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.

Keywords: conjugated polymers; N-heteroacenes; on-surface synthesis; scanning probe microscopies.

Scheme 1

Reaction pathways of 4Br4AzaPn on Au(111), Ag(111), and Ag(100).

Figure 1

Structural and electronic characterization of P1 on Ag(111). a) Chemical scheme illustrating the on‐surface synthesis of P1. b) Overview STM image after depositing a submonalayer coverage of 4Br4AzaPn on Ag(111) at RT. V _b = 200 mV, I _t = 10 pA, scale bar = 1 nm. c) nc‐AFM image of (b). V _b = 3 mV, scale bar = 1 nm. d) HR‐STM image of P1. V _b = −1 V, I _t = 350 pA, and scale bar = 6 Å. e) Nc‐AFM image of P1 revealing a bright protrusion at the middle of the ethynylene bridge, which is attributed to a triple bond. V _b = 3 mV, scale bar = 6 Å. f) DFT model of the ethynylene‐bridged N‐containing pentacene polymer with two Ag adatoms bonded to the nitrogen atoms per unit cell (dark, blue, white, and light grey balls correspond to carbon, nitrogen, silver surface atoms, and silver adatoms, respectively). g) Constant‐height STM image of P1 showing an increased density of unoccupied states located perpendicularly to the triple bond highlighted by a white square. V _b = 3 mV, scale bar = 6 Å. h) Simulated constant‐height STM image of P1. i) Scanning tunneling spectra acquired on the positions depicted in (d) by the purple and cyan stars and reference spectrum taken on the bare Ag(111) surface (grey line).

Figure 2

Structural and electronic characterization of P2 on Au(111). a) Chemical scheme illustrating the on‐surface synthesis of P2. b) Overview STM image of the deposition of a submonolayer coverage of 4Br4AzaPn precursor on Au(111) after annealing at 100 °C. V _b = 1 V, I _t = 25 pA, scale bar = 3 nm. c) HR‐STM image of P2. V _b = 20 mV, I _t = 5 pA, scale bar = 2 nm. d) nc‐AFM image of P2 showing the non‐planar twisted geometry. e) Laplace filtered nc‐AFM image confirming the formation of the pyrrolopyrrole bridge. f) Superposition of chemical scheme on (e). d–f) V _b = 5 mV, scale bar = 5 Å. g) Scanning tunneling spectra acquired on the position depicted in (c) and on Au(111).

Figure 3

Structural characterization of P1 and P3 on Ag(100). a) Chemical scheme of the on‐surface synthesis of P1 and P3 on Ag(100). b) Overview STM image of the deposition a submonolayer coverage of 4Br4AzaPn precursor on Ag(100) revealing the formation of P1 (green square) and P3 (blue square). V _b = 20 mV, I _t = 5 pA, scale bar = 5 nm. c) HR‐STM image of P1. V _b = 20 mV, I _t = 5 pA, scale bar = 6 Å. d) Laplace filtered nc‐AFM image of P1. V _b = 3 mV, scale bar = 6 Å. e) HR‐STM image of P3. V _b = 20 mV, I _t = 5 pA, scale bar = 8 Å. f) Laplace filtered nc‐AFM image of P3. V _b = 3 mV, scale bar = 5 Å.

Figure 4

Reaction mechanisms on Ag(111) and Au(111). a) The coupling mechanisms between two dehalogenated monomers on Ag(111) and Au(111), comparing the direct formation of a pyrrolopyrrole connection with the stepwise formation of the pyrrolopyrrole via the ethynylene intermediate. Effective barriers for the different processes are shown and the details of the reactions are given in the Supporting Information. b) The reaction mechanism on Ag(111) of removing adatoms (S0 to S2) and the concomitant ladderization of an ethynylene bridge into a pyrrolopyrrole group (S2 to S3), showing local minima (S0‐S3) and transition states (TS1‐TS3) of the pathway and the associated energy profile. In both (a) and (b) energies are given in units of eV.