Electron-accepting Unit Research Articles

Facile direct arylation polycondensation of random semiconducting terpolymers for single-junction non-fullerene organic solar cells

Herein, we designed and synthesized two novel donor-acceptor1-acceptor2-acceptor1 (D-A1-A2-A1) type random conjugated terpolymers, DTP-TPD-DPP and DTP-TPD-BT, as a donor component for single-junction non-fullerene organic solar cells. The terpolymers were randomly combined from N-aryldithienopyrrole (DTP) donor unit and N-alkylthienopyrroledione (TPD) acceptor unit with electron-accepting dibrominated acceptor units including diketopyrrolopyrrole (DPP) or bithiophenebenzothiadiazole (TBT) by direct arylation polycondensation. Compared to two different acceptor units in the polymer backbone, DTP-TPD-DPP containing DPP exhibits stronger intramolecular charge transfer. Thus, the single-layer organic solar cells based on DTP-TPD-DPP:ITIC (1:1) demonstrated a power conversion efficiency (PCE) of 2.92 % and a high open-circuit voltage Voc of 1.0 V while devices using DTP-TPD-BT obtained a PCE of 2.64 % under the same condition fabrication in the ambient condition without any additives. The results showed that random conjugated terpolymer is an efficient and straightforward synthetic strategy to explore promising donor materials for high-performance single-junction organic solar cells.

Exploring the impact of electron acceptor tuning in D-π-A′-π-A photosensitizers on the photovoltaic performance of acridine-based DSSCs: A DFT/TDDFT perspective

Nine novel organic molecules with D-π-A′-π-A motifs, designated as (Ac-T-Qs-T-CA) and (Ac-T-Bx-T-CA), were proposed as potential photosensitizers for use in dye-sensitized solar cells (DSSCs). These novel organic photosensitizers are comprised of an acridine electron donor (Ac) and a thiophene π-spacer (T), while the electron acceptor unit (A′) is either a substituted quinoxaline (Qs) or a benzo[2,5]diazole-1-heteroatom (Bx), and the anchoring group (A) is 2-cyanoacrylic acid (CA). The influence of electron acceptor (A′) moieties on the electronic, photophysical, thermodynamic and photovoltaic properties of the photosensitizers was analyzed using DFT and TD-DFT calculations. The MM3 force field and CONFLEX conformational search methods were employed to study all possible structural conformations of the photosensitizers. The goal was to identify the lowest energy conformer and gain a better understanding of how the structure conformation and torsional angles of (Ac-T-Qs-T-CA) and (Ac-T-Bx-T-CA) compared to the reference photosensitizer RK1 affect electron injection ability and overall performance of DSSCs. The thermodynamic analysis revealed that the photosensitizers (Ac-T-Q2,3-diOMe-T-CA) and (Ac-T-BtA-T-CA), which contain a 2,3-dimethoxyquinoxaline (Q2,3-diOMe) and a benzo[d][1,2,3]triazole (BtA) acceptor moieties, respectively, exhibit a noteworthy enhancement in their electron-injection free energy (ΔGinj) and a reduction in their recombination-free energy (ΔGrec) values, leading to an improvement in the overall photovoltaic properties of the solar cell when compared to the other photosensitizers. The photovoltaic characteristics of the photosensitizers showed that the photosensitizer (Ac-T-Q2,3-diOMe-T-CA) demonstrated the highest values of open-circuit photovoltage (Voc = 1.41 eV) and light harvesting efficiency (LHE = 0.80 eV) compared to the other (Ac-T-Qs-T-CA) photosensitizers. Similarly, (Ac-T-BtA-T-CA) photosensitizer demonstrated the highest Voc (1.08 eV) and LHE (0.75 eV) among (Ac-T-Bx-T-CA) photosensitizers. The photovoltaic performance of these new photosensitizers outperformed the reference RK1 photosensitizer, which only achieved a Voc of 1.2 eV and LHE of 0.61 eV. Finally, the electronic structures and optical properties of photosensitizers adsorbed on the TiO2 surface were studied to provide more accurate information about their adsorption on the semiconductor surface.

