Entropy-controlled biradical–quinoid isomerization of a π-conjugated delocalized biradical†
Biradicaloid species have been extensively studied for their characteristic features in electric conductivity, magnetism, and optical nonlinearity. Theoretical investigations of rigid biradicaloid species have been suggesting that they are represented as a resonance hybrid of open-shell biradical and closed-shell quinoid structures. However, much is still unknown about flexible biradicaloid species whether the activation free energy barrier between these states exists or not. Herein, we investigated the thermal isomerization from the photogenerated unstable biradical to the stable quinoid species observed for the photochromic dimer of a bisimidazolyl radical and found that the large negative activation entropy for the valence isomerization causes the activation free energy barrier between these two states.