Abstract:
Artificial light-harvesting systems have until now not been able to self-assemble into
structures with a large photon capture cross-section that upon a stimulus reversibly can
switch into an inactive state. Here we describe a simple and robust FLFL-dipeptide construct
to which a meso-tetraphenylporphyrin has been appended and which self-assembles to fibrils,
platelets or nanospheres depending on the solvent composition. The fibrils, functioning as
quenched antennas, give intense excitonic couplets in the electronic circular dichroism
spectra which are mirror imaged if the unnatural FDFD-analogue is used. By slightly increasing
the solvent polarity, these light-harvesting fibres disassemble to spherical structures with
silent electronic circular dichroism spectra but which fluoresce. Upon further dilution with the
nonpolar solvent, the intense Cotton effects are recovered, thus proving a reversible
switching. A single crystal X-ray structure shows a head-to-head arrangement of porphyrins
that explains both their excitonic coupling and quenched fluorescence.