Aptamer-Modified Homogeneous Catalysts, Heterogenous Nanoparticle Catalysts, and Photocatalysts: Functional "Nucleoapzymes", "Aptananozymes", and "Photoaptazymes".

Conjugation of aptamers to homogeneous catalysts ("nucleoapzymes"), heterogeneous nanoparticle catalysts ("aptananozymes"), and photocatalysts ("photoaptazymes") yields superior catalytic/photocatalytic hybrid nanostructures emulating functions of native enzymes and photosystems. The concentration of the substrate in proximity to the catalytic sites ("molarity effect") or spatial concentration of electron-acceptor units in spatial proximity to the photosensitizers, by aptamer-ligand complexes, leads to enhanced catalytic/photocatalytic efficacies of the hybrid nanostructures. This is exemplified by sets of "nucleoapzymes" composed of aptamers conjugated to the hemin/G-quadruplex DNAzymes or metal-ligand complexes as catalysts, catalyzing the oxidation of dopamine to aminochrome, oxygen-insertion into the Ar─H moiety of tyrosinamide and the subsequent oxidation of the catechol product into aminochrome, or the hydrolysis of esters or ATP. Also, aptananozymes consisting of aptamers conjugated to Cu2+ - or Ce4+ -ion-modified C-dots or polyadenine-stabilized Au nanoparticles acting as catalysts oxidizingdopamine or operating bioreactor biocatalytic cascades, are demonstrated. In addition, aptamers conjugated to the Ru(II)-tris-bipyridine photosensitizer or the Zn(II) protoporphyrin IX photosensitizer provide supramolecular photoaptazyme assemblies emulating native photosynthetic reaction centers. Effective photoinduced electron transfer followed by the catalyzed synthesis of NADPH or the evolution of H2 is demonstrated by the photosystems. Structure-function relationships dictate the catalytic and photocatalytic efficacies of the systems.

New application of an old dye: Bay-annulated indigo (BAI) as an organic photocatalyst for the oxidation of organic sulfides

Bay-annulated indigo (BAI), an old red dye first produced in 1914, which shows good performance as electron-accepting units in photovoltaic and semiconducting applications, was utilized as an organic photocatalyst for the first time. Under the irradiation of blue LEDs, organic sulfides can be selectively oxidized to corresponding sulfoxides with good to excellent yields. In addition, the photocatalytic oxidations of arylboronic acids and 2,3-dihydroquinazolinone derivatives were also investigated, indicating that it is a very promising organic photocatalyst for much more applications.

Porphyrin Acceptors Improve the Crystallization of Y6 and the Exciton Dissociation in Ternary Organic Solar Cells

The development of the third component is very important for ternary organic solar cells (OSCs). Herein, we synthesized two porphyrin electron acceptors (Zn-Por-IC4F and Zn-Por-IC) with an A-2π-D-2π-A (A: electron acceptor unit, D: electron donor unit) structure as the third component for PTB7-Th:Y6 devices. Zn-Por-IC4F and Zn-Por-IC in the ternary OSCs can improve the crystallization of Y6 and the exciton dissociation, therefore enhancing their fill factors (FF). As a result, the power conversion efficiencies (PCEs) of the PTB7-Th:Y6:Zn-Por-IC4F and PTB7-Th:Y6:Zn-Por-IC ternary OSCs are improved to 11.6 and 10.67%, which are increased by 21 and 11.5%, respectively, compared with that of PTB7-Th:Y6 binary devices with a PCE of 9.57%. This is the first report that porphyrins are employed as electron acceptors in ternary OSCs, though porphyrins have been widely used as donor materials of OSCs, which may open up an avenue for the applications of porphyrins in OSCs and provide guidelines for designing photovoltaic materials.

