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Abstract In this study, the occurrence of Diels–Alder reaction of cyclopentadiene yielding dicyclopentadiene within a confined closed space provided by octa acid (OA) in water at room temperature is established. The Diels–Alder reaction within the OA capsule occurs at least 2000 times faster than in water. Catalysis of Diels–Alder reaction by hosts such as cyclodextrin, cucurbituril, and Fujita's Pd nano–host occurs in water. Despite their similarity, these three hosts provide an open environment where the reactant molecules are exposed to aqueous environment. The onlyfullyclosed host known to catalyze the Diels–Alder reaction in water is OA. Although Rebek's host is established to catalyze Diels–Alder reaction it occurs in an organic solvent. The closed environment explored in this presentation provides an opportunity to better understand the origin of non–covalent catalysis in a restricted space and in water. Because the product binds stronger than the reactant, disappointingly, the capsule can't be recycled. We recognize that this aspect needs to be addressed for the OA capsule to become synthetically useful. We are in the process of understanding the origin of catalysis and finding ways to make reaction recyclable. 

“Orientational isomerism” is a concept necessary for deeper understanding of the selective reactivities in a host-guest system. This concept has been rarely explored in the context of supramolecular host guest chemistry. We designed a model system including four cyclohexene derivatives and a water-soluble host Octa Acid (OA), with hydrophobic inner cavity. The overall length of the guest molecules (~ 12 Å) was limited by manipulating the alkyl substituents at 1- and 4-positions on the cyclohexene ring. 1D 1H /2D COSY and NOESY NMR and photooxygenation reaction were used to understand the observations with this model system. Specific packaging or “orientational isomerism” of each guest molecule, induced by the host OA led to specific, in one case enhanced product selectivity. With this model system we show the important role of “orientational isomerism” in explaining enhanced product selectivity in a host-guest supramolecular system. 

Thiocarbonyls exhibit unique photophysical properties, characterized by rapid intersystem crossing (ISC) due to favorable singlet−triplet energetics and enhanced spin−orbit coupling. However, the role of hydrogen bonding in modulating the ISC remains underexplored. This study investigates the effect of solvent−solute hydrogen bonding on the ISC dynamics of 7-(diethylamino)-4- methyl-2-sulfanylidene-2H-chromen-2-one (thiocoumarin 1, TC1) using steadystate and time-resolved spectroscopy, complemented by theoretical calculations. Experimental data reveal that in methanol, hydrogen bonding leads to increased fluorescence quantum yield, prolonged singlet-state lifetime, and reduced triplet yield compared to aprotic acetonitrile. Time-resolved spectroscopy identifies an additional long-lived emissive singlet state in methanol, attributed to a hydrogen-bonded state, which slows ISC. Theoretical calculations demonstrate that hydrogen bonding alters the electronic structure and constrains ISC along key nuclear coordinates, including the C S bond vibration and dihedral angles, leading to decreased triplet formation. These findings provide mechanistic insights into hydrogen-bonding-mediated control of ISC in thiocoumarins, with implications for designing functional materials with tunable photophysical properties. 

ndene, a hydrophobic molecule, exhibits complex behavior in water due to its tendency to aggregate. This study combines NMR spectroscopy, molecular dynamics simulations, and ab initio calculations to investigate indene’s dynamic interactions with monomeric and aggregated states. NMR results reveal dynamic chemical exchange between monomer and aggregate states, and further studies show a preferential aggregation pathway akin to Ostwald ripening. Molecular dynamics simulations provide insights into indene behavior in water and acetonitrile, with a pronounced preference for aggregation in water. Geometry optimization and thermochemistry calculations reveal the formation of stable dimers, with water favoring aggregation energetically. These findings advance our understanding of hydrophobic molecule behavior in water and have implications for organic compound–aqueous environment interactions and photochemistry research. 

Water is an under-appreciated reaction medium that has been shown to facilitate photodimerization reactions. Despite having a low formal concentration, hydrophobic nature of organic compounds could lead to higher local concentrations, thereby favoring their cycloaddition. In contrast, photodimerization reactions are unlikely to occur in organic solvents under similar conditions. This study explores the supramolecular assembly of small organic molecules in water, focusing on their role in promoting photodimerization reactions. NMR spectroscopy, molecular dynamics simulations, and ab initio calculations were used to examine the dynamic interactions between indene and its aggregated state in water. Quantum mechanical calculations suggest that the stacking of indene with an antiparallel-displaced orientation is the most stable configuration, and MD simulations support the role of water in promoting aggregation. NMR results confirm the existence of indene aggregates, and 2D NMR reveals dynamic exchange between monomer and aggregate states. The study elucidates the complex dynamics of indene aggregation and its impact on photodimerization, providing insights into designing other photoreactions in water. 

ortho-Nitrosocumene (o-NC) exhibits dynamic N,N bonding, interchanging monomer and E/Z-azodioxide dimer structures, the extent of which depends on the environment. As a solid, o-NC is a Z-dimer; in organic solvent, the monomer is favored; and in water, dimers are favored. A supramolecular assembly of o-NC is observed as a separate species by NMR in water, shown to be a novel nanometer-sized aggregate containing B2000 molecules. 

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. Such supramolecular characteristics usher in new features to the photorelease methodology. The thiocarbonyl chromophore extends the absorption of coumarinyl trigger to visible range while enhancing the intersystem crossing (ISC) to the triplet state, making it the reactive state. Despite the non-polar environment within the OA capsules the photocleavage occurs in a heterolytic fashion to release the conjugate base and the used trigger as triplet carbocation in an adiabatic process. Interestingly, the triplet carbocation crosses to the ground singlet surface (closed shell singlet carbocation) with the help of water molecules, possibly aided by C = S chromophore. Utilizing the known excited state dynamics of related thiocoumarinyl and coumarinyl systems, we have identified a few of the important mechanistic features of the photorelease process of 7-diethylaminothio-4-coumarinyl derivatives. Ultrafast excited state dynamic studies and quantum chemical calculations planned should help us better understand the photorelease process so as to effectively exploit the proposed system for potential applications. 

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 wellknown 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. Enclosing TC1 within an organic capsule indicates its potential applications, even in aqueous environments. Ultrafast studies reveal a dominant subpicosecond intersystem crossing process, indicating the importance of upper excited singlet and triplet states in the molecule’s photochemistry. The distinct fluorescence and phosphorescence origins, along with the presence of closely spaced singlet excited states, support the observed efficient intersystem crossing. The sulfur atom alters the excited-state behavior, shedding light on reactive triplet states and paving the way for further investigations. 

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) 1H NMR spectroscopic experiments, we have established a relationship between structure, dynamics and reactivity of molecules confined in an OA capsule. Introduction of CF3 and CH3 substitution at the 4-position of the aryl group of AAIO reveals that in addition to space confinement, weak interactions between the guest and the OA capsule control the dynamics and reactivity of guest molecules. 1H NMR studies revealed that there is a temperature-dependence to guest molecules tumbling (180° rotation along the capsular short axis) within an OA capsule. While 1H NMR points to the occurrence of tumbling motion, MD simulations and simulation of the temperature-dependent NMR signals provide an insight into the mechanism of tumbling within OA capsules. Thermal and photochemical isomerization of AAIO were found to occur within an OA capsule just as in organic solvents. The observed selectivity noted during thermal and photo induced isomerization of OA encapsulated AAIOs can be qualitatively understood in terms of the well-known concepts due to Bell−Evans− Polanyi (BEP principle), Hammond and Zimmerman.