Quest for the Most Aromatic Pathway in Charged Expanded Porphyrins.

Despite the central role of aromaticity in the chemistry of expanded porphyrins, the evaluation of aromaticity remains difficult for these extended macrocycles. The presence of multiple conjugation pathways and different planar and nonplanar π-conjugation topologies makes the quantification of global and local aromaticity even more challenging. In neutral expanded porphyrins, the predominance of the aromatic conjugation pathway passing through the imine-type nitrogens and circumventing the amino NH groups is established.

Enhancing Dual-State Emission in Maleimide Fluorophores through Fluorocarbon Functionalisation.

Herein, a library of trifluoroethyl substituted aminomaleimide derivatives are reported with small size and enhanced emissions in both solution and solid-state. A diCH CF substituted aminochloromaleimide exhibits the most efficient dual-state emission (Φ >50 % in solution and solid-state), with reduced quenching from protic solvents. This is attributed to the reduction of electron density on the maleimide ring and suppressed π-π stacking in the solid-state.

Reply to the Correspondence on "How Aromatic Are Molecular Nanorings? The Case of a Six-Porphyrin Nanoring".

A recent article by Anderson and co-workers challenges our conclusions on the aromaticity of the four oxidation states of a butadyine-linked six-porphyrin nanoring, based on the experimental H-NMR data and some recent calculations they have performed using the BLYP35 functional. Here, we show that BLYP35 should be taken with caution and demonstrate that the indirect evidence of a ring current from experimental H-NMR data is not a definite proof of aromaticity.

Rigidochromism by imide functionalisation of an aminomaleimide fluorophore.

Fluorescent dyes that exhibit high solid state quantum yields and sensitivity to the mechanical properties of their local environment are useful for a wide variety of applications, but are limited in chemical diversity. We report a trityl-functionalised maleimide that displays rigidochromic behaviour, becoming highly fluorescent when immobilised in a solid matrix, while displaying negligible fluorescence in solution. Furthermore, the dye's quantum yield is shown to be sensitive to the nature of the surrounding matrix.

How Aromatic Are Molecular Nanorings? The Case of a Six-Porphyrin Nanoring*.

Large conjugated rings with persistent currents are novel promising structures in molecular-scale electronics. A six-porphyrin nanoring structure that allegedly sustained an aromatic ring current involving 78π electrons was recently synthesized. We provide here compelling evidence that this molecule is not aromatic, contrary to what was inferred from the analysis of H-NMR data and computational calculations that suffer from large delocalization errors.

Fluorescent Imidazo[1,2-a]pyrimidine Compounds as Biocompatible Organic Photosensitizers that Generate Singlet Oxygen: A Potential Tool for Phototheranostics.

Photodynamic therapy has been used to treat a variety of diseases, however, there is continuing search for new biocompatible photosensitizers. Herein, we demonstrate for the first time that imidazo[1,2-a]pyrimidine compounds are able to generate singlet oxygen species and can act as photosensitizers in the intracellular environment. Our results show that this class of compounds absorb and emit in the 400-500 nm region, present low cytotoxicity in the dark, are efficiently uptaken by cells, are fluorescent in intracellular medium, and generate singlet oxygen upon irradiation, killing cancer cells within 2 h at low concentration (2.

Manipulating the fluorescence lifetime at the sub-cellular scale via photo-switchable barcoding.

Fluorescent barcoding is a pivotal technique for the investigation of the microscale world, from information storage to the monitoring of dynamic biochemical processes. Using fluorescence lifetime as the readout modality offers more reproducible and quantitative outputs compared to conventional fluorescent barcoding, being independent of sample concentration and measurement methods. However, the use of fluorescence lifetime in this area has been limited by the lack of strategies that provide spatiotemporal manipulation of the coding process.

How do the Hückel and Baird Rules Fade away in Annulenes?

Two of the most popular rules to characterize the aromaticity of molecules are those due to Hückel and Baird, which govern the aromaticity of singlet and triplet states. In this work, we study how these rules fade away as the ring structure increases and an optimal overlap between orbitals is no longer possible due to geometrical restrictions. To this end, we study the lowest-lying singlet and triplet states of neutral annulenes with an even number of carbon atoms between four and eighteen.

