Probing the Hydrophobic Interactions of a Series of Pyrene End-Labeled Poly(ethylene oxide)s in Aqueous Solution Using Time-Resolved Fluorescence

2013-03-05T00:00:00Z (GMT) by Shaohua Chen Jean Duhamel
The hydrophobic association of a series of poly­(ethylene oxide)­s covalently labeled at both ends with pyrene (PEO­(X)-Py2 where X represents the number average molecular weight (Mn) of the PEO chains equal to 2, 5, 10, and 16.5 kDa) in aqueous solutions was investigated at different polymer concentrations (CP) using steady-state and time-resolved fluorescence measurements. Phase separation was observed with PEO­(2 kDa)-Py2 and PEO­(5 kDa)-Py2 samples at high CP. The steady-state fluorescence spectra showed that the ratios of excimer-to-monomer fluorescence intensities (IE/IM) of all PEO samples remained constant when CP was below 4 × 10–5 M and decreased dramatically with increasing PEO chain length due to a decrease in intramolecular pyrene excimer formation. The IE/IM ratio in this regime was found to scale as Mn–2.3±0.2. For CP > 4 × 10–5 M, pyrene excimer is formed by both intra- and intermolecular interactions and the IE/IM ratio increases linearly with increasing CP except for PEO­(2 kDa)-Py2 which undergoes phase separation. The decays obtained at various polymer concentrations were fitted according to a “sequential model” (SM) which assumes that the pyrene excimer is formed in a sequential manner. The molar fractions of all excited pyrene species and the rate constants for pyrene excimer formation were determined from the global analysis of the monomer and excimer fluorescence decays. The fraction of pyrenes that formed excimer from ground-state pyrene aggregates (fE0) was found to increase with CP in the regime where the pyrene excimer is formed both intra- and intermolecularly and decrease with Mn in the regime where the pyrene excimer is formed only intramolecularly. The fraction of pyrene pendants subject to hydrophobic interactions were used to determine the hydrophobic capture radius (Rc) of pyrene in water from the distribution of PEO end-to-end distances. Rc was found to equal 2.2 ± 0.2 nm using fE0.