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

Abstract

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.

Ultrafast Electronic Energy Transfer Beyond the Weak Coupling Limit in a Proximal but Orthogonal Molecular Dyad

“…In contrast, lateral translation of the entire T4 donor with respect to the acceptor, leading to loss of the T-shaped geometry, would facilitate throughspace EET. 24 This distortive motion, together with modest curvature of the T4 unit, reduces the angle between the transition dipole moment vectors to 72° in the most extreme geometry. shows the response which would be recorded for an instantaneous rise, indicating that the observed PL formation occurs after the pulse.…”

“…Application of Förster theory requires knowledge of the mutual alignment of these vectors and also their directions with respect to the molecular axis. When the transition dipole moment vectors are aligned perpendicular to each other, 19 and especially if one of these is orthogonal to the molecular axis, 10,20 significant extra challenges are presented to fully explain resonant energy transfer between closely spaced chromophores.…”

Hole-transfer induced energy transfer in perylene diimide dyads with a donor–spacer–acceptor motif

“…Futhermore this illustrates the crucial role of the spacer on the energy and electron transfer as also investigated for other donor-spaceracceptor systems. 12,15,[43][44][45][46][47][48][49][50][51][52][53][54][55] The ab initio calculations show that the hole-transfer induced FRET mechanism is also present in the molecules D-(Ph-Yn2-Ph)-A, D-(Ph-Yn-Ph)-A and D-(Ph-Ph)-A, even though the spacer is an aromatic π system without an aliphatic Dexter blocker. Geometry optimizations show that in the ground state equilibrium structures the π-planes of the donor, spacer and acceptor have significant torsional angles to each other, which breaks any donor-spacer-acceptor π-conjugation.…”

Providing power for miniaturized medical implants: triplet sensitization of semiconductor surfaces

“…The direction of EET along the molecular axis can be switched by rapid protonation of the terminal acceptor using a photo-acid [19]. The mechanism of the EET process has been resolved in many cases, while the relative contributions of coulombic EET and electron exchange have been delineated by high-pressure techniques [20]. The molecular arrays are readily dispersed in plastic films without harm to the overall EET efficacy.…”