In line with reported data, exposure of a solution of (Z)-1 to various wavelengths of light did not cause any UV-vis spectral changes (Supplementary section 8), which confirms that no isomerization takes place. Remarkably, after addition of TFA, irradiation at 455 nm afforded a bathochromically shifted absorbance profile similar to the one observed for the thermally generated (E)-1 . H + species (Fig. ). It is important to note that the irradiation studies were performed at lower temperature (≤ 0 °C) and higher dilution than the isomerization experiments described above to suppress the thermally activated process. The spectral changes thus reveal that photoisomerization can occur in the protonated state. Likewise, in the 1 H NMR spectrum, the same set of signals that was observed upon acid-induced thermal isomerization appeared after irradiation of a solution of Z isomer in presence of TFA, supporting that the E isomer can be generated photochemically upon protonation (Supplementary section 9). Interestingly, when a sample of (E)-1 . H + (generated using 455 nm light) was subsequently exposed to other wavelengths, including those at which the (Z)-1 . H + species does not absorb (>625 nm), no further spectral changes were noted. This suggests that backwards photoisomerization is inhibited, presumably through an ESIPT reaction, as was similarly reported for (Z)-1 and the E isomer of an indole-derived hemi-thioindigo. Nevertheless, after addition of excess Et3N, now giving the absorption spectrum characteristic for (E)-1 (Fig. ), full isomerization back to original (Z)-1 was achieved upon irradiation at 455 nm. Also here, thermal isomerization was negligible under the experimental conditions used.