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. 2019 Dec 16;4(27):22332–22344. doi: 10.1021/acsomega.9b02396

Poly(1,4-phenylene vinylene) Derivatives with Ether Substituents to Improve Polymer Solubility for Use in Organic Light-Emitting Diode Devices

Crystal A Young †, Audrey Hammack ‡, Han Ju Lee §, Huiping Jia ‡, Tianlang Yu §, Maria D Marquez §, Andrew C Jamison §, Bruce E Gnade ∥, T Randall Lee §,*
PMCID: PMC6941186  PMID: 31909316

Abstract

graphic file with name ao9b02396_0012.jpg

New ether-substituted poly(1,4-phenylene vinylene) (PPV) derivatives were synthesized via Horner–Emmons coupling. The structures of the monomers and the resultant oligomers were confirmed by 1H and 13C NMR spectroscopies. The molecular weights of the oligomers were characterized by gel permeation chromatography, giving the number-average and weight-average molecular weights and the corresponding polydispersity indices. Measurements of UV–vis absorption and fluorescence were used to characterize the optical properties of the oligomers. Estimation of the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels and other electrochemical characteristics of the oligomers were investigated by cyclic voltammetry. Dialkyl and dialkoxy PPV oligomers were also prepared and characterized following the same instrumental methods used for the ether-substituted oligomers, providing a known reference system to judge the performance of the new conjugated oligomers. Devices were fabricated to analyze the electroluminescent characteristics of the oligomers in organic light-emitting diodes.

Introduction

Conjugated semiconducting polymers have been the focus of much research since the first green electroluminescent (EL) device, generated from poly(phenylene vinylene) (PPV), was presented.1−5 Since then, many modifications have been made to improve polymer processability, device efficiency, emission wavelength, and turn-on voltage.6−10 However, unsubstituted PPV is known to be insoluble and infusible, and because the oft-cited advantage of polymeric materials over small molecules in electronic devices is ease of processability, the search for soluble conjugated polymers has been extensive. Methods to improve solubility include the introduction of solubilizing side groups on the PPV structure, introduction of soluble polymers to yield PPV copolymers, and introduction of flexible blocks.11−15 One of the most common methods used to improve PPV solubility is the incorporation of solubilizing long alkyl chains—linear, branched, or alkyl chains with polar functional groups—onto the phenyl rings, see Figure 1 for examples.16−25 Synthetically, the solubilizing moieties can easily be attached through a phenoxy precursor. The increased acidity of the phenolic hydrogen facilitates alkylation of the aromatic ring, opening avenues for the synthesis of a variety of functionalized PPV derivatives.

Figure 1.

Figure 1

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Common PPV derivatives with solubilizing side chains.

Previous research by Lee and co-workers focused on improving device lifetime by eliminating the use of electron-donating alkoxy substituents on the aromatic ring.26 This not only decreases the oxidation potential by lowering the energy of the highest occupied molecular orbital (HOMO) but also introduces radical scavenging benzylic positions associated with the replacement of a solubilizing substituent. The proposed mechanism of PPV degradation shows singlet oxygen cleaving the vinyl bonds, reducing the conjugation length, and also introducing carbonyl defects that have been shown to be fluorescence quenchers.27,28 On the other hand, cleavage of the solubilizing alkyl chains from the benzylic position does not have a significant impact on the optical properties of the polymer; cleavage at the benzylic position ensures that the polymer backbone remains intact, with no change in conjugation length.

However, in a previous study, removal of the oxygen atoms attached to the aromatic ring not only had the expected consequence of producing a blue shift in the emission spectrum (from orange to green), because of the replacement of the strong electron-donating oxygen with the weakly electron-donating alkyl group, but also produced a marked reduction in solubility from the dialkoxy PPV derivative—poly(2,5-didecyloxy-1,4-phenylene vinylene) (PDOPV)—to the new dialkyl PPV derivative—poly(2,5-didecyl-1,4-phenylene vinylene) (PDDPV), Figure 2.25,29 The decrease in solubility complicated comparisons in device performance because of the inability of PDDPV to form films of adequate thickness. While the oxygen present in PDOPV is detrimental to device lifetime, it appears to be necessary for improved device efficiency based on the role of solubility in device preparation. Apparent from the previous study, the presence of oxygen, directly attached to the aromatic ring, significantly affected the optical properties of the polymer. However, when the solubility-enhancing oxygen was not present, or in a position to donate electron density through resonance, the optical properties were less red-shifted, which is more amenable to realizing blue emitters. Therefore, herein we report the synthesis and characterization of new PPV derivatives in which the solubilizing chains are varied to study the effect on performance in organic light-emitting diode (OLED) devices. Meanwhile, because of the improved performance of the alternating copolymer poly(2,5-didecyl-p-phenylene vinylene-alt-2,5-didecyloxy-p-phenylene vinylene) (PDDPV-alt-DOPV) relative to the homopolymers in a previous study, the alternating copolymer incorporating the new heptyloxypropyl moiety was also studied.26 The oligomers prepared for this study include poly(2,5-didecyloxy-1,4-phenylene vinylene) (PDOPV) and poly(2,5-didecyl-1,4-phenylene vinylene) (PDDPV) and the new PPV derivatives poly(2,5-bis(3-(heptyloxy)propyl)-1,4-phenylene vinylene) (PHOPPV), poly(2,5-bis(3-(2-butoxyethoxy)propyl)-1,4-phenylene vinylene) (PBEPPV), poly(2,5-bis(3-(heptyloxy)propyl)-1,4-phenylene vinylene-alt-2,5-didecyl-1,4-phenylene vinylene) (PHOP-alt-DDPV), and poly(2,5-bis(3-(heptyloxy)propyl)-1,4-phenylene vinylene-alt-2,5-didecyloxy-1,4-phenylene vinylene) (PHOP-alt-DOPV); the molecular structures of the oligomers used in this study are shown in Figure 2.

Figure 2.

Figure 2

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PPV derivatives targeted in this study.

The incorporation of oxygen in the alkyl side chains should enhance the solubility of the new PPVs, leading to improved processing in the fabrication of OLED devices. To synthesize the targeted oligomers, the Horner–Emmons condensation was employed. This technique has been used to create polymers with no evidence of saturated defects, as can be seen with the Gilch polymerization.30 Irregularities introduced in the polymer reduce the conjugation length and diminish the device efficiency by acting as charge traps.31,32 In addition, unlike the Wittig reaction, the Horner–Emmons condensation has been shown to produce no cis-defects, which can also reduce charge transfer by limiting conjugation.33 Furthermore, unlike the Heck coupling or acyclic diene metathesis (ADMET) polymerization, the Horner–Emmons condensation does not rely on expensive metal catalysts.34,35

Experimental Section

Details on instrumentation and device fabrication can be found in the Supporting Information.

