Aromatic ether-containing fluorene monomers, processes for their preparation and polymerization thereof

Novel aromatic ether-containing monomers are described along with processes for their preparation and their polymerization into corresponding aromatic ether-containing polyfluorenes. These polyfluorenes exhibited stable blue-emission and therefore have application in polymer light-emitting devices.

FIELD OF THE DISCLOSURE

The present disclosure relates to aromatic-ether containing fluorine monomers and polymers derived therefrom. The present disclosure further relates to uses of the novel aromatic-ether containing polyfluorenes as materials for photoluminescence and electroluminescence in light emitting devices.

BACKGROUND OF THE DISCLOSURE

Blue-emitting polyfluorene (PF) polymers are being pursued as active materials in polymer light-emitting diodes,1lasers,2-6and sensors.7-12To meet the requirements of practical device application, high-molecular-weight blue-emitting poly(p-phenylene) (PPP) materials including, ladder-type PPPs,13polyfluorenes (PF),14polyindenofluorenes (PlFs),15and polytetrahydrophenanthrene (PTHP)16have being widely investigated. Alkyl-substituted PFs, such as poly(9,9′-dioctylfluorene) (PFO), are among the most promising candidates for optoelectronic applications.17Still, the inherent spectral instability of alkyl-substituted PFs (i.e. green emission upon exposure to thermal stressing) remains a significant challenge limiting full realization of their device potential.

It is now well-established that the primary source of the undesirable green emission is fluoreneone defects formed during and/or after polymer synthesis.18Attempts to prevent defect formation have included derivitization at the 9-position with trifluoromethyl,19silole,20siloxane,21silsesquioxane,22polyphenylene,23and dendritic benzyl-ether24moieties. These studies clearly show that controlling the molecular structure of PF at the 9-position affords one solution toward improving material performance.

Two classical methods for preparing aromatic ethers (AEs) are the copper-mediated Ullmann-ether synthesis29, 30and electron-withdrawing-group (EWG)-facilitated nucleophilic-aromatic-substitution (SNAr) protocols.25, 31These approaches are generally ineffective in preparing high purity materials for organic electronics (e.g., PLEDs) because elevated temperatures, copper salts, and EWGs are required for the reaction to go to completion and are difficult to remove from the product. In addition, it is difficult to prepare materials in a controlled stepwise fashion, thereby limiting control over subtle changes in molecular structure. Fine structural control is well known to dramatically impact material properties.

SUMMARY OF THE DISCLOSURE

Two unique fluorene monomers (PTE I(a) and MTE (Ib)) and polyfluorene-based homopolymers (PPTE III(a) and PMTE III(b)) containing covalently linked aromatic-ether (AE) moieties were synthesized via microwave Ni(0)-mediated Yamamoto coupling reactions. The monomers and polymers demonstrated thermal stabilities, as determined by TGA, much higher than that of the status quo poly(9,9′-dioctylfluorene) (PFO) (i.e. greater than 100° C.), and most importantly, the spectral emission remained stable after annealing in ambient and inert atmospheres. PPTE and PMTE were annealed in an N2atmosphere at 200° C. for 72 hours and at 150 ° C. in ambient atmosphere for 1 hour showing no evidence of green emission, in stark contrast to PFO. The results show that PFs with AE moieties present in the 9-position exhibit stable blue-emission with potential application in polymer light-emitting diodes (PLEDs).

Accordingly, the present disclosure includes novel aromatic ether-containing fluorene monomers of the Formula I:

wherein X is a polymerization-enabling leaving group; andn is 0 or 1.

There is also included in the present disclosure a process for the preparation of compounds of Formula I:

wherein X is a polymerization-enabling leaving group; andn is 0 or 1,comprising:(a) reacting a compound of the Formula IV:

wherein X is a polymerization-enabling leaving group, with a compound of the Formula V:

wherein R1═H for compounds of Formula I wherein n is 0 and R1═Cl for compounds of Formula I wherein n is 1,
under conditions to form a compound of the Formula VI

wherein X is a polymerization-enabling leaving group andR1is H for compounds of Formula I wherein n is 0 and R1is Cl for compounds of Formula I wherein n is 1;(b) when R1is H, reacting the compounds of Formula VI under conditions to remove the CpFe+group to form a compound of Formula I wherein n is 0; orwhen R1is Cl, reacting the compounds of the Formula VI with a compound of the Formula VII:

under conditions to form a compound of the Formula VIII:

wherein R2is O-Ph; and(c) treating the compounds of the Formula VIII under conditions to remove the CpFe+group to form a compound of Formula I wherein n is 1.

