Intense phosphorescence triggered by alcohols upon formation of a cyclodextrin ternary complex

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Abstract

Intense phosphorescence is observed when alcohols (ROH) are introduced to aqueous solutions containing 1-bromonaphthalene (1-BrNp) and a glucosyl-modified cyclodextrin (Gβ-CD). Steady-state and time-resolved luminescence measurements and equilibrium constant data are consistent with phosphorescence arising from 1-BrNp as part of a 1-BrNp·Gβ-CD·ROH ternary complex. The association of the 1-BrNp to Gβ-CD is increased in the presence of the alcohols (K = 800 M-1 in the absence of ROH and K = 1900-3400 M-1 in the presence of ROH with the exception of cyclohexanol where K = 760 M-1). However the phosphorescence quantum yields show no obvious correlation with the apparent formation constants of the ternary complex. For instance, the ternary complex of cyclohexanol exhibits the highest phosphorescence quantum yield (φe = 0.035) despite possessing the smallest formation constant. Stern-Volmer analysis shows that the phosphorescence enhancement induced by alcohol is related to its effectiveness in shielding photoexcited 1-BrNp from oxygen. The rate constants for oxygen quenching decrease generally as the bulkiness of the alcohol increases. Accordingly, tert-butyl alcohol and cyclohexanol give rise to the smallest oxygen quenching rate constants and the highest emission quantum yields. The ability of alcohols to trigger an intense luminescence response is a first step in the development of an optical scheme to detect alcohols. The advantage of strategies using a 1-BrNp·Gβ-CD·ROH ternary complex is that alcohol detection occurs by the appearance of bright green phosphorescence relative to a photonically silent background. © 1993 American Chemical Society.

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