Chlorophylls as Molecular Semiconductors: Introduction and State of Art (original) (raw)

Advanced Materials Technologies

The generic name Chlorophyll (Chl) indicates a group of cyclic tetrapyrroles constituting the most abundant pigments present in nature, even visible from the outer space. Such pigments play a pivotal role in the photosynthesis, the metabolic process that fuels the entire biosphere by converting the energy associated to the solar radiation into chemical energy through the fixation of CO 2 into carbohydrates. [1] Chlorophylls are involved in the three major reactions of the photosynthetic process, namely i) absorption of the light radiation acting as antennas in the light harvesting complexes, ii) transfer of the excitation energy to the socalled reaction center proteins, iii) accomplishment of the photoinduced primary charge separation across the photosynthetic membrane. Photosynthetic organisms are found among Eukarya and Bacteria, with large differences in their photosynthetic apparatus (Figure 1). [2] Plants (superior and inferior) and algae are the two main groups of eukaryotic photosynthetic organisms, both performing oxygenic photosynthesis using water as source of electrons and evolving oxygen, by means of two kinds of reaction centers (RC) named photosystem I (PSI) and II (PSII) acting in sequence in the so-called Z scheme. [1a] Bacteria has the largest variety of photosynthetic organisms that can be grouped into oxygenic organisms, Cyanobacteria, possessing both photosystems, and anoxygenic organisms that do not produce oxygen (pigments biosynthesis is inhibited by its presence) and have only one photosystem. [3] In turn, anoxygenic bacteria can be classified according to the kind of photosystem used: purple bacteria, have PSII-like reaction centers, while green sulfur bacteria have RCs similar to PSI. [4] All these photosystems contain Chls arranged in precise positions within the protein scaffolding and in particular, when arranged as dimers they fulfill the function of primary electron donors. Intact photosynthetic proteins and even whole photosynthetic microorganisms are extensively employed in a large variety of devices as recently reviewed. [5] A non-exhaustive list includes photoelectrochemical cells [6] (possessing also selfhealing properties and operating at sub-zero temperatures), [7] dye-sensitized solar cells, [8] transistors, [9] tactile sensors, [10] and photodetectors. [11] Combining the approach of organic Chlorophylls (Chls) and their derivatives are the most common pigments used for light absorption, energy transfer, and charge separation in photo synthetic organisms. Their functions change upon different aggregation states and specific pigment-protein interactions. Differences in the aromatic πsystem and substituents finely tune Chls electronic, spectroscopic, and supramolecular characteristics. The varieties of Chls species and the pos sibility 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 semisynthetic 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 devel oping 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 lightresponsive molecular semiconductors known so far. Exploiting the high efficiency of these renewable, biocompat ible, and recyclable natural systems can pave the way for next generation biooptoelectronics, including artificial light energy converters, photodetec tors, and, more in general, forwardthinking technologies.