Proof of Energy Transfer and Absence of Electron Injection in Spectral Sensitization of Evaporated AgBr by Oxacarbocyanine

Spectral sensitization by monomolecular layers of cyanine dyes is observed when the dyes are separated from the surface of evaporated silver bromide layers by monomolecular interlayers. The experimental data of the dependence of spectral sensitization on the distance between the sensitizer layer and the silver bromide surface are interpreted by an energy-transfer model with localized acceptor centers in the silver bromide at about 30 Å below the surface. The sensitization is proportional to [1 + (d / d0)n]−1, where d is the distance between the planes of the sensitizers and the acceptors and n = 3.8 ± 0.3; d0 = 35 ± 3 Å for N,N′-distearyl-oxacarbocyanine, n = 3.9 ± 0.4; d0 = 55 ± 2 Å for N,N′-distearyl-4,4′-carbocyanine. This relation with exponent n = 4 is expected for energy transfer. Using an energy-accepting dye competing with the acceptors in AgBr, the sensitizing action of the oxacarbocyanine dye is diminished and by a quantitative investigation absence of electron injection is demonstrated in the case of direct contact between the sensitizer and the evaporated silver bromide.