Emission Spectra of Chlorophyll-a in Polar and Nonpolar Solvents

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Abstract
Emission spectra of chlorophyll‐a, dissolved in ethanol and various nonpolar solvents, were measured at 300°, 77°, and 4°K, in both dilute and concentrated solutions. For this purpose, a new technique utilizing fiber optics was employed. In dilute ethanolic solutions, the observed fluorescence bands emanate from chlorophyll‐a monomers, solvated at the central magnesium. At 4°K the fluorescence yield in this solvent is 0.85. In highly concentrated ethanolic solutions, a low‐temperature band occurs at about 724 nm, which may be attributed to solvated dimers, formed by a π—π interaction between the chlorophyll monomers. In nonpolar solvents, a low‐temperature luminescence band is observed at about 755 nm. This emission is attributed to π—π* phosphorescence from unsolvated monomers. Also in nonpolar solvents, highly concentrated solutions have a weak band in the neighborhood of 733 nm at 77°K, which originates from chlorophyll dimers formed by interaction between the magnesium of one monomer with the cyclopentanone carbonyl of another. The 733‐nm band is either π—π* phosphorescence or fluorescence. In CCl4 and benzene the dimers are believed to be unsolvated. At room temperature unsolvated dimers fluoresce feebly at 713 nm. Cooling dilute solutions of chlorophyll a in benzene and 1,2‐dichloroethane causes dimer formation, but this is less in CCl4 and absent in ethanol. A calculation of the upper limit for the rate constant for intersystem crossing to the triplet state, in ethanol, gives a value of 9.0×106/sec.