Materials

1,4-Dichlorobenzene, 1,3-bis(diphenylphosphino)propane nickel(II) chloride [NiCl2(dppp)], 1-bromodecane, bromine, iodine, copper cyanide (CuCN), potassium tert-butoxide (t-BuOK), diisobutylaluminium hydride (DIBAL-H in toluene), paraformaldehyde, 33% hydrobromic acid in acetic acid, triethyl phosphite, 1,4-dihydroxybenzene, triethylphosphonoacetate, triethyl 2-phosphonopropionate, terephthalaldehyde, sodium hydride (NaH), 10% palladium on carbon (Pd/C), lithium aluminum hydride (LiAlH4), 1-iodoheptane, pyridinium chlorochromate (PCC), thionyl chloride (SOCl2), potassium iodide (KI), methanesulfonyl chloride (MsCl), 2-butoxyethanol, triethylamine, 9,10-diphenylanthracene, quinine sulfate, triethylene glycol, ortho-dichlorobenzene (ODCB), potassium t-butoxide, ammonium hydroxide, and Celite were purchased from either Acros or Sigma-Aldrich and used without further purification, unless otherwise noted. For synthetic preparations, diethyl ether, dichloromethane (CH2Cl2), and tetrahydrofuran (THF) were acquired from Sigma-Aldrich and dried by distillation from CaH2 under nitrogen. Sulfuric acid (98.0%), anhydrous dimethyl sulfoxide, anhydrous benzene, and anhydrous dimethylformamide (DMF) were purchased from Sigma-Aldrich and used as received. Column chromatography was performed using silica gel (Sorbent Technologies, 40–63 μm). Fused quartz microscope slides were purchased from AdValue Technology. For the preparation of the devices, Clevios PEDOT:PSS was purchased from Heraeus Precious Metals. Tris(8-hydroxyquinolinato)-aluminum (Alq3; sublimed) was acquired from Luminescence Technology Corp., and lithium fluoride (LiF; anhydrous) was purchased from Sigma-Aldrich, while the 1.0 mm diameter aluminum wire (99.999%) was purchased from Kurt J. Lesker Co.

Synthesis of Monomers and Oligomers

The chemical structures of the targeted oligomeric products are illustrated in Figure 2. The synthetic routes used to prepare the monomers, oligomers, and alternating co-oligomers are outlined in Schemes 1, 2, 3.

Scheme 1. Synthetic Route Followed to Obtain PHOPPV.

Scheme 1

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Scheme 2. Synthetic Route to Synthesize PBEPPV.

Scheme 2

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Scheme 3. Horner–Emmons Condensation Used to Synthesize the Alternating Co-Oligomers.

Scheme 3

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Poly(2,5-didecyl-p-phenylene vinylene) (PDDPV)26

The synthesis of PDDPV was conducted following the procedures from the literature. The final product was dried under vacuum to afford the oligomer, PDDPV, as a bright yellow-green solid (0.59 g, 65%). 1H NMR (500 MHz, CDCl3): δ 7.41 (2H, aromatic), 7.23 (peak obscured by residual CHCl3), 2.74–2.80 (m, 4H, α-CH2), 1.62–1.72 (m, 4H, β-CH2), 1.20–1.48 (m, 28H, CH2), 0.82–0.87 (m, 6H, CH3). 1H NMR (500 MHz, THF-d8): δ 7.44 (2H, aromatic), 7.28 (s, 2H, vinylene), 2.74–2.84 (m, 4H, α-CH2), 1.62–1.72 (peak obscured by residual THF), 1.18–1.48 (m, 28H, CH2), 0.82–0.88 (m, 6H, CH3).

Poly(2,5-didecyloxy-p-phenylene vinylene) (PDOPV)

PDOPV was prepared following a procedure similar to that of PDDPV.26 The final product was dried under vacuum to afford the oligomer PDOPV as a red solid (0.054 g, 84%). 1H NMR (400 MHz, CDCl3): δ 7.45 (s, 2H, aromatic), 7.15 (d, J = 12 Hz, 2H, trans-vinylene), 3.95–4.09 (m, 4H, α-CH2), 1.77–1.91 (m, 4H, β-CH2), 1.25–1.53 (m, 24H, CH2), 1.45–1.66 (m, 4H, CH2), 0.84–0.88 (m, 6H, CH3).

(2E,2′E)-Diethyl 3,3′-(1,4-Phenylene)diacrylate (2)26,36

In a 1 L round-bottomed flask equipped with a condenser dried and flushed with N2 gas, 60% sodium hydride (10.6 g, 0.265 mol) in mineral oil, previously rinsed with petroleum ether, and 500 mL of anhydrous benzene were added. Triethylphosphonoacetate (1) (19.6 mL, 0.125 mol) was slowly added to the suspension and stirred for 1 h in an ice bath. After that, terephthalaldehyde (8.3824 g, 62.495 mmol), in 200 mL of anhydrous benzene, was added to the chilled suspension with vigorous stirring. The reaction was heated to 60 °C for 5 d, and after cooling to room temperature (rt), the suspension was filtered. The filtrate was concentrated and recrystallized in hexanes to give light-yellow crystals (3.777 g, 22%). 1H NMR (400 MHz in CDCl3): δ 7.66 (d, J = 16.0 Hz, 2 H, α-CH), 7.53 (s, 4 H, aromatic), 6.46 (d, J = 16.0 Hz, 2 H, β-CH), 4.27 (q, J = 7.32 Hz, 4 H, CH2CH3), 1.34 (t, J = 7.32 Hz, 6 H, CH2CH3).

Diethyl 3,3′-(1,4-Phenylene)dipropanoate (3)37

In a 1 L pressure vessel flushed with N2, 10 wt % palladium on carbon (3.678 g, 3.456 mmol) was added with a minimal amount of methanol. The system was then vacuum-purged three times. After 30 min under an atmosphere of hydrogen (1 atm), 2 (9.500 g, 34.66 mmol) in degassed benzene (350 mL) was transferred into the pressure vessel by cannula and stirred at rt overnight. Completion of the reaction was verified by 1H NMR spectroscopy. After completion, the suspension was filtered through Celite using CH2Cl2 as the solvent. The solvent was evaporated to give the product as a white powder (9.7046 g, quantitative). 1H NMR (500 MHz, CDCl3): δ 7.11 (s, 4H, aromatic), 4.12 (q, J = 6.90 Hz, 4H, OCH2), 2.91 (t, J = 7.45 Hz, 4H, α-CH2), 2.59 (quintet, J = 5.15 Hz, 4H, β-CH2), 1.39 (t, J = 6.85 Hz, 6H, OCH2CH3).

3,3′-(1,4-Phenylene)dipropan-1-ol (4)36

In an oven-dried 500 mL round-bottomed flask (previously flushed with N2 gas), lithium aluminum hydride (9.3 g, 0.25 mol) and freshly distilled diethyl ether (100 mL) were stirred and chilled in an ice bath. A solution of 3 (11.391 g, 40.925 mmol) in 300 mL of diethyl ether was slowly added. The reaction mixture was stirred at rt overnight. The reaction was then chilled in an ice bath before quenching with 20 mL of 2 M HCl. The mixture was filtered, the residual H2O was removed with Na2SO4, followed by concentration and drying of the filtrate. The product was obtained as a white powder (8.93 g, quantitative). 1H NMR (500 MHz, CDCl3): δ 7.12 (s, 4H, aromatic), 3.67 (q, J = 6.25 Hz, J′ = 5.75 Hz, 4H, γ-CH2), 2.68 (t, J = 7.45 Hz, 4H, α-CH2), 1.89 (quintet, J = 6.85 Hz, 4H, β-CH2), 1.25 (t, J = 5.15 Hz, 2H, OH).

3,3′-(2,5-Dibromo-1,4-phenylene)dipropan-1-ol (5)26

In a round-bottomed flask wrapped in foil, a solution of iodine (0.1374 g, 0.5414 mmol) and 4 (6.486 g, 33.39 mmol) in freshly distilled CH2Cl2 (25 mL) and bromine (13.255 g, 82.943 mmol) was added dropwise under rigorous exclusion of light. After 20 h at rt, a 20% KOH solution was added carefully until the red color of the solution disappeared. The mixture was filtered, and the solid was recrystallized in CH2Cl2. The product, 17, was obtained as a white powder (10.1737 g, 87%). 1H NMR (500 MHz, CDCl3): δ 7.40 (s, 2H, aromatic), 3.70 (q, J = 6.30 Hz, J′ = 5.15 Hz, 4H, γ-CH2), 2.76 (t, J = 8.05 Hz, 4H, α-CH2), 1.86 (quintet, J = 8.00 Hz, 4H, β-CH2), 1.33 (t, J = 5.15 Hz, 2H, OH).

