Supramolecular chirality induced by a weak thermal force.

We show that the unexpected chirality of aggregated structures based on an uncharged achiral porphyrin system originates from small temperature gradients that act as an asymmetrical physical perturbation; the consequent thermal force gives rise to the thermophoretic motion of the aggregates. We establish that the induced optical activity can be controlled, and even vanished, by minimizing the thermal force.