Combining form with function--the dawn of phosphole-based functional materials

Phosphole-based π-conjugated compounds have recently attracted significant attention due their unique electronic properties. It is now well established that the versatile phosphorus chemistry offers great opportunities for efficient fine-tuning of the properties of π-conjugated systems from a fundamental point of view; a feature that pure carbon-based π-conjugated materials cannot provide. This perspective highlights the recent progress using phosphole-based π-conjugated building blocks towards applied materials with multiple and diverse functionalities.     

Pluripotent Features of Doubly Thiophene-Fused Benzodiphospholes as Organic Functional Materials

Linear ladder-type π-conjugated molecules have attracted much interest because of their intriguing physicochemical properties. To modulate their electronic structures, an effective strategy is to incorporate main-group elements into ladder-type π-conjugated molecules. In line with this strategy, a variety of ladder-type π-conjugated molecules with main-group elements have been synthesized to explore their potential utility as organic functional materials. In this context, phosphole-based π-conjugated molecules are highly attractive, owing to their unique optical and electrochemical properties, which arise from the phosphorus atom. Herein, the synthesis and physicochemical properties of doubly thiophene-fused benzodiphospholes, as a new class of phosphole-based ladder-type π-conjugated molecule, are reported. Systematic investigations into the physicochemical properties of doubly thiophene-fused benzodiphospholes revealed their pluripotent features: intense near-infrared fluorescence, excellent two-photon absorption property, and remarkably high electron-transporting ability. This study demonstrates the potential utility of doubly thiophene-fused benzodiphospholes as organic functional materials for biological imaging, nonlinear optics, organic transistors, and organic photovoltaics.     

Structural,photoelectrical and thermol properties of ultra-stable Benzo[ghi]perylene trimide dimer anion

Perylene diimide and its various derivatives tend to form stable anion radicals, which can usually form a variety of aggregates with special properties. Herein, we reported synthesis and properties of the benzo[ghi]perylene trimide and its anion radical π-bonded dimer. The dimer exhibited high stable in solid status and in most organic solvents, but conversion of L-type π-bonded dimer to face to face type π-bonded dimer or π-stacked oligomer occurs in dimethyl formamide solution. The temperature and crystallization influenced the π-bonded length in the dimer and the partial memory can be reserved and probed in solution at room temperature.     

A semiempirical SCF–LCAO–MO treatment of some oxygen- and nitrogen-containing heterocyclic molecules

The π-electronic properties of furan, oxazole, benzofuran, benzoxazole, anthranil, and dibenzofuran are calculated by the semiempirical self-consistent-field molecular orbital method. A single set of parameters is found which satisfactorily reproduces the π → π* electronic transition energies and other π-electronic properties.     

吡咯与HX (X＝F,Cl, Br)分子间多种氢键的电子密度拓扑研究

采用密度泛函B3LYP/6-311＋＋G(d,p)方法, 对吡咯与HX (X＝F, Cl, Br)形成的经典氢键和π型氢键, 从其几何参数、电子密度的拓扑性质和电子积分等方面进行了研究. 在对π型氢键的讨论中我们将π电子与σ电子分离, 得到了π型氢键体系的π电子的密度等值线和拉普拉斯量等值线图以及各原子的π电子积分, 形象地说明了π型氢键的作用本质.     

Insulated π-conjugated metallopolymers

Recently, there has been a progressive development of insulated π-conjugated metallopolymers with accumulated features of π-conjugated bridging units, transition metal complexes, and encapsulating moieties, as higher-order functionalized materials. A number of insulated conjugated metallopolymers have been successfully synthesized and their fascinating properties have been reported. In addition to the conventional features derived from π-conjugation and transition metals, their insulated structures can compensate for solubility, a disadvantage in conventional metallopolymers, and enhance their functionalities, such as sensing, luminescence, and conduction. In this review, we summarize the synthetic methodologies, structural characteristics, and functionalities of one-dimensional insulated π-conjugated metallopolymers, while focusing on the effect of transition metals and insulation on their properties.     

