Supramolecular-Surface Photochemistry: Assembly and Photochemistry of Host–Guest Capsules on Silica Surface

Host cavitands and organic guest molecules independently adsorbed on silica particles when mixed and shaken in the presence of a few drops of water underwent intra- and interparticle migration to form capsular complexes that were not formed either in water or organic solvents. Importance of cavitand migration and tumbling on silica surface was established by demonstrating that covalently linked cavitands do not form capsular complexes. The encapsulated guests exhibited selective photochemistry as they do within an organic capsule in solution

Volume Conserving Geometric Isomerization of Encapsulated Azobenzenes in Ground and Excited States and as Radical Ion

To probe the role of the supramolecular steric effects and free volume on photoreactions, geometric isomerization of neutral azobenzenes (ABs) and their radical ions, generated by electron transfer with gold nanoparticles, included within an octa acid capsule, was investigated. A comparison of the isomerization of ABs that proceed by volume conserving pyramidalization and stilbene analogues that proceed by volume demanding one bond flip has indicated the differing influence of 4-alkyl groups on these two processes

Reversible Disassembly–Assembly of Octa Acid–Guest Capsule in Water Triggered by a Photochromic Process

Octa acid (OA), a calixarene-based cavitand, forms a 1:2 capsular assembly with neutral 1,3,3-trimethyl-6′-nitrospiro­[2<i>H</i>-1]­benzopyran-2,2′-indoline and 1:1 cavitandplex with its open zwitterionic merocyanine form. Photochromic interconversion between the spiropyran and merocyanine leads to unprecedented reversible capsular disassembly and assembly. OA provides stability to the merocyanine in both the ground and excited states. The photochemically controlled disassembly and assembly process established here points toward the opportunity of using the OA capsule in delivering small molecules at the desired locations

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 ¹H 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 <i>trans</i>,<i>trans</i>-DPB and <i>trans</i>,<i>trans</i>-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

Competitive Binding of Organic Dyes between Cucurbiturils and Octa Acid

Employing six cationic water-soluble organic dye molecules as probes, we have attempted to qualitatively understand the factors that govern the attraction between such molecules and the anionic water-soluble host, octa acid (OA). Examination of the competitive host–guest complexation between cucurbit[8]­uril (CB[8]) and OA using absorption and emission spectroscopy revealed that the dye molecules included within CB[8] could be “pulled out” by OA. However, an order of magnitude higher concentration of OA was required to shift the equilibrium toward OA, suggesting that attraction between the anionic host OA and the cationic dye molecules such as cresyl violet perchlorate and methylene blue is weaker than the hydrophobic and cation–dipolar interaction between these dye molecules and CB[8]. The importance of Coulombic attraction between OA and dye molecules is also revealed by monomer-to-dimer conversion upon addition of OA to an aqueous solution of monomeric dye molecules. Under conditions where the dye-to-OA ratio is high, freely dissolved monomeric dye molecules are attracted to the exterior of OA and aggregate as dimers on the exterior wall of OA. On the other hand, at high ratios of OA to dye molecules, the dye molecules adsorb as monomers on the exterior of OA. Thus, the monomer-to-dimer ratio in aqueous solution can be controlled by adjusting the ratio of dye to OA molecules. The results presented are of value in qualitatively understanding the relative binding properties of ionic guests with ionic hosts. Studies are qualitative in nature, and further detailed quantitative studies planned for the future are likely to provide deeper understanding of the interaction between water-soluble dye molecules, OA, and CB

Hydrocarbons Depending on the Chain Length and Head Group Adopt Different Conformations within a Water-Soluble Nanocapsule: ¹H NMR and Molecular Dynamics Studies

In this study we have examined the conformational preference of phenyl-substituted hydrocarbons (alkanes, alkenes, and alkynes) of different chain lengths included within a confined space provided by a molecular capsule made of two host cavitands known by the trivial name “octa acid” (OA). One- and two-dimensional ¹H NMR experiments and molecular dynamics (MD) simulations were employed to probe the location and conformation of hydrocarbons within the OA capsule. In general, small hydrocarbons adopted a linear conformation while longer ones preferred a folded conformation. In addition, the extent of folding and the location of the end groups (methyl and phenyl) were dependent on the group (H₂C–CH₂, HCCH, and CC) adjacent to the phenyl group. In addition, the rotational mobility of the hydrocarbons within the capsule varied; for example, while phenylated alkanes tumbled freely, phenylated alkenes and alkynes resisted such a motion at room temperature. Combined NMR and MD simulation studies have confirmed that molecules could adopt conformations within confined spaces different from that in solution, opening opportunities to modulate chemical behavior of guest molecules

CIDEP from a Polarized Ketone Triplet State Incarcerated within a Nanocapsule to a Nitroxide in the Bulk Aqueous Solution

