Picosecond fluorescence dynamics of dioctadecylrhodamine B at air/water interface: micropolarity and cluster formation

Fluorescence decay dynamics and fluorescence anisotropy of N,N′-dioctadecylrhodamine B (DORB) at the air/water interface with surface molecular areas of ∼105 Å2/molecule were examined by picosecond single photon timing spectroscopy. The interfacial dielectric constant around DORB was estimated to be ∼21 by the dependence of nonradiative rate constant on polarity parameter ET(30). The rotational relaxation of DORB at the air/water interface showed two-exponential behavior with lifetimes of 3.15 ± 0.14 ns and 420 ± 30 ps. The longer component was interpreted in terms of the formation of a DORB cluster, and the average number of molecules in clusters was estimated to be 7∼8.