Steady-state and time-resolved studies of 2,7-carbazole-based conjugated polymers in solution and as thin films: determination of their solid state fluorescence quantum efficiencies

The UV–Vis absorption spectra and the luminescence properties of poly(N-octyl-2,7-carbazole) (POC) and poly(N-octyl-2,7-carbazole-alt-9,9-dioctyl-2,7-fluorene) PCF have been investigated in solution and in the solid state (thin films). No aggregate and/or excimer formation has been detected in these polymeric systems. From time-resolved fluorescence measurements in solution and in the solid state, the fluorescence efficiencies of the thin films have been estimated. It is found that the fluorescence efficiencies of these polycarbazoles in the solid state are quenched, as compared to those measured in fluid solutions, but remain relatively high (φ_F0.40), making them promising materials for electroluminescent devices.     

含苯并硒二唑聚咔唑/芴类共轭聚电解质及其前驱体的合成与性能研究

采用Suzuki偶合反应合成了一系列新型的咔唑、芴和2,1,3-苯并硒二唑的共聚物——聚[3,6-(N-(2-乙基己基))咔唑-2,1,3-苯并硒二唑-9,9-双(N,N-二甲基胺丙基)芴](PCzN-BSeD)及其相应的聚电解质衍生物——聚[3,6-(N-(2-乙基己基))咔唑-2,1,3-苯并硒二唑-9,9-(双(3′-(N,N-二甲基)-N-乙基铵)丙基)芴]二溴(PCzNBr-BSeD).在聚咔唑和芴中引入不同比例的2,1,3-苯并硒二唑(BSeD)单元,引起了由咔唑和芴链段向窄带隙苯并硒二唑(BSeD)单元有效的能量转移.通过对聚合物电致发光性能的研究,发现用聚(3,4-亚乙基二氧基噻吩)(PEDOT)或聚(3,4-亚乙基二氧基噻吩)/聚乙烯咔唑(PEDOT/PVK)作为空穴传输层时,器件的性能相差不大,表明咔唑的引入较明显的改善了聚合物的空穴注入性能.而且几乎所有的聚合物用高功函数铝作阴极的器件和用钡/铝作阴极的器件具有相近的发光性能,表明这类聚合物具有良好的电子注入性能.     

Selective dispersion of single-walled carbon nanotubes with specific chiral indices by poly(N-decyl-2,7-carbazole)

Physico-chemical methods to sort single-walled carbon nanotubes (SWNTs) by chiral index are presently lacking but are required for in-depth experimental analysis and also for potential future applications of specific species. Here we report the unexpected selectivity of poly(N-decyl-2,7-carbazole) to almost exclusively disperse semiconducting SWNTs with differences of their chiral indices (n - m) ≥ 2 in toluene. The observed selectivity complements perfectly the dispersing features of the fluorene analogue poly(9,9-dialkyl-2,7-fluorene), which disperses semiconducting SWNTs with (n - m) ≤ 2 in toluene. The dispersed samples are further purified by density gradient centrifugation and analyzed by photoluminescence excitation spectroscopy. All-atom molecular modeling with decamer model compounds of the polymers and (10,2) and (7,6) SWNTs suggests differences in the π-π stacking interaction as origin of the selectivity. We observe energetically favored complexes between the (10,2) SWNT and the carbazole decamer and between the (7,6) SWNT and the fluorene decamer, respectively. These findings demonstrate that subtle structural changes of polymers lead to selective solvation of different families of carbon nanotubes. Furthermore, chemical screening of closely related polymers may pave the way toward simple, low-cost, and index-specific isolation of SWNTs.     

Acceptor–acceptor conjugated copolymers based on perylenediimide and benzothiadiazole for all‐polymer solar cells

Donor–acceptor (D–A) conjugated copolymers are one of known classes of organic optoelectronic materials and have been well developed. However, less attention has been paid on acceptor–acceptor (A–A) conjugated analogs. In this work, two types of A–A conjugated copolymers, namely P1‐Cn and P2‐Cn (n is the carbon number of their alkyl side chains), were designed and synthesized based on perylenediimide ( PDI ) and 2,1,3‐benzothiadiazole ( BT ). Different from P1‐Cn , P2‐Cn polymers have additional acetylene π‐spacers between PDI and BT and thus hold a more planar backbone configuration. Property studies revealed that P2‐Cn polymers possess a much red‐extended UV–vis absorption spectrum, stronger π–π interchain interactions, and one‐order larger electron mobility in their neat film state than P1‐Cn . However, all‐polymer solar cells using P1‐Cn as acceptor component and poly(3‐hexyl thiophene) or poly(2,7‐(9,9‐didodecyl‐fluoene)‐alt?5,5′‐(4,7‐dithienyl‐2‐yl‐2,1,3‐benzothiadiazole) as donor component exhibited much better performance than those based on P2‐Cn . Apart from their backbone chemical structure, the side chains were found to have little influence on the photophysical, electrochemical, and photovoltaic properties for both P1‐Cn and P2‐Cn polymers. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1200–1215     

