Orbital Specific Chirality and Homochiral Self-Assembly of Achiral Molecules Induced by Charge Transfer and Spontaneous Symmetry Breaking

We study the electronic mechanisms underlying the induction and propagation of chirality in achiral molecules deposited on surfaces. Combined scanning tunneling microscopy and ab initio electronic structure calculations of Cu-phthalocyanines adsorbed on Ag(100) reveal the formation of chiral molecular orbitals in structurally undistorted molecules. This effect shows that chirality can be manifest exclusively at the electronic level due to asymmetric charge transfer between molecules and substrate. Single molecule chirality correlates with attractive van der Waals interactions, leading to the propagation of chirality at the supramolecular level. Ostwald ripening provides an efficient pathway for complete symmetry breaking and self-assembly of homochiral supramolecular layers.

Figure 1

STM images of CuPc adsorbed on Ag(100). (a) Molecules rotated by $+/-30°$ with respect to the [011] crystallographic direction (inset) show $r/l$ chirality at bias voltage $V_b=-0.3\mathrm{V}$. (b) STM image of $r$-CuPc obtained at $V_b=-1\mathrm{V}$ and (c) $V_b=+0.3\mathrm{V}$ compared to the corresponding (d) and (e) STM images calculated using DFT.

Figure 2

(a) Top- and (b) diagonal side view of the differential charge density induced by molecular bonding to the substrate. The contour value is $\pm 3.8\times {10}^{-3}\mathrm{e}/{\AA }^3$. The excess (depletion) of charge is shown in yellow (red). (c) Calculated DOS of $r$-$\mathrm{CuPc}/\mathrm{Ag}(100)$ projected onto the unperturbed CuPc molecular orbitals. Positive (negative) units refer to spin up (down) states; a Gaussian broadening of 0.25 eV has been employed. (d),(e) Theoretical conductance maps at the peak position of the (d) HOMO and (e) LUMO.

Figure 3

Chiral self-assembly of CuPc. (a) $rR$ and (b) $lL$ homochiral domains recorded with $V_b=+0.52\mathrm{V}$ and $-0.10\mathrm{V}$ respectively. (c) $rR$ and $rL$ dimers shown together with the relative probability of each configuration (d) Homochiral $rR$ domain extending over a terrace and crossing a screw dislocation. Image size $77.3\times 77.3\mathrm{nm}$.