Molecular aggregation, solvato- and acidochromic properties of symmetrical ketocyanines

Abstract

The absorption and fluorescence spectral characteristics of some symmetrical ketocyanine dyes have been investigated in different solvents of various polarities using steady-state absorption and emission spectroscopic techniques. The absorption spectral features are less sensitive to the solvent characteristics than the corresponding fluorescence spectra. A large excited state dipole moment parallel to the smaller ground state dipole moment was calculated. The large dipole moment change with the strongly solvatofluorochromic behaviour, as the solvent polarity increases, demonstrates the formation of an intramolecular charge-transfer (ICT) excited state, which is confirmed by theoretical calculation using semiempirical PM3 method. Also, the effect of dye concentration on the electronic absorption spectra for the investigated dyes has been studied in ethanol. In addition, the acidochromic behaviour of these dyes has been studied using sulphuric acid at different H o values. A progressive red-shift in the absorption and emission spectra has been observed upon increasing the concentration of sulphuric acid with dramatic changes in colour. This suggests the design of potential optical sensors for probing acidity of the medium or water content in sulphuric acid. The calculated excited state protonation constants pKaast are lower than p Ka values calculated for the ground state, indicating that the investigated ketocyanines become more basic in the first excited singlet state. [Figure not available: see fulltext.] © 2013 Indian Academy of Sciences.