Fluorescence anisotropy decay and solvation dynamics in a nanocavity: coumarin 153 in methyl beta-cyclodextrins

Fluorescence anisotropy decay and solvation dynamics of coumarin 153 (C153) are studied in dimethyl beta-cyclodextrin (DIMEB) and trimethyl beta-cyclodextrin (TRIMEB) nanocavity in water. C153 binds to DIMEB and TRIMEB to form both 1:1 and 1:2 (C153:cyclodextrin) complexes. The anisotropy decays of C153 in DIMEB and TRIMEB are found to be biexponential. The fast component of anisotropy decay (approximately 1000 ps) is attributed to the 1:1 complex and the slower one (approximately 2500 ps) to the 1:2 complex. From the components of the anisotropy decay, the length of the 1:1 and 1:2 complexes are estimated. Solvation dynamics of C153 in DIMEB exhibits a very fast (2.4 ps) component (41%) and two slower components of 50 ps (29%) and 1450 ps (30%). Solvation dynamics in TRIMEB is described by three slow components of 10.3 ps (24%), 240 ps (45%), and 2450 ps (31%). Possible origins of the ultraslow components are discussed.