Ultrafast fluorescence resonance energy transfer in the micelle and the gel phase of a PEO-PPO-PEO triblock copolymer: Excitation wavelength dependence

Fluorescence resonance energy transfer (FRET) from coumarin 480 (C480) to rhodamine 6G (R6G) is studied in the micelle and the gel phase of a triblock copolymer, (PEO)(20)-(PPO)(70)-(PEO)(20) (Pluronic P123 (P123)) by picosecond and femtosecond emission spectroscopy. The time constants of FRET were obtained from the rise time of the acceptor (R6G) emission. In a P123 micelle, FRET occurs in multiple time scales: 2.5, 100, and 1700 ps. In the gel phase, three rise components are observed: 3, 150, and 2600 ps. According to a simple Forster model, the ultrafast (2.5 and 3 ps) components of FRET correspond to donor-acceptor distance R-DA=13 +/- 2 angstrom. The ultrafast FRET occurs between a donor and an acceptor residing at close contact at the corona (PEO) region of a P123 micelle. With increase in the excitation wavelength (lambda(ex)) from 375 to 435 nm, the relative contribution of the ultrafast component of FRET (similar to 3 ps) increases from 13% to 100% in P123 micelle and from 1% to 100% in P123 gel. It is suggested that at lambda(ex) = 435 nm, mainly the highly polar peripheral region is probed where FRET is very fast due to close proximity of the donor and the acceptor. The 100 and 150 ps components correspond to R-DA = 25 +/- 2 angstrom and are ascribed to FRET from C480 deep inside the micelle to an acceptor (R6G) in the peripheral region. The very long component of FRET (1700 ps in micelle and 2600 ps component in gel) may arise from diffusion of the donor from outside the micelle to the interior followed by fast FRET.