Charge-Segregated Stacking Structure with Anisotropic Electric Conductivity in NIR-Absorbing and Emitting Positively Charged π-Electronic Systems

Squarylium-based π-electronic cation with an augmented dipole was synthesized by methylation of zwitterionic squarylium. The cation formed various ion pairs in combination with anions, and the ion pairs exhibited distinct photophysical properties in the dispersed state, ascribed to the formation of J- and H-aggregates. The ion pairs provided solid-state assemblies based on cation stacking. It is noteworthy that complete segregation of cations and anions was observed in a pseudo-polymorph of the ion pair with pentacyanocyclopentadienide as a π-electronic anion. In the crystalline state, the ion pairs exhibited photophysical properties and electric conductivity derived from cation stacking. In particular, the charge-segregated ion-pairing assembly induces an electric conductive pathway along the stacking axis. The charge-segregated mode and fascinating properties were derived from the reduced electrostatic repulsion between adjacent π-electronic cations via dipole–dipole interactions.     

Description of the radiative properties of unsaturated molecules containing a triple bond based on a quasi π-nodel

In this paper we provide a model for the theoretical calculation of radiative triplet properties of aromatic molecules containing a triple bond. Such molecules have special low-lying electronic states with the symmetry properties of a σπ* -state. These states axe caused by electron promotion from an “in-plane π-system” (quasi π-system), which in highly localized at the triple bond, to the aromatic π-system and vice versa. It is shown that these states lead to one-center spin-orbit integrals and therefore are highly active in radiative triplet deny. Theoretical and spectroscopic investigations of ethynylbenzene show that the radiative properties of the lowest triplet state, such as phosphorescence yield, emission spectrum and degree of polarization can be explained not only qualitatively but also quantitatively by restriction to these states (quasi π-model). The theoretical evaluation of the radiative rate was done with respect to individual promoting modes. It is shown that the first order rate (at the equilibrium position of the molecule), though allowed by symmetry, has nearly no influence on the radiative lifetime. As a consequence the first transition in the phosphorescence emission cannot be assigned as the 0-0 transition.     

Theoretical Studies on the Electronic Structures and Spectral Properties of 4,4′-（Ethane-1,2-diylbis-（oxy））dibenzaldehyde and Its Polymorph

The ground state geometries, frontier molecular orbital properties, and absorption properties of 4,4′-(ethane-1,2-diylbis(oxy))dibenzaldehyde (EDO-DBDHD) and its polymorph have been studied theoretically. The density functional theory (DFT) method was employed to optimize the ground state geometries, and theoretical data reveal that EDO-DBDHD features the planar molecular conformations, in contrast to V-shaped structures of its polymorph, which agrees with the experimental data. Additionally, the absorption spectra of both compounds were predicted using time-dependent density functional theory (TDDFT). The calculated results show that the lowest lying absorption bands of these compounds have the transition configurations of HOMO → LUMO, resulting in the transition character of π→π*/n→π*. The transition of HOMO → LUMO+3 mainly contributes to the highest lying absorption bands of two compounds at 225 nm with the character of π→π*/n→π*.     

A Symmetrically π-Expanded Carbazole Incorporating Fluoranthene Moieties

A novel doubly cyclopentannulated carbazole which is accessible through a successive π-expansion of di(1-naphthylamine) is disclosed. The carbazole moiety is generated in the final step through intramolecular oxidative coupling. The π-expansion of carbazole resulted in strongly altered optoelectronic and electrochemical properties. The solid-state structure features an interesting packing motif with alternating face-to-face π⋅⋅⋅π and edge-to-face C−H⋅⋅⋅π interactions. The experimental findings were corroborated by theoretical calculations.     

Four-coordinate organoboron compounds with a π-conjugated chelate ligand for optoelectronic applications

Four-coordinate organoboron compounds with a π-conjugated chelate backbone have emerged recently as highly attractive materials for a number of applications including use as emitters and electron-transport materials for organic light-emitting diodes (OLEDs) or organic field transistors, photoresponsive materials, and sensory and imaging materials. Many applications of this class of boron compounds stem from the electronic properties of the π-conjugated chelate backbone. Charge-transfer transitions from an aromatic substituent attached to the boron center of the π-conjugated chelate backbone and steric congestion have also been found to play important roles in the luminescent and photochromic properties of the four-coordinate boron compounds. This article provides an update-to-date account on the application aspects of this important class of compounds in materials science with the emphasis on OLED applications and photochromic switching.     

