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

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

Deep-Cavity Cavitand Octa Acid as a Hydrogen Donor: Photofunctionalization with Nitrenes Generated from Azidoadamantanes

1-azidoadamantane and 2-azidoadamantane form a 1:1 complex with hosts octa acid (OA) and cucurbit[7]­uril (CB7) in water. Isothermal titration calorimetric measurements suggest these complexes to be very stable in aqueous solution. The complexes have been characterized by ¹H NMR in solution and by ESI-MS in gas phase. In both phases, the complexes are stable. Irradiation of these complexes (λ > 280 nm) results in nitrenes via the loss of nitrogen from the guest azidoadamantanes. The behavior of nitrenes within OA differs from that in solution. Nitrenes included within octa acid attack one of the four tertiary benzylic hydrogens present at the lower interior part of OA. While in solution intramolecular insertion is preferred, within OA intermolecular C–H insertion seems to be the choice. When azidoadamantanes included in CB7 were irradiated (λ > 280 nm) the same products as in solution resulted but the host held them tightly. Displacement of the product required the use of a higher binding guest. In this case, no intermolecular C–H insertion occurred. Difference in reactivity between OA and CB7 is the result of the location of hydrogens; in OA they are in the interior of the cavity where the nitrene is generated, and in CB7 they are at the exterior. Reactivity of nitrenes within OA is different from that of carbenes that do not react with the host

Ultrafast Electron Transfer across a Nanocapsular Wall: Coumarins as Donors, Viologen as Acceptor, and Octa Acid Capsule as the Mediator

Results of our study on ultrafast electron transfer (eT) dynamics from coumarins (coumarin-1, coumarin-480, and coumarin-153) incarcerated within octa acid (OA) capsules as electron donors to methyl viologen dissolved in water as acceptor are presented. Upon photoexcitation, coumarin inside the OA capsule transfers an electron to the acceptor electrostatically attached to the capsule leading to a long-lived radical–ion pair separated by the OA capsular wall. This charge-separated state returns to the neutral ground state via back electron transfer on the nanosecond time scale. This system allows for ultrafast electron transfer processes through a molecular wall from the apolar capsular interior to the highly polar (aqueous) environment on the femtosecond time scale. Employing femtosecond transient absorption spectroscopy, distinct rates of both forward (1–25 ps) and backward eT (700–1200 ps) processes were measured. Further understanding of the energetics is provided using Rehm–Weller analysis for the investigated photoinduced eT reactions. The results provide the rates of the eT across a molecular wall, akin to an isotropic solution, depending on the standard free energy of the reaction. The insights from this work could be utilized in the future design of efficient electron transfer processes across interfaces separating apolar and polar environments

Cucurbituril Adamantanediazirine Complexes and Consequential Carbene Chemistry

Adamantanediazirines, precursors of adamantanylidenes, form 1:1 complexes (guest to host) with cucurbit[7]­uril and cucurbit[8]­uril and a 3:1 complex with a Pd nanocage in water. ¹H NMR spectra suggested that these complexes are stable in water on the NMR time scale. While photolysis of adamantanediazirines in water gave mainly adamantanone and adamantanol via adamantanylidene as intermediate, the 1:1 complexes of adamantanediazirine with cucurbiturils gave intramolecular C–H insertion products of adamantanylidene in >90% yield. The study establishes that significant control of carbene reactivity can be achieved when the precursor is encapsulated within a tight inert cavity. While the general characteristics of molecular containers can be understood on the basis of concepts such as “confinement” and “weak interactions”, each one is unique and deserves careful scrutiny

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

Ultrafast Electron Transfer from Upper Excited State of Encapsulated Azulenes to Acceptors across an Organic Molecular Wall

In the context of generating reactive organic radical cations within a confined capsule and exploring photoinduced electron transfer from encapsulated organic molecules to organic and inorganic acceptors through an organic molecular wall, we have investigated electron transfer from the upper excited state (S₂) of azulene (Az) and guaiazulene (GAz) enclosed within an octa acid (OA) capsule to water-soluble 4,4′-dimethyl viologen²⁺ (MV²⁺) and pyridinium⁺ (Py⁺) salts or colloidal TiO₂. S₂ fluorescence of OA encapsulated Az and GAz was quenched by electron acceptors such as MV²⁺ and Py⁺ salts. That electron transfer is responsible for S₂ fluorescence quenching was established by recording the transient absorption spectrum of the MV^●+ in the femtosecond time regime. Femtosecond time-resolved fluorescence experiments suggested that the time constant for the forward and reverse electron transfer from encapsulated Az and GAz to MV²⁺ is 4 and 3.6 ps, and 55.7 and 36.9 ps, respectively. The observed S₂ fluorescence quenching by colloidal TiO₂ in aqueous buffer solution is attributed to electron transfer from encapsulated Az and GAz to TiO₂. Lack of quenching by the wider band gap material ZrO₂ supported the above conclusion. FT-IR-ATR experiments confirmed that OA capsules containing Az and GAz can be adsorbed on TiO₂ films, and excitation of these resulted in S₂ fluorescence quenching. The observations presented here are important in the context of establishing the value of OA type cavitands where charge separation and donor shielding are critical

Synthetic versus Natural Receptors: Supramolecular Control of Chemical Sensing in Fish

The encapsulation of odorants by the synthetic receptor cucurbit[7]­uril (CB[7]) reduces the response of olfactory receptors in Mozambique tilapia (<i>Oreochromis mossambicus</i>) <i>in vivo</i>. For example, the olfactory receptor response to the odorant adamantan-1-amine, as measured by electro-olfactography, was suppressed by 92% in the presence of CB[7]. A reduction in olfactory response of 88% was observed for pentane-1,5-diamine (cadaverine), an odorant associated with carrion avoidance in some fish. The results reveal how the association constants and the concentrations of natural and synthetic receptors play a determinant role and show that synthetic receptors can be used to remove bioactive molecules from fish olfaction