Fluorescence lifetimes (τf) of bacteriochlorophyll a (BChl a) have been measured by the method of time‐correlated single‐photon counting on dilute (1 μM) solutions of the pigment in 15 solvents. There is a pronounced dependence of τf on the nature of the solvent. Specifically, τf, 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 (øf) 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.
Fluorescence lifetimes (TO of chlorophyll a (Chi a ) have been measured by the single-photoncounting technique over a wide range of concentrations ( -10-7--10-4 M ) in deoxygenated pyridine, diethyl ether. toluene and methanol. At pigment concentrations > 1 pcM, reabsorption of fluorescence induces significant artifacts on measured values of T~ which are dependent on detection wavelength and the specific geometry of the experiment. There is a clear dependence of T( on the nature and degree of solvation, including both coordination of the central magnesium and hydrogen-bonding of the solvent (vir. alcohols) to the macrocycle. Quenching of the excited singlet state by molecular oxygen was measured quantitatively in ether. and a bimolecular rate constant markedly slower than the diffusioncontrolled limit was obtained.
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