Flourimetric and prototropic studies on the inclusion complexation of 2-amino and 4-aminodiphenyl ethers with β-cyclodextrin: Unusual behavior of 4-aminodiphenyl ether

The fluorescence characteristics of diphenyl ether (DPE), 2-aminodiphenyl ether (2ADPE) and 4-aminodiphenyl ether (4ADPE) and prototropic behavior of 2ADPE and 4ADPE on inclusion complexation with β-cyclodextrin have been investigated. DPE forms 1:1 complex whereas 2ADPE and 4ADPE form 1:2 complex with β-CDx. The fluorimetric and prototropic behaviors of 4ADPE in β-CDx are different from those in aqueous solution. The dual fluorescence of 4ADPE in β-CDx is found to be due to twisted intramolecular charge transfer (TICT) character induced by inclusion complexation. The two equilibria viz. monocation↔monocation solvent exciplex↔neutral reported for 4ADPE in aqueous solution are not observed in presence of β-CDx. The ground and excited state pK_a values for monocation-neutral equilibrium of 2ADPE and 4ADPE have been reported.