TEMPERATURE-DEPENDENT ELECTRON-TRANSFER QUENCHING OF DYE MONOMER FLUORESCENCE ON OCTAHEDRAL AGBR GRAINS

At very low coverage, in the range of 0.005, fluorescence decay of dye molecules, 3,3'-diethylthiacarbocyanine iodide, is independent of coverage on the (111) face of monodisperse octahedral AgBr grains in a gelatin emulsion. Fluorescence decay of the adsorbed monomers is monoexponential from room temperature down to 5 K. The decay time increases from 56 ps at room temperature to 550 ps at 100 K, and it remains constant below 100 K. When the decay is fast, ground-state dye molecules are consumed and silver clusters are formed in the AgBr grains. The short decay times above 100 K are attributed to electron transfer from the excited singlet state of adsorbed dye monomers to the empty conduction band of AgBr. The rate constant shows an exponential increase with rising temperature, i.e., Arrhenius behavior. The activation energy is 0.09 eV and the preexponential factor is 3.2 X 10(11) s-1. In the nonadiabatic limit we arrive at an electron-transfer integral of 27 cm-1. The excited donor orbital of the dye monomers lies at the lower edge of the conduction band of the octahedral AgBr grains.