Fluorene Copolymers Research Articles

Synthesis, Photo- and Electroluminescence of New Polyfluorene Copolymers Containing Dicyanostilbene and 9,10-Dicyanophenanthrene in the Main Chain.

Using palladium-catalyzed Suzuki polycondensation, we synthesized new light-emitting fluorene copolymers containing the dicyano derivatives of stilbene and phenanthrene and characterized them by gel permeation chromatography, UV-vis absorption spectroscopy, spectrofluorimetry, and cyclic voltammetry. The photoluminescence spectra of the synthesized polymers show significant energy transfer from the fluorene segments to the dicyanostilbene and 9,10-dicyanophenanthrene units, which is in agreement with the data of theoretical calculations. OLEDs based on these polymers were fabricated with an ITO/PEDOT-PSS (35 nm)/p-TPD (30 nm)/PVK (5 nm)/light emitting layer (70-75 nm)/PF-PO (20 nm)/LiF (1 nm)/Al (80 nm) configuration. Examination of their electroluminescence revealed that copolymers of fluorene with dicyanostilbene show yellow-green luminescence, while polymers with 9,10-dicyanophenanthrene have a greenish-blue emission. The 9,10-dicyanophenanthrene units have a more rigid structure compared to dicyanostilbene and, in OLEDs based on them, an increase in maximum brightness is observed with an increase in the content of the additive to the polymer chain. In particular, the device using fluorene copolymer with 9,10-dicyanophenanthrene (2.5 mol%) exhibited a maximum brightness of 9230 cd/m2 and a maximum current efficiency of 3.33 cd/A.

Synthesis of a fluorene and quinoxaline-based co-polymer for organic electronics

Quinoxaline has recently gained interest as monomer in conjugated copolymers because of its easy synthetic accessibility and successful use in highly efficient organic solar cells. In this contribution, we introduce a quinoxaline–fluorene-co-polymer, PFQ10, synthesized by copolymerization of 5,8-dibromo-6,7-difluoro-2-[(2-hexyldecyl)oxy]quinoxaline and 9,9-dioctyl-9H-9-fluorene-2,7-bis(boronic acid pinacol ester) using the Suzuki–Miyaura reaction. By optimization of the reaction conditions, polymers with molecular weights up to 17.2 kDa and a low dispersity of 1.3 were obtained. PFQ10 showed blue photoluminescence with an emission maximum at 459 nm and a relative fluorescence quantum yield of 0.37. As proof of principle, PFQ10 was employed in organic light-emitting diodes and showed a blue–green electroluminescence.Graphical abstract

Impact of Conjugated Polymer Characteristics on the Enrichment of Single-Chirality Single-Walled Carbon Nanotubes

Polymer characteristics influence the chiral selectivity and yield in the enrichment of (7,6) from Signis SG76 in toluene by various fluorene homopolymers and copolymers. Fluorene homopolymers such as poly(9,9-di-n-dodecylfluorene) (PFDD) and copolymers with anthracene such as poly[(9,9-dihexylfluorenyl-2,7-diyl)-co-(9,10-anthracene)] (PFH-A) result in high (7,6) selectivities of up to 67 and 72%, respectively. The selectivity is calculated by optical absorption using a known extinction coefficient and also evaluated by Raman scattering and photoluminescence excitation. In contrast, poly[(9,9-dioctyl-2,7-divinylenefluorenylene)-alt-co-(9,10-anthracene)] (PFO=A), with a double-bond spacer, results in (9,5) dominant extractions with the purity of (9,5) up to 80% after ultracentrifugation, which is believed to be the first report on the successful enrichment of (9,5) by conjugated polymers. The chiral selectivity is also greatly impacted by the length of alkyl side chain of fluorene homopolymers: hexyl group has low selectivity, while octyl, dodecyl, and tetradecyl (PFTD) groups show high selectivity toward (7,6), and the octadecyl group is prone to include bigger tubes. Interestingly, a chiral selection transition from (7,6) to (8,6) occurred using PFTD after a few enrichment cycles. The chiral purity of (8,6) was greatly improved from 50 to 86% after slow filtration. Polymer molecular weight (MW, here PFDD) also plays a role in the selectivity and yield. Low MW leads to a low yield, and high MW leads to a low selectivity, with the optimal range (Mn) being ca. 20–35 kDa when the polydisperse index is ca. 2–3. Molecular dynamics simulation confirms that different lengths of side chain of polyfluorene have different wrapping stabilities toward specific chirality, where PFTD shows greater stability toward (8,6) than (7,6).

