QUENCHING DYNAMICS AND DIFFUSION OF SMALL HYDROPHOBIC MOLECULES IN LONG RODLIKE MICELLES

The previously presented model (J. Phys. Chem. 1988, 92, 4479) for the deactivation of long-lived excited singlet or triplet states by a diffusion-controlled mechanism in very long cylindrical micelles has been extended to the case where the deactivator exchange between the micelles and the surrounding aqueous phase is important. This and the nonexchange case have been explored experimentally by both fluorescence quenching (1-μs time scale) and triplet deactivation (20-μs time scale), with a number of probe/quencher pairs in solutions of cetyltrimethylammonium chloride (CTAC) + sodium chlorate. From these measurements, the relative diffusion coefficients for the pairs were primarily obtained, and by combination of the results, the individual diffusion coefficients for the probes and quenchers were deduced. These are in the range (5-2.5) × 10-11 m2 s-1 for aromatic hydrocarbons like pyrene, 9-methylanthracene, azulene, and benzophenone and substantially smaller, (1.5-0.5) × 10-11 m2 s-1, for compounds with alkyl chain substituents, hexylbenzophenone, hexadecylpyridinium chloride, and octadecyl anthracene-9-carboxylate. The results from the deactivation in the long micelles were compared to the quenching constants, kq sph, determined separately for the same pairs in small globular micelles of CTAC. A correlation expressed by kq sph = 1.1(D/R2), where D is the mutual diffusion coefficient and R = 21 Å, the spherical micelle radius, was established for most of the pairs, with the excimer formation of pyrene and pyrene + 9-methylanthracene as exceptions. The dissociation of pyrene excimers and the inseparable fluorescence from pyrene and 9-methylanthracene are the probable explanations to the behavior in these cases. © 1990 American Chemical Society.