Protonation and thermal E/Z isomerization were accompanied by distinct color changes and were additionally monitored using UV-vis spectroscopy (Fig. and Supplementary section 7). All compounds absorbed in the visible region having absorption maxima between 478-508 nm. In the case of pyridyl derivative (Z)-1, addition of TFA was accompanied by a large bathochromic shift (max = 482 to 504 nm). When the solution was allowed to equilibrate, the absorption shifted to longer wavelength (max = 549 nm) and decreased in intensity. This redshifted absorption is in agreement with time-dependent DFT calculations for (Z)-1 . H + (max = 505 nm) and (E)-1 . H + (max = 554 nm) and thus, supportive of E→Z isomerization (Supplementary section 10). Subsequent addition of excess Et3N to basify the solution led to a hypsochromic shift (max = 489 nm) and over time, the original UV-vis spectrum was recovered, demonstrating reversibility of the protonation-induced isomerization.