Phase behavior of poly(9,9-di-n-hexyl-2,7-fluorene)

Here we report the phase behavior of poly(9,9-di-n-hexyl-2,7-fluorene) (PFH), which previously received little attention as compared to its homologues poly(9,9-di-n-octyl-2,7-fluorene) (PFO) and poly(9,9-di-(2'-ethylhexyl)-2,7-fluorene) (PFEH). By means of differential scanning calorimetry, X-ray diffraction, and electron microscopy, we show that there exist four different phases in PFH. The as-cast film is mainly composed of a mesomorphic beta phase with layer spacing of ca. 1.4 nm. This beta phase is inherently metastable and, upon heating above 175 degrees C, transforms into a crystalline (alpha) form that melts into a nematic (N) liquid above 250 degrees C. Upon stepwise cooling, the nematic melt crystallizes into the alpha phase first, followed by solid-solid transformation into another crystalline (alpha') form. Unit cell structure of the alpha form is monoclinic whereas that of the alpha' form is triclinic, but departures from strict orthogonality are slight (by ca. 6 degrees). These observations not only support our previous assignment of two crystalline forms (both orthorhombic in structure) in PFO but also provide insights to the crystalline nature of the polyfluorene series.     

The effect of molecular weight on the separation of semiconducting single‐walled carbon nanotubes using poly(2,7‐carbazole)s

The use of selective interactions between conjugated polymers and single‐walled carbon nanotubes has emerged as a promising method for the separation of nanotubes by electronic type. Although much attention has been devoted to investigating polyfluorenes and their ability to disperse semiconducting carbon nanotubes under specific conditions, other polymer families, such as poly(2,7‐carbazole)s, have been relatively overlooked. Poly(2,7‐carbazole)s have been shown to also preferentially interact with semiconducting carbon nanotubes, however a detailed investigation of polymer parameters, such as molecular weight, has not been performed. We have prepared seven different molecular weights of a poly(2,7‐carbazole), from short chain oligomers to high molecular weight polymers, and have investigated their effectiveness at dispersing semiconducting single‐walled carbon nanotubes. Although all polymer chain lengths were able to efficiently exfoliate carbon nanotube bundles using a mild dispersion protocol, only polymers above a certain threshold molecular weight (M_n ～ 27 kDa) were found to exhibit complete selectivity for semiconducting nanotubes, with no observable signals from metallic species. Additionally, we found the quality of separation to be strongly dependent on the ratio of polymer to carbon nanotube. Contrary to previous reports, we have found that an excess of poly(2,7‐carbazole) leads to incomplete removal of metallic carbon nanotubes. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2510–2516     

Rylene diimide and dithienocyanovinylene copolymers for polymer solar cells

Two polymers containing(E)-2,3-bis(thiophen-2-yl)acrylonitrile(CNTVT) as a donor unit, perylene diimide(PDI) or naphthalene diimide(NDI) as an acceptor unit, are synthesized by the Stille coupling copolymerization, and used as the electron acceptors in the solution-processed organic solar cells(OSCs). Both polymers exhibit broad absorption in the region of 300–850 nm. The LUMO energy levels of the resulted polymers are ca. –3.93 eV and the HOMO energy levels are –5.97 and –5.83 eV. In the binary blend OSCs with PTB7-Th as a donor, PDI polymer yields the power conversion efficiency(PCE) of up to 1.74%, while NDI polymer yields PCE of up to 3.80%.     

