New biphenylvinylanthracene-based polymers for organic electronics applications: effect of the acceptor group on optoelectronic properties

Abstract

In this study, the molecular design of new semiconducting biphenylvinylvinylanthracene-type polymers with (Poly-BPAn) and without CN (Poly-BPAn-CN) groups in the main chain has been investigated. The two polymers (Poly-BPAn and Poly-BPAn-CN) were synthesized with high yields using Wittig and Knoevenagel polycondensation; respectively. Structural and thermal analyses (NMR, FTIR, TGA, DSC) confirmed the molecular structure and thermal stability up to 275 °C. Similar absorption profiles were revealed in optical analyses, indicating that steric effects are more significant than the electron-withdrawing influence of the cyano group. However, fluorescence efficiency and electron affinity were both affected by the CN group incorporation. The optimized macromolecular structure of both polymers was estimated using DFT, and there was a significant change in planarity based on the incorporated withdrawing group. Single-layer devices of the (ITO/Polymer/Al) configuration have been elaborated and showed relatively low turn-on voltages. Also, the electrical characterization showed a significantly remarkable improvement in mobility following the incorporation of the CN group into the conjugated system of Poly-BPAn-CN. The theoretical insertion of monolayer of optimized single walled carbon nanotubes (SWNTs) (5.5) permitting to reduce the barrier injection (ΔE_e) from the (Ca, Mg) cathode to the synthesized active layer Poly-BPAn and Poly-BPAn-CN, which improve the efficiency of the designed OLEDs.

Introduction

Organic light-emitting diode (OLED) displays has undergone significant evolution in recent decades, driven by advances in materials science and device engineering. While most existing technologies currently rely on the use of inorganic materials, consumers are using organic electronic components likely without being aware of it¹. Indeed, technologies based on organic chemistry offer the possibility of new applications that do not require high electrical outputs². For example, they present the advantage of large-area technology, as compared to their inorganic counterparts, which is limited by the size of the silicon substrate (wafer)³. Moreover, this technology offers the potential to produce devices at very low temperatures on any kind of rigid, flexible or even bio-sourced substrates⁴. In this context, the performance/cost ratio of the organic electronics concept is one of the main reasons that explain researchers to explore this field and to develop the performance of devices based on this technology⁵. Currently, the fruits of this research can be discerned in the industrial production of a variety of applications⁶. Notably, there are large-area flat-panel displays that are commercially available. For example, at the Consumer Electronics Show (CES) in Las Vegas in 2020, LG presented its first OLED-based rollable TVs and began marketing them in 2021.

In terms of organic molecular architecture, poly(phenylene vinylene) (PPV) derivatives have predominantly drawn significant attention due to their potential for light-emitting devices with lower power consumption⁷. They represent an important class of electroluminescent (EL) materials that exhibit high photoluminescence (PL) efficiency and laser properties⁸. On the other hand, anthracene compounds demonstrated promising characteristics⁹. They are among the first aromatic materials employed in OLED elaboration¹⁰. However, due to their rigid and planar structure, anthracene derivatives tend to favor π-π stacking. This can lead to PL quenching in the aggregated state, thereby weakening the electroluminescence efficiency¹¹. Therefore, researchers often seek to integrate groups such as biphenyl into the macromolecular structure to reduce molecular symmetry and disrupt the effects of interchain interactions in the solid state¹².

Within this framework, this study proposes a potential strategy for the synthesis and the design of new biphenylanthracene-based semiconducting polymers incorporating a coplanar CN (cyano) group for organic thin-film electroluminescent materials. The optical and electrochemical behaviors of these organic semiconducting materials were studied. In previously published work¹³, the substitution effects of the electron-withdrawing cyano group within the π-conjugated system were investigated. It has been shown that the CN group can significantly affect the performance of the corresponding conducting polymer. In fact, it can reduce the band gap of the polymer by lowering the LUMO energy level, thereby improving the performance of the corresponding conducting polymer¹⁴.

Experimental

Materials and measurements

All chemicals and solvents were used as supplied by Acros Organics (France) without further purification. NMR (¹H: 300 MHz; ¹³C: 75.5 MHz) spectra were recorded on a Bruker AV300 spectrometer. FT-IR spectra were acquired on a Perkin-Elmer BX FT-IR system spectrometer by dispersing samples in KBr pellets (range). UV–Vis absorption spectra were performed on a Cary 2300 spectrophotometer. Photoluminescence (PL) spectra were obtained on a Jobin– Yvon pectrometer HR460 coupled to a nitrogen cooled Si Charged-Coupled Device (CCD) detector. Samples were excited with a 450 W Xenon lamp at 370 nm. The film thicknesses were measured by a Dektak profilometer and were about 60 nm. The PL quantum yields were measured in dilute chloroform solution according to a relative method using quinine. Cyclic Voltammetry (CV) was performed on a CHI660B electrochemical station in three electrode cell and using material films that were drop- cast onto an indium tin oxide (ITO) as working electrode. The current–voltage (I–V) characteristics of the devices were recorded with a Keithley 236 source. In our previous work, we have detailed the experimental details of the various techniques used and the methods for calculating fluorescence yields¹³.

