The crystal and molecular structure of the acetic acid solvate of 5,5'-dichloro-3,3',9-triethylthiacarbocyanine bromide, a photographic sensitizing dye

Abstract

The crystal structure of the title compound, a known spectral sensitizer of silver bromide photographic emulsions, has been determined by a single-crystal three-dimensional X-ray diffraction study. The compound crystallizes in space group P1 with two C23H2aBrC12N2S2. CH3COOH formula units in each unit cell of dimensions a=9408 (5), b= 16-342 (9), c= 8-919 (6) A, ~= 10147 (4), fl=9125 (4) and 7'= 7945 (4) °. Calculated and observed densities are 1-514 and 1.51 (1) g cm -3, respectively. Intensity data were obtained using an Enraf-Nonius CAD-3 automated diffractometer (Mo K~ radiation, 2= 0.71069 A). The structure was solved by the heavy-atom method and refined by full-matrix least-squares methods to a conventional R value of 0.073 for 1991 independent reflections. The structure consists of columns of C23H23CI2N2S + dye cations separated by bromide ions and acetic acid molecules. Bromine-oxygen distances indicate the presence of a weak hydrogen bond between the latter two species. Within a given column, cations are stacked in two non-equivalent ways resulting in mean perpendicular distances between cation least-squares planes of 3-63 and 4.09 A. The cations consist of two structurally equivalent halves, each planar to approximately + 0.05 A; however, the dihedral angle between these planes is 8.6 ° and the cation is bowed, rather than twisted. Lastly, sulfur atoms are cis, and the nonbonding S . . .S distance [6-168 (5) A] is considerably longer than those (5.3, 54 A) calculated for similar planar or twisted dyes without the central 9-ethyl substituent. The cation geometry is used to examine possible geometric constraints on the epitaxic attachment of thiacarbocyanine dyes to the {111 ) and {100} surfaces of AgBr. For dye cations attached edge-on along [110] Ag + rows, it is shown that weak Ag +.. .S ligand bonds between the dye and the crystal surface can form only if a certain fraction of surface silver sites are vacant. Using ideal models for the {111} and {100} surfaces, a regular attachment of dye molecules stacked edge-on to the {111 } crystal surface with pairs of Ag +.-. S ligand bonds contributing to the binding of each molecule appears to be possible. It is not now obvious how such an attachment with Ag +.. .S bonds could occur on the {100) surface. A detailed consideration of surface packing requirements suggests that particular aggregates should form preferentially.