Trimethylsilyl Substituents Research Articles

Crystals of Diphenyl-Benzothiadiazole and Its Derivative with Terminal Trimethylsilyl Substituents: Growth from Solutions, Structure, and Fluorescence Properties.

Linear conjugated molecules consisting of benzothiadiazole (BTD) and phenyl rings are highly efficient organic luminophores. Crystals based on these compounds have great potential for use as light-emitting elements, in particular, scintillation detectors. This paper compares the peculiarities of growth, structure, and fluorescent properties of crystals based on 4,7-diphenyl-2,1,3-benzothiadiazole (P2-BTD) and its organosilicon derivative 4,7-bis(4-(trimethylsilyl)phenyl) BTD ((TMS-P)2-BTD). The conditions for the formation of centimeter-scale single crystals were found for the former, while it was possible to prepare also bulky faceted individual crystals for the latter. The structures of P2-BTD and (TMS-P)2-BTD crystals at 85 and 293 K were investigated by single-crystal X-ray diffraction. The crystal structure of P2-BTD has been refined (sp. gr. P1̅, Z = 4), and for (TMS-P)2-BTD crystals, the structure has been solved for the first time (sp. gr. P21/c, Z = 32). Experimental and theoretical investigations of the absorption-fluorescent properties of solutions and crystals of the molecules have been carried out. The luminophores are characterized by a large Stokes shift for both solutions and crystals with a high fluorescence quantum yield of 75-98% for solutions and 50-85% for the crystals. A solvatochromic effect was observed for solutions of both luminophores: an increase in the values of the fluorescence quantum yield and the excited state lifetime were established with increasing the solvent polarity. Fluorescence properties of solutions and crystals have been analyzed using the data on crystal structure and conformation structure of the molecules as well as density functional theory calculations of their electronic structure. The results have shown that the crystal packing of P2-BTD molecules exhibits uniformity in conformational states, while (TMS-P)2-BTD molecules display a variety of conformational structures in the crystals. This unique combination of features makes them a remarkable example among the other molecular systems for identifying the relationship between the structure and absorption-fluorescence properties through comparative analysis.

Substituent Positioning Effects on the Magnetic Properties of Sandwich-Type Erbium(III) Complexes with Bis(trimethylsilyl)-Substituted Cyclooctatetraenyl Ligands.

Lanthanide complexes with judiciously designed ligands have been extensively studied for their potential applications as single-molecule magnets. With the influence of ligands on their magnetic properties generally established, recent research has unearthed certain effects inherent to site differentiation due to the different types and varying numbers of substituents on the same ligand platform. Using two new sandwich-type Er(III) complexes with cyclooctatetraenyl (COT) ligands featuring two differently positioned trimethylsilyl (TMS) substituents, namely, [Li(DME)Er(COT1,5-TMS2)2]n (Er1) and [Na(DME)3][Er(COT1,3-TMS2)2] (Er2) [COT1,3-TMS2 and COT1,5-TMS2 donate 1,3- and 1,5-bis(trimethylsilyl)-substituted cyclooctatetraenyl ligands, respectively; DME = 1,2-dimethoxyethane], and with reference to previously reported [Li(DME)3][Er(COT1,4-TMS2)2] (A) and [K(DME)2][Er(COT1,4-TMS2)2] (B), any possible substituent position effects have been explored for the first time. The rearrangement of the TMS substituents from the starting COT1,4-TMS2 to COT1,3-TMS2 and COT1,5-TMS2, by way of formal migration of the TMS group, was thermally induced in the case of Er1, while for the formation of Er2, the use of Na+ in the placement of its Li+ and K+ congeners is essential. Both Er1 and Er2 display single-molecule magnetic behaviors with energy barriers of 170(3) and 172(6) K, respectively. Magnetic hysteresis loops, butterfly-shaped for Er1 and wide open for Er2, were observed up to 12 K for Er1 and 13 K for Er2. Studies of magnetic dynamics reveal the different pathways for relaxation of magnetization below 10 K, mainly by the Raman process for Er1 and by quantum tunneling of magnetization for Er2, leading to the order of magnitude difference in magnetic relaxation times and sharply different magnetic hysteresis loops.

