Topological wetting states of microdroplets on closed-loop structured surfaces: Breakdown of the Gibbs equation at the microscale.

Microdroplets are a class of soft matter that has been extensively employed for chemical, biochemical, and industrial applications. However, fabricating microdroplets with largely controllable contact-area shape and apparent contact angle, a key prerequisite for their applications, is still a challenge. Here, by engineering a type of surface with homocentric closed-loop microwalls/microchannels, we can achieve facile size, shape, and contact-angle tunability of microdroplets on the textured surfaces by design. More importantly, this class of surface topologies (with universal genus value = 1) allows us to reveal that the conventional Gibbs equation (widely used for assessing the edge effect on the apparent contact angle of macrodroplets) seems no longer applicable for water microdroplets or nanodroplets (evidenced by independent molecular dynamics simulations). Notably, for the flat surface with the intrinsic contact angle ~0°, we find that the critical contact angle on the microtextured counterparts (at edge angle 90°) can be as large as >130°, rather than 90° according to the Gibbs equation. Experiments show that the breakdown of the Gibbs equation occurs for microdroplets of different types of liquids including alcohol and hydrocarbon oils. Overall, the microtextured surface design and topological wetting states not only offer opportunities for diverse applications of microdroplets such as controllable chemical reactions and low-cost circuit fabrications but also provide testbeds for advancing the fundamental surface science of wetting beyond the Gibbs equation.

Spontaneous Oxidation of Thiols and Thioether at the Air-Water Interface of a Sea Spray Microdroplet.

The transport of dissolved organic sulfur, including thiols and thioethers, from the ocean surface to the atmosphere through sea spray aerosol (SSA) is of great importance for the global sulfur cycle. Thiol/thioether in SSA undergoes rapid oxidation that is historically linked to photochemical processes. Here, we report the discovery of a non-photochemical, spontaneous path of thiol/thioether oxidation in SSA. Among 10 investigated naturally abundant thiol/thioether, seven species displayed rapid oxidation in SSA, with disulfide, sulfoxide, and sulfone comprising the major products. We suggest that such spontaneous oxidation of thiol/thioether was mainly fueled by thiol/thioether enrichment at the air-water interface and generation of highly reactive radicals by the loss of an electron from ions (e.g., glutathionyl radical produced from ionization of deprotonated glutathione) at or near the surface of the water microdroplet. Our work sheds light on a ubiquitous but previously overlooked pathway of thiol/thioether oxidation, which could contribute to an accelerated sulfur cycle as well as related metal transformation (e.g., mercury) at ocean-atmosphere interfaces.

Accelerated Photolysis of H2O2 at the Air-Water Interface of a Microdroplet.

Photochemical homolysis of hydrogen peroxide (H2O2) occurs widely in nature and is a key source of hydroxyl radicals (·OH). The kinetics of H2O2 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 H2O2 photolysis at the air-water interface of microdroplets, with a rate 1.9 × 103 times faster than that in bulk water. Our simulations show that due to the trans quasiplanar conformational preference of H2O2 at the air-water interface compared to the bulk or gas phase, the absorption peak in the spectrum of H2O2 is significantly redshifted by 45 nm, corresponding to greater absorbance of photons in the sunlight spectrum and faster photolysis of H2O2. This discovery has great potential to solve current problems associated with ·OH-centered heterogeneous photochemical processes in aerosols. For instance, we show that accelerated H2O2 photolysis in microdroplets could lead to markedly enhanced oxidation of SO2 and volatile organic compounds.

Nature of excited-state dependent hydrogen bonds and their critical role in determining the photophysical properties of aromatic thioketones.

In this work, how the excited-state dependent hydrogen bond (H-bond) interactions control photophysical processes have been uncovered by accurate electronic structure calculations for the five lowest-lying states (S0, S1, S2, T1, and T2) of three aromatic thioketones and their isomers. The difference in the H-bond nature between S2 and S1 gives rise to ultrafast S2 → S1 internal conversion via the two-state conical intersection. Strong S2 fluorescence observed usually in thiocarbonyl compounds is absent in aromatic thioketones with intramolecular H-bonds. Meanwhile, the relatively weak H-bond interactions in S1 and T1 states make the S1, T2, and T1 states degenerate or quasi-degenerate. As a result, the T2 state acts as a relay and enables both forward S1 → T1 and reverse T1 → S1 processes to occur efficiently, which provides new insights into the mechanism of thermally activated delayed fluorescence (TADF), and could be used to improve the design principle of purely organic TADF materials.

Efficient exploration of complex free energy landscapes by stepwise multi-subphase space metadynamics.

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.

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.

Time-Dependent Afterglow Color in a Single-Component Organic Molecular Crystal.