Discovering Biomolecules with Activity: Designed Repeat Proteins as Biocatalysts for (3 + 2) Cycloadditions.

Designed repeat proteins catalyze the 1,3-dipolar reaction between an imine and a π-deficient dipolarophile in THF solution to form unnatural nitroproline esters, a reaction that no enzyme can catalyze. NMR studies and mutation experiments show that both acidic and basic residues can catalyze the reaction. The diastereocontrol of the reaction depends on the flexibility of the protein and on the number and location of the active lysine and glutamate residues, which can participate independently or forming dyads that promote the formation of unusual diastereomeric cycloadducts.

Rational design of substituted maleimide dyes with tunable fluorescence and solvafluorochromism.

A series of maleimide derivatives were systematically designed and synthesized with tunable fluorescent properties. The facile modifications herein provide a simple methodology to expand the scope of maleimide-based dyes and also provide insight into the relationship between substitution pattern and optical properties.

New electron delocalization tools to describe the aromaticity in porphyrinoids.

The role of aromaticity in porphyrinoids is a current subject of debate due to the intricate structure of these macrocycles, which can adopt Hückel, Möbius and even figure-eight conformers. One of the main challenges in these large π-conjugated structures is identifying the most conjugated pathway because, among aromaticity descriptors, there are very few that can be applied coherently to this variety of conformers. In this paper, we have investigated the conjugated pathways in nine porphyrinoid compounds using several aromaticity descriptors, including BLA, BOA, FLU and HOMA, as well as the recently introduced AV1245 and AV indices.

Density Functional Theory Study on the Demethylation Reaction between Methylamine, Dimethylamine, Trimethylamine, and Tamoxifen Catalyzed by a Fe(IV)-Oxo Porphyrin Complex.

In this work, we studied computationally the N-demethylation reaction of methylamine, dimethylamine, and trimethylamine as archetypal examples of primary, secondary, and tertiary amines catalyzed by high-field low-spin Fe-containing enzymes such as cytochromes P450. Using DFT calculations, we found that the expected C-H hydroxylation process was achieved for trimethylamine. When dimethylamine and methylamine were studied, two different reaction mechanisms (C-H hydroxylation and a double hydrogen atom transfer) were computed to be energetically accessible and both are equally preferred.

Relevance of the DFT method to study expanded porphyrins with different topologies.

Meso-aryl expanded porphyrins present a structural versatility that allows them to achieve different topologies with distinct aromaticities. Several studies appeared in the literature studying these topological switches from an experimental and theoretical point of view. Most of these publications include density functional theory calculations, being the B3LYP the most used methodology.

Angular momentum-induced delays in solid-state photoemission enhanced by intra-atomic interactions.

Attosecond time-resolved photoemission spectroscopy reveals that photoemission from solids is not yet fully understood. The relative emission delays between four photoemission channels measured for the van der Waals crystal tungsten diselenide (WSe) can only be explained by accounting for both propagation and intra-atomic delays. The intra-atomic delay depends on the angular momentum of the initial localized state and is determined by intra-atomic interactions.

Two-State Reactivity of Histone Demethylases Containing Jumonji-C Active Sites: Different Mechanisms for Different Methylation Degrees.

The N-demethylation reactions of N,N,N-trimethylpropan-1-ammonium and N,N-dimethyl- and N-methylpropan-1-aminium cations in the presence of [(AcO) (imidazole) (H O)Fe=O] complex have been studied by density functional theory. These transformations are suitable models for the N-demethylation of tri-, di-, and monomethylated lysine residues of histones in the presence of Jumonji-C containing histone demethylases. It has been found that the N-demethylation reaction is stepwise and occurs on triplet and quintet potential energy hypersurfaces.

Carbon Dioxide Capture and Release by Anions with Solvent-Dependent Behaviour: A Theoretical Study.

Recently, Clyburne and co-workers [Science, 2014, 344, 75-78] reported the novel synthesis of the elusive cyanoformate anion, NCCO . The stability of this anion is dependent on the dielectric constant of the local environment (polarity-switchable solvent): it is stable in low-polarity media and unstable in high-polarity solvents; hence, capturing and then releasing CO . The possibility of extending such behaviour to other anions is explored herein.

