Effects of energy dissipation on motional dynamics in unilamellar vesicles.

We have investigated the molecular reorientation dynamics of perylene incorporated into the nonpolar acyl chain region of 100 nm diameter phosphocholine unilamellar vesicles. When perylene is excited to the S(1) electronic manifold, it exhibits a different fluorescence anisotropy decay than when it is excited to its S(2) electronic manifold. In addition to differences in the polarization of the two transitions, there is a difference in the amount of excitation energy dissipated nonradiatively, because for both excitation conditions radiative emission is from the S(1) state. The data reveal a faster rotational diffusion constant for excitation to the S(2) manifold, and we interpret this finding in the context of local heating of the chromophore immediate environment by the nonradiative dissipation of energy associated with rapid relaxation from the S(2) to S(1) manifolds. Our results are consistent with transient heating on the order of 4 K, with the actual temperature increase depending subtly on whether the phospholipid acyl chains are in the gel phase or the fluid phase.