Resumen de Synthesis of fullerene-based phthalocyanine and subphthalocyanine hybrids and their electron transfer reactivity
Increasing global demand for energy, along with dwindling fossil fuel resources and a better understanding of the hidden costs associated with these energy sources, have spurred substantial political, academic and industrial interest in alternative energy resources.
18 Among them, solar energy is one of the most promising in the long term. Every hour, the Earth receives enough solar power to supply humans’ energy consumption for one year.19 Meanwhile, the energy consumption of the world reached 16 terawatts in 2006 and is predicted to rise to ≈30 terawatts by 2050.20 The development of species that can harvest and convert sunlight efficiently is thus an extremely important mission. Organic chemistry plays a crucial role in the search for such systems, as proper positioning of the active components is a key point for the achievement of efficient devices.
In this regard, the photosynthetic system found in nature is the supramolecular photoactive device by excellence that has served as inspiration to many scientists. Much effort has been devoted to the understanding of the mechanism of light conversion in this system, with the aim of designing artificial systems that can behave similarly in human’s profit. Photosynthesis21 is the process employed by plants, algae and cyanobacteria to convert the radiant energy from the sun into chemical energy to fuel the activities of these organisms. The success of this conversion relies upon the efficient absorption and conversion of sunlight.
The main players in the process are chlorophylls and carotenoids with characteristic absorption features (Figure 1a). While the latter play mainly a photoprotective role, chlorophylls are involved in light harvesting and charge separation processes.22 Photosynthetic systems present 18 a) S. Berardi, S. Drouet, L. Francas, C. Gimbert-Surinach, M. Guttentag, C. Richmond, T. Stolla, A.
Llobet, Chem. Soc. Rev. 2014, 43, 7501-7519; b) R. Wengenmair, T. Bührke, Renewable energy: sustainable energy concepts for the future, Wiley - VCH, Weinheim, 2013, pp. 1-170; c) issue on “Artificial Photosynthesis and Solar Fuels”, Acc. Chem. Res. 2009, 42, 1859-2029.
19 Q. Schiermeier, J. Tollefson, T. Scully, A. Witze, O. Morton, Nature, 2008, 454, 816-823.
20 M. I. Hoffert, K. Caldeira, A. K. Jain, E. F. Haites, L. D. D. Harvey, S. D. Potter, M. E. Schlesinger, S. H.
Schneider, R. G. Watts, T. M. L. Wigley and D. J. Wuebbles. Nature, 1998, 395, 881-884.
21 B. Gobets, R. van Grondelle, Biochim. Biophys. Acta, 2001, 1507, 80-99.
22 a) B. Alberts, A. Johnson, J. Lewis, M. Raff, K. K. Roberts, P. Walter, Molecular Biology of the Cell, 5th ed., Garland Science, New York, 2007, pp. 813-878; b) R. E. Blankenship, Molecular Mechanisms of Photosynthesis, Blackwell Science, 2002, pp. 1-321; c) R. Berera, R. van Grondelle, J. T. M. Kennis, Photosynth. Res, 2009, 101, 105-108.
12 Introduction and objectives two basic components: an antenna complex for light harvesting, and a reaction center for charge separation.
Figure 1. a) UV-vis absorption spectra of Chlorophylls (green and red lines) and carotenoids (yellow line); bSchematic representation of photosystem I.
The first step in photosynthesis is light absorption by the antenna complex. Photoinduced energy transfer is a process of great importance in the light harvesting complex, and, moreover, it acts as the trigger of charge separation process. The aim of this event is to progressively direct the energy from sunlight to the reaction centre. Energy transfer is a photophysical process where the excitation of a chromophore is transferred to a radiationless relaxation. Spectral overlap between the emission of the donor and the absorption of the acceptor is required for energy transfer to occur.
In the case of photosynthesis, energy transfer takes place through the dipolar coupling mechanism described by Theodore Förster in 1914. Förster resonance energy transfer (FRET) takes place when non-radiative excitation transfer occurs between two molecular entities separated by distances that exceed the sum of their Van der Waals radii.23 This energy exchange happens via the electromagnetic field associated to the electron in the LUMO of the donor, which causes a perturbation on the electrons in the HOMO of the acceptor. The effectiveness of the energy transfer depends on the distance between the chromophores (the rate is proportional to R-6 ) and the relative orientation of the chromophores. In this regard, organization of the chromophores is very important for an efficient photosynthetic process. A common feature of photosynthetic systems is a ring-like organization of the antenna (Figure 1 b) complex 23 Helms, Volkhard. "Fluorescence Resonance Energy Transfer". Principles of Computational Cell Biology.
Weinheim: Wiley-VCH. 2008. p. 202.
13 Introduction and objectives around the reaction center. Such degree of organization of the photosynthetic pigments addressed to ensure formation of efficient antennas and reaction centers is based on supramolecular interactions involving not only the pigments but also proteins and protein dimers.
Conversion of sunlight energy into chemical energy takes place through a cascade of unidirectional electron transfer reactions that will lead to the synthesis of carbohydrates. The success of this process relies on the effectiveness of these electron transfers and the lack of recombination reactions that would interrupt the process and cause a waste of the absorbed energy. In a similar manner, artificial self-assembly of supramolecular structures from photoactive components may be expected to modify the ground-state and/or excitedstate behaviour of the individual molecules. This fact may give rise to a number of processes (energy transfer, charge separation, perturbation of optical transitions and polarizabilities, modification of redox potentials, regulation of binding properties, photochemical reactions, etc.) that will depend on the arrangement of components in the supramolecular structure.
Resuming, in natural photosynthetic systems, cascades of energy- and electron-transfer reactions are triggered either directly by photoexcitation or indirectly by energy transfer from light-harvesting antenna systems. In terms of charge separation, the characteristics of the individual electron acceptors and electron donors are decisive to modulate its overall efficiency.
In terms of charge recombination, the environment of the photosynthetic reaction center plays a crucial role to slow it down. Therefore, designing, synthesizing, and probing efficient energy capacitors as well as porphyrinoid chromophores featuring unique panchromatic absorptive, redox, and electrical properties is of a crucial importance.
Electron transfer and Marcus theory In recent decades, the scientific community revealed a great interest towards non-covalent energy- and electron transfer processes, continuosly analyzing how the underlying interactions vary as a function of environment.24 24 a) S. Fukuzumi, K. Ohkubo, F. D'Souza, J. L. Sessler. Chem Commun. 2012, 48, 9801-9