Main/side chain asymmetric molecular design enhances charge transfer of two-dimensional conjugated polymer/g-C3N4 heterojunctions for high-efficiency photocatalytic sterilization and degradation

Heterojunctions based on conjugated polymers (PHJs) are of promise as photocatalysts. Here, we fabricate the two-dimensional benzodithiophene (BDT) and thieno[2,3-f]benzofuran (TBF) based conjugated polymers/g-C3N4 PHJs creatively using the symmetry-breaking strategy. PD1 and PD3 with the asymmetric backbone TBF have better crystallinity. Moreover, PD3 utilizing fluorinated benzotriazole as the electron acceptor unit possesses more compact π − π stacking and higher charge mobility. The conjugated polymer PD5 with asymmetric side chains in the donor unit BDT guarantees more efficient charge transfer in the corresponding PD5/g-C3N4 PHJ while maintaining comparable light utilization rate. Consequently, PD5/g-C3N4 shows the champion performance with photocatalytic sterilization rates reaching 99.1% and 97.3% for S. aureus and E. coli. Notably, the reaction rate constant for Rhodamine B degradation of PD5/g-C3N4 is 8 times that of g-C3N4, a record high among conjugated polymers/g-C3N4. This study aims to reveal the structure − property correlation of asymmetric conjugated polymers/g-C3N4 for potential photocatalysis applications.

Diquat Based Dyes: A New Class of Photoredox Catalysts and Their Use in Aerobic Thiocyanation.

A series of new organic donor-π-acceptor dyes incorporating a diquat moiety as a novel electron-acceptor unit have been synthesized and characterized. The analytical data were supported by DFT calculations. These dyes were explored in the aerobic thiocyanation of indoles and pyrroles. Here they showed a high photocatalytic activity under visible light, giving isolated yields of up to 97 %. In addition, the photocatalytic activity of standalone diquat and methyl viologen through formation of an electron donor acceptor complex is presented.

Fluorinated Dihydropentalene-1,4-Dione: A Strong Electron-Accepting Unit with Organic Semiconductor Characteristics.

The development of electron-accepting units is of significant importance because the construction of donor (D)-acceptor (A) configurations is an effective strategy for tuning the electronic properties of π-conjugated systems. Although doubly fused pentagons represented by diketopyrrolopyrrole (DPP) have been used as an effective electron-accepting unit, the relatively high-lying frontier molecular orbital levels (FMOs) leave room for further improvement. We report herein the synthesis of a fluorinated dihydropentalene-1,4-dione (FPD) derivative as a strong electron-accepting unit and the development of D-A-D π-extended molecules. X-ray analyses revealed that the presence of fluorine atoms contributed to the formation of high planar structures and slipped-stacked packing. Electrochemical measurements indicated that the FPD derivatives showed relatively lower FMO energy levels than the corresponding DPP-containing derivatives. The D-A-D molecule based on terthiophene and FPD showed semiconducting responses. This study demonstrates that the FPD unit can function as a new acceptor unit for organic semiconductors.

Selenium˗decorated nitrogen-rich honeycomb-like g-C3N4 as anode materials for lithium ion batteries

Heteroatom doping is an efficient method for modifying the electronic properties of carbon, especially its lithium storage capacity. On the other hand, selenium (Se) doping in various forms of heteroatoms has received insignificant attention despite its excellent physiochemical features. We report a straight-forward approach for the synthesis of selenium decorated honeycomb-like g-C3N4 (Se–H-g-C3N4) through pyrolysis and selenization for the serviceability of the anode of a Li-ion battery. Structural analysis using X-ray photoelectron spectroscopy (XPS) indicated that Se can serve as a stronger electron acceptor unit than carbon and nitrogen. Selenium doping also had a big effect on the electrical structure of the carbon skeleton. This is because Se atoms significantly widened the gap between carbon sheets, which made more active sites for storing Li-ions and sped up the rate of diffusion. Because of these effects, Se–H-g-C3N4 outperformed bulk g-C3N4 and honeycomb-like g-C3N4 (H-g-C3N4) as anode material in terms of rate capability and specific capacity. This method of powerful synergistic effects is a new way to find lithium ion battery anode materials with high electrochemical performance, as shown by their long cycle life and high capacity.