Thioxanthone and benzil derivatives were incarcerated into an octa acid nanocapsule. Photoexcitation of these ketones generated electronic triplet excited states, which become efficiently quenched by positively charged nitroxides adsorbed outside on the external surface of the negatively charged nanocapsule. Although the triplet excited ketone and quencher are separated by a molecular wall (nanocapsule), quenching occurs on the nanosecond time scale and generates spin-polarized nitroxides, which were observed by time-resolved EPR spectroscopy. Because opposite signs of spin polarization of nitroxides were observed for thioxanthone and benzil derivatives, it is proposed that the electron spin polarization transfer mechanism of spin-polarized triplet states to nitroxides is the major mechanism of generating nitroxide polarization

Role of Free Space and Conformational Control on Photoproduct Selectivity of Optically Pure α‑Alkyldeoxybenzoins within a Water-Soluble Organic Capsule

Optically pure α-alkyl deoxybenzoins resulting in products of Norrish Type I and Type II reactions upon excitation has been investigated within the octa acid (OA) capsule in water. The product distribution was different from that in an organic solvent and was also dependent on the length of the α-alkyl chain. Most importantly, a rearrangement product not formed in an organic solvent arising from the triplet radical pair generated by Norrish Type I reaction was formed, and its yield was dependent on the alkyl chain length. In an organic solvent, since the cage lifetime is shorter than the time required for intersystem crossing (ISC) of the triplet radical pair to the singlet radical pair the recombination with or without rearrangement of the primary radical pair (phenylacetyl and benzyl) does not occur. Recombination without rearrangement within the capsule as inferred from monitoring the racemization of the optically pure α-alkyl deoxybenzoins suggesting the capsule’s stability for at least 10^–8 s (the time required for ISC) is consistent with our previous photophysical studies that showed partial opening and closing of the capsule in the time range of microseconds

Photoinduced Electron Transfer Across a Molecular Wall: Coumarin Dyes as Donors and Methyl viologen and TiO₂ as Acceptors

Coumarins C-153, C-480, and C-1 formed 1:2 (guest:host) complexes with a water-soluble cavitand having eight carboxylic acid groups (OA) in aqueous borate buffer solution. The complexes were photoexcited in the presence of electron acceptors (methyl viologen, MV²⁺, or TiO₂) to probe the possibility of electron transfer between a donor and an acceptor physically separated by a molecular wall. In solution at basic pH, the dication MV²⁺ was associated to the exterior of the complex C-153@OA₂, as suggested by diffusion constants (∼1.2 × 10^–6 cm²/s) determined by DOSY NMR. The fluorescence of C-153@OA₂ was quenched in the presence of increasing amounts of MV²⁺ and Stern–Volmer plots of <i>I</i>_o/<i>I</i> and τ_o/τ vs [MV²⁺] indicated that the quenching was static. As per FT-IR-ATR spectra, the capsule C-153@OA₂ was bound to TiO₂ nanoparticle films. Selective excitation (λ_exc = 420) of the above bound complex resulted in fluorescence quenching. When adsorbed on insulating ZrO₂ nanoparticle films, excitation of the complex resulted in a broad fluorescence spectrum centered at 500 nm and consistent with C-153 being within the lipophilic capsule interior. Consistent with the above results, colloidal TiO₂ quenched the emission while colloidal ZrO₂ did not

Supramolecular Surface Photochemistry: Cascade Energy Transfer between Encapsulated Dyes Aligned on a Clay Nanosheet Surface

Three coumarin derivatives (7-propoxy coumarin, coumarin-480, and coumarin-540a, <b>2</b>, <b>3</b>, and <b>4</b>, respectively) having different absorption and emission spectra were encapsulated within a water-soluble organic capsule formed by the two positively charged ammonium-functionalized cavitand octaamine (OAm, <b>1</b>). Guests <b>2</b>, <b>3</b>, and <b>4</b> absorb in ultraviolet, violet, and blue regions and emit in violet, blue, and green regions, respectively. Energy transfer between the above three coumarin@(OAm)₂ complexes assembled on the surface of a saponite clay nanosheet was investigated by steady-state and time-resolved emission techniques. Judging from their emission and excitation spectra, we concluded that the singlet–singlet energy transfer proceeded from <b>2</b> to <b>3</b>, from <b>2</b> to <b>4</b>, and from <b>3</b> to <b>4</b> when OAm-encapsulated <b>2</b>, <b>3</b>, and <b>4</b> were aligned on a clay surface as two-component systems. Under such conditions, the energy transfer efficiencies for the paths <b>2</b>* to <b>3</b>, <b>2</b>* to <b>4</b>, and <b>3</b>* to <b>4</b> were calculated to be 33, 36, and 50% in two-component systems. When all three coumarins were assembled on the surface and <b>2</b> was excited, the energy transfer efficiencies for the paths <b>2</b>* to <b>3</b>, <b>2</b>* to <b>4</b>, and <b>3</b>* to <b>4</b> were estimated to be 32, 34, and 33%. A comparison of energy transfer efficiencies of the two-component and three-component systems revealed that excitation of <b>2</b> leads to emission from <b>4</b>. Successful merging of supramolecular chemistry and surface chemistry by demonstrating novel multi-step energy transfer in a three-component dye encapsulated system on a clay surface opens up newer opportunities for exploring such systems in an artificial light-harvesting phenomenon