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.     

Photophysics of π-conjugated oligomers and polymers that contain transition metal complexes

There has been a surge of interest concerning the synthesis, optical and electronic properties of π-conjugated polymers that contain transition metal complexes. The integration of transition metal chromophores that feature metal to ligand charge transfer (MLCT) excited states into a π-conjugated polymer permits easy variation of the material’s optical and electronic properties. In this review, we survey a number of recent photophysical studies that examine π-conjugated oligomer or polymer/transition metal complex hybrids. The effects of the types of π-conjugated backbone, oligomer and polymer structure, the conjugation length and coordination to a variety of metal chromophores on the photophysics of the organic-metal hybrids are discussed. The degree of interaction between the polymer (or oligomer) and metal complex based excited states dramatically modulates the observed photophysics.     

Tunable third-order NLO properties of acene derivatives with molecular structural modification

The substitution of heteroatoms and the expansion of π-conjugated units have significant effects on the photoelectric properties of polycyclic aromatic hydrocarbons (PAHs). In this study, based on the experimental molecule PBC, 10 acene derivatives containing the pyrrole group were designed by three strategies: (1) changing the connection position of the pyrrole group, (2) exchanging the position of the N atom, and (3) increasing the length of the π-linker. Density functional theory (DFT) was used to optimize the molecular geometric structure. Time-dependent density functional theory (TD-DFT) was used to calculate the relevant parameters of the excited states. The results show that both of the pyrrole group and the N atom in the para-position, and the addition of the π-linker can reduce the energy gap, cause redshift of the linear absorption peak, and increase the two photon absorption (TPA) cross-section. The analyses of the charge density difference (CDD) and the charge-transfer matrix (CTM) proved that the electron transfer is mainly concentrated in the π-linker. Moreover, the participation of the multi-ring skeleton on both sides decreases gradually with the increase of the length of the π-linker. The length of the π-linker changes the dominant transition channel and intramolecular charge-transfer (ICT) characteristics of TPA, thus affecting the transition dipole moments and nonlinear absorption properties. The second hyperpolarizability of the designed molecule PBI5-p5 is also significantly superior to that of similar materials reported. It is expected that the above molecular design strategies and comprehensive analysis of nonlinear optical (NLO) properties could provide theoretical support for the research on acene derivatives.     

Enhance photocatalytic properties of TiO2 using π-π* conjugate system

Abstract

A novel π-π* conjugate system photo-catalyst was successfully constructed using aniline, pyrrole and TiO₂. The structures of photo-catalyst were measured by XRD, SEM, BET, TEM, FT-IR, XPS and TGA in detail. The photocatalytic properties were investigated in rhodamine B and methyl orange solution, respectively. And the enhance mechanism of π-π* conjugate system was discussed in depth. The weight percentage of Ti weight about 47% in the compound and the π-π* conjugate system did not change the crystal structure of TiO₂. The photocatalytic degradation properties of the π-π* conjugate system photo-catalyst could get 99% for rhodamine B and methyl orange after 10 and 15?min, respectively. The introduction of the π-π* conjugate system into TiO₂ was beneficial to improve light harvest, photoelectric response and separation of electron-holes.     

A way to stable, highly emissive fluorubine dyes: tuning the electronic properties of azaderivatives of pentacene by introducing substituted pyrazines

Pentacene and its derivatives are among the most important examples of π-electron-rich molecules used in organic field effect transistors. The replacement of CH groups by nitrogen atoms opens an elegant way to generate highly electron-deficient molecules, known as oligoazaacenes. We describe the synthesis and spectroscopic properties of two novel derivatives of this family, namely the zwitterionic and quinoidal conjugated forms of dihydro-5,6,7,12,13,14-hexaazapentacene (fluorubine). We outline a powerful strategy to tune the electronic properties of these redox-active azaacenes by the selective introduction of substituted pyrazines. Their acidochromic and solvatochromic behaviour is investigated experimentally and interpreted with the help of theoretical calculations. The simple "exchange" of substituents or protonation is shown to significantly alter the spectroscopic and electronic properties of these remarkably stable π-systems. Their exceptional optical properties, such as high fluorescence quantum yields combined with a redox-active behaviour, make them promising candidates for sensor materials. Additional marked features in the solid state, such as herringbone packing in combination with short π-π distances, will open access to electronic materials.     

