EFFECTS OF SOLVENT ON THE FLUORESCENCE PROPERTIES OF BACTERIOCHLOROPHYLL a

Fluorescence lifetimes (τ_f) of bacteriochlorophyll a (BChl a ) have been measured by the method of time-correlated single-photon counting on dilute (1 μ M ) solutions of the pigment in 15 solvents. There is a pronounced dependence of τ_f on the nature of the solvent. Specifically, τ_f, is longer when the central magnesium is hexacoordinated than when pentacoordinated and shorter when the macrocycle is hydrogen-bonded than when it is not, but the latter effect is more pronounced. Both trends were confirmed by parallel studies on bacteriopheophytin a (BPheo a ). Because of the short lifetimes (˜ 2.2–3.6 ns), quenching of fluorescence by molecular oxygen is not a significant factor in aerated solutions of the bacterial pigments. However, reabsorption artifacts are non-negligible, which necessitates studies on dilute solutions. Fluorescence quantum yields (ø_f) have been estimated for BChl a in 13 solvents by comparing the observed fluorescence lifetimes with the radiative lifetimes calculated from the integrated absorption spectra.