Nonconjugated Spacer Research Articles

Synthesis, optical properties and DFT-TDDFT computational study of phenothiazine dye: donor-acceptor molecules

PurposeThe purpose of this study aims to synthesize a novel donor–acceptor dye based on phenothiazine as a donor (D) and nonconjugated spacer was devised and synthesized by condensing of 2,2'-(1H-indene-1,3(2H)-diylidene) dimalononitrile with aldehyde and the practical synthesis methodology as given in Scheme 1.Design/methodology/approachThe prepared phenothiazine dye was systematically experimentally and theoretically examined and characterized using nuclear magnetic resonance spectroscopy (1H,13C NMR), Fourier-transform infrared spectroscopy (IR) and high-resolution mass spectrometry. Density functional theory (DFT) and time-dependent density functional theory DT-DFT calculations were implemented to determine the electronic properties of the new dyeFindingsThe UV-Vis absorption and fluorescence spectroscopy of the synthesized dye was investigated in a variety of solvents with varying polarities to demonstrate positive solvatochromism correlated with intramolecular charge transfer (ICT). The probe’s quantum yields (Фf) are experimentally measured in ethanol, and the Stokes shifts are found to be in the 4846–9430 cm−1 range.Originality/valueThe findings depicted that the novel (D-π-A) chromophores may act as a significant factor in the organic optoelectronics.

Role of Linker Functionality in Polymers Exhibiting Main-Chain Thermally Activated Delayed Fluorescence.

Excellent performance has been reported for organic light‐emitting diodes (OLEDs) based on small molecule emitters that exhibit thermally activated delayed fluorescence. However, the necessary vacuum processing makes the fabrication of large‐area devices based on these emitters cumbersome and expensive. Here, the authors present high performance OLEDs, based on novel, TADF polymers that can be readily processed from a solution. These polymers are based on the acridine‐benzophenone donor–acceptor motif as main‐chain TADF chromophores, linked by various conjugated and non‐conjugated spacer moieties. The authors’ extensive spectroscopic and electronic analysis shows that in particular in case of alkyl spacers, the properties and performance of the monomeric TADF chromophores are virtually left unaffected by the polymerization. They present efficient solution‐processed OLEDs based on these TADF polymers, diluted in oligostyrene as a host. The devices based on the alkyl spacer‐based TADF polymers exhibit external quantum efficiencies (EQEs) ≈12%, without any outcoupling‐enhancing measures. What's more, the EQE of these devices does not drop substantially upon diluting the polymer down to only ten weight percent of active material. In contrast, the EQE of devices based on the monomeric chromophore show significant losses upon dilution due to loss of charge percolation.

S-Tetrazine donor-acceptor electrodeposited layer with properties controlled by doping anions generally considered as interchangeable

Two donor-acceptor (D-A) s-tetrazine derivatives are reported, their electrochemical, optical and spectroelectrochemical (UV-Vis and EPR) analysis is carried out. Thiophene ring, in the case of TzTH and terthiophene unit for Tz3TH serves as a donor group which are linked with s-tetrazine ring via nonconjugated spacer. Investigations have shown that such design of molecule lets to tune the band-gap by changing in the strength of donor part without affecting of the reduction process of the s-tetrazine ring. Additionally, Tz3TH undergoes electrochemical polymerization and depending on doping anion (BF4−orPF6−) used for electropolymerization, the electrodeposited layers exhibit different properties. By selection of proper electrolyte salt, the stability and conjugation level of obtained layer can be tuned.

Toughening of a Soft Polar Polythiophene through Copolymerization with Hard Urethane Segments.

Polar polythiophenes with oligoethylene glycol side chains are exceedingly soft materials. A low glass transition temperature and low degree of crystallinity prevents their use as a bulk material. The synthesis of a copolymer comprising 1) soft polythiophene blocks with tetraethylene glycol side chains, and 2) hard urethane segments is reported. The molecular design is contrary to that of other semiconductor‐insulator copolymers, which typically combine a soft nonconjugated spacer with hard conjugated segments. Copolymerization of polar polythiophenes and urethane segments results in a ductile material that can be used as a free‐standing solid. The copolymer displays a storage modulus of 25 MPa at room temperature, elongation at break of 95%, and a reduced degree of swelling due to hydrogen bonding. Both chemical doping and electrochemical oxidation reveal that the introduction of urethane segments does not unduly reduce the hole charge‐carrier mobility and ability to take up charge. Further, stable operation is observed when the copolymer is used as the active layer of organic electrochemical transistors.

