Understanding the Photoprocesses in Biological Systems: Need for Accurate Multireference Treatment

Light-matter interaction is crucial to life itself and revolves around many of the central processes in biology. The need for understanding these photochemical and photophysical processes cannot be overemphasized. Interaction of light with biological systems starts with the absorption of light and subsequent phenomena that occur in the excited states of the system. However, excited states are typically difficult to understand within the mean field approximation of quantum chemical methods. Therefore, suitable multireference methods and methodologies have been developed to understand these phenomena. In this Perspective, we will describe a few methods and methodologies suitable for these descriptions and discuss some persisting difficulties.

Substitution enables significant new decay channels for a non-canonical amino acid

UV-Vis absorption spectra and emission peaks of indole and 7-fluoroindole are measured and it is observed that 7-fluoroindole quenches the fluorescence signals significantly compared to indole. This observation is elucidated via reconstruction of the potential energy surfaces, determination of the conical intersections, and dynamical studies. It is observed that a single fluorine substitution on indole leads to the appearance of several accessible low-energy conical intersections that cause fast nonradiative decay. In this paper, we have investigated the nonradiative processes of Ind and 7F-Ind theoretically using high-level methods, such as EOM-EE-CCSD, SA-CASSCF, MS-CASPT2/6-311++G(d,p) levels of theory, to study these pathways and their feasibility.

Non-Born-Oppenheimer effects in molecular photochemistry: an experimental perspective

Non-adiabatic couplings between Born-Oppenheimer (BO)-derived potential energy surfaces are now recognized as pivotal in describing the non-radiative decay of electronically excited molecules following photon absorption. This opinion piece illustrates how non-BO effects provide photostability to many biomolecules when exposed to ultraviolet radiation, yet in many other cases are key to facilitating 'reactive' outcomes like isomerization and bond fission. The examples are presented in order of decreasing molecular complexity, spanning studies of organic sunscreen molecules in solution, through two families of heteroatom containing aromatic molecules and culminating with studies of isolated gas phase H₂O molecules that afford some of the most detailed insights yet available into the cascade of non-adiabatic couplings that enable the evolution from photoexcited molecule to eventual products. This article is part of the theme issue 'Chemistry without the Born-Oppenheimer approximation'.

Elucidating Inner Workings of Naturally Sourced Organic Optoelectronic Materials with Ultrafast Spectroscopy

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.

Ultrafast Dynamics and Photoresponse of a Fungi-Derived Pigment Xylindein from Solution to Thin Films

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.

Effect of Dimerization on the Nonradiative Processes of Eumelanin Monomer

Eumelanin is a polymeric structure made of dihydroxyindole (DHI) as the basic motif. Since the oxidative polymerization of DHI forms the core of eumelanin, understanding the effect of polymerization on its optical and photoprotective properties is crucial to elucidate the structure-function relationship of eumelanin. In this work, we investigate the effect of dimerization of DHI on the photoprocesses of eumelanin. We observe that there are several low-energy conical intersections and energetically favorable pathways for deactivation of photoexcited dimeric DHI species. While the original deactivation modes of the monomers are still important, in dimers the intermonomer dihedral angles seem to play a central role.

Effect of microsolvation on the non-radiative decay of the eumelanin monomer

A plethora of various low energy accessible deactivation modes of DHI were explored.