Benzo-Fused Periacenes or Double Helicenes? Different Cyclodehydrogenation Pathways on Surface and in Solution

Controlling the regioselectivity of C–H activation in unimolecular reactions is of great significance for the rational synthesis of functional graphene nanostructures, which are called nanographenes. Here, we demonstrate that the adsorption of tetranaphthyl-p-terphenyl precursors on metal surfaces can completely change the cyclodehydrogenation route and lead to obtaining planar benzo-fused perihexacenes rather than double [7]helicenes during solution synthesis. The course of the on-surface planarization reactions is monitored using scanning probe microscopy, which unambiguously reveals the formation of dibenzoperihexacenes and the structures of reaction intermediates. The regioselective planarization can be attributed to the flattened adsorption geometries and the reduced flexibility of the precursors on the surfaces, in addition to the different mechanism of the on-surface cyclodehydrogenation from that of the solution counterpart. We have further achieved the on-surface synthesis of dibenzoperioctacene by employing a tetra-anthryl-p-terphenyl precursor. The energy gaps of the new nanographenes are measured to be approximately 2.1 eV (dibenzoperihexacene) and 1.3 eV (dibenzoperioctacene) on a Au(111) surface. Our findings shed new light on the regioselectivity in cyclodehydrogenation reactions, which will be important for exploring the synthesis of unprecedented nanographenes.

The rotation barrier (in eV) of a naphthyl group of 1a on the bond connected to the terphenyl base in solution, on the Au(111) surface and the Cu(110) surface, and the adsorption energy of 1a on Au(111) and Cu(110) that is estimated from the reactive force field Page S9 Table S2 Figure S7 Adsorption energies (∆Ea, in eV) of 3a and 2a-1 on Cu(110) from DFT calculations Spin density of the radical cation of precursor 1b Page S10 Page S10

Figure S8
Proposed the reaction processes Page S11 Schemes S1, S2 Figures S9-S13 Synthesis and characterization of precursors 1b and 5 Page S12-19 Figure S14 Electronic properties of dibenzoperihexacenes 3a and 3b and dibenzoperioctacene 6 measured on Cu(111) Page S20 References Pages S21       We conducted reactive force field simulations to estimate the rotation barrier of 1a at different conditions. The rotation barrier of a naphthyl group of 1a is 0.32 eV in solution. On Au(111), the rotation barrier is 0.44 eV, which is 0.12 eV higher than that of the solution. On Cu(110), the rotation barrier is 0.46 eV, which is 0.14 eV higher that of the solution. Thus, the presence of a metal substrate increases the rotation barrier, thereby making the rotation on the surfaces more difficult.
We also estimated the adsorption energy of 3a on Au(111) and Cu(110), which is defined as follows: ΔE = Etotal -(E1a + Esurface) (S1) S10 We estimated the relative stability by calculating the adsorption energy (∆Ea) of 3a and 2a-1 from DFT calculations, as follows: (2) Here, Esur+ads is the energy of surface with an adsorbent, Esur is the energy of metal slab and Eads is the energy of isolated adsorbent. Our DFT calculations show that ∆Ea of 3a is -8.79 eV, ∆Ea of 2a-1 is -4.14 eV, as shown in Table S2., which indicates that the metal surface prefers to stabilize the flatten structure (3a with larger binding energy) than jagged structure (2a-1). We consider these DFT calculations provide evidence to explain the experimentally observed different trends in selectivity between solution reaction and metal surface catalyzed reaction.
Spin density of the radical cation species of precursor 1b was calculated by DFT at the UB3LYP/6-31G (d,p) level of theory, using the Gaussian 09 software package. 3 The higher spin density at the α-position over the β-position of the naphthyl moiety explains the regioselectivity of the oxidative cyclodehydrogenation in solution.  Figure S8 Proposed the reaction processes. The reactions may start from C1-C6 and C9-C10 coupling as shown in Figure 2 (structure 1a to structure 4). At 620 K, structure 4 (shown in Fig.   2) directly transforms into structure 3a, which indicates that no stable intermediate exists between structure 4 and structure 3a at this temperature. No other intermediate structure was

S11
found, indicating that the planarization of each naphthyl group from structure 4 was accompanied by subsequent or simultaneous formation of three C-C bonds, e.g., C4-C5. C2-C3 and C13-C14, leading to structure S1, and then to the final product structure 3a.

General Methods
All starting materials were purchased from Aldrich, Acros, and Alfa Aesar and were used as received. Preparative column chromatography was performed using silica gel from Merck with a grain size of 0.063-0.200 mm (silica gel). NMR spectra were recorded in CD2Cl2 on AVANCE 300 MHz or AVANCE 500 MHz Bruker spectrometers. Abbreviations: s = singlet, d = doublet, t = triplet, and m = muliplet. High-resolution mass spectrometry (HRMS) was performed on a SYNAPT G2 Si high resolution time-of-flight mass spectrometer (Waters Corp., Manchester, UK) using matrix-assisted laser desorption/ionization (MALDI).