Electrochiroptical response of a hexaarylethane derivative with a helical π-skeleton: drastic UV–Vis and CD spectral changes upon electrolysis of 4′,5′-dibromodispiro[xanthene-9,9′(9′H,10′H)-phenanthrene-10′,9″-xanthene]

Abstract

Upon two-electron oxidation of the title electron donor 1a, the elongated central C₉–C₁₀ bond [1.656(5) Å (X-ray)] is cleaved to give the biphenyl-2,2′-diyl bis(xanthenylium) dye 2a²⁺, which regenerates the colorless dispiro compound 1a by two-electron reduction. The presence of isosbestic points in the UV–Vis spectra during the electrochemical oxidation of 1a to 2a²⁺ indicates the negligible steady-state concentration of the intermediate cation radical. Interconversion between optically resolved 1a and 2a²⁺ is accompanied by drastic changes in the CD spectra again with several isosbestic points, and racemization of (P)- and (M)-1a and (S)- and (R)-2a²⁺ does not occur at ambient conditions. This redox pair represents a new motif for the multi-output response system, where the electrochemical input is transduced into two independent spectral outputs.