On-surface light-induced generation of higher acenes and elucidation of their open-shell character

Acenes are an important class of polycyclic aromatic hydrocarbons which have recently gained exceptional attention due to their potential as functional organic semiconductors. Fundamentally, they are important systems to study the convergence of physico-chemical properties of all-carbon sp2-frameworks in the one-dimensional limit; and by virtue of having a zigzag edge topology they also provide a fertile playground to explore magnetism in graphenic nanostructures. The study of larger acenes is thus imperative from both a fundamental and applied perspective, but their synthesis via traditional solution-chemistry route is hindered by their poor solubility and high reactivity. Here, we demonstrate the on-surface formation of heptacene and nonacene, via visible-light-induced photo-dissociation of α-bisdiketone precursors on an Au(111) substrate under ultra-high vacuum conditions. Through combined scanning tunneling microscopy/spectroscopy and non-contact atomic force microscopy investigations, together with state-of-the-art first principles calculations, we provide insight into the chemical and electronic structure of these elusive compounds.

(500 ml). The mixture was stirred for 5 days at rt. The reaction was quenched with sodium hydrosulfite, and the reaction mixture was stirred for 10 min. After removing acetone under reduced pressure, the organic phase was extracted with AcOEt, and then washed with water and brine, and then dried over Na 2 SO 4
After the reaction, the solvent was removed under reduced pressure. The residue was dissolved in AcOEt, and the solution was washed with NaHCO 3 -aq, water and brine. The   Compound 7a. Pyridine (28 ml) was added to thionyl chloride (33 ml) at 0 °C under Ar atmosphere. After stirring for 10 min, 6a (24 g, 105 mmol) was added portionwise to the reaction mixture. Additional thionyl chloride (46 ml) was slowly added, and the reaction mixture was stirred for 30 min at 0 °C. The mixture was slowly heated to reflux for 2 h. After cooling to 0 °C, the solution was diluted with DCM (300 ml), and poured into ice water.
The reaction mixture was extracted with DCM, and the combined organic phase was washed with NaHCO 3 -aq, 3 M HCl and brine. The organic phase was dried over Na 2 SO 4 , and was concentrated under reduced pressure. The residue was washed with MeOH to af-  Compound 11. The suspension of 10 (597 mg, 1.62 mmol), p-chloranil (1.6 g, 6.59 mmol) and K 2 CO 3 (3.1 g) in dry-toluene (300 ml) was heated to reflux for 12 h under Ar atmosphere. The hot-reaction mixture was filtered, and the filtrate was washed with hot-toluene.
After cooling to rt, the filtrate was washed with 20% NaOH-aq, water, and then dried over Na 2 SO 4 . The organic phase was concentrated under reduced pressure, and the residue was dissolved in DCM, and anti-isomer was filtrated off. The filtrate was purified by silica gel column chromatography (hexane/DCM = 4:3, R f = 0.6), and then further purified by recrystallization with DCM and MeOH to afford 11 (263 mg, 0.50 mmol, 45%) as a white solid. Compound 12. OsO 4 (270 mg, in microcapsule 10% w/w, 0.1 mmol) was added portionwise to a suspension of 11 (615 mg, 1.2 mmol), NMO (1.5 g, 12.8 mmol) in acetone (100 ml). The mixture was stirred for 3 days at rt. Then, the reaction was quenched by addition of an aqueous solution of sodium hydrosulfite, and then the mixture was stirred for 10 min.
After concentrated in vacuo, the brown residue was dissolved in AcOEt, and then was washed with water, brine, and then dried over Na 2 SO 4 . The organic phase was concentrated under reduced pressure, and the residue was purified by silica gel column chromatography  Supplementary Figure 12. a) Computed Fermi-Dirac distribution curves in blue, red and black for 100, 300 and 400 K, respectively, illustrating the temperature-dependent width of the Fermi edge. b) Experimental Fermi-Dirac distribution curves obtained by Ultraviolet Photoelectron Spectroscopy (UPS) measurements of the Au(111) substrate. Blue, red, green and black symbols correspond to measurements performed with the sample cooled to low temperature (LT, 50 K), without cooling or heating at room temperature (RT, 282 K), during exposure to 470 nm light for three hours at room temperature (3h, 288 K), and with the sample heated to 120 °C (395 K)), respectively. Temperatures indicated in brackets are those measured with a silicon diode located close to the sample holder, and are thus lower/higher than the actual sample temperature in case of sample cooling/heating, respectively. The curves are vertically offset for clear differentiation of each curve. Fitting of the experimental data in the range of -0.3 eV to +0.05 eV binding energy (black, green and red curves) or -0.3 eV to +0.04 eV binding energy (blue curve) has been done with the usual Fermi-Dirac function on a linear background. The resulting fits are shown as thin lines. c) Experimental values of the Fermi energy (chemical potential) and the sample surface temperature, determined from the fits shown in (b). Supplementary Figure 18. Frontier orbitals of heptacene and nonacene. a,d) Constant height LDOS (plane 4 Å above molecular plane) for the HOMO of the N+1 charged system (top) and for the LUMO of the N-1 charged system (bottom) for heptacene and nonacene, respectively. b,e) Constant height LDOS (plane 4 Å above molecular plane) for HOMO (top) and LUMO (bottom) of the CS neutral system for heptacene and nonacene, respectively. c,f) isosurface of the probability amplitude (red and blue correspond to positive and negative isovalues) for HOMO (top) and LUMO (bottom) of the CS neutral system of heptacene and nonacene, respectively. The fact that the HOMO and LUMO of the CS neutral system are similar to the HOMO of the N+1 and N-1 charged systems respectively indicate that a CS character is still present in the ground state of the two systems.

Computational details.
To obtain the equilibrium geometries of the molecules adsorbed on the Au(111) substrate and to compute corresponding STM images we used the CP2K code 3,4 implementing DFT within a mixed Gaussian plane waves approach. 5 The surface/adsorbate systems where modeled within the repeated slab scheme, 6 i.e., a simulation cell contained 4 atomic layers of Au along the [111] direction and a layer of hydrogen atoms to passivate one side of the slab in order to suppress one of the two Au(111) surface states. 40 Å of vacuum were included in the simulation cell to decouple the system from its periodic replicas in the direction perpendicular to the surface. The electronic states were expanded with a TZV2P Gaussian basis set 7 for C and H species and a DZVP basis set for Au species. A cutoff of 600 Ry was used for the plane waves basis set. Norm-conserving Goedecker-Teter-Hutter 8 (GTH) pseudopotentials were used to represent the frozen core electrons of the atoms. We used the PBE parameterization for the general gradient approximation of the exchange correlation functional. 9 To account for van der Waals interactions we used the scheme proposed by Grimme. 10  For the GW calculations, we used the CP2K code 11 . We employed eigenvalue-self consistent GW 12,13 to compute the HOMO-LUMO gap of gas phase acenes based on PBE wavefunctions. We employed GTH pseudopotentials, the aug-DZVP basis from Ref. 11 and analytic continuation with a two-pole model. We apply an image charge model 14 to account for the screening by the metal surface. A distance of 1.58 Å between the image plane and the planar acene molecules has been used. This distance has been computed as difference between the adsorption height of 3.1 Å above the first substrate layer, see the main manuscript, and the distance of 1.42 Å between the image plane and the first substrate layer which has been taken from Ref. 15 .