Dynamic covalent and noncovalent assembly of -nitrosocumene in organic solvents and water.

-Nitrosocumene (-NC) exhibits dynamic N,N bonding, interchanging monomer and /-azodioxide dimer structures, the extent of which depends on the environment. As a solid, -NC is a Z-dimer; in organic solvent, the monomer is favored; and in water, dimers are favored. A supramolecular assembly of -NC is observed as a separate species by NMR in water, shown to be a novel nanometer-sized aggregate containing ∼2000 molecules.

A supramolecular approach towards the photorelease of encapsulated caged acids in water: 7-diethylaminothio-4-coumarinyl molecules as triggers.

Herein, we establish the release of aliphatic acids in water upon excitation of 7-diethylaminothio-4-coumarinyl derivatives encapsulated within the organic host octa acid (OA). The 7-diethylaminothio-4-coumarinyl skeleton, employed here as the trigger, photoreleases caged molecules from the excited triplet state, in contrast to its carbonyl analogue, where the same reaction is known to occur from the excited singlet state. Encapsulation in OA solubilizes molecules in water that are otherwise water-insoluble, and retains the used trigger within itself following the release of the aliphatic acid.

Photochemical Oxidation of Polyethylene Terephthalate Microplastics Adsorbed on Sand and Silica Surfaces.

The environmental contamination by plastics, microplastics, and related compounds is a major concern. While the detection and release of micro- and nanoparticles from these materials have been widely studied, the formation and release of molecules resulting from their degradation in the environment have been overlooked. This work presents a study of the products released from poly(ethylene terephthalate) (PET) irradiated as pure particles and adsorbed on silica and sand surfaces under different irradiation conditions.

Ultrafast Processes in Upper Excited Singlet States of Free and Caged 7-Diethylaminothiocoumarin.

The photochemistry and photophysics of thiocarbonyl compounds, analogues of carbonyl compounds with sulfur, have long been overshadowed by their counterparts. However, recent interest in visible light reactions has reignited attention toward these compounds due to their unique excited-state properties. This study delves into the ultrafast dynamics of 7-diethylaminothiocoumarin (TC1), a close analogue of the well-known probe molecule coumarin 1 (C1), to estimate intersystem crossing rates, understand the mechanisms of fluorescence and phosphorescence, and evaluate TC1's potential as a solvation dynamics probe.

Structure, Dynamics, and Reactivity of Encapsulated Molecules in Restricted Spaces: Arylazoisoxazoles within an Octa Acid Capsule.

In this study, a well-defined organic capsule assembled from two octa acid (OA) molecules acting as host and select arylazoisoxazoles (AAIO) acting as guests were employed to demonstrate that confined molecules have restricted freedom that translates into reaction selectivity in both ground and excited states. The behavior of these AAIO guests in confined capsules was found to be different from that found in both crystals, where there is very little freedom, and in isotropic solvents, where there is complete freedom. Through one-dimensional (1D) and two-dimensional (2D) H NMR spectroscopic experiments, we have established a relationship between structure, dynamics and reactivity of molecules confined in an OA capsule.

Environmental Markers of Plastics and Microplastics.

The slow reaction rates to chemical and photochemical degradation are well-known properties of plastics. However, large plastic surfaces exposed to environmental conditions release particles and compounds that affect ecosystems and human health. The aim of this work was to identify compounds associated with the degradation of polyethylene (PE), polystyrene (PS), and polyvinyl chloride (PVC) microplastics (markers) on silica and sand and evaluate their use to screen microplastics on natural sand.

Application of molecular dynamic simulations in modeling the excited state behavior of confined molecules.

Relative to isotropic organic solvent medium, the structure and conformation of a reactant molecule in an organized and confining medium are often different. In addition, because of the rigidity of the immediate environment, the reacting molecule have a little freedom to undergo large changes even upon gaining energy or modifications in the electronic structure. These alterations give rise to differences in the photochemistry of a molecular and supramolecular species.

Reversible Photoisomerization of Norbornadiene-Quadricyclane within a Confined Capsule.

With the desire to develop a sustainable green method to store and release solar energy via a chemical reaction, we have examined the well-investigated norbornadiene-quadricyclane (NBD-QC) system in water. In this context, we have employed octa acid (OA) as the host that forms a capsule in water. According to H NMR spectra and diffusion constants, OA forms a stable 2:2 complex with both NBD and QC and 1:1:2 mixed complex in the presence of equal amounts of both NBD and QC.

