Chlorophylls as Molecular Semiconductors: Introduction and State of Art

Chlorophylls (Chls) and their derivatives are the most common pigments used for light absorption, energy transfer, and charge separation in photosynthetic organisms. Their functions change upon different aggregation states and specific pigment-protein interactions. Differences in the aromatic pi-system and substituents finely tune Chls electronic, spectroscopic, and supramolecular characteristics. The varieties of Chls species and the possibility of chemical manipulation, together with their exceptional absorption cross section, make them attractive materials for applications as sensitizer in energy conversion devices. Moreover, when deposited as thin films, Chls and their semi-synthetic derivatives exhibit the typical behavior of molecular organic semiconductors with good charge carrier mobility and Fermi level suitable for their use in optoelectronic devices like phototransistors. This review aims at bridging the possible knowledge and language gap between biochemists and biophysicists working on Chls, and material scientists developing organic optoelectronic devices. Starting from the structural features, and proceeding towards optoelectronic devices, the review offers a critical overview on the uses of Chls as light-responsive molecular semiconductors known so far. Exploiting the high efficiency of these renewable, biocompatible, and recyclable natural systems can pave the way for next generation biooptoelectronics, including artificial light energy converters, photodetectors, and, more in general, forward-thinking technologies.