Photophysics of rhodamine dimers in Langmuir-Blodgett films Original Research Article

Temperature dependent dimerization processes of octadecylrhodamine B (RB) and octadecylrhodamine 6G (R6G) in Langmuir-Blodgett (LB) films were studied by steady-state and time-resolved fluorescence methods. The geometry of the dimers in LB films is equal for both dyes, but different to the geometry of the dimers found in solutions. The sandwich-type dimers with lifetimes of 710 ps for RB and 620 ps for R6G have their fluorescence maxima at 635 and 620 nm for RB and R6G, respectively. The dimer with an oblique geometry has its fluorescence maximum at 675 nm for both dyes, and its fluorescence lifetime is 4.6 ns for RB and 4.9 ns for R6G. The proportion of fluorescent dimers increases with decreasing temperature, when the nonfluorescent H dimers reorganize to fluorescent J dimers. The activation energy for this temperature induced process is 1.4 and 2.6 kJ mol−1 for RB and R6G, respectively.