Method

Synthesis of intermediaries and monomers

The monomers 9,10-bis (triphenylphosphoniomethyl)anthracene dichloride (AnP) and 9,10-cyanomethylanthracene (AnCN) were synthesized according to our previously reported work (Scheme 1) ¹⁵.

Scheme 1.

Synthesis of AnCN and AnP.

Synthesis of 4,4′-diéthoxy -1,1′-biphényle (BPC₂)

A solution of 4,4’-biphenol (BP) (1.862 g, 10 mmol), potassium carbonate (2.76 g, 30 mmol) and (2.22 mL, 30 mmol) ethyl bromide was stirred in 5 mL DMF at room temperature. The progress of reaction was monitored by TLC using ethyl acetate and hexane as an eluant (1:9). After the complete of the reaction, the reaction mixture was poured into distilled water and extracted with chloroforme. The organic layer was washed several times with distilled water and dried over anhydrous magnesium sulfate. The obtained solution was then evaporated. The product was purified by recrystallization from ethanol. Yield: 95%; ¹H NMR (CDCl₃), δ, ppm: δ 7.49 (d, 4H, ³ J = 8.7 Hz, Ar–H), 6.94 (d, 4H, ³ J = 8.7 Hz, Ar–H), 3.99 (q, 4H, ³ J = 6.9 Hz, O-CH₂), 0.95 (t, 6H, ³ J = 6.6 Hz, OCH₂CH₃); ¹³C NMR (75.5 MHz, CDCl₃) : δ 157.05, 129.05, 126.80, 115.87, 68.45, 22.66; FT-IR (cm⁻¹): 3065, 3030 (w, aromatic C–H stretching), 2982, 2877 (w, aliphatic C–H stretching), 1605, 1392 (s, C = C stretching), 1260 (s, C–O–C asymmetric stretching), 1045 (s, C–O–C symmetric stretching), 820 (s, aromatic C–H out-of-plane bending).

Synthesis of 4,4′-diéthoxy-3,3′-dichlorométhyl-1,1′-biphényle (BPC₂Cl)

BPC₂ (2.42 g, 10 mmol), paraformaldehyde (2.50 g, 80 mmol) and 37% aqueous HCl (8.5 mL, 102 mmol) are mixed in 30 mL glacial acetic acid at room temperature. The progress of the reaction is monitored by TLC (eluent: ethyl acetate/hexane: 3/7). After 10 h the resulting mixture was poured into distilled water and extracted with diethyl ether. Then product was dried with anhydrous sodium sulphate. The solvent was removed in vacuum and the crude was recrystallized from ethanol. We obtained BPC₂Cl as white solid. Yield: 85%; ¹H NMR (CDCl₃), δ, ppm : δ 7.36 (d, 2H, ⁴ J = 2.4 Hz, Ar–H), 7.25 (dd, 2H, ⁴ J = 2.4 Hz, ³ J = 8.4 Hz, Ar–H), 6.81 (d, 2H, ³ J = 8.4 Hz, Ar–H), 4.59 (s, 4H), 4.07 (q, 4H, ³ J = 6.9 Hz, O-CH₂), 1.44 (t, 3H, ³ J = 6,9 Hz, OCH₂-CH₃); ¹³C NMR (75.5 MHz, CDCl₃): δ 156.28, 133.61, 133.19, 127.06, 127.03, 112.63, 64.24, 41.34, 14.90; FT-IR (cm⁻¹): 3065–3030 (w, aromatic C–H stretching), 2985–2875 (w, aliphatic C–H stretching), 1394 (s, C = C stretching), 1254 (s, C–O–C asymmetric stretching + CH₂Cl out-of-plane bending), 1047 (s, C–O–C asymmetric stretching), 822 (s, aromatic C–H out-of-plane bending), 603 (s, C–Cl stretching).