Progress in organic fluorescent probes for detecting metal ions in environment

The massive emission of various pollutants leads to severe environmental problems, and the air and water issues attract enormous attention. Heavy metal ions highly affect our life and may cause many diseases, which is a critical issue to be solved. As reported, copper ion is highly related to some neurodegenerative diseases, such as Menkes and Wilson’s diseases. Children are susceptible to mercury ion during the early stages of development, leading to a variety of diseases relating to the brain, kidney and central nervous system. Thus, it is of vital importance to develop facile and fast detection methods for life-threatening metal ions. Organic dyes with various functional groups have shown great potential for detecting various metal ion pollutants through apparent color and fluorescence changes, exhibiting excellent selectivity and sensitivity. In terms of photophysical properties and chemical structures of organic dyes, we summarize recent research progress of the commonly seen detection mechanisms for metal ions. The detection mechanisms include photoinduced electron transfer (PET), intramolecular charge transfer (ICT), fluorescence resonance energy transfer (FRET), through-bond energy transfer (TBET), aggregation-disaggregation effect and rearrangement of molecular structures. As one of the mostly studied mechanisms, PET has been widely used in the construction of metal ion probes. These metal ion probes generally consist of three parts including fluorophores, spacers, and recognition sites. Most probes based on the PET mechanism are easily designed with high sensitivity and selectivity. However, most of these probes have no built-in correction, resulting in severe external environment interference. Compared with the PET mechanism, ICT mechanism has great superiority for the construction of ratiometric probes with built-in correction, which can minimize external interference and make quantitative detection more reliable. In addition, ICT mechanism provides another way for detecting some electron-donating and electron-accepting metal ions, such as Fe2+, Cu2+ and Zn2+. To achieve FRET within the probe, fluorophores are usually linked by a nonconjugated spacer, and the energy transfers from the donor to acceptor. In this scenario, spectral overlap is required. On the contrary, in TBET-based fluorescent chemosensors, a rigid conjugated linker is needed to chemically connect fluorophores and thus spectral overlap is not required. However, most of the reported FRET and TBET chemosenmsors are based on rhodamine dyes. It is highly necessary to develop novel energy transfer systems for expanding the variousness of metal ions probes. Naked-eye detection is the one of the most convenient methods for environmental metal ion sensing. Modulating the π-conjugated system of probes result in huge absorption spectrum change and lead to apparent color change of the probe, which is a promising strategy for developing naked-eye metal ions probes with high contrast. Moreover, mechanisms with aggregation-induced signal change including aggregation-caused quenching (ACQ) and aggregation-induced emission (AIE) are studied for environmental pollutant metal ion detection. Above all, these sensing strategies guarantee the successful development of metal ion probes. In this review, the basic design principles and responding mechanisms of the metal ion probes are discussed. Most of the probes can chelate with metal ions and some special reactions between probes and metal ions are also introduced including desulfurization reaction, redox reaction and Heck reaction etc. At last, the developing trends and the prospects of metal ion probes are presented.

