy lowers the energy profile, making the overall reaction exergonic.
While we would not like to make claims about the precise entropy of the adatoms, it is clear that it does affect the energy landscape and can explain the ladderization at elevated temperatures.   ) and by including the translational entropy ( ) of the removed adatoms. The entropy was evaluated at a temperature of 200 and a surface concentration of adatoms of 0.1 nm -2 using a complete potential energy sampling. In Figure SI26 the translational entropy is plotted for different temperatures and surface concentrations. Units in eV. Figure SI28. Calculated results for a hypothetical P1 oligomer consisting of four molecular units on Au(111) with the nitrogen atoms coordinated to Au adatoms (S0), demonstrating the energy cost of removing one (S1) and two (S2) adatoms from the structure. In (a) the top and side views of the different structures are shown, while the energies of the respective local energy minima with respect to S0 are displayed in (b) in units of eV. The energies could be compared to the corresponding states in Figure 4 for Ag(111). On Au(111) it requires 0.90 eV to remove one adatom and 1.88 eV to remove two adatoms, while the corresponding numbers for Ag(111) are 1.13 eV and 2.35 eV, respectively. I.e. adatoms bind weaker in the P1 polymer for Au than Ag, which could be a contributing factor why the P1 polymer is not observed on Au(111).