naphthalene-like emission remains approximately constant with increasing cyclodextrin concentration. The observation of two lifetimes is probably due to two complexes of different geometry.The exciplex lifetimes are 5, 7, and 11 ns for ethanol, acetonitrile, and 0-cyclodextrin solutions, respectively (Table I).The data support a model where 1 forms an emissive 1:1 complex with 0-cyclodextrin in water. Surprisingly, the 0-cyclodextrin cavity for exciplex has a polarity similar to that of ethanol, considerably higher than anticipated. The results suggest that the intramolecular exciplex of 1 formed must be situated near the top of the 0-cyclodextrin cavity in the proximity of OH groups, an environment considerably different from that of excimers. Exciplexes, which possess intrinsic charge-transfer character, are more polar than excimers. Their formation is governed by a delicate balance between their stabilization and dissociation to ion pairs by the polar environment.The failure to observe intramolecular exciplex emission of 1 in -cyclodextrin and in 7-cyclodextrin may be attributed to steric factors. The former possesses too small a cavity for association with 1, while the latter possesses too large a cavity, where polar water molecules may enter freely to cause the dissociation of the exciplex to ion pairs. The effect of cyclodextrins on other intramolecular exciplex systems is being investigated.
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