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

Electron Spin Polarization Transfer from a Nitroxide Incarcerated within a Nanocapsule to a Nitroxide in the Bulk Aqueous Solution

A thioxanthone derivative containing a covalently attached ¹⁵N-labeled nitroxide was incarcerated into an octaacid nanocapsule. Photoexcitation of the thioxanthone chromophore generated electron spin polarization of the nitroxide. This spin polarization of the ¹⁵N-labeled nitroxide was transferred through the walls of the carcerand to a ¹⁴N-labeled nitroxide in external bulk solvent, a process that was directly observed by time-resolved EPR spectroscopy. The efficiency of the communication between the incarcerated guest and molecules in the bulk solvent was shown to be controlled by supramolecular factors such as Coulombic attraction and repulsion between the guest@host complex and charged molecules in the bulk solvent phase

Photoinduced Electron Transfer Reactions of Highly Conjugated Thiophenes for Initiation of Cationic Polymerization and Conjugated Polymer Formation

Photoinduced electron transfer reactions of highly conjugated thiophene derivatives, 4,7-di­(2,3-dihydrothieno­[3,4-<i>b</i>]­[1,4]­dioxin-5-yl)­benzo­[1,2,5]­thiadiazole (DTDT) and 5,8-bis­(2,3-dihydrothieno­[3,4-<i>b</i>]­[1,4]­dioxin-5-yl)-2,3-di­(thiophen-2-yl)­quinoxaline (DTDQ), with diphenyl­iodonium hexafluorophosphate (Ph₂I⁺PF₆^–) and triphenylsulfonium hexafluorophosphate (Ph₃S⁺PF₆^–) were investigated by laser flash photolysis, fluorescence and phosphorescence spectroscopy, and polymerization studies. High fluorescence quantum yields, long fluorescence lifetimes (∼10 ns in aprotic solvents), and absence of detectable phosphorescence at 77 K for both compounds indicate inefficient intersystem crossing into the triplet state and dominant role of singlet excited state. Photolysis of DTDT or DTDQ in the presence of iodonium salt with visible light results in the formation of radical cations of DTDT and DTDQ as detected by laser flash photolysis. Sulfonium salts do not undergo such electron transfer reactions due to the unfavorable redox potentials. The importance of the described photoinduced electron transfer process with respect to the initiation of cationic polymerization and formation of conjugated polymers was demonstrated

Photoinduced Electron Transfer Reactions of Highly Conjugated Thiophenes for Initiation of Cationic Polymerization and Conjugated Polymer Formation

Photoinduced electron transfer reactions of highly conjugated thiophene derivatives, 4,7-di­(2,3-dihydrothieno­[3,4-<i>b</i>]­[1,4]­dioxin-5-yl)­benzo­[1,2,5]­thiadiazole (DTDT) and 5,8-bis­(2,3-dihydrothieno­[3,4-<i>b</i>]­[1,4]­dioxin-5-yl)-2,3-di­(thiophen-2-yl)­quinoxaline (DTDQ), with diphenyl­iodonium hexafluorophosphate (Ph₂I⁺PF₆^–) and triphenylsulfonium hexafluorophosphate (Ph₃S⁺PF₆^–) were investigated by laser flash photolysis, fluorescence and phosphorescence spectroscopy, and polymerization studies. High fluorescence quantum yields, long fluorescence lifetimes (∼10 ns in aprotic solvents), and absence of detectable phosphorescence at 77 K for both compounds indicate inefficient intersystem crossing into the triplet state and dominant role of singlet excited state. Photolysis of DTDT or DTDQ in the presence of iodonium salt with visible light results in the formation of radical cations of DTDT and DTDQ as detected by laser flash photolysis. Sulfonium salts do not undergo such electron transfer reactions due to the unfavorable redox potentials. The importance of the described photoinduced electron transfer process with respect to the initiation of cationic polymerization and formation of conjugated polymers was demonstrated

Synthesis, Isomer Count, and Nuclear Spin Relaxation of H₂O@Open-C₆₀ Nitroxide Derivatives

