On-surface synthesis of non-benzenoid conjugated polymers by selective atomic rearrangement of ethynylarenes

Here, we report a new on-surface synthetic strategy to precisely introduce five-membered units into conjugated polymers from specifically designed precursor molecules that give rise to low-bandgap fulvalene-bridged bisanthene polymers. The selective formation of non-benzenoid units is finely controlled by the annealing parameters, which govern the initiation of atomic rearrangements that efficiently transform previously formed diethynyl bridges into fulvalene moieties. The atomically precise structures and electronic properties have been unmistakably characterized by STM, nc-AFM, and STS and the results are supported by DFT theoretical calculations. Interestingly, the fulvalene-bridged bisanthene polymers exhibit experimental narrow frontier electronic gaps of 1.2 eV on Au(111) with fully conjugated units. This on-surface synthetic strategy can potentially be extended to other conjugated polymers to tune their optoelectronic properties by integrating five-membered rings at precise sites.


SPM experiments
The experiments were carried out in an ultra-high vacuum (UHV) system, with a base pressure below 5 x 10 -10 mbar, hosting a low temperature (4.2 K) scanning tunneling microscope/non-contact atomic force microscope (Createc). Images were captured with a Pt/Ir tip attached to a qPlus sensor (resonant frequency ≈ 30 kHz; stiffness ≈ 1800 N m -1 ). A bias voltage was applied to the sample.
Sharp metallic tips were obtained by gentle indentations on the bare surface. For AFM imaging, a single CO molecule is picked up by the tip-apex previously dosed on the cold sample (T 10 K).
The Au(111) substrate was prepared by standard cycles of Ar + sputtering and annealing. Molecular precursors 1 and 4 were thermally sublimated in UHV onto the clean Au(111) substrate (kept at room temperature) from a tantalum crucible maintained at 80 ºC and 200 ºC, respectively. After annealing step, the sample was transferred to the STM stage held at 4.2 K. Conductance dI/dV point spectra and maps were acquired with conventional lock-in technique with a modulation of 1 and 10 mV, respectively (at frequency of 952 Hz). All data were subject of standard processes using the WSxM software 1 .
For data analysis approximately 100 polymers were considered in dozens of STM overviews of 50 x 50 nm 2 . The bond-length analysis considered a total set of 12 cis-and trans-monomers.

A. dI/dV simulations in the gas phase
Density functional theory (DFT) calculations were carried out using the all electron FHI-AIMS code 2 . For the description of the electronic structure of the freestanding unit cell, the hybrid exchange-correlation functional B3LYP 3 was employed in the calculations. The freestanding unit cell consists of 44 atoms. In all calculations, the default light settings were used for the numerical atomic basis sets. In all total energy calculations, the atomic structures were thoroughly relaxed until the Hellman-Feynman forces were smaller than 10 -2 eV Å -1 . To sample the Brillouin zone, all geometrical optimizations of the infinite systems with a one-dimensional periodic boundary condition used a Monkhorst-Pack grid of 18 x 1 x 1 k-points. The band structure plots were obtained with 50 k-points along the direction of 1D periodicity.
Theoretical dI/dV maps were calculated by the Probe Particle Scanning Tunneling Microscopy (PPSTM) code 4 for a CO-like orbital tip, represented by a combination of P x P y (90%) and s-like wave character (10%) without tip relaxation.

B. AFM Simulations
The AFM images were calculated with Probe Particle code 5 using the effective lateral stiffness k = 0.7 N/m for the flexible probe-particle tip model. The electrostatic interaction between the molecule and the probe particle was calculated using Hartree potential of polymer and differential electron density of CO-tip obtained from fully optimized DFT calculations. Pauli repulsion was calculated from the total density of both polymer and CO-tip.

C. Aromaticity
Gaussian16 6 program was used for the ACID and NICS calculations with B97X-D 7 hybrid functional and def2-SVP 8 basis set. NICS values were calculated 1 Å above molecular plane and only zz component was gathered. Only p z orbitals were included in the ACID calculations.

Synthesis of molecular precursors
Unless otherwise noted, commercially available reagents, solvents and anhydrous solvents were used as purchased without further purification. Dry THF was freshly distilled over Na/benzophenone. TLC plates were purchased from Sigma-Aldrich (silica gel matrix, with fluorescent indicator 254 nm) and were stained with phosphomolybdic acid (5% ethanol solution), or observed under UV light. Flash column chromatography was performed with Silica gel 60 (230-400 mesh, Scharlab, Spain). Silica gel G preparative TLC plates (20x20 cm, 500 micron) were purchased from ANALTECH. 1 H and 13 C NMR spectra were recorded at room temperature on a Bruker Avance and referenced to the signal of the residual protiated solvent ( 1 H: δ = 7.26 for CDCl 3 at room temperature) or the 13 C signal of the solvents ( 13 C: δ = 77.16 for CDCl 3 ) or to the signal of the residual TMS ( 1 H: δ = 0.00). Coupling constant (J) values are given in Hz. Abbreviations indicating multiplicity were used as follow: t = triplet, d = doublet, s = singlet. Assignment of the 13 C NMR multiplicities was accomplished by DEPT techniques. MALDI-TOF mass spectra were recorded on a Bruker Ultraflex III mass spectrometer. IR spectra were recorded with a Perkin-Elmer Spectrum Two FTIR ATR spectrometer.

Synthesis procedures and characterization details
Compound 1:

Single crystal X-Ray crystallography
X-ray diffraction-quality single crystals of compound 4 were obtained by vapor diffusion of acetonitrile into a chloroform solution of 4. The X-ray diffraction measurement was carried out on a Bruker D8 Venture diffractometer with a Photon III detector using Mo Kα radiation. SHELXT 10 was used to solve the structure, which was then refined applying the full-matrix least-squares against F 2 procedure with SHELX 2018 11 using the WinGX32 12 software. During the refinement, hydrogen atoms were placed in idealized positions (U eg (H) = 1.2U eg (C) or U eg (H) = 1.5U eg (C)) and were allowed to ride on their parent atoms.
Summary of the X-ray diffraction measurement and refinement data: Chemical formula,