An organic crystal of 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (pCBP) exhibits time-dependent afterglow color from blue to orange over 1 s. Both experimental and computational data confirm that the color evolution results from well-separated, long-persistent thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence (RTP) with different but comparable decay rates. TADF is enabled by a small S1 -T1 energy gap of 0.7 kcal mol-1 . The good separation of TADF and RTP is due to a 11.8 kcal mol-1 difference in the S0 energies of the S1 and T1 structures, indicating that apart from the excited-state properties, tuning the ground state is also important for luminescence properties. This afterglow color evolution of pCBP allows its applications in anticounterfeiting and data encryption with high security levels.

QM/MM Studies on the Photophysical Mechanism of a Truncated Octocrylene Model.

Methyl 2-cyano-3,3-diphenylacrylate (MCDPA) shares the same molecular skeleton with octocrylene (OCR) that is one of the most common molecules used in commercially available sunscreens. However, its excited-state relaxation mechanism is unclear. Herein, we have used the QM(CASPT2//CASSCF)/MM method to explore spectroscopic properties, geometric and electronic structures, relevant conical intersections and crossing points, and excited-state relaxation paths of MCDPA in methanol solution. We found that in the Franck-Condon (FC) region, the V(1ππ*) state is energetically lower than the V'(1ππ*) state only by 2.8 kcal/mol and is assigned to experimentally observed maximum absorption band. From these two initially populated singlet states, there exist three nonradiative relaxation paths to repopulate the S0 state. In the first one, when the V(1ππ*) state is populated in the FC region, the system diabatically evolves along the V(1ππ*) state into its minimum where the internal conversion to S0 occurs. In the second one, the V'(1ππ*) state is populated in the FC region and the system adiabatically overcomes a barrier of ca. 3.0 kcal/mol to approach the V(1ππ*) minimum eventually leading to a V(1ππ*)-to-S0 internal conversion. In the third one, the V'(1ππ*) state first hops via the intersystem crossing to the T2 state, which then decays through the internal conversion to the T1 state. The T1 state is finally converted to the S0 state via the T1/S0 crossing point. Our present work contributes to understanding the photophysics of OCR and its variants.

Photophysics of a UV-B Filter 4-Methylbenzylidene Camphor: Intersystem Crossing Plays an Important Role.

4-Methylbenzylidene camphor (4MBC) is a frequently used ultraviolet (UV) filter in commercial sunscreens, which is experimentally found to undergo efficient intersystem crossing to triplet manifolds followed by predominant radiationless decay to the ground state. However, its photophysical mechanism is unclear. Herein, we have employed combined CASPT2 and CASSCF methods to study the spectroscopic properties, geometric and electronic structures, conical intersections and crossing points, and excited-state deactivation channels of 4MBC. We have found that the V(1 ππ*) state is populated with large probability in the Franck-Condon region. Starting from this state, there are two efficient nonradiative relaxation processes to populate the 3 ππ* state. In the first one, the V(1 ππ*) state decays to the V'(1 ππ*) state. The resultant V'(1 ππ*) state further jumps to the 1 nπ* state by internal conversion at the 1 ππ*/1 nπ* conical intersection. Then, the 1 nπ* state hops to the 3 ππ* state through an efficient 1 nπ*→3 ππ* intersystem crossing process. In the second one, the V(1 ππ*) state can diabatically relax along the photoisomerization reaction coordinate. In this process, a 1 ππ*/3 nπ* crossing point helps the 1 ππ* system decay to the 3 nπ* state, which further decays to the 3 ππ* state through internal conversion at the 3 nπ*/3 ππ* conical intersection. Once the 3 ππ* state is formed, a nearly barrierless relaxation path drives the 3 ππ* system to hop to the S0 state via the 3 ππ*/S0 crossing point. Our current work not only rationalizes recent experimental observations but also enriches our photophysical knowledge of UV filters.

QM/MM nonadiabatic dynamics simulations on photoinduced Wolff rearrangements of 1,2,3-thiadiazole.

