Light‐Harvesting Antennae

Abstract

An antenna for light harvesting (Figure 6.1) is an organized multicomponent system in which several chromophoric molecular species absorb the incident light and channel the excitation energy to a common acceptor component [1]. For artificial systems, the term antenna effect was first used [2] to discuss the case of strongly emitting but weakly absorbing lanthanide ions surrounded by strongly absorbing ligands, where the luminescence of the lanthanide ion was sensitized by excitation in the ligand-centered (LC) excited states. Research in this area is still very active [3]. Antenna systems are widely used by Nature to solve the problem of light-harvesting efficiency in the photosynthetic process, where light is converted into chemical energy [4]. Collecting light by an antenna system, however, may also be useful for other purposes, such as signal amplification in luminescence sensors [5], photodynamic cancer therapy [6], and upconversion processes [7]. A large system, where an array of chromophoric units absorb light and transfer energy to a luminescent center, can also be considered a spatial and spectral energy concentrator (molecular lens) [8].The antenna effect can only be obtained in supramolecular arrays suitably organized in the dimensions of time, energy, and space. Each molecular component has to absorb the incident light, and the excited state so obtained (donor) has to transfer electronic energy to a nearby component (acceptor), before undergoing radiative or nonradiative deactivation (organization in the time dimension). For energy transfer to occur, the energy of the acceptor excited state has to be lower or, at most, equal to the energy of the excited state of the donor (organization in the energy dimension). Finally, the successive donor-to-acceptor energy-transfer steps must result in an overall energy-transfer process leading the excitation energy toward a selected component of the array (organization in the space dimension).In the course of evolution, Nature has succeeded in building up antenna systems that fully satisfy the above requirements. In green plants, such natural antennae j135 Molecular Devices and Machines. Concepts and Perspectives for the Nanoworld. 2 nd Ed.