Illuminating Excited-State Intramolecular Proton Transfer of a Fungi-Derived Red Pigment for Sustainable Functional Materials

Proton transfer is an important player that contributes to functional properties of light-sensitive organic molecules from photostability to energy transfer. For an organic pigment secreted by the wood-spalting fungus Scytalidium cuboideum, little is known about Draconin Red despite its discovery decades earlier. With steady-state and time-resolved spectroscopic techniques as well as quantum calculations, two tautomers of Draconin Red with different orientations of hydroxy groups were found to comprise most of the equilibrium population. These tautomers may underlie major species in fluorescent and nonfluorescent needle-like crystals, the former showing waveguide properties. Femtosecond transient absorption measurements revealed a dynamic equilibrium due to excited-state intramolecular proton transfer (ESIPT) between the two tautomers on faster (<120 fs) and slower (similar to 750 fs) time scales, supplemented by computationally scanning two nonequivalent ESIPT coordinates. Ground and excited-state femtosecond stimulated Raman spectroscopy (FSRS) confirmed the presence of both tautomers in solution with key frequency shifts of vibrational marker bands upon photoexcitation, tracking an initial ultrafast unidirectional tautomerization. We envision the rational design of the highly symmetric red pigment by incorporation of electron donating and/or withdrawing groups to elevate the electronic and photonic performance of this naturally derived small molecule going from solution to the solid state.