Abnormal fluorescence behavior of some cyanine dyes in a cyanine dye–TiO2 colloids system

The absorption and fluorescence spectra of five typical cyanine dyes adsorbed on the surface of TiO2 colloids have been studied. The absorption spectra show that there are two kinds of isomer produced by intramolecular torsional motion of these dyes in their ground states. The fluorescence quantum yields of these dyes may be enhanced or reduced in the presence of TiO2 colloids. This behavior can be understood by a consideration of whether the reduction of radiationless transition quantum yields of intramolecular torsional motion in the excited singlet states 1dye∗, (φTr, nr0−φTr, nr), are larger or smaller than φET, the quantum yield for electron transfer from their 1dye∗ to the conduction band of TiO2. ESR results indicate that the dye shows an ESR signal at g=2.005 if (φTr, nr0−φTr, nr) is smaller than φET, otherwise it gives no ESR signal in the presence of TiO2. The relationships between the enhancement or reduction in fluorescence quantum yields and structures of these dyes are discussed.