Supramolecular photochemistry of encapsulated caged ortho-nitrobenzyl triggers

ortho-Nitrobenzyl (oNB) triggers have been extensively used to release various molecules of interest. However, the toxicity and reactivity of the spent chromophore, o-nitrosobenzaldehyde, remains an unaddressed difficulty. In this study we have applied the well-established supramolecular photochemical concepts to retain the spent trigger o-nitrosobenzaldehyde within the organic capsule after release of water-soluble acids and alcohols. The sequestering power of organic capsules for spent chromophores during photorelease from ortho-nitrobenzyl esters, ethers and alcohols is demonstrated with several examples.National Science FoundationNational Science Foundation (NSF) [CHE-1807729]Kansas University Endowment AssociationFCT - Foundation for Science and TechnologyPortuguese Foundation for Science and Technology [UID/Multi/04326/2019, EMBRC.PT ALG-01-0145-FEDER-022121

Caged Molecules Released from Capsules: Photochemical and Photophysical Studies of Coumarin Triggers

This thesis comprises a detailed investigation on the photochemical and photophysical properties of various organic molecules encapsulated in molecular containers. Exploiting these photochemical and photophysical properties of fascinating organic molecules are often frustrating and tricky as they tend to get influenced by several environmental factors. For instance, in the solution phase, solvent can influence the properties of short-lived and reactive intermediates that are generated upon excitation. In order to control these properties, one of the promising approaches is to encapsulate them in an inert and confined environment. This strategy involves the encapsulation of the desired molecule in a stable enclosure, which subsequently drives the distribution of reactive intermediates into a variety of new environments or media. This avenue opens up new vistas and offers tremendous opportunities for systematic exploration of environmental effects on the photochemical and photophysical properties of organic molecules and enables their mechanistic investigations. In addition, the inert medium offers an additional advantage such as extending the lifetime of reactive intermediates. Furthermore, the enclosed medium reduces the relaxation process that usually occurs in the solvent phase. In this thesis a molecular container with a deep cavity called octa acid (OA) was exploited as host for various guest molecules such as 7-methoxy and 7-diethyl amino coumaryl esters, o-nitrobenzyl esters and their photochemical behavior has been investigated subsequently. Further, BODIPY derivatives and oilgothiophenes molecules were also used as guest molecules to investigate their photophysical properties. Remarkably, when in an aqueous environment, two of the host (OA) molecules can self-assemble each other to form a capsule by encapsulating a guest molecule inside them. The inner cavity of the OA capsule is hydrophobic and inert in nature, which is favorable to conduct and control various photochemical and photophysical reactions of insoluble guest molecules in water. Especially, studying the photochemical properties of coumaryl photocages within OA capsule provided a pathway to radical-derived products inside the OA capsule and ion-derived products outside of it. We also synthesized water-soluble cavitand functionalized gold nanoparticles and explored the photophysical properties of AuNPs surface assisted small organic molecules.</p

Melding caged compounds with supramolecular containers: photogeneration and miscreant behavior of the coumarylmethyl carbocation

By merging well-established concepts of supramolecular chemistry, protecting group strategy, and photochemistry, we have solubilized in water hydrophobic organic molecules consisting of a photoactive protecting group and masked carboxylic acids, released the desired acid, and confined a reactive carbocation intermediate within a capsule. Confinement of the photogenerated carbo cation brought out the latent radical-like behavior. This observation is consistent with the recent theoretical prediction of the 7-(diethylamino)coumarinyl-4-methyl carbocation having a triplet diradical ground-state electronic contribution.National Science Foundation [CHE-1411458]FCT-Foundation for Science and Technology [UID/Multi/04326/2013]Kansas University Endowment Associationinfo:eu-repo/semantics/publishedVersio

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. In our study, the two sets of coumarins serve different purposes: one is employed to explore electron transfer across the capsule and the other to release photoprotected acids into the aqueous environment. The capsule is made up of two molecules of octa acid (OA) and is soluble in an aqueous medium under slightly basic conditions. Molecular modeling studies revealed that while the OA capsule is fully closed with no access to water in the case of smaller coumarins, with the larger molecules, the capsule is not tight and the guest is in contact with water molecules, the number being dependent on the size of the coumarin. We have used the ultrafast time-dependent Stokes shift method to understand the solvent dynamics around the above guest molecules encapsulated within an OA capsule in an aqueous medium. Results depict that for the smaller sets of coumarins, water cannot access the guests within the OA cavity during their excited state lifetime. However, the case is completely different for the larger coumaryl esters. Distorted capsule structure exposes the guest to water, and a dynamics Stokes shift is observed. The average solvation time decreases with the increasing size of guests that clearly indicates accessibility of the encapsulated guests toward greater number of water molecules as the capsule structure distorts with increasing size of the guests. Results of the ultrafast solvation dynamics are consistent with that of molecular dynamics simulation

Photorelease of incarcerated caged acids from hydrophobic coumaryl esters into aqueous solution

Photolysis of aqueous solutions of carboxylic acid esters of 7-(methoxycoumaryl)-4-methanol included within the capsule made up of two molecules of octaacid released the acids in water. The trigger 7-(methoxycoumaryl)-4-methyl chromophore remains within octaacid either as the alcohol or as an adduct with the host octaacid through a hydrogen abstraction process. The method established here offers a procedure to release hydrophobic acid molecules in water at will in a timely manner with light. In addition, the system offers an unanticipated opportunity to probe the mechanistic dichotomy of a diradicaloid intermediate expressing both radical and ionic behavior when generated by coumarylmethyl ester photolysis in a hydrophobic environment

Synthesis, Characterization, Guest Inclusion, and Photophysical Studies of Gold Nanoparticles Stabilized with Carboxylic Acid Groups of Organic Cavitands

Water-soluble gold nanoparticles (AuNP) stabilized with cavitands having carboxylic acid groups have been synthesized and characterized by a variety of techniques. Apparently, the COOH groups similar to thiol are able to prevent aggregation of AuNP. These AuNP were stable either as solids or in aqueous solution. Most importantly, these cavitand functionalized AuNP were able to include organic guest molecules in their cavities in aqueous solution. Just like free cavitands (e.g., octa acid), cavitand functionalized AuNP includes guests such as 4,4′-dimethylbenzil and coumarin-1 through capsule formation. The exact structure of the capsular assembly is not known at this stage. Upon excitation there is communication between the excited guest present in the capsule and gold atoms and this results in quenching of phosphorescence from 4,4′-dimethylbenzil and fluorescence from coumarin-1