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Photoconvertible fluorescent proteins (pcFPs) have enabled exquisite images of cellular structures due to their genetic encodability and red-shifted emission with high brightness, hence receiving increased traction in the field. However, the red form of Kaede-like pcFPs after photoconversion remains underexplored. We implemented ultrafast electronic and vibrational spectroscopies on the red Kaede chromophore in solution vs the protein pocket of the least-evolved ancestor (LEA, a Kaede-like green-to-red pcFP) to gain crucial insights into the photophysical processes of the chromophore. The measured fluorescence quantum yield (FQY) values were correlated with ultrafast dynamics to reveal that hydrogen-bonding interactions with the solvent can quench the excited-state Kaede in solution. A viscosity-dependent sub-ps decay indicates nonradiative relaxation involving swift chromophore conformational motions. Femtosecond transient absorption and stimulated Raman spectroscopy (FSRS) reveal an additional ∼1 ps decay of the photoconverted red form of LEA that is absent in green LEA before photoconversion. Transient structural dynamics from FSRS elucidate this decay to involve the phenolate and imidazolinone ring twists that are implicated during cis → trans isomerization and on → off photoswitching in phototransformable fluorescent proteins (FPs). Compared to green-emitting species, the FQY of red LEA (∼0.58) and many other red FPs are often reduced, limiting their applications in modern bioimaging techniques. By shining more light on the often overlooked photoconverted form of pcFPs with ultrafast spectroscopies, we envision such essential mechanistic insights to enable a bottom-up approach for rationally improving the brightness of red-emitting LEA and many other controllable bioprobes, including FPs. 

Abstract Hypericin from St. John's wort has been used as a potent photosensitizer, but its working mechanism remains elusive which hinders its rational design for improved functionality. We implement ultrafast spectroscopy and quantum calculations to track the excited‐state dynamics in an intricate hydrogen‐bonding network of hypericin in solution. Using femtosecond transient absorption (fs‐TA), we track excited state intramolecular proton transfer (ESIPT) via a previously unreported blueshift of a long‐wavelength stimulated emission (SE) band with excitation‐dependent dynamics in various solvents, owing to the dominant Q^7,14tautomer that undergoes bidirectional ESIPT. This finding is corroborated by ground‐state femtosecond stimulated Raman spectroscopy (GS‐FSRS) and density functional theory (DFT) calculations. Moreover, contrasting the neutral and anionic forms of hypericin enables us to reveal an intramolecular charge transfer step underlying ESIPT. We demonstrate UV and visible excitations as an integral platform to provide direct insights into the photophysics and origin for phototoxicity of hypericin. Such mechanistic insights into the excited state of hypericin will power its future development and use. 

The Front Cover illustrates ultrafast spectroscopic insights into the photoexcited energy relaxation pathways of St. John's wort-derived fluorescent photosensitizer hypericin in solution. The bidirectional excited-state intramolecular proton transfer (ESIPT) gains prominence after UV excitation with enhanced photoprotection in a “proton pachinko”, whereas visible excitation results in more phototoxicity. More information can be found in the Research Article by C. Fang and co-workers (DOI: 10.1002/chem.202500639). Cover design by S. Johnson and C. Fang. 

Abstract Iron is a promising candidate for a cost‐effective anode for large‐scale energy storage systems due to its natural abundance and well‐established mass production. Recently, Fe‐ion batteries (FeIBs) that use ferrous ions as the charge carrier have emerged as a potential storage solution. The electrolytes in FeIBs are necessarily acidic to render the ferrous ions more anodically stable, allowing a wide operation voltage window. However, the iron anode suffers severe hydrogen evolution reaction with a low Coulombic efficiency (CE) in an acidic environment, shortening the battery cycle life. Herein, a hybrid aqueous electrolyte that forms a solid‐electrolyte interphase (SEI) layer on the Fe anode surface is introduced. The electrolyte mainly comprises FeCl₂and ZnCl₂as cosalts, where the Zn‐Cl anionic complex species of the concentrated ZnCl₂allows dimethyl carbonate (DMC) to be miscible with the aqueous ferrous electrolyte. SEI derived from DMC's decomposition passivates the iron surface, which leads to an average CE of 98.3% and much‐improved cycling stability. This advancement shows the promise of efficient and durable FeIBs. 

