Abstract
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.