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

The absorption and fluorescence spectra of five typical cyanine dyes adsorb ed on the surface of TiO2 colloids have been studied. The absorption spectr a show that there are two kinds of isomer produced by intramolecular torsio nal motion of these dyes in their ground states. The fluorescence quantum y ields of these dyes may be enhanced or reduced in the presence of TiO2 coll oids. This behavior can be understood by a consideration of whether the red uction of radiationless transition quantum yields of intramolecular torsion al motion in the excited singlet states (1)dye*, (phi(Tr,nr)(0) - phi(Tr,nr )), are larger or smaller than phi(ET), the quantum yield for electron tran sfer from their (1)dye* to the conduction band of TiO2. ESR results indicat e that the dye shows an ESR signal at g = 2.005 if (phi(Tr,nr)(0) - phi(Tr, nr)) is smaller than phi(ET), otherwise it gives no ESR signal in the prese nce of TiO2. The relationships between the enhancement or reduction in fluo rescence quantum yields and structures of these dyes are discussed. (C) 199 9 Elsevier Science B.V. All rights reserved.