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

Abstract

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.

Designing Dual-Functional Metal–Organic Frameworks for Photocatalysis

“…Energy loss can be due to charge recombination, the formation of a deep trap state, or multiple intermediate energy dissipation steps within or between different excited states. Femtosecond transient absorption (fs-TA) spectroscopy can be used to gain insights into how the photogenerated charge carrier dynamics affect the overall charge transfer and energy usage. − As an ultrafast electronic spectroscopic technique, fs-TA employs a pump pulse and a time-delayed probe pulse to track the excited-state dynamics with characteristic time constants. For example, the solvation-aided excited-state stabilization could improve charge transfer efficiency, in accordance with a blue-shifted excited-state absorption (ESA) peak. , …”

Fluorescence Modulation of ortho-Green Fluorescent Protein Chromophores Following Ultrafast Proton Transfer in Solution

“…Proton transfer is among the most abundant and fundamental processes in organic, inorganic, and biological systems − which can occur between molecules and solvents (intermolecular), leading to a change of the protonation state; or within the molecule itself (intramolecular), resulting in the formation of a constitutional (structural) isomer. Excited-state proton transfer (ESPT) and excited-state intramolecular proton transfer (ESIPT) represent functional processes with distinct underlying mechanisms that have powered broad applications from research labs to industrial settings and from biological to energy fields, which include bioimaging and materials advances. , Recently, there have been extensive studies into the potential of ESIPT reactions from optoelectronic devices, fluorescence sensors, organic light-emitting diodes, lasers, and optical data storages to displays, among others. ,− Many of the molecules possessing ESPT or ESIPT capabilities are naturally occurring, such as the green fluorescent protein (GFP) from Aequorea victoria, , a blue-green pigment xylindein from the fungi Chlorociboria aeruginosa, and a red pigment Draconin red from the fungi Scytalidium cuboideum . The fundamental knowledge and insights obtained that delineate the proton transfer mechanisms in various natural systems have since inspired rational design inquiries and targeted molecular engineering endeavors.…”

Quantum Coherent Phenomena in Energy Harvesting and Storage

“…Other chemical systems examining the role of quantum coherences involving nuclear degrees of freedom include measurement of coherences during metal-to-ligand charge transfer in platinum complexes as depicted in Figure , coherences involved with ligand-to-metal charge transfer in azurin, and coherence impacts on excited state proton transfer in model dye molecules …”

“…In the development of our understanding of the possible uses of quantum coherences to increase light harvesting and energy conversion, experimental results aimed at probing the functional role of coherences have been crucial. These include 2-dimensional electronic spectroscopy (2DES), ,,, which is the leading experimental approach in this field, as well as ultrafast transient absorption spectroscopy ,,,, and femtosecond stimulated Raman spectroscopy. , This VSI includes a thoughtful review on 2DES, as well as several papers highlighting the ability to simulate 2DES measurements. ,, The role of careful experimental measurements such as temperature-dependent and magnetic field-dependent absorption and emission spectroscopy are critical in adding to our fundamental understanding. Recent breakthroughs have also come from advances in theoretical and computational methods, such as the ability to accurately simulate complex spectra from complex and heterogeneous chemical systems, as well as to model interactions between chromophores, polymers, and donor–acceptor complexes.…”

“…6 Other chemical systems examining the role of quantum coherences involving nuclear degrees of freedom include measurement of coherences during metal-to-ligand charge transfer in platinum complexes as depicted in Figure 1, 7 coherences involved with ligand-to-metal charge transfer in azurin, 8 and coherence impacts on excited state proton transfer in model dye molecules. 9 Dephasing and decoherence processes are of great importance in maintaining and manipulating quantum coherences. This also leads to a need to understand longerrange charge transfer and spatial dependencies on the generation and downstream chemical reactivity of quantum coherences.…”