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.

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 carbocation 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.

We report the synthesis and photophysical characterization of a triplet donor−acceptor dyad (3), which was constructed from a quinoidal naphthalene thioamide triplet sensitizer 1 and a perylene derivative 2 as the acceptor. The dyad of interest exhibits convoluted photophysical characteristics from the donor and acceptor chromophores. The kinetic analysis of the phosphorescence of 3 produced two lifetimes τ 1 = 1.1 ms and τ 2 = 20 ms. The shorter component was attributed to the phosphorescence lifetime of the sensitizer 3 (1)* moiety (within the dyad) and the longer lifetime can be assigned to either the triplet state of the acceptor 3 (2)* or a charge transfer (CT)* species. This result was further corroborated with time-resolved transient absorption studies which revealed the CT band in the transient spectrum of the dyad. In this picture, the 1•••2 interactions, which led to the formation of the CT species, was found to be the dominant photodynamics. Alternatively, dyad 3 was used to sensitize free perylene acceptor; the bimolecular 3 (3)* → perylene triplet energy transfer allowed triplet−triplet annihilation based photon upconversion to generate anti-Stokes photoluminescence of perylene using a 532 nm excitation source.

Photoinduced electron transfer across an organic capsular wall between excited donors and ground-state acceptors is established to occur with rate constants varying in the range 0.32–4.0 × 1011 s–1 in aqueous buffer solution. The donor is encapsulated within an anionic supramolecular capsular host, and the cationic acceptor remains closer to the donor separated by the organic frame through Coulombic attraction. Such an arrangement results in electron transfer proceeding without diffusion. Free energy of the reaction (ΔG°) and the rate of electron transfer show Marcus relation with inversion. From the plot, λ and V el were estimated to be 1.918 and 0.0058 eV, respectively. Given that the donor remains within the nonpolar solvent-free confined space, and there is not much change in the environment around the acceptor, the observed λ is believed to be because of “internal” reorganization rather than “solvent” reorganization. A similarity exists between the capsular assembly investigated here and glass and crystals at low temperature where the medium is rigid. The estimated electronic coupling (V el) implies the existence of interaction between the donor and the acceptor through the capsular wall. Existence of such an interaction is also suggested by 1H NMR spectra. Results of this study suggest that molecules present within a confined space could be activated from outside. This provides an opportunity to probe the reactivity and dynamics of radical ions within an organic capsule.

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.

We report the synthesis and photophysical characterization of π‐expanded quinoidal triplet chromophores which exhibit attractive light‐harvesting properties. The kinetic of the triplet excited state of quinoidal benzotetraphene 2 was found to be one order of magnitude higher than the lifetime of 3(1)* from the less conjugated parent chromophore 1. Furthermore, the evaluation of the optoelectronic properties indicates that π‐expansion helps narrow the optoelectronic band gap, but the influence of the additional aromatic rings in the structure of 2 and 3 compromises the stability of the p‐quinoidal ring. QDM 2 was isolated and fully characterized; however, it was found to rearomatize to a mixture of uncharacterized radical species.

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.

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