Symmetrical and Asymmetrical Thiophene-Coumarin-Based Organic Semiconductors.

Organic semiconductors are a valuable material class for optoelectronic applications due to their electronic and optical properties. Four new symmetric and asymmetric thiophene-coumarin derivatives were designed and synthesized via Pd-catalyzed Suzuki and Stille Cross-Coupling reactions. Single crystals of all synthesized thiophene-coumarin derivatives were obtained, and π···π interactions were observed among them. The π···π interactions were supported by UV-vis, transmission electron microscopy, and atomic force microscopy analyses. The photophysical and electrochemical properties of the coumarins were investigated and supported by density functional theory studies. Fluorescence quantum yields were recorded between 36 and 66%. Moreover, mega Stokes shifts (175 nm or 8920 cm-1) were observed in these new chromophore dyes. The emission and absorption colors of the thiophene-coumarin compounds differed between their solution and film forms. Electrochemically, the highest occupied molecular orbital levels of the coumarins increased with the 3,4-ethylenedioxythiophene group, leading to a narrowing of the band gap, while the phenyl bridge weakened the donor-acceptor interaction, expanding the band gap.

The Role of Electron-Donating Subunits in Cross-Linked BODIPY Polymer Films.

A new method for synthesizing cross-linked 4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes (BODIPYs) using a radical-based thiol-ene click reaction is developed. This method is simple, efficient, and cost-effective, and it produces polymers with unique optical, electrochemical, and surface morphology properties. Significant blue shifts in absorption and photoinduced electron transfer in emissions are observed in the cross-linked BODIPY thin films. Cross-linking also leads to the restriction of conjugation, which results in the breakage of the terminal vinyl group, an increase in the oxidation potential, and a slight upshift in the HOMO position. As a result, the electrochemical band gap is widened from 1.88 to 1.94 eV for polymer bearing N,N-dimethylamino-BODIPY and from 1.97 to 2.02 eV for polymer bearing N,N-diphenylamino-BODIPY moieties. Monomer thin films form planar surfaces due to crystallinity, while amorphous cross-linked BODIPY polymers form more rough surfaces. Additionally, photopatterning on the film surface is successfully performed using different patterned masks. This new method for synthesizing cross-linked BODIPYs has the potential to be used in a variety of applications, including organic electronics, bioimaging, and photocatalysis.

Viologen-Based Covalent Organic Frameworks toward Metal-Free Highly Efficient Photocatalytic Hydrogen Evolution.

Covalent organic frameworks (COFs) have shown promise in the field of photocatalysts for hydrogen evolution. Many studies have been carried out using various electroactive and photoactive moieties such as triazine, imide, and porphyrin to produce COFs with different geometric structures and units. Electron transfer mediators like viologen and their derivatives can accelerate the transfer of electrons from photosensitizers to active sites. Herein, the combination of a biphenyl-bridged dicarbazole electroactive donor skeleton with a viologen acceptor structure is reported for the photocatalytic hydrogen evolution of novel COF structures with various alkyl linkers {TPCBP X-COF [X = ethyl (E), butyl (B), and hexyl (H)]}. The structures became more flexible and exhibited less crystal behavior as the length of the alkyl chain increased according to scanning and transmission electron microscopy images, X-ray diffraction analyses, and theoretical three-dimensional geometric optimization. In comparison, the H2 evolution rate of the TPCBP B-COF (12.276 mmol g-1) is 2.15 and 2.38 times higher than those of the TPCBP H-COF (5.697 mmol h-1) and TPCBP E-COF (5.165 mmol h-1), respectively, under visible light illumination for 8 h. The TPCBP B-COF structure is one of the best-performing catalysts for the corresponding photocatalytic hydrogen evolution in the literature, producing 1.029 mmol g-1 h-1 with a high apparent quantum efficiency of 79.69% at 470 nm. Our strategy provides new aspects for the design of novel COFs with respect to future metal-free hydrogen evolution by using solar energy conversion.

Synthesis and photovoltaic properties of novel ferrocene-substituted metallophthalocyanines.

