Measurements are reported on some characteristics of photohyphen;conduction and semiconduction in pure crystalline chlorophyllsaandb.The semiconduction activation energy is 1.12 ev in chlorophyllaand 1.44 ev in chlorophyllb(E/2kT). The latter value is in excellent agreement with the triplet state energy of 1.43 ev measured by the phosphorescence method in chlorophyllb.The former value suggests that a possible cause of the lack of detectable phosphorescence in chlorophyllais 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 chlorophyllband less easily in chlorophylla, because of the much larger dark current. The photoconduction activation energy in chlorophyllbis 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 oxygenhyphen;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 oxygenhyphen;chlorophyllacomplex is about 1.4 ev; that of the oxygenhyphen;chlorophyllbcomplex 0.63 ev. There is no detectable activation energy for the formation of the complex. It is not a photohyphen;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 explanationmdash;that of an oxygen compound being responsible for thermoluminescence and a thermal spike in conductivitymdash;rather than their first assumption, that of electron trapping and recombination in chlorophyll itself.
展开▼
