Differential encapsulation of pyrene-end-capped poly(ethyleneoxide) by cyclodextrins in aqueous solutions. A fluorometric study

Abstract

The photophysical behavior of pyrene-end-capped poly(ethylene oxide) (PYPY) has been studied in water and aqueous α-, β- and γ-cyclodextrin (CD) environments through steady-state and time-resolved spectroscopy. Both absorption and fluorometric studies reveal that α-, β- and γ-CD interact differently with the fluorophore. While PYPY forms 1:1 inclusion complexes with β- and γ-CD as reflected from the presence of isosbestic points in the absorption spectra and isoemissive points in the fluorescence spectra, it does not show similar complexation with α-CD. The emission spectrum of PYPY in aqueous solution shows characteristic dual emission; with a structured band assigned to the pyrene monomer emission and a broad and unstructured one at lower energy assigned to the excimer emission. Addition of β- or γ-CD to the aqueous solution of PYPY modifies the fluorescence spectrum markedly. In β-CD solution, the excimer emission yield is reduced with a concomitant enhancement of the monomer emission while in γ-CD solution the excimer emission increases at the cost of the other band. The differences in photophysical behavior of the fluorophore in different CD environments are explained in the light of different modes of encapsulation of PYPY in CDs with different cavity dimensions. The present study points to the probable location of the probe within the microheterogeneous environment. The relevance of these results to the use of cyclodextrins in modulating the aggregation behavior of hydrophobically end-capped poly(ethyleneoxide)s is discussed.