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The interaction of a ketocyanine dye with a cobalt(II) ion has been studied in solution by monitoring the electronic absorption and emission spectral characteristics of the dye. A new absorption band at a longer wavelength appears in solutions containing cobalt(II) ions. An isosbestic point is observed for systems containing a fixed dye concentration and varying Co(2+) ion concentration, pointing to the formation of a complex. The stoichiometry of the complex has been found to be 1:1. Equilibrium constant has been determined from the observed data. The nature of interaction between the dye (S(0) state) and the Co(II) ion is mostly electrostatic. Spectroscopic results have been supported by DFT/TDDFT calculation. The fluorescence band is characterized by a small blue shift. In the concentration range of 10(-3)-10(-4) M of the Co(II) ion, a quenching of the dye fluorescence is noticed. The Stern-Volmer plot points to the operation of both static and dynamic mechanisms of quenching. For a micromolar concentration of the Co(II) ion, however, an enhancement of fluorescence intensity with a slight blue shift has been observed, which has been explained in terms of formation of a different type of complex in the S(1) state at this concentration level. The value of lifetime increases at the micromolar level of concentration of the Co(II) ion, where the intensity increases and then remains practically unchanged as more salt is added to the system. Values of the decay constant for the different photophysical processes have been calculated. Complexation in the S(1) state is characterized by a slower decay of the excited dye by a nonradiative path.

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