Conjugation through Si-O-Si bonds, silsesquioxane (SQ) half cage copolymers, extended examples via SiO0.5/SiO1.5 units: multiple emissive states in violation of Kasha's rule

We previously reported that phenyl- and vinyl-silsesquioxanes (SQs), [RSiO1.5]8,10,12 (R = Ph or vinyl) functionalized with three or more conjugated moieties show red-shifted absorption- and emission features suggesting 3-D conjugation via a cage centered LUMOs. Corner missing [PhSiO1.5]7(OSiMe3)3 and edge opened, end capped [PhSiO1.5]8(OSiMe2)2 (double decker, DD) analogs also offer red shifted spectra again indicating 3-D conjugation and a cage centered LUMO. Copolymerization of DD [PhSiO1.5]8(OSiMevinyl)2 with multiple R-Ar-Br gives copolymers with emission red-shifts that change with degree of polymerization (DP), exhibit charge transfer to F4TNCQ and terpolymer averaged red-shifts suggesting through chain conjugation even with two (O-Si-O) end caps possibly via a cage centered LUMO. Surprisingly, ladder (LL) SQ, (vinylMeSiO2)[PhSiO1.5]4(O2SiMevinyl) copolymers offer emission red-shifts even greater for analogous copolymers requiring a different explanation. Here we assess the photophysical behavior of copolymers of a more extreme SQ form: the half cage [PhSiO1.5]4(OSiMe2Vinyl)4, Vy4HC SQs. We again see small red-shifted absorptions coupled with significant red-shifted emissions, even with just a half cage, thus further supporting the existence of pπ-dπ and/or σ*-π* conjugation through Si-O-Si bonds and contrary to most traditional views of Si-O-Si linked polymers. These same copolymers donate an electron to F4TCNQ generating the radical anion, F4TCNQ-. as further proof of conjugation. Column chromatographic separation of short from longer chain oligomers reveals a direct correlation between DP and emission λmax red-shifts as another indication of conjugation. Further, one- and two-photon absorption and emission spectroscopy reveals multiple excited fluorescence-emitting states in a violation of Kasha's rule wherein emission occurs only from the lowest excited state. Traditional modeling studies again find HOMO LUMO energy levels residing only on the aromatic co-monomers rather than through Si-O-Si bonds as recently found in related polymers.

Boron nitride aerogels incorporated with metal nanoparticles: Multifunctional platforms for iodine capture and detection

Radioactive iodine vapors produced by nuclear fission can pose a significant risk to human health and the environment. Effective monitoring of iodine vapor leakage, capture and storage of radioactive iodine vapor are of great importance for the safety of the nuclear industry. Herein, we report a novel structure-function integrated solid iodine vapor adsorbent based on metal-modified boron nitride (BN) aerogel. Metal-modified BN aerogels incorporated with Cu/Ag nanoparticles (named as BN-Cu and BN-Ag, respectively) are successfully prepared by a metal-induced, ultrasonic-assisted, and in-situ transformation method. The metal-modified BN aerogels show improved mechanical properties in both of the maximum stress and residual deformation. Remarkably, due to the greatly enhanced "host-guest" and "guest-guest" effects by the introduction of metal nanoparticles, the BN-Cu and BN-Ag aerogels exhibit record-breaking iodine vapor adsorption capacities among inorganic adsorbents (1739.8 and 2234.13 wt% respectively), which are even higher than that of most organic adsorbents. Furthermore, an integrated iodine adsorption detection device based on metal-modified aerogels is constructed to realize real-time detection of the electrical properties of aerogels during iodine adsorption. This work provides a foundation for the development of BN aerogels as multifunctional platforms for effective iodine capture and detection. It also introduces new ideas for the use of structural-functional integrated materials in the prevention and control of radioactive iodine pollution.

Unconventional Conjugation in macromonomers and polymers

Multiple reviews have been written concerning conjugated macromonomers and polymers both as general descriptions and for specific applications. In most examples, conjugation occurs via elec-tronic communication via continuous overlap of...

