Steric Hindrance Groups Research Articles

Accessing Highly Efficient Photothermal Conversion with Stable Open‐Shell Aromatic Nitric Acid Radicals

AbstractIt is very challenging to prepare stable radicals as they are usually thermodynamically or kinetically unstable in air. Herein, a series of star‐shaped aromatic nitric acid radicals were prepared via facile demethylation and consequent oxidation. As phenol radicals without steric hindrance group protection, they exhibit high electrochemical and thermal stability due to their rich resonance structures including closed‐shell nitro‐like and open‐shell nitroxide structure with unpaired electrons delocalized in conjugated backbones. Among them, TPA‐TPA‐O6 powder exhibited extremely wide absorption from 300 to 2000 nm covering the whole solar spectral irradiance, high photothermal conversion efficiency, and negligible photobleaching effect in seawater desalination. Under the irradiation of one sunlight, the water evaporation efficiency of TPA‐TPA‐O6 is recorded to be as high as 89.41 % and the water evaporation rate is 1.293 kg m−2 h−1, which represents the top performance in pure organic small molecule photothermal materials.

Accessing Highly Efficient Photothermal Conversion with Stable Open‐Shell Aromatic Nitric Acid Radicals

It is very challenging to prepare stable radicals as they are usually thermodynamically or kinetically unstable in air. Herein, a series of star-shaped aromatic nitric acid radicals were prepared via facile demethylation and consequent oxidation. As phenol radicals without steric hindrance group protection, they exhibit high electrochemical and thermal stability due to their rich resonance structures including closed-shell nitro-like and open-shell nitroxide structure with unpaired electrons delocalized in conjugated backbones. Among them, TPA-TPA-O6 powder exhibited extremely wide absorption from 300 to 2000 nm covering the whole solar spectral irradiance, high photothermal conversion efficiency, and negligible photobleaching effect in seawater desalination. Under the irradiation of one sunlight, the water evaporation efficiency of TPA-TPA-O6 is recorded to be as high as 89.41 % and the water evaporation rate is 1.293 kg m-2 h-1 , which represents the top performance in pure organic small molecule photothermal materials.

Endowing imidazole derivatives with thermally activated delayed fluorescence and aggregation‐induced emission properties for highly efficient non‐doped organic light‐emitting diodes

AbstractThe development and enrichment of high‐performance organic fluorophores that simultaneously possess thermally activated delayed fluorescence (TADF) and aggregation‐induced emission (AIE) properties is going pursued but remains a great challenge due to severe exciton quenching. Herein, a systematical investigation on imidazole moiety has successfully given rise to a series of highly efficient imidazole‐based TADF‐AIE luminogens for the first time. The attachment of two cyano functionalities on imidazole moiety can significantly increase the electron‐withdrawing ability, so as to realize TADF emissions with small singlet‐triplet energy gaps (ΔEST) values. Meanwhile, the installation of a steric hindrance group at N1 position of imidazole moiety can twist the geometry between imidazole and phenyl bridge, thus transforming imidazole derivative from an aggregation‐caused quenching emitter into an AIE luminogen. Consequently, the non‐doped organic light‐emitting diodes (OLEDs) utilizing these TADF‐AIE luminogens as emitters exhibit outstanding sky‐blue and green emissions, with external quantum efficiency (EQE) as high as 20.0% and low efficiency roll‐off (EQE at 1000 cd m−2, 16.1%). These values represent the state‐of‐the‐art performance for all imidazole‐based OLED devices, which illustrates the significant potential of imidazole derivatives in assembling high‐performance OLEDs.

Efficient Intramolecular Charge‐Transfer Fluorophores Based on Substituted Triphenylphosphine Donors

AbstractTriphenylphosphine (TPP)‐based luminescent compounds are rarely investigated because of the low photoluminescence quantum yield (PLQY). Here, we demonstrate that introducing steric hindrance groups to the TPP moiety and separating the orbitals involved in the transition can drastically suppress the non‐radiative decay induced by structural distortion of TPP in the excited state. High PLQY up to 0.89 as well as thermally activated delayed fluorescence are observed from the intramolecular charge‐transfer (ICT) molecules with substituted TPP donors (sTPPs) in doped films. The red organic light‐emitting diodes employing these emitters achieve comparable external quantum efficiencies to the control device containing a classical phosphorescent dye, revealing the great potential of the ICT emitters based on electrochemically stable sTPPs.

