The excited-state reaction of push-pull styryl system was studied in this work. In the excited state, compounds of 2-StP-NMe2, 2-StQ-NMe2, 4-StQ-NMe2 and 4-StP-NMe2 possess a sufficiently large acidity change for the proton transfer process to occur. For compounds 2-StT-NMe2 and 2-StN-NMe2 the acidity change is too small to induce the proton transfer process. A new type of excited-state de-hydrogen bond (ESDHB) process was observed in this study. It is possible to explore the polarity and HB effect of solvent simultaneously by using the ESDHB mechanism. (c) 2005 Elsevier B.V. All rights reserved.