Optical Sensitizing of Silver Halides by Dyes III. The Relation of Sensitizing to the Absorption Spectra and Constitution of Dyes

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Abstract
Optical sensitizing is related to dye structure in general, not confined to particular classes of dyes. While sufficiently strong adsorption to silver halide, and relative insolubility of the adsorbed dye are essential, they are not sufficient conditions. The absorption spectra of adsorbed dyes correspond to their spectra in solution, and are affected in the same way by structural changes, but are not identical therewith. The displacement of the spectrum should be referred to the absorption of the dye in the gaseous state at low pressures; examples are given. This displacement corresponds to an adsorption energy available for sensitizing, but this energy difference may not be available, and is not generally sufficient. For ``molecular'' sensitizing, planarity of the dye molecule appears to be essential, and the following conditions are deduced as necessary and probably sufficient for sensitizing: (i) Planar configuration of dye molecules, respectively of adsorbed dye ions, (ii) edge‐on adsorption of planar molecules oriented possibly orthogonally but more probably at an angle of 70° to a (111) plane of the crystal, and (iii) electronic transition in dye ion or dipole on absorption of a photon polarized in an azimuth defined by (i) and (ii). On these bases a hypothesis of coplanar coupling of electronic displacements in the dye and in a congruent plane of the silver halide lattice is advanced. It is suggested that this is a key factor in optical sensitizing by dyes, both for molecular and aggregate sensitizing. Certain other relations of constitution to sensitizing are discussed, including the influence of nuclear (perichrome) changes and of substitution.

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