Photophysical properties of aqueous solutions of 3,3'-diethyl-thiacarbocyanine iodide (DTCI) was studied in the presence of cucurbit[7]uril (CB7) by means of absorption and fluorescence spectroscopy. The addition of CB7 decreases the 550 nm peak of the DTCI absorption spectrum and concomitantly transforms the familiar shoulder at 510 nm into a pronounced peak; fluorescence was found to decay monoexponentially with a lifetime of 0.12 ns in aqueous DTCI solution without CB7 and 1.07 ns in the presence of CB7. The association constant of 1: 1 complexes of DTCI with CB7 has been determined to be K = (2.8 +/- 0.2) x 10(4) M (1). The Rulliere model of photoisoomerisation of cyanines was used for explaining the main features of the effect observed. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.