The photoinduced rearrangement reaction mechanism of 1,2,3-thiadiazole remains experimentally elusive. Two possible mechanisms have been proposed to date. The first is a stepwise mechanism via a thiocarbene intermediate; the second is an excited-state concerted rearrangement mechanism. Herein we have adopted both the electronic structure calculations and nonadiabatic dynamics simulations to study the photoinduced rearrangement reactions of 1,2,3-thiadiazole in the S2, S1, and S0 states in solution. On the basis of QM(CASPT2)/MM [quantum mechanics(complete active space self-consistent field second-order perturbation theory)/molecular mechanics] calculations, we have found that (1) the thiocarbene intermediate is not stable; thus, the stepwise mechanism should be unfavorable; (2) the excited-state decay from the S2 via S1 to S0 state is ultrafast and completed within ca. 200 fs; therefore, both the S2 and S1 states should not have a long enough time for the excited-state rearrangements. Instead, we have computationally proposed a modified photoinduced rearrangement mechanism. Upon irradiation, the S2 state is first populated (114.0 kcal/mol), followed by an ultrafast S2 → S1 → S0 excited-state decay along the S-N bond fission, which eventually leads to a very "hot" intermediate with the S-N bond broken (18.3 kcal/mol). Then, thermal rearrangements to thioketene, thiirene, and ethynethiol occur in a concerted asynchronous way. This mechanistic scenario has been verified by full-dimensional trajectory-based nonadiabatic dynamics simulations at the QM(CASPT2)/MM level. Finally, our present computational work provides experimentally interesting mechanistic insights into the photoinduced rearrangement reactions of cyclic and acyclic diazo compounds.

Two-dimensional ice-like water adlayers on a mica surface with and without a graphene coating under ambient conditions.

Water tends to wet all hydrophilic surfaces under ambient conditions, and the first water adlayers on solids are important for a broad range of physicochemical phenomena and technological processes, including corrosion, wetting, lubrication, anti-icing, catalysis, and electrochemistry. Unfortunately, challenges in characterizing the first water adlayer in the laboratory have hampered molecular-level understanding of the contact water structure. Herein, we present the first ab initio molecular dynamics simulation evidence of a previously unreported ice-like adlayer structure (named as Ice-AL-II) on a prototype mica surface under ambient conditions. Calculation showed that the newly identified Ice-AL-II structure is more stable than the widely recognized ice-adlayer structure on mica surfaces (named as Ice-AL-I). Ice-AL-II exhibited a face-centered corner-cut tetragon (or a face-centered irregular pentagon) pattern of a hydrogen-bonded network. The center of the corner-cut tetragon was occupied by either a K+ cation or a water molecule with two H atoms pinned by the mica (100) via double hydrogen bonds. Our simulation also suggested that bilayer Ice-AL-II favors AA stacking rather than AB stacking. Interestingly, when a graphene sheet was coated on top of the ice-like adlayer, the stability of Ice-AL-II was further enhanced. In contrast, due to its strongly puckered structure, the Ice-AL-I structure could be crushed into a near-Ice-AL-II structure by the graphene coating. Ice-AL-II is thus proposed as a promising candidate for the ice-like structure on a mica surface detected by scanning polarization force microscopy and by atomic force microscopy between a graphene coating and a mica surface.

Mechanistic insight into the competition between interfacial and bulk reactions in microdroplets through N2O5 ammonolysis and hydrolysis.

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.

Thieno[3,4-d]pyrimidin-4(3H)-thione: an effective, oxygenation independent, heavy-atom-free photosensitizer for cancer cells.

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.

A mono-copper doped undeca-gold cluster with up-converted and anti-stokes emissions of fluorescence and phosphorescence.

We have synthesized single crystals of a highly stable Cu-doped undeca-gold cluster protected by both triphenylphosphine (PPh3) and 2-pyridinethiol (-SPy) ligands, formulated as [Au11Cu1(PPh3)7(SPy)3]+. This cluster (Au11Cu1 NCs for short) has a metallic core of C3v Au@Au10 with the Cu atom capped on one of the nine triangular facets and it is triply-coordinated to three N atoms of the SPy ligands of which the sulfur atom simultaneously binds to three adjacent Au atoms via singly-coordinated S-Au bonds, respectively. The other seven gold atoms form a crown structure by a link of three orthogons with common sides and are protected by seven PPh3 ligands. Besides the well-organized coordination, this Au11Cu1 nanocluster is demonstrated to exhibit superatom stability of the metallic core within 8 valence electrons (assuming that the 3 electrophilic-SPy ligands capture 3 electrons from the metal center). More interestingly, this Au11Cu1 nanocluster shows interesting emissions in both ultraviolet visible (UV-Vis) and near infrared (NIR) regions, and the emissions display novel anti-Stokes up-conversion lasing characteristics. TD-DFT calculated UV-vis and emission spectra well reproduce the experimental results, shedding light on the nature of excitation states and underlying mechanism of electronic transitions between diverse energy levels of such a monolayer-protected bimetallic cluster.

Excited-State Properties and Relaxation Pathways of Selenium-Substituted Guanine Nucleobase in Aqueous Solution and DNA Duplex.

