Fluorescence Probes for Cyclodextrin Interiors

Abstract

6-Propionyl-2-(dimethylamino)naphthalene (PRODAN) emits two fluorescence bands, at ∼510 and ∼435 run, when dissolved in γ-cyclodextrin (CD) aqueous solutions. The relative contributions of these two bands were found to depend on time and temperature. These emissions are attributed to the inclusion of PRODAN with the dimethylamino group toward the larger and smaller rims inside the γ-CD cavities, respectively. The first position corresponds to a slightly polar and slightly rigid environment, while the second corresponds to a hydrophobic and rigid environment relative to the aqueous polar bulk. In contrast, PRODAN in either α-CD or β-CD aqueous solutions emits a single fluorescence band at 525 and 510 nm, respectively. The emission of PRODAN in α-CD is similar to that in water and indicates no inclusion at all. In β-CD, only one kind of inclusion is possible with the dimethylamino group of PRODAN toward the larger rims of β-cavities. These results are supported by fluorescence decay lifetime measurements and are consistent with our previous observations made for 4-dimethylaminobenzonitrile (DMABN) and 4-diethylaminobenzonitrile (DEABN) in α- and β-CD aqueous solutions [23,24]. Therefore the possibility of twisted intramolecular charge transfer (TICT) state formation in PRODAN in terms of environmental polarity and local free volume of CD cavities is discussed. These observations put PRODAN, DMABN, and other TICT compounds as fluorescence probes for CD interiors.