Structure of muoniated trimethylsilylvinyl radicals

Muoniated vinyl radicals can be produced by muonium (Mu) addition to triple bonds. Rhodes et al. observed a muoniated radical formed by Mu addition to trimethylsilylacetylene but were unable to determine the structure (J. Chem. Soc., Chem. Commun. 1987, 447). We have performed transverse field muon spin rotation measurements on trimethylsilylacetylene at 298 K and two different applied magnetic fields and observed that two types of muoniated radical were formed: a major product with muon hyperfine coupling constant (A µ ) of 587.3(1) MHz and a minor product with A µ of 570.9(5) MHz. DFT calculations show that Mu addition is preferred to the terminal carbon of the triple bond and that the two resulting radicals are isomers of the Me3SiĊ =CH(Mu) radical. Calculated muon hyperfine coupling constants were used to assign the observed radicals, which have Mu cis and trans to the trimethylsilyl substituent, respectively.

Solvent-controlled formation of alkali and alkali-earth-secured cucurbituril/guest trimers.

Cucurbit[7]uril (CB[7]) encapsulates adamantyl and trimethylsilyl substituents of positively charged guests in dimethyl sulfoxide (DMSO). Unlike in water or deuterium oxide, addition of a selection of alkali and alkali-earth cations with van der Waals radii between 1.0 and 1.4 Å (Na+, K+, Ca2+, Sr2+, Ba2+ and Eu3+) to the CB[7]/guest complexes triggers their cation-mediated trimerization, a process that is very slow on the nuclear magnetic resonance (NMR) time scale. Smaller (Li+, Mg2+) or larger cations (Rb+, Cs+ or NH4+) are inert. The trimers display extensive CH-O interactions between the equatorial and pseudo-equatorial hydrogens of CB[7] and the carbonyl rim of the neighboring CB[7] unit in the trimer, and a deeply nested cation between the three interacting carbonylated CB[7] rims; a counteranion is likely perched in the shallow cavity formed by the three outer walls of CB[7] in the trimer. Remarkably, a guest must occupy the cavity of CB[7] for trimerization to take place. Using a combination of semi-empirical and density functional theory techniques in conjunction with continuum solvation models, we showed that trimerization is favored in DMSO, and not in water, because the penalty for the partial desolvation of three of the six CB[7] portals upon aggregation into a trimer is less unfavorable in DMSO compared to water.

Highly Efficient Deep-Blue Phosphorescent OLEDs Based on a Trimethylsilyl-Substituted Tetradentate Pt(II) Complex.

Compared to Ir(III) complexes with octahedral geometries, Pt(II) complexes with square planar geometries show superior optical properties because their flat shapes lead to an orientation that enhances the outcoupling of organic light-emitting diodes (OLEDs). However, the flat shapes of Pt(II) complexes typically induce a bathochromic shift, limiting their application in high-performance deep-blue phosphorescent OLEDs with high color purity. In this study, bulky trimethylsilyl (TMS)-substituted blue phosphorescent Pt(II) complex (PtON7-TMS) is successfully synthesized to improve color purity. The TMS substituent containing Si atom effectively suppresses intermolecular interaction and aggregation even when the complex concentration in the film state is higher than 30 wt %. As a result, the PtON7-TMS-based OLEDs exhibit a maximum external quantum efficiency of 21.4%, along with a pure-blue color of CIE (0.14, 0.09) at 20 wt % doping concentration and a full-width at half maximum of 30 nm. The pure blue color is maintained at a higher doping concentration (>30 wt %).

Borole/Borapyramidane Relationship.

Boroles and borapyramidanes are classical and nonclassical constitutional isomers, respectively. It is here shown that they can indeed be interconverted. Treatment of the bis(alkynyl)B(C6F5) SMe2 adduct 3·SMe2 with HB(C6F5)2 gave borole 1·SMe2, featuring trimethylsilyl substituents in both α positions to boron, by means of a 1,1-hydroboration/alkenylboration sequence. Photolysis of the classical borole adduct 1·SMe2 resulted in rearrangement to its nonclassical structural isomer, borapyramidane 2, in high yield, which exhibits a vicinal pair of trimethylsilyl substituents at the square pyramidane base. Neutral borapyramidane 2 is a rare example of an isoster of the (CH)5+ pyramidane cation. Thermolysis of borapyramidane 2 in the presence of SMe2 at 60 °C re-formed borole 1·SMe2, which converted at 100 °C to 2,3-bis-silyl-substituted borole isomer 8·SMe2. Its photolysis also gave borapyramidane 2. Prolonged photolysis of 2 at elevated temperatures converted this to borapyramidane isomer 10 containing a pair of trimethylsilyl groups in 1,3-position at its square C4-pyramidal base. The borole and borapyramidane isomers were characterized by X-ray diffraction, and the system was analyzed by density functional theory (DFT) calculations.