In this paper, a series of new metallophthalocyanines, including ferrocene groups, were designed, synthesized and, characterized, and their photovoltaic properties were investigated as alternative electron-donor materials in bulk heterojunction (BHJ) solar cells. These products were synthesized by a Sonogashira cross-coupling reaction between tetraiodophthalocyanine and ethynyl ferrocene. The newly synthesized phthalocyanines (4-6) were characterized by FT-IR, UV-Vis, 1H NMR, and MALDI-TOF spectroscopic methods and elemental analysis. The electrochemical characterizations were carried out by cyclic voltammetry as well as differential pulse voltammetry. Density functional theory calculations were realized to prove the charge separation between ferrocene as an electron-donor and the phthalocyanine ring as an acceptor. According to UV-Vis measurements, a 25 nm red-shift was observed for complex 4 compared with complexes 5 and 6. Finally, the photovoltaic performance of these compounds used as an electron-donor moiety in a BHJ device were investigated. A function of different blend ratios was tested by fabricating a series of BHJ devices with the architecture of FTO/PEDOT:PSS/4-6: PCBM blend/Ag with an identical thickness of the active layer. The results indicated that the photovoltaic conversion efficiency of BHJ devices exhibited a strong blend-ratio dependence. The maximum power conversion efficiency was obtained by 5-based devices, as 3.65%, with a blend ratio of 1.5 : 1.0 under standard AM 1.5 illumination.

Effect of thiophene linker addition to fluorene-benzotriazole polymers with the purpose of achieving white emission in OLEDs.

With the purpose of obtaining white emission from single layer organic light emitting diodes (OLEDs), fluorene benzotriazole based polymers with double bond subunits (namely TP2 and SP3 with and without thiophene linker, respectively) were synthesized by a Suzuki cross-coupling polymerization reaction. SP3 and TP2 were used as an emissive layer of the OLED devices due to their outstanding solubility in organic solvents, photoluminescence intensity and morphological suitability for fine thin film-forming capability. The optical, electrochemical, light emission and electroluminescence characteristics, Density Functional Theory (DFT) calculations and admittance spectroscopic analysis of OLEDs based on SP3 and TP2 were realized in detail to understand the effects of thiophene linker addition as a donor unit to the main chain of fluorene benzotriazole based polymers. As a result, TP2 emitted a bright yellow emission with a maximum brightness of 243 cd m-2 at 40 mA cm-2, and a maximum current efficiency of 1.38 cd A-1 with more broad electroluminescence characteristics than SP3 polymer without the thiophene linker. SP3 emitted a greenish yellow emission with a maximum brightness of 1001 cd m-2 at 845 mA cm-2, and a maximum current efficiency of 0.33 cd A-1. Carrier transport properties, charge carrier transit time and the equivalent circuit modelling studies were obtained through admittance spectroscopy. An equivalent circuit model with a combination of two resistors and one capacitor explained the charge conduction mechanism of SP3 and TP2 based OLEDs. SP3 and TP2 OLED devices represented typical p-type transporting characteristics with mobilities of 0.073 and 0.017 cm2 V-1 s-1, respectively with simplified device configuration. All the results indicate that thiophene addition to the main chain of fluorene benzotriazole based polymers with double bond subunits lead to a promising candidate for white emissive materials used in single layer white OLEDs.

Photoinduced synthesis of poly(N-ethylcarbazole) from phenacylium salt without conventional catalyst and/or monomer.

We report a new method for the photoinduced synthesis of poly(N-ethylcarbazole) (PEC). Inspired by the photochemical decomposition and electron transfer reactions of N-phenacylethyl carbazolium hexafluorophosphate (ECPhAc) salt, we used ECPhAc as the single molecule possessing both initiator and monomer structures to form PEC with relatively higher yield and higher molecular weight compared to that obtained from the bare monomer. This step-growth approach, eliminating the use of additional oxidizing agent, involves successive photoexcitation, cleavage, electron transfer, proton release, and coupling steps. The structural characteristics, electrochemical properties, and surface morphologies of the obtained PEC before and after the dedoping process were investigated by UV-vis, FT-IR, fluorescence, NMR, MALDI-TOF, SEC, AFM, cyclic voltammetry (CV), and differential pulse voltammetry (DPV). Doped PEC exhibited white light emission covering all visible regions offering a potential application in white light OLEDs.

Photoinduced Step-Growth Polymerization of N-Ethylcarbazole.

A novel method for photochemical step-growth synthesis of poly( N-ethylcarbazole) (PEC) via consecutive diphenyl iodonium hexafluorophospate (Ph2I+PF6-) mediated electron transfer and coupling reactions is reported. The photoinduced electron transfer reaction of the excited N-ethylcarbazole (EC) in the presence of Ph2I+PF6- as the oxidizing salt proceeded efficiently giving EC radical cations (EC+•). Subsequently, the protons released concomitantly with coupling of two EC radical cations. The successive reactions involving excitation, electron transfer, proton release, and coupling lead to the formation of PEC. The electrochemical properties and surface morphology of the thin films of the formed polymers before and after dedoping were investigated by cyclic voltammetry, differential pulse voltammetry, and atomic force microscopy techniques, respectively.