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

The absorption and fluorescence spectra of five typical cyanine dyes adsorbed on the surface of TiO colloids have been 2 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 TiO colloids. This behavior can be understood by a consideration of whether the reduction of radiationless transition 2 1 ) Ž 0 . quantum yields of intramolecular torsional motion in the excited singlet states dye , f yf , are larger or smaller Tr, nr Tr, nr than f , the quantum yield for electron transfer from their dye) to the conduction band of TiO . ESR results indicate that ET 2 Ž 0 . the dye shows an ESR signal at gs2.005 if f yf is smaller than f , otherwise it gives no ESR signal in the Tr, nr Tr, nr ET presence of TiO . The relationships between the enhancement or reduction in fluorescence quantum yields and structures of 2 these dyes are discussed. q 1999 Elsevier Science B.V. All rights reserved.