INTRAMOLECULAR PROTON-TRANSFER IN INCLUSION COMPLEXES OF CYCLODEXTRINS - ROLE OF WATER AND HIGHLY POLAR NONAQUEOUS MEDIA

The molecule 4-methyl-2,6-dicarbomethoxyphenol (CMOH) which undergoes ultrafast excited-state intramolecular proton transfer (ESIPT) has bee n employed as a guest to probe ESIPT within the interior of the host a lpha-and beta-cyclodextrins (CDx) in water and highly polar nonaqueous solvents. The ESIPT reaction of CMOH is favored in its microencapsula ted form in both aqueous and nonaqueous media. As a consequence, resul tant enhancement of tautomer emission, following ESIPT, relative to th at in homogeneous bulk media is observed. CMOH forms 1:2 and 1:1 compl exes with alpha- and beta- CDx's (host), respectively, in both water a nd highly polar nonaqueous media. However, tautomer emission due to ES IPT 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 compris ing CMOH, second guest, and cyclodextrin in nonaqueous media where the nonaqueous solvent acts as the second guest molecule.