Effect of triptycene unit on the performance of porphyrin-based dye-sensitized solar cells

Triptycene has been first introduced into porphyrin sensitizers (JY74 and JY75) for dye sensitized solar cells. Compared to reference dye YD26, the homoaromatic electron delocalization of triptycene makes the designed dyes a broadened absorption. Meanwhile, the rigid shape-persistent character of triptycene endows JY74 and JY75 an improved ability to diminish the charge recombination between electrolyte and semiconductor TiO2. As a result, after structural modification, larger short-circuit current and higher open-circuit voltage are achieved for dyes JY74 and JY75. The power conversion efficiency of JY75 is increased by 26.8 % compared to that of YD26. Consequently, triptycene may be a promising bulky steric hindrance group for decorating photosensitizer to get an attractive photovoltaic performance.

Improving the thermoelectric performance of solution-processed polymer nanocomposites by introducing platinum acetylides with tailored intermolecular interactions

Hybrid thermoelectric (TE) materials incorporated with the conjugated polymers and single-walled carbon nanotubes (SWCNTs) are promising to decouple the intractable interrelation of the key TE parameters, and therefore yield considerable composites for solution-processable flexible TE generators. However, the disordered intermolecular interactions and inefficient overlap of the frontier molecular orbitals between the polymer backbones and the SWCNTs largely degenerate the TE properties of the composites. Herein, we demonstrate an efficient strategy to regulate their intermolecular π-π interactions by introducing heavy metal atoms (platinum) along with tailored π-ligands to reconstruct the main chain of the incorporated polymers. Consequently, both the calculated binding energy and the observed Dexter energy transfer process between the SWCNTs and the polymers are remarkably increased with the insertion of platinum, indicating significantly enhanced π-π interactions between the two. Meanwhile, their self-aggregation can be effectively restrained by the large steric hindrance groups of tributyl phosphine ligands synchronously. Therefore, a high power factors of over 464 μW m−1 K−2 (r.t.) and a high output power approaching to 2200 nW (ΔT = 70 K, 10 legs) can be achieved by the P(HTh-Pt)/SWCNT-based TE generators, which is almost two times as high as the homologous P3HT one without platinum.

Achieving enhanced solid-state photochromism and mechanochromism by introducing a rigid steric hindrance group.

For photochromic molecules, effective isomerization usually requires conformational freedom, which is usually unavailable under solvent-free conditions. In this work, we report a new method, which can realize the reversible switching of spiropyran molecules by introducing a rigid aromatic ring group and this method can provide the required free volume to transform from a closed-ring to an open-ring form. This new molecule can quickly change color in the solid state under ultraviolet light, and can be erased after being heated at 60 °C for about 5 minutes. Furthermore, this new compound presents mechanochromicity when a mechanical force is applied. What is more, it can be used for at least 30 cycles of print-erase operations without apparent fatigue. This new molecule exhibits improved photochromic and anti-fatigue properties in the solid state, which can promote its application in both ultraviolet printing and anti-counterfeiting materials.

Enhancement of Solid‐State Reversible Photochromism by Incorporation of Rigid Steric Hindrance Groups

AbstractSolid‐state materials with reversible properties are highly attractive, owing to versatile applications in security, sensors, bioimaging, and information storage. Unfortunately, obtaining these materials in the solid state remains a huge challenge. Efficient isomerization usually requires sufficient conformational freedom and cannot be achieved without solvents or matrices. In this study, four structurally simple spiropyran derivatives are prepared that show excellent reversible photochromism under solvent‐free conditions. The large free volumes induced by different rigid aromatic ring groups enable an efficient photoswitching between closed‐ring compounds and their photoisomers. Furthermore, these compounds exhibit mechanochromic properties upon application of a mechanical force. Their color can be maintained for at least 2 days without fading under visible light irradiation at 30 °C, and can be easily erased by heating at 60 °C for 5 min. At least 30 consecutive print–erase cycles can be performed without apparent fatigue. The excellent solid‐state photochromic properties of the present compounds can find application in both UV printing and rewritable paper.

A novel quaternary ammonium salts derived from α-amino acids with large steric hindrance group and its application in asymmetric Mannich reaction

Quaternary ammonium salts derived from amino acids with large steric hindrance group have been designed and synthesized, which promoted the asymmetric Mannich reaction of glycine imines with N-Boc-aldimines in excellent yields with high enantio- and diastereo-control compared with known catalysts of this type reported by our group.