In a further embodiment of the present disclosure, there is included an aromatic-ether-containing polyfluorene comprising repeating monomeric units of the Formula II:

wherein n is 0 or 1.

It another embodiment of the present disclosure, there is included an aromatic-ether-containing polyfluorene of the Formula III:

In yet another embodiment of the present disclosure, there is included a light-emitting solid state device comprising an aromatic-ether containing polyfluorene of the present disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

(II) Monomers of the Disclosure

The present disclosure includes novel aromatic ether-containing fluorene monomers of the Formula I:

wherein X is a polymerization-enabling leaving group; andn is 0 or 1.

In an embodiment of the disclosure, X is bromo. In a further embodiment of the disclosure, n is 1. In another embodiment of the disclosure, the aromatic ethers are attached at positions that are meta or para to each other.

In yet another embodiment, the monomer of Formula I has the following structure:

wherein X is a polymerization-enabling leaving group, for example bromo.

In another embodiment, the monomers of Formula I are selected from:

Table 1 summarizes andFIG. 1shows that the onset of decomposition (10% weight loss) of I(a) (PTE) and I(b) MTE in N2and air occurs at substantially higher temperatures (ca. 150° C.) than that for DOF. This increased stability is attributed to the stabilizing influence of the AE moieties. It is also noted that the onset of decomposition of I(a) and I(b) is approximately the same in both N2and air, in stark contrast to DOF. While note wishing to be limited by theory, this may indicate the lack of a readily accessible decomposition pathway for AE-containing I(a) and I(b).

The present disclosure also describes SNAr protocols utilizing transition-metal-mediated activation of aromatic rings as a method for preparing AE bonds in AE containing fluorene monomers. In particular, the process for preparing monomers of Formula I comprise iron-based methodologies utilizing cyclopentadienyliron (CpFe+) as an activating group,32which is readily removed, leaving behind wholly AE containing material.

Accordingly, there is included in the present disclosure a process for the preparation of compounds of Formula I:

wherein X is a polymerization-enabling leaving group; andn is 0 or 1,comprising:(a) reacting a compound of the Formula IV:

wherein X is a polymerization-enabling leaving group, with a compound of the Formula V:

wherein R1═H for compounds of Formula I wherein n is 0 and R1═Cl for compounds of Formula I wherein n is 1,
under conditions to form a compound of the Formula VI

wherein X is a polymerization-enabling leaving group andR1is H for compounds of Formula I wherein n is 0 and R1is Cl for compounds of Formula I wherein n is 1;

(b) when R1is H, reacting the compounds of Formula VI under conditions to remove the CpFe+group to form a compound of Formula I wherein n is 0; orwhen R1is Cl, reacting the compounds of the Formula VI with a compound of the Formula VII:

under conditions to form a compound of the Formula VIII:

wherein R2is O-Ph; and(c) treating the compounds of the Formula VIII under conditions to remove the CpFe+group to form a compound of Formula I wherein n is 1.

In an embodiment of the disclosure, the conditions to form a compound of the Formula VI comprise reacting the compounds of Formula IV and V in the presence of a base in an inert solvent, such as dimethylformamide (DMF) at room temperature for about 12 hours to about 36 hours, suitably about 24 hours. In an embodiment, the base is K2CO3

In a further embodiment of the present disclosure, the conditions to remove the CpFe+group comprise reacting the compound of Formula VI or VIII in a suitable high boiling solvent and heating, suitably in a microwave reactor, to a temperature of about 150° C. to about 250° C., suitably about 200° C., for about 10 to about 15 minutes, suitably about 12 minutes.

In another embodiment of the present disclosure the conditions to form a compound of the Formula VIII comprise reacting the compounds of the Formula VI and VII presence of a base in an inert solvent, such as dimethylformamide (DMF) at room temperature for about 48 to about 96 hours, suitably about 72 hours. In an embodiment, the base is K2CO3.

