From indigo to isoindigo: Rationalizing the high efficiency of photoprotective molecular mechanisms

Abstract

Indigo (IND) and its natural derivatives, including Tyrian purple and indirubin (INR), are examples of molecules with a history of millions of years. They also illustrate molecular evolution and longevity, linked to their extraordinary molecular stability associated with excited state decay mechanisms. In the excited state, these molecules efficiently undergo a radiationless deactivation process resulting in negligible fluorescence and triplet state formation (less than 0.1 % of the quantum loss in indigo for both processes). Here, we demonstrate that in isoindigo (ISO) this process is even more efficient than in IND, resulting in nearly 100 % of the excited state deactivation. In IND, the molecular mechanism behind this decay process involves intramolecular proton transfer (ESPT). In the case of INR, ESPT efficiently competes with the formation of a conformational species –the syn-rotamer– for deactivation to the ground state. With ISO, the ESPT process is absent. Instead, the deactivation mechanism involves a rotation of a few degrees around the central C–C bond, aRotational Isomerization (RI). This rotation leads to a sloped conical intersection (CI), making the radiationless deactivation process more efficient than the ESPT found with IND.