Lithium-Templated Formation of Polyhedral Oligomeric Silsesquioxanes (POSS)

A coordination complex, lithium hepta(i-butyl)silsesquioxane trisilanolate (1; Li-T₇), a stable intermediate in silsesquioxane (SQ) syntheses, was successfully isolated in 65% yield and found to be highly soluble in nonpolar solvents such as hexane. The structure of Li-T₇ was confirmed by NMR, IR spectroscopy, matrix-assisted laser desorption ionization time-of-flight mass spectrometry, electrospray ionization mass spectrometry, and computational simulation, providing detailed elucidation of the intermolecular self-association of the SQ cage with a box-shaped Li₆O₆ polyhedron through strong coordination bonds. After acid treatment, Li-T₇ undergoes lithium-proton cationic exchange, yielding hepta(i-butyl)silsesquioxane trisilanol (2; H-T₇) quantitatively. The high yield of H-T₇ seems to be influenced by Li-O bonding in the Li-T₇ complex that affects the selective formation of hepta(i-butyl)silsesquioxane trisilanolate and the bulky i-butyl groups which may prevent decomposition or SQ cage-rearrangement even at reflux under alkaline conditions. Single-crystal X-ray crystallography confirms the presence of the dumbbell-shaped SQ partial cages through strong intermolecular hydrogen bonds. Interestingly, lowering the polarity of the reaction solution by adding dichloromethane results in formation of the cubic octa(i-butyl)silsesquioxane (3; T₈) cage in a good yield (47%), which is isolated by crystallization from the reaction solution.

Porous Silsesquioxane-Imine Frameworks as Highly Efficient Adsorbents for Cooperative Iodine Capture

The efficient capture and storage of radioactive iodine (¹²⁹I or ¹³¹I), which can be formed during nuclear energy generation or nuclear waste storage, is of paramount importance. Herein, we present highly efficient aerogels for reversible iodine capture, namely, porous silsesquioxane-imine frameworks (PSIFs), constructed by condensation of octa(3-aminopropyl)silsesquioxane cage compound and selected multitopic aldehydes. The resulting PSIFs are permanently porous (Brunauer-Emmet-Teller surface areas up to 574 m²/g), thermally stable, and present a combination of micro-, meso-, and macropores in their structures. The presence of a large number of imine functional groups in combination with silsesquioxane cores results in extremely high I₂ affinity with uptake capacities up to 485 wt %, which is the highest reported to date. Porous properties can be controlled by the strut length and rigidity of linkers. In addition, PSIF-1a could be recycled at least four times while maintaining 94% I₂ uptake capacity. Kinetic studies of I₂ desorption show two strong binding sites with apparent activation energies of 77.0 and 89.0 kJ/mol. These energies are considerably higher than the enthalpy of sublimation of bulk I₂.

[PhSiO1.5]8,10,12 as nanoreactors for non-enzymatic introduction of ortho, meta or para-hydroxyl groups to aromatic molecules

Traditional electrophilic bromination follows long established "rules": electron-withdrawing substituents cause bromination selective for meta positions, whereas electron-donating substituents favor ortho and para bromination. In contrast, in the [PhSiO_1.5]_8,10,12 silsesquioxanes, the cages act as bulky, electron withdrawing groups equivalent to CF₃; yet bromination under mild conditions, without a catalyst, greatly favors ortho substitution. Surprisingly, ICl iodination without a catalyst favors (>90%) para substitution [p-IC₆H₄SiO_1.5]_8,10,12. Finally, nitration and Friedel-Crafts acylation and sulfonylation are highly meta selective, >80%. In principle, the two halogenation formats coupled with the traditional electrophilic reactions provide selective functionalization at each position on the aromatic ring. Furthermore, halogenation serves as a starting point for the synthesis of two structural isomers of practical utility, i.e. in drug prospecting. The o-bromo and p-iodo compounds are easily modified by catalytic cross-coupling to append diverse functional groups. Thereafter, F^-/H₂O₂ treatment cleaves the Si-C bonds replacing Si with OH. This represents a rare opportunity to introduce hydroxyl groups to aromatic rings, a process not easily accomplished using traditional organic synthesis methods. The as-produced phenol provides additional opportunities for modification. Each cage can be considered a nanoreactor generating 8-12 product molecules. Examples given include syntheses of 4,2'-R,OH-stilbenes and 4,4'-R,OH-stilbenes (R = Me, CN). Unoptimized cleavage of the Br/I derivatives yields 55-85% phenol. Unoptimized cleavage of the stilbene derivatives yields 35-40% (3-5 equivalents of phenol) in the preliminary studies presented here. In contrast, meta R-phenol yields are 80% (7-10 mol per cage).

D 5h [PhSiO1.5]10 synthesis via F− catalyzed rearrangement of [PhSiO1.5]n. An experimental/computational analysis of likely reaction pathways

Fluoride catalyzed rearrangement of PhSiO_1.5 favoring [PhSiO_1.5]₁₀.