Influence of applied pressure on the probability of electronic energy transfer across a molecular dyad

A pair of covalently linked molecular dyads is described in which two disparate boron dipyrromethene dyes are separated by a tolane-like spacer. Efficient electronic energy transfer (EET) occurs across the dyad; the mechanism involves important contributions from both Förster-type coulombic interactions and Dexter-type electron exchange processes. The energy acceptor is equipped with long paraffinic chains that favor aggregation at high concentration or at low temperature. The aggregate displays red-shifted absorption and emission spectral profiles, relative to the monomer, such that EET is less efficient because of a weaker overlap integral. The donor unit is insensitive to applied pressure but this is not so for the acceptor, which has extended π-conjugation associated with appended styryl groups. Here, pressure reduces the effective π-conjugation length, leading to a new absorption band at higher energy. With increasing pressure, the overall EET probability falls but this effect is nonlinear and at modest pressure there is only a small recovery of donor fluorescence. This situation likely arises from compensatory phenomena such as restricted rotation and decreased dipole screening by the solvent. However, the probability of EET falls dramatically over the regime where the π-conjugation length is reduced owing to the presumed conformational exchange. It appears that the pressure-induced conformer is a poor energy acceptor.