The target artificial light-harvesting antenna, comprising 21 discrete chromophores arranged in a logical order, undergoes photochemical bleaching when dispersed in a thin plastic film. The lowest-energy component, which has an absorption maximum at 660 nm, bleaches through first-order kinetics at a relatively fast rate. The other components bleach more slowly, in part, because their excited-state lifetimes are rendered relatively short by virtue of fast intramolecular electronic energy transfer to the terminal acceptor. Two of the dyes, these being close to the terminal acceptor but interconnected through a reversible energy-transfer step, bleach by way of an autocatalytic step. Loss of the terminal acceptor, thereby switching off the energy-transfer route, escalates the rate of bleaching of these ancillary dyes. The opposite terminal, formed by a series of eight pyrene-based chromophores, does not bleach to any significant degree. Confirmation of the various bleaching steps is obtained by examination of an antenna lacking the terminal acceptor, where the autocatalytic route does not exist and bleaching is very slow.

The front cover artwork is provided by the groups of Tony Harriman (Newcastle University) and Raymond Ziessel (ECPM-Strasbourg). The image shows the processes following illumination of an artificial light harvester that slowly fades under continuous exposure to white light. Read the full text of the article at 10.1002/cphc.201500150.

This essay is based on research leading to the identification of catalysts capable of the selective oxidation of water to molecular oxygen. The real need for such materials relates to the large-scale photochemical dissociation of water into its constituent elements, as opposed, for example, to the electrochemical decomposition of water at macroscopic electrodes. Combining the catalyst with the essential components needed for efficacious photochemistry brings special challenges, as does the ultimate need to scale up the system by a massive amount. Nature has developed a highly successful process for O₂ evolution under ambient illumination that makes use of a cubane tetramanganese cluster having a closely associated calcium cation in attendance. This catalyst is surprisingly delicate and it is debatable as to whether we could adapt such a system for use with artificial photosystems. Historically the latter have used colloidal metal oxides, and consideration is given here as to which materials might offer the most promising catalytic performance. Moving towards heterogeneous systems has more practical meaning, but the same materials come to mind. Recent attention to cobalt-based catalysts is highlighted as a possible breakthrough that might lead to interesting electrochemical systems when combined with wind turbines. Molecular catalysts provide interesting opportunities for photochemical O₂ evolution but suffer from problems of scale-up. This field has witnessed the most important progress over the past decade or so but still needs urgent attention if advanced materials are to be identified in a timely manner. Finally, consideration is given to the actual status of the field in specific terms of developing an effective artificial photosynthetic apparatus. Moving progressively from using sacrificial redox agents as a simple means to isolate the oxidative photochemical cycle towards full water cleavage will stimulate the development of demonstration models suitable for public display. The reward should be increased investment.

The photophysical properties of a prototypic donor–acceptor dyad, featuring a conventional boron dipyrromethene (Bodipy) dye linked to a dicyanovinyl unit through a meso-phenylene ring, have been recorded in weakly polar solvents. The absorption spectrum remains unperturbed relative to that of the parent Bodipy dye but the fluorescence is extensively quenched. At room temperature, the emission spectrum comprises roughly equal contributions from the regular π, π* excited-singlet state and from an exciplex formed by partial charge transfer from Bodipy to the dicyanovinyl residue. This mixture moves progressively in favor of the locally excited π, π* state on cooling and the exciplex is no longer seen in frozen media; the overall emission quantum yield changes dramatically near the freezing point of the solvent. The exciplex, which has a lifetime of approximately 1 ns at room temperature, can also be seen by transient absorption spectroscopy, in which it decays to form the locally excited triplet state. Under applied pressure (P<170 MPa), formation of the exciplex is somewhat hindered by restricted rotation around the semirigid linkage and again the emission profile shifts in favor of the π, π* excited state. At higher pressure (170<P<550 MPa), the molecule undergoes reversible distortion that has a small effect on the yield of π, π* emission but severely quenches exciplex fluorescence. In the limiting case, this high-pressure effect decreases the molar volume of the solute by approximately 25 cm³ and opens a new channel for nonradiative deactivation of the excited-state manifold.

