Photophysical and photochemical properties of merocyanine 540

Absorption and corrected fluorescence emission spectra of merocyanine 540 (MC 540) were recorded in solvents with different physico-chemical parameters - dielectric constant epsilon(r) refractive index n, dipolarity/polarizability pi*, hydrogen-bonding ability alpha. A pronounced hypsochromic shift of absorption spectra was linearly correlated with increasing epsilon(r). Calibration of solvatochromic behaviour of MC 540 was used for probing of local polarity of phospholipid liposomes. Stokes shifts of their maxima positions did not obey Lippert-Mataga equation. However, a correlation with pi* and alpha was observed. Fluorescence quantum yields and fluorescence lifetimes for MC 540 in a number of solvents were also determined and a ddependence on hydrogen bonding ability was indicated. The effect of pH on MC 540 absorption and fluorescence spectra was shown. A pK value of 1.60 between the neutral protonated form and the anionic form of MC 540 was determined. Increasing the pH above 7.8 resulted in irreversible changes in both absorption and fluorescence spectra. The reversible effect of ions from physiological solutions on MC 540 absorption and fluorescence spectra was observed. The effect of salts was found to mostly depend on their cationic charge. Increasing salt concentration in the liposome suspensions resulted in increased incorporation of the MC 540 dye into phospholipid liposomes.