Synthesis, silver (I) extraction and silver (I) binding studies of novel N1,N3-bis(2-(benzylthio)ethyl)propanediamide derivatives

Solvent or liquid–liquid extraction represents a highly valuable technique for the selective recovery of metals from the aqueous phase due to the ease of operation and short turnaround times. Ligands bearing soft donor atoms including nitrogen and sulfur are ideal candidates for selective silver recovery due to their preference for silver binding. Herein, novel N1,N3-bis(2-(benzylthio)ethyl)propanediamide derivatives bearing sulfur and nitrogen donor atoms were prepared in low to high yields and tested for Ag+ extraction from ternary aqueous solutions also containing Cu2+ and Pb2+ following a well-established solvent extraction protocol. It was observed that electronics effects at the 4-aryl position in the propanediamide (or malondiamide) derivatives had a significant effect on the selectivity, but little effect on the efficiency of Ag+ extraction with the 4-methoxy analogue proving the most selective. Steric hindrance provided by dimethyl substitutions at the α-positions to the sulfur atoms had negative effects on Ag+ extraction efficiency and selectivity, while diethyl steric hindrance at the methylene center lowered selectivity but increased extraction efficiency for Ag+. Detailed binding studies reveal that one of the malondiamide derivatives which lacked the electronic and steric hindrance groups studied coordinated Ag+ in a 1:1 fashion suggesting a tetrahedral complex geometry. Overall, the results show that simple modification of the electronics and sterics of the N1,N3-bis(2-(benzylthio)ethyl)propanediamides, can improve their selectivity for Ag+ recovery from the aqueous phase.

Constructing a Local Hydrophobic Cage in Dye-Doped Fluorescent Silica Nanoparticles to Enhance the Photophysical Properties.

Aggregation-caused quenching (ACQ) and poor photostability in aqueous media are two common problems for organic fluorescence dyes which cause a dramatic loss of fluorescence imaging quality and photodynamic therapy (PDT) failure. Herein, a local hydrophobic cage is built up inside near-infrared (NIR) cyanine-anchored fluorescent silica nanoparticles (FSNPs) in which a hydrophobic silane coupling agent (n-octyltriethoxysilane, OTES) is doped into FSNPs for the first time to significantly inhibit the ACQ effect and inward diffusion of water molecules. Therefore, the obtained optimal FSNP-C with OTES-modification can provide hydrophobic repulsive forces to effectively inhibit the π–π stacking interaction of cyanine dyes and simultaneously reduce the formation of strong oxidizing species (•OH and H2O2) in reaction with H2O, resulting in the best photostability (fluorescent intensity remained at 90.1% of the initial value after 300 s of laser scanning) and a high PDT efficiency on two- and three-dimensional (spheroids) HeLa cell culture models. Moreover, through molecular engineering (including increasing covalent anchoring sites and steric hindrance groups of cyanine dyes), FSNP-C exhibits the highest fluorescent intensity both in water solution (12.3-fold improvement compared to free dye) and living cells due to the limitation of molecular motion. Thus, this study provides an effectively strategy by combining a local hydrophobic cage and molecular engineering for NIR FSNPs in long-term bright fluorescence imaging and a stable PDT process.

Enhancement of electro-optic properties of nonlinear optical chromophores by introducing pentafluorobenzene group into the donor and π-bridge

Two isophorone-based chromophores A-B has been synthesized and investigated based on diethylaminophenyl donor and tricyanovinyldihydrofuran or phenyl-trifluoromethyltricyanofuran acceptors. The chromophores were modified with a bulky pentafluorobenzene group covalently attached to both the donor and bridge section of the chromophores to prevent close packing of molecules, which benefit the polarization efficiency. The ultraviolet absorption, Density functional theory (DFT) calculation, thermal stabilities and electro-optic activities of these chromophores were systematically studied. The chromophores A-B exhibited good thermal stabilities with the decomposition temperatures (Td) higher than 220 °C. Most importantly, the rigid steric hindrance groups can effectively reduce intermolecular electrostatic interactions to translate the hyperpolarizabilities of chromophore into large electro-optic (EO) coefficients (r33). Polymers doped with 30 wt% chromophore A-B have been poled to afford ultrahigh electro-optic coefficient (r33) of 115 pm/V and 213 pm/V, respectively at 1.31 μm, which is higher than that of analogue chromophores which without isolation groups.