A small series of donor–acceptor molecular dyads has been synthesized and fully characterized. In each case, the acceptor is a dicyanovinyl unit and the donor is a boron dipyrromethene (BODIPY) dye equipped with a single styryl arm bearing a terminal amino group. In the absence of the acceptor, the BODIPY-based dyes are strongly fluorescent in the far-red region and the relaxed excited-singlet states possess significant charge-transfer character. As such, the emission maxima depend on both the solvent polarity and temperature. With the corresponding push–pull molecules, there is a low-energy charge-transfer state that can be observed by both absorption and emission spectroscopy. Here, charge-recombination fluorescence is weak and decays over a few hundred picoseconds or so to recover the ground state. Overall, these results permit evaluation of the factors affecting the probability of charge-recombination fluorescence in push–pull dyes. The photophysical studies are supported by cyclic voltammetry and DFT calculations.

In order to examine how changes in the size of the rotary group affect the efficacy of molecular probes for monitoring changes in local viscosity, a boron dipyrromethene dyebearing a meso-phenanthrene unit has been synthesized and fully characterized. ¹⁹F NMR spectroscopy, together with molecular modelling, indicates that the bulky phenanthryl unit cannot rotate completely around the connecting C–C linkage but can oscillate over a reasonably large dihedral angle. This situation is to be contrasted with the corresponding dye having a meso-phenylene ring. The latter dye functions as a molecular probe for changes in viscosity of the surrounding solvent but remains essentially insensitive to changes in the polarity of the solvent. The opposite situation is found for the phenanthryl derivative, where a charge-transfer state lies at higher energy than the emissive π,π* excited state but can be accessed thermally. The results are considered in terms of energy-level diagrams taking into account rotational freedom. Photophysical properties are reported for both dyes in a range of solvents, and temperature-dependent studies are described.

Synthetic strategies have been devised that allow the rational design and isolation of highly coloured boron dipyrromethene (BODIPY) dyes that absorb across much of the visible region. Each dye has an aryl polycycle (usually pyrene or perylene) connected to the central BODIPY core through a conjugated tether at the 3,5-positions. Both mono- and difunctionalised derivatives are accessible, in certain cases containing both pyrene and perylene residues. For all new compounds, the photophysical properties have been recorded in solution at ambient temperature and in a glassy matrix at 77 K. The presence of the aryl polycycle(s) affects the absorption and emission maxima of the BODIPY nucleus, thereby confirming that these units are coupled electronically. Indeed, the band maxima and oscillator strengths depend on the conjugation length of the entire molecule, whereas there is no sign of fluorescence from the polycycle. As a consequence, the radiative rate constant tends to increase with each added appendage. The nature of the linkage (styryl, ethenyl, or ethynyl) also exerts an effect on the photophysical properties and, in particular, the absorption spectrum is perturbed in the region of the aryl polycycle. The perylene-containing BODIPY derivatives absorb over a wide spectral range and emit in the far-red region in almost quantitative yield. A notable exception to this generic behaviour is provided by the anthracenyl derivative, which exhibits charge-transfer absorption and emission spectra in weakly polar media at ambient temperature. Regular BODIPY-like behaviour is restored in a glassy matrix at 77 K. Overall, these new dyes represent an important addition to the range of strongly absorbing and emitting reagents that could be used as solar concentrators.