Carbon dioxide capture with the ozone-like polynitrogen molecule Li3N3.

In a very recent article (Chem.-Eur. J.

Effect of the meso-substituent in the Hückel-to-Möbius topological switches.

Expanded porphyrins have emerged as a new promising class of molecules for the creation of new Hückel-to-Möbius topological switches with distinct aromaticities and magnetic and electric properties. In this work, we report a theoretical investigation of the conformational switch between the Hückel planar and the singly twisted Möbius structure for eight different meso-substituted [28]-hexaphyrins (with different steric effects and electron-withdrawing and -releasing character). Our results show how a change in the nature of the meso-substituent is able to turn an endothermic interconversion process with a high energy barrier into an exothermic and almost barrierless Hückel-Möbius transition.

Sulfuric acid as autocatalyst in the formation of sulfuric acid.

Sulfuric acid can act as a catalyst of its own formation. We have carried out a computational investigation on the gas-phase formation of H(2)SO(4) by hydrolysis of SO(3) involving one and two water molecules, and also in the presence of sulfuric acid and its complexes with one and two water molecules. The hydrolysis of SO(3) requires the concurrence of two water molecules, one of them acting as a catalyzer, and our results predict an important catalytic effect, ranging between 3 and 11 kcal·mol(-1) when the catalytic water molecule is substituted by a sulfuric acid molecule or one of its hydrates.

Evaluation of the nonlinear optical properties for an expanded porphyrin Hückel-Möbius aromaticity switch.

The conformational flexibility of the expanded porphyrins allows them to achieve different topologies with distinct aromaticities and nonlinear optical properties (NLOP). For instance, it is possible to switch between Möbius and Hückel topologies applying only small changes in the external conditions or in the structure of the ring. In this work, we evaluate the electronic and vibrational contributions to static and dynamic NLOP of the Hückel and Möbius conformers of A,D-di-p-benzi[28]hexaphyrin(1.

Is the HO4- anion a key species in the aqueous-phase decomposition of ozone?

The role of the HO(4)(-) anion in atmospheric chemistry and biology is a matter of debate, because it can be formed from, or be in equilibrium with, key species such as O(3) + HO(-) or HO(2) + O(2) (-). The determination of the stability of HO(4)(-) in water therefore has the greatest relevance for better understanding the mechanism associated with oxidative cascades in aqueous solution. However, experiments are difficult to perform because of the short-lived character of this species, and in this work we have employed DFT, CCSD(T) complete basis set (CBS), MRCI/aug-cc-pVTZ, and combined quantum mechanics/molecular mechanics (QM/MM) calculations to investigate this topic.

Effects of a single water molecule on the OH + H2O2 reaction.

The effect of a single water molecule on the reaction between H(2)O(2) and HO has been investigated by employing MP2 and CCSD(T) theoretical approaches in connection with the aug-cc-PVDZ, aug-cc-PVTZ, and aug-cc-PVQZ basis sets and extrapolation to an ∞ basis set. The reaction without water has two elementary reaction paths that differ from each other in the orientation of the hydrogen atom of the hydroxyl radical moiety. Our computed rate constant, at 298 K, is 1.

Evaluation of the Nonlinear Optical Properties for Annulenes with Hückel and Möbius Topologies.

Recently, much attention has been focused on the design and synthesis of molecules with aromatic Möbius topology. One of the most promising applications is the manufacture of Hückel-to-Möbius topological optical switches with high nonlinear optical properties. In this work, we evaluate the electronic and vibrational contributions to static and dynamic nonlinear optical properties of the CS Hückel and C2 Möbius topologies synthesized by Herges and co-workers (Ajami, D.

Protonation of water clusters induced by hydroperoxyl radical surface adsorption.

We have investigated the HO(2) adsorption and acid dissociation process on the surface of (H(2)O)(20) and (H(2)O)(21) clusters by using quantum-chemistry calculations. Our results show that the radical forms a stable hydrogen-bond complex on the cluster. The HO(2) acid dissociation is more favorable in the case of the (H(2)O)(21) cluster, for which the inner water molecule plays a crucial role.