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.