Modulation of Ground- and Excited-State Dynamics of [2,2'-Bipyridyl]-3,3'-diol by Micelles

The binding of [2,2'-bipyridyl]-3,3'-diol (BP(OH) 2 ) with ionic and n eutral surfactants like cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and Triton X-100 (TX-100) has been studied by steady-state and time-resolved fluorescence spectroscopy. The absorption as well as emission spectra of BP(OH) 2 are highly sensitive toward the variation of surfactant concentration and hydrophobicity of the environment. The fluorescent state of the diketo form gains stability in surfactant assemblies, leading to a red-shifted emission spectra. A sharp increase in the fluorescence quantum yield near critical micellar concentration (CMC) is encountered followed by saturation. This indicates a complete encapsulation of BP(OH) 2 in the micelles. The maximum fluorescence quantum yield in anionic SDS is rationalized by the formation of cationic fluorophore at the Stern layer. The increase in quantum yield in neutral TX-100 is attributed to higher microviscosity experienced by the fluorophore in the palisade layer. A direct support in favor of this argument in TX-100 is provided by the viscosity dependence exhibited by the probe in different concentrations of sucrose solutions. CTAB exhibiting only hydrophobic effect shows least increase in qunatum yield ofBP(OH) 2 among all the surfactants. Time-resolved fluorescence study of BP(OH) 2 in micelles is used as a tool to monitor the extent of micellization in the lipophilic cavity. An increase in fluorescence quantum yield as well as lifetime of BP(OH) 2 upon micellization indicates an enhanced extent of ESIDPT in hydrophobic medium.