A Labile Metallo-Porphyrin as Tool to Control J-Aggregates

The zinc(II) metal derivative of 5,10,15,20- tetrakis (4-sulfonatophenyl)porphyrin (TPPS ) is quite labile and readily demetallates under acidic conditions, aﬀording the parent diacid porphyrin in a monomeric form. The rate of this process is ﬁrst order on [ZnTPPS ] and second order on [H ], allowing a precise control of the monomer release in solution. Under high ionic strength, this latter species is able to self-assemble into J-aggregates,

The zinc(II) metal derivative of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS ) is quite labile and readily demetallates under acidic conditions, affording the parent diacid porphyrin in a monomeric form. The rate of this process is first order on [ZnTPPS ] and second order on [H ], allowing a precise control of the monomer release in solution. Under high ionic strength, this latter species is able to self-assemble into J-aggregates, whose kinetics of growth are largely modulated by pH. The aggregation kinetics have been treated according to a well-established model, in which the formation of an initial nucleus is the rate determining step preceding the autocatalytic growth of the whole assembly. The extinction spectra of the aggregates suggest the occurrence of a dipolar coupling mechanism very similar to that operating in metal nanoparticles. Spontaneous symmetry breaking takes place in these aggregates as evidenced by unusual circular dichroism spectra. The intensity and sign of the effect is controlled by the aggregation rate and therefore can be tuned through a proper choice of initial conditions. 1 reproducibility of the experiments, since the distribution of species in solution at the beginning of aggregation could be unpredictable (monomers, dimers, oligomers etc.). In complex systems, all these factors are able to change the aggregation pathway and eventually influence the resulting structure of the final supramolecular assembly . For these reasons, it is very important to develop approaches to minimize the potential sources of uncertainty. In a previous study, we showed that controlled amounts of the zinc(II) metal derivative of TPPS when added to the metal-free porphyrin decrease consistently the formation of J-aggregates and influence their chiroptical properties . This particular metal complex is quite labile under acidic conditions and the metal ion can be easily removed, affording an easy and convenient way to obtain in situ the monomeric diacid H TPPS , that is the building block for the subsequent aggregate growth (see Scheme 1).

Kinetic analysis
Our kinetic experiments have shown that when ZnTPPS is used as starting material, the rates are independent on the mixing order, and sigmoidal profiles for extinction vs time have been observed in all cases. A precise control on the release of metal free porphyrin could be achieved at pH higher than 1, a condition that requires rather high ionic strength to trigger aggregation.

Chiroptical properties
Samples of the prepared J-aggregates exhibit rather peculiar spectroscopic features: i) a broad J-band, suggesting a dipolar coupling mechanism, instead of the usual Frenkel exciton model , ii) resonance enhanced light scattering (RLS) corresponding to the absorption bands due to their size and the strong electronic coupling among the monomers . The increase of intensity of RLS on increasing [H ] and ionic strength suggests that aggregates become larger at lower pH and higher ionic strength.
As already reported in literature, spontaneous symmetry breaking can occur in these systems, leading to the observation of circular dichroism (CD) spectra . On increasing [H ] or the ionic strength, i.e.
increasing the aggregation rates (see Figure 1), the absolute intensity of the dissymmetry g-factor decreases and the bands eventually switch from negative to positive Cotton effect.

Figure 1.
Plot of dissymmetry g-factor as function of the corresponding rate constants k and two representative CD spectra for positive and negative Cotton effect (inset).
All our experimental evidence points to a specific role played by the zinc(II) ions, at molecular level and on the mesoscopic structure of the aggregates, in analogy to other polycationic species At difference with J-aggregates prepared by simply adding inorganic acids, the sign of the Cotton effect in these species can be changed from negative to positive by varying pH and ionic strength . All together, these findings shed further light onto these fascinating supramolecular assemblies and pave the way to new approaches in order to modulate their spectroscopic and chiroptical properties for potential applications.