Synthesis,characterization,photophysical properties,and computational studies on N -hexylphenothiazine/cyanopyridine based π-conjugated copolymers

Abstract

In this paper, π-conjugated copolymers, namely N-hexylphenothiazine/cyanopyridine/phenyl/benzothiadiazole, N-hexylphenothiazine/cyanopyridine/phenyl/9,9-dihexylfluorene, and N-hexylphenothiazine/cyanopyridine/phenyl/9,9-diethylhexylfluorene were readily synthesized via Pd-catalyzed Suzuki cross-coupling reaction. The polymer structures and their photophysical properties were characterized by elemental analysis, ¹H NMR, GPC, TGA, XRD, UV-vis absorption and PL spectroscopy measurements. The coupling agent effect on photophysical properties of copolymers was investigated to rationally design polymers with particular physical properties to be employed in optoelectronic devices. The UV-vis absorption spectroscopy of copolymers showed λ_max at a range of ∼334–474 nm and red-shifted in their films to a range of ∼342–381 nm. These copolymers displayed highly intense fluorescence in their solutions and films. The PL spectra of copolymers indicated red and near-infrared light, rendering them a prospect for being red and near-infrared light-emitting materials for PLEDs. XRD analysis demonstrated a d-spacing range of ∼3.79–4.32 Å, reflecting π-π stacking and some degree of crystallinity in some polymers, and only P1 and P2 showed peaks in the small-angle region, indicating lamellar structures. To understand the relationship between molecular structures of target materials and their photophysical and photovoltaic properties, density functional theory (DFT) and its time-dependent form (TD-DFT) were employed.

Keywords

D–A copolymers cyanopyridine/phenothiazine-based copolymers benzothiadaizole Suzuki cross-coupling DFT and TD-DFT theoretical calculations

Introduction

The ever-increasing demand for clean energy makes it highly significant to develop new technologies to increase renewable energy sources.^1–3 In this regard, organic semiconductors are impressive materials to be recognized for their prospective applications in optoelectronic devices.^4–6 In particular, π-conjugated polymers have attracted considerable interest due to their simple processing, low operating voltages, quick response times, and color tuning over the entire UV-vis range, making this kind of polymers ideally suited for a wide range of optoelectronic devices.^7–16 Thus, design and synthesis of new conjugated polymers with various optoelectronic properties hold great promise in display technology area.^17–19

Selecting electron-donating (donor; D) and withdrawing units (acceptor; A) in an alternate mode in the polymer chain is considered to be one of the most competent methods of depreciating optical and electrochemical band gaps of conjugated polymers.^20–23 Increasing the degree of delocalization of electrons acquired by increasing the coplanarity of structure causes the band gap to be further depreciated.^10,24 This can be attributed to intramolecular charge transfer in the excited state. However, the excited electron in such a case is supposed to be delocalized over electron-deficient segments.^25–27 Conjugated polymers of D–A type such as DPP-based D–A type copolymers have exhibited excellent properties in various fields including OFET, OPVs, OLED, in addition to nonlinear optics (NLO).^28–32

In the past years, fluorene-based conjugated polymers attracted great interest as very auspicious candidates for blue light-emitting diodes because of their very pure blue and efficient luminescence combined with rising thermal and chemical stability, high mobility, hole-transporting properties and upward film-forming capability.^33–36 Benzothiadiazole has been featured as an outstanding electron-deficient acceptor core for synthesizing D–A copolymers and small molecules for organic electronics applications.³⁷ On the other hand, pyridine has attracted much attention from many researchers, particularly for developing new conjugated polymers because of its intriguing heterocyclic system with thermally-stable N-type.^1,13,38–44

Besides, pyridine acts as a powerful electron-withdrawing moiety with excellent electron-transporting properties, and localization of lone pair of sp² orbital electrons of nitrogen atom rendered it with outstanding optical properties.^40–44 Moreover, decreasing the band gap facilitates higher open-circuit voltage due to presence of cyano substituent group as an efficient electron-withdrawing in polymer repeating units, reducing both HOMO and LUMO energy levels.^41,45,46 However, a decline position of the lowest unoccupied molecular orbital reduces environmental oxidation of polymer, leading to stable polymers.^47,48 As a result of their controlled spectroscopic, electrochemical and thermal stability and a good charge transporting ability, thiophene and/or benzene have been previously utilized effectively in the polymer backbone.^49,50

Due to their small ionization potential, phenothiazine derivatives were used as stronger electron-donating species and can produce high stable radical cations. They are also characterized by high electrical and thermal stability, with high excellent electro-optical characteristics in devices.^51–53 Phenothiazine was used as electron donor, with benzothiadiazole, possessing a robust electron affinity as electron acceptor, and thiophene with rich electrons linked to each end to increase π-conjugation efficiency. This provides a design of a basic molecular structure, affording a low band gap polymer.⁵⁴

Based on previous findings and our interest in synthesizing π-conjugated organic and polymeric materials for optoelectronic applications,^55–60 we introduced a straightforward synthetic design to prepare a set of π-conjugated copolymers based on D–A type via Pd-catalyzed Suzuki cross-coupling method. Comonomer, composed of 10-hexyl-10H-phenothiazine, cyanopyridine, and phenyl units, was readily synthesized and then copolymerized with various coupling agents to study the effect of varying coupling agent on conjugation and related photophysical properties. Density functional theory (DFT) and its time-dependent form (TD-DFT) were employed to understand the relationship between molecular structures of target materials and their photophysical and photovoltaic properties.

