Fluorescence properties of 1-heptanoylpyrene:: a probe for hydrogen bonding in microaggregates and biological membranes

Fluorescence properties of 1-heptanoylpyrene in homogeneous solutions and incorporated into liposome membranes have been measured. The fluorescence intensity of the alkanoylpyrene is small in neat hydrocarbon solutions (Q(f) = 0.006) but increases with increasing solvent polarity. On the contrary, Of of the 1-alkylpyrene is larger by a factor of 10 in a hydrocarbon solvent and decreases slightly in polar solvents. in alcohols the fluorescence yield of 1-heptanoylpyrene becomes considerably larger in comparison to a nonhydroxylic environment and nearly identical to that of heptylpyrene in the same solvent. The fluorescence spectra shift bathochromically in that polar environment. The fluorescence properties of 1-heptanoylpyrene incorporated in a liposome membrane indicate that the probe molecules are well embedded within the hydrophobic core of the bilayer. Semiempirical calculations were performed for 1-ethanoylpyrene and 1-ethylpyrene as model compounds in order to analyze the influence of solvent polarity and of hydrogen bonding on excited state energies and fluorescence properties of I-acyl derivatives of pyrene. Hydrogen-bonding increases the energy gap between the lowest singlets, which are nearly degenerated, and the (3)pi*-n triplet and reduced consequently the intersystem crossing efficiency. But also a change in the sequence of the excited (1)pi*-pi states is concluded from experimental data and should contribute to the increase of the fluorescence yield upon hydrogen bonding. (C) 1999 Elsevier Science B.V. All rights reserved.