Mapping structure-property relationships in a 6-oxo-verdazyl radical by variable pressure crystallography and density functional theory.

Abstract

The response of the 1,5-tolyl-3-phenyl-6-oxo-verdazyl radical (pTolOV) to pressure in the crystalline state has been investigated using both high-pressure X-ray diffraction techniques and density functional theory (DFT) calculations. Changes in structure, electronic properties such as band gap and density of states, and magnetic exchange interactions have been explored. Structural analysis reveals a phase transition between the P2₁/n and C2/c space groups between 2.30 and 2.73 GPa, and an accompanying increase in molecular point group symmetry in the solid state structure at higher pressures. Modelling using periodic DFT-based methods reveals no significant energy penalty to this phase transition with a small reduction in the band gap upon increasing pressure due to changes in the intermolecular distances, before a sharp increase in band gap with the changes in intermolecular alignment in the C2/c phase. These variations fall in the range 2.02-1.94 eV, and the radical remains a wide-gap semiconductor in the solid state across the pressure range examined. Further DFT calculations with gas-phase models reveal weak antiferromagnetic exchange interactions occurring in antiparallel π-stacked chains in both phases.