Abstract Here, four MOFs, namely Sc-TBAPy, Al-TBAPy, Y-TBAPy, and Fe-TBAPy (TBAPy: 1,3,6,8-tetrakis(p-benzoic acid)pyrene), were characterized and evaluated for their ability to remediate glyphosate (GP) from water. Among these materials, Sc-TBAPy demonstrates superior performance in both the adsorption and degradation of GP. Upon light irradiation for 5 min, Sc-TBAPy completely degrades 100% of GP in a 1.5 mM aqueous solution. Femtosecond transient absorption spectroscopy reveals that Sc-TBAPy exhibits enhanced charge transfer character compared to the other MOFs, as well as suppressed formation of emissive excimers that could impede photocatalysis. This finding was further supported by hydrogen evolution half-reaction (HER) experiments, which demonstrated Sc-TBAPy’s superior catalytic activity for water splitting. In addition to its faster adsorption and more efficient photodegradation of GP, Sc-TBAPy also followed a selective pathway towards the oxidation of GP, avoiding the formation of toxic aminomethylphosphonic acid observed with the other M³⁺-TBAPy MOFs. To investigate the selectivity observed with Sc-TBAPy, electron spin resonance, depleted oxygen conditions, and solvent exchange with D₂O were employed to elucidate the role of different reactive oxygen species on GP photodegradation. The findings indicate that singlet oxygen (¹O₂) plays a critical role in the selective photodegradation pathway achieved by Sc-TBAPy. 

In the present study, we demonstrated that the introduction of a 1,4-diethyl-1,2,3,4-tetrahydroquinoxalin moiety into the arylidene part of GFP chromophore-derived compounds results in the formation of environment-sensitive fluorogens. The rationally designed and synthesized compounds exhibit remarkable solvent- and pH-dependence in fluorescence intensity. The solvent-dependent variation in fluorescence quantum yield makes it possible to use some of the proposed compounds as polarity sensors suitable for selective endoplasmic reticulum fluorescent labeling in living cells. Moreover, the pH-dependent emission intensity variation of other fluorogens makes them selective fluorescent labels for the lysosomes in living cells. 

Metal–organic frameworks (MOFs) have emerged as a highly tunable class of porous materials with wide-ranging applications from gas capture to photocatalysis. Developing these exciting properties to their fullest extent requires a thorough mechanistic understanding of the structure–function relationships. We implement an ultrafast spectroscopic toolset, femtosecond transient absorption and femtosecond stimulated Raman spectroscopy (FSRS), to elucidate the correlated electronic and vibrational dynamics of two isostructural 1,3,6,8-tetrakis(p-benzoic acid)pyrene (TBAPy)-based MOFs, which manifest drastically different photocatalytic behaviors. Systematic comparisons between the M3+-TBAPy MOFs and bare ligands in various environments reveal the unproductive dimer formation in Al-TBAPy, whereas Sc-TBAPy is dominated by a catalytically active charge-transfer (CT) process. Two ground-state FSRS marker bands of the TBAPy ligand at ∼1267 and 1617 cm−1 probe the chromophore environment at thermal equilibrium. For comparison, the excited-state FSRS of Sc-TBAPy suspended in neutral water unveils a key ∼300 fs twisting motion of the TBAPy peripheral phenyl groups toward planarity, promoting an efficient generation of CT species. This motion also exhibits high sensitivity to solvent environment, which can be a useful probe; we also showed the CT variation for ultrafast dynamics of Sc-TBAPy in the glyphosate aqueous solution. These new insights showcase the power of table-top tunable FSRS methodology to delineate structural dynamics of functional molecular systems in action, including MOFs and other photosensitive “nanomachines.” We expect the uncovered ligand motions (ultrafast planarization) to enable the targeted design of new MOFs with improved CT state characteristics (formation and lifetime) to power applications, including photocatalysis and herbicide removal from waterways. 

The incorporation of noncanonical amino acids (ncAAs) into fluorescent proteins is promising for red-shifting their fluorescence and benefiting tissue imaging with deep penetration and low phototoxicity. However, ncAA-based red fluorescent proteins (RFPs) have been rare. The 3-aminotyrosine modified superfolder green fluorescent protein (aY-sfGFP) represents a recent advance, yet the molecular mechanism for its red-shifted fluorescence remains elusive while its dim fluorescence hinders applications. Herein, we implement femtosecond stimulated Raman spectroscopy to obtain structural fingerprints in the electronic ground state and reveal that aY-sfGFP possesses a GFP-like instead of RFP-like chromophore. Red color of aY-sfGFP intrinsically arises from a unique “double-donor” chromophore structure that raises ground-state energy and enhances charge transfer, notably differing from the conventional conjugation mechanism. We further developed two aY-sfGFP mutants (E222H and T203H) with significantly improved (∼12-fold higher) brightness by rationally restraining the chromophore's nonradiative decay through electronic and steric effects, aided by solvatochromic and fluorogenic studies of the model chromophore in solution. This study thus provides functional mechanisms and generalizable insights into ncAA-RFPs with an efficient route for engineering redder and brighter fluorescent proteins.