Symmetry, Chirality and Unidirectional Motion

Several aspects of chirality are presented: Curie principle, types of chirality, atropoisomerism, true and false chirality. Then, considering the quantum mechanical aspects, we discuss Hund’s paradox–namely why, although the complete Hamiltonian of a molecule is parity-invariant, broken symmetry enantiomeric states appear and can be frequently isolated as macroscopic samples. A first approach is based on the existence of a potential energy curve with two minima, which can be rationalized in the frame of Pseudo Jahn Teller effect. But a more complete description is now available through the environment induced decoherence of superposed states. Coming back to the standard (semi-classical) Born-Oppenheimer description based on potential energy surfaces, we analyze qualitatively a simple example, 2,2’-dibromobiphenyl to point out the interaction between different degrees of freedom. This is the starting point for a rational construction of a system with built-in symmetry allowing unidirectional motion, like Feringa’s molecule. The role of chirality as a necessary ingredient is stressed, as well as the occurrence of conical intersections in the photochemical excitation step. Then, “flashing ratchet” mechanisms, involving a potential energy curve with sawtooth asymmetric profile, are introduced. Experimental examples of molecular systems deposited on surfaces and excited by the tunneling current of a STM are discussed.