Enhancement of Visible-Light-Driven Hydrogen Evolution Activity of 2D π-Conjugated Bipyridine-Based Covalent Organic Frameworks via Post-Protonation

Photocatalytic hydrogen (H₂) evolution represents a promising and sustainable technology. Covalent organic frameworks (COFs)-based photocatalysts have received growing attention. A 2D fully conjugated ethylene-linked COF (BTT-BPy-COF) was fabricated with a dedicated designed active site. The introduced bipyridine sites enable a facile post-protonation strategy to fine-tune the actives sites, which results in a largely improved charge-separation efficiency and increased hydrophilicity in the pore channels synergically. After modulating the degree of protonation, the optimal BTT-BPy-PCOF exhibits a remarkable H₂ evolution rate of 15.8 mmol g⁻¹ h⁻¹ under visible light, which surpasses the biphenyl-based COF 6 times. By using different types of acids, the post-protonation is proved to be a potential universal strategy for promoting photocatalytic H₂ evolution. This strategy would provide important guidance for the design of highly efficient organic semiconductor photocatalysts.     

Dismantling the Hyperconjugation of π-Conjugated Phosphorus Heterocycles

The development of π-extended phosphorus heterocycles has been rapidly increasing because of their unique optoelectronics properties, which are very often considered to be a consequence of special hyperconjugative interactions. However, the latter interactions have primarily been investigated within the five-membered species, phospholes, and they are often conceptually extrapolated to the rest of π-extended phosphorus heterocycles (including six-membered P-heterocycles) despite evident structural differences. Herein, we report, for the first time, a detailed investigation that sheds light on the hyperconjugative effects of a series of phosphorus heterocyclic systems by means of EDA and NBO calculations within a DFT framework. Our results lay the foundations for the future design of π-extended phosphorus heterocycles with improved optoelectronics properties.     

Deposition of π-Conjugated Polycyanate Ester Thin Films and Their Dielectric Properties

Two kinds of novel π-conjugated polycyanate esters, namely the plasma-polymerized 4-methoxyphenol cyanate ester (PPMPCE) and the plasma-polymerized 4-phenylphenol cyanate ester (PPPPCE), were successfully prepared by plasma polymerization for the first time. The structure and compositions of both plasma polycyanate esters were investigated by Fourier Transform Infrared (FT-IR), X-ray Photoelectron Spectroscopy (XPS) and UV–Visible Absorption Spectra (UV–Vis). The results show that extensively conjugated C=N double bonds were formed in the plasma-deposited cyanate ester thin films, the plasma polymerization of both monomers proceeded mainly via the opening of π-bonds of the O–C≡N functional groups which are further on being formed into a large π-conjugated system, this unique process is noticeably different from the conventional thermal polymerization reaction of cyanate ester monomers. Further dielectric measurement shows that PPPPCE thin film gives a lower dielectric constant comparing to that of the PPMPCE film, and the dielectric constant of both plasma deposited thin films decreased with an increase in measurement frequency.     

Benchmarking of Density Functionals for the Description of Optical Properties of Newly Synthesized π-Conjugated TADF Blue Emitters

Computational modeling of the optical characteristics of organic molecules with potential for thermally activated delayed fluorescence (TADF) may assist markedly the development of more efficient emitting materials for organic light-emitting diodes. Recent theoretical studies in this area employ mostly methods from density functional theory (DFT). In order to obtain accurate predictions within this approach, the choice of a proper functional is crucial. In the current study, we focus on testing the performance of a set of DFT functionals for estimation of the excitation and emission energy and the excited singlet-triplet energy gap of three newly synthesized compounds with capacity for TADF. The emitters are designed specifically to enable charge transfer by π-electron conjugation, at the same time possessing high-energy excited triplet states. The functionals chosen for testing are from various groups ranging from gradient-corrected through global hybrids to range-separated ones. The results show that the monitored optical properties are especially sensitive to how the long-range part of the exchange energy is treated within the functional. The accurate functional should also be able to provide well balanced distribution of the π-electrons among the molecular fragments. Global hybrids with moderate (less than 0.4) share of exact exchange (B3LYP, PBE0) and the meta-GGA HSE06 are outlined as the best performing methods for the systems under study. They can predict all important optical parameters correctly, both qualitatively and quantitatively.     

Gas-Phase Ion Fluorescence Spectroscopy of Tailor-made Rhodamine Homo- and Heterodyads: Quenching of Electronic Communication by π-Conjugated Linkers

While many key photophysical features are understood for electronic communication between chromophores in neutral compounds, there is limited information on the effect of charges in practically relevant ionic chromo/fluorophores. Here we have chosen positively charged rhodamines and prepared a selection of homo- and heterodimers with alkyl or π-conjugated, acetylenic bridges. Protonated molecules were transferred as isolated ions to gas phase where there is no solvent screening of charges, and fluorescence spectra were measured with a custom-made ion-trap setup. Our work reveals strong polarization of the π-spacer (induced dipole/quadrupole) when it experiences the electric field from one/ two dyes. Hence, π-spacers provide efficient shielding of charges by reducing the Coulomb interaction, whereas two dye cations polarize each other when connected by an alkyl. The screening influences the Förster resonance energy transfer efficiency that relies on the dipole–dipole interaction.     