Experimental

Reagents and materials

All manipulations and reactions involving air-sensitive reagents were conducted under a dry oxygen-free nitrogen atmosphere unless otherwise stated. All reagents and solvents were obtained from commercial sources and dried using standard procedures before use. 4-Bromoacetophenone, 4,7-diboronic ester-2,1,3-benzothiadiazole, 9,9-dihexylfluorene-2,7-bis(trimethyleneborate) and 9,9-diethylhexylfluorene-2,7-bis(trimethyleneborate) were purchased from Sigma-Aldrich Company and used without any further purification.

Instrumentation and methods

¹H and ¹³C-NMR spectra were measured on a Brucker spectrometer (400 MHz for ¹H and 100 MHz for ¹³C) in CDCl₃ or DMSO-d₆ at 25°C with TMS as internal standard. Chemical shifts were recorded in ppm units, and coupling constants (J) are given in Hz. All reactions were monitored by TLC until completion, and all products were purified by flash column chromatography with Merck silica gel 60 (particle size 60–120 mesh ASTM) and UV-254 fluorescent indicators. The thermal degradation temperature was measured using Shimadzu thermogravimetric analyzer TGA-50 under nitrogen atmosphere with a heating rate of 20°C/min. The XRD measurements were performed on a polymer thin film (20 mg/mL) using PHILIPS X’Pert diffractometer. The gel permeation chromatography (GPC) analysis was conducted with a Shimadzu (LC-20A Prominence Series) instrument using THF as a carrier solvent (flow rate: 1 mL/min, at 35°C), and calibration curves were made with standard polystyrene samples. Steady-state electronic absorption spectra were recorded on JASCO double-beam UV-vis-IR scanning spectrophotometer (UV-780). Fluorescence emission spectra of polymer solutions were recorded using fluorescence spectrofluorimeter JASCO FP-8300, while fluorescence spectra of films were recorded on Kimmon Koha IK Series He-Cd Laser (320 nm).

Synthesis of 10-hexyl-10H-phenothiazine (2)

To a mixture of phenothiazine (1, 2.0 g, 10 mmol), 50 mg of tetraoctylammonium bromide (TOAB) and sodium hydroxide (20 mL, 50%) in DMSO (30 mL) were added in a three-necked flask followed by stirring the reaction mixture at room temperature for 30 min under a nitrogen atmosphere. 1-Bromohexane (2 mL, 10 mmol) was then added to the reaction mixture, followed by stirring overnight at room temperature. After reaction completion (TLC), the reaction mixture was neutralized with HCl (2 M) and extracted with hexane. The organic layers were collected, dried over anhydrous MgSO_4, and concentrated under reduced pressure to give the crude product. The crude product was purified by silica gel column chromatography eluted with hexane. The pure compound 2 was obtained as a colorless liquid (2.38 g; 85% yield).

¹H NMR (CDCl₃, 400 MHz, δ/ppm): 7.04–7.08 (m, 4H), 6.83 (t, 2H, J = 6.1 Hz), 6.77 (d, 2H, J = 7.9 Hz), 3.73 (t, 2H, J = 7.2 Hz), 1.69–1.75 (m, 2H), 1.32–1.38 (m, 2H), 1.21–1.25 (m, 4H), 0.84 (t, 3H, J = 6.9 Hz). ¹³C NMR (CDCl₃, 100 MHz, δ/ppm): 145.2, 127.2, 127.0, 124.8, 122.1, 115.2, 47.2, 31.3, 26.7, 26.5, 22.5, and 13.9. C₁₈H₂₁NS (283.43): calcd. C 76.28, H 7.47, N 4.94, S 11.31; found C 76.55, H 7.34, N 4.89, S 11.23.

Synthesis of 10-hexyl-10H-phenothiazine-3-carbaldehyde (3)

A volume of POCl₃ (7.15 mL) was added dropwise under stirring to an ice-cooled flask containing DMF (11.4 mL), followed by stirring at room temperature for 90 min. The reaction mixture was cooled in an ice-cold bath, and 10-hexyl-10H-phenothiazine (2, 2.83 g, 10 mmol) dissolved in 50 mL CH₂Cl₂ was added dropwise. The reaction mixture was warmed gradually up to ∼75°C, under a reflux system with stirring, and kept under these conditions for 24 h. The reaction mixture was then cooled to room temperature and poured into ice-cold water, followed by adding saturated aqueous K₂CO₃ solution until reaching alkalinity and then extracted with CHCl₃ (4 × 30 mL). The organic layers were collected, washed with water, dried over MgSO₄, evaporated, and the obtained residue was purified by flash silica gel column chromatography using hexane/ethyl acetate (8/2) as eluent system, affording the desired pure product as yellow solid (2.86 g; 92% yield).

¹H NMR (400 MHz, CDCl₃): δ = 9.78 (s, 1H, –CHO), 7.63 (d, J = 2 Hz, 2H), 7.56 (s, 1H), 7.19 (t, 1H), 7.11 (d, 2 Hz, 1H), 6.98 (t, 1H), 6.94 (t, 1H), 6.87 (d, 2H), 3.85 (t, 2H, –CH₂–N), 1.82–1.76 (m, 2H), 1.43–1.40 (m, 2H), 1.30–1.28 (m, 4H), 0.86 (t, 3H) ppm. ¹³C NMR (CDCl₃, 100 MHz, δ/ppm): 13.97, 22.57, 26.51, 26.74, 31.38, 48.02, 114.79, 115.96, 123.56, 123.79, 125.01, 127.55, 128.34, 130.08, 131.04, 143.43, 150.72, and 189.98. C₁₉H₂₁NOS (311.44): calcd. C 73.27, H 6.80, N 4.50, O 5.14, S 10.29; found C 73.35, H 6.92, N 4.40, S 10.09.

