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All-organic, heavy-atom-free photosensitizers based on thionation of nucleobases are receiving increased attention because they are easy to make, noncytotoxic, work both in the presence and absence of molecular oxygen, and can be readily incorporated into DNA and RNA. In this contribution, the DNA and RNA fluorescent probe, thieno[3,4-d]pyrimidin-4(1H)-one, has been thionated to develop thieno[3,4-d]pyrimidin-4(3H)-thione, which is nonfluorescent and absorbs near-visible radiation with about 60% higher efficiency. Steady-state absorption and emission spectra are combined with transient absorption spectroscopy and CASPT2 calculations to delineate the electronic relaxation mechanisms of both pyrimidine derivatives in aqueous and acetonitrile solutions. It is demonstrated that thieno[3,4-d]pyrimidin-4(3H)-thione efficiently populates the long-lived and reactive triplet state generating singlet oxygen with a quantum yield of about 80% independent of solvent. It is further shown that thieno[3,4-d]pyrimidin-4(3H)-thione exhibits high photodynamic efficacy against monolayer melanoma cells and cervical cancer cells both under normoxic and hypoxic conditions. Our combined spectroscopic, computational, and in vitro data demonstrate the excellent potential of thieno[3,4-d]pyrimidin-4(1H)-thione as a heavy-atom-free PDT agent and paves the way for further development of photosensitizers based on the thionation of thieno[3,4-d]pyrimidine derivatives. Collectively, the experimental and computational results demonstrate that thieno[3,4-d]pyrimidine-4(3H)-thione stands out as the most promising thiobase photosensitizer developed to this date.

Water microdroplets have been demonstrated to exhibit extraordinary chemical behaviors, including the abilities to accelerate chemical reactions by several orders of magnitude and to trigger reactions that cannot occur in bulk water. One of the most striking examples is the spontaneous generation of hydroxyl radical from hydroxide ions. Alcohols and alkoxide ions, being structurally similar to water and hydroxide ions, might exhibit similar behavior on microdroplets. Here, we report the spontaneous generation of alkoxide radicals from alcohols (RCH2OH) in aqueous microdroplets through quantum chemical calculations, quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations, ab initio MD simulations, and mass spectrometry. Our results show that an electric field (EF) on the order of 10−1 V/Å and partial solvation at the air‐water interface jointly promote the dissociation of RCH2OH into RCH2O− and H3O+ ions. QM/MM MD simulations indicate that RCH2O− can be ionized to produce RCH2O⋅ radicals on the microdroplet surface. Furthermore, partial solvation and the EF collaboratively catalyze the isomerization of the RCH2O⋅ radical into a more stable tautomer, R⋅CHOH. This study highlights the molecular mechanisms underlying the widespread generation of radicals at the microdroplet surface and provides insights into the importance of fundamental alcohol chemistry in the atmosphere. [ABSTRACT FROM AUTHOR]

We present an efficient method based on an extension of metadynamics for exploring complex free energy landscapes (FELs). The method employs two-step metadynamics simulations. In the first step, rapid metadynamics simulations using broad and tall Gaussians are performed to identify a free energy pathway (FEP) connecting the two states of interest. The FEP is then divided into a series of independent subphase spaces that comprise selected discrete images of the system. Using appropriate collective variables (CVs) chosen according to the FEP, the accurate FEL of each subphase space is separately calculated in subsequent divide-and-conquer metadynamics simulations with narrow and low Gaussians. Finally, all FELs calculated in each subphase space are merged to obtain the full FEL. We show that the method greatly improves the performance of the metadynamics approach. In particular, we are able to efficiently model chemical systems with complex FELs, such as chemical reactions at the air/water interface. We demonstrate the performance of this method on two model reactions: the hydrolysis of formaldehyde in the gas phase and at the air/water interface. [ABSTRACT FROM AUTHOR]

Reactive uptake of dinitrogen pentaoxide (N2O5) into aqueous aerosols is a major loss channel for NOx in the troposphere; however, a quantitative understanding of the uptake mechanism is lacking. Herein, a computational chemistry strategy is developed employing high-level quantum chemical methods; the method offers detailed molecular insight into the hydrolysis and ammonolysis mechanisms of N2O5 in microdroplets. Specifically, our calculations estimate the bulk and interfacial hydrolysis rates to be (2.3 ± 1.6) × 10−3 and (6.3 ± 4.2) × 10−7 ns−1, respectively, and ammonolysis competes with hydrolysis at NH3 concentrations above 1.9 × 10−4 mol L−1. The slow interfacial hydrolysis rate suggests that interfacial processes have negligible effect on the hydrolysis of N2O5 in liquid water. In contrast, N2O5 ammonolysis in liquid water is dominated by interfacial processes due to the high interfacial ammonolysis rate. Our findings and strategy are applicable to high-chemical complexity microdroplets.The authors report a computational strategy to simulate the hydrolysis and ammonolysis of N2O5 in aerosols using high-level quantum chemical methods. The computational results reveal a complete picture of the reactive uptake of N2O5 by atmospheric aerosols with or without NH3. [ABSTRACT FROM AUTHOR]

The activation of halogens (X = Cl, Br, I) by N 2 O 5 is linked to NO x sources, ozone concentrations, NO 3 reactivity, and the chemistry of halide-containing aerosol particles. However, a detailed chemical mechanism is still lacking. Herein, we explored the chemistry of the N 2 O 5 ···X - systems at the air-water interface. Two different reaction pathways were identified for the reaction of N 2 O 5 with X - at the air-water interface: the formation of XNO 2 or XONO, along with NO 3 - . In the case of the Cl - system, the ClNO 2 generation pathway is more favorable, while for the Br - and I - systems, the formation of BrONO and IONO is barrierless, making them the predominant products. Furthermore, the mechanisms of formation of X 2 from XNO 2 and XONO were also investigated. The high energy barriers of reactions and the high free energies of the products compared to those of the reactants indicate that ClNO 2 is stable at the air-water interface. Contrary to the widely held belief regarding X 2 producing from the reaction of XNO 2 with X - , our calculations demonstrate that BrONO and IONO initially form stable BrONO···Br - and IONO···I - complexes, which then subsequently react with Br - and I - to form Br 3 - and I 3 - , respectively. Finally, Br 3 - and I 3 - decompose to form Br 2 and I 2. These findings have significant implications for experimental interpretation and offer new insights into halogen cycling in the atmosphere.

Photochemical homolysis of hydrogen peroxide (H 2 O 2 ) occurs widely in nature and is a key source of hydroxyl radicals (·OH). The kinetics of H 2 O 2 photolysis play a pivotal role in determining the efficiency of ·OH production, which is currently mainly investigated in bulk systems. Here, we report considerably accelerated H 2 O 2 photolysis at the air-water interface of microdroplets, with a rate 1.9 × 10 3 times faster than that in bulk water. Our simulations show that due to the trans quasiplanar conformational preference of H 2 O 2 at the air-water interface compared to the bulk or gas phase, the absorption peak in the spectrum of H 2 O 2 is significantly redshifted by 45 nm, corresponding to greater absorbance of photons in the sunlight spectrum and faster photolysis of H 2 O 2 . This discovery has great potential to solve current problems associated with ·OH-centered heterogeneous photochemical processes in aerosols. For instance, we show that accelerated H 2 O 2 photolysis in microdroplets could lead to markedly enhanced oxidation of SO 2 and volatile organic compounds.