The excited-state properties and relaxation mechanisms after light irradiation of 6-selenoguanine (6SeG) in water and in DNA have been investigated using a quantum mechanics/molecular mechanics (QM/MM) approach with the multistate complete active space second-order perturbation theory (MS-CASPT2) method. In both environments, the S1 1(nSeπ5*) and S2 1(πSeπ5*) states are predicted to be the spectroscopically dark and bright states, respectively. Two triplet states, T1 3(πSeπ5*) and T2 3(nSeπ5*), are found energetically below the S2 state. Extending the QM region to include the 6SeG-Cyt base pair slightly stabilizes the S2 state and destabilizes the S1, due to hydrogen-bonding interactions, but it does not affect the order of the states. The optimized minima, conical intersections, and singlet-triplet crossings are very similar in water and in DNA, so that the same general mechanism is found. Additionally, for each excited state geometry optimization in DNA, three kind of structures ("up", "down", and "central") are optimized which differ from each other by the orientation of the C═Se group with respect to the surrounding guanine and thymine nucleobases. After irradiation to the S2 state, 6SeG evolves to the S2 minimum, near to a S2/S1 conical intersection that allows for internal conversion to the S1 state. Linear interpolation in internal coordinates indicate that the "central" orientation is less favorable since extra energy is needed to surmount the high barrier in order to reach the S2/S1 conical intersection. From the S1 state, 6SeG can further decay to the T1 3(πSeπ5*) state via intersystem crossing, where it will be trapped due to the existence of a sizable energy barrier between the T1 minimum and the T1/S0 crossing point. Although this general S2 → T1 mechanism takes place in both media, the presence of DNA induces a steeper S2 potential energy surface, that it is expected to accelerate the S2 → S1 internal conversion.

Hydrogen-Bonding Interaction Regulates Photoisomerization of a Single-Bond-Rotation Locked Photoactive Yellow Protein Chromophore in Protein.

We have employed the QM(CASPT2//CASSCF)/MM method to explore the excited-state isomerization and decay mechanism of a single-bond-rotation locked photoactive yellow protein (PYP) chromophore in wild-type and mutant proteins. The S1 state is a spectroscopically bright state in the Franck-Condon region. In this state, there exist two excited-state isomerization pathways separately related to the clockwise and anticlockwise rotations of the C=C bond. The clockwise path is favorable because of a small barrier of 2 kcal/mol and uses a novel bicycle-pedal unidirectional photoisomerization mechanism in which the involved two dihedral angles rotate asynchronously because of the reinforced hydrogen-bonding interaction between the chromophore and Cys69. Near the twisted S1 minimum, the chromophore hops to the S0 state via the S1/S0 conical intersection. Finally, the R52A mutation has small effects on the excited-state properties and photoisomerization of the locked PYP chromophore. The present work provides new insights for understanding the photochemistry of PYP chromophores in protein surroundings.

MS-CASPT2 Studies on the Photophysics of Selenium-Substituted Guanine Nucleobase.

The MS-CASPT2 method has been employed to optimize minimum-energy structures of 6-selenoguanine (6SeGua) and related two- and three-state intersection structures in and between the lowest five electronic states, i.e., S2(1ππ*), S1(1 nπ*), T2(3 nπ*), T1(3ππ*), and S0. In combination with MS-CASPT2 calculated linearly interpolated internal coordinate paths, the photophysical mechanism of 6SeGua has been proposed. The initially populated S2(1ππ*) state decays to either S1(1 nπ*) or T2(3 nπ*) states through a three-state S2/S1/T2 intersection point. The large S2/T2 spin-orbit coupling of 435 cm-1, according to the classical El-Sayed rule, benefits the S2 → T2 intersystem crossing process. The S1(1 nπ*) state that stems from the S2 → S1 internal conversion process at the S2/S1/T2 intersection point can further jump to the T2(3 nπ*) state through the S1 → T2 intersystem crossing process. This process does not comply with the El-Sayed rule, but it is still related to a comparatively large spin-orbit coupling of 39 cm-1 and is expected to occur relatively fast. Finally, the T2(3 nπ*) state, which is populated from the above S2 → T2 and S1 → T2 intersystem crossing processes, decays to the T1(3ππ*) state via an internal conversion process. Because there is merely a small energy barrier of 0.11 eV separating the T1(3ππ*) minimum and an energetically allowed two-state T1/S0 intersection point, the T1(3ππ*) state still can decay to the S0 state quickly, which is also enhanced by a large T1/S0 spin-orbit coupling of 252 cm-1. Our proposed mechanism explains experimentally observed ultrafast intersystem crossing processes in 6SeGua and its 835-fold acceleration of the T1 state decay to the S0 state compared with 6tGua. Finally, we have found that the ground-state electronic structure of 6SeGua has more apparent multireference character.