Enhancement of excited state nonradiative deactivation of Nile Red in γ-cyclodextrin:: evidence for multiple inclusion complexes

The effect of cyclodextrins in influencing the photophysics of a polarity sensitive dye Nile Red (NR) is investigated using steady-state and time-resolved fluorescence methods. The singlet excited state of Nile Red is deactivated very effectively in gamma-cyclodextrin and a 40 ps fluorescence lifetime has been measured. The optical absorption spectrum of Nile Red is affected in gamma-CD significantly. Spectroscopic evidence suggests that multiple inclusion complexes are formed both in beta- and gamma-CDs. Studies in D2O showed small isotope effects indicating that hydrogen bonding interactions are relatively unimportant for the deactivation of the singlet excited state. It is hypothesized that the twisted intramolecular charge transfer (TICT) state of NR is strongly affected in gamma-CD but not in beta-CD. (C) 1999 Published by Elsevier Science B.V. All rights reserved.