Fluorene based electrochromic conjugated polymers: A review

This review briefly describes electrochromic conductive polymers containing fluorene in the main chain or as a subunit/core. According to the arrangement of the fluorene unit and its chemical location on the chain, polyfluorene family was expressed in three classes; fluorene EC homopolymers, copolymers and functional EC polymers. With its sometimes surprising properties and countless advantages, fluorene monomer has many contributions to the electrochromic conductive polymer chain. Although there are many fluorene derived polymers in the literature that have been functionalized and/or used in various applications, this review only found it appropriate to focus on conductive electrochromic polymers with possessing fluorene chemical units. For this purpose, it has been tried to review and comment on fluorene polymers by comparing their chemical, electrochemical and optical properties in a logical framework. One of the biggest problems of electrochromic conductive polymer researchers is to analyze how the redox and electrochromic properties of polymers, which consist of a functional group, change according to the position on the conjugated polymer chain. For this reason, it is thought that the contribution and guideness of this study to the literature will be considerable.

Correlation of electronic and vibrational properties with the chiro-optical activity of polyfluorene copolymers

The rationale of this paper is to shed some light on the origin of the optical response of two similar chiral fluorene copolymers in correlation with their vibrational modes, to understand how a chiral center placed in a ramification affects the optical properties of the main chain. Various spectroscopic ellipsometric techniques, in the scope of the Stokes theory were used to characterize the optical-vibrational behavior of the polyfluorenes: ellipsometry in emission (EE), transmission (TE), and Raman (ERS). The results showed that the optical activity and the emission of the circularly polarized light depends substantially on the interaction of the chiral carbon in the ramification and the main chain through specific optically active vibrational modes, for each sample. One interesting achievement was to find the absolute dextrorotatory configuration of the studied molecules, that could induce a helicoidal structure to the entire material.

Facile, Real‐Time Identification of Blood Components with Self‐Powered Organic–Inorganic Heterostructure Photodetectors

AbstractA rapid and efficient opto‐electrical method for the determination of blood components is demonstrated through a tin oxide (SnO2) quantum dot (QD) fluorene copolymer heterojunction photodetector (FP‐PD). The photo‐active materials, the device structure, and the sensing system are carefully designed to provide miniaturization at a low cost. SnO2 QDs are synthesized with a one‐step solvothermal reaction and a photodetector, and then fabricated in a simple sequential spin‐coating process. Under optimal conditions, the SnO2 QD/FP‐PD shows remarkable responsivity, detectability, fast responses, and stability under self‐powered operation. In this research, optical characteristics of blood components, and interactions between the photodetector and blood components from illuminating two different wavelengths of ultraviolet light, are systemically investigated. Furthermore, three different blood component mixtures are successfully determined by the SnO2 QD/FP‐PD based on specific absorption phenomena, which provides a simple and efficient technique for emergency medical care and health monitoring systems.

Comparing Commercial Metal-Coated AFM Tips and Home-Made Bulk Gold Tips for Tip-Enhanced Raman Spectroscopy of Polymer Functionalized Multiwalled Carbon Nanotubes.