Oligoorganogermanes: Interplay between Aryl and Trimethylsilyl Substituents.

Derivatives of main group elements containing element–element bonds are characterized by unique properties due to σ-conjugation, which is an attractive subject for investigation. A novel series of digermanes, Ar3Ge-Ge(SiMe3)3, containing aryl (Ar = p-C6H4Me (1), p-C6H4F (2), C6F5 (3)) and trimethylsilyl substituents, was synthesized by the reaction of germyl potassium salt, [(Me3Si)3GeK*THF], with triarylchlorogermanes, Ar3GeCl. The optical and electronic properties of such substituted oligoorganogermanes were investigated spectroscopically by UV/vis absorption spectroscopy and theoretically by DFT calculations. The molecular structures of compounds 1 and 2 were studied by XRD analysis. Conjugation between all structural fragments (Ge-Ge, Ge-Si, Ge-Ar, where Ar is an electron-donating or withdrawing group) was found to affect the properties.

Cyclometalated Platinum(II) β‐Diketonate Complexes with Extremely High External Quantum Efficiency for White Organic Light‐Emitting Diodes

AbstractThree cyclometalated Pt(II) β‐diketonate compounds with sky‐blue to green phosphorescence are prepared by modifying the substituents of the 2′,6′‐dimethoxy‐2,3′‐bipyridine (OMe2pypy, C^N) chelate ligand. These molecules have the general formula Pt(C^N)(O^O), where O^O = acetylacetonate; one is unsubstituted (1), one possesses a trimethylsilyl (TMS) substituent (2), and the other is substituted with dimesitylboron (BMes2) (3). The intermolecular interactions of 2 are greater than those of 1 and 3, based on color mapping and surface area results from Hirshfeld analyses. Time‐dependent density functional theory calculations reveal that the electronic transitions of monomeric 1 and 3 are predominantly intraligand or ligand‐to‐ligand charge transfer mixed with metal‐to‐ligand charge transfer transitions, while for the dimeric forms, dimeric 2, an additional strong metal–metal‐to‐ligand charge transfer transition is observed. White organic light‐emitting diode (WOLED) devices using an 1,3‐bis(N‐carbazolyl)benzene (mCP) or mCP:TSPO1(diphenyl[4‐(triphenylsilyl)phenyl]phosphine oxide) mixed host and 1–3 as dopants are fabricated. With a dopant concentration of 20 wt% in the emissive layer, compound 2 exhibited bright yellow–green emission with a maximum external quantum efficiency (EQE) of 24.9%. This result corresponds to one of the highest EQEs among the reported C^N chelated Pt(II) β‐diketonate‐based WOLEDs.

Growth of p-Sexiphenyl Crystals and its Trymethylsilyl Derivative from the Vapor Phase

Abstract—Large single-crystal films of p-sexiphenyl and its derivative with terminal trimethylsilyl substituents are grown under physical-vapor-transport conditions. The structure of the single-crystal films is studied by X-ray diffraction. In the crystals of the trimethylsilyl derivative there is a significant shift of molecular π systems relative to each other inside the (001) layers with “herringbone” packing in comparison with the crystal structure of p-sexiphenyl. The optical absorption and photoluminescence spectra of the studied substances in tetrahydrofuran solutions are obtained and analyzed. The presence of –Si(CH3)3 terminal substituents in the molecule is found to cause a slight bathochromic shift in the absorption and photoluminescence spectra of the solutions.

Abnormally aggregation-induced emissions observed from hydrogen- and silyl-substituted siloles

Abstract Silacyclopentadienes (siloles) are currently of great interest because of their intriguing aggregation-induced emission (AIE) characteristics. In this work, abnormally AIE phenomena were observed from silyl- and hydrogen-substituted siloles respectively. We propose silyl-substituent and hydrogen atom effects to explain this via both experiments and theoretical calculations. It was discovered that trimethylsilyl substituents destroyed the p-interactions with the silole core and resulted in the non-fluorescence. Additionally, 1-chloro-1-H-2,3,4,5-tetraphenylsilole featuring strong fluorescences in both solid and solution states with high fluorescence quantum yields represents a new potential photoelectric material. This work not only reports the silole with strong fluorescences in both solid and solution states for the first time but also contributs to enrich the AIE research of siloles.