A conformationally restricted molecular dyad has been synthesized and subjected to detailed photophysical examination. The dyad comprises a borondipyrromethene (Bodipy) dye covalently linked to a buckminsterfullerene C₆₀ residue, and is equipped with hexadecyne units at the boron centre in order to assist solubility. The linkage consists of a diphenyltolane, attached at the meso position of the Bodipy core and through an N-methylpyrrolidine ring at the C₆₀ surface. Triplet states localised on the two terminals are essentially isoenergetic. Cyclic voltammetry indicates that light-induced electron transfer from Bodipy to C₆₀ is thermodynamically favourable and could compete with intramolecular energy transfer in the same direction. The driving force for light-induced electron abstraction from Bodipy by the singlet excited state of C₆₀ depends critically on the solvent polarity. Thus, in non-polar solvents, light-induced electron transfer is thermodynamically uphill, but fast excitation energy transfer occurs from Bodipy to C₆₀ and is followed by intersystem crossing and subsequent equilibration of the two triplet excited states. Moving to a polar solvent switches on light-induced electron transfer. Now, in benzonitrile, the charge-transfer state (CTS) is positioned slightly below the triplet levels, such that charge recombination restores the ground state. However, in CH₂Cl₂ or methyltetrahydrofuran, the CTS is slightly higher in energy than the triplet levels, and decays, in part, to form the triplet state localized on the C₆₀ residue. This step is highly specific and does not result in direct formation of the triplet excited state localized on the Bodipy unit. Subsequent equilibration of the two triplets takes place on a relatively slow timescale.

A novel boron dipyrromethene (BODIPY) dye has been synthesized in which the F atoms, usually bound to the boron center, have been replaced with 1-ethynylperylene units and a 4-pyridine residue is attached at the meso-position. The perylene units function as photon collectors over the wavelength range from 350 to 480 nm. Despite an unfavorable spectral overlap integral, rapid energy transfer takes place from the singlet-excited state of the perylene unit to the adjacent BODIPY residue, which is itself strongly fluorescent. The mean energy-transfer time is 7 ± 2 ps at room temperature. The dominant mechanism for the energy-transfer process is Dexter-type electron exchange, with Förster-type dipole–dipole interactions accounting for less than 10 % of the total transfer probability. There are no indications for light-induced electron transfer in this system, although there is evidence for a nonradiative decay channel not normally seen for F-type BODIPY dyes. This new escape route is further exposed by the application of high pressure. The meso-pyridine group is a passive bystander until protons are added to the system. Then, protonation of the pyridine N atom leads to complete extinction of fluorescence from the BODIPY dye and slight recovery of fluorescence from the perylene units. Quenching of BODIPY-based fluorescence is due to charge-transfer to the pyridinium unit whereas the re-appearance of perylene-based emission is caused by a reduction in the Förster overlap integral upon protonation. Other cations, most notably zinc(II) ions, bind to the pyridine N-atom and induce similar effects but the resultant conjugate is weakly fluorescent.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

The rate constant for triplet energy transfer (k_TET) has been measured in fluid solution for a series of mixed-metal Ru–Os bis(2,2′:6′,2′′-terpyridine) complexes built around a tethered biphenyl-based spacer group. The length of the tether controls the central torsion angle for the spacer, which can be varied systematically from 37 to 130°. At low temperature, but still in fluid solution, the spacer adopts the lowest-energy conformation and k_TET shows a clear correlation with the torsion angle. A similar relationship holds for the inverse quantum yield for emission from the Ru–terpy donor. Triplet energy transfer is more strongly activated at higher temperature and the kinetic data require analysis in terms of two separate processes. The more weakly activated step involves electron exchange from the first-excited triplet state on the Ru–terpy donor and the size of the activation barrier matches well with that calculated from spectroscopic properties. The pre-exponential factor derived for this process correlates remarkably well with the torsion angle and there is a large disparity in electronic coupling through π and σ orbitals on the spacer. The more strongly activated step is attributed to electron exchange from an upper-lying triplet state localized on the Ru–terpy donor. Here, the pre-exponential factor is larger but shows the same dependence on the geometry of the spacer. Strangely, the difference in coupling through π and σ orbitals is much less pronounced. Despite internal flexibility around the spacer, k_TET shows a marked dependence on the torsion angle computed for the lowest-energy conformation.