Tip-enhanced Raman spectroscopy (TERS) combines the high specificity and sensitivity of plasmon-enhanced Raman spectroscopy with the high spatial resolution of scanning probe microscopy. TERS has gained a lot of attention from many nanoscience fields, since this technique can provide chemical and structural information of surfaces and interfaces with nanometric spatial resolution. Multiwalled carbon nanotubes (MWCNTs) are very versatile nanostructures that can be dispersed in organic solvents or polymeric matrices, giving rise to new nanocomposite materials, showing improved mechanical, electrical and thermal properties. Moreover, MWCNTs can be easily functionalized with polymers in order to be employed as specific chemical sensors. In this context, TERS is strategic, since it can provide useful information on the cooperation of the two components at the nanoscale for the optimization of the macroscopic properties of the hybrid material. Nevertheless, efficient TERS characterization relies on the geometrical features and material composition of the plasmonic tip used. In this work, after comparing the TERS performance of commercial Ag coated nanotips and home-made bulk Au tips on bare MWCNTs, we show how TERS can be exploited for characterizing MWCNTs mixed with conjugated fluorene copolymers, thus contributing to the understanding of the polymer/CNT interaction process at the local scale.

Cross-linking fluorene copolymers via thiol-ene click chemistry

A series of 9,9-bis(allyl)fluorene copolymers (PFAF) with allyl groups on the side chain were synthesized by Suzuki coupling reaction. The cross-linked fluorene copolymers were realized via thiol-ene click reaction between allyl groups and thiol groups from crosslinker under ultraviolet light. Fourier transform infrared spectroscopy (FTIR), cyclic voltammetry (CV), UV–Vis absorption spectroscopy and photoluminescence spectroscopy (PL) were used to investigate the crosslinking efficiency of the thiol-ene click reaction and the impacts of crosslinking on the performances of fluorene copolymers. The results showed that allyl fluorene copolymers with high molecular weight and different content of 9,9-bis(allyl) fluorene units were successfully synthesized. Fluorene copolymers containing allyl fluorene structural units can be effectively crosslinked by the thiol-ene click reaction. The crosslinking conditions and the factors which influence the crosslinking were investigated in detail. The fluorene copolymer films which were composed of a fluorene copolymer with 10 mol% allyl fluorene structural units (PF10AF) and 39 wt% pentaerythritol tetra(3-mercaptopropionate) (PETMP) can be crosslinked to 85% crosslinking degree under irradiation of 1.8 mW/cm2 LED UV light at 390 nm for 50 s in air. It was found that the thiol-ene click cross-linking had little effects on the photoluminescence spectra of the fluorene copolymers. The thiol-ene click crosslinking showed a much faster crosslinking rate and higher crosslinking efficiency for fluorene copolymers than those of conventional free radical polymerization crosslinking. The results of this study provide a feasible method for cross-linking conjugated polymers in various applications.

Synthesis of benzothiadiazole-9,9-bis(perfluorohexylethyl propionate) fluorene copolymers with enhanced photoluminance stability

Synthesis of benzothiadiazole-9,9-bis(perfluorohexylethyl propionate) fluorene copolymers with enhanced photoluminance stability

Benzo crown ether functionalized conjugated polyfluorenes with anthracene fragment for sustainable light-emitting device technology

Dibenzo crown ether-substituted units of fluorene and acene-bearing copolymers have been successfully synthesized. FTIR, 1H and 13C NMR spectroscopy, DSC, TGA and GPC were used to characterize the resulting copolymers (P1–P5). With fascinating quantum yield, the polymers showed large band gaps. With a polydispersity index of 1.58–2.21, these polymers showed a higher molecular weight (16,080–20,480). Strong thermal stability was indicated by DSC and TGA studies. The cyclic voltammetry analysis found that these polyfluorenes were stable in the range of 0.43–0.53 eV under redox conditions with HOMO and LUMO. For light-emitting materials, thermal stability and emission studies display strong results that are desirable.

