Resolving the contribution due to Forster-type intramolecular electronic energy transfer in closely coupled molecular dyads

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 Forster-type EET within the system, leaving the electron-exchange contribution. For the target dyad, the Forster component is ca. 5% for pyrene and ca. 25% for perylene. Such contributions are not inconsistent with calculations made on the basis of Forster 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 Forster rates and ranges from ca. 4 degrees for the ideal dipole approximation to only 0.5 degrees 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.