Figure 2.

UV/vis absorption spectra of (top) SP and (bottom) NSP (62 μM in acetonitrile, black lines) upon addition of 1 equiv CF₃SO₃H (red lines). Irradiation at 365 nm induces a red shift to 420 nm (blue lines), which is reversed by irradiation at 455 nm (for E-MCH⁺ to Z-MCH⁺ see Figure 3).