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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. 

Abstract Red fluorescent proteins (RFPs) represent an increasingly popular class of genetically encodable bioprobes and biomarkers that can advance next‐generation breakthroughs across the imaging and life sciences. Since the rational design of RFPs with improved functions or enhanced versatility requires a mechanistic understanding of their working mechanisms, while fluorescence is intrinsically an ultrafast event, a suitable toolset involving steady‐state and time‐resolved spectroscopic techniques has become powerful in delineating key structural features and dynamic steps which govern irreversible photoconverting or reversible photoswitching RFPs, and large Stokes shift (LSS)RFPs. The pertinentcis‐transisomerization and protonation state change of RFP chromophores in their local environments, involving key residues in protein matrices, lead to rich and complicated spectral features across multiple timescales. In particular, ultrafast excited‐state proton transfer in various LSSRFPs showcases the resolving power of wavelength‐tunable femtosecond stimulated Raman spectroscopy (FSRS) in mapping a photocycle with crucial knowledge about the red‐emitting species. Moreover, recent progress in noncanonical RFPs with a site‐specifically modified chromophore provides an appealing route for efficient engineering of redder and brighter RFPs, highly desirable for bioimaging. Such an effective feedback loop involving physical chemists, protein engineers, and biomedical microscopists will enable future successes to expand fundamental knowledge and improve human health. 

Abstract Recent advances in sustainable optoelectronics including photovoltaics, light‐emitting diodes, transistors, and semiconductors have been enabled by π‐conjugated organic molecules. A fundamental understanding of light‐matter interactions involving these materials can be realized by time‐resolved electronic and vibrational spectroscopies. In this Minireview, the photoinduced mechanisms including charge/energy transfer, electronic (de)localization, and excited‐state proton transfer are correlated with functional properties encompassing optical absorption, fluorescence quantum yield, conductivity, and photostability. Four naturally derived molecules (xylindein, dimethylxylindein, alizarin, indigo) with ultrafast spectral insights showcase efficient energy dissipation involving H‐bonding networks and proton motions, which yield high photostability. Rational design principles derived from such investigations could increase the efficiency for light harvesting, triplet formation, and photosensitivity for improved and versatile optoelectronic performance. 

Methylation occurs in a myriad of systems with protective and regulatory functions. 8-methoxypyrene-1,3,6-trisulfonate (MPTS), a methoxy derivative of a photoacid, serves as a model system to study effects of methylation on the excited state potential energy landscape. A suite of spectroscopic techniques including transient absorption, wavelength-tunable femtosecond stimulated Raman spectroscopy (FSRS), and fluorescence quantum yield measurements via steady-state electronic spectroscopy reveal the energy dissipation pathways of MPTS following photoexcitation. Various solvents enable a systematic characterization of the H-bonding interaction, viscosity, and dynamic solvation that influence the ensuing relaxation pathways. The formation of a charge-transfer state out of the Franck–Condon region occurs on the femtosecond-to-picosecond solvation timescale before encountering a rotational barrier. The rotational relaxation correlates with the H-bond donating strength of solvent, while the rotational time constant lengthens as solvent viscosity increases. Time-resolved excited-state FSRS, aided by quantum calculations, provides crucial structural dynamics knowledge and reveals the sulfonate groups playing a dominant role during solvation. Several prominent vibrational motions of the pyrene ring backbone help maneuver the population toward the more fluorescent state. These ultrafast correlated electronic and nuclear motions ultimately govern the fate of the photoexcited chromophore in solution. Overall, MPTS in water displays the highest probability to fluoresce, while the aprotic and more viscous dimethyl sulfoxide enhances the nonradiative pathways. These mechanistic insights may apply robustly to other photoexcited chromophores that do not undergo excited-state proton transfer or remain trapped in a broad electronic state and also provide design principles to control molecular optical responses with site-specific atomic substitution. 

Abstract Organic semiconductor materials have recently gained momentum due to their non‐toxicity, low cost, and sustainability. Xylindein is a remarkably photostable pigment secreted by fungi that grow on decaying wood, and its relatively strong electronic performance is enabled by π–π stacking and hydrogen‐bonding network that promote charge transport. Herein, femtosecond transient absorption spectroscopy with a near‐IR probe was used to unveil a rapid excited‐state intramolecular proton transfer reaction. Conformational motions potentially lead to a conical intersection that quenches fluorescence in the monomeric state. In concentrated solutions, nascent aggregates exhibit a faster excited state lifetime due to excimer formation, confirmed by the excimer→charge‐transfer excited‐state absorption band of the xylindein thin film, thus limiting its optoelectronic performance. Therefore, extending the xylindein sidechains with branched alkyl groups may hinder the excimer formation and improve optoelectronic properties of naturally derived materials.