Selective Reduction Approach in Cocrystal Systems for Highly Conducting Organic Radical Crystals

Many polycyclic aromatic hydrocarbon molecules are electrically inactive despite their high degree of π-conjugation. We solve this problem by forming molecular cocrystals with radical-generating molecules which upon post-treatment become highly conducting organic radical crystals. Fluoranthene (FA)–tetracyanoquinodimethane (TCNQ) cocrystals were prepared from a mechanochemically prepared FA–TCNQ mixture precursor by the physical vapor transport (PVT) process. While both FA crystals and as-prepared FA–TCNQ cocrystals show low electrical conductivity, FA–TCNQ cocrystals exposed to hydrazine show dramatically increased conductivity as hydrazine generates radicals in TCNQ which work as charge carriers in FA–TCNQ cocrystals. The systematic studies of electron paramagnetic resonance spectroscopy, Raman spectroscopy, and electrical device measurement support that the generation of radical species from TCNQ by the hydrazine reaction is responsible for the high conductivity.