Ultrafast Excited State Dynamics of Spatially Confined Organic Molecules.

This Feature Article highlights the role of spatial confinement in controlling the fundamental behavior of molecules. Select examples illustrate the value of using space as a tool to control and understand excited-state dynamics through a combination of ultrafast spectroscopy and conventional steady-state methods. Molecules of interest were confined within a closed molecular capsule, derived from a cavitand known as octa acid (OA), whose internal void space is sufficient to accommodate molecules as long as tetracene and as wide as pyrene.

Photochemistry in a capsule: controlling excited state dynamics confinement.

Exerting control on excited state processes has been a long-held goal in photochemistry. One approach to achieve control has been to mimic biological systems in Nature (, photosynthesis) that has perfected it over millions of years by performing the reactions in highly organized assemblies such as membranes and proteins by restricting the freedom of reactants and directing them to pursue a select pathway. The duplication of this concept at a smaller scale in the laboratory involves the use of highly confined and organized assemblies as reaction containers.

Keeping the name clean: [2 + 2] photocycloaddition.

Crossed [2 + 2] photocycloaddition is a specific case of intramolecular photocycloaddition reaction. Recently, the term "crossed [2 + 2] photocycloaddition" is interchangeably used to represent intermolecular [2 + 2] photocycloaddition reactions of two dissimilar double bonds/alkenes. To avoid confusion and to help researchers use the correct terminologies, this perspective clarifies the terminology used for different [2 + 2] photocycloaddition processes based on prior literature with the hope of establishing a standard for addressing the diverse set of photocycloaddition reactions that will be helpful to the chemical community.

Vibration-Assisted Intersystem Crossing in the Ultrafast Excited-State Relaxation Dynamics of Halocoumarins.

Due to its numerous applications, triplet formation and resulting phosphorescence remain a frontier area of research for over eight decades. Facile intersystem crossing (ISC) is the primary requirement for triplet formation and observation of phosphorescence. The incorporation of a heavy atom in molecules is one of the common approaches employed to facilitate ISC.

Organic Host Encapsulation Effects on Nitrosobenzene Monomer-Dimer Distribution and C-NO Bond Rotation in an Aqueous Solution.

The intermolecular (monomer-dimer equilibrium) and intramolecular (C-NO and C-NMe rotations) dynamics of 4-nitrosocumene () and 4-(,-dimethylamino)nitrosobenzene (), respectively, were found to be controlled by the medium (water) and the host environment (organic capsules and cavitands). The ability of water to shift the equilibrium toward the dimer appears to result from dipolar stabilization of the polar dimer structure and has a resemblance to water's known ability to favor organic cycloaddition reactions. In an aqueous medium, a range of organic hosts selectively include only the nitrosocumene monomer .

An Organic Capsule as a Matrix to Capture and Store Reactive Molecules at Room Temperature in Aqueous Solution: 7-cis-β-Ionone.

Low-temperature m atrix isolation method is the most popular one to generate and store reactive molecules and characterize them by in situ IR spectroscopy. Recognizing the need for a simpler method to trap and store such molecules and characterize by NMR spectroscopy at room temperature in solution, we have performed experiments exploring the value of water-soluble octa acid (OA) capsule as a storage vessel. The molecule we have chosen to illustrate the feasibility is the highly hindered 7-cis-β-ionone, which has been established to exist in equilibrium with its cyclic form with the later favored at room temperature.

Spatial confinement alters the ultrafast photoisomerization dynamics of azobenzenes.

Ultrafast transient absorption spectroscopy reveals new excited-state dynamics following excitation of -azobenzene (-Az) and several alkyl-substituted -Az derivatives encapsulated in a water-soluble supramolecular host-guest complex. Encapsulation increases the excited-state lifetimes and alters the yields of the → photoisomerization reaction compared with solution. Kinetic modeling of the transient spectra for unsubstituted -Az following nπ* and ππ* excitation reveals steric trapping of excited-state species, as well as an adiabatic excited-state → isomerization pathway for confined molecules that is not observed in solution.

Guest/Host Complexes of Octa Acid and Amphiphilic Benzylidene-3-methylimidazolidinones Exchange Hosts within the NMR Time Scale.

Cavitand octa acid (OA) is established to form a stable capsular assembly with one or two hydrophobic guest molecules (1:2 or 2:2 guest/host complex). Examples are known in which the guest molecule tumbles within the capsule without disrupting the structure of the capsuleplex. This process makes the two OA molecules that form the capsule magnetically equivalent.