Synthesis of 6-(4-bromophenyl)-4-(10-hexyl-10H-phenothiazin-3-yl)-2-hydroxy-nicotinonitrile (5)

A mixture of 10-alkyl-10H-phenothiazine-3-carbaldehyde (2, 0.62 g, 2 mmol) and 4-bromoacetophenone (4, 0.4 g, 2 mmol) was dissolved in 20 mL distilled ethanol. Ammonium acetate (1.32 g, 17 mmol) was then added to the reaction mixture, followed by adding ethyl cyanoacetate (0.3 mL, 3 mmol). The reaction mixture was stirred under reflux for 24 h, and completeness of reaction was monitored by TLC using elution system of ethyl acetate/hexane (1/4). After reaction completion, the reaction mixture was cooled to room temperature, forming an orange precipitate. The solid product was filtered off and washed with ethanol, followed by drying under vacuum (0.6 g, 51% yield).

¹H NMR (400 MHz, CDCl₃): δ = 12,73 (s, 1H, –OH), 7.83 (d, 2H), 7.70 (d, J = 2,2 Hz, 2H), 7.59 (d, 1H), 7.52 (s, 1H), 7.15 (s, 1H), 7.23 (t, 1H), 7.15 (d, J = 2 Hz, 1H), 7.12 (d, J = 2.4 Hz, 1H), 7.04 (d, J = 2.1 Hz, 1H), 6.99 (t, 1H), 6.85 (s, 1H), 3.87 (t, 2H), 1.7–1.23 (m, 6H), 0.85 (t, 3H) ppm. ¹³C NMR (CDCl₃, 100 MHz, δ/ppm), 14.26, 22.50, 26.23, 26.59, 31.26, 47.12, 106.70, 115.84, 116.61, 117.15, 123.21, 123.56, 124.16, 125.31, 127.32, 127.69, 128.34, 128.61, 129.90, 130.22, 132.27, 144.14, 147.31, 150.60, 158.52, and 162.71. C₃₀H₂₆BrN₃OS (556.52): calcd. C 64.75, H 4.71, Br 14.36, N 7.55, O 2.87, S 5.76; found C 64.70, H 4.88, Br 14.25, N 7.52, S 5.70.

Synthesis of 6-(4-bromophenyl)-4-(10-hexyl-10H-phenothiazin-3-yl)-2-(hexyloxy)nicotinonitrile (6)

To a solution of 6-(4-bromophenyl)-4-(10-hexyl-10H-phenothiazin-3-yl)-2-(hexyloxy) nicotinonitrile (5, 0.5 mmol, 0.28 g) in DMF (10 mL), K₂CO₃ (0.5 mmol, 0.07 g) was added with stirring for few minutes, followed by adding 1-bromohexane (0.5 mmol, 0.083 g). The reaction mixture was stirred under reflux overnight. The completeness of reaction was monitored by TLC (eluent; ethyl acetate/hexane; 1/3). After completeness of the reaction, reaction mixture was cooled to room temperature, followed by quenching with ice-cooled water with stirring. The obtained yellow precipitated solid was filtered off, washed with ethanol several times, and dried under vacuum (0.3 g, 95% yield).

¹H NMR (400 MHz, CDCl₃): δ = 7.86 (d, J = 2.2 Hz, 2H), 7.55 (d, J = 2.2 Hz, 2H), 7.44 (d, J = 2.2 Hz, 2H), 7.27 (s, 1H), 7.17 (s, 1H), 7.08 (t, 2H), 6.86 (d, J = 5 Hz, 2H), 4.5 (t, 2H), 3.8 (t, 2H), 1.8–1.4 (m, 12H), 1.37–1.28 (t, 6H) ppm. ¹³C NMR (CDCl₃, 100 MHz, δ/ppm): 13.99, 14.05, 22.59, 22.61, 25.68, 26.65, 26.84, 28.77, 29.71, 31.45, 31.57, 31.74, 47.76, 67.68, 92.79, 112.54, 115.25, 115.56, 115.69, 123.01, 123.96, 124.96, 125.25, 125.52, 126.93, 127.50, 127.72, 128.78, 128.88, 130.04, 132.04, 132.15, 136.42, 144.22, 146.98, 155.45, 156.60, and 165.09. C₃₆H₃₈BrN₃OS (640.68): calcd. C 67.49, H 5.98, Br 12.47, N 6.56, O 2.50, S 5.00; found C 67.45, H 5.92, Br 12.55, N 6.48, S 5.05.

Synthesis of 4-(7-bromo-10-hexyl-10H-phenothiazin-3-yl)-6-(4-bromophenyl)-2-hydroxynicotinonitrile (7)

To an ice-cold stirred solution of 6-(4-bromophenyl)-4-(10-hexyl-10H-phenothiazin-3-yl)-2-(hexyloxy) nicotinonitrile (6, 0.5 mmol, 0.3 g) dissolved in dry THF (40 mL) under N₂ atmosphere, NBS (0.705 mmol, 0.125 g) was added in one portion, and the reaction mixture was stirred at 0°C for ∼1 h, followed by stirring overnight at room temperature. The completeness of reaction was monitored by TLC (eluent; EA/Hex; 1/9). After reaction completion, it was quenched with ice-cooled water, followed by extraction with methylene chloride. The organic layers were collected, washed with water, and finally dried over anhydrous MgSO₄. The crude product was purified by column chromatography (eluent; EA/Hex; 1/9), affording a viscous oily orange product (0.27 g, 79% yield).