Copolymers comprising 2,7-carbazole and bis-benzothiadiazole units for bulk-heterojunction solar cells

On the basis of theoretical considerations of the intramolecular charge transfer (ICT) effect, we have designed a series of donor (D)–acceptor (A) conjugated polymers based on bis‐benzothiadiazole (BBT). A PPP‐type copolymer of electron‐rich 2,7‐carbazole (CZ) and electron‐deficient BBT units poly[N‐(2‐decyltetradecyl)‐2,7‐carbazole‐co‐7,7′‐{4,4′‐bis‐(2,1,3‐benzothiadiazole)}] ( PCZ‐BBT ), a PPV‐type copolymer poly[N‐(2‐decyltetradecyl)‐2,7‐carbazolevinylene‐co‐7,7′‐{4,4′‐bis‐(2,1,3‐benzothiadiazolevinylene)}] ( PCZV‐BBTV ), and a tercopolymer based on carbazole, thiophene, and BBT poly[N‐(2‐decyltetradecyl)‐2,7‐(di‐2‐thienyl)carbazole‐co‐7,7′‐{4,4′‐bis‐(2,1,3‐benzothiadiazole)}] ( PDTCZ‐BBT ) have been synthesized to understand the influence of BBT acceptor structure and linkage on the photovoltaic characteristics of the resulting materials. Both the HOMO and LUMO of the resulting polymers are found to be deeper‐lying than those of benzothiadiazole‐based polymers. The measured electrochemical band gaps (eV) are in the following order: PDTCZ‐BBT (1.65 eV) < PCZV‐BBTV (1.69 eV) < PCZ‐BBT (1.75 eV). All the polymers provide a photovoltaic response when blended with a fullerene derivative as an electron acceptor. The best cell reaches a power conversion efficiency of 2.07 % estimated under standard solar light conditions (AM1.5G, 100 mW cm^?2). We demonstrate for the first time that BBT‐based polymers are promising materials for use in bulk‐heterojunction solar cells.     

Synthesis and characterization of deep blue emitters from starburst carbazole/fluorene compounds

A series of well-defined, highly fluorescent starburst compounds with a carbazole core and oligo(2,7-fluorene ethynylene) arms have been synthesized by Sonogashira coupling reaction and fully characterized. These conjugated compounds exhibit good solubility, high thermal stability, and excellent fluorescence quantum yields (up to 0.99). The incorporation of carbazole core interrupted the main-chain conjugation and resulted in blue-shifted absorption and emission. Moreover, deep blue light has been approached from organic light-emitting diodes (OLEDs) adopting these compounds as emitting layer.     

Manipulating photogenerated radical ion pair lifetimes in wirelike molecules using microwave pulses: molecular spintronic gates

We have studied spin-dependent charge transfer dynamics in wirelike donor-bridge-acceptor (D-B-A) molecules comprising a phenothiazine (PTZ) donor, an oligo(2,7-fluorene) (FL(n)) bridge, and a perylene-3,4:9,10-bis(dicarboximide) (PDI) acceptor, PTZ-FL(3)-PDI (1) and PTZ-FL(4)-PDI (2), dissolved in the magnetic field-aligned nematic phase of 4-cyano-4'-n-pentylbiphenyl (5CB) at 295 K. Time-resolved EPR spectroscopy using both continuous wave and pulsed microwaves shows that the photogenerated radical pairs (RPs), PTZ(+?)-FL(3)-PDI(-?) and PTZ(+?)-FL(4)-PDI(-?), recombine much faster from the singlet RP manifold than the triplet RP manifold. When a strong resonant microwave π pulse is applied following RP photogeneration in 1 and 2, the RP lifetimes increase about 50-fold as indicated by electron spin-echo detection. This result shows that the RP lifetime can be greatly extended by rapidly switching off fast triplet RP recombination.     

Synthesis and photoluminescence properties of some novel fluorenophanes

Fluorenophanes derived from 2,7-bis-(bromomethyl)-9,9′-diheptyl-9H-fluorene and various diols exhibit excellent photoluminescence properties.     

Significant influence of doping effect on photovoltaic performance of efficient fullerene-free polymer solar cells

The modification mechanism of the water/alcohol cathode interlayer is one of the most complicated problems in the field of organic photovoltaics,which has not been clearly elucidated yet;this greatly restricts the further enhancement of the PCE for polymer solar cells.Herein,we clarified the different effects of PFN and its derivatives,namely,poly[(9,9-bis(3'-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)](PFN-Br) in modifying fullerene-free PSCs.It is found for the first time that doping on IT-4F by the amino group of PFN leads to the unfavorable charge accumulation,and hence,forms a dense layer of electronegative molecule due to the poor electron transport capacity of the non-fullerene acceptor IT-4F.The electronegative molecular layer can block the electron transfer from the active layer to the interlayer and cause serious charge recombination at the active layer/cathode interface.This mechanism could be verified by the ESR measurement and electron-only devices.By replacing PFN with PFN-Br,the excessive doping effect between the cathode interlayer and IT-4F is eliminated,by which the charge transport and collection can be greatly improved.As a result,a high PCE of 13.5%was achieved in the fullerene-free PSCs.     

