J . Phys. Chem. 1989, 93, 4615-4619
@d[fiMS(o)l
Conclusions The photophysics of TBPe in solvents, vesicles and lyotropic liquid crystals is very similar to that of perylene in solvents. TBPe’s rotational motion in solution shows, in contrast to that of perylene, sticking boundary conditions over a wide temperature region. TBPe’s rotation in solution is well described by a single correlation time. The orientational distribution of TBPe is broad in membranes and lyotropic liquid crystals since TBPe preferentially solubilizes in the hydrophobic region. The rotational correlation function of TBPe solubilized in vesicles and cubic phases is very well described by the model relevant for the rotation of an oblate ellipsoid in a solvent. Unlike in solvents, the diffusion coefficients about (Dll) and perpendicular to (0,) the symmetry axis are significantly different. As compared to most fluorescent probes used in membrane research, TBPe is photostable with a monoexponential fluorescence decay. The fluorescence anisotropy fits nicely to simple motional models that neglect the local anisotropy of TBPe.
Acknowledgment. We are very grateful to Eva Vikstrom for skillful technical assistance. This work was supported by the Swedish Natural Research Council.
Appendix We consider a cylindrically symmetric chromophore in a solvent cage. The molecule rotates due to fast (for example, librational) motions and slow (diffusive) motions. If these motions are independent, then eq 3 can be separated as36 ?
2
4615
Dgr[fiM’S(t)l dnMS dfiMS (A21 AfiMs(o)] is the initial orientational distribution of a chromophore relative to a fictive frame (S)which is fixed in the solvent cage. g[nMS(o)lfiMSt(t)] is the conditional probability density. The exponential terms describe the slow rotational motion of the molecule and the angular correlation functions in the brackets, ( ), account for fast motions in the solvent cage. If g[fiMS(o)(fiMtS(t)] decays very rapidly, as compared to the rate of fluorescence, to an equilibrium orientational distribution A f i M S t ] , then eq A2 becomes (D%[fiMS(o)l
g[nMS(o)lnMS(t)l
= (DEd(fiM.5)) (D%*(fiM!S)) (A31
@r[nM’S(t)l)
We here assume that the chromophore in its solvent cage can rotate through an angle fa,,, about the symmetry axis and that
(
f(nM‘S) = f ( n M S )
=
; @ - T /2amax 2,T-o)
0C
CY
< amax and
amax