Synthesis, Characterization and Properties of Novel Star-Shaped π-Conjugated Oligomers with Triphenylamine Core

Four new star‐shaped π‐conjugated oligomers ( TPA‐CZ3 , TPA‐TPA3 , TPA‐PTZ3 and TPA‐BT3 ) with triphenylamine as a core and different electron‐donating ability groups, carbazole, triphenylamine, phenothiazine and bithiophene, as peripheral units have been designed and synthesized via the Heck reaction. These oligomers show good solubility in common organic solvents. Their photophysical, electrochemical, electronic structure and charge transfer properties between these star‐shaped π‐conjugated oligomers and N,N′‐bis(1‐ethylpropyl)‐3,4:9,10‐perylene bis(tetracarboxyl diimide) (EP‐PDI) have been investigated by UV‐vis absorption spectra, photoluminescence (PL) spectra, cyclic voltammetry (CV) measurement, theoretical calculations and fluorescence quenching. The results show that the absorptions and fluorescences of TPA‐CZ3 , TPA‐TPA3 and TPA‐PTZ3 are red shifted with the electron‐donating ability of the peripheral unit increasing from carbazole to triphenylamine and phenothiazine. In addition, although the bithiophene group has a weaker electron‐donating ability than carbazole, triphenylamine and phenothiazine, the absorption and fluorescence of TPA‐BT3 have a red shift than those of TPA‐CZ3 , TPA‐TPA3 and TPA‐PTZ3 because TPA‐BT3 has a longer conjugation length than TPA‐CZ3 , TPA‐TPA3 and TPA‐PTZ3 . The triphenylamine core and the peripheral units can constitute a large conjugated structure. The fluorescence quenching properties indicate that efficient charge transfer can happen between the star‐shaped oligomers and EP‐PDI.     

Electronic structure and aromaticity of planar singlet π-electron monocyclic π2 carbenes

Induced -electron ring currents and relative diamagnetic susceptibilities of planar singlet -electron monocyclic ² carbenes are calculated in the coupled variant of perturbation theory in the CNO method. It is shown that the parity theorem holds for charged planar ² and ² carbenes formally generated by the detachment of a proton or a hydride ion from alternant hydrocarbons. This leads, in such carbenes, to the same energies of singlet-singlet and singlet-triplet transitions, electric polarizabilities, conjugation energies, -electron ring currents, diamagnetic susceptibilities and current contributions to chemical shifts, whereas the charge contributions to the latter are of opposite signs.Donbas State Academy of Building and Architecture, 1 Derzhavina St., 339023 Makeevka, Donetsk Region, Ukraine. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 32, No. 3, pp. 157–162, May–June, 1996. Original article submitted December 5, 1994.     

Development of Functionalized Cyclotriveratrylene Analogues: Introduction of Withdrawing and π-Conjugated Groups

Cyclotriveratrylene analogues (CTVs) are supramolecular bowl-shaped molecules known for their ability to complex organic and organometallic guests, to form liquid crystals, polymers, or nanostructures. In this Article, we report the synthesis of new cyclotriveratrylene analogues with fluorescence properties in which various electron-withdrawing or π-extended conjugated groups are appended to the wide rim ortho to the methoxy-donating groups. Synthetically, these functionalized CTVs cannot be obtained as CTVs with electron-rich functions by the typical method (i.e., the trimerization of the corresponding benzyl alcohol) but are prepared from a common key intermediate, the C(3)-triiodocyclotriveratrylene (CTV-I(3)), in good yields. Despite the synthetic difficulties encountered due to the presence of three reactive centers, we have demonstrated the possibility of performing Sonogashira coupling and Huisgen cycloaddition reactions directly to the CTV core for the first time. CTVs with π-extended conjugated groups reveal interesting fluorescence profiles. More broadly, this study utilizes CTV-I(3) to introduce novel functionalities into CTVs to keep exploring their potential applications.     

One-Pot Synthesis of 3-to 15-Mer π-Conjugated Discrete Oligomers with Widely Tunable Optical Properties

A series of all-donor type alternating (D1-D2) monodisperse oligomers based on cyclopentadithiophene (CPDT) and 1,4-difluorobenzene (DFB) with gradually increas...     

