Light harvesting in dendrimer materials: Designer photophysics and electrodynamics
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Multichromophoric dendrimers are increasingly being considered for solar energy systems. To design materials with suitably efficient photon collection demands a thorough understanding of crucial photophysical conditions and electrodynamic mechanisms, many of which prove to emulate photosynthetic systems. Key parameters include the chromophore absorption properties, the generation, branching and folding of the dendrimer, and the presence of a spectroscopic gradient. Driving excitation towards a trap, resonance energy transfer favors migration between nearest neighbor chromophores. In modeling the progress of excitation from antenna chromophores towards the trap, a propensity matrix method has broad applicability, giving physical insights of generic validity. Calculations on specific dendrimers are best served by quantum chemistry models; again, links with photobiological systems can be discerned. Two important optically nonlinear features are cooperative energy pooling, and two-photon energy transfer. Branch multiplicity and the polar or polarizable nature of the chromophores also play important roles in determining energy harvesting characteristics. I. INTRODUCTION
For a variety of reasons, and with a sense of urgency heightened by rising concerns about the security of crude oil availability and the safety of nuclear installations, there has been a recent shift in emphasis towards a reevaluation of solar energy as a potentially major contributor to meet the global energy demand. It is congruous to recall that the rate of supply of energy from the Sun to the Earth is almost 104 times greater than current global power requirements1; yet traditional photovoltaic solar cells remain disappointingly expensive and inefficient. Nature, however, presents us with numerous examples of high-efficiency molecular systems to harness the energy of sunlight, built on complex arrays of light-harvesting chromophores. The successful determination of many of their structures has spurred on efforts to produce synthetic materials that can emulate the photon capture principles, achieving similarly high levels of efficiency. In consequence, there has been a new focus on developing relatively inexpensive and synthetically reproducible molecular materials designed to emulate the organizational and mechanistic principles that operate in photosynthetic systems.2–7 One key to many of the successes in this area has been the design, synthesis, and characterization of dendrimers, a)
Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2011.436 J. Mater. Res., Vol. 27, No. 4, Feb 28, 2012
which are also known as functional cascade molecules.6–17 These materials have a large number of chemically similar chromophores bonded together in a quasi fractal geometry. Often built around a core that acts as a trap for the excitation, they approach an ideal type of structure for solar energy harvesting, expediting highly efficient, ultrafast energy transport.11,17
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