Kinetic Control of Chirality in Porphyrin J‑Aggregates Andrea Romeo, †,⊥ Maria Angela Castriciano,* ,‡,⊥ Ilaria Occhiuto, † Roberto Zagami, † Robert F. Pasternack, § and Luigi Monsù Scolaro* ,†,⊥ ‡ Istituto per lo Studio dei Materiali Nanostrutturati ISMN-CNR and † Dipartimento di Scienze Chimiche, University of Messina and CIRCMSB, 98166 Messina, Italy § Department of Chemistry and Biochemistry, Swarthmore College, Swarthmore, Pennsylvania 19081, United States * S Supporting Information ABSTRACT: Detailed kinetic investigations demonstrate the fundamental role of kinetic parameters in the expression and transmission of chirality in supramolecular systems. The rate of the aggregation process leading to the formation of J-aggregates strongly affects the size of these nanoassemblies and the chiral induction. R eports of optical activity for assemblies of achiral entities in the absence of templates were greeted at first with skepticism. However, now that these findings have been confirmed, considerable attention has been focused on this phenomenon. The possibility that what is being observed in these systems is a spontaneous mirror-symmetry breaking has led to speculation about the relationship of these processes to those responsible for the ubiquitous homochirality in our universe. 1 Some intriguing theories have been proposed for the putative role of external influences such as circularly polarized light, 2-4 electroweak interaction, 5,6 vortex motion, 7,8 stir- ring, 8-19 catalysis at prochiral crystal surfaces, 20,21 and combinations of external fields. 22,23 Alternatively, others have suggested that “hidden” templates such as trace impurities in solvents are responsible for the observed chirality. 24 All these experimental findings and speculations finally resolve on the question of how chirality is transferred from a chiral bias, either physical or chemical, to a growing assembly. Achiral chromophores, especially porphyrins, have been of some considerable importance for such symmetry-breaking studies due to their rich spectral properties and their ability (under appropriate conditions) to self-assemble into chiral supra- molecular structures. 25-30 In particular, meso-4-sulfonato- phenyl- and aryl-substituted porphyrins have been widely used as starting materials. 31,32 Recently, control of the handedness of chiral J-aggregates obtained from such achiral porphyrin monomers has been achieved by applying rotational, gravitational, and orienting forces at the beginning of the assembly process. 23 This experimental result underscores the role that even small, macroscopic chiral perturbations applied simultaneously with nucleation steps can have in directing the handedness of initially formed seeds, and thereby of the final supramolecular structure (“amplification effect”). The supra- molecular aggregation processes are based on hierarchical self- assembly showing different thermodynamically and kinetically controlled paths related to medium properties and exper- imental conditions such as concentration, pH, and ionic strength. 25,33-35 TPPS J-aggregates, obtained in aqueous solution in the absence of any added chiral templating agent, show an unpredictable chirality, resulting in controversial proposals for their basis. Since achiral monomers were used, it had been assumed that aggregates would be formed as a racemic mixture, exhibiting no optical activity. 8 When detected, TPPS J-aggregates’ optical activity had been proposed to arise through (among others) the adventitious presence of chiral impurities (i.e., “hidden” templates) or an intrinsically chiral arrangement of porphyrins in the unit cell. 30 In the presence of chiral templating reagents, such as tartaric acid, the situation might have been expected to be more straightforward, but kinetic investigations on the formation of J-aggregates of meso- tetrakis(4‑sulfonatophenyl)porphyrin (TPPS) have shown distinct kinetic patterns and a corresponding variance in the amplification of chirality for the two enantiomers. 25 In the current report, we attempt to resolve some of the confounding issues related to optical activity of assembly formation by providing an answer to a controversial question: Is there a correlation between the different methodologies used to initiate the aggregation and the rate constants for the process as well as the final observed optical activity? Here, we report on the kinetics of the self-assembly of TPPS in the absence of an added chiral template, investigated through a combination of UV/vis, circular dichroism (CD), and resonance light scattering (RLS) spectroscopies, and show that the supramolecular chirality is related to (i) reagent mixing protocol, (ii) kinetic rates, and (iii) aggregate size. The aggregation process was triggered in an aqueous solution by adding the porphyrin as first (PF) or last reagent (PL) (see Supporting Information (SI) for experimental details). The acidification of porphyrin solutions induces the self-assembling of the zwitterionic monomers (Soret band at 434 nm) into J-aggregated species characterized by an extinction band at 490 nm. Within these aggregates, the porphyrins are stacked side-by-side, are stabilized by electrostatic, hydrogen-bonding, and dispersive interactions, 15,36-41 and exhibit interesting exciton delocaliza- tion properties. 42 Previous reports on porphyrin aggregation have pointed out the importance of establishing a well-defined protocol for reagent mixing in order to obtain reproducible results. This issue becomes particularly relevant when dealing with kinetic studies because the kinetics/mechanism of growth drive the Received: October 14, 2013 Published: December 10, 2013 Communication pubs.acs.org/JACS © 2013 American Chemical Society 40 dx.doi.org/10.1021/ja410514k | J. Am. Chem. Soc. 2014, 136, 40-43