Effect of solvents and cyclodextrin complexation on acid–base and photophysical properties of dapoxyl dye

• Cyclodextrin induces dapoxyl dye deprotonation and modulates the excited state properties upon encapsulation.
• Solvent polarity dependent photophysical properties of dapoxyl dye to understand micropolarity of cyclodextrin cavity.
• Computational study to get the better insight into mode of binding of dapoxyl dye and cyclodextrin.

A pH dependent encapsulation of dapoxyl sodium sulfonate (DSS), a charge transfer (CT) dye, with α- and β-cyclodextrin (CD), macrocyclic molecules made up with sugar units, has been performed using steady-state and time-resolved fluorescence measurements; both confirm the formation of an efficient inclusion complex with both non-protonated and protonated form of DSS. Moreover, the fluorescence intensity is affected due to modulation of acid-dissociation constant by CD encapsulation as a consequence of preferential binding affinities with different prototopic forms of DSS. In particular, we observed that the relocation of DSS from water into the less polar hydrophobic cavity of CD causes huge fluorescence enhancement in water. Our investigation reveals that the micro-polarity inside β-CD cavity is akin to methanol–water (1:1) mixtures.

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