¹H NMR (400 MHz, CDCl₃): δ = 7.87 (d, 2H, J = 2), 7.55 (d, 2H, J = 2), 7,43 (s, 1H), 7.27 (d, 2H, J = 2.2), 7.18 (d, 2H, J = 2.8), 6.62 (s, 1H), 6.85 (s, 1H), 4.49 (t, 2H), 3.87 (t, 2H), 1.8–1.5 (m, 12H), 1.4–1.29 (t, 6H) ppm. ¹³C NMR (CDCl₃, 100 MHz, δ/ppm): 13.99, 14.05, 22.60, 23.00, 25.68, 26.57, 28.77, 28.95, 29.71, 30.39, 31.42, 31.57, 38.76, 67.72, 68.17, 92.90, 112.61, 115.50, 116.82, 125.02, 126.91, 127.93, 128.79, 130.16, 130.88, 132.06, 136.34, 155.13, 156.66, and 165.06. C₃₈H₄₀Br₂N₂ (700.56): calcd. C 65.15, H 5.76, Br 22.81, N 4.00, O 2.28; found C 65.22, H 5.71, Br 22.77, N 3.92.

Suzuki cross-coupling polymerization (typical procedure for the synthesis of copolymers P1)

Into a highly dried three-necked round-bottomed flask, equimolar amounts of 4-(7-bromo-10-hexyl-10H-phenothiazin-3-yl)-6-(4-bromophenyl)-2-hydroxynicotinonitrile (7, 0.5 mmol, 0.36 g) and 2,1,3-benzothiadiazole-4,7-bis(boronic acid pinacol ester) (8, 0.5 mmol, 0.194 gm) were dissolved in freshly distilled toluene (15 mL) and purged with nitrogen for 30 min. Aqueous K₂CO₃ (2 M, 15 mL) and Pd(PPh₃)₄ (5 mol% relative to Br) were then added, and the reaction mixture was degassed again with nitrogen for 30 minutes. The reaction mixture was heated under reflux at 90∼95°C and stirred vigorously for 48 h, under nitrogen atmosphere. The end-capping process was performed in separated two steps: a solution of phenylboronic acid pinacol ester (5 mol%) in 2 mL toluene was first added to the reaction mixture, followed by heating under reflux at 90∼95°C for 6 h, under nitrogen atmosphere. The same process was repeated by adding bromobenzene (5 mol%) solution in 2 mL toluene. After the second end-capping process, the reaction mixture was cooled to room temperature, followed by quenching with distilled water (∼10 mL), and extracted with toluene three times. The collected organic layers were separated, concentrated, and the polymer precipitated out from its toluene solution by dropwise addition into methanol with vigorous stirring. The filtered crude copolymer was re-precipitated twice by dissolving the filtered and dried polymer residue in ∼10 mL THF, followed by dropwise addition of THF solution into methanol with vigorous stirring. The resulting crude copolymer was collected by filtration and washed consecutively with methanol. The residual solid was loaded into an extraction thimble and washed successively with methanol (24 h) followed by acetone (24 h). The polymeric product was then dried under reduced pressure at ∼50°C for at least 24 h and used for analytical methods. Copolymers P2 and P3 were prepared according to same procedure but we replaced 2,1,3-benzothiadiazole-4,7-bis(boronic acid pinacol ester) (8) by 9,9-dihexylfluorene-2,7-bis(trimethyleneborate) (9, 0.5 mmol, 0.25 g) for P2 and 9,9-diethylhexylfluorene-2,7-bis(trimethyleneborate) (10, 0.5 mmol, 0.26 g) for P3. The chemical yields, molecular weights (M_n) as well as molecular weight distributions (PDI) of the resulting copolymers P1-P3 are summarized in Table 1.

Table 1.

Polymerization results of copolymers P1-P3.

	Yield (%)^a	M_w (kg/mol)^b	M_n (kg/mol)^b	PDI (M_w/M_n)^b	T_{d, onset} (°C)^c	T_{d, max} (°C)^c	Char yield (%)^c
P1	55%	39.88	19.55	2.04	319	780	5.80
P2	88%	42.32	22.63	1.87	365	536	0.20
P3	84%	37.61	21.49	1.75	327	550	0.00

^a Based on the weight of the polymer obtained after Soxhlet extraction and drying under vacuum.

^b Determined by GPC with polystyrene as standard and THF as an eluent at 35°C.

^c Determined by TGA under nitrogen atmosphere at a heating rate of 20°C/min.

Poly 6-(4-(benzo[c][1,2,5]thiadiazol-4-yl)phenyl)-4-(10-hexyl-10H-phenothiazin-3-yl)-2-hydroxynicotinonitrile (P1)

¹H NMR (400 MHz, CDCl₃): δ = 8.39–8.18 (br, 2H), 8.10–8.08 (br, 2H), 8.04–7.98 (br, 2H), 7.86–7.84 (br, 1H), 7.58–7.52 (br, 2H), 7.46–7.25 (br, 2H), 7.18–6.62 (br, 2H), 4.52–4.47 (br, 2H, OCH₂), 3.83–3.75 (br, 2H, NCH₂), 1.80–1.73 (br, 4H), 1.44–1.17 (br, 12H), 0.83–0.82 (br, 6H, 2 × CH₃) ppm. C₄₂H₃₉N₅OS₂ (693.93): calcd. C 72.70, H 5.67, N 10.09, O 2.31, S 9.24; found C 72.84, H 5.69, N 10.01, S 9.11.