Achiral Zeolites as Reaction Media for Chiral Photochemistry.

Obtaining enantiomerically-enriched photoproducts from achiral reactants has been a long-sought goal. The various methods developed to achieve chiral induction in photoproducts during the last fifty years still suffer from a lack of predictability, generality, and simplicity. With the current emphasis on green chemistry, obtaining enantiomerically enriched products via photochemistry is a likely viable alternative for the future.

Reversal of Regioselectivity during Photodimerization of 2-Anthracenecarboxylic Acid in a Water-Soluble Organic Cavitand.

The value of octa acid (OA) as a reaction vessel in steering a photoreaction toward a less favored product was established. Photodimerization of 2-anthracenecarboxylic acid within OA yields exclusively dimers, while in media such as solution, cyclodextrins (CD) and related hosts yield predominantly dimers. Further, OA enhances the chiral selectivity on the product dimers.

Ultrafast Solvation Dynamics Reveal that Octa Acid Capsule's Interior Dryness Depends on the Guest.

Coumarins are well-known to exhibit environment-dependent excited-state behavior. We have exploited this feature to probe the accessibility of solvent water molecules to coumarins (guest) encapsulated within an organic capsule (host). Two sets of coumarins, one small that fits well within the capsule and the other larger that fits within an enlarged capsule, are used as guests.

Visible-Light-Triggered Selective Intermolecular [2+2] Cycloaddition of Extended Enones: 2-Oxo-3-enoates and 2,4-Dien-1-ones with Olefins.

Photosensitization has recently re-emerged owing to the current interest in visible-light catalysis. One of the photoreactions investigated in this context, namely, photo[2+2]cycloaddition of olefins, is established to show high selectivity and wide generality. Here, we describe the results of our studies on selective intermolecular cycloaddition between extended enones (2,4-dien-1-ones and 2-oxo-3-enoates) and olefins under visible-light sensitization.

Modulation of Reduction Potentials of Bis(pyridinium)alkane Dications through Encapsulation within Cucurbit[7]uril.

Supramolecular modulation of reduction potentials of two series of bis(pyridinium)alkane salts is described. Study of the encapsulation of bis(pyridinium)alkane guests within the CB[7] cavity revealed the critical influence of the linker length and the position of the heteroatom on the reduction potentials of encapsulated guests. CB[7] complexation of pyridinium salts induced reduction potential changes ranging between +50 and -430 mV.

Ultrafast trans → cis Photoisomerization Dynamics of Alkyl-Substituted Stilbenes in a Supramolecular Capsule.

Ultrafast spectroscopy reveals the effects of confinement on the excited-state photoisomerization dynamics for a series of alkyl-substituted trans-stilbenes encapsulated in the hydrophobic cavity of an aqueous supramolecular organic host-guest complex. Compared with the solvated compounds, encapsulated trans-stilbenes have broader excited-state absorption spectra, excited-state lifetimes that are 3-4 times longer, and photoisomerization quantum yields that are 1.7-6.

Ultrafast Dynamics of Encapsulated Molecules Reveals New Insight on the Photoisomerization Mechanism for Azobenzenes.

Spatial confinement can have a profound impact on the dynamics of chemical reactions, especially for isomerization reactions that involve large-amplitude structural rearrangement of a molecule. This work uses ultrafast spectroscopy to probe the effects of confinement on trans → cis photoisomerization following ππ* excitation of 4-propyl stilbene and 4-propyl azobenzene encapsulated in a supramolecular host-guest complex. Transient absorption spectroscopy of the encapsulated azobenzene derivative reveals the formation of two distinct excited-state species with spectral signatures resembling the cis and trans isomers.

Chemo- and Regioselective Synthesis of Alkynyl Cyclobutanes by Visible Light Photocatalysis.

Here is the first visible light catalytic intermolecular cross [2 + 2] cycloaddition of enynes with alkenes to alkynyl cyclobutanes established with good functional group tolerance and high reaction efficiency and selectivity. Detailed studies reveal that enynes, including nonaromatic ones, can be sensitized by fac-Ir(ppy) via an energy transfer pathway. Addition of the Lewis acid PPhAuNTf enables the cross photo[2 + 2] cycloaddition reaction to take place under both direct visible light irradiation or sensitization by Ru(bpy)(PF).