It is a further embodiment of the present disclosure, that in the preparation of the compounds of the Formula VIII, the compounds of the Formula VI, wherein R1is Cl are not isolated. In this embodiment, once the formation of the compounds of the Formula VI wherein R1is Cl is complete, the compound of the Formula VII and base are added directly to the reaction mixture and the reaction allowed to continue to provide the compounds of the Formula VIII.

(III) Polymers of the Disclosure

Polymerization of the AE-containing fluorene monomers of the present disclosure, utilizing microwave initiated Yamamoto-coupling,37yielded blue-emitting AE-functionalized PFs. Thermogravimetric analysis (TGA) and thermal-oxidative degradation studies at ambient temperature confirm that the incorporation of AE units at the 9-position substantially improves the thermal, oxidative, and color stability of these new materials when compared to status quo alkyl-functionalized PFs (i.e. PFO). The inclusion of AE structural units provides a straightforward approach towards eliminating adverse affects of thermal degradation of PF materials.

Accordingly, the present disclosure includes an aromatic-ether-containing polyfluorene comprising repeating monomeric units of the Formula II:

wherein n is 0 or 1.

In an embodiment of the present disclosure the aromatic-ether-containing polyfluorene comprises repeating monomeric units selected from

It another embodiment of the present disclosure, there is included an aromatic-ether-containing polyfluorene of the Formula III:

In an embodiment of the disclosure, n is 1. In a further embodiment of the disclosure R3is methyl or ethyl, suitably methyl and o is 1, 2 or 3, suitably 2. In a further embodiment of the disclosure, o is 2 and the 2 R3groups are located at the 3 and 5 positions of the phenyl ring.

The value of m varies as desired to achieve desired properties such as solubility, proccessability, formability and the like as would be known to a person skilled in the art.

In a further embodiment of the disclosure, the aromatic-ether-containing polyfluorene of the Formula III is selected from:

The polymers of Formula III are prepared using Yamamoto-coupling initiated in a microwave reactor. It is appreciated that any suitable end-capping agent can be used in the polymerization reaction. Such agents are known to those skilled in the art.

In an embodiment of the disclosure the AE-containing polyfluorenes are homopolymers. In a further embodiment the AE-containing polyfluorenes are copolymerized with other monomers. Usually copolymerization is used to reduce the cost of materials by reducing the fluorene content. Representative comonomeric materials include olefin units such as ethylene, propylene and the like, aromatic units such as styrene and the like and ester units. If comonomeric units are present, the leaving groups X, are selected to accommodate the additional comonomers in the polymerization. The relative weight proportion of the comonomeric units will range from 100:0 (for pure polyfluorene homopolymer) to about 10:90 for a highly diluted material.

Compounds III(a) (PPTE) and III(b) (PMTE), shown above, exhibit significantly higher thermal decomposition temperatures than PFO, indicating AE moieties substantially improve the thermal and oxidative stability of PFs.

Thermal annealing of PPTE, PMTE, and PFO in air show very different affects on the PL spectra. PFO shows a dramatic increase in green emission (ca. 550 nm) after annealing in air for 60 minutes at 150° C. and the resulting green emission visibly dominates film color after only 20 minutes of thermal stressing.45In contrast, the PL spectra of PPTE and PMTE exhibit negligible change under identical conditions which severely degrade PFO

Proof-of-concept polymer light-emitting diodes (PLEDs) with PMTE III(b) as the emitting layer were fabricated with the following sandwich structure: ITO/PEDOT-PSS/PMTE/Ca/Al (ITO=indium tin oxide; PEDOT-PSS=polyethylenedioxythiophene polystyrene sulfonate). The electroluminescence spectrum acquired in ambient conditions did not shift over device testing time (ca. 30 min). PMTE exhibited turn-on voltages of ca. 4.5 V and exhibited blue electroluminescence as shown inFIG. 8.

Accordingly, in yet another embodiment of the present disclosure, there is included a light-emitting solid state device comprising an aromatic-ether containing polyfluorene of the present disclosure, i.e. an an aromatic-ether-containing polyfluorene comprising repeating monomeric units of the Formula II or an aromatic-ether-containing polyfluorene of the Formula III. In an embodiment, the light-emitting solid state device is configured as a light-emitting diode or a light-emitting electrochemical cell.

