Strong and Tunable Near-Infrared Circular Dichroism in Helical Tetrapyrrole Complexes

The selective synthesis of nickel and copper complexes of 19-benzoyl-5,10,15-triphenyl-bilatrien-1-one (H₂ TPBT) is reported, a molecule which crystallizes as a molecular helix of one-and-a-quarter which turns with a 5.7 Å radius and a 3.2 Å pitch, and all 26 participating atoms are sp² -hybridized. UV/vis, ECD, ESR and cyclic voltammetry experiments reveal a strong interaction between metal and ligand and partial radical character when copper is coordinated instead of nickel. Strong ECD absorption in the 800 nm range is found which, using TD-DFT calculations as well as literature spectra, is shown to be highly tunable both by metal coordination and variation of the aryl groups in the TPBT periphery. The radical character of the ligand in Cu(TPBT) enables rapid interconversion between (M)- and (P)-enantiomers, possibly via intermittent breakage of a Cu-N bond. The 19-benzoyl group kinetically stabilizes enantiopure (M/P)-Ni(TPBT). The results are interpreted with regard to the application as circularly polarized light (CPL) detectors as well as to the chirality-induced spin-selectivity (CISS) effect which is currently lacking a concise theoretical model.