Poly 6-(4-(9,9-dihexyl-9H-fluoren-2-yl)phenyl)-4-(10-hexyl-10H-phenothiazin-3-yl)-2-(hexyloxy)nicotinonitrile (P2)

¹H NMR (400 MHz, CDCl₃): δ = 8.12 (br, 4H), 7.73 (br, 6H), 7.56 (br, 4H), 7.40–7.38 (br, 2H), 6.90–6.82 (br, 1H), 4.54 (br, 4H, OCH₂ + NCH₂), 1.98 (br, 4H), 1.84 (br, 4H), 1.47 (br, 4H), 1.31–1.18 (br, 8H), 1.06 (m, 10H), 0.85–0.84 (m, 6H), 0.68 (m, 12H, 4 × CH₃) ppm. C₆₁H₆₉N₃OS₂ (892.30): calcd. C 82.11, H 7.79, N 4.71, O 1.79, S 3.59; found C 82.35, H 7.88, N 4.57, S 3.53.

Poly 6-(4-(9,9-diethyl hexyl-9H-fluoren-2-yl)phenyl)-4-(10-hexyl-10H-phenothiazin-3-yl)-2-(hexyloxy)nicotinonitrile (P3)

¹H NMR (400 MHz, CDCl₃): δ = 8.24 (br, 1H), 8.11 (br, 2H), 7.84–7.82 (m, 1H), 7.75–7.64 (m, 4H), 7.58–7.47 (m, 2H), 7.39–7.24 (m, 4H), 7.07–7.05 (m, 1H), 6.88 (m, 2H), 4.54 (s, 2H, OCH₂), 3.92–3.86 (br, 2H, N–CH₂), 2.01–2.00 (br, 8H), 1.84–1.83 (br, 6H), 1.47–1.46 (br, 4H), 1.31 (br, 6H), 0.85–0.80 (m, 14H), 0.57–0.47 (m, 18H, 6 × CH₃) ppm. C₆₅H₇₉N₃OS (950.43): calcd. C 82.14, H 8.38, N 4.42, O 1.68, S 3.37; found C 82.27, H 8.44, N 4.38, S 3.30.

Results and discussion

Synthesis of monomers and copolymers

Figure 1 describes the synthetic pathways for the main comonomer 7. For this purpose, phenothiazine (1) was readily first N-alkylated through reaction with 1-bromohexane in N, N-dimethylformamide (DMF) solvent in the presence of potassium tert-butoxide as a base, affording the desired product 10-hexyl-10H-phenothiazine (2, HPT) in 88% yield. Formylation of HPT (2) with POCl₃ in DMF afforded the corresponding 10-hexyl-10H-phenothiazine-3-carbaldehyde (3) in 92% yield. Condensation of 10-alkyl-10H-phenothiazine-3-carbaldehyde (3) and 4-bromoacetophenone (4) in the presence of ammonium acetate and ethyl cyanoacetate afforded the desired product 6-(4-bromophenyl)-4-(10-hexyl-10H-phenothiazin-3-yl)-2-hydroxy-nicotinonitrile (5) in 51% yield. The cyclization mechanism of the above-mentioned reaction was believed to occur via condensation of aromatic aldehydes, aromatic ketones, ethyl cyanoacetate, followed by cyclization in presence of ammonium acetate, leading to the corresponding substituted 2-hydroxy-3-cyanopyridine derivative via Knoevenagel condensation reaction.^47,48

Figure 1.

Synthetic pathway of comonomer (7).

The reaction of 6-(4-bromophenyl)-4-(10-hexyl-10H-phenothiazin-3-yl)-2-hydroxynicotinonitrile (5) with 1-bromohexane in basic ethanolic solution afforded the corresponding product 6-(4-bromophenyl)-4-(10-hexyl-10H-phenothiazin-3-yl)-2-(hexyloxy)nicotinonitrile (6) in 95% yield. Bromination of compound 6 with N-brmosuccinimde (NBS) in THF afforded the corresponding 4-(7-bromo-10-hexyl-10H-phenothiazin-3-yl)-6-(4-bromophenyl)-2-hydroxynicotinonitrile (7) in 79% yield. It is worth mentioning that all synthesized compounds 2-7 were purified by flash silica gel column chromatography before use in the next reactions. Moreover, their structures were characterized and confirmed by elemental analysis as well as ¹H- and ¹³C-NMR spectroscopy. Their spectral analyses are mentioned in supporting information.

Comonomer 7 was used as a building block to synthesize π-conjugated P1-P3 via the commonly used Pd-catalyzed Suzuki cross-coupling reaction, as shown in Figure 2. However, Pd (PPh₃)₄ was used as a catalyst in the polymerization reaction under thermal reaction conditions in toluene.

Figure 2.

Synthetic pathways of copolymers P1-P3.

Copolymer P1 was synthesized in ∼55% yield by Suzuki cross-coupling polymerization of equimolar amounts of comonomer 7 with 4,7-diboronicester-2,1,3-benzothiadiazole (8) in toluene and Pd(0) catalyst and 2 M K₂CO₃ (volume ratios 2:1), under nitrogen atmosphere. Under the same Pd-catalyzed Suzuki cross-coupling polymerization conditions, copolymerization of comonomer 7 with 9,9-dihexylfluorene-2,7-bis(trimethyleneborate) (9) and 9,9-diethylhexylfluorene-2,7-bis(trimethyleneborate) (10) afforded the corresponding copolymers P2 and P3 in 88% and 84% yields, respectively.

After precipitation in methyl alcohol, the crude copolymers were filtered off and washed extensively with methanol, followed by sequential Soxhlet extractions with methanol and acetone to remove any catalyst residues, byproducts and oligomers. Analyzing the refined copolymers showed that palladium catalyst was removed completely. The final polymers were dried under vacuum for at least 24 h and were then subjected to demanded analysis. All copolymers exhibited excellent solubilities (>5 mg mL⁻¹) at room temperature in most common organic solvents such as THF, chloroform, toluene, xylene, and chlorobenzene. The chemical yields, number average molecular weights (M_n) as well as molecular weight distributions (PDI) of the resulting copolymers P1-P3 were determined by gel permeation chromatography (GPC) in CHCl₃ using polystyrene standards and summarized in Table 1.

