2-StQ-20H possessing a proton donor group (hydroxy) and a proton acceptor group (quinoline) was synthesized in order to study the influence of structural effects on the excited-state proton transfer (ESPT) and tautomerization. The ESPT were observed for ground-state hydrogen-bonding complex between 2-StQ-20H and 2,2.2-trifluoroethanol (TFE) or with triethylamine. In aqueous solution, ESPT can only be observed in a-cyclodextrin (alpha-CD) inclusion complex. The dielectric constant values of the alpha-CD cavities probed by 2-StQ-20H and 2-StQ-2OMe are 8.5 and 6.3, respectively, because of the difference in size of the probe molecule inside the cavity. (c) 2005 Elsevier B.V. All rights reserved.