Exploring the Mechanism of Fluorescence Quenching and Aggregation-Induced Emission of a Phenylethylene Derivative by QM (CASSCF and TDDFT) and ONIOM (QM:MM) Calculations

We report a QM (including TD-DFT and CASSCF) and ONIOM (QM:MM) study on the fluorescence quenching in methanol solution and fluorescence enhancement in crystal for a styrene derivative, namely 4-diethylamino-2 benzylidene malonic acid dimethyl ester (BIM) that possesses push–pull structure and AIE properties. The results showed that in methanol solution the weakening of ethylenic CC bond after photoexcitation initiates a barrierless relaxation via one-bond rotation around it, until the reactive molecule reaches a low-energy intermediate with strong charge-transfer character, then a S₁/S₀ conical intersection optimized near the charge-transfer intermediate is responsible for the fluorescence quenching in the dilute solution. The existences of charge-transfer intermediate as well as the conical intersection in the vicinity, which has not been observed in other symmetric (or less polar) phenylethylene-based luminophores, are the major features of BIM in solution. While in crystalline phase, the excited-state deactivation channels via torsional motions, either via one-bond rotation or via hula-twist mechanism, are restricted by steric hindrance and electrostatic repulsion from surrounding molecules, and thus fluorescence is enhanced

Theoretical Study of Trimethylacetic Acid Adsorption on CeO₂(111) Surface

Trimethylacetic acid (TMAA) adsorption on stoichiometric and oxygen-deficient CeO₂(111) surfaces was investigated using density functional theory that accounts for the on-site Coulomb interaction via a Hubbard term (DFT+U) and long-range dispersion correction. Both the molecular state and dissociative state (TMAA → TMA^– + H⁺) were identified on stoichiometric and oxygen-deficient CeO₂(111) surfaces. For the stoichiometric surface, two thermodynamically favorable configurations with adsorption energies of the order of −30 kcal/mol are identified; one is a molecule adsorption state, and the other one is a dissociative state. For the oxygen-deficient surface, dissociative states are more favorable than molecular states. The most favorable configuration is the dissociative adsorption of TMAA with the adsorption energy of the order of −77 kcal/mol. The dissociated TMA moiety takes the position of oxygen vacancy, forming three Ce–O bonds. The signature vibrational frequencies for these thermodynamically stable structures are reported as well as their electronic structures. The effects of long-range dispersion interactions are found to be negligible for geometries but important for adsorption energies

Effects of Protonation and C5 Methylation on the Electrophilic Addition Reaction of Cytosine: A Computational Study

The mechanism for the effects of protonation and C5 methylation on the electrophilic addition reaction of Cyt has been explored by means of CBS-QB3 and CBS-QB3/PCM methods. In the gas phase, three paths, two protonated paths (N3 and O2 protonated paths B and C) as well as one neutral path (path A), were mainly discussed, and the calculated results indicate that the reaction of the HSO₃^– group with neutral Cyt is unlikely because of its high activation free energy, whereas O2-protonated path (path C) is the most likely to occur. In the aqueous phase, path B is the most feasible mechanism to account for the fact that the activation free energy of path B decreases compared with the corresponding path in the gas phase, whereas those of paths A and C increase. The main striking results are that the HSO₃^– group directly interacts with the C5C6 bond rather than the N3C4 bond and that the C5 methylation, compared with Cyt, by decreasing values of global electrophilicity index manifests that C5 methylation forms are less electrophilic power as well as by decreasing values of NPA charges on C5 site of the intermediates make the trend of addition reaction weaken, which is in agreement with the experimental observation that the rate of 5-MeCyt reaction is approximately 2 orders of magnitude slower than that of Cyt in the presence of bisulfite. Apart from cis and trans isomers, the rare third isomer where both the CH₃ and SO₃ occupy axial positions has been first found in the reactions of neutral and protonated 5-MeCyt with the HSO₃^– group. Furthermore, the transformation of the third isomer from the cis isomer can occur easily

Lateral substituent effects on UV stability of high-birefringence liquid crystals with the diaryl-diacetylene core: DFT/TD-DFT study