Room‐Temperature Phosphorescence Invoked Through Norbornyl‐Driven Intermolecular Interaction Intensification with Anomalous Reversible Solid‐State Photochromism

AbstractHerein, norbornyl (NB), a bulky annular nonconjugated spacer, is melded into π systems to construct two groups of ladder‐type room‐temperature phosphorescence (RTP) luminogens. The effect of the NB on π‐π interactions, packing modes and RTP performance is explored systematically. The experimental and computational results demonstrate the versatility of NB in reducing π‐π distances and synergistically intensifying the intermolecular interactions, which not only induces intersystem crossing from S1 to Tn but also diminishes the nonradiative decay of triplet excitons. Impressively, 1800‐fold phosphorescence lifetime enhancement is achieved in comparison with the reference compounds without NB. The molecular packing and RTP performance can be further modulated by the length of the backbones and terminal end‐groups. It is quite peculiar that NB‐annulated phthalic acid exhibits reversible photochromism in the solid state, likely due to the formation of persistent radical pairs. Our study paves an ingenious avenue towards enhancing intermolecular interactions and provides significant implications for a better comprehensive understanding of the origin of their RTP and the inherent photophysical mechanism.

Room-Temperature Phosphorescence Invoked Through Norbornyl-Driven Intermolecular Interaction Intensification with Anomalous Reversible Solid-State Photochromism.

Herein, norbornyl (NB), a bulky annular nonconjugated spacer, is melded into π systems to construct two groups of ladder-type room-temperature phosphorescence (RTP) luminogens. The effect of the NB on π-π interactions, packing modes and RTP performance is explored systematically. The experimental and computational results demonstrate the versatility of NB in reducing π-π distances and synergistically intensifying the intermolecular interactions, which not only induces intersystem crossing from S1 to Tn but also diminishes the nonradiative decay of triplet excitons. Impressively, 1800-fold phosphorescence lifetime enhancement is achieved in comparison with the reference compounds without NB. The molecular packing and RTP performance can be further modulated by the length of the backbones and terminal end-groups. It is quite peculiar that NB-annulated phthalic acid exhibits reversible photochromism in the solid state, likely due to the formation of persistent radical pairs. Our study paves an ingenious avenue towards enhancing intermolecular interactions and provides significant implications for a better comprehensive understanding of the origin of their RTP and the inherent photophysical mechanism.

Electrochromic behavior of films and «smart windows» prototypes based on π-conjugated and non–conjugated poly(pyridinium triflate)s

This study is focused on two electrochromic poly(pyridinium triflate)s. They are aromatic rigid-rod polymer PV1 and PV12 containing alkyl linkages. Electrochromic behavior of both polymers was comparatively investigated by employing a three-electrode cell and «smart windows» prototypes. The electrochromic properties of polymer films were examined by electrochemical and spectroelectrochemical methods. Polymers with π-conjugate and non-conjugate structure demonstrate reversible redox process accompanied by a reversible color change both in three-electrode cell and electrochromic device (ECD). In general, introduction of the non-conjugated spacers to the PV12 backbone structure leads to deterioration in the optical contrast and switching time in comparison with π-conjugated PV1. The same effect was also detected for ECD. The reason behind this is thought to be a complication of electron transport in the system due to non-conjugated PV12 backbone structure.

Effect of Nonconjugated Spacers on Mechanical Properties of Semiconducting Polymers for Stretchable Transistors

AbstractNonconjugated segments in polymer semiconductors have been utilized to improve the processability of semiconducting polymers. Recently, several reports have described the improvement of stretchability of polymer semiconductors by incorporating nonconjugated spacers. However, the effect of relative flexibility of such conjugation breakers on mechanical and electrical properties has not yet been studied systematically. Here, conjugation breakers with different chain length and rigidity are incorporated into the backbone of diketopyrrolopyrrole‐based semiconductors. Interestingly, it is observed that the longer and more flexible conjugation breakers result in greater ductility and lower elastic modulus without significantly affecting mobility. The enhancement of stretchability is attributed to the reduced modulus and the decrease in crystallinity, as confirmed by X‐ray diffraction. With this newly established molecular design, transistors are prepared with a semiconducting polymer containing dodecyl segments as conjugation breakers. It is observed that this polymer retains a mobility of >0.36 cm2 V−1 s−1 at 100% strain, and after 100 cycles at 50% strain. Finally, its high stability against strain is also observed with a fully stretchable transistor fabricated. Taken together, the above results indicate that molecular engineering of conjugated polymers, i.e., by incorporating suitable conjugation breakers, can effectively tune mechanical properties without significantly compromising their electrical properties.

