Theoretical studies on the photochemistry of 2-nitrofluorene in the gas phase and acetonitrile solution

Insight into the environmental photochemistry of nitrated polycyclic aromatic hydrocarbons in water and in ice: kinetics, pathways and photo-modified toxicity

Nitrated polycyclic aromatic hydrocarbons (nitro-PAHs) are contaminants of emerging concern due to their various sources and widespread existence in the environment. This study demonstrated an in-depth comparison of the aqueous and ice photochemistry of three nitro-PAHs: 1-nitropyrene (1-Npyr), 2-nitrofluorene (2-Nflu) and 9-nitrophenanthrene (9-Nphe). Upon exposure to the simulating solar irradiation (λ > 290 nm), their apparent photolysis followed pseudo-first-order kinetics, with apparent quantum yields (Φs) and half-lives (t1/2) depending on the chemical structures or the reaction media (water/ice). Based on the ROS scavenging experiments, 1-Npyr was found to suffer from self-sensitized photo-oxidation by hydroxyl radicals (·OH), while 2-Nflu and 9-Nphe underwent singlet-oxygen (1O2) mediated self-sensitized photolysis. Moreover, the contributions of the self-sensitized photolysis via ·OH/1O2 in ice were lower than in water for all the nitro-PAHs (p < 0.05), which may be ascribed to the lower fluidity of the molecules in ice and insufficient ·OH/1O2 generated to participate in the reactions. The product identification by HPLC-MS/MS indicated that the main photodegradation pathways involved photoinduced hydroxylation, photooxidation and isomerization. Interestingly, isomerization reaction only occurred in the ice phase, attributing to the freezing concentration effect that led to the enrichment of solutes at the crystal boundaries of the ice crystals and facilitated the generation of isomers in ice. Furthermore, the photo-modified toxicities of the nitro-PAHs to Vibrio fischeri were examined in the two phases, indicating that the higher or comparable toxicities persisted in their intermediates. The toxicities of the individual intermediates to multiple trophic-level organisms were further assayed by the ECOSAR software, indicating consistency with the results of the bioassay using Vibrio fischeri. These results showed that the similarities and differences between aqueous and ice photochemistry of nitro-PAHs, which has crucial implications for how we undertake assessments of environmental persistence for the group of chemicals in cold regions.

Excited-State Decay and Photolysis of O-Nitrophenol before Proton Transfer. I: A Theoretical Investigation in the Microsolvated Atmospheric Environment

As a potential source of the hydroxyl (OH) radical and nitrous acid (HONO), photolysis of o-nitrophenol (ONP) is of significant interest in both experimental and theoretical studies. In the atmospheric environment, the number of water molecules surrounding ONP changes with the humidity of the air, leading to an anisotropic chemical environment. This may have an impact on the photodynamics of ONP and provide a mechanism that differs from previously reported ones in the gas phase or in solution. Herein, the high-level MS-CASPT2//CASSCF method was performed to elucidate the excited-state decay and the generation of the OH radical for ONP before proton transfer in the microsolvated surrounding. We found that the varying number of water molecules affects the ground-state structures and alters the energy levels of nπ* and ππ* at the Franck-Condon (FC) region. Nevertheless, this is not the case for the excited-state minima, which exhibit very similar adiabatic excitation properties. In addition, the presence of water molecules also significantly influences the intersection structures since hydrogen bonds will hinder or alleviate the rotation or pyramidalization of the nitro (NO2) group. This will, in turn, change the excited-state relaxation mechanism of ONP. Finally, we speculated that the OH radical might be formed in the hot ground state of ONP in the microsolvated surrounding after exploring all possible electronic states.

A theoretical insight into excited-state decay and proton transfer of p-nitrophenylphenol in the gas phase and methanol solution

The excited-state decay (ESD) and proton transfer (EPT) of p-nitrophenylphenol (NO₂-Bp-OH), especially in the triplet states, were not characterized with high-level theoretical methods to date. Herein, the MS-CASPT2//CASSCF and QM(MS-CASPT2//CASSCF)/MM methods were employed to gain an atomic-level understanding of the ESD and EPT of NO₂-Bp-OH in the gas phase and its hydrogen-bonded complex in methanol. Our calculation results revealed that the S₁ and S₂ states of NO₂-Bp-OH are of ¹ππ* and ¹nπ* characters at the Franck-Condon (FC) point, which correspond to the ICT-EPT and intramolecular charge-transfer (ICT) states in spectroscopic experiments. The former state has a charge-transfer property that could facilitate the EPT reaction, while the latter one might be unfavorable for EPT. The vertical excitation energies of these states are almost degenerate at the FC region and the electronic configurations of ¹ππ* and ¹nπ* will exchange from the S₁ FC region to the S₁ minimum, which means that the ¹nπ* state will participate in ESD once NO₂-Bp-OH departs from the S₁ FC region. Besides, we found that three triplets lie below the first bright state and will play very important roles in intersystem crossing processes. In terms of several pivotal surface crossings and relevant linearly interpolated internal coordinate (LIIC) paths, three feasible but competing ESD channels that could effectively lead the system to the ground state or the lowest triplet state were put forward. Once arrived at the T₁ state, the system has enough time and internal energy to undergo the EPT reaction. The methanol solvent has a certain effect on the relative energies and spin-orbit couplings, but does not qualitatively change the ESD processes of NO₂-Bp-OH. By contrast, the solvent effects will remarkably stabilize the proton-transferred product by the hydrogen bond networks and assist to form the triplet anion. Our present work would pave the road to properly understand the mechanistic photochemistry of similar hydroxyaromatic compounds.