H₂O@C₆₀ derivatives covalently linked to a nitroxide radical were synthesized. The ¹H NMR of the guest H₂O revealed the formation of many isomers with broad signals. Reduction to the diamagnetic hydroxylamines sharpened the ¹H NMR signals considerably and allowed for an “isomer count” based on the number of observed distinct signals. For H₂O@K-8, 17 positional isomeric nitroxides are predicted, not including additional numbers of regioisomers; indeed, 17 signals are observed in the ¹H NMR spectrum

EPR Analysis and DFT Computations of a Series of Polynitroxides

Polynitroxides with varying numbers of nitroxide groups (one to four) derived from different aromatic core structures show intramolecular electron spin–spin coupling. The scope of this study is to establish an easy methodology for extracting structural, dynamical, and thermodynamical information from the EPR spectra of these polynitroxides which might find use as spin probes in complex systems, such as biological and host/guest systems, and as polarizing agents in dynamic nuclear polarization (DNP) applications. Density functional theory (DFT) calculations at the B3LYP/6-31G(d) level provided information on the structural details such as bond lengths and angles in the gas phase, which were compared with the single crystal X-ray diffraction data in the solid state. Polarizable continuum model (PCM) calculations were performed to account for solvent influences. The electron paramagnetic resonance (EPR) spectra of the polynitroxides in chloroform were analyzed in detail to extract information such as the percentages of different conformers, hyperfine coupling constants <i>a</i>, and rotational correlation times τ_c. The temperature dependence on the line shape of the EPR spectra gave thermodynamic parameters Δ<i>H</i> and Δ<i>S</i> for the conformational transitions. These parameters were found to depend on the number and relative positions of the nitroxide and other polar groups

Distance-Dependent Paramagnet-Enhanced Nuclear Spin Relaxation of H₂@C₆₀ Derivatives Covalently Linked to a Nitroxide Radical

A series of H₂@C₆₀ derivatives covalently linked to a nitroxide radical has been synthesized. We report distance-dependent nuclear spin relaxivity of H₂ in these derivatives. The results clearly indicate that the relaxivity of H₂ is distance-dependent and in good agreement with the Solomon−Bloembergen equation, which predicts a 1/<i>r</i>⁶ dependence

Paramagnet Enhanced Nuclear Spin Relaxation in H₂O@Open‑C₆₀ and H₂@Open‑C₆₀

Relaxation rates of <i>endo</i>-H₂O in H₂O@Open-C₆₀ in the presence of a nitroxide radical and of their nitroxide derivatives have been measured and are compared with effects for <i>endo</i>-H₂ in similar cages. <i>T</i>₁ relaxation enhancement of the <i>endo</i>-H₂O and H₂ induced by either intra- or intermolecular interaction is relatively insensitive to the presence of a cage opening. Enhancement of intermolecular relaxation is observed, however, when the cage opening has an OH group

Capsular Complexes of Nonpolar Guests with Octa Amine Host Detected in the Gas Phase

Nanocapsules, made up of the deep cavitand octa amine and several guests, were prepared in aqueous acidic solution and were found to be stable in the gas phase as detected by electrospray ionization mass spectrometry (ESI-MS). The observed gas phase host–guest complexes contained five positive charges and were associated with several acid molecules (HCl or HBr)

On the Mechanisms of Cyanine Fluorophore Photostabilization

Cyanine fluorophores exhibit greatly improved photostability when covalently linked to stabilizers, such as cyclooctatetraene (COT), nitrobenzyl alcohol (NBA), or Trolox. However, the mechanism by which photostabilization is mediated has yet to be determined. Here, we present spectroscopic evidence that COT, when covalently linked to Cy5, substantially reduces the lifetime of the Cy5 triplet state and that the degree of triplet-state quenching correlates with enhancements in photostability observed in single-molecule fluorescence measurements. By contrast, NBA and Trolox did not quench the Cy5 triplet state under our conditions, suggesting that their mechanism of photostabilization is different from that of COT and does not target the fluorophore triplet state directly. These findings provide insights into the mechanisms of fluorophore photostabilization that may lead to improved fluorophore designs for biological imaging applications