Controlling J-Aggregates Formation and Chirality Induction through Demetallation of a Zinc(II) Water Soluble Porphyrin

Abstract

Under acidic conditions and at high ionic strength, the zinc cation is removed from its metal complex with 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS4) thus leading to the diacid free porphyrin, that subsequently self-organize into J-aggregates. The kinetics of the demetallation step and the successive supramolecular assembly formation have been investigated as a function of pH and ionic strength (controlled by adding ZnSO4). The demetallation kinetics obey to a rate law that is first order in [ZnTPPS4] and second order in [H+], according to literature, with k2 = 5.5 ± 0.4 M−2 s−1 at 298 K (IS = 0.6 M, ZnSO4). The aggregation process has been modeled according to an autocatalytic growth, where after the formation of a starting seed containing m porphyrin units, the rate evolves as a power of time. A complete analysis of the extinction time traces at various wavelengths allows extraction of the relevant kinetic parameters, showing that a trimer or tetramer should be involved in the rate-determining step of the aggregation. The extinction spectra of the J-aggregates evidence quite broad bands, suggesting an electronic coupling mechanism different to the usual Frenkel exciton coupling. Resonance light scattering intensity in the aggregated samples increases with increasing both [H+] and [ZnSO4]. Symmetry breaking occurs in these samples and the J-aggregates show circular dichroism spectra with unusual bands. The asymmetry g-factor decreases in its absolute value with increasing the catalytic rate kc, nulling and eventually switching the Cotton effect from negative to positive. Some inferences on the role exerted by zinc cations on the kinetics and structural features of these nanostructures have been discussed.

Role of Cobalt(III) Cationic Complexes in the Self-Assembling Process of a Water Soluble Porphyrin

“…These values are very similar to those reported for the self-assembling of H 2 TPPS 4 under a variety of experimental conditions [65,77]. The values of the power exponent n range between 2 ÷ 4 (Figure 3c), again very similarly to other systems reported in literature [65,77]. As in other systems, the values for the rate constants of the uncatalyzed pathway, k 0 , contribute only to a small extent to the overall kinetic process and will not be further discussed.…”

“…Concerning the size of the critical nucleus, the values of m are in the range 2 ÷ 3, so a dimer or trimer should be considered as the initial seeding aggregate (Figure 3b). These values are very similar to those reported for the self-assembling of H 2 TPPS 4 under a variety of experimental conditions [65,77]. The values of the power exponent n range between 2 ÷ 4 (Figure 3c), again very similarly to other systems reported in literature [65,77].…”

“…Different studies pointed out the important role played by hydrogen bonding [62][63][64], counter-anions [63], and specific cationic species [16,34,42,[65][66][67][68][69][70][71][72][73] in governing the kinetics of growth and, consequently, the general optical and chiroptical features of these systems. Some cations, such as Na + or K + exert the mere role of increasing the ionic strength thus decreasing the electrostatic repulsion between adjacent porphyrins and favoring the formation of an initial dimeric aggregate [58,60].…”

“…Some cations, such as Na + or K + exert the mere role of increasing the ionic strength thus decreasing the electrostatic repulsion between adjacent porphyrins and favoring the formation of an initial dimeric aggregate [58,60]. Ni 2+ or Zn 2+ are effective in stabilize these species [74,75], even if some specific role in the build-up of a mesoscopic network and in the chiroptical response has been recently pointed out for this latter metal ion [65]. Some chiral complex metal cations are able to specifically interact with the growing J-aggregates imprinting their chirality in the final structures [70,71].…”

Effects of the Mixing Protocol on the Self-Assembling Process of Water Soluble Porphyrins

“…Upon protonation, the diacid form H 4 TPPS 4 2− , a dianion, initiates a self-assembling process mediated by electrostatic hydrogen bonding and solvophobic interactions. An intriguing feature in some of these aggregated species is the emergence of chirality, either imprinted through chemical [ 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 ] or physical bias [ 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 ] or deriving from spontaneous symmetry breaking [ 27 , 53 , 54 ]. Anyway, diverse results of the observed morphology, sizes, kinetics, and chirality of aggregates obtained under a variety of experimental conditions have focused attention on the importance of the mixing protocols adopted in the different investigations [ 23 , 27 , 55 ].…”