Thermogravimetric analysis (TGA)

The thermal properties of copolymers P1-P3 were evaluated by thermogravimetric analysis (TGA) under nitrogen atmosphere. Copolymers P1-P3 showed thermal decomposition temperatures (T_d, 95 weight % residues) in the range of 319∼365°C (Table 1), indicating high thermal stabilities due to the presence of aromatic heterocyclic ring in polymers.⁴² In TGA curves, we determined T_{d, onset}, T_{d, max,} and charing TGA of copolymers, revealing that residual weights of copolymers P1-P3 are greater than 50% when temperature was raised to 400°C (Figure 3 and Table 1).

Figure 3.

Thermogravimetric analysis thermograms of copolymers P1-P3.

The copolymers P1-P3 showed decomposition process with onset decomposition temperatures (T_{d, onset}) at 319, 365, and 327°C corresponding to 95, 96, and 94% weight residues, respectively (Table 1 and Figure 3), indicative of high thermal stabilities, which could be assigned to side-chain decomposition upon heating processes.^49,50 The weight loss might be explained due to decomposition of hexyloxy side chains on cyanopyridine rings in polymer backbones and N-hexyl side chains of phenothiazine. On the other hand, thermal decomposition maximum temperatures (T_d, _max; corresponding to the maximum rate of weight loss) of copolymers P1-P3 were found to be located at 780, 536, and 550°C, with remaining weights of 8, 6, and 1%, respectively. It should be mentioned that P1 showed high thermal stability compared to P2 and P3 due to the presence of rigid cyanopyridine rings linked to stiff benzo[c][1,2,5]thiadiazole units in the polymer chain.^42,50 Interestingly, TGA revealed that at the end of process, remaining weights of P1-P3 were 5.8, 0.2, and 0% of the total weight of original polymer samples (represented by char yield %).

Optical properties

The UV-visible absorption spectra of copolymers P1-P3 in chloroform solution and spin-coated films prepared from their chlorobenzene solutions on glass slides are shown in Figure 4. The absorption maxima and estimated optical band gaps ( $E_{g}^{o p}$ ) are summarized in Table 2.

Figure 4.

UV-visible absorption of copolymers P1-P3 in chloroform solution and in thin films.

Table 2.

Optoelectronic properties of the synthesized copolymers P1-P3.

UV-visible absorption							Photoluminscence^b,c
	Solution			Film			Solution^b	Film
	λ_max (nm)	λ_onset (nm)	$E_{g}^{o p}$ (eV)^a	λ_max (nm)	λ_onset (nm)	$E_{g}^{o p}$ (eV)^a	λ_max (nm)	λ_max (nm)
P1	334	484	2.56	342	533	2.32	551	650
P2	374	483	2.56	381	497	2.49	544	562
P3	367	494	2.51	373	507	2.44	540	554

^a Optical band gap was calculated from the onset absorption of copolymers ( $E_{g}^{o p}$ = 1240/λ_onset).

^b All samples were measured in chloroform as a solvent at a concentration of 10⁻⁶ M.

^c PL λ_max is the fluorescence emission peak maxima.

As shown, the absorption maximum wavelength of polymers in solution appeared at 334, 374, and 367 nm for copolymers P1-P3, respectively. These values appeared in thin films at 342, 381, and 373 nm, demonstrating that polymers absorptions are red-shifted in their thin films. All copolymers exhibited boarder absorptions than their corresponding absorption in solutions due to molecular aggregations of polymer chains in the solid-state.⁶¹ Also, polymers absorption onset wavelengths in solutions appeared at 484, 484, and 494 nm for copolymers P1-P3, respectively, while these values appeared at 533, 497, and 507 nm in thin films, indicating a red shift. Optical band gap was calculated from the onset absorption of copolymers ( $E_{g}^{o p}$ = 1240/λ_onset), giving values of 2.32, 2.49, and 2.44 eV for copolymers P1-P3, respectively. However, as expected, these values were lower than those calculated from corresponding solution values, demonstrating absorption of red shift and broadening in films than in solutions. From their onset absorption wavelengths, optical band gap values in all polymers films were significantly reduced to lower values relative to their corresponding values in solution (Table 2). The significantly red-shifted thin-film absorption spectra for all copolymers could be attributed to enhanced planarity and interchain π-π stacking of polymers.^61,62 Photoluminescence of copolymers P1-P3 was measured for their solutions in chloroform and spin-coated polymer films prepared by drop-casting of their chlorobenzene solutions. The fluorescence emission spectra of copolymers P1-P3 were acquired by irradiative excitation at their respective wavelengths of the absorption maxima in solution at a concentration of 10⁻⁶ M, and their spin-coated films on glass slides are given in Figure 5. The fluorescence emission peak maxima (PL max) of copolymer solutions and films are also shown in Table 2. Upon photoexcitation of copolymers P1-P3 (1 μM; at λ_ex = 370 nm) in chloroform solution, they exhibited characteristic intense emission maxima peaks at 551, 544, and 540 nm, respectively. On the other side, emission maxima peaks of spin-coated polymer films of P1-P3 are 650, 562, and 554 nm, respectively. These values are red-shifted by 99 nm for P1, by 18 nm for P2 and by 18 nm P3 compared to their emission maxima peaks in solutions, indicating an increase of the conjugation length upon chain desolvation. This evidence may be attributed to a considerable degree of self-organization experienced by the polymer in the solid-state.

Figure 5.

Photoluminescence of copolymers P1-P3 in chloroform solution and in thin films.