Synthesis and performances of 9,9‐bis(perfluorohexylethyl propionate) fluorene copolymers

Abstract2,7‐dibromo‐9,9‐bis(perfluorohexylethyl propionate) fluorene was synthesized by Michael addition reaction using 2,7‐dibromofluorene and perfluorohexyl ethyl acrylate as the reactants. 9,9‐Bis(perfluorohexylethyl propionate) fluorene copolymers were then synthesized by Suzuki coupling reaction with 2,7‐bis(4,4,5,5‐tetramethyl‐1,3,2‐dioxaborane‐diyl)‐9,9 ‐dioctyl fluorene, 2,7‐dibromo‐9,9‐dioctyl fluorene and 2,7‐dibromo‐9,9‐bis(perfluorohexylethyl propionate) fluorene as the monomers. The fluorinated fluorene copolymers were characterized and investigated via Fourier infrared spectroscopy (FTIR), hydrogen nuclear magnetic resonance spectroscopy (1HNMR), ultraviolet absorption spectroscopy (UV–vis), cyclic voltammetry (CV) and photoluminance spectroscopy (PL). Because of the long fluorine‐containing alkyl side chain, the ultraviolet absorption peak of 9,9‐bis(perfluorohexylethyl propionate) fluorene is blue‐shifted compared with poly(9,9‐dioctylfluorene)(PF8). The LUMO energy level increases and the energy band gap of copolymers widens. Due to the self‐assembly of the long fluoroalkyl side chain, the photoluminance spectra of 9,9‐bis(perfluorohexylethyl propionate) fluorene copolymers exhibit a new excimer emission peak at 550 nm. The photoluminance stabilities of the copolymers under irradiation and humid conditions are significantly improved due to protective effect from the long fluoroalkyl side chain. After being irradiated under 500 W iodine tungsten lamp or kept under 70% relative humid conditions, the 9,9‐bis(perfluorohexylethyl propionate) fluorene copolymers showed much better photoluminance stability than that of poly(9,9‐dioctylfluorene) (PF8). These show that introducing long fluoroalkyl side chain into conjugated polymer main chain is a promising strategy to improve environmental stability of devices based on organic conjugated polymers.

Insights into the π - π interaction driven non-covalent functionalization of carbon nanotubes of various diameters by conjugated fluorene and carbazole copolymers.

We investigate the interaction of polyfluorene and fluorene/carbazole copolymers bearing various functional groups and side chains with small to large diameter-from 1.7 nm to 9 nm-carbon nanotubes (CNTs) in vacuo. We use variable-charge molecular dynamics simulations based on the reactive force field ReaxFF. We show that non-covalent functionalization of nanotubes, driven by π - π interactions, is effective for all the polymers studied, thanks to their conjugated backbone and regardless of the presence of specific functional groups. The geometry at equilibrium of these polymer/CNT hybrids is analyzed in detail at the scale of each fluorene or carbazole unit. The role of both the functional groups and the alkyl chain length is analyzed in detail. Adsorption of the polymers on the nanotube sidewalls is shown to be either complete-with the whole chain physisorbed-or partial-due to intrachain coiling or interchain repulsion-depending on the initial geometry, number of polymers, and nanotube diameter. Energetic arguments supplement the described geometric features. Both energetic and geometric adsorption features are derived here for the first time for large diameter carbon nanotubes (up to 9 nm) and fluorene/carbazole copolymers having up to 30 monomers and bearing different functional groups. The force field ReaxFF and its available parameterization used for the simulations are validated, thanks to a benchmark and review on higher-level quantum calculations-for simple π - π interacting compounds made up of polycyclic aromatic molecules adsorbed on a graphene sheet or bilayer graphene. Although it is shown that the influence of the nanotube chirality on the adsorption pattern and binding strength cannot be discussed with our method, we highlight that an available force field such as ReaxFF and its parameterization can be transferable to simulate new systems without specific re-parameterization, provided that this model is validated against reference methods or data. This methodology proves to be a valuable tool for optimal polymer design for nanotube functionalization at no re-parameterization cost and could be adapted to simulate and assist the design of other types of molecular systems.

Solvent influence on molecular interactions in the bulk of fluorene copolymer films.