EXAMPLES

η6-1,4-dichlorobenzene-η5-cyclopentadienylironhexafluorophosphate (4a) and η6-1,3-dichlorobenzene-η5-cyclopentadienylironhexafluorophosphate (4b) were prepared according to literature procedures.41(5b) was not isolated. Microwave syntheses were carried out with a Biotage Initiator system.1H-NMR and13C-NMR spectra were recorded with a Varian Inova 400 (400 MHz and 100 MHz, respectively) and Varian 500 (500 MHz and 125 MHz, respectively) spectrometers. Elemental analysis was performed with a Carlo Erba CHNS-O EA1108 elemental analyzer. Photoluminescence (PL) and excitation spectra were obtained with a Varian Cary Eclipse Fluorescence Spectrophotometer. UV-vis spectra were obtained with an Agilent 8453 UV-vis spectrophotometer. Low resolution mass spectrometry was performed with an Applied Biosystems Voyager Elite matrix-assisted laser desorption time-of-flight (MALDI-TOF) system and high resolution mass spectrometry was performed with a Bruker 9.4T Fourier-transform ion-cyclotron resonance (FTICR) and an Applied Biosystems Mariner orthogonal acceleration time-of-flight (ao-TOF) systems. Thermogravimetric analysis (TGA) was performed on a Perkin Elmer Pyris 1 system at a heating rate of 10° C./min. Differential scanning calorimetry (DSC) was performed with a TA Instruments Q1000 system at various heating and cooling rates. GPC analysis was performed on an Agilent 1100 series system equipped with a Waters Styragel® HR 4E column.

Indium-tin-oxide (ITO) coated glass substrates (8-12 Ω/sq., Delta Tech.) were sonicated in IPA, dried at 120° C., and exposed to an O2plasma for one minute. Hole injection PEDOT/PSS (Aldrich) was applied from a 2.8% w/v aqueous solution and heated at 60° C. for 10 minutes in a class 10 cleanroom. Active layers were prepared from 0.5% w/v toluene solutions and heated at 70° C. for 15 minutes in an N2filled glovebox. Electrical contacts were fabricated by sequentially depositing ca. 5 nm of Ca and ca. 150 nm of Al. PLED electroluminescence spectra were obtained in air using a Varian Cary Eclipse fluorescence spectrophotometer. IV curves were collected using a computer-controlled Keithley 2400 source.

Monomer Experimental

The monomers of the disclosure were prepared in as shown in Scheme 2. Details of these syntheses are provided below.

(c) K2CO3, dissolve 3 and 4a or 4b in DMF, stir at room temperature for 24 hours, isolation of 5a and 5b is not necessary prior to proceeding to the next synthetic step;(d) K2CO3, phenol, more DMF, stir at room temperature for 72 hours; (e) dissolve 6a and 6b in DMF and CH3CN, dimethylglyoxime and heat for 12 minutes at 200° C. in a microwave reactor.

Polymer Experimental

The monomers of the disclosure were prepared in as shown in Scheme 3. Details of these syntheses are provided below.

Results and Discussion for Examples 1 and 2

Monomer Synthesis

Intermediate compound 2,7-Dibromofluoren-9-one38(2) and target precursor compound 2,7-Dibromo-9,9′-di-(4-hydroxyphenyl)-9H-fluorene39(3), as shown in Scheme 1, were prepared according to literature procedures and characterized with1H and13C NMR spectroscopy, and electron-impact (EI) mass spectrometry. The crystal structure of (3) was reported previously.40

(3) was subsequently reacted with 4a or 4b,41as outlined in Scheme 2, to generate p-intermediate-bimetallic-complex (PIBC) 5a or m-intermediate-bimetallic-complex (MIBC) 5b. Isolation of PIBC or MIBC was not necessary; both were further reacted with additional phenol to produce p-final-bimetallic-complex (PFBC) 6a or m-final-bimetallic-complex (MFBC) 6b, respectively. The CpFe+moieties on the bimetallic complexes PFBC and MFBC were removed via microwave irradiation to generate p-tetra-ether (PTE) I(a) and m-tetra-ether (MTE) I(b), respectively. Compounds 5-7a,b were characterized with1H- and13C-NMR spectroscopy, high-resolution mass spectrometry (HRMS), and elemental analysis (EA).

