Chemical size-effects on fluorescence lifetime of Rhodamine 6G in ethylene glycol/water microdroplets dispersed in polydimethylsiloxane matrix

The fluorescence dynamics of Rhodamine 6G (R6G) in ethylene glycol/water (EG/W) microdroplets dispersed in a polydimethylsiloxane (PDMS) solid matrix were studied by picosecond time-resolved microspectroscopy. The large decrease in the fluorescence lifetime of R6G was observed for the droplets with the diameter (d) below 10 μm and at the R6G concentrations ([R6G]) above 0.1 mM. Absorption microspectroscopy revealed that the dimer formation of R6G in EG/W microdroplets was facilitated with decreasing d and increasing [R6G]. The size-dependent fluorescence lifetime of R6G was related to quenching of the monomer excited state by the R6G dimer. The origin of size-dependent dimer formation and, thus, the size-dependent fluorescence lifetime of R6G in micrometer-sized droplets in the PDMS matrix was discussed in detail.