Phase Change-Driven Radical Population and Electrical Conductivity Increases of Organic Radical Crystals by Mild Heating

Carrier doping in conductive radical crystal systems is one of the most important and challenging issues in employing radicals as charge carriers, i.e., electronic radicals for high-performance organic electronics. Here, we report an experimental demonstration of the increase in the radical population and the corresponding increase in electrical conductivity by water desorption-involved crystal structure change upon mild heating. The electrical conductivity of the dihydrophenazinium radical cation (H₂PNZ·⁺) crystal is enhanced by 4 orders of magnitude with an increase in radical population by 1.5 times according to the electron paramagnetic resonance signal after heating at 333 K. Single-crystal X-ray analyses show that the increase in radical population originates from the phase transformation from dimer-like radical units to monomer-like radical units upon mild heating in a narrow temperature range. The increase in intermolecular distance results in producing more independent radicals and reducing the band gap. Therefore, the electrical conductivity is increased by the synergistic effects of the radical population increase and band gap reduction.