Synthesis of 4,4′-diéthoxy-3,3′-diacétylméthyl-1,1′-biphényle (BPC₂Ac)

(3.39 g, 10 mmol) of (BPC₂Cl) and (3.28 g, 40 mmol) of anhydrous sodium acetate are mixed in 20 mL of DMF. The mixture is heated to 90 °C for 2 h and then allowed to return to room temperature and decanted into excess distilled water. The product is thus separated as a white precipitate easily recoverable by filtration. Yield: 90%; ¹H NMR (CDCl₃), δ, ppm : δ 7.31 (d, 2H, ³ J = 7.8 Hz, Ar–H), 7.26 (dd, 2H, ³ J = 6.9 Hz, ⁴ J = 3.0 Hz, Ar–H), 6.81 (d, 2H, ³ J = 7.8 Hz, Ar–H), 5.10 (s, 4H, CH₂O), 4.08 (t, 4H, ³ J = 6.3 Hz, OCH₂CH₃), 2.09 (s, 6H, OCOCH₃), 1.42 (t, 6H, ³ J = 6.9 Hz, OCH₂CH₃); ¹³C NMR (75.5 MHz, CDCl₃): δ 170.97, 156.12, 132.68, 132.22, 126.89, 125.32, 113.48, 63.79, 61.79, 21.06, 15.06; FT-IR (cm⁻¹): (C = O) 174, (C-O) 1221.

Synthesis of 4,4′-diéthoxy-3,3′-dihydroxyméthyl-1,1′-biphényle (BPC₂OH)

To 100 mL ethanol containing (1.6 g, 40 mmol) NaOH, (3.86 g, 10 mmol) (BPC₂Ac) was added. The reaction mixture is then refluxed for 4 h. It is then concentrated in a rotary evaporator and decanted into an excess of distilled water. The hydroxymethylated product was obtained as a white precipitate which is recovered by filtration and dried under vacuum. Yield: 95%; ¹H NMR (CDCl₃), δ, ppm : δ 7.48 (d, 2H, ³ J = 7.8 Hz, Ar–H), 7.46 (dd, 2H, ³ J = 6.9 Hz, ⁴ J = 3.0 Hz, Ar–H), 6.94 (d, 2H, ³ J = 7.8 Hz, Ar–H), 4.76 (s, 4H, CH₂OH), 4.17 (t, 4H, ³ J = 6.3 Hz, OCH₂CH3), 2.53 (s, 2H, OH), 1.48 (t, ³ J = 6.9 Hz, 6H, OCH₂CH₃). ¹³C NMR (75.5 MHz, CDCl₃): δ 156.12, 132.68, 132.22, 126.89, 125.32, 113.48, 63.79, 61.79, 15.06; FT-IR (cm⁻¹): ν(OH) 3342, ν(C-O) 1023.

Synthesis of 4,4’-diéthoxy-3,3’-diformyl-1,1’-biphényle (BPC₂Al)

In a solution of (3.2 g, 10 mmol) of (BPC₂OH) in 20 mL of anhydrous dichloromethane. (8.62 g, 40 mmol) of pyridinium chlorochromate (PCC) are added per portion at 0 °C for 1 h. The mixture is then allowed to return to room temperature and the progress of the reaction is monitored by TLC until the starting product disappears (about 4 h). The reaction mixture is then filtered, decanted into an excess of distilled water and extracted with dichloromethane. The recovered organic phases are then concentrated. Yellow crystals were obtained by recristallization in ethyl acetate (Scheme 2). Yield : 80%; ¹H NMR (CDCl₃), δ, ppm : δ 10.51 (s, 2H, CHO), 8.02 (d, 4H, J = 8,1 Hz, Ar–H), 7.80 (d, 4H, J = 8.1 Hz, Ar–H), 7.04 (d, 4H, J = 8.1 Hz, Ar–H), 4.07 (q, 4H, ³ J = 6.9 Hz, OCH₂CH₃), 1.44 (t, 6H, ³ J = 6.9 Hz, OCH₂CH₃); ¹³C NMR (75.5 MHz, CDCl₃): δ 190.97, 161.12, 134.68, 131.52, 126.17, 125.32, 113.48, 64.79, 15.06; FT-IR (cm⁻¹): ν(C = O) 1685.

Scheme 2.

The synthetic route of BPC₂Al.

Synthesis of anthracene-based polymers

The polymer Poly-BPAn was prepared via Wittig polycondensation. 10 mL of freshly prepared anhydrous THF are introduced into 1 mmol of the dialdehyde (BPC₂Al) and 1 mmol of the diphosphonium salt (AnP). Then 10 mL of a 0.5 M t-BuOK/THF solution (5 mmol) is added dropwise. The mixture immediately turns orange, indicating the formation of the corresponding phosphorus ylide; it is kept at room temperature for 24 h and then heated at reflux for 4 h. The temperature of the medium was then allowed to return to room temperature and the reaction was stopped by acidification with 3% HCl. The reaction mixture is then transferred to distilled water and extracted several times with chloroform. The organic phases are then collected, washed several times with distilled water and concentrated. The polymer was isolated by precipitation in methanol, dissolved in dichloromethane and purified by successive precipitations in methanol. Orange powder was finally obtained (Scheme 3). Yield: 45%; ¹H NMR (CDCl₃), δ, ppm : δ 8.82–6.3 (m, aromatic and vinylic protons), 4.08 (m, O-CH₂CH₃), 3.12 (s, An-CH₃, end-group), 1.49–0.95 (m, aliphatic protons); FT-IR (cm⁻¹): ν(C–H)_aromatic 3056; ν(C–H) 2976–2918; ν(C = C)_aromatic 1604; ν_as(C–O–C) 1238; ν_s(C–O–C) 1118; (Z-CH = CH)_vinylic 930 ; δ(E-CH = CH)_vinylic 806; δ(C–H)_aromatic out of-plane 750.