1,4-Dibromo-2,5-bis(3-(heptyloxy)propyl)benzene (6)38

Anhydrous DMF (10 mL) was added to a round-bottomed flask containing 60% NaH in mineral oil (2.0436 g, 85.150 mmol), which was previously washed three times with petroleum ether. A solution of 5 (3.5657 g, 10.128 mmol) in DMF (20 mL) was transferred by a syringe into the round-bottomed flask. After stirring for 1 h at rt, iodoheptane (15.0 mL, 91.5 mmol) was added. The mixture was heated to 120 °C overnight. After cooling to rt, the reaction was quenched with H2O and extracted with CH2Cl2. After vacuum distillation at 100 °C, a purple oil was obtained. Column chromatography in a 24:1 ratio (hexanes/EtOAc) afforded a colorless oil (3.5132, 63%). 1H NMR (400 MHz, CDCl3): δ 7.38 (s, 2H, aromatic), 3.41 (t, J = 6.44 Hz, 4H, γ-CH2), 3.38 (t, J = 6.88 Hz, 4H, OCH2), 2.73 (t, J = 7.56 Hz, 4H, α-CH2), 1.86 (quintet, J = 6.40 Hz, 4H, β-CH2), 1.57 (quintet, J = 7.80 Hz, OCH2CH2), 1.22–1.38 (m, 16H, OCH2CH2(CH2)4), 0.88 (t, J = 6.30 Hz, 6H, O(CH2)6CH3).

2,5-Bis(3-(heptyloxy)propyl)terephthalonitrile (7)26

In a round-bottomed flask equipped with a stir bar and a condenser (oven-dried and flushed with nitrogen gas), 6 (1.0368 g, 1.8905 mmol) and CuCN (0.5910 g, 6.598 mmol) in anhydrous DMF (12 mL) were heated to 162 °C for 3 d. The solution was then cooled to rt and poured into a 25% NH4OH solution (24 mL) to precipitate the product. The product was then washed with NH4OH and water. The remaining material was extracted in a Soxhlet apparatus with acetone for 1 h. After evaporation of the solvent, a light brown solid was isolated (0.7789 g, 93%). 1H NMR (500 MHz, CDCl3): δ 7.58 (s, 2H, aromatic), 3.42 (t, J = 6.30 Hz, 4H, γ-CH2), 3.41 (t, J = 6.85 Hz, 4H, OCH2), 2.93 (t, J = 7.45 Hz, 4H, α-CH2), 1.93 (quintet, J = 6.30 Hz, 4H, β-CH2), 1.53 (m, OCH2CH2, obscured by H2O), 1.21–1.32 (m, 16H, OCH2CH2(CH2)4), 0.88 (t, J = 6.30 Hz, 6H, O(CH2)6CH3).

2,5-Bis(3-(heptyloxy)propyl)terephthalic Acid (8)39

To a round-bottomed flask equipped with a condenser, 7 (0.8101 g, 1.838 mmol), KOH (1.2527 g, 22.328 mmol), and triethylene glycol (18 mL) were added. The mixture was heated to 200 °C overnight. After cooling to rt, the reaction mixture was poured into a 50% H2SO4 solution (600 mL) and then filtered. The solid was rinsed with H2O and dried under vacuum to yield a brown solid (0.8083 g, 92%). 1H NMR (500 MHz, CDCl3): δ 7.80 (s, 2H, aromatic), 3.45 (t, J = 6.30 Hz, 4H, γ-CH2), 3.38 (t, J = 6.90 Hz, 4H, OCH2), 3.03 (t, J = 6.85 Hz, 4H, α-CH2), 1.95 (quintet, J = 7.45 Hz, 4H, β-CH2), 1.53 (m, OCH2CH2), 1.27 (m, 16H, OCH2CH2(CH2)4), 0.87 (t, J = 6.90 Hz, 6H, O(CH2)6CH3).

Diethyl 2,5-Bis(3-(heptyloxy)propyl)terephthalate (9)

In a round-bottomed flask, a solution of 8 (0.8831 g, 1.845 mmol), concentrated H2SO4 (1.41 mL), and absolute ethanol (14 mL) were heated to 88 °C for 12 h. After cooling the reaction mixture to rt, it was poured into H2O. The solid was then filtered, washed with H2O, and dried under vacuum to yield a dark brown solid (0.8957 g, 91%). 1H NMR (500 MHz, CDCl3): δ 7.71 (s, 2H, aromatic), 4.40 (q, J = 6.90 Hz, 4H, C(O)OCH2), 3.42 (t, J = 6.85 Hz, 4H, γ-CH2), 3.40 (t, J = 6.85 Hz, 4H, OCH2CH2), 2.98 (t, J = 8.00 Hz, 4H, α-CH2), 1.87 (quintet, J = 6.30 Hz, 4H, β-CH2), 1.57 (quintet, J = 6.85 Hz, OCH2CH2), 1.39 (t, J = 6.85 Hz, 6H, OCH2CH3), 1.24–1.35 (m, 16H, OCH2CH2(CH2)4), 0.87 (t, J = 6.85 Hz, 6H, O(CH2)6CH3).

(2,5-Bis(3-(heptyloxy)propyl)-1,4-phenylene)dimethanol (10)34

Lithium aluminum hydride (0.3772 g, 9.939 mmol) and dry diethyl ether (5 mL) were stirred in an oven-dried 50 mL round-bottomed flask (previously flushed with N2 gas) at 0 °C. A solution of 9 (0.8957 g, 1.675 mmol) in dry diethyl ether (29 mL) was then added slowly. The reaction mixture was stirred at rt for 12 h and then chilled in an ice bath before quenching with 2 M HCl (4.5 mL). The mixture was filtered, the residual H2O was removed with Na2SO4, followed by concentration and drying of the filtrate, and the product was obtained as a white powder (0.6668 g, 88%). 1H NMR (400 MHz, CDCl3): δ 7.18 (s, 2H, aromatic), 4.65 (s, 4H, CH2OH), 3.39 (t, J = 6.88 Hz, 4H, γ-CH2), 3.37 (t, J = 6.88 Hz, 4H, OCH2), 2.75 (t, J = 7.36 Hz, 4H, α-CH2), 1.90 (quintet, J = 6.88 Hz, 4H, β-CH2), 1.55 (quintet, J = 6.88 Hz, OCH2CH2), 1.24–1.34 (m, 16H, OCH2CH2(CH2)4), 0.87 (t, J = 6.40 Hz, 6H, O(CH2)6CH3).

2,5-Bis(3-(heptyloxy)propyl)terephthalaldehyde (11)40

To a dry 25 mL flask, Celite (0.7337 g) and PCC (0.6403 g, 2.970 mmol) were added. A solution of 10 (0.3111 g, 0.6903 mmol) in freshly distilled CH2Cl2 (7 mL) was transferred to this suspension and stirred for 3 h at rt. The mixture was then filtered through a pad of Celite and rinsed with additional CH2Cl2. After removing the solvent, the product was obtained as a pale-yellow solid (0.2721 g, 88%). 1H NMR (500 MHz, CDCl3): δ 10.4 (s, 2H, C(O)H), 7.75 (s, 2H, aromatic), 3.41 (t, J = 6.30 Hz, 4H, γ-CH2), 3.38 (t, J = 6.90 Hz, 4H, OCH2), 3.14 (t, J = 8.05 Hz, 4H, α-CH2), 1.90 (quintet, J = 6.30 Hz, 4H, β-CH2), 1.57 (quintet, J = 6.90 Hz, OCH2CH2), 1.24–1.36 (m, 16H, OCH2CH2 (CH2)4), 0.88 (t, J = 6.9 Hz, 6H, O(CH2)6CH3).

