Efficient chirality transcription utilizing a cerium(IV) double decker porphyrin: a prototype for development of a molecular memory system

Abstract

A cerium(IV) double decker porphyrin 2 bearing two pairs of 4-pyridyl groups and two pairs of 3,5-dimethoxyphenyl groups was synthesized. In the presence of chiral dicarboxylic acid guests with a two carbon spacer [e.g., (1R,2R)-cyclohexane-1,2-dicarboxylic acid and Boc-L-aspartic acid] 2 gave a CD-active species. The plots of the CD intensity vs. the guest concentration showed a sigmoidal curvature, a sign of homotropic, positive allosterism. Analysis according to the Hill equation indicated that the guests were bound autoacceleratively. Even after the chiral guests were removed by the addition of excess pyridine, 2 remained CD-active because of its inherent chirality. Thereafter, the CD intensity decreased very slowly as a result of internal rotation of the porphyrin subunits. Thermodynamic analysis of this racemization process gave ΔG ^‡₂₉₈ = 23.0 kcal mol^–1, ΔH ^‡₂₉₈ = 18.1 kcal mol^–1 and ΔS ^‡₂₉₈ = –16.4 cal mol^–1 K^–1. Observations and calculations indicate that the chiral memory can be preserved for 3 days at 0 °C and for one year at –37 °C. In conclusion, this is a rare artificial system for which a homotropic, positive allosterism is observable and in which the guest chirality is transcribed and stored.