Fig. 4. UV spectra for all species in the study at 293 K (except for 2, measured at 280 K) in methylcyclohexane. Spectra for alkenes 1 and the corresponding [5]helicene 4 were recorded directly while spectra for 2 and 3 were extracted from mixtures (see ESI, Section S5.8†). Error bars represent the estimated uncertainty (±1 s.d.) in the extinction coefficients at that wavelength.