1,4-Bis(chloromethyl)-2,5-bis(3-(heptyloxy)propyl)benzene (12)30

Thionyl chloride (0.75 mL, 0.010 mol) was added to a solution of 10 (0.3121 g, 0.7216 mmol) in freshly distilled CH2Cl2 (7 mL) in a dry 50 mL flask. After stirring for 3 h, the reaction mixture was poured into H2O (40 mL). The aqueous layer was extracted with CH2Cl2. The combined organic layers were washed with H2O and dried over Na2SO4. After removal of the solvent, the product was obtained as a yellow oil (0.3612 g, quantitative). 1H NMR (500 MHz, CDCl3): δ 7.19 (s, 2H, aromatic), 3.14 (s, 4H, CH2Cl), 3.43 (t, J = 6.30 Hz, 4H, γ-CH2), 3.41 (t, J = 6.90 Hz, 4H, OCH2), 2.77 (t, J = 7.45 Hz, 4H, α-CH2), 1.90 (quintet, J = 6.30 Hz, 4H, β-CH2), 1.59 (quintet, J = 6.90 Hz, OCH2CH2), 1.24–1.38 (m, 16H, OCH2CH2 (CH2)4), 0.88 (t, J = 6.9 Hz, 6H, O(CH2)6CH3).

1,4-Bis(methylenediethylphosphonate)-2,5-bis(3-(heptyloxy)propyl)benzene (13)

In a round-bottomed flask attached to a condenser, 12 (0.3612 g, 0.7408 mmol) and triethyl phosphite (0.85 mL, 5.0 mmol) were heated at 160 °C for 24 h. Excess triethyl phosphite was removed by vacuum distillation at 120 °C to give 13 as a brown solid (0.4389 g, 86%). 1H NMR (500 MHz, CDCl3): δ 7.11 (s, 2H, aromatic), 3.97 (q, J = 6.85 Hz, 8H, POCH2), 3.41 (t, J = 6.3 Hz, 4H, γ-CH2), 3.39 (t, J = 6.85 Hz, 4H, OCH2), 3.14 (d, J = 20.7 Hz, 4H, CH2P), 2.71 (t, J = 8.0 Hz, 4H, α-CH2), 1.83 (quintet, J = 7.45 Hz, 4H, β-CH2), 1.58 (m, 4H, OCH2CH2) (obscured by H2O), 1.20–1.36 (m, 16H, OCH2CH2(CH2)4), 1.21 (t, J = 6.85 Hz, 6H, OCH2CH3), 0.87 (t, J = 6.85 Hz, 6H, O(CH2)6CH3).

Poly(2,5-bis(3-(heptyloxy)propyl)-1,4-phenylene vinylene) (PHOPPV)

The dialdehyde 11 (0.1250 g, 0.2799 mmol), the diphosphonate 13 (0.1930 g, 0.2799 mmol), and potassium t-butoxide (0.1568 g, 1.397 mmol) were added to a 25 mL two-neck round-bottomed flask with a condenser that had been flame-dried, flushed with N2 gas, and wrapped in aluminum foil. Anhydrous DMF (18 mL, purged 20 min with N2 gas) was added via a syringe. The solution was stirred at 80 °C for 12 h and then cooled to rt. Methanol (24 mL) was added to precipitate the product. After transferring to a centrifuge tube, the suspension was chilled to promote precipitation. The sample was centrifuged and the supernatant was discarded. After adding a minimal amount of THF, additional reprecipitations were carried out using methanol (twice), isopropanol, and pentane. After vacuum-drying, the product was obtained as a yellow-green solid (0.0241 g, 19%). 1H NMR (500 MHz, CDCl3): δ 7.44 (s, 2H, aromatic), 7.22 (s, 2H, vinylene; the peak contains contributions from residual CHCl3 in the NMR solvent), 3.47 (m, 4H, γ-CH2), 3.39 (m, 4H, OCH2), 2.87 (m, 4H, α-CH2), 1.93 (m, 4H, β-CH2), 1.59 (m, 20H, CH2), 1.27 (m, 6H, CH3).

3,3′-(2,5-Dibromo-1,4-phenylene)bis(propane-3,1-diyl) Dimethanesulfonate (14)

Diol 5 (10.0 g, 28.4 mmol) was introduced into a 500 mL round-bottomed flask containing 250 mL of THF, and then an aliquot of triethylamine (14.0 mL, 100 mmol) was slowly added into the solution. The reaction mixture was stirred at rt for 30 min. The reaction flask was placed in an ice bath, and methanesulfonyl chloride (8.0 mL, 100 mmol) was slowly transferred into the solution while stirring. The ice bath was then removed, and the reaction was stirred for 4 h at rt. After completion of the reaction, excess methanesulfonyl chloride was destroyed by addition of 100 mL of water. The mixture was extracted with diethyl ether (3 × 100 mL). The combined organic layers were washed with 2.0 M HCl solution (100 mL) and water (2 × 100 mL). The organic layer was dried over Na2SO4, filtered, and concentrated in vacuo to produce 14 (11.0 g, 21.6 mmol), which was used in the next step without further purification (76% yield). 1H NMR (500 MHz, CDCl3): δ 7.40 (s, 2H), 4.25 (t, J = 5.7 Hz, 4H), 3.03 (s, 6H), 2.80 (t, J = 8.0 Hz, 4H), 2.06 (m, 4H).

1,4-Dibromo-2,5-bis(3-iodopropyl)benzene (15)

The crude mesylate 14 (11.0 g, 21.6 mmol) and KI (9.00 g, 54.1 mmol) were dissolved in reagent-grade acetone (250 mL) and refluxed for 12 h. The solvent was evaporated by rotary evaporation. Water (100 mL) and diethyl ether (50 mL) were added to dissolve the salts and the product. The mixture was extracted with diethyl ether (2 × 100 mL). The combined organic layers were washed with brine (2 × 100 mL), dried over Na2SO4, filtered, and concentrated in vacuo to obtain the product (8.9 g, 16 mmol) as a brown solid (72% yield). 1H NMR (500 MHz, CDCl3): δ 7.40 (s, 2H), 3.20 (t, J = 6.9 Hz, 4H), 2.78 (t, J = 7.5 Hz, 4H), 2.10 (m, 4H).

1,4-Dibromo-2,5-bis(3-(2-butoxyethoxy)propyl)benzene (16)36

An aliquot of 2-butoxyethanol (6.0 g, 50 mmol) was added dropwise to a stirred solution of potassium hydride (2.00 g, 50.0 mmol) in THF (20 mL) at 0 °C under nitrogen. After 30 min, 15 (3.0 g, 5.3 mmol) in THF (10 mL) was added. The reaction mixture was refluxed for 24 h. After evaporation of the solvent, the crude product was purified by column chromatography (hexanes:ether = 9.8:0.2) to give 0.40 g (0.72 mmol) of 16 (13% yield). 1H NMR (500 MHz in CDCl3): δ 7.38 (s, 2H, aromatic), 3.59 (s, 8H, OCH2CH2O), 3.49 (t, J = 6.85 Hz, 4H, γ-CH2), 3.48 (t, J = 6.30 Hz, 4H, OCH2), 2.73 (t, J = 7.45 Hz, 4H, α-CH2), 1.87 (quintet, J = 7.45 Hz, 4H, β-CH2), 1.58 (quintet, J = 6.3 Hz, 4H, OCH2CH2), 1.58 (sextet, J = 7.45 Hz, 4H, OCH2CH2CH2), 0.91 (t, J = 7.45 Hz, 6H, CH3).

