Homochiral to heterochiral transition in pentahelicene monolayer on the Pb(111) surface

The chiral self-assembly of racemic pentahelicene molecules on the Pb(111) surface has been systematically investigated using low-temperature scanning tunneling microscopy combined with density functional theory calculations. At low-temperature (∼100 K), molecular diffusion is strongly suppressed, yielding only isolated molecules and small clusters. In contrast, room-temperature deposition enables a well-defined coverage-dependent phase evolution: from a disordered gaseous phase to a homochiral honeycomb lattice, followed by a chiral-alternated checkerboard superlattice, and ultimately to an ordered racemic phase composed of periodically alternating upright heterochiral (M-P) dimers at monolayer coverage. The transition is cooperatively driven by molecular reorientation (flat → upright) and chiral modulation (homochiral → heterochiral), demonstrating a dynamic and tip-mediated reversible chiral segregation process. These findings offer fundamental insights into two-dimensional chiral crystallization and pave the way for designing chiral functional materials with tailored superstructures.