FREE-VOLUME EFFECT OF ORGANIC-DYE MONOMERS IN THE ADSORBED STATE

The temperature dependence ranging from 298 to 4 K of the monomer fluorescence lifetimes of a thiacarbocyanine, pseudoisocyanine, and malachite green adsorbed on silica gel and on quartz has been examined. The fluorescence lifetimes at 298 K are about 20 times longer than the corresponding lifetimes in low-viscosity solvents. Weakly activated or completely activationless behavior of the rate constants of internal conversion has been observed. Asymmetric potential surfaces, induced by unidirectional forces exerted by the surface, are proposed that can explain the observed temperature dependences. The fluorescence lifetimes at 4 K are substantially shorter than the corresponding ones in rigid matrix and are attributed to a large free-volume effect in the adsorbed state. © 1990 American Chemical Society.