2,5-Bis(3-(2-butoxyethoxy)propyl)terephthalonitrile (17)

2,5-Bis(3-(2-butoxyethoxy)propyl)terephthalonitrile (17) was prepared following an analogous procedure used for the preparation of 7: 0.1368 g, 65% yield. 1H NMR (400 MHz in CDCl3): δ 7.60 (s, 2H, aromatic), 3.58 (s, 8H, OCH2CH2O), 3.49 (t, J = 5.96 Hz, 4H, γ-CH2), 3.48 (t, J = 6.88 Hz, 4H, OCH2), 2.93 (t, J = 7.80 Hz, 4H, α-CH2), 1.94 (quintet, J = 5.92 Hz, 4H, β-CH2), 1.57 (quintet, J = 4.56 Hz, 4H, OCH2CH2), 1.36 (sextet, J = 7.80 Hz, 4H, OCH2CH2CH2), 0.91 (t, J = 7.36 Hz, 6H, CH3).

2,5-Bis(3-(2-butoxyethoxy)propyl)terephthalic Acid (18)

2,5-Bis(3-(2-butoxyethoxy)propyl)terephthalic acid (18) was prepared following an analogous procedure used for the preparation of 8: 0.1489 g, quantitative yield. 1H NMR (400 MHz in CDCl3): δ 7.70 (s, 2H, aromatic), 3.62 (m, 4H, OCH2CH2O(CH2)3CH3), 3.57 (m, 4H, OCH2CH2O(CH2)3CH3), 3.52 (t, J = 6.88 Hz, 4H, γ-CH2), 3.43 (t, J = 5.96 Hz, 4H, OCH2), 3.07 (t, J = 7.32 Hz, 4H, α-CH2), 1.89 (quintet, J = 6.40 Hz, 4H, β-CH2), 1.63 (quintet, J = 7.76 Hz, 4H, OCH2CH2), 1.38 (sextet, J = 7.80 Hz, 4H, OCH2CH2CH2), 0.92 (t, J = 7.32 Hz, 6H, CH3).

Diethyl 2,5-Bis(3-(2-butoxyethoxy)propyl)terephthalate (19)

Diethyl 2,5-bis(3-(2-butoxyethoxy)propyl)terephthalate (19) was prepared following an analogous procedure used for the preparation of 9: 0.1425 g, 86% yield. 1H NMR (400 MHz in CDCl3): δ 7.71 (s, 2H, aromatic), 4.35 (q, J = 6.88 Hz, 4H, C(O)OCH2), 3.58 (s, 8H, OCH2CH2O), 3.49 (t, J = 6.40 Hz, 4H, γ-CH2), 3.46 (t, J = 6.84 Hz, 4H, OCH2), 2.98 (t, J = 7.36 Hz, 4H, α-CH2), 1.89 (quintet, J = 7.32 Hz, 4H, β-CH2), 1.56 (quintet, J = 6.84 Hz, 4H, OCH2CH2), 1.39 (t, J = 6.84 Hz, 6H, CH2CH3), 1.37 (sextet, J = 7.32 Hz, 4H, OCH2CH2CH2), 0.90 (t, J = 7.36 Hz, 6H, CH3).

(2,5-Bis(3-(2-butoxyethoxy)propyl)-1,4-phenylene)dimethanol (20)

(2,5-Bis(3-(2-butoxyethoxy)propyl)-1,4-phenylene)dimethanol (20) was prepared following an analogous procedure used for 10: 0.1053 g, 88% yield. 1H NMR (400 MHz in CDCl3): δ 7.19 (s, 2H, aromatic), 4.66 (s, 4H, CH2OH), 3.56 (s, 8H, OCH2CH2O), 3.47 (t, J = 6.88 Hz, 4H, γ-CH2), 3.45 (t, J = 5.96 Hz, 4H, OCH2), 2.76 (t, J = 7.45 Hz, 4H, α-CH2), 1.92 (quintet, J = 6.44 Hz, 4H, β-CH2), 1.57 (quintet, J = 7.80 Hz, 4H, OCH2CH2), 1.36 (sextet, J = 7.32 Hz, 4H, OCH2CH2CH2), 0.91 (t, J = 7.36 Hz, 6H, CH3).

2,5-Bis(3-(2-butoxyethoxy)propyl)terephthalaldehyde (21)

2,5-Bis(3-(2-butoxyethoxy)propyl)terephthalaldehyde (21) was prepared following an analogous procedure used for 11: 0.0346 g, 66% yield. 1H NMR (500 MHz in CDCl3): δ 10.3 (s, 2H, C(O)H), 7.76 (s, 2H, aromatic), 3.57 (s, 8H, OCH2CH2O), 3.48 (t, J = 6.30 Hz, 4H, γ-CH2), 3.47 (t, J = 6.30 Hz, 4H, OCH2), 3.14 (t, J = 7.80 Hz, 4H, α-CH2), 1.92 (quintet, J = 6.30 Hz, 4H, β-CH2), 1.58 (quintet, J = 6.30 Hz, 4H, OCH2CH2), 1.37 (sextet, J = 7.45 Hz, 4H, OCH2CH2CH2), 0.91 (t, J = 7.45 Hz, 6H, CH3).

1,4-Bis(3-(2-butoxyethoxy)propyl)-2,5-bis(chloromethyl)benzene (22)

1,4-Bis(3-(2-butoxyethoxy)propyl)-2,5-bis(chloromethyl)benzene (22) was prepared following an analogous procedure used for 12: 0.0605 g, quantitative yield. 1H NMR (500 MHz in CDCl3): δ 7.19 (s, 2H, aromatic), 4.60 (s, 4H, CH2Cl), 3.59 (s, 8H, OCH2CH2O), 3.50 (t, J = 5.95 Hz, 4H, γ-CH2), 3.48 (t, J = 6.90 Hz, 4H, OCH2), 2.77 (t, J = 7.80 Hz, 4H, α-CH2), 1.92 (quintet, J = 6.40 Hz, 4H, β-CH2), 1.58 (quintet, J = 6.90 Hz, 4H, OCH2CH2), 1.37 (sextet, J = 7.80 Hz, 4H, OCH2CH2CH2), 0.91 (t, J = 7.30 Hz, 6H, CH3).

1,4-Bis(methylenediethylphosphonate)-2,5-bis(3-(2-butoxyethoxy))benzene (23)

1,4-Bis(methylenediethylphosphonate)-2,5-bis(3-(2-butoxyethoxy))benzene (23) was prepared following an analogous procedure used for 13: 0.0745 g, 88% yield. 1H NMR (400 MHz, CDCl3): δ 7.10 (d, J = 1.36 Hz, 2H, aromatic), 3.96 (dq, J = 7.32 Hz, J′ = 2.28 Hz, 8H, POCH2), 3.57 (s, 8H, OCH2CH2O), 3.48 (t, J = 6.4 Hz, 4H, γ-CH2), 3.46 (t, J = 6.88 Hz, 4H, OCH2), 3.13 (d, J = 20.6 Hz, 4H, CH2P), 2.70 (t, J = 7.80 Hz, 4H, α-CH2), 1.85 (quintet, J = 7.80 Hz, 4H, β-CH2), 1.57 (quintet, J = 7.76 Hz, 4H, OCH2CH2), 1.35 (sextet, J = 7.80 Hz, 4H, OCH2CH2CH2), 1.21 (t, J = 6.88 Hz, 6H, OCH2CH3), 0.90 (t, J = 7.80 Hz, 6H, CH3).

