Abstract

Excitation transfer in photosynthetic pigments is generally described as incoherent Förster hopping, but the exciton concept has also been applied for photosynthetic antenna systems (1,2). In that case excitation energy is delocalized over a number of pigment molecules and the dynamics occurs through the relaxation between different exciton states. However, often it is not so straightforward to unambiguously distinguish in experiment the two qualitatively different kinetic processes, incoherent hopping and exciton relaxation. The actual dynamics can be a combination of these limiting cases. For example, smaller sections of the full system might behave as a small exciton whereas the dynamics on a larger scale may correspond to the hopping-like transfer of this small exciton. An argument against the pure incoherent Förster mechanism is the recent observation of coherent nuclear motions in the antenna complexes of photosynthetic bacteria (3). The vibrational coherence is preserved for at least the same time as the estimated single step transfer time in these systems. This implies that one of the main assumptions of the Förster theory that the vibronic relaxation occurs much faster than the excitation transfer is not fulfilled. Furthermore, the structural data of several light-harvesting proteins (4,5) show densely packed pigment systems, where exciton interactions are quite strong.

© 1996 Optical Society of America