Synthesis of an A2BC-type asymmetric zinc phthalocyanine derivative for efficient visible/near-infrared-driven H2 evolution on g-C3N4

By using π-electron-rich thiophene unit to replace one of the benzenoid rings as electron donor and a carboxyl-naphthalene unit as electron acceptor, a novel A2BC-type asymmetric zinc phthalocyanine derivative (Zn-di-PcNcTh-2) bearing four diphenylthiophenol substituents as steric hindrance groups is synthesized. The A2BC-type asymmetric structure of Zn-di-PcNcTh-2 causes a strongly split Q-band and wide spectral absorption in the visible/near-infrared region. By using it as a sensitizer of Pt-loaded g-C3N4, the spectral response region of g-C3N4 is extended from ∼450 nm to more than 800 nm, and the corresponding Zn-di-PcNcTh-2-sensitized product delivers an average H2 evolution activity of 232 μmol h−1 with an extremely high turnover number of 23187 h−1 under visible light (λ ≥ 420 nm) irradiation. Moreover, robust apparent quantum yield values of 5.53%, 4.09% and 2.35% are attained at 760, 780 and 800 nm near-infrared monochromatic light, respectively. The prominent bathochromic and electronic push-pull effects due to the introduced diphenylthiophenol substituents, π-electron-rich thiophene and carboxyl-naphthalene units benefit the efficient red/near-infrared-driven H2 evolution of the Zn-di-PcNcTh-2-sensitized g-C3N4.

Synthesis and Characterization of Novel 1,4‐Di(o‐thioaryl)benzene Buta‐1,3‐diynes

AbstractThe dimers of terminal alkynes were synthesized through a room temperature reaction which used (diacetoxyiodo)benzene as oxidizing agent and acetonitrile as solvent. The structures were characterized by 1H NMR, 13C NMR. The electrochemical and spectrochemical properties of the compounds were also characterized by cyclic voltammetry and UV spectroscopy. Meanwhile, the single crystal X‐ray analysis of the model compound was discussed.Introduction of the thioaryl group in ortho position lead to the red shift of the UV absorption peaks, and the introduction of a large steric hindrance group reduced the influences of the intermolecular π‐π interactions on their electronic properties. Moreover the sulfur atom can form the stable S−Au bond with gold. These data indicated that compounds 3 b‐3 dare expected to be applied in the field of molecular wires.

Enhanced electro-optic activity and thermal stability by introducing rigid steric hindrance groups into double-donor chromophore

A series of novel chromophores based on same bis (N,N-diethyl) aniline donor have been synthesized and systematically characterized. Chromophores with rigid steric hindrance groups showed superb thermal stabilities with high thermal decomposition temperatures above 260 °C. Besides, compared with the chromophore (y2) without isolation group, these new chromophores show better intramolecular charge-transfer absorption. Most importantly, the introduction of rigid steric hindrance groups can effectively reduce dipole–dipole interactions to translate their relatively β values into bulk large electro-optic activities. The electro-optic coefficient of poled films containing 25 wt % of these new chromophores doped in amorphous polycarbonate afforded values of 163, 129, 183 and 196 pm V−1 at 1310 nm for chromophores y1-y4 respectively. These results indicated that introducing rigid isolation group to control the shape of chromophores can efficiently reduce intermolecular electrostatic interactions, thus enhancing the macroscopic electro-optic activity. These properties, together with good solubility, suggest the potential use of these chromophores as advanced material devices.

Improved electro-optical property by introducing stronger acceptor to thermal stable chromophores using modified julolidine as donor

Two novel second order nonlinear optical chromophores based on large steric hindrance group modified julolidine donor and CF3-Ph-TCF acceptor linked via thiophene bridge were synthesized and characterized. Density functional theory (DFT) calculations suggested that the molecular quadratic hyperpolarizability (β) value of CF3-Ph-TCF based chromophore is 18.1% and 18.8% larger than that of TCF based chromophores, respectively. The optimized structure shows that chromophore z4-3f with stronger acceptor and larger modification group presents the best molecular configuration. UV–Vis results show great red-shift about 130 nm for chromophores z2-3f and z4-3f compared with z2 and z4. According to cyclic voltammetry measurements, chromophores z2-3f and z4-3f showed lower energy gap indicating better intramolecular charge-transfer (ICT) transition. Especially, chromophore z4-3f displayed good thermal stability with onset decomposition temperature at about 265 °C. The poled films containing 25 wt% chromophores in amorphous polycarbonate (APC) afforded electro-optical(EO) coefficients (r33) of 125 and 128 pm/V at 1310 nm. Further, film containing larger group modified chromophore was more efficient to realize good EO property with an order parameter (Φ) up to 18.4%. It's confirmed that chromophores z2-3f and z4-3f, which had good thermal stability and high r33 values after poling in films, was a promising candidate for practical use.

