Electronic energy transfer

Around 60 years ago, Förster described a theoretical model explaining how excitation energy could be transferred over large distances between molecules lying well outside orbital contact. According to Förster (Fig. 1), the critical feature underpinning such transfers involves Coulombic coupling between electrons of the excited-state donor and the ground-state acceptor; the actual coupling matrix element corresponding to coupling between the respective transition dipole moment vectors of the two reactants. This model holds for strongly-allowed optical transitions and is most appropriate for highly-fluorescent singlet-excited states. The theory was soon applied to electronic energy transfer (EET) events in solutions, solids, monomolecular layers, microheterogeneous media and (most significantly) biological environments. With advances in synthetic chemistry came intramolecular EET over shorter, well-defined distances, making use of tailor-made molecular dyads and higher-order arrays. Next came conducting polymers, perhaps the ultimate challenge for understanding the dynamics of EET, and quantum dots. Improved experimental and theoretical techniques have permitted ever deeper examination of EET events on faster timescales and with precise spatial resolution.

Fig. 1 Theodor Förster

Certain results emerging from the intramolecular EET studies for singlet-excited states gave rate constants faster than could be explained by Förster theory. Additional concerns were raised with regard to the fact that the rates were heavily dependent on the topology and composition of the bridge separating the active terminals. From this work came the realisation that Dexter electron exchange could dominate the EET process at short separations and/or in systems employing relatively low-energy spacer units. Indeed, for triplet-excited states the main mechanism involves electron exchange. The problem nowadays is to identify intramolecular EET cases where the Förster mechanism holds exclusively so that details of the theory can be isolated and subjected to critical examination. This is not a simple operation!

Applications of EET are both common and ingenious. There is a long-standing history associated with using Förster theory as a molecular ruler by which to measure distances in biological media. Such studies are amenable to follow protein folding and unfolding in real time. Many fluorescent sensors have been developed for in situ monitoring of microscopic environmental changes, including slight variations in temperature, pressure, acidity, polarity or phase. Systems have been developed as molecular switches where EET induces an important conformational change or accompanies activation of a photochromic reaction. Such work has led to exploration of artificial neural networks and preferred EET to particular sites distributed within a network of seemingly similar sites. There are reports of EET along channels, leading to the discovery of optical light guides operating on the molecular scale, and across phase boundaries. Spatial resolution can be achieved with confocal microscopes while up-conversion techniques have facilitated measuring EET events on sub-picosecond timescales as a matter of routine. On a molecular front, EET forms the critical step in the development of solar concentrators, or artificial photon collectors, and advanced bio-labels. Related systems operate as sensitisers for solar cells and as the basis for highly sophisticated anti-forgery devices. The importance of EET in organic light-emitting diodes is now recognised.

So, where are we now in terms of EET in molecular and biological systems? The Förster theory, a standard photophysical tool for many decades, is now under detailed scrutiny. Questions are being raised with respect to the actual form of the distance dependence at short separations, the nature of the screening factor (usually taken as the inverse square of the solvent refractive index), and the scale of the dipole length for molecules with extended conjugation such as poly(aryls). The use of point dipoles is no longer acceptable for molecular dyads. Following from recent electrochemical studies, can we build an EET microscope able to focus light to the single-molecule level? Can we develop robust theoretical models that successfully incorporate spin? There is the need to devise systems that make use of unusual intermediates, such as exciplexes, or involve unconventional substrates, like plasmons. There are but few reports of EET involving radicals or radical ions, although the utilisation of upper-lying excited states as donor or acceptor is far from uncommon. In natural systems, EET is a means to an end and we might think about how to construct molecular systems able to accumulate multiple electronic charges at catalytic sites—this is a massive challenge that must be solved if we are to develop so-called solar fuels. In any practical system there needs to be regulation of the photon flux but there are no artificial systems capable of self regulation (or self protection against high photon densities). An even bigger challenge is to devise artificial systems that undergo coherent EET over extended distances without dissipation of the excitation energy.

This themed issue brings together many experts in the field of EET and covers a broad spectrum of the subject. Energy transfer in natural photosynthetic organisms is covered by van Grondelle (DOI: 10.1039/c003025b) and Fleming (DOI: 10.1039/c003389h) while Albinsson (DOI: 10.1039/c003805a) reports a comprehensive examination of EET in artificial donor-bridge-acceptor systems. Aspects of EET theory are described by Andrews (DOI: 10.1039/c002313m) and Voityuk (DOI: 10.1039/c003131c). Molecular dyads displaying various features of intramolecular EET form part of the contributions from Bittner (DOI: 10.1039/c003113e), D’Souza (DOI: 10.1039/c002757j), Curutchet (DOI: 10.1039/c003496g) and Campagna (DOI: 10.1039/c003789c). Studies on EET in complexes formed via supramolecular chemistry are reported by Ng (DOI: 10.1039/c004373g) while Xiao (DOI: 10.1039/c001504k) describes new applications of EET in chemical sensors. Bagchi (DOI: 10.1039/c003217d) describes EET in conjugated polymers and Curry (DOI: 10.1039/c003179h) highlights EET in hybrid organic–inorganic nanocrystals.

Anthony Harriman, Newcastle University, UK.

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