Ultrafast fluorescence resonance energy transfer in a bile salt aggregate: Excitation wavelength dependence

Fluorescence resonance energy transfer (FRET) from Coumarin 153 (C153) to Rhodamine 6G (R6G) in a secondary aggregate of a bile salt (sodium deoxycholate, NaDC) is studied by femtosecond up-conversion. The emission spectrum of C153 in NaDC is analysed in terms of two spectra-one with emission maximum at 480 nm which corresponds to a non-polar and hydrophobic site and another with maximum at ∼530 nm which arises from a polar hydrophilic site. The time constants of FRET were obtained from the rise time of the emission of the acceptor (R6G). In the NaDC aggregate, FRET occurs in multiple time scales — 4 ps and 3700 ps. The 4 ps component is assigned to FRET from a donor (D) to an acceptor (A) held at a close distance (R _DA ∼ 17 ?) inside the bile salt aggregate. The 3700 ps component corresponds to a donor-acceptor distance ∼48 ?. The long (3700 ps) component may involve diffusion of the donor. With increase in the excitation wavelength (λ _ex) from 375 to 435 nm, the relative contribution of the ultrafast component of FRET (∼4 ps) increases from 3 to 40% with a concomitant decrease in the contribution of the ultraslow component (∼3700 ps) from 97 to 60%. The λ _ex dependence is attributed to the presence of donors at different locations. At a long λ _ex (435 nm) donors in the highly polar peripheral region are excited. A short λ _ex (375 nm) ‘selects’ donor at a hydrophobic location.