Crystal Structures and Physical Properties of 1,6-Diaminopyrene-p-chloranil (DAP-CHL) Charge-Transfer Complex. Two Polymorphs and Their Unusual Electrical Properties

Abstract

Abstract A combination of 1,6-diaminopyrene and p-chloranil gives two kinds of 1 : 1 charge-transfer complex crystals from a benzene solution. The α-form is triclinic, space group , a = 8.963(1), b = 9.112(2), c = 6.730(1) Å, α = 99.59(1), β = 99.65(1), γ = 114.43(1)°, V = 476.0(1) Å3, and Z = 1; the β-form is also triclinic, space group , a = 8.123(3), b = 9.708(3), c = 6.791(2) Å, α = 110.73(2), β = 108.89(2), γ = 79.11(3)°, V = 472.3(3) Å3, and Z = 1. In both crystals, the donors and acceptors are neutral, and stack alternately. The main difference between the α- and β-forms is in the overlapping mode. Both crystals are electrically insulating at the initial states. The α-form single crystal, however, undergoes and irreversible conductivity change upon mild heating (< 380 K) or fracture of the crystal into powder. In both cases, the conductivity change amounts to 106 to 108, and the resultant solids are highly conducting semiconductors. Structural, optical, magnetic, and solid-state NMR studies suggest that the low-resistance state mainly comprises neutral molecules, despite its high conductivity. The resistivity of β-DAP-CHL also decreases upon heating to above 400 K, while fracture of the crystal has no effects. The resistivity change in β-DAP-CHL is considered as simple ionization of a part of the components.