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.