Ultrafast Pathways of the Photoinduced Insulator–Metal Transition in a Low‐Dimensional Organic Conductor

Abstract

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.

Nuclear dynamics of singlet exciton fission in pentacene single crystals

“…1B , with the incident electron beam aligned normal to the crystal’s ab plane. Pentacene being a pure hydrocarbon, the overall diffraction intensity is low compared to metal-organic compounds or organic-inorganic charge-transfer (CT) complexes previously studied with FED ( 21 , 22 , 27 , 28 ). A diffuse scattering cloud is observed in the background, which we attribute to defects in the crystal and inelastic scattering.…”

“…Timeresolved Raman spectroscopy may give some structural information on a molecular level but is limited if additional molecule-lattice coupling is involved (15,19). In contrast, ultrafast diffraction experiments with x-rays and electrons, established in the last decade, allow observation of structural changes on atomic time and length scales in crystalline materials with unit cell sizes up to several hundred atoms (20)(21)(22).…”

Two Ongoing Magnetic Transitions Originating from Spin-Peierls Dimerization and Cation Orientation Transformation in an S = 1/2 Spin Chain System

“…It is very important and helpful for the design of a new functional material with phase transition to understand the mechanism of the phase transition occurring. In the contexts of organic conducting and magnetic compounds, Peierls or spin-Peierls transition has been observed in a series of 1D half-filled band metallic − or S = 1/2 uniform antiferromagnetic spin chain − systems, in which crystal structure the planar and electron-delocalized radical ions form 1D stack with stronger intermolecular interactions along the stacking direction. In the molecular crystal engineering, on the other hand, it is worth noting that the isotropically reorienting molecules/ions, that is, so-called globular-shaped structural units, commonly show isotropic rotation with little hindrance in lattice and may occur in several conformations with little difference in strain energy, allowing for the possibility of metastable states.…”

Nuclear dynamics of singlet exciton fission: a direct observation in pentacene single crystals

“…An exemplary transmission electron diffraction pattern is shown in Figure 1(b), with the incident electron beam aligned along the crystal's c-axis. Pentacene being a pure hydrocarbon, the overall diffraction intensity is low compared to metal-organic compounds or organic-inorganic charge transfer complexes previously studied with FED [30][31][32][33]. A diffuse scattering cloud is observed in the background, which we attribute to defects in the crystal and inelastic scattering.…”