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. In this study, we have examined the dynamics of capsules that host amphiphilic benzylidene-3-methylimidazolidinone molecules as guests. In these capsuleplexes, although the guest does not tumble, the two OA molecules become magnetically equivalent because the two OA molecules that form the capsule exchange their positions in the NMR time scale. This is equivalent to the content of the capsule remaining stationary while the capsule swirls around it. Benzylidene-3-methylimidazolidinones form both 1:1 and 1:2 supramolecular complexes with cavitand OA. Two-dimensional NMR, ROESY, and NOESY data suggest that in a 300 ms time scale, the two halves of the capsule exchange between themselves and with free OA. The conclusion drawn here provides valuable information concerning the stability of the OA capsuleplex and cavitandplex that is used as the well-defined space to control the excited-state chemistry and dynamics of confined guest molecules.

Acid-Triggered, Acid-Generating, and Self-Amplifying Degradable Polymers.

We describe the 3-iodopropyl acetal moiety as a simple cleavable unit that undergoes acid catalyzed hydrolysis to liberate HI (p Ka ∼ -10) and acrolein stoichiometrically. Integrating this unit into linear and network polymers gives a class of macromolecules that undergo a new mechanism of degradation with an acid amplified, sigmoidal rate. This trigger-responsive self-amplified degradable polymer undergoes accelerated rate of degradation and agent release.

Photoisomerization and photooxygenation of 1,4-diaryl-1,3-dienes in a confined space.

Geometric isomerization of light-activated olefins plays a significant role in biological events as well as in modern materials science applications. In these systems, the isomerization occurs in highly confined spaces, and concepts derived from solution investigations are only partially applicable. This study makes contributions in understanding the excited-state behavior of olefins in confined spaces by investigating the excited-state behavior of 1,4-diphneyl-13-butadiene (DPB) and 1,4-ditolyl-1,3-butadiene (DTB) encapsulated in a well-defined organic capsule made up of the octa acid (OA) host. Both of these dienes that exist in three isomeric forms (trans,trans; trans,cis; and cis,cis) formed 1:2 guest-host complexes with OA in aqueous borate buffer. Competition experiments monitored by (1)H NMR signals revealed that among the three isomers the cis,cis isomer of DPB and DTB formed the most stable complex with OA. Molecular modeling studies suggested that all six isomers of DPB and DTB preferred the cisoid conformation within the OA capsule. Irradiation (>280 nm) of the diene-OA complex (diene@OA2) resulted in geometric isomerization, and the photostationary state consisted of cis,trans isomer as major and cis,cis as minor products. The photostationary state could be enriched with the cis,cis isomer in yields close to 70% with proper cutoff filters because the cis,cis isomer absorbs at shorter wavelength than the other two isomers. Consistent with the MD simulation prediction that trans,trans-DPB and trans,trans-DTB existed in cisoid conformation within OA capsule, the generation of singlet oxygen in the presence of OA encapsulated DPB or DTB resulted in facile [4 + 2] addition between the diene and the singlet oxygen.

A latent reaction in a model GFP chromophore revealed upon confinement: photohydroxylation of ortho-halo benzylidene-3-methylimidazolidiones via an electrocylization process.

Excited state behavior of halogen substituted model GFP chromophores was investigated in an acetonitrile solution and in a confined environment provided by an octa acid capsule in water. Of the ortho, meta, and para halogen substituted GFP chromophores only the ortho compounds gave a new product resulting from an unprecedented photosubstitution of halogens by the hydroxyl group. This unusual reaction highlights the importance of confined spaces in bringing about some unattainable photoreactions.

Copper(II)/tertiary amine synergy in photoinduced living radical polymerization: accelerated synthesis of ω-functional and α,ω-heterofunctional poly(acrylates).

Photoinduced living radical polymerization of acrylates, in the absence of conventional photoinitiators or dye sensitizers, has been realized in "daylight'"and is enhanced upon irradiation with UV radiation (λ(max) ≈ 360 nm). In the presence of low concentrations of copper(II) bromide and an aliphatic tertiary amine ligand (Me6-Tren; Tren = tris(2-aminoethyl)amine), near-quantitative monomer conversion (>95%) is obtained within 80 min, yielding poly(acrylates) with dispersities as low as 1.05 and excellent end group fidelity (>99%). The versatility of the technique is demonstrated by polymerization of methyl acrylate to a range of chain lengths (DP(n) = 25-800) and a number of (meth)acrylate monomers, including macromonomer poly(ethylene glycol) methyl ether acrylate (PEGA480), tert-butyl acrylate, and methyl methacrylate, as well as styrene. Moreover, hydroxyl- and vic-diol-functional initiators are compatible with the polymerization conditions, forming α,ω-heterofunctional poly(acrylates) with unparalleled efficiency and control. The control retained during polymerization is confirmed by MALDI-ToF-MS and exemplified by in situ chain extension upon sequential monomer addition, furnishing higher molecular weight polymers with an observed reduction in dispersity (D = 1.03). Similarly, efficient one-pot diblock copolymerization by sequential addition of ethylene glycol methyl ether acrylate and PEGA480 to a poly(methyl acrylate) macroinitiator without prior workup or purification is also reported. Minimal polymerization in the absence of light confers temporal control and alludes to potential application at one of the frontiers of materials chemistry whereby precise spatiotemporal "on/off" control and resolution is desirable.

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.

Release of guests from encapsulated masked hydrophobic precursors by a phototrigger.

Examples of release of organic acids from encapsulated p-methoxyphenacyl esters provided here demonstrate the value of a phototrigger strategy to release chemicals of interest in water from hydrophobic precursors. In this study, a photochemical ß-cleavage process centered on the p-methoxyphenacyl group is exploited to release the acid of interest from a water-soluble capsule made up of octa acid.

Gold nanoparticles functionalized with deep-cavity cavitands: synthesis, characterization, and photophysical studies.

In this report, we present methods of functionalization of AuNP's with deep-cavity cavitands that can include organic molecules. Two types of deep-cavity cavitand-functionalized AuNP's have been synthesized and characterized, one soluble in organic solvents and the other in water. Functionalized AuNP soluble in organic solvents forms a 1:1 host-guest complex where the guest is exposed to the exterior solvents. The one soluble in water forms a 2:1 host-guest complex where the guest is protected from solvent water. Phosphorescence from thiones and benzil included within heterocapsules attached to AuNP was quenched by gold atoms present closer to the guests included within deep-cavity cavitands. During this investigation, we have synthesized four new deep-cavity cavitands. Of these, two thiol-functionalized hosts allowed us to make stable AuNP's. However, AuNP's protected with two amine-functionalized cavitands tended to aggregate within a day.

Steric and electronic effects in capsule-confined green fluorescent protein chromophores.

The turn-on of emission in fluorescent protein chromophores sequestered in an "octaacid" capsule is controlled by stereoelectronic effects described by a linear free energy relationship. The stereochemical effects are governed by both the positions and volumes of the aryl substituents, while the electronic effects, including ortho effects, can be treated with Hammett σ parameters. The use of substituent volumes rather than A values reflects packing of the molecule within the confines of the capsule.

Activation of fluorescent protein chromophores by encapsulation.

Chromophores related to fluorescent proteins, when sequestered into the "octaacid" capsule, recover their fluorescence. The fluorescence recovery is related to the inhibition of torsional motions within the cavity, implicating the single-bond torsion as an important contributor to internal conversion within this important class of chromophores.