Unified model for singlet fission within a non-conjugated covalent pentacene dimer

When molecular dimers, crystalline films or molecular aggregates absorb a photon to produce a singlet exciton, spin-allowed singlet fission may produce two triplet excitons that can be used to generate two electron–hole pairs, leading to a predicted ∼50% enhancement in maximum solar cell performance. The singlet fission mechanism is still not well understood. Here we report on the use of time-resolved optical and electron paramagnetic resonance spectroscopy to probe singlet fission in a pentacene dimer linked by a non-conjugated spacer. We observe the key intermediates in the singlet fission process, including the formation and decay of a quintet state that precedes formation of the pentacene triplet excitons. Using these combined data, we develop a single kinetic model that describes the data over seven temporal orders of magnitude both at room and cryogenic temperatures.

Charge carrier mobility through vacuum–sublimed glassy films of s-triazine- and carbazole-based bipolar hybrid and unipolar compounds

Instead of physical mixtures as bipolar charge transport media for organic electronics, chemical hybrids comprising non-conjugated spacers are explored to strive for miscibility and morphological stability. Bipolar TRZ-3Cz(MP)2 and TRZ-1Cz(MP)2 as well as unipolar C3-2TRZ(2tBu), C2-2TRZ(2tBu) and C3-2Cz(MP)2 were synthesized with propylene or ethylene spacers serving to decouple the Cz(MP)2 and TRZ moieties. Glassy films were prepared by vacuum sublimation for the characterization of transport properties using the photocurrent time-of-flight technique. The results indicate that both the TRZ:Cz(MP)2 ratio and the spacer length enable charge-carrier mobility to be modulated across orders of magnitude. The C2-2TRZ(2tBu) film exhibits the highest electron mobility of all the unipolar TRZ-based glassy films reported to date.

Effects of the number of chromophores and the bulkiness of a nonconjugated spacer in a dye molecule on the performance of dye-sensitized solar cells

Three organic photosensitizers, Single-, Double- and Triple-CBZ, containing one, two or three carbazole-based chromophores, respectively, and a nonconjugated spacer in each molecule, were designed and successfully synthesized. The electro-optical and photovoltaic properties of the photosensitizers were investigated in terms of molecular microstructure, i.e., the number of the chromophores and the bulkiness of the nonconjugated spacer. A dye-sensitized solar cell (DSSC) based on Triple-CBZ containing three carbazole chromophores and a bulky tris(hexyloxyphenyl)ethane (THPE) spacer showed a considerably improved power conversion efficiency (PCE) of over 55%, mainly due to the enhanced open-circuit voltage (Voc) and short circuit current, when compared with that of the DSSC with Double-CBZ (or Single-CBZ) containing two (or one) carbazole chromophores and a hexyl spacer. More chromophores in the dye molecule increased the molar extinction coefficient, thereby enhancing the energy harvesting efficiency. In addition, a bulkier spacer was favorable for effectively protecting the charge recombination between TiO2 and electrolytes, thereby improving Voc by the longer electron lifetime.

Effect of dendron generation on properties of self-host heteroleptic green light-emitting iridium dendrimers

A series of self-host heteroleptic green light-emitting iridium (Ir) dendrimers G1 and G2 have been synthesized under mild conditions with high yields, and their photophysical, electrochemical and electroluminescent properties are investigated in detail. Compared with the model compound G0, both G1 and G2 exhibit similar photophysical and electrochemical properties, indicating that the incorporation of carbazole dendrons via a flexible non-conjugated spacer can retain the independence of the emissive Ir core. However, the device performance gradually increases with the increasing dendron generation due to the reduced intermolecular interactions. As a result, a peak luminous efficiency of 17.2cd/A has been obtained for the G2-based non-doped device, which is about 6 times that of G0. Further dispersing the dendrimer G2 into a host matrix, the efficiency can be improved to 29.2cd/A.

