Spin polarization in the lowest triplet state of chlorophyll

Abstract

Chlorophyll-b in glassy solution has a spin-polarized lowest triplet state at and above 77 K. The magnitude of the effect is different for MTHF and ethanol as solvents, in contrast to what is found for the porphin free base. Chlorophyll-a does not exhibit spin-polarization under identical conditions as for chlorophyll-b. Zero-field parameters are found to be: chlorophyll-a (MTHF) D = (281 ± 6) × 10 −4 cm −1; E = (39 ± 3) × 10 −4 cm −1; chlorophyll-b (MTHF) D = (289 ± 4) × 10 −4 cm −1; E = (49 ± 3) × 10 −4 cm −1, From ESR signal kinetics it follows that for chlorophyll-b, population and depopulation mainly involve the spin level y⪢, describing a spin moving in a plane perpendicular to the molecular plane: P y ⪢ P x ⪢ P z ; k x = 240 ± 40 s −1; k y = 600 ± 120 s −1; k z ⩽ 75 s −1, where P i and k i denote populating and decay rates. Thus, the kinetic scheme for the chlorophyll triplet is different from that of porphyrins with heavier metal ions, but very similar to that of the porphin free base. The spin-lattice relaxation time is found to be anisotropic and shorter than the decay rates of individual spin levels. Nevertheless, spin polarization can be observed, essentially because the ESR signal amplitude depends on population differences.