<p>To study the effect of the lateral substituents on the UV stability of high birefringence liquid crystals (LCs), computational chemistry was used to examine a series of high birefringence LCs based on a diphenyl-diacetylene (DPDA) central core, thiophene segments as elongated π-conjugated units and four electron-withdrawing groups (-F, -CF₃, -OCF₃, -CN) as lateral substituents. In the present study, geometry optimisations have been performed using the DFT/B3LYP/6-311G (d, p) method. Out of a series of functional and basis sets examined, the functional ωB97X-D and basis set 6-31G (d, p) are most successful in predicting charge transfer absorption. The theoretical study indicates that the enhancement of UV stability is related with the types, numbers and positions of the lateral substituents. The calculated results indicate that the electron-withdrawing groups can shorten triple bond length, decrease energy gap value and increase the absorption maxima of the high-Δ<i>n</i> LCs, which is beneficial for good UV stability. With the introduction of increasing lateral electron-withdrawing substituent numbers, the DPDA derivatives would further improve UV stability. This work may provide an effective solution for the obstacle existed in the high-Δ<i>n</i> LCs with DPDA structures and pave a way for their applications in LC photonics.</p

Photochemistry of the Simplest Criegee Intermediate, CH₂OO: Photoisomerization Channel toward Dioxirane Revealed by CASPT2 Calculations and Trajectory Surface-Hopping Dynamics

The photochemistry of Criegee intermediates plays a significant role in atmospheric chemistry, but it is relatively less explored compared with their thermal reactions. Using multireference CASPT2 electronic structure calculations and CASSCF trajectory surface-hopping molecular dynamics, we have revealed a dark-state-involved <i>A</i>¹A → <i>X</i>¹A photoisomerization channel of the simple Criegee intermediate (CH₂OO) that leads to a cyclic dioxirane. The excited molecules on the <i>A</i>¹A state, which can have either originated from the <i>B</i>¹A state via <i>B</i>¹A → <i>A</i>¹A internal conversion or formed by state-selective electronic excitation, is driven by the out-of-plane motion toward a perpendicular <i>A</i>/<i>X</i>¹A minimal-energy crossing point (MECI) then radiationless decay to the ground state with an average time constant of ∼138 fs, finally forming dioxirane at ∼254 fs. The dynamics starting from the <i>A</i>¹A state show that the quantum yield of photoisomerization from the simple Criegee intermediate to dioxirane is 38%. The finding of the <i>A</i>¹A → <i>X</i>¹A photoisomerization channel is expected to broaden the reactivity profile and deepen the understanding of the photochemistry of Criegee intermediates

Competition between HO₂ and H₂O₂ Reactions with CH₂OO/<i>anti</i>-CH₃CHOO in the Oligomer Formation: A Theoretical Perspective

Understanding Criegee chemistry has become one of the central topics in atmospheric studies recently. Ozonolysis of unsaturated hydrocarbons is believed to be an important pathway of secondary organic aerosol (SOA). However, the SOA formation mechanisms via Criegee chemistry are still poorly understood. Here, we systematically study the competition between HO₂ and H₂O₂ reactions with CH₂OO/<i>anti</i>-CH₃CHOO in the oligomer formations. The calculated results show that oligomers having Criegee intermediates as the chain units are produced by the sequential addition of Criegee intermediates (CIs) to HO₂ and H₂O₂ molecules. The addition reactions are predicted to be strongly exothermic, and the apparent activation barriers are estimated to be negative, suggesting that these reactions are feasible both thermochemically and dynamically. Compared to the barriers of 4CH₂OO + HO₂ and 4CH₂OO + H₂O₂ reactions, it can be found that the first two CH₂OO addition reactions in the former case are favored, while the last two CH₂OO addition reactions in the latter case are preferable. A similar conclusion is also obtained from those of the 4<i>anti</i>-CH₃CHOO + HO₂/H₂O₂ systems. The mechanistic insights can motivate future experimental studies of the effect of longer-chain CIs on the formation of SOA, which plays an important role on air quality and climate change