Charge Manipulation in a Series of π-Stacked Pillared-Layer Frameworks by Tuning Electron Donation Ability of Building Blocks

One advantage of charge-transfer metal–organic frameworks comprising electron-donor (D) and electron-acceptor (A) units is their controllable electronic state. Herein, a new charge-ordered state with the formula Z+[D+D0A2–]− was revealed in π-stacked pillared-layer frameworks (π-PLFs), [FeCp*2][{Ru2(RxPhCO2)4}2(TCNQ)]·n(solv) (RxPhCO2– = R-substituted benzoate with R = m-F (1), and 3,5-F2 (2); [FeCp*2]+ = decamethylferrocenium; TCNQ = 7,7,8,8-tetracyano-p-quinodimethane; solv = interstitial crystallization solvents), where [FeCp*2]+ (Z+) acts as a pillar connecting the two-dimensional [D2A]− layers of [{Ru2(RxPhCO2)4}2(TCNQ)]− in the π-stacking mode. The prototype π-PLF with R = 2,3,5,6-F4 (0) had another charge-ordered state Z+[D02A–], whereas the Z+[D+D0A2–]−-type charge-ordered state in 1 and 2 was achieved by significantly enhancing the electron-donating ability of the [Ru2] units compared to that in 0, inducing electron transfer from [Ru2II,II] to TCNQ•– to produce a set of [Ru2II,III]+ and TCNQ2– in the layer. Compounds 1 and 2 were paramagnetic because of the isolation of paramagnetic species [Ru2II,II] (S = 1), [Ru2II,III]+ (S = 3/2), and [FeCp*2]+ (S = 1/2) shielded by diamagnetic TCNQ2– linkers. This work demonstrates the feasibility of charge manipulation, even in the π-PLF system, by controlling the electron-donating ability of the [Ru2] unit.

Donor to Acceptor Charge Transfer in Carbazole‐based Conjugated Microporous Polymers for Enhanced Visible‐Light‐Driven Photocatalytic Water Splitting

AbstractNowadays, conjugated microporous polymers (CMPs) of the donor‐acceptor (D‐A) type show excellent photocatalytic performance in water splitting due to their efficient isolation of light‐induced excitons. The key to this activity lies in the appropriate selection of electron donor and acceptor units. In this paper, we synthesized three D‐A carbazole‐based CMPs and investigated the influence of planarity, electronic structure, crystallinity, and surface area on their photocatalytic efficiency in hydrogen evolution from water. Our CMPs have excellent light collection ability, fast separation rates, and minimal recombination rates for the electron/hole pairs, so an excellent hydrogen evolution rate (HER) was achieved by water reduction. Moreover, the most efficient semiconductor Cz‐4Py CMP, which had the lowest dihedral angle, more suitable LUMO level, semi‐crystalline nature, and largest surface area, showed the best HER up to 8813 μmol g−1 h−1 under visible light with an apparent quantum yield (AQY) of 4.51 % at λ=420 nm. While the HERs for Cz‐4TPT and Cz‐4TzTz CMPs with higher dihedral angle, amorphous nature, and lower surface area were 1702 and 1541 μmol g−1 h−1, respectively. The efficiencies obtained exceed those of many polymeric materials reported for photocatalytic H2O reduction. This work confirms the importance of the molecular design of CMP photocatalysts to optimize their performance in hydrogen release by water dissociation.

Molecular structure tuning impact on optical linearity and nonlinearity of novel push-pull conjugated organic systems for photonic applications