Oligo(2,7-fluorene ethynylene)s with pyrene moieties: synthesis, characterization, photoluminescence, and electroluminescence

A series of highly fluorescent, pyrene-modified oligo(2,7-fluorene ethynylene)s were synthesized and fully characterized. Different emissions were observed between pyrene-end-capped and pyrene-centered oligo(2,7-fluorene ethynylene)s. Moreover, these oligomers were applied to fabricate organic light-emitting diodes (OLEDs), and bright blue or green electroluminescence (EL) with good device performances was achieved in multilayer OLEDs. These pyrene-modified oligo(2,7-fluorene ethynylene)s could be used as optoelectronics materials or models to investigate fluorescent structure-property relationship of fluorene derivatives.     

Synthesis and Characterization of Thermotropic Liquid Crystalline Poly(ester-imide)s

A series of thermotropic liquid crystalline poly(ester-imide)s was synthesized by melt polymerization of diacetoxynaphthalene acid and n-(ω-carboxyalkylene) trimellitic imides. All polymers with 2,6 substituent positions (n-2,6 PEIM) on the napthalene ring exhibit liquid crystalline phases, whereas polymers with 2,7 substituent positions (n-2,7 PEIM) do not. This result suggests that the kink structure of n-2,7 PEIMS would hinder the formation of liquid crystalline polymer. The copoly(ester-imide)s with an irregular sequence of aliphatic units and aromatic mesogens showed the liquid crystallinity with the lower transition temperatures and a lesser tendency to crystallize than homopoly(ester-imide)s. The semicrystalline polymers with more regular monomeric sequence in the main chain showed the hysteresis of viscoelastic property in the temperature cycle. A nematic glassy copolymer gave the higher molecular orientation to the fiber than a semicrystalline polymer. © 1997 John Wiley & Sons, Ltd.     

Tuning the electronic coupling in a low-bandgap donor-acceptor copolymer via the placement of side-chains

We present a spectroscopic and theoretical investigation of the effect of the presence and position of hexyl side-chains in the novel low-bandgap alternating donor-acceptor copolymer poly[bis-N,N-(4-octylphenyl)-bis-N,N-phenyl-1,4-phenylenediamine-alt-5,5'-4',7',-di-2-thienyl-2',1',3'-benzothiadiazole] (T8TBT). We use electronic absorption and Raman spectroscopic measurements supported by calculations of chain conformation, electronic transitions, and Raman modes. Using these tools, we find that sterically demanding side-chain configurations induce twisting in the electronic acceptor unit and reduce the electronic interaction with the donor. This leads to a blue-shifted and weakened (partial) charge-transfer absorption band together with a higher photoluminescence efficiency. On the other hand, sterically relaxed side-chain configurations promote coupling between donor and acceptor units and exhibit enhanced absorption at the expense of luminescence efficiency. The possibility of tuning the donor-acceptor character of conjugated polymers by varying the placement of side-chains has very important ramifications for light emitting diode, Laser, display, and photovoltaic device optimization.     

9H-Fluorene-9-carbodithioic acids and dithioates. First isolation and characterization of a gem-enedithiol

[2,7-Bis(octyloxy)fluoren-9-yliden]methanedithiol (1a) and its tautomer 2,7-bis(octyloxy)-9H-fluorene-9-carbodithioic acid (2a) can be isolated in pure form from the reaction of monolithiated 2,7-bis(octyloxy)-9H-fluorene with CS₂ followed by protonolysis with aqueous HCl. Compound 1a is the first isolated and unambiguously characterized gem-enedithiol. When 9H-fluorene, 2,7-di-tert-butyl-9H-fluorene or 2,7-dimethoxy-9H-fluorene are used, the same synthetic procedure yields the unstable dithioic acids, which can be converted to the piperidinium salts.     

Self-encapsulation of poly-2,7-fluorenes in a dendrimer matrix.

The synthesis and characterization of complex dendritic, rigid rod poly-2,7-fluorene homopolymers and copolymers via a macromonomer approach is reported. Several 2,7-dibromofluorene monomers containing benzyl ether dendrons (generations 1, 2, and 3) in the 9,9'-position of the fluorene ring were prepared and employed in condensation polymerizations to yield both homopolymers and copolymers with diethylhexylfluorene. Fluorescence measurements of the materials reveal extensive conjugation along the polymer backbone. The determination of the solid-state PL spectra and quantum efficiencies showed that there is an apparent optimum size of the dendritic side groups with the [G-2]-derivatives showing high reactivity with associated site isolation of the conjugated chain. AFM analysis and DSC results confirmed that the hybrid polymers and copolymers did not show any sign of a microphase-separated morphology. First EL-results demonstrated that the homopolymers have higher turn-on voltages then the corresponding copolymers.     