Thermal and Oxidative Stability of PTE and MTE Monomers

TGA of PTE (I(a)), MTE (I(b)), and purified 2,7-dibromodioctylfluorene (DOF), in N2and air, provided insight into the thermal and oxidative stability of AE functionalized monomers as well as direct comparison with DOF. Table 1 summarizes andFIG. 1shows the onset of decomposition (10% weight loss) of MTE and PTE in N2and air occurs at substantially higher temperatures (ca. 150° C.) than for DOF. This increased stability is attributed to the stabilizing influence of the AE moieties. It is also noted that the onset of decomposition of PTE and MTE is approximately the same in both N2and air, in stark contrast to DOF. This may indicate the lack of a readily accessible decomposition pathway for AE-containing PTE and MTE.

Thermal Phase Behavior of PTE and MTE Monomers

Differential scanning calorimetry (DSC) provides insight into the phase behavior of PTE (I(a)) and MTE (I(b)) monomers. Table 1 summarizes the melting temperatures (Tm), purity, and glass transition temperatures (Tg) of PTE and MTE as determined by DSC. PTE and MTE exhibit well-defined melting temperatures (Tm) of 166 and 181° C., respectively, when heated from 35 to 250° C. on the first heating cycle. Upon cooling (10° C./min) and reheating (10° C./min) the sample, the well-defined melting temperatures are replaced by PTE and MTE glass transition temperatures of 78 and 53° C., respectively. The appearance of PTE and MTE glass transitions indicate that the cooling rate employed (10° C./min) during the DSC run was faster than nucleation and subsequent crystal growth. Similar behavior has been noted for glass forming, high Tg, bulky spiro compounds in which the crystallization kinetics are slowed.42Importantly, the higher Tg's exhibited by PTE and MTE are expected to translate into AE-based polymers with higher Tg's than PFO,43and increased emission stability.

The role of microwave technology in organic synthesis is expanding greatly and is now beginning to be utilized in organic polymer syntheses. One of the primary advantages of this synthetic technique is significantly decreased reaction times that make high throughput material synthesis viable. Highlighting this point, the first report of a PF-type polymer synthesis using microwave irradiation appeared in 2002.44Carter outlined the successful polymerization of alkyl-substituted PF materials in 10 minutes; a dramatic advancement over the traditional 3-4 days required for conventional thermal heating. A microwave-based approach was employed for the polymerization of the AE-functionalized PTE and MTE monomers of the present disclosure, and poly-p-tetra-ether (PPTE) III(a) and poly-m-tetra-ether (PMTE) III(b), as shown in Scheme 3. PPTE and PMTE were characterized with1H- and13C-NMR spectroscopy, gel permeation chromatography (GPC), MALDI-TOF mass spectrometry and elemental analysis (EA).

Thermal Phase Behavior of PPTE III(a) and PMTE III(b) Polymers

The glass transition temperature (Tg) of PPTE III(a) and PMTE III(b) were evaluated with DSC at a heating rate of 10° C./min as shown inFIG. 2and Table 2. The glass transition temperature of PPTE (184° C.) is substantially higher than PMTE (98° C.), which is attributed to PPTEs higher molecular weight and more symmetric side chains.

Thermal Stability Characterization of PPTE III(a) and PMTE III(b) Polymers

TGA investigations of PPTE III(a), PMTE III(b), and commercially available PFO, in N2and air atmospheres, were conducted to gain direct comparison of their thermal and oxidative stability.FIG. 3and Table 2 show PPTE and PMTE lose approximately 10 and 20% of their weight prior to plateauing at ca. 320° C. These polymers subsequently undergo another weight loss, which is attributed to decomposition of the polymer, at 528 and 515° C. respectively. If PPTE and PMTE are first heated to 320° C., cooled to 25° C., and reheated to 700° C., similar decomposition temperatures (534 and 515° C.) are observed (SeeFIG. 4). While the weight loss up to 320° C. could be attributed to the liberation of volatile impurities (e.g., solvent) remaining following synthesis and workup, placing PPTE and PMTE in a vacuum oven at ca. 100° C. for 24 hours did not address this issue. Elemental analysis of PPTE and PMTE revealed carbon content to be ca. 5% and 10% below theoretically predicted values and nitrogen contamination of ca. 0.5 and 2.0%, respectively was noted.