Poly(2,5-bis(3-(2-butoxyethoxy)propyl)-1,4-phenylene vinylene) (PBEPPV)

Monomers 21 (0.0788 g, 0.175 mmol) and 23 (0.1226 g, 0.176 mmol) were used to prepare the oligomer PBEPPV following an analogous procedure used for the preparation of PHOPPV: 0.0303 g, 43% yield. 1H NMR (500 MHz, CDCl3): δ 7.43 (s, 2H, aromatic), 7.26 (s, 2H, vinylene; the peak contains contributions from residual CHCl3 in the NMR solvent), 3.57 (m, 8H, OCH2CH2O), 3.44 (t, J = 6.85 Hz, 8H, γ-CH2, OCH2), 2.88 (m, 4H, α-CH2), 1.96 (m, 4H, β-CH2), 1.55 (sextet, J = 6.85 Hz, 4H, OCH2CH2), 1.34 (sextet, J = 7.45 Hz, 4H, OCH2CH2CH2), 0.89 (t, J = 7.45 Hz, 6H, CH3).

1,4-Bis(methylenediethylphosphonate)-2,5-didecylbenzene (24)

The synthesis of 24 was conducted following procedures from the literature.23 A total of 3.60 g of white solid was obtained (99% yield). 1H NMR (500 MHz, CDCl3): δ 7.07 (d, J = 1.2 Hz, 2H, aromatic), 3.89–4.10 (m, 8H, −OCH2CH3), 3.11 (d, 4H, J = 20.1, CH2P), 2.59 (t, J = 8.0 Hz, 4H, α-CH2), 1.48–1.55 (m, 4H, β-CH2), 1.20–1.37 (m, 28H, CH2), 1.20 (t, J = 6.9 Hz, 12H, −OCH2CH3), 0.85 (t, J = 6.9 Hz, 6H, CH3).

2,5-Bis(decyloxy)benzene-1,4-dialdehyde (25)

The synthesis of 25 was conducted following procedures from the literature.41 The final product was obtained as a yellow solid (59% yield). 1H NMR (400 MHz, CDCl3): δ 10.51 (s, 2H, CHO), 7.42 (s, 2H, aromatic), 4.07 (t, J = 6.4 Hz, 4H, OCH2R″), 1.82 (quintet, J = 7.8 Hz, 4H, OCH2CH2R′), 1.42–1.49 (m, 4H, OCH2CH2CH2R), 1.21–1.39 (m, 24H, OCH2CH2CH2(CH2)6CH3), 0.87 (t, J = 6.8 Hz, 6H, CH3).

Poly(2,5-bis(3-(heptyloxy)propyl)-1,4-phenylene vinylene-alt-2,5-didecyl-1,4-phenylene vinylene) (PHOP-alt-DDPV)

Monomers 11 (0.0683 g, 0.165 mmol) and 24 (0.1006 g, 0.1671 mmol) were used to prepare the co-oligomer PHOP-alt-DDPV following an analogous procedure used for the preparation of PHOPPV: 0.0317 g, 12% yield. 1H NMR (500 MHz, CDCl3): δ 7.42 (s, 8H, aromatic), 7.26 (s, 8H, vinylene; the peak contains contributions from residual CHCl3 in the NMR solvent), 3.48 (m, 4H, γ-CH2), 3.41 (m, 4H, OCH2), 2.86 (m, 4H, α-CH2), 2.77 (m, 4H, α-CH2), 1.94 (m, 4H, β-CH2), 1.65 (m, 4H, β-CH2), 1.26 (m, 48H, CH2), 0.87 (m, 12H, CH3).

Poly(2,5-bis(3-(heptyloxy)propyl)-1,4-phenylene vinylene-alt-2,5-didecyloxy-1,4-phenylene vinylene) (PHOP-alt-DOPV)

Monomers 11 (0.0502 g, 0.121 mmol) and 25 (0.0772 g, 0.112 mmol) were used to prepare the co-oligomer PHOP-alt-DOPV following an analogous procedure used for the preparation of PHOPPV: 0.0233 g, 12% yield. 1H NMR (500 MHz, CDCl3): δ 7.42 (s, 8H, aromatic), 7.26 (s, 8H, vinylene; the peak contains contributions from residual CHCl3 in the NMR solvent), 3.47 (m, 4H, γ-CH2), 3.39 (m, 4H, OCH2), 2.85 (m, 4H, α-CH2), 1.93 (m, 4H, β-CH2), 1.86 (m, 4H, OCH2CH2), 1.26 (m, 48H, CH2), 0.87 (m, 12H, CH3).

Results and Discussion

Monomer and Oligomer Synthesis

As outlined in Schemes 1, 2, 3 and detailed in the Experimental Section, the Horner–Emmons polycondensation was carried out in DMF to accomplish the polymerizations. All the investigated oligomers were isolated in modest yields with those formed from alkyl substituents as yellow-green solids and those including alkoxy substituents as red solids. The anticipated chemical structures of the oligomers were verified by analysis of their 1H NMR spectra (Figures S1–S6).

For the synthesis of the diether precursors, our initial approach was to use the same Kumada coupling used to obtain didecylbenzene in the synthesis of PDDPV.42 However, coupling of the ether side chains onto 1,4-dichlorobenzene failed, and thus, the route seen in Scheme 1 was then developed to obtain the dibromobenzene, intermediate 5, used in the synthesis of both diether precursors to PHOPPV and PBEPPV in good yield. While the purification of the dibromobenzene with the ether side chains (6) in Scheme 1 was achieved in moderate yields, the route outlined in Scheme 2 was more successful for the PBEPPV precursors, with substituents bearing two oxygen atoms. The increased solubility of PBEPPV contributed to low yields in the polymerization step as significant portions of the product failed to precipitate during the purification process. Even with two glycol-like moieties per repeat unit, PBEPPV still remained insoluble in water, a necessary characteristic for purification methods as well as for future device fabrication. Additionally, the 1H NMR spectra for this series of oligomers/co-oligomers confirmed that the Horner–Emmons route led to PPV derivatives with no saturated defects.30,32,43 While derivatives with benzylic hydrogens exhibit peaks below 3.0 ppm, and the hydrogens adjacent to oxygen are observed above 3.3 ppm, no saturated defects were observed at 3.1 ppm in the PPV oligomers prepared here (please see the 1H NMR spectra in Figures S1–S4),30 which is consistent with other reported syntheses of PPV derivatives using Horner–Emmons methods.32,43 We also note that a reduced signal-to-noise ratio was observed for PDDPV and to a lesser extent for PHOP-DDPV because of reduced solubility of the PDDV-containing derivatives; however, all other derivatives were readily soluble when analyzed by 1H NMR spectroscopy.

Molecular Weight Measurements

The number-average molecular weight (Mn), the weight-average molecular weight (Mw), and the polydispersity index (PDI) are shown in Table 1.

Table 1. Molecular Weight Data As Determined by GPC Relative to Polystyrene Standards for PPV Derivatives.

oligomer Mna Mwa PDIb DPc yield (%)
PDDPV 5200 8800 1.7 21 65
PDOPV 3500 5700 1.6 13 84
PHOPPV 3300 5400 1.6 12 19
PBEPPV 4500 6900 1.5 15 43
PHOP-alt-DDPV 3200 5700 1.7 14 12
PHOP-alt-DOPV 4200 8000 1.9 19 12
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a

Mn = number-average molecular weight; Mw = weight-average molecular weight. Molecular weights of THF-soluble materials determined by GPC.

b

PDI = Mw/Mn.

c

DP = number of monomer units per chain.

