Interaction of alkali, alkaline earth and transition metal ions with a ketocyanine dye: A comparative electronic spectroscopic study Sanjib Kr. Sardar a , Kambalapalli Srikanth a , Prasun K. Mandal a,⇑ , Sanjib Bagchi b,⇑ a Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741 252, Nadia, West Bengal, India b Department of Chemistry and Biochemistry, Presidency University, Kolkata 700 073, West Bengal, India highlights " A ketocyanine dye with D–A configuration forms 1:1 complex with metal ions. " DFT calculation has been done on dye–metal ion complexation. " Slight blue shift of the absorption spectrum for low concentration of metal ions. " Life time and fluorescence intensity increase for low concentration of metal ions. graphical abstract -6 -5 -4 -3 -2 150 200 250 300 350 Florescence Intensity Mg 2+ Ca 2+ Sr 2+ Ba 2+ Li + Log (Cation Conc / M) 200 300 400 500 -6.5 -6.0 -5.5 -5.0 -4.5 Fluorescence Intensity log (Cation Canc / M) Mn 2+ Co 2+ article info Article history: Received 22 May 2012 Received in revised form 16 August 2012 Accepted 4 September 2012 Available online 10 September 2012 Keywords: Ketocyanine dye Metal ions Fluorescence enhancement abstract Interaction of a dye which is structurally similar to a ketocyanine dye with metal ions (alkali, alkaline earth and transition metal) has been studied by monitoring the electronic absorption, steady state and time resolved fluorescence parameters of the dye. The dye (S 0 state) forms a 1:1 complex with cations as indicated by the appearance of a new band at a longer wavelength. Equilibrium constant and other thermodynamic parameters for complexation have been determined. The interaction between the dye and the cation is mostly electrostatic in nature. Spectroscopic results have been supplemented by DFT calculation. For very low concentration of cations, where complexation is insignificant, the absorption band of the dye undergoes a slight blue shift. Enhancement of fluorescence intensity has been observed in the same concentration range. Both phenomena have been explained in terms of formation of a weak association complex where one/more cation replace equivalent solvent molecules in the cybotatic region around the dye. The binding constant of the weak association complex involving cation and the dye (S 1 state) has been determined and has been found to depend on the charge-to-size ratio of the cations. Mea- surement of fluorescence lifetime of the dye indicates that the association complex is slowly decaying relative to solvated dye. At higher concentration of metal ions, however, fluorescence of the dye is quenched by the metal ions. A red shift of fluorescence maximum has also been observed in this concen- tration range. Ó 2012 Elsevier B.V. All rights reserved. Introduction Photophysical and spectroscopic properties of dyes containing both donor and acceptor in the molecule has been the subject of intensive investigation [1]. Ketocyanine dyes provide an interest- ing system belonging to this class [2–4]. Due to the presence of an electron donor (D) e.g. amino group and an electron acceptor (A) carbonyl group, in the molecule, the electronic transition is 1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.09.015 ⇑ Corresponding authors. Tel.: +91 9434238073 (S. Bagchi), tel.: +91 9732730377 (P.K. Mandal). E-mail addresses: prasunchem@iiserkol.ac.in (P.K. Mandal), bsanjibb@yahoo.com (S. Bagchi). Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 99 (2012) 37–45 Contents lists available at SciVerse ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa