Intramolecular Proton Transfer in Inclusion Complexes of Cyclodextrins: Role of Water and Highly Polar Nonaqueous Media

Abstract

The molecule 4-methyl-2,6-dicarbomethoxyphenol (CMOH) which undergoes ultrafast excited-state intramolecular proton transfer (ESIPT) has been employed as a guest to probe ESIPT within the interior of the host α- and β-cyclodextrins (CDx) in water and highly polar nonaqueous solvents. The ESIPT reaction of CMOH is favored in its microencapsulated form in both aqueous and nonaqueous media. As a consequence, resultant enhancement of tautomer emission, following ESIPT, relative to that in homogeneous bulk media is observed. CMOH forms 1:2 and 1:1 complexes with α- and β- CDx's (host), respectively, in both water and highly polar nonaqueous media. However, tautomer emission due to ESIPT is significantly enhanced in water compared to nonaqueous solvents, despite formation of stronger guest−host (inclusion) complexes with much higher binding constant in the nonaqueous media. The results have been interpreted as due to the formation of ternary complexes comprising CMOH, second guest, and cyclodextrin in nonaqueou...