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 (∼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.

Proton transfer processes of organic molecules are key to charge transport and photoprotection in biological systems. Among them, excited-state intramolecular proton transfer (ESIPT) reactions are characterized by quick and efficient charge transfer within a molecule, resulting in ultrafast proton motions. The ESIPT-facilitated interconversion between two tautomers (PS and PA) comprising the tree fungal pigment Draconin Red in solution was investigated using a combination of targeted femtosecond transient absorption (fs-TA) and excited-state femtosecond stimulated Raman spectroscopy (ES-FSRS) measurements. Transient intensity (population and polarizability) and frequency (structural and cooling) dynamics of –COH rocking and –C=C, –C=O stretching modes following directed stimulation of each tautomer elucidate the excitation-dependent relaxation pathways, particularly the bidirectional ESIPT progression out of the Franck–Condon region to the lower-lying excited state, of the intrinsically heterogeneous chromophore in dichloromethane solvent. A characteristic overall excited-state PS-to-PA transition on the picosecond timescale leads to a unique “W”-shaped excited-state Raman intensity pattern due to dynamic resonance enhancement with the Raman pump–probe pulse pair. The ability to utilize quantum mechanics calculations in conjunction with steady-state electronic absorption and emission spectra to induce disparate excited-state populations in an inhomogeneous mixture of similar tautomers has broad implications for the modeling of potential energy surfaces and delineation of reaction mechanisms in naturally occurring chromophores. Such fundamental insights afforded by in-depth analysis of ultrafast spectroscopic datasets are also beneficial for future development of sustainable materials and optoelectronics.