X-ray diffraction (XRD) study

Solid-state morphology and molecular organization of copolymers were inspected by XRD analysis, which was performed on deposited films of copolymers from their chlorobenzene solutions. Figure 6 shows XRD patterns for thin films of copolymers P1-P3. The d-spacing was obtained according to Bragg’s equation, nλ = 2d_hkl sin θ, where λ is the radiation wavelength, d_hkl is the specific lattice spacing, and θ is the diffraction angle. All copolymers (P1-P3) show a broad peak at ∼23.39°, 20.66°, and 20.52°, corresponding to a d-spacing of 3.79, 4.29, and 4.32 Å, respectively. This is believed due to π-π stacking of polymer backbone.⁶³ However, copolymers P1, P2 and P3 showed second peaks at 41.91°, 40.04°, and 38.08°, corresponding to a d-spacing of 2.15, 2.25 and 2.36 Å, respectively. The presence of these second peaks in all copolymers indicates the presence of some crystallinity. There is no apparent peak in the small-angle region for copolymer P1, while copolymers P2 and P3 show a sharp peak around 9.40° and 9.18°, illustrating that the distance between main polymer chains separated by alkyl side chains (lamellar structure) is 9.39, and 9.62 Å, respectively.⁶⁴

Figure 6.

X-ray diffraction patterns of the copolymers P1-P3 as solid films.

Overall, the presence of low intense diffraction π-stacking peak in combination with broad shape (between 12° to 40°) in copolymer P1 suggests that this polymer has a relatively low crystallinity when compared with P2 and P3.⁶⁵ However, the presence of π-π stacking distance at the wide-angle region is related to flexible side chains and electrostatic interaction between D and A moieties.⁶⁶

A computational study

DFT and TD-DFT calculations were performed to understand the effect of replacing benzothiadiazole in P1 by 9,9-dialkyl-substituted fluorene with long alkyl chains, as in P2 and P3, on their spectroscopic and photovoltaic properties. The Becke3-Lee-Yang-Parr (B3LYP) exchange-correlation functional^67–70 with the Pople basis set, 6-31G(d, p), was first applied in the full geometry optimization to locate ground state (S₀) configurations. Subsequently, spectroscopic properties related to electronically excited states of all molecular systems were carried out at Cam-B3LYP/6-311+G(d)⁷¹ within TD-DFT approaches. Meanwhile, solvent effects of chloroform, using SMD variation of IEFPCM of Truhlar and workers,⁷² were considered. All calculations were performed using Gaussian 16 package.⁷³ The long hexyl and ethylhexyl side chains were simplified to methyl and isobutyl groups, respectively, to reduce the computational load. Figure 7 displays optimized molecular geometries of the three comonomers. The structural changes among polymers could not greatly change the overall geometry. For example, the dihedral angle at molecule center is relatively smaller (∼17°) than the two-terminal dihedral angles (∼43° and 36°), as depicted in Figure 7. This means that molecules twist more at their terminals.

Figure 7.

B3LYP-optimized geometries of the title comonomers. Black and red colors, respectively, labeled the dihedral angles among the moieties in the ground and excited states. The atom symbols are provided below the figure.

Upon excitation, all dihedral angles were reduced to make molecules approaching more to coplanarity, especially in the case of P1. For example, Φ₁, Φ₂, and Φ₃ of P1 were reduced by 7.90°, 26.01°, and 18.57°, respectively. The inter-distance between every two adjacent subunits in all compounds is exactly 1.48 Å. Upon excitation, the left inter-distance is raised by 0.01 Å, while the middle and right ones are reduced by 0.02 Å and 0.01 Å, respectively. These bond lengths have a double bond character describing the ICT across the molecule.

The energies and electron density distribution of FMO of conjugated polymers are a playmaker for managing the ICT, intermolecular charge transfer, light absorption/emission, charge injection/extraction/trapping and electrochemical processes.⁷⁴ The HOMO wave function of all materials is mainly localized over the donor unit phenothiazine (Figure 8). In contrast, the LUMO wave function of P1 is predominantly localized on the rest of molecule. On the other hand, the LUMO wave function of P2 and P3 are concentrated on the phenyl-pyridine unit with a small contribution of 9,9-dialkylfluorene. This is the second mark for observing the ICT character, but visually. Furthermore, the degree of photoinduced ICT is related to the difference between ground and excited state dipole moments (Δμ).^75,76 Upon excitation, the dipole moment increases in all studied compounds indicating that the excited state is more polar than the ground state (Table 3). Moreover, Δμ of investigated compounds and their degree of ICT increase in this direction P2 > P3 > P1.

Figure 8.

The electron density distribution of the frontier orbitals of the studied comonomers.

Table 3.

Data of E_HOMO, E_LUMO, the maximum open-circuit voltage (V_oc) by eV and dipole moment (Debye).

	E_HOMO (eV)	E_LUMO (eV)	E_g (eV)	V _oc	μ_g (Debye)	μ_e (Debye)	Δμ (Debye)
P1	−5.19	−2.57	2.62	1.79	4.77	5.26	0.49
P2	−5.17	−1.97	3.20	1.77	6.35	7.58	1.23
P3	−5.17	−1.97	3.20	1.77	6.01	7.21	1.20
PCBM(C₆₀)	−6.10	−3.70

TD-DFT is the most suitable method for predicting optical properties of organic materials with a considerable size. Table 4 presents some spectroscopic properties that describe nature of electronic transitions. The UV-vis spectra of studied molecules are plotted in Figure 9. As expected, spectroscopic properties of P2 and P3 are similar. This can be seen from their spectral profile and the position of $λ_{max}^{c a l c}$ . The main intense band of lower energies can be assigned to ICT transitions due to electronic excitation to S1. The $λ_{max}^{c a l c}$ of this band mainly corresponds to the transition from HOMO to LUMO ( $π - π^{*}$ ) for all compounds except for P1, which corresponds to H-1 → L electronic transition. Limited blue shift (18 nm) was observed upon structural modification from P1 to P2 or P3. This may be attributed to more tendency of P1 to coplanarity than other compounds upon excitation. Consequently, the rate of electron delocalization through molecular backbone increases.

