The synthesis of a disulfide-strapped viologen derivative is described starting from 4,4'-bipyridinyl-3,3'-diol. The first two one-electron reduction potentials, as determined by cyclic voltammetry, occur at E(1/2) = -0.03 V and E(1/2) = -0.16 V vs Ag/AgCl. This is accompanied by two more well separated one-electron reductions at E(1/2) = -1.26 V and E(1/2) = -1.54 V vs Ag/AgCl and the breaking of the disulfide bridge. To alleviate electrostatic repulsion between the two thiolate ions the molecular system must twist or "spring open" to accommodate the final two electrons.

A D–A3 structure is used to enhance the triplet harvesting rate of a purely organic molecule. However, excited state symmetry breaking dynamics plays an detrimental role causing localisation of the electronic structure and reducing this rate.

This work examines the electronic energy-transfer (EET) processes inherent to a molecular dyad in which aryl polycycles attached to a boron dipyrromethene (Bodipy) dye act as ancillary light harvesters for near-UV photons. The solvent, being methyltetrahydrofuran, is compressed under applied pressure to such an extent that, over the accessible pressure range, there is a 25% decrease in molar volume. This effect serves to increase the effective concentration of the solute and increases fluorescence from Bodipy when this chromophore is excited directly. Illumination into the aryl polycycles, namely pyrene and perylene derivatives, leads to rapid intramolecular EET to Bodipy but fluorescence from these units is partially restored under high pressure. The argument is made that applied pressure restricts torsional motions around the linkages and imposes a near orthogonal geometry for transition dipole moment vectors on the reactants. In turn, this pressure-induced conformational restriction switches off F€ orstertype EET within the system, leaving the electron-exchange contribution. For the target dyad, the F€ orster component is ca. 5% for pyrene and ca. 25% for perylene. Such contributions are not inconsistent with calculations made on the basis of F€ orster theory but modelling is rendered difficult by the absence of accurate information about the nature of the conformational motion. Two possibilities have been considered. In the first case, the appendages remain stiff but pressure reduces the extent of displacement from the lowest-energy position. The results can be accounted for in a quantitative sense on the basis of small deviations from the lowest-energy conformation; the actual amount of displacement needed to explain the pressure effect depends on the method used to compute the F€ orster rates and ranges from ca. 4 for the ideal dipole approximation to only 0.5 for the extended dipole method. Secondly, pressure is assumed to bend each appendage into a banana-like shape. Again, the full effect of applied pressure can be accounted for by way of minor curvature of the linkage.

The rate constant (k EET ) for electronic energy transfer between well-defined chromophores can be employed as a means to determine structural information about the system under investigation.[1] The most notable examples arise from biochemistry where protein-bound reactants are separated by 30 or more and where there are only very weak electronic interactions.[2] Under such conditions, the Fçrster coulombic mechanism [3] is likely to hold and multipole interactions can be ignored. In certain cases, the separation distance and/or mutual orientation of the reactant pair can be deduced from spectroscopic observations. It is often considered [4] that this ideal dipole approximation will breakdown at shorter separations but hard experimental evidence for such behavior is scarce. In fact, it has been shown [5] that Fçrster theory gives an acceptable account of experimental k EET values at 20 separations, at least in certain situations, while even smaller separations become possible [6] when the reactants possess unusually short transition dipole moment vectors. It has also been shown [7] that supposedly rigid organic frameworks are subject to considerable out-of-plane bending in fluid solution at ambient temperature. Such structural fluctuations might contribute significantly to the observed k EET values if the lowest-energy conformation imposes orthogonality on the respective transition dipole moment vectors.[8] Alternative protocols for expressing electronic energy transfer (EET) between closely spaced but weakly coupled reactants are available but have not been well-tested with molecular dyads. Such treatments include the extended dipole approach, introduced by Kuhn and co-workers, [9] where the point dipoles inherent to Fçrster theory are replaced with a linear dipole of fixed length. More rigorous treatments include the atomistic approach, [10] where the transition dipole moments are broken down into contributions for each atom, and the transition density cube [11] that does a similar job over the entire wave function.Herein, we compare experimental and computed k EET values for three carefully selected molecular dyads that differ in terms of the geometry of the central connector. In each case, the donor (D) is a diketopyrrolopyrrole (DPP) dye [12] while the corresponding acceptor (A) is an extended boron dipyrromethene (Bodipy) dye fitted with ethenylthiophene units as the conjugation extenders (Scheme 1); [13] see the Supporting Information for synthesis of D [14] and A. The critical part of these molecular dyads relates to the bridges that serve to impose structural integrity while minimizing electronic connectivity between the terminals. Two of these bridges are formed by carborane clusters [15] that provide either para or ortho linkages while the shortest dyad has a phenylethynylenethienyl connector (Scheme 2).The target dyads were synthesized step-by-step from the pre-organized para-and ortho-carborane and modules D and A. For the synthesis of DPP-Cp-BOD, two orthogonal functions (a terminal alkyne and a tr...

A small series of partially alkylated Bodipy dyes were prepared containing F n -aryl (n = 1,2,3,5) groups at the meso position. The effect of increasing the number of fluorines, and their position in the aryl ring, on the electrochemical and photophysical properties of the dyes is discussed. The highly electron withdrawing pentafluoroaryl group makes reduction of the Bodipy segment especially easy when compared to the basic phenylene derivative. High quantum yields of fluorescence in toluene solution are seen for derivatives with fluorine(s) substituted in the ortho position of the meso aryl group. This effect is also mirrored in the fluorescence lifetimes for the molecules. Pressure dependent fluorescence measurements carried out reveal subtle differences in the behaviour for the series of Bodipy derivatives.