PPTE II(a) and PMTE II(b), shown inFIG. 4, exhibit significantly higher thermal decomposition temperatures than PFO, indicating AE moieties substantially improve the thermal and oxidative stability of PFs. The improvement in thermal stability of the AE-containing polymers (PPTE and PMTE) of the present disclosure over PFO is smaller (ca. 100° C.) than the difference in stability between the AE-containing monomers and DOF (ca. 150° C.), indicating that monomer stability does not necessarily quantitatively correlate with polymer stability.

FIG. 5and Table 2 show solution UV-vis and photoluminescence (PL) spectra of PPTE III(a) and PMTE III(b). These spectra are consistent with polymer formation and similar to those obtained for alkyl-substituted PF materials (e.g. PFO). Films of PPTE, PMTE, and PFO drop-coated and/or spin-coated from 1% w/v toluene solutions onto quartz substrates were evaluated for thermal and morphological stability upon thermal annealing in an N2atmosphere. In an effort to ensure removal of excess toluene, films were dried in ambient prior to drying in vacuo (ca. 30 mTorr) for 24 hours. Upon annealing a PFO film at 200° C., in an inert N2atmosphere, the PL spectrum continuously evolved over 80 hours resulting in three changes to the spectrum: (1) the contribution to the spectrum from each of the vibronic components change; (2) a shift in the relative positions of these transitions; (3) the intensity of the PL, in the green spectral region (ca. 550 nm), increases.45These changes are well established and have been attributed to polymer chain reordering and excimer formation.46In contrast to commercial PFO, PPTE and PMTE display spectral stability when annealed under identical conditions, as shown inFIG. 6.

Thermal annealing of PPTE, PMTE, and PFO in air show very different affects on the PL spectra. PFO shows a dramatic increase in green emission (ca. 550 nm) after annealing in air for 60 minutes at 150° C. and the resulting green emission visibly dominates film color after only 20 minutes of thermal stressing.45In contrast, the PL spectra of PPTE and PMTE exhibit negligible change under identical conditions which severely degrade PFO response, as shown inFIG. 7. We attribute the increased stability to the stabilizing influence of the AE moieties at the 9-position in PPTE and PMTE.

Proof-of-concept polymer light-emitting diodes (PLEDs) with PMTE III(b) as the emitting layer were fabricated with the following sandwich structure: ITO/PEDOT-PSS/PMTE/Ca/Al. The electroluminescence spectrum acquired in ambient conditions did not shift over device testing time (ca. 30 min). PMTE exhibited turn-on voltages of ca. 4.5 V and exhibited blue electroluminescence as shown inFIG. 8.

Conclusions

The direct incorporation of AE functionality into PF materials at the 9-position has allowed for the preparation of thermally stable materials with excellent oxidative stability. Initial electroluminescence results support the use of this material system for PLED applications.

While the present disclosure has been described with reference to what are presently considered to be the preferred examples, it is to be understood that the disclosure is not limited to the disclosed examples. To the contrary, the disclosure is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

TABLE 1Melting Temperature (Tm), Purity and DecompositionTemperature (Td) under both N2and air ofMTE and PTE monomersTd(° C.)Td(° C.)TgeMonomerTm(° C.)aPuritybN2caird(° C.)MTE I(a)17595.341841553PTE I(b)16696.544344578aTmwas determined with DSC at a heating rate of 0.5° C./min with 1-3 mg of sample.bCalorimetric purity determinations were made using 1-3 mg samples heated at 0.5° C./min with a DSC. The DSC purity determination software constructs a van't Hoff plot for the calculation of purity.c,dOnset decomposition temperatures (10% mass loss) were determined with a TGA in N2and air atmospheres, respectively.eTgwas determined by heating at 10° C./min from 35° C. to 250° C. followed by cooling to −50° C. and heating again at 10° C./min to 250° C.