Porphyrins Through the Looking Glass: Spectroscopic and Mechanistic Insights in Supramolecular Chirogenesis of New Self-Assembled Porphyrin Derivatives

Abstract

The solvent driven aggregation of porphyrin derivatives, covalently linked to a L- or D-prolinate enantiomer, results in the stereospecific formation of species featuring remarkable supramolecular chirality, as a consequence of reading and amplification of the stereochemical information stored in the proline-appended group. Spectroscopic, kinetic, and topographic SEM studies gave important information on the aggregation processes, and on the structures of the final chiral architectures. The results obtained may be the seeds for the construction of stereoselective sensors aiming at the detection, for example, of novel emergent pollutants from agrochemical, food, and pharmaceutical industry.

Keywords: chirality; circular dichroism; porphyrins; self-assembly; supramolecular chemistry; supramolecular chirogenesis.

Scheme 1

(i) EDCl, HOBT, dry CH₂Cl₂, 0°C, 1 h; then RT, 48 h. (ii) TFA/CH₂Cl₂ (2/3, v/v), 1.5 h, then aqueous NaHCO₃. (iii) Zn(OAc)₂·2H₂O, CHCl₃/MeOH, RT, 1 h.

Figure 1

(A) UV-Visible spectroscopic pattern of (D)ZnP(–) 5.0 μM in various media at 298 K: (a) ethanol; (b) EtOH/H₂O (25:75 v:v) after mixing (Type-I aggregates); (c) EtOH/H₂O (25:75 v:v) at equilibrium (Type-II aggregates). The inset reports the variations of the corresponding Q bands. (B) Extinction vs. time evolution of Type-I into Type-II aggregates. The term “Extinction” is consistently used instead of the more usual “Absorbance,” as the intensities of the UV-Vis features are unavoidably affected by the contribution of the RLS component (Micali et al., 2001).

Figure 2

CD spectra of (D)- and (L)ZnP(–) 5 μM (EtOH/H₂O 25:75 v:v): (A) Type-I aggregates (t = 0); (B) Type-II aggregates (at equilibrium). (C) Enlargement of the Type-II aggregate Q bands region.

Scheme 2

Schematic drawing of the proposed mechanism of aggregate formation (EtOH/H₂O 25:75 v:v; T = 298 K).

Figure 3

(A) CD spectral changes with time of (L)ZnP(–), 5 μM (EtOH/H₂O 25:75 v:v). (B) Corresponding calculated fit (Equation 1) at the indicated wavelength maxima of the lower energy bands.

Figure 4

(A) UV-Visible spectroscopic changes with time of the Soret band of (L)ZnP(–) 1 μM (EtOH/H₂O 25:75 v:v). (B) Corresponding calculated fit according to Equation 2 (λ = 422 nm). The point at t = 0 is measured in non-aggregative conditions, due to the fast rate of the process.

Figure 5

SEM images of drop casted equilibrium solutions of porphyrin aggregates (5 μM (EtOH/H₂O 25:75 v:v). (A): (L)ZnP(–); (B): (L)ZnP(–) enlargement of 5A; (C): (D)ZnP(–); (D) racemic mixture.