Quantum mechanics/molecular mechanics studies on the mechanistic photophysics of sunscreen oxybenzone in methanol solution

Herein, we have employed the QM(CASPT2//CASSCF)/MM method to explore the photophysical and photochemical mechanism of oxybenzone (OB) in methanol solution. Based on the optimized minima, conical intersections and crossing points, and minimum-energy reaction paths related to excited-state intramolecular proton transfer (ESIPT) and excited-state decay paths in the ¹ππ*, ¹nπ*, ³ππ*, ³nπ*, and S₀ states, we have identified several feasible excited-state relaxation pathways for the initially populated S₂(¹ππ*) state to decay to the initial enol isomer' S₀ state. The major one is the singlet-mediated and stretch-torsion coupled ESIPT pathway, in which the system first undergoes an essentially barrierless ¹ππ* ESIPT process to generate the ¹ππ* keto species, and finally realizes its ground state recovery through the subsequent carbonyl stretch-torsion facilitating S₁ → S₀ internal conversion (IC) and the reverse ground-state intramolecular proton transfer (GSIPT) process. The minor ones are related to intersystem crossing (ISC) processes. At the S₂(¹ππ*) minimum, an S₂(¹ππ*)/S₁(¹nπ*)/T₂(³nπ*) three-state intersection region helps the S₂ system branch into the T₁ state through a S₂ → S₁ → T₁ or S₂ → T₂ → T₁ process. Once it has reached the T₁ state, the system may relax to the S₀ state via direct ISC or via subsequent nearly barrierless ³ππ* ESIPT to yield the T₁ keto tautomer and ISC. The resultant S₀ keto species significantly undergoes reverse GSIPT and only a small fraction yields the trans-keto form that relaxes back more slowly. However, due to small spin-orbit couplings at T₁/S₀ crossing points, the ISC to S₀ state occurs very slowly. The present work rationalizes not only the ultrafast excited-state decay dynamics of OB but also its phosphorescence emission at low temperature.

Theoretical Studies on the Excited-State Decay Mechanism of Homomenthyl Salicylate in a Gas Phase and an Acetonitrile Solution

Here, we employ the CASPT2//CASSCF and QM(CASPT2//CASSCF)/MM approaches to explore the photochemical mechanism of homomenthyl salicylate (HMS) in vacuum and an acetonitrile solution. The results show that in both cases, the excited-state relaxation mainly involves a spectroscopically "bright" S₁(¹ππ*) state and the lower-lying T₁ and T₂ states. In the major relaxation pathway, the photoexcited S₁ keto system first undergoes an essentially barrierless excited-state intramolecular proton transfer (ESIPT) to generate the S₁ enol minimum, near which a favorable S₁/S₀ conical intersection decays the system to the S₀ state followed by a reverse ground-state intramolecular proton transfer (GSIPT) to repopulate the initial S₀ keto species. In the minor one, an S₁/T₂/T₁ three-state intersection in the keto region makes the T₁ state populated via direct and T₂-mediated intersystem crossing (ISC) processes. In the T₁ state, an ESIPT occurs, which is followed by ISC near a T₁/S₀ crossing point in the enol region to the S₀ state and finally back to the S₀ keto species. In addition, a T₁/S₀ crossing point near the T₁ keto minimum can also help the system decay to the S₀ keto species. However, small spin-orbit couplings between T₁ and S₀ at these T₁/S₀ crossing points make ISC to the S₀ state very slow and make the system trapped in the T₁ state for a while. The present work rationalizes not only the ultrafast excited-state decay dynamics of HMS but also its low quantum yield of phosphorescence at 77 K.

Quantum Mechanics/Molecular Mechanics Studies on the Photophysical Mechanism of Methyl Salicylate

Methyl salicylate (MS) as a subunit of larger salicylates found in commercial sunscreens has been shown to exhibit keto-enol tautomerization and dual fluorescence emission via excited-state intramolecular proton transfer (ESIPT) after the absorption of ultraviolet (UV) radiation. However, its excited-state relaxation mechanism is unclear. Herein, we have employed the quantum mechanics(CASPT2//CASSCF)/molecular mechanics method to explore the ESIPT and excited-state relaxation mechanism of MS in the lowest three electronic states, that is, S₀, S₁, and T₁ states, in a methanol solution. Based on the optimized geometric and electronic structures, conical intersections and crossing points, and minimum-energy paths combined with the computed linearly interpolated Cartesian coordinate paths, the photophysical mechanism of MS has been proposed. The S₁ state is a spectroscopically bright ¹ππ* state in the Franck-Condon region. From the initially populated S₁ state, there exist three nonradiative relaxation paths to repopulate the S₀ state. In the first one, the S₁ system (i.e., ketoB form) first undergoes an ESIPT path to generate an S₁ tautomer (i.e., enol form) that exhibits a large Stokes shift in experiments. The generated S₁ enol tautomer further evolves toward the nearby S₁/S₀ conical intersection and then hops to the S₀ state, followed by the backward ground-state intramolecular proton transfer (GSIPT) to the initial ketoB form S₀ state. In the second one, the S₁ system first hops through the S₁ → T₁ intersystem crossing (ISC) to the T₁ state, which then further decays to the S₀ state via T₁ → S₀ ISC at the T₁/S₀ crossing point. In the third path, the T₁ system that stems from the S₁ → T₁ ISC process via the S₁/T₁ crossing point first takes place a T₁ ESIPT to generate a T₁ enol tautomer, which can further decay to the S₀ state via T₁-to-S₀ ISC. Finally, the GSIPT occurs to back the system to the initial ketoB form S₀ state. Our present work could contribute to understanding the photophysics of MS and its derivatives.