Electronic excitation transfer in the photosynthetic unit: Reflections on work of William Arnold

Among his many contributions to photosynthesis, William Arnold made critical suggestions about the mechanism of the initial stages of excitation energy transfer and its measurement. Thus he helped found not only the general concept of the photosynthetic unit but also the key idea behind the detailed functional aspects of its 'chlorophyll antenna'. We review the development of these ideas and the modern form in which they have emerged.