In the present work, we have designed and synthesized push-pull conjugated (symmetric d-π-A-π-D type) organic chromophores 2-(2,6-bis((E)-2-(anthracen-9-yl) vinyl)-4H-pyran-4-ylidene)malononitrile (M2) and 2-(2,6-bis((E)-2-(pyren-1-yl) vinyl)-4H-pyran-4-ylidene) malononitrile (M3). These chromophores are the derivatives of 2-(2, 6-dimethyl-4H-pyran-4-ylidene) malononitrile (DPM) with DPM acting as an electron acceptor unit whereas anthracene and pyrene substituents are represented as the electron donor units in M2 and M3 respectively. The structural and photophysical properties of these compounds have been analyzed using IR, NMR, UV–Vis absorption and emission spectroscopies. We comparatively investigated the third-order NLO properties of these novel compounds in CHCl3 for different dye solution concentrations and various laser powers. Moreover, the effect of substitution (electron acceptor unit anthracene and pyrene) on the nonlinear optical properties have been discussed based on the structural modification. The quantum chemical calculations (DFT) were performed to obtain optimized geometry, frontier molecular orbitals, natural bond orbitals (NBO), and NLO parameters of these compounds. The time-dependent density functional theory (TD-DFT) was employed for insight into the electronic structure, photophysical, and NLO properties of these compounds. Experimental results have been supported by the obtained theoretical results.

A polymer donor based on difluoro-quinoxaline with a naphthalimide substituent unit exhibits a low-lying HOMO level for efficient non-fullerene polymer solar cells.

The design and synthesis of polymer donors with a low-lying highest occupied molecular orbital (HOMO) level are crucial for increasing open-circuit voltages (VOC) and achieving high-performance non-fullerene polymer solar cells. Here, we developed two copolymers using non-fluorinated or fluorinated thienyl-conjugated benzodithiophenes as electron donor units, and difluoro-quinoxaline with a naphthalimide substituent (DNB) as the electron acceptor unit. These copolymers, namely PDNB and PDNB-2F, exhibited deep HOMO levels owing to the strong electron-withdrawing ability of the naphthalimide substituent. Density-functional theory calculations demonstrated that the skeletons of the two copolymers featured good coplanarity. Owing to the fluorination, PDNB-2F displayed an increased absorption coefficient and deeper HOMO level than PDNB. Moreover, the blended film based on PDNB-2F:Y6 demonstrated enhanced carrier mobility, decreased bimolecular recombination as well as favorable phase-separation regions. Consequently, the PDNB-2F:Y6-based device yielded a superior power conversion efficiency (PCE) of 12.18%, whereas the device based on PDNB:Y6 showed a comparatively lower PCE of 8.83%. These results indicate that difluoro-quinoxaline with a naphthalimide substituent is a prospective electron-deficient building block to develop donor polymers with low-lying HOMO levels to achieve efficient non-fullerene polymer solar cells.

A Molecularly Tailored Photosensitizer with an Efficiency of 13.2% for Dye-Sensitized Solar Cells.

Photosensitizers yielding superior photocurrents are crucial for copper-electrolyte-based highly efficient dye-sensitized solar cells (DSCs). Herein, two molecularly tailored organic sensitizers are presented, coded ZS4 and ZS5, through judiciously employing dithieno[3,2-b:2″,3″-d]pyrrole (DTP) as the π-linker and hexyloxy-substituted diphenylquinoxaline (HPQ) or naphthalene-fused-quinoxaline (NFQ) as the auxiliary electron-accepting unit, respectively. Endowed with the HPQ acceptor, ZS4 shows more efficient electron injection and charge collection based on substantially reduced interfacial charge recombination as compared to ZS5. As a result, ZS4-based DSCs achieve a power conversion efficiency (PCE) of 13.2% under standard AM1.5G sunlight, with a high short-circuit photocurrent density (Jsc ) of 16.3mAcm-2 , an open-circuit voltage (Voc ) of 1.05V and a fill factor (FF) of 77.1%. Remarkably, DSCs sensitized with ZS4 exhibit an outstanding stability, retaining 95% of their initial PCE under continuous light soaking for 1000h. It is believed thatthis is a new record efficiency reported so far for copper-electrolyte-based DSCs using a single sensitizer. The work highlights the importance of developing molecularly tailored photosensitizers for highly efficient DSCs with copper electrolyte.

Naphthalimide-based conjugated macrocycles possessing tunable self-assembly and supramolecular binding behaviours.

