INFLUENCE OF THE PHASE STATE OF PHOSPHATIDYLCHOLINE MATRIX MONOLAYERS ON THE PHOTOPHYSICAL CHARACTERISTICS OF AN AMPHIPHILIC OXACYANINE DYE AT THE AIR-WATER-INTERFACE

The photophysical characteristics of an amphiphilic oxacyanine dye embedded in phospholipid matrix monolayers at the air/water interface are reported. The matrix molecule used in the present study was dimyristoyl phosphatidyl-choline. Surface pressure-, reflection- and fluorescence- area isotherms along with fluorescence and reflection spectra were measured as a function of dye/lipid molar ratio and subphase temperature. It is observed that the photophysical properties of oxacyanine dye depend sensitively on the molecular organization parameters of the matrix monolayer. The surface density normalized fluorescence intensity shows a strong increase with increasing surface pressure. The reflection-area isotherms along with the reflection spectra show no evidence of dimer formation up to a dye/lipid molar ratio of 1:5. Further increase of dye concentration leads to changes in reflection spectra indicating dimer and higher aggregate formation. The fluorescence intensity varies linearly with dye concentration up to a dye/lipid molar ratio of 1:50. The fluorescence quantum yield increases strongly with decreasing temperature (275-300 K) of the subphase. The dependence of photophysical properties of the dye on various molecular organization parameters is discussed in terms of phase transitions and fluidity of the matrix monolayer.