Figure 9.

Simulated UV-vis spectra of title compounds using the Cam-B3LYP/6-311+G(d) method.

Table 4.

Spectroscopic properties of the investigated compounds.

	$λ_{max}^{c a l c}$ (nm)	$λ_{max}^{e x p}$ (nm)	f	Main configuration	Excited state	C (%)
P1	367	334	0.93	H-1 → L	S₁	89
	347		0.31	H → L+1	S₂	71
	316		0.54	H-1 → L+1	S₃	81
P2	349	374	1.05	H → L	S₁	71
	336		1.28	H → L+1	S₂	73
	265		0.64	H-1 → L+1	S₈	57
P3	349	367	1.14	H → L	S₁	70
	337		1.20	H-1 → L	S₂	71
	264		0.34	H-2 → L	S₈	44

The Bulk Heterojunction organic solar cells (BHJ) rely on intermolecular charge transfer in a blend produced from donor organic materials such as conjugated polymers and acceptor materials such as fullerene. [6,6]-Phenyl-C₆₁-butyric acid methyl ester (PCBM(60)) is one of the most widely used acceptors in solar cell devices.⁷⁷ Here, the DFT method was applied to predict photovoltaic properties of compounds as donors blended with PCBM(60). The FMO energies of donor and acceptor components of BHJ are a critical factor to determine power of injection of photoelectron from LUMO of polymer to LUMO of PCBM(60). Figure 10 shows the energy of FMO of both donor and acceptor materials. Both HOMO and LUMO levels of the studied molecules were well-matched with the requirement for an efficient photosensitizer.

Figure 10.

The absolute energy of the FMO of comonomers and PCBM(60).

The difference in LUMO energy levels of studied compounds (P1-P3) and PCBM(60) was in the range of 1.13–1.73 eV, suggesting that photoexcited electron transfer from the studied molecule to the acceptor PCBM(60) could be sufficiently efficient to be deployed in photovoltaic devices.⁷⁸ On the other hand, the power conversion efficiency (PCE) can be calculated according to the following equation⁷⁹:

PCE = \frac{J_{SC} V_{OC} FF}{P_{inc}}

where P_inc is the incident power density, J_sc is the short-circuit current, V_oc is the open-circuit voltage, and FF denotes the fill factor.

The maximum open-circuit voltage (V_oc) of the BHJ solar cell is related to the difference between HOMO of electron-donating polymer and LUMO of electron-accepting fullerene, taking into account energy loss during photo-charge generation.^77,80 The theoretical values of open-circuit voltage V_oc have been calculated from the following expression:

V_{OC} = |E_{HOMO}^{donor}| - |E_{LUMO}^{acceptor}| - 0.3

The calculated V_oc of P1-P3 ranges from 1.77 eV to 1.79 eV. These values are higher (should be larger than 0.3 eV), which guarantees efficient exciton split and charge dissociation at the donor/acceptor interface⁸¹ and suggests that polymers under study are good candidates for photovoltaic applications.

Conclusions

Three phenothiazine/cyanopyridine/phenyl-based copolymers (P1-P3) were synthesized by alternating the type of coupling agent incorporated in polymerization with same monomeric unit using Suzuki cross-coupling methodology. The copolymers showed good solubility in most common organic solvents with good film-forming properties. These polymers showed good thermal stability with thermal decomposition temperatures (T_d, 95% wt) at a range of 319–365°C. In our investigation of photophysical properties of these polymers, they showed absorption maximum peaks in the range of 334∼374 nm, which are red-shifted and broadened in their thin-film absorption. Moreover, their PL spectra showed red and near-infrared light, rendering them with good red and near-infra red-light emitting materials for OLEDs. XRD of these polymers indicated that P2 and P3 had lamellar structures with 2θ = 9.40° and 9.18° for P2 and P3, respectively. All copolymers showed peaks at a range 38.1∼41.9°, indicating some crystallinity in all synthesized copolymers. The DFT findings suggested that photoexcited electron transfer from studied polymers to the acceptor PCBM(60) could be sufficiently efficient to be employed in photovoltaic devices.

Supplemental material

Supplemental Material, sj-pdf-1-hip-10.1177_0954008320988757 - Synthesis, characterization, photophysical properties, and computational studies on N-hexylphenothiazine/cyanopyridine based π-conjugated copolymers

Supplemental Material, sj-pdf-1-hip-10.1177_0954008320988757 for Synthesis, characterization, photophysical properties, and computational studies on N-hexylphenothiazine/cyanopyridine based π-conjugated copolymers by Ashraf A El-Shehawy, Adel M Attia, Abdul-Rahman IA Abdallah and Morad M El-Hendawy in High Performance Polymers

Footnotes

Acknowledgments

Ashraf A El-Shehawy would like to thank Kafrelsheikh University research program for kind support.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was financially supported by the Science & Technology Development Fund (STDF) (Egypt) through the STDF-NRG Project (ID Project Number: 7973) and by the Academy of Science and Research & Technology (ASRT), Egypt, Grant No. 6371 under the project Science UP.

ORCID iD

Ashraf A El-Shehawy

Supplemental material

Supplemental material for this article is available online.

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