Isomers of B←N-Fused Dibenzo-azaacenes: How B←N Affects Opto-electronic Properties and Device Behaviors?

The B←N unit has a large dipole and it is isoelectronic to C−C moiety with no dipole. Incorporating B←N units into π-conjugated system is a powerful strategy to design organic small molecules and polymers with intriguing opto-electronic properties and excellent opto-electronic device performance. However, it is unclear how the B←N unit affects electronic structures and opto-electronic properties of large π-conjugated molecules. In this work, to address this question, we developed three dibenzo-azaacene molecules in which two B←N units were introduced at different positions. Although the dibenzo-azaacene skeleton is fully π-conjugated, the effect of B←N unit on the electronic structures of the adjacent rings is much stronger than that of the distant rings. As a result, the three molecules with isomerized B←N incorporation patterns possess different electronic structures and exhibit tunable opto-electronic properties. Among the three molecules, the centrosymmetrical molecule exhibits higher LUMO/HOMO energy levels than those of the two axisymmetrical molecules. When used as the active layer in organic field-effect transistors (OFETs), while the two axisymmetrical molecules show unipolar electron transporting property, the centrosymmetrical molecule exhibits ambipolar hole and electron transporting behavior. This work not only deepens our understanding on organoboron π-conjugated molecules, but also indicates a new strategy to tune opto-electronic properties of organic semiconductors for excellent device performance.     

O-Doped Nanographenes: A Pyrano/Pyrylium Route Towards Semiconducting Cationic Mixed-Valence Complexes

Herein we report an efficient synthesis to prepare O-doped nanographenes derived from the π-extension of pyrene. The derivatives are highly fluorescent and feature low oxidation potentials. Using electrooxidation, crystals of cationic mixed-valence (MV) complexes were grown in which the organic salts organize into face-to-face π-stacks, a favorable solid-state arrangement for organic electronics. Variable-temperature electron paramagnetic resonance (EPR) measurements and relaxation studies suggest a strong electron delocalization along the longitudinal axis of the columnar π-stacking architectures. Electric measurements of single crystals of the MV salts show a semiconducting behavior with a remarkably high conductivity at room temperature. These findings support the notion that π-extension of heteroatom-doped polycyclic aromatic hydrocarbons is an attractive approach to fabricate nanographenes with a broad spectrum of semiconducting properties and high charge mobilities.     

Ladder-type P,S-bridged trans-stilbenes

Phosphole-containing π-systems have emerged as building blocks with enormous potential as electronic materials because of the tunability of the phosphorus center. Among these, asymmetric P-bridged trans-stilbenes are still rare, and here an elegant and efficient synthesis toward such fluorescent molecular frameworks is described. Fine-tuning of the photophysical properties is attempted by enforcing the planarization of the phosphorus tripod and thus increasing the interaction between the phosphorus lone pair and the π-system. The electronic structure of the π-conjugated frameworks is analyzed with NMR, UV-vis and fluorescence spectroscopy, and time-dependent density functional theory (TD-DFT) calculations.     

π-Conjugated Unit-Dependent Optical Properties of Linear Conjugated Oligomers

The optical properties of three linear conjugated oligomers （F-P, F-P-F, and P-F-P-F-P）, where phenothiazine （P） and fluorene （F） groups arrange alternately, are investigated. With the enhancement of the π-conjugated system, their absorption and emission bands both gradually red shift, and their two-photon properties are also improved. Meanwhile, their fluorescence dynamic traces are analyzed with continuous rate distribution model, exhibiting that their decay rates gradually accelerate and the rate distribution width become narrower. The quantum chemical calculation offers their molecular structures and transition mechanism, showing that the enhancement of π-conjugated system should be responsible for tile improvement of two-photon properties.