Synthesis and properties of partially conjugated hyperbranched light‐emitting polymers

AbstractVia A2 + B4 and A2 + B3 [where A2 is 1,4‐distyrylol‐2,5‐butoxybenzene, B3 is 1,1,1‐tris‐(p‐tosyloxymethyl)‐propane, and B4 is pentaerythritol tetra(methyl benzene sulfonate)] approaches, we synthesized two kinds of partially conjugated hyperbranched polymers, hyperbranched polymer with 3 arms (HP1) and hyperbranched polymer with 4 arms (HP2), which had rigid conjugated segments [oligo‐poly(phenylene vinylene)] and flexible, nonconjugated spacers arranged alternately through ether bonds in the skeleton. The conjugated segments were modified by pendant butoxy groups, which imparted the resulting polymers with excellent solubility in common organic solvents and excellent film‐forming abilities. Fourier transform infrared and nuclear magnetic resonance spectroscopy were used to identify the structure of the monomers and polymers. Thermal property investigations showed that two polymers both had good thermal stability with their decomposition temperatures in the range 396–405°C and high glass‐transition temperatures, which are of benefit to the fabrication of high‐performance light‐emitting devices. The photophysical properties were studied, and the relative photoluminescence quantum efficiencies of HP1 and HP2 in dilute chloroform solution amounted to 56.8 and 49.3%, respectively. A brief light‐emitting diode device with a configuration of indium tin oxide/HP1/Ca/Al was fabricated, and its electroluminescence performance was studied. The brightness of the device reached an optimistic maximum of 190 cd/m2 at 8.2 V. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Film Properties of Polyazomethines (2). Poly(imide-azomethine)s Derived from Ester-containing Dialdehydes, Tetracarboxylic Dianhydrides, and a Fluorinated Diamine

Poly(imide-azomethine)s (PIAzMs) were prepared from 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA) and terephthalaldehyde (TPAL) with 2,2'-bis(trifluoromethyl)benzidine (TFMB) to improve the dielectric constants and the film transparency while keeping low CTE characteristics. However, this approach using CBDA instead of pyromellitic dianhydride (PMDA) was less effective for improving the film transparency than initially expected. The results were explained by a rather strong contribution around the azomethine units to the electronic conjugation. As an alternative approach, novel dialdehyde monomers, 4-formylphenyl-4'-formylbenzoate (FPFB) and bis(4-formylphenyl)terephthalate (FPTP) were synthesized in which two formyl groups of the monomers were separated via some nonconjugated para-ester spacers. The PIAzM film derived from FPFB, PMDA, and TFMB achieved excellent combined properties, an extremely low CTE (—0.81 ppmK-1), a low water absorption (0.33%), a very high tensile modulus (6.3 GPa), sufficient film flexibility ( εb = 14%), good thermal stability, and improved transparency. A mechanism for the generation of a negative CTE was also proposed. The use of FPTP with a longer ester spacer allowed further improvement of the film transparency (cut-off wavelength = 362 nm) and the optically estimated dielectric constant ( εcal = 2.97) on the basis of reduced electronic conjugation. The results suggest that the ester-containing PIAzMs can be promising candidates as a new type of heat-resistant dielectric materials.

Synergetic Effect of Efficient Energy Transfer and 3D π−π Stack for White Emission Based on the Block Copolymers Containing Nonconjugated Spacer

A series of block copolymers containing nonconjugated spacer and 3D pi-pi stacking structure with simultaneous blue-, green-, and yellow-emitting units has been synthesized and characterized. The dependence of the energy transfer and electroluminescence (EL) properties of these block copolymers on the contents of oligo(phenylenevinylene)s has been investigated. The block copolymer (GEO8-BEO-YEO4) with 98.8% blue-emitting oligomer (BEO), 0.8% green-emitting oligomer (GEO), and 0.4% yellow-emitting oligomer (YEO) showed the best electroluminescent performance, exhibiting a maximum luminance of 2309 cd/m(2) and efficiency of 0.34 cd/A. The single-layer-polymer light-emitting diodes device based on GEO2-BEO-YEO4 emitted greenish white light with the CIE coordinates of (0.26, 0.37) at 10 V. The synergetic effect of the efficient energy transfer and 3D pi-pi stack of these block copolymers on the photoluminescent and electroluminescent properties are investigated.