One donor–two acceptor (D–A1)‐(D–A2) random terpolymers containing perylene diimide,naphthalene diimide,and carbazole units

A series of one donor–two acceptor (D–A₁)‐(D–A₂) random terpolymers containing a 2,7‐carbazole donor and varying compositions of perylene diimide (PDI) and naphthalene diimide (NDI) acceptors was synthesized via Suzuki coupling polymerization. The optical properties of the terpolymers are weighted sums of the constituent parent copolymers and all show strong absorption over the 400 to 700 nm range with optical bandgaps ranging from 1.77 to 1.87 eV, depending on acceptor composition. The copolymers were tested as acceptor materials in bulk heterojunction all‐polymer solar cells using poly[(4,8‐bis‐(2‐ethylhexyloxy)‐benzo[1,2‐b;4,5‐b′]dithiophene)‐2,6‐diyl‐alt‐(4‐(2‐ethylhexanoyl)‐thieno[3,4‐b]thiophene)‐2,6‐diyl] (PBDTTT‐C) as the donor material. In contrast to the optoelectronic properties, the measured device parameters are not composition dependent, and rather depend solely on the presence of the NDI unit, where the devices containing any amount of NDI perform half as well as those using the parent polymer containing only carbazole and PDI. Overall this is the first example of a one donor–two acceptor random terpolymer system containing perylene diimide (PDI) and naphthalene diimide (NDI) acceptor units, and demonstrates a facile method of tuning polymer optoelectronic properties while minimizing the need for complicated synthetic and purification steps. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3337–3345     

N-苯基咔唑取代的蓝光聚(2,7-咔唑)的合成与表征

采用Suzuki聚合方法并通过改变催化剂的投料量,合成了一系列具有不同分子量的N-苯基咔唑取代的蓝光聚(2,7-咔唑),并详细研究了分子量对聚合物的光物理、电化学和电致发光器件性质的影响.结果表明,刚性N-苯基咔唑侧链的引入,能够有效地调控主链之间的相互作用,提高蓝光的色纯度,并使得该系列聚合物的固态荧光量子效率高达0.40.同时,在热退火及器件工作过程中,没有观察到类似聚芴中绿光峰的出现,表明具有良好的光谱稳定性.单层聚合物电致发光器件的最大发光效率为0.33 cd/A,色坐标为(0.18,0.12).     

Effect of a Pendant Acceptor on Thermally Activated Delayed Fluorescence Properties of Conjugated Polymers with Backbone‐Donor/Pendant‐Acceptor Architecture

Three sets of conjugated polymers with backbone‐donor/pendant‐acceptor architectures, named PCzA3PyB, PCzAB2Py, and PCzAB3Py, are designed and synthesized. The three isomeric benzoylpyridine‐based pendant acceptor groups are 6‐benzoylpyridin‐3‐yl (3PyB), 4‐((pyridin‐2‐yl)carbonyl)phenyl (B2Py) and 4‐((pyridin‐3‐yl)carbonyl)phenyl (B3Py), whereas the identical backbone consists of 3,6‐carbazolyl and 2,7‐acridinyl rings. One acridine ring and each acceptor group constitute a definite thermally activated delayed fluorescence (TADF) unit, incorporated into the main chain of the polymers through the 2,7‐position of the acridine ring with the varied content. All of the polymers display legible TADF features with a short microsecond‐scale delayed lifetime (0.56–1.62 μs) and a small singlet/triplet energy gap (0.10–0.19 eV). Progressively redshifted emissions are observed in the order PCzAB3Py, PCzA3PyB, and PCzAB2Py owing to the different substitution patterns of the pyridyl group. Photoluminescence quantum yields can be improved by regulating the molar content of the TADF unit in the range 0.5–50 %. The non‐doped organic light‐emitting devices (OLEDs) fabricated by solution‐processing technology emit yellow‐green to orange light. The polymers with 5 mol % of the TADF unit exhibit excellent comprehensive electroluminescence performance, in which PCzAB2Py5 achieves a maximum external quantum efficiency (EQE) of 11.9 %, low turn‐on voltage of 3.0 V, yellow emission with a wavelength of 573 nm and slow roll‐off with EQE of 11.6 % at a luminance of 1000 cd m^?2 and driving voltage of 5.5 V.