Photo- and Semiconduction in Crystalline Chlorophylls a and b

Abstract

Measurements are reported on some characteristics of photo-conduction and semiconduction in pure crystalline chlorophylls a and b. The semiconduction activation energy is 1.12 ev in chlorophyll a and 1.44 ev in chlorophyll b (E/2kT). The latter value is in excellent agreement with the triplet state energy of 1.43 ev measured by the phosphorescence method in chlorophyll b. The former value suggests that a possible cause of the lack of detectable phosphorescence in chlorophyll a is that it occurs in the infrared at about 11 000 A. The evidence indicates that the mobility of the predominant charge carriers (positive holes) is the same in both crystals. Photoconduction is easily measured in chlorophyll b and less easily in chlorophyll a, because of the much larger dark current. The photoconduction activation energy in chlorophyll b is 0.36 ev. This supports Rabinowitch's view that photoconduction in chlorophyll is not involved in the primary act of photosynthesis. The oxygen adsorbed on the surface of the crystals forms an oxygen-chlorophyll complex. This leads to an increase in both the dark current and the photocurrent probably by increasing the mobility of the surface charge carriers. There is no detectable effect of oxygen upon the semiconduction activation energy, since the increase of the magnitude of the current is small (a factor of 4 to 10). The binding energy of the oxygen-chlorophyll a complex is about 1.4 ev; that of the oxygen-chlorophyll b complex 0.63 ev. There is no detectable activation energy for the formation of the complex. It is not a photo-oxidation process. Some evidence suggests that a more tightly bound oxygen complex exists which has no effect on electronic conductivity. This more stable complex is converted by light into the weakly bound complex described above (probably by photoreduction of chlorophyll in the complex). These results are similar to those of Arnold and Sherwood's work on dried chloroplasts. They favor their alternative explanation—that of an oxygen compound being responsible for thermoluminescence and a thermal spike in conductivity—rather than their first assumption, that of electron trapping and recombination in chlorophyll itself.