The special geometric configurations and optoelectronic properties of p-conjugated macrocycles have always been the focus of materials science. The incorporation of building moieties with different features into macrocycles can not only change their geometric configurations, but also realize the regulation of intramolecular charge transfer, which is expected to bring unusual performance in supramolecular chemistry and optoelectronic devices. Herein, four novel p-conjugated macrocycles based on typical electron acceptor units naphthalimide (NMI) with aryl or alkyl substitutions were reported. The different substitutions on NMI had greatly affected the self-assembly behaviours of these macrocycles. Alkyl substituted NP2b and NP3b showed obvious self-aggregation in solution, while similiar phenomenon was not found in aryl substituted macrocycles NP2a and NP3a, which can be attributed to the steric hindrance caused by rigid aryl groups that could affect the aggregation of macrocycles in solution. In addition, all the macrocycles exhibited supramolecular encapsulation with C70, in which the larger macrocycles NP3a and NP3b with twisted geometries showed stronger binding affinity towards C70 than the corresponding small-size macrocycles NP2a and NP2b with near-planar geometries. Our studies have greatly extended the family of macrocycles based on NMI, pointing out the direction for further supramolecular studies and applications on p-conjugated macrocycles.

Synthesis, Properties, and Photovoltaic Characteristics of Fluoranthenedione-containing Nonfullerene Acceptors for Organic Solar Cells

The progress of organic solar cells (OSCs) largely depends on the development of nonfullerene acceptors (NFAs) based on electron-accepting π-conjugated compounds. Therefore, the creation of its building unit is important to tune the properties and increase the OSC performance. In this contribution, a new electron-accepting building unit, fluoranthenedione (FDO), was designed by extending the π-conjugation of representative indene-1,3-dione (IDO) framework. The electron-accepting π-conjugated compound (FL-FDO) composed of thiophene-linked fluorene (FL) as a central unit and FDO as terminal units was designed and synthesized. Compared to the corresponding IDO-containing NFAs, the introduction of FDO led to the red-shifted absorption and increased frontier orbital energy levels. The OSCs based on FL-FDO and poly(3-hexylthiophene) (P3HT) as a donor showed improved power conversion efficiency, compared to the P3HT/FL-IDO-based devices. These results indicate that the FDO unit has the potential to act as a promising terminal unit of NFAs.

Exploring the Optoelectronic Properties of D-A and A-D-A 2,2'-bi[3,2-b]thienothiophene Derivatives.

The synthesis of some novel donor-acceptor and acceptor-donor-acceptor systems containing a 2,2'-bi[3,2-b]thienothiophene donor block and various electron-accepting units is described alongside their photophysical properties studied using electrochemistry, optical spectroscopy and theoretical calculations. The obtained results show that the energy levels can be modulated by changing the strength of the acceptor unit. Among the three investigated end-groups, 1,1-dicyanomethylene-3-indanone exhibited the largest bathochromic shift and the lowest band gap suggesting the strongest electron-withdrawing character. Moreover, the emissive properties of the investigated systems vary greatly with the nature of the terminal group and are generally lower compared to their precursor aldehyde derivatives.

Thiadiazole-Based Covalent Organic Frameworks with a Donor-Acceptor Structure: Modulating Intermolecular Charge Transfer for Efficient Photocatalytic Degradation of Typical Emerging Contaminants.

As novel metal-free photocatalysts, covalent organic frameworks (COFs) have great potential to decontaminate pollutants in water. Fast charge recombination in COFs yet inhibits their photocatalytic performance. We found that the intramolecular charge transfer within COFs could be modulated via constructing a donor-acceptor (D-A) structure, leading to the improved photocatalytic performance of COFs toward pollutant degradation. By integrating electron donor units (1,3,4-thiadiazole or 1,2,4-thiadiazole ring) and electron acceptor units (quinone), two COFs (COF-TD1 and COF-TD2) with robust D-A characteristics were fabricated as visible-light-driven photocatalysts to decontaminate paracetamol. With the readily excited electrons in 1,3,4-thiadiazole rings, COF-TD1 exhibited efficient electron-hole separation through a push-pull electronic effect, resulting in superior paracetamol photodegradation performance (>98% degradation in 60 min) than COF-TD2 (∼60% degradation within 120 min). COF-TD1 could efficiently photodegrade paracetamol in complicated water matrices even in river water, lake water, and sewage wastewater. Diclofenac, bisphenol A, naproxen, and tetracycline hydrochloride were also effectively degraded by COF-TD1. Efficient photodegradation of paracetamol in a scaled-up reactor could be achieved either by COF-TD1 in a powder form or that immobilized onto a glass slide (to further ease recovery and reuse) under natural sunlight irradiation. Overall, this study provided an effective strategy for designing excellent COF-based photocatalysts to degrade emerging contaminants.

