Effect of γ-cyclodextrin on the intramolecular charge transfer processes in aminocoumarin laser dyes

Abstract

Steady-state and time-resolved studies on the emission properties of three 7-(diethylamino)coumarin laser dyes (I-III) in aqueous γ-cyclodextrin (γ-CD) solutions are reported. On addition of γ -CD, fluorescence maxima of all the three dyes shift to higher energy, with a huge enhancement of the yield of intramolecular charge transfer (ICT) emission for the flexible ones (I and II) and a very slight enhancement for the rigid analogue III. The results are explained in terms of the concept of polarity-dependent twisted ICT rates. It is proposed that twisted ICT, the main nonradiative process in the excited ICT states of these dyes, is severely restricted inside the γ-cyclodextrin cavity because of the reduced polarity. This is responsible for the enhancement of fluorescence from the ICT state.