The oxygen-organic molecule photosystem: revisiting the past, recalibrating the present, and redefining the future

Singlet Oxygen Photophysics: From Liquid Solvents to Mammalian Cells

Molecular oxygen, O2, has long provided a cornerstone for studies in chemistry, physics, and biology. Although the triplet ground state, O2(X3Σg-), has garnered much attention, the lowest excited electronic state, O2(a1Δg), commonly called singlet oxygen, has attracted appreciable interest, principally because of its unique chemical reactivity in systems ranging from the Earth's atmosphere to biological cells. Because O2(a1Δg) can be produced and deactivated in processes that involve light, the photophysics of O2(a1Δg) are equally important. Moreover, pathways for O2(a1Δg) deactivation that regenerate O2(X3Σg-), which address fundamental principles unto themselves, kinetically compete with the chemical reactions of O2(a1Δg) and, thus, have practical significance. Due to technological advances (e.g., lasers, optical detectors, microscopes), data acquired in the past ∼20 years have increased our understanding of O2(a1Δg) photophysics appreciably and facilitated both spatial and temporal control over the behavior of O2(a1Δg). One goal of this Review is to summarize recent developments that have broad ramifications, focusing on systems in which oxygen forms a contact complex with an organic molecule M (e.g., a liquid solvent). An important concept is the role played by the M+•O2-• charge-transfer state in both the formation and deactivation of O2(a1Δg).

1O2-Relevant Afterglow Luminescence of Chlorin Nanoparticles for Discriminative Detection and Isotopic Analysis of H2O and D2O

Discriminative detection between D₂O and H₂O is important for diverse fields but challenging due to their high similarity in chemical and physical properties. Current molecular sensors for D₂O detection generally rely on the spectral change of fluorophores with suitable pK_a in response to D₂O and H₂O with slightly different pH acidity. Herein, we report a new and facile D₂O sensor by using singlet oxygen (¹O₂)-relevant afterglow luminescence of chlorin e4 nanoparticles (Ce4-NPs) to achieve distinguishable detection between D₂O and H₂O. As ¹O₂ is a key initiator involved in the afterglow luminescence process, it displays a 22-fold longer lifetime in D₂O relative to H₂O and thereafter generates more dioxetane intermediates after laser irradiation to lead to ultimate afterglow brightness of Ce4-NPs in D₂O. In addition, Ce4-NPs are capable of quantitatively detecting the amount of H₂O in D₂O with a limit of detection (LOD) of 1.45%. Together, this study broadens the utility of afterglow materials and presents a facile strategy for isotopic purity analysis of heavy water.

Spin-Orbit Couplings of Open-Shell Systems with Restricted Active Space Configuration Interaction

In this work we perform electronic structure calculations to unravel the origin of spin-orbit couplings (SOCs) in open-shell molecules. For that, we select systems displaying di or polyradical character, e.g., trimethylene, and analyze the changes in the magnitude of SOC constants along molecular distortions of ethylene and in the presence of intermolecular interactions between open and closed-shell moieties in the O₂-C₂H₄ system. Calculations were performed by using nonrelativistic wave functions obtained with the restricted active space configuration interaction (RASCI) method, in conjunction with a recent implementation for the calculation of SOC based on the spin-orbit mean field approximation. Our results demonstrate the suitability of RASCI in the calculation of SOCs of open-shell systems, while providing a deep understanding of the relationship between couplings and the nature of the electronic states. Moreover, we introduce a new definition of the SOC constant for the study of molecular aggregates.

Perturbed and Activated Decay: The Lifetime of Singlet Oxygen in Liquid Organic Solvents

Singlet oxygen, O₂(a¹Δ_g), the lowest excited electronic state of molecular oxygen, plays an important role in a range of chemical and biological processes. In liquid solvents, the reactions of singlet oxygen with a solute kinetically compete with solvent-mediated deactivation that yields the ground electronic state of oxygen, O₂(X³Σ_g^-). In this regard, the key parameter is the solvent-mediated lifetime of singlet oxygen, which embodies fundamental physical principles ranging from intermolecular interactions that perturb the forbidden O₂(a¹Δ_g) → O₂(X³Σ_g^-) transition to the transfer of oxygen's excitation energy into the vibrational modes of a solvent molecule M. Extensive research performed by the global community on this oxygen-related issue over the past ∼50 years reflects its significance. Unfortunately, a satisfactory quantitative understanding of this unique solvent effect has remained elusive thus far. In temperature-dependent studies, we have quantified the singlet oxygen lifetime in common aromatic and aliphatic organic solvents, including partially deuterated molecules that exploit the H/D solvent isotope effect on the lifetime. We now account for experimental data, including previously intractable data, using a model that exploits both weak and strong coupling in the M-O₂ complex to accommodate the roles that M plays to (1) induce the forbidden O₂(a¹Δ_g) → O₂(X³Σ_g^-) transition and (2) accept the excitation energy of O₂(a¹Δ_g). As such, our approach brings us appreciably closer to an accurate and predictive ab initio solution for the long-standing oxygen-dependent problem that, in turn, should be relevant for a host of other molecular systems.

X3Σg- → b1Σg+ Absorption Spectra of Molecular Oxygen in Liquid Organic Solvents at Atmospheric Pressure

Spectra and absorption coefficients of the forbidden 765 nm X³Σ_g^- → b¹Σ_g⁺ transition of molecular oxygen dissolved in organic solvents at atmospheric pressure were recorded over a 5 m path length using a liquid waveguide capillary cell. The results show that it is possible to investigate this weak near-infrared absorption transition in a common liquid hydrocarbon solvent without the need for a potentially dangerous high oxygen pressure. Proof-of-principle data from benzene, toluene, chlorobenzene, bromobenzene, and iodobenzene reveal a pronounced heavy atom effect on this spin-forbidden transition. For example, the absorption coefficient at the band maximum in iodobenzene, (28.9 ± 3.3) × 10^-3 M^-1 cm^-1, is approximately 21 times larger than that in benzene, (1.4 ± 0.1) × 10^-3 M^-1 cm^-1. These absorption measurements corroborate results obtained from O₂(X³Σ_g^-) → O₂(b¹Σ_g⁺) excitation spectra of O₂(a¹Δ_g) → O₂(X³Σ_g^-) phosphorescence, which depended on data from a plethora of convoluted experiments. Spectroscopic studies of molecular oxygen in liquid solvents can help evaluate aspects of the seminal Strickler-Berg approach to treat the effect of solvent on Einstein's A and B coefficients for radiative transitions. In particular, our present results are a key step toward using the O₂(X³Σ_g^-) → O₂(b¹Σ_g⁺) transition to evaluate the speculated limiting condition of applying the Strickler-Berg treatment to a highly forbidden process. This latter issue is but one example of how an arguably simple homonuclear diatomic molecule continues to aid the scientific community by providing fundamental physical insight.