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 (Ea= 0.33 eV). Crystals of the Et3NH+ salt (3a) are monoclinic: a= 19.035(4)A, b= 8.476(3)A, c= 17.131(4)A, β= 101.85(2)°, space group P21/n, Z= 4, R= 7.7 (Rw= 7.3)%; those of the Ph4P+ salt (3b) are monoclinic: a= 12.634(6)A, b= 11.802(9)A, c= 12.875(9)A, β= 95.69°, space group P2/c, Z= 2, R= 6.4 (Rw= 6.1)%; and those of the Me2NH+CH2CH2O—CO—C(Me)CH2 salt (3c) are monoclinic: a= 31.313(5)A, b= 7.436(3)A, c= 25.495(4)A, β= 100.63(7)°, space group C2/c, Z= 8, R= 7.7 (Rw= 6.5)%.