Synthesis and Electro-Optical Properties of Carbazole Containing Copolymers with Different Conjugated Structures for Polymer Light-Emitting Devices

New copolymers such as Poly(9-(4-(2-ethyl-hexyloxy)-3,5-dimethylphenyl)carbazole-alt-aminonaphtalene) [ P1 ] and Poly(4-(bis-methylbiphenyl-alt-9H-carbazol-9-yl)benzenamine) [ P2 ] with chromophores separated by non-conjugated methylene spacers, and Poly(4-(dioctylfluorene-alt-9H-carbazole-9yl)benzenamine) [ P3 ] based on alternating fluorene and amine units in the polymer backbone were synthesized by palladium catalyzed polycondensation reaction. Optical properties of these polymers were characterized in terms of UV/visible absorption and photoluminescence (PL) emission in solution and film state. Maximum PL emission (λmax,PL) of synthesized copolymers were observed in the blue region of 426 nm–460 nm in solution state, but the three polymers all showed large red shift in film state with the green emission. The electro-optical properties of these polymers in polymer light-emitting diode (PLED) devices were investigated from the viewpoint of utilizing the new polymers as emitting layer and hole injection/transport layer. The polymer P3 was found to function both as hole injection and hole transport layer with performance better than that of well known PEDOT:PSS hole injection layer material.

Molecular engineering leading to better processability of conjugated chromophores: The optical properties of new soluble copolymers containing alternative oligo-octylthiophene and oligo-methylene blocks

A series of conjugated/non-conjugated copolymers and their corresponding oligomers were prepared and their optical and physical properties were investigated for understanding the merit of blocking the conjugated chromophores with non-conjugated spacers. It was found that compared to the oligomers, copolymers have the advantages of amorphous nature, high thermal stability, and good thin film processability without sacrificing the quantum efficiency and purity of the emission color. Furthermore, based on the optical data of these copolymers and oligomers, the conjugation length of the polyoctylthiophene was estimated to be extended over 23–31 octylthiophene rings.

Synthesis and property investigations: A partially conjugated hyperbranched polymer for light-emitting application

An A 2 + B 4 approach was adopted to synthesize a partially conjugated hyperbranched polymer HPBPV in which rigid fluorescent conjugated segments and flexible non-conjugated spacers arranged alternately through ether bonds in the skeleton. FT-IR, NMR and elemental analysis were employed to identify the structure of the monomers and polymer. The optimized conformation of HPBPV (one generation) demonstrated three-dimensional structure instead of two-dimensional structure of the fully conjugated hyperbranched polymer. Property investigations revealed that the polymer had good solubility in common organic solvents, good thermal stability and high glass transition temperature. It is also found that the end groups have an obvious impact on the solubility and thermal properties of the polymers, which was discussed in detail in the paper. The relative PL quantum efficiency of the polymer in dilute chloroform solution amounts to 62%. A brief LED device was fabricated and its electroluminescent performance was studied.

Synthesis and optical properties of fluorene and p-phenylenevinylene copolymers containing non-conjugated spacer

A series of alternating fluorene and p-phenylenevinylene copolymers containing non-conjugated spacer have been synthesized through the Wittig polycondensation reaction. These amorphous copolymers are highly soluble in common organic solvents and can be spin-cast to obtain transparent films. The effects of non-conjugated spacers in the main chain and the methoxyl groups on the side chain on the thermal behavior, photoluminescence (PL) and electroluminescence (EL) properties of these copolymers have been investigated in detail. Single-layered light-emitting diodes (LEDs) have been fabricated in the configuration of ITO/PEDOT/copolymer/Ca/Al and emitted blue light in the range of 456–492 nm. The measurements of current vs voltage show turn-on voltages at 6.2–12.4 V. Among the LEDs based on the six copolymers, the maximum EL brightness and efficiency of the LED based on P1 containing 4CH 2 aliphatic segment length in the main chain and without methoxyl groups on side chain are reached 3936 cd/m 2 and 0.70 cd/A, respectively.