Data File 1: Improving UV stability of tolane-liquid crystals in photonic applications by the ortho fluorine substitution

Phase transition temperatures (T/ oC) and associated transition enthalpy values (kJ mol-1) in parentheses for compounds 3Fn and 4Fn. Originally published in Optical Materials Express on 01 January 2016 (ome-6-1-97

Photochemistry of the Simplest Criegee Intermediate, CH₂OO: Photoisomerization Channel toward Dioxirane Revealed by CASPT2 Calculations and Trajectory Surface-Hopping Dynamics

The photochemistry of Criegee intermediates plays a significant role in atmospheric chemistry, but it is relatively less explored compared with their thermal reactions. Using multireference CASPT2 electronic structure calculations and CASSCF trajectory surface-hopping molecular dynamics, we have revealed a dark-state-involved <i>A</i>¹A → <i>X</i>¹A photoisomerization channel of the simple Criegee intermediate (CH₂OO) that leads to a cyclic dioxirane. The excited molecules on the <i>A</i>¹A state, which can have either originated from the <i>B</i>¹A state via <i>B</i>¹A → <i>A</i>¹A internal conversion or formed by state-selective electronic excitation, is driven by the out-of-plane motion toward a perpendicular <i>A</i>/<i>X</i>¹A minimal-energy crossing point (MECI) then radiationless decay to the ground state with an average time constant of ∼138 fs, finally forming dioxirane at ∼254 fs. The dynamics starting from the <i>A</i>¹A state show that the quantum yield of photoisomerization from the simple Criegee intermediate to dioxirane is 38%. The finding of the <i>A</i>¹A → <i>X</i>¹A photoisomerization channel is expected to broaden the reactivity profile and deepen the understanding of the photochemistry of Criegee intermediates

Tailor-Made pH-Responsive Poly(choline phosphate) Prodrug as a Drug Delivery System for Rapid Cellular Internalization

Rapid cellular uptake and efficient drug release in tumor cells are two of the major challenges for cancer therapy. Herein, we designed and synthesized a novel pH-responsive polymer–drug conjugate system poly­(2-(methacryloyloxy)­ethyl choline phosphate)-<i>b</i>-poly­(2-methoxy-2-oxoethyl methacrylate-hydrazide-doxorubicin) (PCP-Dox) to overcome these two challenges simultaneously. It has been proved that PCP-Dox can be easily and rapidly internalized by various cancer cells due to the strong interaction between multivalent choline phosphate (CP) groups and cell membranes. Furthermore, Dox, linked to the polymer carrier via acid-labile hydrazone bond, can be released from carriers due to the increased acidity in lysosome/endosome (pH 5.0–5.5) after the polymer prodrug was internalized into the cancer cells. The cell viability assay demonstrated that this novel polymer prodrug has shown enhanced cytotoxicity in various cancer cells, indicating its great potential as a new drug delivery system for cancer therapy

How To Make a Glycopeptide: A Synthetic Biology Approach To Expand Antibiotic Chemical Diversity

Modification of natural product backbones is a proven strategy for the development of clinically useful antibiotics. Such modifications have traditionally been achieved through medicinal chemistry strategies or via in vitro enzymatic activities. In an orthogonal approach, engineering of biosynthetic pathways using synthetic biology techniques can generate chemical diversity. Here we report the use of a minimal teicoplanin class glycopeptide antibiotic (GPA) scaffold expressed in a production-optimized Streptomyces coelicolor strain to expand GPA chemical diversity. Thirteen scaffold-modifying enzymes from 7 GPA biosynthetic gene clusters in different combinations were introduced into S. coelicolor, enabling us to explore the criteria for in-cell GPA modification. These include identifying specific isozymes that tolerate the unnatural GPA scaffold and modifications that prevent or allow further elaboration by other enzymes. Overall, 15 molecules were detected, 9 of which have not been reported previously. Some of these compounds showed activity against GPA-resistant bacteria. This system allows us to observe the complex interplay between substrates and both non-native and native tailoring enzymes in a cell-based system and establishes rules for GPA synthetic biology and subsequent expansion of GPA chemical diversity