Ultrafast Pathways of the Photoinduced Insulator-Metal Transition in a Low-Dimensional Organic Conductor.

Among functional organic materials, low-dimensional molecular crystals represent an intriguing class of solids due to their tunable electronic, magnetic, and structural ground states. This work investigates Cu(Me,Br-dicyanoquinonediimine)2 single crystals, a charge transfer radical ion salt which exhibits a Peierls insulator-to-metal transition at low temperatures. The ultrafast electron diffraction experiments observe collective atomic motions at the photoinduced phase transition with a temporal resolution of 1 ps. These measurements reveal the photoinduced lifting of the insulating phase to happen within 2 ps in the entire crystal volume with an external quantum efficiency of conduction band electrons per absorbed photon of larger than 20. This huge cooperativity of the system, directly monitored during the phase transition, is accompanied by specific intramolecular motions. However, only an additional internal volume expansion, corresponding to a pressure relief, allows the metallic state for long times to be optically locked. The identification of the microscopic molecular pathways that optically drive the structural Peierls transition in Cu(DCNQI)2 highlights the tailored response to external stimuli available in these complex functional materials, a feature enabling high-speed optical sensing and switching with outstanding signal responsivity.