The effect of intermolecular interactions between the chains of the amorphous PFO–MEH-PPV films built from toluene and tetrahydrofuran (THF) were studied by atomistic molecular dynamics simulations, applying a successive solvent removal procedure. In the good solvent toluene, the incidence of topological entanglements is more significant. While in the poor solvent, coplanar interactions between neighbouring segments of the chains were also found, which is characteristics of cohesional entanglements. Structure factor curves of the films showed three peaks associated with the microstructure of the film, as previously reported by WAX diffractogram measurements. Moreover, the good solvent promotes more flexibility in dihedral angles, and the chains become nearer to each other.

Molecular base and interfacial enrichment of chain entangle in amorphous conjugated polymers: Role of racemic alkyl side groups

Abstract Clear structure characterization is the basics of performance analysis, which is difficult for the complex structure of amorphous conjugated polymers. In this work, we have given solid evidence in copolymer of fluorene and carbazole that the carbazole with racemic branched alkyl side chain tend to form chain entanglement in high concentration of solution and thin film. The single crystal structure of two racemic carbazole dimer with stronger π-π interaction, which 1HNMR peaks is well corresponding to that of racemic polymer at high concentration and proves the existence of chain entanglement in racemic polymer. The chain entanglement segment with high molecular weight and low energy level (estimated as about 3% and absent in the chiral polymer) have enriched at the interface during film fabrication, which induce strong charge trapping and excited state quenching during the working of optoelectronic devices. These chain entanglement would be well restrained by chiral substitution, which induce one order of magnitudes improvement of device brightness and efficiency. These results indicate that the chain entangle at interface and charge trapping play the key role of bad device performance, which help to explore the instability origin and performance promotion.

Molecular Design Principles for Achieving Strong Chiroptical Properties of Fluorene Copolymers in Thin Films

Understanding the molecular level origin of chiroptical properties in enantiopure π-conjugated polymers is essential for tailoring these materials for application in organic light-emitting diodes and photonic devices. Here we have studied poly(9,9′-dialkylfluorene-alt-2,5-dialkoxyphenyl)s as prototypical copolymers to investigate their structure-chiroptical property relationship. The effect of a systematic variation of the location and configuration (S or R) of chiral and achiral side chains in the repeating units on the liquid crystalline ordering and its relationship to chiroptical properties were investigated. The results clearly indicate that enantiopure side chains on the fluorene units are critical for obtaining the cholesteric liquid crystalline ordering and thereby strong chiroptical properties in both absorption and photoluminescence in annealed thin films. Finally, solution-processed organic light-emitting diodes constructed from a liquid crystalline polymer show strong emission of circularly polarized light, demonstrating the potential application of these systems.

A series of novel BODIPY-fluorene copolymers: Synthesis, characterization, optical-electronic and nonlinear optical properties.

A series of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) with different connection sites and conjugation length and fluorene alternating conjugated polymers have been synthesized via palladium-catalyzed Suzuki coupling reaction. The polymers were fully characterized by 1H NMR spectroscopy and gel permeation chromatography (GPC), and their properties were measured by thermogravimetric analysis (TGA), UV-vis absorption spectroscopy, cyclic voltammetry (CV), photoluminescence (PL) spectroscopy, the Z-scan technique, and theoretical calculation using density-functional theory (DFT) method. The results indicate that the connection sites and conjugation length of BODIPY have great effect on the optical-electronic and nonlinear optical properties of the conjugated BODIPY- fluorene copolymers although they don't affect the polymerization process. The absorption and emission peaks of P1 exhibit red shifts compared to those of P2 and P3. The polymers show good nonlinear optical properties that are closely related to the connection sites of the BODIPY pendants. P1 exhibits relatively richer nonlinear optical (NLO) properties than P2 and P3. The third order nonlinear optical coefficient (χ(3)) of P1 is 3.57 × 10-11 esu, which is ~4 times larger than those of P2 and P3.