P‐180: Efficient Blue Phosphorescent Organic Light‐Emitting Diode with Extremely High External Quantum Efficiency

Two blue phosphorescent materials were synthesized by combining main ligands of bipyridine with trimethylsilyl(TMS) substituent, and ancillary ligand of acetylacetonate(acac). Each material has different TMS positions to bipyridine. Both materials have π‐π interactions and strong intermolecular interactions via C‐H·F. These are important for the performance of phosphorescent organic light‐emitting diodes (PhOLED). Material 1 has stronger intermolecular interactions than material 2. The substituent and ancillary ligand decrease the energy gap compared with the unsubstituted reference. Two blue PhOLEDs of both materials as dopants were fabricated to determine the device performance of the materials. The device of material 2 shows very high external quantum efficiency of 30.9% as blue phosphorescent emitter with single host.

Sterically facilitated meta-lithiation of arenes, containing electron-donating groups

The influence of the bulky trimethylsilyl substituent on the selectivity of metallation of dimethylaniline, anisole and 1-dimethylaminonaphthalene is studied. The neighboring SiMe3 group forces dimethylamino and methoxy groups to occupy a conformation with an unshared electron pair turned towards silicon atom. This forced conformation prevents NMe2 and OMe groups from providing the DOM-effect, thus facilitating meta-metallation with the use of bulky LiCKOR. While the inverted NMe2 group completely suppresses ortho-metallation, the less bulky and more electron withdrawing OMe group demonstrates more rotation freedom allowing selective ortho-metallation with smaller reagents such as n-BuLi or tert-BuLi.

Blue Phosphorescent Ir(III) Complexes Achieved with Over 30% External Quantum Efficiency

AbstractTwo heteroleptic blue phosphorescent Ir(III) complexes are synthesized based on the main C^N chelate bipyridine ligands, with either a 4‐positon (1) or 5‐position (2) of bulky trimethylsilyl (TMS) substituent, and acetylacetonate (acac) as an ancillary ligand. Their structure–electroluminescent property relationships are systematically investigated. Both complexes have strong intermolecular interactions via C–H···F and π–π interactions that affect the performance of phosphorescent organic light‐emitting diodes (PHOLEDs). Complex 1 possesses fewer intermolecular interactions than complex 2 based on the Hirshfeld analysis. The combination of TMS substituent and acac ligand decreases the energy gap relative to the nonsubstituted counterpart: Ir(dfpypy)(acac), where dfpypy is 2′‐6′‐difluoro‐2,3′‐bipyridine. Blue PHOLEDs which use both complexes as a dopant, and 9‐(3‐(9H‐carbazol‐9‐yl)phenyl)‐9H‐carbazol‐3‐yl)diphenylphosphine oxide as a host material, are fabricated successfully. Complex 1 has the highest performance among the reported blue PHOLEDs without co‐host, making it an outstanding blue triplet emitter in PHOLEDs with over 30% of external quantum efficiency. Furthermore, the efficiency roll‐off dramatically improved in the 1‐based device.

Synthesis of a Contrapositionally Substituted Cyclohexa-meta-phenylene: A Ready-to-Use Precursor for Cyclohexa-meta-phenylene-Based Materials

A contrapositionally substituted derivative of cyclohexa-meta-phenylene ([6]CMP) was synthesized by an intramolecular Yamamoto coupling reaction of an appropriate terphenyl unit containing a trimethylsilyl substituent. Iododesilylation of the trimethylsilyl groups of the product with iodine monochloride was used to incorporate iodo groups, an important functionality for metal-catalyzed coupling reactions. The iodo groups were also converted into a (pinacolato)boryl groups, another important functionality for coupling reactions. The diborylated [6]CMP is expected to be a versatile potential comonomer and a precursor for the synthesis of CMP-based materials. The synthetic route to the disubstituted [6]CMP included lithiation, Pd-catalyzed borylation, Suzuki coupling, and Yamamoto coupling. The structure of the product was established by NMR spectroscopy and mass spectrometry.

