We performed multiple cycles of irradiation alternating between a 305 nm and 455 nm LED (Fig. ). Although the irradiation repeatedly led to the formation of 18 in the PSS and backformation of 15, analysis of the UV/vis absorption maxima at 390 nm over several cycles indicated a moderate switching fatigue caused by the formation of trace side products in the photoswitching process (see Supplementary Fig. ). We hypothesize that the formation of rearranged 18 is driven by a homoaromatic stabilization effect and thus turned our attention to the computational electronic and magnetic structure analysis of 15 and 18. Fig. | Photoswitchable [1,11] sigmatropic rearrangement . a) Homoannulene ester 15 undergoes reversible [1,11] sigmatropic rearrangement at  = 305 nm to ester 18 (only one possible enantiomer displayed) and back-reaction to 15 at  = 455 nm; b) Cycling experiments of the photoreaction. Absorbance at 390 nm shown under alternating irradiation between 305 nm (blue shaded) and 455 nm (red shaded); c) UV/vis absorption of 15 (cyan) in cyclohexane (c(15) = 5.5×10 -5 mol L -1 ) upon irradiation with a 305 nm LED (grey, every 5 s, 80 s total) and till the PSS305 nm is reached (green); d) back-irradiation into 15 using a 455 nm LED (grey, spectra recorded every 60 s, 11 min total). Insets show HPLC chromatograms at the start and end of the irradiation.