Reversible Switching between Molecular and Charge Transfer Phases in a Liquid Crystalline Organic Semiconductor

We report the first experimental example, to our knowledge, of reversible switching between a molecular and a charge transfer phase in an organic semiconductor. An oriented film of liquid crystal perylene diimide molecules reversibly switches between a red phase with narrow conduction and valence bands and a large bandwidth black phase as the pi-stacked chromophores shift just 1.6 angstrom relative to their neighbors. This shift causes a substantial change in the intermolecular electronic overlap between molecules. The polarization of maximum absorbance rotates similar to 90 degrees, from an apparently molecule centered transition to an intermolecular charge transfer (CT) transition polarized along the pi-pi stacking axis. The experimental results are further explored via density functional theory calculations on a dimer model that demonstrate the variations in energy and oscillator strength of the molecular (Frenkel) and CT transitions as the longitudinal molecular offset is varied. These results demonstrate the exquisite sensitivity of the electrical properties of organic semiconductors to slight variations in molecular stacking.