Self-Assembly of Porphyrin Nanostructures at the Interface between Two Immiscible Liquids

Abstract

One of the many evolved functions of photosynthetic organisms is to synthesize light harvesting nanostructures from photoactive molecules such as porphyrins. Engineering synthetic analogues with optimized molecular order necessary for the efficient capture and harvest of light energy remains challenging. Here, we address this challenge by reporting the self-assembly of zinc(II) meso-tetrakis(4-carboxyphenyl)porphyrins into films of highly ordered nanostructures. The self-assembly process takes place selectively at the interface between two immiscible liquids (water|organic solvent), with kinetically stable interfacial nanostructures formed only at pH values close to the pKa of the carboxyphenyl groups. Molecular dynamics simulations suggest that the assembly process is driven by an interplay between the hydrophobicity gradient at the interface and hydrogen bonding in the formed nanostructure. Ex situ XRD analysis and in situ UV/vis and steady state fluorescence indicates the formation of chlathrate type nanostructures that retain the emission properties of their monomeric constituents. The self-assembly method presented here avoids the use of acidic conditions, additives such as surfactants and external stimuli, offering an alternative for the realization of light-harvesting antennas in artificial photosynthesis technologies. 1.

Reversible Electrochemical Ion Intercalation at an Electrified Liquid|liquid Interface Functionalised with Porphyrin Nanostructures

“…This shift confirms a negative excess charge at the interface due to presence of deprotonated carboxyl groups at this pH. 36 The peak at -0.15 V was seen at all frequencies across a range from 5 to 80 Hz ( Figure S4b On the contrary, the total concentration…”

“…For clarity, a zoomed in version of the CV obtained with [R2N + ] = 1 mM (dashed blue line) in (d) is shown inFigure S6.To probe the effects of the bulk aqueous pH and R2N + concentration, both were varied independently(Figures 2c and d). As we described previously,36 the ZnPor-INs were stabilised by cooperative hydrogen bonding. Thus, pH conditions more alkali than the carboxyl groups pKa of 5.8 were ruled out as the ZnPor-IN film would destabilise and dissolve.…”

Pathway Complexity in Supramolecular Porphyrin Self-Assembly at an Immiscible Liquid|Liquid Interface

“…Spectroscopically monitoring the pH-and concentration-dependency of ZnTPPc interfacial self-assembly. As discussed in our previous work, 35 ZnTPPc self-assembles at the interface between water and an immiscible organic solvent, such as -trifluorotluene (TFT), to form highly ordered nanostructures. The self-assembly process is selective to the interface, only taking place when the aqueous pH is within +/-0.2 units of the pKa of the porphyrin's carboxyl groups (pH 5.8).…”

“…Of the parameters evaluated, the self-assembly process was most sensitive to the aqueous pH, in agreement with our previous findings. 35 A range of pH values were investigated between pH 5.0 and 6.8 with 8 M ZnTPPc added to the bulk aqueous phase ( Figure S3). For control experiments in the absence of ZnTPPc, no UV/vis signal was detected in the region of interest.…”

“…33 Recently, our group demonstrated that these interfacial ZnTPPc nanostructures are stabilized by cooperative hydrogen bonding and likely represent metastable or kinetically trapped non-equilibrium nanostructures. 34,35 Despite such insights, our understanding of the assembly mechanism of nanostructures at immiscible liquid|liquid interfaces remains limited. Due to easily detectable spectral changes arising from exciton coupling of their transition dipole moments, dye molecules are ideal candidates to study the mechanisms and thermodynamics of interfacial self-assembly processes by UV/vis spectroscopy.…”