The symmetry breaking and the spontaneous induction of optical activity from achiral entities is a very fascinating topic due to its close relationship with biological systems. The quest to unravel the origin of homochirality has triggered an intense search to identify possible external influences that are able to cause enantioselection. In this framework, representative examples are reported for large electrostatic assemblies of achiral chromophoric compounds such as cyanine dyes and porphyrin.1 In particular, meso-4-sulfonatophenyl- and arylsubstituted porphyrins have been widely used as molecular components because they are able to self-assemble into J-aggregates depending on the concentration, pH and ionic strength. Their aggregation processes are based on hierarchical self-assemblies showing different thermodynamically and kinetically controlled paths.2 Quite recently the combination of magnetic and rotational forces has been used to control the handedness of growing aggregates of the achiral tris-(4-sulfonatophenyl)phenylporphyrin (TPPS3), showing that, under the samespinning sense, by reverting the effective gravity experienced by the samples is possible to invert the chirality of the supramolecular assemblies.3 Chiral J-aggregates of tetrakis-(4- sulfonatophenyl)porphyrin (TPPS4) have been obtained through the employment of physical fields such as vortices able to cause symmetry breaking and spontaneous resolution of chiral J-aggregates.4 Though the adventitious presence of chiral impurities has been proposed, lately a flexible two-dimensional chiral sheet of zwitterionic porphyrins has been reported to explain the optical activity of these J-aggregates.5 Here, we report on the dichroic properties of chiral TPPS4 J-aggregates induced by different inorganic achiral acid. Detailed kinetic investigations on the self-assembling process have been performed under different experimental conditions (pH, ionic strength, mixing order of the reagents and porphyrin concentrations). We anticipate that, depending on the overall rate of the process, a distinctive kinetic difference, together with a difference variance in the extent of the chiral transfer, is clearly evident for the various acids and strictly connected with medium properties.
KINETIC EFFECTS OF ACHIRAL ACIDS ON THE GROWTH OF CHIRAL J-AGGREGATES OF TETRAKIS(4- SULFONATOPHENYL)PORPHYRIN
A ROMEO;R ZAGAMI;L MONSU' SCOLARO
2013
Abstract
The symmetry breaking and the spontaneous induction of optical activity from achiral entities is a very fascinating topic due to its close relationship with biological systems. The quest to unravel the origin of homochirality has triggered an intense search to identify possible external influences that are able to cause enantioselection. In this framework, representative examples are reported for large electrostatic assemblies of achiral chromophoric compounds such as cyanine dyes and porphyrin.1 In particular, meso-4-sulfonatophenyl- and arylsubstituted porphyrins have been widely used as molecular components because they are able to self-assemble into J-aggregates depending on the concentration, pH and ionic strength. Their aggregation processes are based on hierarchical self-assemblies showing different thermodynamically and kinetically controlled paths.2 Quite recently the combination of magnetic and rotational forces has been used to control the handedness of growing aggregates of the achiral tris-(4-sulfonatophenyl)phenylporphyrin (TPPS3), showing that, under the samespinning sense, by reverting the effective gravity experienced by the samples is possible to invert the chirality of the supramolecular assemblies.3 Chiral J-aggregates of tetrakis-(4- sulfonatophenyl)porphyrin (TPPS4) have been obtained through the employment of physical fields such as vortices able to cause symmetry breaking and spontaneous resolution of chiral J-aggregates.4 Though the adventitious presence of chiral impurities has been proposed, lately a flexible two-dimensional chiral sheet of zwitterionic porphyrins has been reported to explain the optical activity of these J-aggregates.5 Here, we report on the dichroic properties of chiral TPPS4 J-aggregates induced by different inorganic achiral acid. Detailed kinetic investigations on the self-assembling process have been performed under different experimental conditions (pH, ionic strength, mixing order of the reagents and porphyrin concentrations). We anticipate that, depending on the overall rate of the process, a distinctive kinetic difference, together with a difference variance in the extent of the chiral transfer, is clearly evident for the various acids and strictly connected with medium properties.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