Circularly Polarized Light From a Series of Chiral Fluorene Copolymers

An innovative methodology for the study of the chiro-optical properties of chiral materials based in transmission ellipsometry is described, applied to fluorene thin films-based copolymers with high optical activity. The direct chirality evaluation parameter was possible. To the best of our knowledge, this is a first report of a complete experimental procedure for that determination. Four copolymers with systematic variation in chemical structure bearing chiral centers in side chains were used.

Aggregation-induced chiroptical generation and photoinduced switching of achiral azobenzene-alt-fluorene copolymer endowed with left- and right-handed helical polysilanes.

The left and right helicities of azobenzene (Azo)-containing main-chain polymer (PF8Azo) were successfully controlled with an enantiomeric pair of rigid rod-like helical polysilanes carrying (S)- and (R)-2-methylbutyl groups (PSi-S and PSi-R, respectively) as their hetero-aggregates in a mixture of chloroform and methanol solvents and in the solid state. Optimizing the good and poor cosolvents and their volume fractions showed that the molar ratio of PF8Azo to PSi-S/-R and the molecular weight of PF8Azo were crucial to boost the CD amplitudes of PF8Azo/PSi-S and PF8Azo/PSi-R hetero-aggregates. The photoresponsive trans–cis transformation caused noticeable changes in the sign and magnitude of the chiroptical behavior due to the hetero-aggregates. Moreover, the optically active PF8Azo homo-aggregates were produced by complete photoscissoring reactions at 313 nm, which could be assigned to the Siσ–Siσ* transitions of PSi-S and PSi-R.

Highly Luminescent Ladderized Fluorene Copolymers Based on B-N Lewis Pair Functionalization.

A new B-N functionalized polyaromatic building block for conjugated hybrid polymers is developed. Bromine-functionalized dipyridylfluorene is first subjected to Lewis-base-directed electrophilic borylation and subsequently incorporated into conjugated polymers via transition-metal-catalyzed cross-coupling reactions. The borane monomer exhibits bright blue luminescence in solution, as a result of the rigid ladder-type structure generated upon electrophilic borylation. Yamamoto coupling gives rise to a homopolymer and Stille coupling to a vinylene-bridged copolymer. Polymerization of the BN-fused ladder molecules leads to large bathochromic shifts in absorption and emission, which are most pronounced for the vinylene-bridged copolymer. The polymers display strong luminescence in solution with quantum yields of 55% and 78% and sub-ns fluorescence lifetimes; the copolymer also exhibits bright yellow luminescence in the solid state when precipitated from solution.

Controlling Molecular Conformation for Highly Efficient and Stable Deep-Blue Copolymer Light-Emitting Diodes.

We report a novel approach to achievedeep-blue, high-efficiency, and long-lived solution-processed polymer light-emitting diodes (PLEDs) via a simple molecular level conformation change of an emissive conjugated polymer.We introduce rigid β-phase segments into a 95% fluorene-5% arylamine copolymer emissive layer. The arylamine moieties at low density act as efficient exciton formation sites in PLEDs, whereas the conformational change alters the nature of the dominant luminescence from a broad, charge transfer like emission to a significantly blue-shifted and highly vibronically structured excitonic emission. As a consequence, we observe a significant improvement in the Commission International de L'Eclairage ( x, y) coordinates from (0.149, 0.175) to (0.145, 0.123) while maintaining high efficiency and improved stability. We achieve a peak luminous efficiency, η = 3.60 cd/A, and a luminous power efficiency, ηw = 2.44 lm/W, values that represent state-of-the-art performance for single copolymer deep-blue PLEDs. These values are 5-fold better than for otherwise-equivalent, β-phase poly(9,9-dioctylfluorene) PLEDs (0.70 cd/A and 0.38 lm/W). This report represents the first demonstration of the use of molecular conformation as a simple but effective method to control the optoelectronic properties of a fluorene copolymer; previous examples have been confined to homopolymers.