The Phosphorescent State as an ODMR Probe of the Solubilization Site of Naphthalene in Frozen Micelles

To obtain a better understanding of the structural features characteristic of the equilibrated micelle a solubilized molecule acting as probe may be employed. A good probe is one whose selected physical properties are sensitive to the nature of the mioroenvironment it experiences and which are readily measured by experiment. The nature of the microenvironment includes the microvisco3ity, polarity, polarizability and the extent of water penetration in the surfactant aggregates [1]. The solubilization sites of arenes within micelles have been examined by a variety of methods [2]. Pyrene has been used as a probe most extensively [2(b, c), 3]. The study of the systematic variation of selected physical properties of the probe as a function of surfactant chain length can provide information concerning the nature of the solubilization site within a micelle [1]. In a recent Investigation [5] of triplet-triplet energy transfer between benzophenone and naphthalene solubilized in frozen sodium dodecyl sulfate micelles, it was shown that optical detection of magnetic resonance (ODMR) of the triplet excited state is capable of distinguishing differences in the microenvironments experienced by a benzophenone donor and the naphthalene acceptor. This study also indicated that micelles retain their structural integrity when frozen to very low temperatures, a conclusion supported by the results of other researchers [6]. Naphthalene (N) was chosen as the probe in these studies rather than pyrene since it should cause less perturbation of the micelle architecture due to its smaller molecular dimensions. In addition, the sublevel kinetics of the N triplet state are well characterized [7]. In this article low temperature phosphorescence, decay and ODMR results in zero applied magnetic field for naphthalene solubilized in a series of anlonic sodium (n)alkyl sulfate (NaCnS, n 10,12,11) micelles and in cationic (n)-alkyl trimethyl ammonium bromide micelles (CnTABr, n 10,12,14,16) are presented as a function of chain length, n. The results are compared with those observed for N in 201 aqueous glycerol and in n-decane solvents under similar conditions. Results presented here demonstrate that low temperature phosphorescence and ODMR can be exploited to study differences in the microenvironraent experienced by N in cationic CnTABr and in anionic NaCnS micelles of varying alkyl chain length. This study also provides further evidence for the presence of weak specific Interactions [8, 9] of the trimethyl ammonium head groups with the solubilized arene probe N in the cationic micelles.