Evolution of Br⋯Br contacts in enantioselective molecular recognition during chiral 2D crystallization

Halogen-mediated interactions play an important role in molecular recognition and crystallization in many chemical and biological systems, whereas their effect on homochiral versus heterochiral recognition and crystallization has rarely been explored. Here we demonstrate the evolution of Br⋯Br contacts in chiral recognition during 2D crystallization. On Ag(100), type I contacts prevail at low coverage and lead to homochiral recognition and the formation of 2D conglomerates; whereas type II contacts mediating heterochiral recognition are suppressed at medium coverage and appear in the racemates induced by structural transitions at high coverage. On Ag(111), type I contacts dominate the 2D crystallization and generate 2D conglomerates exclusively. DFT calculations suggest that the energy difference between type I and type II contacts is reversed upon adsorption due to the substrate induced mismatch energy penalty. This result provides fundamental understanding of halogen-mediated interactions in molecular recognition and crystallization on surface.

To verify that TBTA remains intact under the conditions of our measurements, we conducted stepwise annealing of TBTA both on Ag(100) and Ag(111) to monitored its chemical transformation. The results demonstrate that no new structure appears on the surface after annealing at room temperature for 15 mins.
After annealing at 370 K, 1D chains with alternating bright and dark dots appear, which are attributed to the oligomers of organometallic intermediates ( Supplementary Fig. 2a). Stepwise annealing of the TBTA/Ag(111) reveals similar results. Specifically, no new structure appears on the surface after annealing the sample at room temperature. After annealing treatment at 370 K, 1D chains of the organometallic intermediates are formed, which are surround with bright dots corresponding to the Br atoms ( Supplementary Fig. 2b). The S3 results indicate that the dehalogenation of TBTA and the formation of organometallic intermediates does not occur up to room temperature both on Ag(100) and Ag(111). Fig. 3 XPS spectra of TBTA on Ag(100). a C 1s spectra of TBTA/Ag(100) at 150 K, room temperature, and 370 K. b Br 3p spectra of TBTA/Ag(100) at 150 K, room temperature, and 370 K.

Supplementary
To elucidate the chemical states of TBTA on Ag(100) at different temperatures, we performed XPS characterizations. As shown in Supplementary Fig. 3, three peaks located at 284.7 eV, 286.6 eV, and 287.2 eV can be resolved in the C 1s spectrum of the sample prepared at 150 K, corresponding to the carbon atoms in C-C, C-N, and C-Br bonds, respectively. Only a peak appears at 183.8 eV in the spectrum of Br 3p, corresponding to the C-Br bond. The XPS signals of C 1s and Br 3p spectra do not show obvious change after annealing the sample at room temperature. The results indicate that the TBTA molecule is intact at 150 K and room temperature. With the increase of the annealing temperature to 370 K, the peaks corresponding to the C-Br bond (both in C 1s and Br 3p spectra) decrease, and the peaks corresponding to the C-Ag bond (283.7 eV) and the Br-Ag bond (181.6 eV) appear simultaneously. The results consist well with the results revealed by STM measurements, which support that the integrity of TBTA and the absence of debromination in the present study. The STM image reveal the coexistence of the porous islands grown from the initially formed molecular rings as well as short molecular chains. It can be seen that the length of the molecular chains is comparable to that observed at low-coverage stage, suggesting they do not grow with the molecular coverage. Supplementary Fig. 5 The comparison of the STM images of the homochiral dimers and the HCBs with their molecular models. a ss-dimer. b rr-dimer. c r-HCBs. Scale bars: a, b 0.5 nm. c 1 nm.
To compare the STM data and the molecular models of the homochiral dimers and the HCBs, we superimposed the molecular models upon the high-resolution STM images, as shown in Supplementary Fig.   5. One thing to note is that the Br atoms appear as very bright spots in STM images; the phenyl moieties, by contrast, appear as faint spots. Therefore, we judge the match between the molecular models and the STM S5 images mainly based on the positions of the Br atoms and the central triazine rings. It can be seen that the molecular models of the homochiral dimers and the r-HCBs consist well with their molecular models.  Supplementary Fig. 11 The optimized adsorption configurations of s-TBTA on Ag(100).
The orientation of the 2D homochiral HCBs with respect to the substrate lattice keeps constant during the expansion of the HCBs into two-dimensional structure, implying that the molecular orientation in the HCBs is the preferred adsorption orientation. According to the experimental results, it is deduced that the rotation axis of the s-TBTA molecule deviates from the <01 � 1> direction of the substrate by -13°. We referred to this molecular orientation within the HCBs in building the initial molecular model for the DFT simulations.
On this basis, we calculated the energy of the TBTA/Ag (100)  We assumed that the s-TBTA adopts the optimized adsorption configuration and explored the effect of the adsorption configuration (adsorption site and orientation relative to s-TBTA) of the other molecule on the energy of the ss-dimer/Ag(100) or the sr-dimer/Ag(100) entity. Considering the orientation with respective to the substrate, three homochiral dimers and three heterochiral dimers could be obtained, named ss-1, ss-2, ss-3, and rs-1, rs-2, rs-3 in Supplementary Fig. 12.

Supplementary Table 1. The energies of the optimized adsorption configuration of TBTA dimers on Ag(100).
The energies are relative to the ss-1.
The molecular models of the trimeric clusters used for DFT calculations were built based on the optimized stable molecular models of ss-1 dimer/Ag(100) and rs-3 dimer/Ag(100) shown in Supplementary   Fig. 12. That is, another s-TBTA molecule was added to the optimized ss-1 dimer/Ag(100) and rs-3 dimer/Ag(100) respectively to be used as the initial molecular models of the sss-trimer/Ag(100) and the srstrimer/Ag(100).