Aggregation processes leading to chirality transfer or induction or amplification phenomena are currently one of the "hot topics" of modern chemical science. The investigation of such processes has influenced research in many areas including studies on the origin of the homochirality of life, various biomimetic systems, materials and polymer science, nonlinear optics, molecular devices, molecular recognition, and the determination of absolute configuration. [1] In the course of our investigations on supramolecular chirogenesis phenomena in porphyrin chromophores, [2] we recently discovered a new type of highly optically active aggregates based on bis(zinc porphyrin) upon interaction with chiral amines in the solid state. [2a] Extension of this research revealed that not only more structurally complex bisporphyrins form chiral aggregated species in the solid state, but that even a conventional and simple monomeric zinc porphyrin may generate chiral aggregates under certain conditions. Here we describe the time-dependent formation of chiral aggregates of zinc octaethylporphyrin (ZnOEP, Figure 1) upon interaction with enantiopure chiral amines in the solid state. This results in the highest optical activity ever reported for a supramolecular chirogenic system based on porphyrin chromophores.
Recently it was shown that ZnOEP exhibits either no or only negligible optical activity in the porphyrin absorption region upon interaction with chiral amines in solution [2d] and in the solid state. [3] However, and exclusively for the case of enantiopure 1-cyclohexylethylamines (CHEAs), further analysis of the solid-state mixture with ZnOEP revealed the gradual development of a supramolecular chirogenic process. The solid-state samples were prepared according to the standard procedure for obtaining a glassy KBr matrix. [4] Then the process of supramolecular chirogenesis was monitored by UV/Vis and circular dichroism (CD) spectroscopy over time.
In the beginning the UV/Vis spectrum of freshly prepared ZnOEP•L (L = (S)-CHEA) in a KBr matrix closely resembles that in CH 2 Cl 2 (Figure 2 b, see also ref.
[2d] and the Support-[*] Dr.