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

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 (M_n) of the PEO chains equal to 2, 5, 10, and 16.5 kDa) in aqueous solutions was investigated at different polymer concentrations (C_P) 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 C_P. The steady-state fluorescence spectra showed that the ratios of excimer-to-monomer fluorescence intensities (I_E/I_M) of all PEO samples remained constant when C_P was below 4 × 10^–5 M and decreased dramatically with increasing PEO chain length due to a decrease in intramolecular pyrene excimer formation. The I_E/I_M ratio in this regime was found to scale as M_n^–2.3±0.2. For C_P > 4 × 10^–5 M, pyrene excimer is formed by both intra- and intermolecular interactions and the I_E/I_M ratio increases linearly with increasing C_P 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 (f_E0) was found to increase with C_P in the regime where the pyrene excimer is formed both intra- and intermolecularly and decrease with M_n 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 (R_c) of pyrene in water from the distribution of PEO end-to-end distances. R_c was found to equal 2.2 ± 0.2 nm using f_E0.