Solution and solid-state studies of some bleachable semi-conducting photographic dyes

Abstract

A number of monomeric and oligomeric hydroxypyridone mono- and poly-methine bridged oxonol and merocyanine dyes have been prepared and their solution and solid-state properties have been investigated. One particular and commercially important feature of the solution chemistry of the oxonol dyes is the reaction with sulphite ion leading to bleaching of the dye. The mechanism of this process has not previously been investigated. A detailed kinetic and spectroscopic study supports the previously proposed mechanism in which the nucleophile adds to the methane bridge in a Michael-type fashion. The reaction intermediates have been detected spectroscopically for several nucleophiles but in general are too unstable to be isolated. Oxonol and merocyanine dyes readily undergo molecular aggregation in both solution and the solid-state. One previous X-ray diffraction analysis of an oxonol dye has been carried out which revealed that the anions formed planar molecular stacks. Such an arrangement is well suited to intermolecular charge transfer and hence electronic conduction. The main objective of the current work has been to investigate how this stacking might be influenced by variation of substituents and counter-ions with a view to modifying the conduction properties. X-ray structure studies reveal that variation of the cation leads to three different types of solid-state structure: herringbone; uniform; and dimeric. The electrical properties have been investigated in detail by d.c. conductivity and a.c. dielectric spectroscopy measurements. They suggest that in general, the simple hydroxypyridone trimethine oxonols are semi-insulating (10

-8 [Delta] 10

-12[omega]-1 cm-1) though some exceptionshave been found. Any observable d.c. conduction is essentially ionic in nature. The merocyanines and pentamethine oxonols are semi-conducting ( as a consequence of added electronic contributions to the conduction mechanism. The most highly conducting oxonol salt, containing the tetrathiafulvalene (TTF) radical cation has a -3 -1 -1 d.c. conductivity, [delta] 10

-3 [omega]-1 cm-1 (compactedpowder, room temperature). In this case electronic conduction dominates.