The Quenching of Pyrochlorophyll Fluorescence by Nitro Compounds

We have examined the quenching of pyrochlorophyll fluorescence by a series of substituted nitrobenzenes in ethanol-pyridine solution and the effect of fluorescence quenching on the quantum yield of sensitized photoreduction of three of the nitro compounds by hydrazobenzene. All nitro compounds examined are at least moderately good quenchers of fluorescence. There is a good correlation between the Stern-Volmer quenching constants and the polarographic quarter-wave reduction potentials of the nitro compounds. The variation of quantum yield of photoreduction with concentration of nitro compound is interpreted as evidence of electron transfer in the quenching process, as proposed by Dilung and Chernyuk. However, the efficiency of photoreduction sensitized by the singlet excited state of pyrochlorophyll is less than that sensitized by the triplet excited state. This can be explained as a consequence of the spin multiplicity of the radical-ion pair created by the electron transfer.