Propeller-Shaped Semi-fused Porphyrin Trimers: Molecular-Symmetry-Dependent Chiroptical Response.

Triple helicene-like semi-fused trimeric NiII porphyrins were constructed by alkyne trimerization of an ethynyl-substituted porphyrin and subsequent three-fold Grignard addition to the formyl groups and acid-catalyzed intramolecular cyclization. The presence of stereogenic sp3 carbons in the central bridge leads to small inter-porphyrin conjugative interactions as was revealed by electrochemical and optical properties. Two diastereomers with stable chiral conformations were optically resolved, and the separated enantiomers displayed considerably intense circular dichroism. Importantly, the chiroptical response of C3 -symmetric helical isomer (|Δϵ|=830 m-1  cm-1 ) is 1.8 times amplified from that of C1 -symmetric one (|Δϵ|=470 m-1  cm-1 ). The observed amplification has been interpreted in terms of different spatial arrangements of the three porphyrins.