Solubility and Crystal Growth of p-Quaterphenyl and Its Derivative with Trimethylsilyl Terminal Substituents

The melting point and enthalpy of fusion are refined for p-quaterphenyl (4P) and measured for the first time for its derivative with two terminal trimethylsilyl substituents (TMS-4P-TMS) by DSC method. The optical absorption spectra of 4P and TMS-4P-TMS solutions in toluene are measured at 25°C. Analysis of the absorption spectra shows the range within which aggregated solutions of the investigated substances exist. The shape of the absorption spectra of concentrated TMS-4P-TMS solutions indicates the J-aggregation of molecules occurs. It is found that the solubility of the TMS-4P-TMS derivative in toluene at room temperature is almost six times higher than that of p-quaterphenyl. Large single-crystal films and plates of the investigated compounds (up to 8 mm in size for 4P and up to 25 mm in size for TMS-4P-TMS) are obtained by growing them from solutions. Data on crystal growth of TMS-4P-TMS are reported for the first time.

Triamidoamine-Supported Zirconium Compounds in Main Group Bond-Formation Catalysis.

The rationale to pursue long-term study of any system must be sound. Quick discoveries and emergent fields are more than temptations. They remind us to ask what are we gaining through continued study of any system. For triamidoamine-supported zirconium, there has been a great deal gained with yet more ahead. Initial study of the system taught much that is applied to catalysis. Cyclometalation of a trimethylsilyl substituent of the ancillary ligand, abbreviated (N3N) when not metalated for simplicity, via C-H bond activation is facile and highly reversible. It has allowed for the synthesis of a range of Zr-E bonds, which are of fundamental interest. More germane, cyclometalation has emerged as our primary product liberation step in catalysis. Cyclometalation also appears to be a catalyst resting state, despite how cyclometalation is a known deactivation step for many a compound in other circumstances. Catalysis with triamidoamine-supported zirconium has been rich. Rather than summarizing the breadth of reactions, a more detailed report on the dehydrocoupling of phosphines and hydrophosphination is provided. Both reactions demonstrate the outward impact that the study of (N3N)Zr-based catalysis has afforded. Dehydrocoupling catalysis, or bond formation via loss of hydrogen, is particular to 3p and heavier main group elements. The reaction has been important in the formation of E-E and E-E' bonds in the main group for molecular species and materials. While study of this reaction at (N3N)Zr compounds provides key insights into mechanism, discoveries in the area of P-P and Si-Si bond formation with (N3N)Zr derivatives as catalysts have greater reach than merely the synthesis of main group element containing products. For example, that work has informed design principles for the identification of catalysts that transfer low-valent fragments. The successful application of these principles was evident in the discovery of a catalyst that transfers phosphinidene ("PR") to unsaturated substrates. Hydrophosphination exhibits perfect atom economy in the formation of P-C bonds. The reaction can proceed without a catalyst, but the purpose of a catalyst is enhanced reactivity and selectivity. Nevertheless, significant challenges in this reaction remain. In particular, (N3N)Zr compounds have demonstrated high activity in hydrophosphination and readily utilize unactivated unsaturated organic molecules, challenging substrates for any heterofunctionalization reaction. This activity has led to not only impressive metrics in the catalysis but access to previously untouched substrates and formation of unique products. The particular properties of the (N3N)Zr system that engage in this reactivity may influence other heterofunctionalization reactions. The recently discovered photocatalytic hydrophosphination with (N3N)ZrPRR' compounds already appears to be general rather than unique and may drive additional bond formation catalysis among early transition-metal compounds.

