A switchable self-assembling and disassembling chiral system based on a porphyrin-substituted phenylalanine–phenylalanine motif

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.

e:The solubility of Fmoc-FLFL-TPP in dry dichloromethane is superior to 1.2 mg in 10 mL after short sonication at room temperature. It is insoluble in dry n-heptane. This difference between a "good" and a "bad" solvent is useful for the injection method to induce selfassembly.

Supplementary Methods
General procedure for coupling phenylalanine with the porphyrin compounds: The Bocor Fmoc-protected phenylalanine (1 equiv) was dissolved in dry dichloromethane and the solution was cooled to 0 C in an ice bath. N,N'-dicyclohexylcarbodiimide (DCC, 1.2 equiv) and 1-hydroxybenzotriazole hydrate (HOBt, 1.2 equiv) were added and the stirring was continued for 30 min at 0 C. Then the porphyrin compound TPP-NH2 (1 equiv) was added and the resulting mixture was stirred at 8 C for 48 h. After addition of CH2Cl2 the solution was washed once with water and the organic layer was dried over Na2SO4, filtered, concentrated, and the residue was purified by chromatography on a silica gel column.

Theoretical calculations on the Boc-FLFL-TPP monomer in vacuum and CH2Cl2
In order to obtain a better insight into the electronic properties of the Boc-FLFL-TPP monomer we performed calculations using 6-31G(d) basis sets. The geometry optimized structure possesses a nearly planar porphyrin ring ( Figure S24). TD-DFT analysis (using dichloromethane as solvent phase) of the Boc-FLFL-TPP monomer porphyrin reveals the presence of an intense Soret band in the region of 410 nm to 417 nm along with two Q bands at 553nm and 587 nm corresponds to porphyrin based p-p* transitions ( Figure S25). The absorptions at 391 nm, 385 nm are due to the transitions from amino acid fragments to the porphyrin moiety as observed from the molecular orbital picture ( Figure S26). The HOMO and LUMO fragments are located mainly on the porphyrin moiety ( Figure S26).

Powder diffraction data and comparison with the single crystal X-ray data
Based on the positions of the first 10 reflections the lattice constants were evaluated. Compared to the CIF-file from the single crystal the powder cell refinement gave slightly different cell lengths and a slightly lower beta-angle (see below): Sqrt(Sum(Th O-C)**2)/(Nref-Npar)) = .000929 Sqrt(Sum(2Th O-C)**2)/(Nref-Npar)) = .000465 The simulated powder data was calculated using Crystallographica 1 and the CIF. (but with the cell parameters based on the above). A comparison of the powder pattern generated by Crystallographica and that generated by Mercury shows high consistency.
For the crystallization trials in the 10 cm tubes, three limpid layers could be discerned: a lower dichloromethane very dark purple coloured layer up to ~8-10 mm, an intermediate layer of a 1:1 (v:v) dichloromethane and cyclohexane up to ~16-20 mm and a third colourless cyclohexane layer filling the tube. If turbidity appeared rapidly within the second or third layer, this usually led to amorphous material which did not diffract X-rays. On the other hand if the three layers remained limpid, slow diffusion over a period of several months usually yielded in microcrystals.