A multicomponent cluster has been synthesised in which four disparate chromophores have been covalently linked through a logical arrangement that favours efficient photon collection and migration to a terminal emitter. The primary energy acceptor is a boron dipyrromethene (Bodipy) dye and different polycyclic aryl hydrocarbons have been substituted in place of the regular fluorine atoms attached to the boron centre. The first such unit is perylene, linked to boron through a 1,4-diethynylphenyl unit, which collects photons in the 320–490 nm region. The other photon collector is pyrene, also connected to the boron centre by a 1,4-diethynylphenyl spacer and absorbing strongly in the 280–420 nm region, which itself is equipped with an ethynylfluorene residue that absorbs in the UV region. Illumination into any of the polycyclic aryl hydrocarbons results in emission from the Bodipy unit. The rates of intramolecular electronic energy transfer have been determined from time-correlated, single-photon counting studies and compared with the rates for Coulombic interactions computed from the Förster expression. It has been necessary to allow for i) a more complex screening potential, ii) multipole–multipole coupling, iii) an extended transition dipole moment vector and iv) bridge-mediated energy transfer. The bridge-mediated energy transfer includes both modulation of the donor transition dipole vector by bridge states and Dexter-type electron exchange. The latter is a consequence of the excellent electronic coupling properties of the 1,4-diethynylphenyl spacer unit. The net result is a large antenna effect that localises the photon density at the primary acceptor without detracting from its highly favourable photophysical properties.

Une supramolécule originale comportant 4 chromophores distincts liés de façon logique sur une plateforme fluorescente permet de collecter une très vaste gamme de photons et de les faire migrer de façon directionnelle vers un chromophore qui sera le seul émetteur de lumière. L'accepteur d'énergie primaire est une sous-unité Bodipy tandis qu'un ensemble des composés polyaromatiques ont été positionné de telle manière à favoriser le transfert d'énergie. L'ensemble des accepteurs secondaires est lié de façon covalente sur l'atome de bore par l'intermédiaire de triples liaisons qui assurent une très bonne connectivité chimique et électronique. Parmis ces unités, on trouve le 1,4-diéthynylperylène qui collecte les photons dans une plage comprise entre 320 et 490 nm. Un autre collecteur de photons est le 1,6-diéthynylpyrène qui est absorbe entre 280 et 420 nm. Ces deux sous-unités sont liés au bore par un fragment 1,4-diéthynylphényle qui est indispensable pour assurer une synthèse optimale. Enfin un résidu éthynylfluorène permet d'élargir la collection de photons jusqu'à l'ultraviolet lointain et d'assurer une excellente solubilité du composé ciblé. Quelque soit la longueur d'onde d'irradiation utilisée l'ensemble des photons collectés sont transférés vers la sous-unité Bodipy qui fluorescence efficacement. Les vitesses de transfert intramoléculaire d'énergie ont été déterminées par des méthodes traditionnelles de comptage de photons et les vitesses ont été comparées aux valeurs calculées en utilisant les équations de Förster appropriées à une interaction Coulombic. Toutefois il a fallu tenir de nombreux autres facteurs comme: i) un potentiel de surface élargi; ii) de multiple interactions multipole-multipole; iii) un moment de transition dipolaire étendu; et iv) des transferts d'énergie à travers les liaisons. Ce dernier incluant à la fois la modulation du vecteur dipolaire du donneur d'énergie et d'états énergétiques localisés sur l'entretoise ainsi que du double échange d'électron de type Dexter. Ce dernier est du aux excellentes propriétés de couplage électronique au travers de l'entité diéthynylphényle. Le résultat final de ces études est un effet d'antenne étendu, une collection de photon exceptionnelle qui localise toute l'énergie excitonique sur l'accepteur final (le Bodipy) sans perturber ces propriétés optiques et de fluorescence exceptionnelles.