Femtosecond Solvation Dynamics in Different Regions of a Bile Salt Aggregate:  Excitation Wavelength Dependence

Solvation dynamics of coumarin 480 (C480) in the secondary aggregate of a bile salt (sodium deoxycholate, NaDC) is studied using femtosecond up-conversion. The secondary aggregate resembles a long (∼40 Å) hollow cylinder with a central water-filled tunnel. Different regions of the aggregate are probed by variation of the excitation wavelength (λex) from 375 to 435 nm. The emission maximum of C480 displays an 8 nm red shift as the λex increases from 345 to 435 nm. The 8 nm red edge excitation shift (REES) suggests that the probe (C480) is distributed over regions of varied polarity. Excitation at a short wavelength (375 nm) preferentially selects the probe molecule in the buried locations and exhibits slow dynamics with a major (84%) slow component (3500 ps) and a small (16%) contribution of the ultrafast component (2.5 ps). Excitation at λex = 435 nm (red end) corresponds to the exposed sites where solvation dynamics is very fast with a major (73%) ultrafast component (≤2.5 ps) and relatively minor (27%) slow (2000 ps) component. In sharp contrast to solvation dynamics, the anisotropy decay becomes slower as λex increases from 375 to 435 nm. It is proposed that the buried locations (λex = 375 nm) offer lower friction because of the rigid sheetlike structure of the bile salt.