Potassium Dimesitylphosphinite Catalyzed Intermolecular Hydrophosphorylation of Alkynes

In this investigation we evaluated the scope of the intermolecular hydrophosphorylation (Pudovik reaction) of alkynes R1–C≡C–R2 (R1 = H, alkyl, Ph; R2 = alkyl, Ph, COOMe, SiMe3, Si(iPr)3) with bis(2,4,6-trimethylphenyl)phosphane oxide (dimesitylphosphane oxide, Mes2P(O)H) in tetrahydrofuran at room temperature or 65 °C, catalyzed with 5 or 10 mol % of potassium dimesitylphosphinite (Mes2P–O–K), yielding alkenyldimesitylphosphane oxides (Mes2P(O)–C(R1)═C(H)R2). This procedure requires substituents with a −I effect at the C≡C triple bond, whereas alkyl-substituted alkynes are inactive under these reaction conditions. The hydrophosphorylation proceeds regioselectively, but E/Z isomer mixtures are obtained. E/Z isomerization occurs at elevated temperatures with an estimated energy barrier of 59 kJ mol–1 (R1 = Me; R2 = Ph). Trimethylsilyl substituents at the alkyne functionality (R1 = H, nBu; R2 = SiMe3) destabilize the product, leading to degradation and formation of Mes2P–O–SiMe3 and R1–C≡C–H.

Substituent Effects on Singlet Exciton Fission in Polycrystalline Thin Films of Cyano-Substituted Diaryltetracenes

Cyano-substituted tetracenes (5,11-dicyano-6,12-diphenyltetracene, Tet) undergo exoergic singlet fission (SF), a spin-allowed photophysical process that generates a pair of triplet excitons from one singlet exciton. To elucidate substituent effects on SF, we have measured the SF dynamics and triplet yields of thin films, formed by Tet bearing hydrogen (H), methyl (Me), fluoro (F), and trimethylsilyl (TMS) substituents on the p-phenyl positions and on the 3 and 9 positions of the tetracene core, by time-resolved spectroscopy in the vis-NIR and IR regions. The H-, Me-, and F-Tet display strong intramolecular electronic coupling (π–π distances <4 A), and SF gives high triplet exciton yields up to 200% (quantitative). In addition, a charge-transfer state mediates SF in F-Tet films, while H-Tet and Me-Tet show no evidence for such a state. Correlations between the SF yields and the crystal structure show that chromophore slippage along both their short and long axes allows efficient SF and that a large degree ...

Synthesis and photostability of 1,4-bis(5-phenyloxazol-2-yl)benzene (POPOP) structural isomers and their trimethylsilyl derivatives

In this work, a versatile synthetic method for preparation of linear phenyloxazoles and their organosilicon derivatives under mild conditions via a combination of van Leusen and direct CH arylation reactions is reported. It was used for the synthesis of a novel trimethylsilyl (TMS) derivative of blue laser dye 1,4-bis(5-phenyloxazol-2-yl)benzene (POPOP) and its structural isomer 1,4-bis(2-phenyloxazol-5-yl)benzene (isoPOPOP) with and without TMS substituents. All of the compounds demonstrated high photoluminescence quantum yields and large molar extinction coefficients, which further increased for their TMS derivatives. Measurements of accelerated photodegradation of the dyes in diluted THF solutions revealed that POPOP-type compounds are twice more stable than their isoPOPOP-type isomers, independent of the presence or absence of TMS substituents. Cyclic voltammetry (CV) investigations revealed that POPOP-type dyes are more electrochemically stable as compared to their isoPOPOP-type isomers. Quantum chemical TD DFT calculations of the frontier energy levels of the dyes were in a good agreement with the experimental data obtained by CV. Single-crystal X-ray diffraction experiments revealed significant structural differences between crystal lattices of the dyes. Due to facile synthesis and excellent optical properties as well as high photo- and thermo- stability the novel luminescent dyes may find wide application in organic photonics.

Improving 1-Hexene Incorporation of Highly Active Cp–Chromium-Based Ethylene Polymerization Catalysts

Single-site chromium catalysts for olefin polymerization with donor functionalized cyclopentadienyl (Cp) ligands have been modified in order to improve their incorporation ability for the comonomer 1-hexene into the polymer chain under maintenance of their very high catalytic activities. A trimethylsilyl substituent in combination with a fused thiophene ring at the Cp ligand has been identified as the best ligand so far, leading to a doubling in 1-hexene incorporation and polyethylene (PE) with up to 27% 1-hexene content (by weight) has been obtained. The complexes lead to PE with molecular weight in the range of 50 000 to 800 000 g mol–1 when used in homogeneous solution, however after supporting the complex on silica ultrahigh molecular weight polyethylene (UHMW-PE) is formed with 9.9% of 1-hexene incorporated into the chain. Although other known catalysts incorporate even more 1-hexene, the presented system is different as it combines considerable α-olefin incorporation with very high polymer molecular...