Because the oligomers were all synthesized under similar reaction conditions, the resultant molecular weights and PDIs are similar. PDDPV, the only oligomer in this series produced without oxygen incorporated in its side chains, produced oligomer chains with the highest degree of polymerization (DP). It is possible that with varying reaction conditions, such as time and concentration, larger molecular weights could be obtained for the more soluble oligomers.44 The poor solubility of the oligomers is also a contributing factor to the low molecular weights obtained. Both PDDPV and PDOPV precipitated during the course of the reaction; however, the other derivatives, PHOPPV, PBEPPV, PHOP-alt-DOPV, and PHOP-alt-DDPV, precipitated upon cooling in methanol. Limitations associated with the gel permeation chromatography (GPC) equipment used in this study also complicate the analysis of the results. The GPC sample preparation methods necessitated multiple filtrations, which only allowed lower molecular weight chains to be analyzed. Regardless, for the current set of polymers, all degrees of polymerization exceed the estimated theoretical limit for delocalization of excitons; therefore, the optical properties should be unaffected by chain length.45

Absorption and Photoluminescence Spectra

The optical properties of the oligomers were measured for dilute solutions in THF and for thin films spin-coated onto quartz slides and are shown in Figures 3 and 4, respectively; a summary of the optical properties with numerical values is given in Table 2. The absorbance maxima in solution for PDDPV, PHOPPV, PBEPPV, and PHOP-alt-DDPV are all similar: 408, 410, 415, and 410 nm, respectively (see Figure 3). A similar trend is also observed for the spin-coated quartz slides, 385, 390, 400, and 375 nm, respectively. The similarity in the absorbance maxima for the aforementioned oligomers can be attributed to the nature of the alkyl linkage at the aromatic ring for the substituent chains. The slight red shift observed for PBEPPV in solution could be attributed to a more rodlike structure from improved solvation of the chains.46 In contrast to all other oligomers, PDOPV exhibited a red shift in the solid state (485 nm) compared to solution (472 nm). This difference can perhaps be attributed to the presence of multiple strong electron-donating alkoxy groups in PDOPV and/or interactions of the multiple phenoxy moieties in PDOPV with the THF solvent and/or additional ordering of PDOPV in the solid state, which has been observed for certain organic semiconductors.47

Figure 3.

Figure 3

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(A) UV–visible absorption and (B) fluorescence spectra for solutions of PDDPV (excitation at 408 nm), PDOPV (excitation at 472 nm), PHOPPV (excitation at 410 nm), PBEPPV (excitation at 415 nm), PHOP-alt-DDPV (excitation at 410 nm), and PHOP-alt-DOPV (excitation at 455 nm); oligomers were prepared by Horner–Emmons polymerization and examined in THF (1.5 × 10–5 M) at rt.

Figure 4.

Figure 4

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(A) UV–visible absorption and (B) fluorescence spectra for films of PDDPV (excitation at 385 nm), PDOPV (excitation at 485 nm), PHOPPV (excitation at 390 nm), PBEPPV (excitation at 400 nm), PHOP-alt-DDPV (excitation at 375 nm), and PHOP-alt-DOPV (excitation at 445 nm); oligomers were prepared by Horner–Emmons polymerization, spin-coated onto quartz slides, and examined at rt.

Table 2. Summary of Optical Properties.

oligomer UV–vis λmax (nm) PL λmax (nm) QYa (%)
PDDPV 408a, 385b 487a, 507b 57
PDOPV 472a, 485b 546a, 582b 34
PHOPPV 410a, 390b 485a, 513b 74
PBEPPV 415a, 400b 489a, 510b 56
PHOP-alt-DDPV 410a, 375b 486a, 514b 69
PHOP-alt-DOPV 455a, 445b 518a, 567b 78
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a

Absorption (UV–vis) and PL maxima on 1.5 × 10–5 M solutions in THF.

b

On quartz slides spin-coated from 10 mg/mL solutions in THF.

c

PL QY in THF relative to 9,10-diphenylanthracene and quinine sulfate.

In both the film and solution photoluminescent (PL) spectra, secondary peaks can be observed, which correspond to vibronic coupling of the excitons.48 All of the investigated oligomers exhibited a slight red shift in the film PL spectra (see Figures 3 and 4), compared to the solution-based samples, because of increased interchain interactions.49 The optical properties for this set of oligomers and co-oligomers are summarized in Table 2, including the PL quantum yields (QYs). From all of the homo-oligomer derivatives, PDOPV exhibited the lowest QY, 34%, while PHOPPV exhibited the highest, 74%. Incorporation of the PHOPPV unit in the investigated co-oligomers led to a higher QY than the other oligomers with alkyl linkages, PDDPV (57%), PDOPV (34%), and PBEPPV (56%). In the alternating co-oligomers (PHOP-alt-DDPV and PHOP-alt-DOPV), the QY was improved significantly relative to the homo-oligomers without the heptyloxypropyl moiety (PDDPV and PDOPV). The higher QY values observed for derivatives containing the heptyloxypropyl moiety can plausibly be attributed to a reduction in the coiled nature of the pendant side chains in these derivatives, leading to an increase in their intrinsic persistence length.50

Electrochemical Properties

Cyclic voltammetry (CV) is widely used in studying the electrochemical behavior of the investigated compounds. The HOMO and lowest unoccupied molecular orbital (LUMO) of the conjugated oligomers were estimated using methods from the literature to verify oligomer compatibility with device structures.51 The onset oxidation measured from thin-film cyclic voltammograms was used to estimate the HOMO, eq 1. Using the low-energy edge of the absorption spectra to calculate the band gap (Eg), eq 2, it is also simple to estimate the LUMO, eq 3.

graphic file with name ao9b02396_m001.jpg 1
graphic file with name ao9b02396_m002.jpg 2
graphic file with name ao9b02396_m003.jpg 3

Eoxd,1 is the first oxidation potential, which was obtained by CV from the oligomer thin film on a glassy carbon working electrode. Eoxd,ferrocene is the onset oxidation potential of ferrocene (0.40 V vs Ag/AgNO3) and was determined by CV in a 0.1 M solution of tetrabutylammonium perchlorate in acetonitrile. Eg is the energy band gap estimated from the low-energy edge of the absorption spectra from the oligomer thin film. A summary of the electronic properties is shown in Table 3. The energy band gaps of the investigated oligomers are similar, ranging from 2.20 to 2.61 eV. The data of PHOP-alt-DOPV, PDDPV, and PDOPV are consistent with values from the previous research.26 While the absence of oxygen in PDDPV increased the band gap relative to PDOPV, moving it to the δ position achieved the same result in PHOPPV, PEBEPV, and PHOP-alt-DDPV.

