Aza-Triangulene: On-Surface Synthesis and Electronic and Magnetic Properties

Nitrogen heteroatom doping into a triangulene molecule allows tuning its magnetic state. However, the synthesis of the nitrogen-doped triangulene (aza-triangulene) has been challenging. Herein, we report the successful synthesis of aza-triangulene on the Au(111) and Ag(111) surfaces, along with their characterizations by scanning tunneling microscopy and spectroscopy in combination with density functional theory (DFT) calculations. Aza-triangulenes were obtained by reducing ketone-substituted precursors. Exposure to atomic hydrogen followed by thermal annealing and, when necessary, manipulations with the scanning probe afforded the target product. We demonstrate that on Au(111), aza-triangulene donates an electron to the substrate and exhibits an open-shell triplet ground state. This is derived from the different Kondo resonances of the final aza-triangulene product and a series of intermediates on Au(111). Experimentally mapped molecular orbitals match with DFT-calculated counterparts for a positively charged aza-triangulene. In contrast, aza-triangulene on Ag(111) receives an extra electron from the substrate and displays a closed-shell character. Our study reveals the electronic properties of aza-triangulene on different metal surfaces and offers an approach for the fabrication of new hydrocarbon structures, including reactive open-shell molecules.


Experimental STM Experiments
STM measurements were performed using a commercial Scienta-Omicron LT-STM at 4.3 K. The system consists of a preparation chamber with a typical pressure in the low 10 -10 mbar regime and a STM chamber with a pressure in the 10 -11 mbar range. The Au(111) and Ag(111) crystals were cleaned via two cycles of Ar + sputtering and annealing (720 K for Au and 700 K for Ag). Ketone substituted triangulene precursor molecule was evaporated from a home-built evaporator at 460 K. Hydrogenation of the sample was achieved with a hydrogen cracking source with a leak valve. The preparation chamber was first filled to a pressure of 2 × 10 -7 mbar, after which the tungsten tube was heated to around 2800 K with a heating power of 80 W. The sample was then placed in front of the source for 2 minutes.
All STM and STS measurements were performed at 4.3 K. To obtain BR-STM images, the tip was functionalized with a CO molecule that was picked up from the metal surfaces. CO was deposited onto the sample via a leak valve at a pressure of approximately 5×10 -9 mbar and a maximum sample temperature of 7.0 K. CO can be picked up with a metallic tip by scanning with a high current and negative bias (e.g. I=1 nA, U=−0.5 V). Functionalization of the tip with a Cl atom is achieved by approaching the tip by 350 pm from initial stabilization conditions of 100 pA and 100 mV with the feedback off, on the top of the deposited NaCl island on Au(111). 1 After pickup, an increase in resolution is seen and a vacancy in the NaCl island can also be observed, which was reported in detail in our previous work. 2 dI/dV measurements were recorded with the internal lock-in of the system. The oscillation frequency used in experiments is 797 Hz. The amplitude for each spectrum is shown in Figure  captions.

DFT calculations
DFT calculations were performed by the Gaussian 16 package 3 using the M06-2X functional and 6-311G(d,p) basis set. Results were visualized by using the software Gaussview 4 and the squares of the wavefunction were generated using the cubman module.
For the positively-charged species, the atomic positions of the neutral N-doped triangulene 5 were used as input for the optimization. Initial optimization set to charge=+1 and spin=1 yielded a D 3h geometry, independent of whether the starting geometries corresponded to D 3h or C 2v geometry. Similarly, starting from any of the neutral C 2v or D 3h geometries led to a D 3h geometry for the negatively-charged species. All geometries and symmetries were confirmed further by re-performing calculations with tight convergence criteria using the keywords opt=tight int=ultrafine. Ionization energies ( Figure S9) were obtained as the electronic energy difference between potential energy surface minima of neutral and cationic species.

Solution synthesis
Starting materials were purchased from TCI and Sigma-Aldrich and used without further purification. O,O',O''-Amino-trisbenzoic acid-trimethylester (S3) and 4H-benzo [9,1]quinolizino[3,4,5,6,7defg]acridine-4,8,12-trione (1) were synthesized following slightly modified literature procedures. 6,7 Reactions were carried out in flame-dried glassware and under an inert atmosphere (Ar) using Schlenk techniques. Thin-layer chromatography (TLC) was performed on Silica Gel 60 F-254 plates (Merck). Column chromatography was performed on silica gel (40-60 µm). NMR spectra were recorded on a Varian Mercury 300 spectrometer. Synthesis of compound 1. A mixture of S3 (0.50 g, 1.2 mmol) and LiOH.H 2 O (0.90 g, 21 mmol) in THF: H 2 O (4:1, 25 mL) was refluxed for 6 h. After cooling to room temperature, the mixture was diluted with ethyl acetate (10 mL) and water (10 mL), and the phases were separated. The aqueous phase was acidified with concentrated HCl until pH = 2 and then extracted with ethyl acetate (3x10 mL). Organic extracts were combined, dried over anhydrous Na 2 SO 4 , filtered and evaporated. The crude product was dissolved in dichloromethane (20 mL) and two drops of DMF were added, followed by SOCl 2 (1.7 mL). The resulting mixture was refluxed for 3 h and then cooled to 0 ºC. Then, SnCl 4 (1.7 mL) was added dropwise. The resulting mixture was refluxed for 16 h and the formed precipitate was collected by filtration and washed with methanol (2x20 mL). The obtained solid was then suspended in 1 M NaOH (20 mL) and stirred for 30 min, filtered, and washed with water (3x20 mL), methanol (2x20 mL) and acetone (2x20 mL) to afford 1 (115 mg, 45%) as a grey solid. 1 9 we extract the linewidth of the Kondo resonance by fitting a Frota function to the experimental data and correct the extracted half width at half maximum (HWHM) to an effective HWHM eff to account for finite temperature of the tip 10 at each temperature point. An effective temperature T eff is used to account for the modulation of the lock-in amplifier. 11 Finally, we extract the Kondo temperature via the empirical found formula HWHM eff =      Figure S13. Constant current dI/dV maps at different energies using a metal tip on Ag(111). Tunneling current for the images starting from lower to higher bias values correspond to: -700pA, -500pA, -500pA, -300pA, 300pA, 500pA, 500pA, 1nA, 700pA, 900pA. All scale bars are 5 Å.