The shapes of many molecules can be transformed by light or heat. Here we investigate collision- and photon-induced interconversions of $EE$, $EZ$, and $ZZ$ isomers of the isolated Congo red (CR) dianion, a double molecular switch containing two $─N═N─$ azo groups, each of which can have the $E$ or $Z$ configuration. We find that collisional activation of CR dianions drives a one-way $ZZ\to EZ\to EE$ cascade towards the lowest-energy isomer, whereas the absorption of a single photon over the 270–600 nm range can switch either azo group from $E$ to $Z$ or $Z$ to $E$, driving the CR dianion to lower- or higher-energy forms. The experimental results, which are interpreted with the aid of calculated statistical isomerization rates, indicate that photoisomerization of CR in the gas phase involves a passage through conical intersection seams linking the excited and ground state potential energy surfaces rather than through isomerization on the ground state potential energy surface following internal conversion.

• Received 21 October 2017

DOI:https://doi.org/10.1103/PhysRevLett.120.223002