Influence of the Thiophene Ring on the Molecular Order of Structurally Simple π-Conjugated Smectogens: 13C NMR Study

Structurally simple rod-like π-conjugated mesogens with thiophene directly connected to phenyl, biphenyl, and fluorenone rings with terminal chains are synthesized respectively. The occurrence of smectic A/smectic C phases is concurred by a hot-stage optical polarising microscope (HOPM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). The static 1D and 2D ¹³C nuclear magnetic resonance (NMR) studies in the liquid crystalline phase are carried out to find the alignment-induced chemical shifts (AIS) and ¹³C−¹H dipolar couplings. The orientational order parameters of the mesogens determined from ¹³C−¹H dipolar couplings disclose that the long axis is not only collinear to the C3−C4 bond of the thiophene ring but also for the local axes of phenyl and biphenyl rings. For fluorenone-based mesogen, the molecular biaxiality is found to be high owing to the increased breadth of the molecule. The study unveils that the orientation of thiophene and the phenyl rings is similar in the current mesogens in stark contrast to mesogens, where thiophene is connected to phenyl rings through linking groups.     

Structural,Electronic and Optical Properties of the Monoclinic α-CoV_2O_6

First-principles calculations based on density functional theory are performed to investigate the structural,electronic and optical properties of monoclinic α-Co V2O6.Firstly,the geometry structures obtained by geometry optimization are consistent with the experimental values.Then,the energy band structure is studied using both GGA and GGA+U methods.It is found that the on-site Coulomb repulsion of the Co 3d orbital plays a key role in describing the electronic properties of α-Co V2O6,and is necessary to open the energy band gap.According to the partial density of states(PDOS),significant Co–O and V–O hybridizations are observed in the valence and conduction bands,respectively.Furthermore,the Co–O and V–O bonds are found to have significant covalent characters.Below the absorption threshold ~1.9 e V,no absorption can be detected.However,there exists a strong and wide absorption band in the energy range from 1.9 to 11 e V.Such novel optical properties imply that the α-Co V2O6 may have some potential optical applications.     

Theoretical Study on the Structural and Electronic Properties of the Reduced SnO2 （110） Surface

The reduced SnO2(110) surface has been investigated by using first-principles method with a slab model. By examining the vacancy formation energy of three kinds of reduced SnO2(110) surfaces, the most energetically favorable defect surface is confirmed to be the surface with the coexistence of bridging and in-plane oxygen vacancies, which is different with the traditional model by only removing bridging oxygen. The results of band structure calculations indicate that the electronic structure of this defect surface is similar to the SnO surface.     

Electrochemical Properties of the Sandwich and Carbonyl π-Complexes of Chromium with Fluoranthene

Redox properties of mono- and binuclear -complexes of Cr with fluoranthene with the composition of (⁶-C₁₆H₁₀)Cr(⁶-C₆H₆), (⁶-C₁₆H₁₀)Cr(CO)₃ and (-⁶,⁶-C₁₆H₁₀)Cr₂(⁶-C₆H₆)(CO)₃ are studied by cyclic voltammetry. Relations between half-wave potentials of redox processes and coordination sites of fragments Cr(⁶-C₆H₆)- and Cr(CO)₃ with the ligand and their nature are found.     

Helical Oligophenylene Linked with [2.2]Paracyclophane: Stereogenic π-Conjugated Dye for Highly Emissive Chiroptical Properties

A stereogenic π-system based on dimer ( 2 ) and trimer ( 3 ) of [2.2]paracyclophane (PC) and biphenyl was prepared and its structural, photophysical, and chiroptical properties were investigated. X-ray analysis revealed that the quaterphenyl moieties in 2 adopt a double helical structure anchoring [2.2]PC from both sides. Furthermore, 3 forms a isosceles triangle structure with a large chiral cavity. A homodesmotic reaction using DFT calculations revealed that 2 has a larger strain energy than 3 owing to its highly twisted phenylene linkers. Electronic and circular dichroic (CD) spectra were recorded in CH₂Cl₂ solution. The spectra of both 2 and 3 are similar, and their longest absorption band accompanying a remarkable Cotton effect is attributed to the transition from HOMO to LUMO, which is delocalized to the quaterphenyl moiety. These compounds exhibit fairly high fluorescence quantum yields (ϕ=0.70–0.83) and moderate dissymmetry factor (|g_CPL|=1.6×10⁻³) in circularly polarized luminescence (CPL).     

Tuning the π-Accepting Properties of Mesoionic Carbenes: A Combined Computational and Experimental Study

Mesoionic imidazolylidenes are recognized as excellent electron-donating ligands in organometallic and main group chemistry. However, these carbene ligands typically show poor π-accepting properties. A computational analysis of 71 mesoionic imidazolylidenes that bear different aryl or heteroaryl substituents in C2 position was performed. The study has revealed that a diphenyltriazinyl (Dpt) substituent renders the corresponding carbene particularly π-acidic. The computational results could be corroborated experimentally. A mesoionic imidazolylidene with a Dpt substituent was found to be a better σ-donor and a better π-acceptor compared to an Arduengo-type N-heterocyclic carbene. To demonstrate the utility of the new carbene, the ligand was used to stabilize a low-valent paramagnetic tin compound.