Exploring Solvent and Substituent Effects on the Excited State Dynamics and Symmetry Breaking of Quadrupolar Triarylamine End-Capped Benzothiazole Chromophores by Femtosecond Spectroscopy.

We investigate herein the excited state dynamics and symmetry breaking processes in three benzothiazole-derived two-photon absorbing chromophores by femtosecond fluorescence and transient absorption (fs-TA) spectroscopies in solvents of various polarity. The chromophores feature a quasi-quadrupolar D-π-A-π-D architecture comprised of an electron-withdrawing benzothiazole core and lateral triphenylamine donors (Qbtz-H), while the acceptor strength of the central unit is enforced by attached cyano groups (Qbtz-CN) and the electron-donating strength of the arylamine moieties by introduction of peripheral methoxy groups (Qbtz'-CN). Steady state spectroscopy reveals positive solvatochromism, which is mostly pronounced for Qbtz'-CN. Femtosecond spectroscopy of Qbtz-H reveals the coexistence of the Franck-Condon (FC) state and states populated after symmetry breaking (SB) in low-polarity solvents such as toluene and tetrahydrofuran, while the SB state becomes favorable in polar acetonitrile. For the other two molecules possessing a stronger electron-accepting unit and thus more polar excited state, SB takes place even in low-polarity solvents, as shown by fs-TA spectroscopy. Global fitting of the fs-TA spectra together with investigation of the evolution associated spectra (EAS) reveals the existence of an initial FC state in Qbtz-H, in all studied solvents, which relaxes toward Intermediate Charge Transfer (I-CT) and SB states. On the other hand, for Qbtz-CN and Qbtz'-CN in more polar solvents, the FC state undergoes ultrafast relaxation toward symmetry-broken charge transfer (SB-CT) states which in turn show very fast recombination to the ground state. Our measurements confirm that the extent of symmetry breaking is larger for D-π-A-π-D systems with the stronger acceptor core and increases further by increasing electron-donating strength of triarylamine moieties, giving rise to symmetry breaking in these nonionic quadrupolar molecules with ethynylene (triple bond) π-spacers also in less polar solvents.

Solvothermal synthesis of donor-acceptor covalent organic framework/coal-based polyaniline composites for three-state electrochromic materials

Conjugated covalent organic frameworks (COFs) with redox activity can be designed for photoelectric materials application. In this paper, COF based electrochromic films were synthesized by solvothermal method in a Teflon-lined reactor, with thieno [3,2-b]thiophene-2,5-dicarbaldehyde (TTDA) as electron donor unit (D) and 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TTA) as electron acceptor unit (A) (COF TT ). Coal-based polyaniline (CPANI) was used as COF TT growth substrate and conductive enhancing agent to improve the photoelectric performance of COF TT . The addition of CPANI results in the morphology transformation of COF TT from nanosheet to nanofiber. Meanwhile, the impedance of COF TT is reduced and the optical contrast is improved. The color of COF TT -CPANI-1 can vary from yellow to dark orange and brown under a voltage window of −3 V–2.2 V. Stability tests show that the contrast retention rate of the electrochromic device is 90.6% after 1600 s switching. Considering the rich selectivity of COF construction units and the designability of topological structure, the development of COF-based electrochromic materials with rich colors and excellent properties has a broad prospect. • A COF electrochromic film was prepared by solvothermal method. • The morphology and photoelectrical properties of COF films was changed by the addition of coal-based polyaniline. • Due to the donor-acceptor properties of the synthesis unit, the COF film presents a three-state electrochromic behavior.