FLUORESCENCE LIFETIMES OF CHLOROPHYLL a: SOLVENT, CONCENTRATION AND OXYGEN DEPENDENCE

Fluorescence lifetimes (τ f ) of chlorophyll a (Chi a ) have been measured by the single‐photon‐counting technique over a wide range of concentrations (˜10 ‐7 ˜10 ‐4 M ) in deoxygenated pyridine, diethyl ether, toluene and methanol. At pigment concentrations ˜1 μ M , reabsorption of fluorescence induces significant artifacts on measured values of τ f which are dependent on detection wavelength and the specific geometry of the experiment. There is a clear dependence of τ f on the nature and degree of solvation, including both coordination of the central magnesium and hydrogen‐bonding of the solvent (viz. 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 diffusion‐controlled limit was obtained.