Oxonol dyes: X-ray crystallographic and solid-state 13C nuclear magnetic resonance studies of some organic semiconductors

Abstract

X-Ray structural studies, together with solution and solid-state multinuclear nuclear magnetic resonance (NMR) spectroscopy and d.c. electrical conductivity data, are reported for the hydroxypyridone trimethine oxonol dyes (3). The structural work reveals that, as in the related cyanine materials, the dye molecules can adopt two different types of packing arrangement: herringbone and infinite parallel stacks. Cations of the type R₄X⁺(X = N, P) favour infinite parallel stacks, whereas less symmetrical cations yield herringbone structures; ¹³C MAS NMR appears to provide a convenient probe for these structural features. All the materials are semi-insulating with the notable exception of one of the phosphonium dyes, which is a moderate semiconductor (E_a= 0.33 eV). Crystals of the Et₃NH⁺ salt (3a) are monoclinic: a= 19.035(4)Å, b= 8.476(3)Å, c= 17.131(4)Å, β= 101.85(2)°, space group P2₁/n, Z= 4, R= 7.7 (R_w= 7.3)%; those of the Ph₄P⁺ salt (3b) are monoclinic: a= 12.634(6)Å, b= 11.802(9)Å, c= 12.875(9)Å, β= 95.69°, space group P2/c, Z= 2, R= 6.4 (R_w= 6.1)%; and those of the Me₂NH⁺CH₂CH₂O—CO—C(Me)CH₂ salt (3c) are monoclinic: a= 31.313(5)Å, b= 7.436(3)Å, c= 25.495(4)Å, β= 100.63(7)°, space group C2/c, Z= 8, R= 7.7 (R_w= 6.5)%.