Bulky side chain effect of poly(N‐vinylcarbazole)‐based stacked polymer electrets on device performance parameters of transistor memories

ABSTRACTThree poly(N‐vinylcarbazole) (PVK)‐based polymer electrets were synthesized through Friedel‐Crafts postfunctionalization for the function of charge storage in nonvolatile organic field effect transistor (OFET) memory devices. The bulky side chain effect of these stacked polymer electrets on the morphology, water contact angles, and memory characteristics were examined with regard to those of precursor PVK. The introduction of steric hindrance groups could interrupt the large length of π‐stacked structures in PVK and block the form of region‐regular structures from region‐random on external electric field. As a result, the memories based on the three modified polymers exhibited approximate memory windows of 32 V increased by 13 V with respect to PVK. Besides, the write‐read‐erase‐read cycles stability of the modified polymers was superior to that of PVK. Furthermore, we found that the holes were mainly located in the region of local π‐stacked structures and bulky π‐conjugated groups also acted as additional electron trapping sites. Molecular engineering of charge trapping site with tunneling polymers will be a promise strategy for the advance of transistor memory. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 3554–3564

Enhanced electro-optic activity from the triarylaminophenyl-based chromophores by introducing different steric hindrance groups

Chromophores L1 and L2 based on modified triphenylamine donors with diverse steric hindrance groups, have been designed in order to achieve large macroscopic r33 value of NLO polymers comparing to traditional triarylaminophenyl chromophore T1. In electro-optic activities, the doped film L1-APC and L2-APC showed r33 value of 37pm/V and 49pm/V at the concentration of 25wt%, which were more than two times higher than that of film T1/APC (16pm/V). These properties, together with the good thermostability, suggest the potential use of the new chromophores as advanced material devices.

Synthesis of novel nonlinear optical chromophores: achieving enhanced electro-optic activity and thermal stability by introducing rigid steric hindrance groups into the julolidine donor

A series of chromophores based on julolidine donors modified by four different rigid steric hindrance groups with the same thiophene bridges and strong tricyanovinyldihydrofuran acceptors were synthesised.

Comparative evaluation of 11 in silico models for the prediction of small molecule mutagenicity: role of steric hindrance and electron-withdrawing groups

The goal of this investigation was to perform a comparative analysis on how accurately 11 routinely-used in silico programs correctly predicted the mutagenicity of test compounds that contained either bulky or electron-withdrawing substituents. To our knowledge this is the first study of its kind in the literature. Such substituents are common in many pharmaceutical agents so there is a significant need for reliable in silico programs to predict precisely whether they truly pose a risk for mutagenicity. The predictions from each program were compared to experimental data derived from the Ames II test, a rapid reverse mutagenicity assay with a high degree of agreement with the traditional Ames assay. Eleven in silico programs were evaluated and compared: Derek for Windows, Derek Nexus, Leadscope Model Applier (LSMA), LSMA featuring the in vitro microbial Escherichia coli–Salmonella typhimurium TA102 A-T Suite (LSMA+), TOPKAT, CAESAR, TEST, ChemSilico (±S9 suites), MC4PC and a novel DNA docking model. The presence of bulky or electron-withdrawing functional groups in the vicinity of a mutagenic toxicophore in the test compounds clearly affected the ability of each in silico model to predict non-mutagenicity correctly. This was because of an over reliance on the part of the programs to provide mutagenicity alerts when a particular toxicophore is present irrespective of the structural environment surrounding the toxicophore. From this investigation it can be concluded that these models provide a high degree of specificity (ranging from 71% to 100%) and are generally conservative in their predictions in terms of sensitivity (ranging from 5% t o 78%). These values are in general agreement with most other comparative studies in the literature. Interestingly, the DNA docking model was the most sensitive model evaluated, suggesting a potentially useful new mode of screening for mutagens. Another important finding was that the combination of a quantitative structure–activity relationship and an expert rules system appeared to offer little advantage in terms of sensitivity, despite of the requirement for such a screening paradigm under the ICH M7 regulatory guideline.