Table 3. Summary of Electronic Properties of the Investigated Oligomers.

oligomer λ (nm) Eg (eV) Eoxd,1 (V) HOMO (eV) LUMO (eV)
PDDPV 487 2.55 1.75 –6.15 –3.60
PDOPV 563 2.20 1.20 –5.60 –3.40
PHOPPV 480 2.58 1.48 –5.88 –3.30
PEBEPV 475 2.61 1.21 –5.61 –3.00
PHOP-alt-DDPV 475 2.61 1.48 –5.88 –3.27
PHOP-alt-DOPV 510 2.43 1.35 –5.75 –3.32
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Device Studies

The OLED energy diagram shown in Figure 5 is a representative example of the devices fabricated for this report. The design consists of a glass substrate patterned with an indium tin oxide (ITO) anode, followed by a conducting layer of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) and then the emissive layer (active layer). Using thermal evaporation, tris(8-hydroxyquinolinato)-aluminum (Alq3), an electron transport layer, and a thin electron injection layer of lithium fluoride (LiF) were deposited prior to the aluminum (Al) cathode.52 The organic component (e.g., PDDPV) is utilized for the emissive layer. Conducting polymers often exhibit higher hole mobility caused by increased HOMO overlap, necessitating electron transport layers (e.g., Alq3) to allow recombination of hole–electron pairs (excitons) to occur in the emissive layer. Energy is released in the form of photons at a wavelength corresponding to the band gap of the emissive material.53 While the characteristics of the polymer are critical to device performance, the effectiveness of the device is also dependent on designing an architecture that has a good band gap overlap between layers.6

Figure 5.

Figure 5

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Energy diagram depicting the relative position of the band gaps for the components of an OLED device representative of those fabricated in this report.54

Previous studies showed marked improvement in the performance of PDOPV and PDDPV devices when using an electron injection layer of Alq3.26 Not only did this promote exciton recombination in the emissive layer but it also increased efficiency by reducing shorts caused by defects in the polymer film. Because of low solubility, pinhole defects in devices without Alq3 caused shorts and pixel burnout. The electronic properties of the new oligomers are within the range between those of PDDPV and PDOPV; therefore, Alq3 was used as the electron injection layer in all of the devices. Based on the above conditions, we designed our device configuration as: ITO/PEDOT:PSS/Oligomer/Alq3 (30 nm)/LiF (8 nm)/Al (80 nm). The properties and performance of the OLED devices fabricated with the investigated oligomers are shown in Figures 6–8.

Figure 6.

Figure 6

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(A) Current density–voltage characteristics and (B) EL spectra of the investigated oligomers in OLED devices.

Figure 8.

Figure 8

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CIE chromaticity diagram of the OLED devices.

The current density–voltage characteristics and the EL spectra of the oligomers in the OLED devices are shown in Figure 6. The luminous efficiency and luminance–voltage characteristics of the OLED devices are shown in Figure 7. The maximum EL brightness of each OLED device obtained with the oligomers is summarized in Table 4. Note that all of the newly synthesized oligomers and co-oligomers exhibited improved solubility in 1,2-dichlorobenzene (ODCB) over PDDPV and PDOPV, allowing for thicker spin-coated films for the devices. Perhaps because of the solubility advantage, the device prepared from PHOP-alt-DDPV was able to outperform the device prepared with PDDPV in EL brightness, an oligomer with poor solubility characteristics, at 157.7 and 101.0 cd/m2, respectively. The maximum efficiency of 0.230 cd/A, for the DDPV co-oligomer while modest, was twice that of the PDDPV homo-oligomer, 0.116 cd/A. Furthermore, the device produced with the alkoxy-substituted PDOPV oligomer had the best results, 210.0 cd/m2, and a maximum efficiency of 0.725 cd/A. However, the most soluble oligomer, PBEPPV, produced a device with the lowest brightness, 14.6 cd/m2. While all other samples produced light in a 0.2 cm2 pixel, the PBEPPV device showed only pinpoints of light within the measured area. It is possible that the increased polarity of PBEPPV might cause unfavorable interactions with the ODCB solvent or the underlying PEDOT:PSS layer.16,55,56 Interestingly, although PBEPPV exhibited similar electronic properties to the other alkyl derivatives, it had a lower turn-on voltage, which allows for milder operating conditions. As expected, the lower energy PDOPV and DOPV co-oligomer exhibited lower turn-on voltages because of their smaller band gaps. The PHOPPV and the DDPV co-oligomer exhibited a similar turn-on voltage as PDDPV because they share correspondingly higher band gaps. The EL maxima are very similar to the film PL maxima and can be seen with a summary of other EL characteristics in Table 4. The observed broadening of the peaks in the EL spectra in Figure 6 when compared to the PL spectra in Figures 3 and 4 can plausibly arise from exciplex emission at the organic solid interface.57

Figure 7.

Figure 7

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(A) Luminous efficiency and (B) luminance–voltage characteristics of the investigated oligomers in OLED devices.

Table 4. EL Characteristics of OLEDs Containing PPV Derivatives.

oligomer turn-on voltagea (V) EL brightness max (cd/m2) maximum efficiency (cd/A) λmaxEL, fwhm (nm) at 8 V 1931 CIE chromaticity (x,y) at 8 V
PDDPV 5.0 101.0 0.116 508, 76 0.31, 0.58
PDOPV 4.0 210.0 0.725 560, 88 0.49, 0.51
PHOPPV 5.0 44.8 0.075 512, 88 0.31, 0.52
PEBEPV 4.0 14.6 0.061 516, 72b 0.29, 0.59b
PHOP-alt-DDPV 5.0 157.7 0.230 512, 76 0.30, 0.59
PHOP-alt-DOPV 4.0 94.0 0.056 588, 112 0.52, 0.47
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a

With EL at 1 cd/m2.

b

Measurement taken at 6 V.

The chromaticity of all fabricated OLED devices is shown in Figure 8. The chromaticity coordinates of each OLED device are also summarized in Table 4. In addition, all of the investigated oligomers, with the exception of PHOP-alt-DOPV, exhibited similar CIE chromaticity coordinates across the measured voltage range and efficiency. The co-oligomer PHOP-alt-DDPV also exhibited twice the efficiency of the PDDPV homo-oligomer, with almost identical optical properties. This shows that the limited performance of PDDPV can be improved by introducing more soluble side chains. Unfortunately, these preliminary results do not clearly delineate the role the position or amount of oxygen atoms present on the side chains has on OLED performance requiring further investigations to determine parameters affecting film formation.

Conclusions

New poly(p-phenylene vinylene) derivatives, PHOPPV and PBEPPV, as well as co-oligomers, PHOP-alt-DDPV and PHOP-alt-DOPV, were synthesized through the well-known Horner–Emmons polycondensation procedure. This simple and inexpensive synthetic route afforded oligomers with low PDI and of sufficiently high molecular weight for use in OLED devices. The introduction of oxygen into the solubilizing alkyl side chains increases the solubility of the oligomers (i.e., PHOPPV was more soluble than PDDPV). Furthermore, increasing the oxygen content in the side chains 2-fold (i.e., PBEPPV) further increases the solubility of the oligomer. The optical properties of these new soluble derivatives were studied using UV–visible absorption and fluorescence emission spectroscopy. The nearly identical optical properties of the oligomers, where oxygen is not attached directly to the ring, were expected because of the inability of the oxygen to participate in resonance. The synthesized oligomers demonstrated the capacity to perform color tuning, as demonstrated by the incorporation of the alkoxy phenylene units in PDOPV and PHOP-alt-DOPV as compared to PHOPPV and the corresponding shift from orange to green as alkoxy units were reduced or removed from the ring. Considering the modest performance of the OLED devices, it is clear that solubility is not the only factor affecting device performance, and film morphology is not totally dependent on good solubility. The study of the lifetime of devices prepared from these oligomers is currently in progress.

Acknowledgments

This work was generously supported by the Robert A. Welch Foundation (E-1320, T.R.L.), the Texas Center for Superconductivity (T.R.L.), and the University of Texas at Dallas (B.E.G.).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b02396.

  • Experimental details regarding the instrumentation used to characterize the oligomers, methods used to construct the LED devices, and 1H NMR spectra of the oligomers (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao9b02396_si_001.pdf (358.3KB, pdf)

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