Scheme 3.

Synthetic route of Poly-BPAn and Poly-BPAnCN.

The polymer Poly-BPAn-CN was prepared via Knoevenagel polycondensation. Into a solvent mixture of 25 mL of freshly prepared anhydrous THF and 12 mL of MeOH, are introduced 1 mmol of monomer (AnCN) and 1 mmol of (BPC₂Al). The resulting solution is then well purged with argon before adding 0.6 mL of a (Bu)₄NOH/MeOH (1 M) solution. The reaction is allowed to continue for 12 h with vigorous stirring and heating to 60 °C. The reaction mixture is then transferred to 50 mL of methanol. The resulting precipitate is recovered, dissolved in dichloromethane and purified by precipitation in methanol and dried in high vacuum (Scheme 3). Yellow powder was finally obtained with a 40% Yield; ¹H NMR (CDCl₃), δ, ppm: δ 8.88–6.22 (m, aromatic and vinylic protons), 5.43(s, CH₂CN, end-group), 4.2–3.88 (m, OCH₂CH₃), 1.49–1 (m, aliphatic protons). IR-TF (cm⁻¹): ν(C–H)_aromatic 3053; ν(C–H)_aromatic 2983–2833; ν(C≡N) 2214; ν(C = C)_aromatic 1511; ν_as(C–O–C) 1252; ν_s(C–O–C) 1040; (E-CH = CH)_vinylic 916; δ(Z–C–H)_vinylic 810; δ(C–H)_aromatic out of-plane 866.

Results and discussion

3.1. Synthesis and structural characterization of Poly-BPAn and Poly-BPAn-CN.

The synthesis steps of the monomers were described in Scheme 2. The new dialdehyde BPC₂Al was prepared from 4,4’-biphenol in five steps¹⁶: we started with the protection of the alcohol functions using bromoethane followed by direct chloromethylation leading to 4,4′-diethoxy-3,3′-difchloromethyl-1,1′-biphenyl (BPC₂Cl)¹⁷, then an acetylation by nucleophilic substitution of the chloride with an acetate group (BPC₂Ac). The last step is the oxidation of the primary alcohols in the presence of pyridinium chlorochromate (PCC)¹⁸. The molecular structure was confirmed by NMR by the appearance of aldehyde proton at 10.5 ppm. The synthesis of monomers (AnP) was reported in our previous work¹³. The 9,10-bis(cyanomethyl)anthracene (AnCN) was prepared in two steps from anthracene (An)¹⁵. The Poly-BPAn polymer was prepared via Wittig reaction by polycondensation of the new aldehyde based on biphenyl: 4,4’-diéthoxy-3,3’-diformyl-1,1’-biphényle (BPC₂Al) and the anthracene triphenyl phosphonium salt (AnP) according to a previously reported procedure using the t-BuOK/THF system (Scheme 3) ¹⁹. The Poly-BPAn polymer was collected as orange powder that is soluble in most common organic solvents with a yield of 45%.

The preparation of Poly-BPAn-CN polymer was achieved through Knoevenagel polycondensation between anthracene-derived dicyanomethyl monomer (AnCN) and 4,4’-diéthoxy-3,3’-diformyl-1,1’-biphényle (BPC₂Al)¹⁶. The synthesis method is based on the use of a mixture of solvents THF/ MeOH (2:1); the tetra-n-butylammonium hydroxide in methanol (Bu)₄NOH/MeOH as a base. The (Poly-BPAn-CN) polymer was collected as yellow powder that is soluble in most common organic solvents with a yield of 40%. The Polymer structures were confirmed by NMR and FT-IR spectroscopic analysis. The ¹H NMR spectra of polymers were described in Fig. 1.

Fig. 1.

¹H NMR Spectra of Poly-BPAn and Poly-BPAn-CN.

The aromatic and vinylic protons appear between 8.8 and 6.30 ppm. The OCH₂ and OCH₃ groups were observed at 4 and 3.1 ppm, respectively. The aliphatic protons showed abroad peak between 1 and 0.9 ppm. The absence of the aldehyde terminal groups was supported by the absence of the corresponding peak from the ¹H NMR spectrum (10 ppm). More, the appearance of a weak signal at 3.10 ppm in ¹H-NMR spectra suggests aromatic methyl terminal groups (Ar-CH₃). The IR spectra of (Poly-BPAn) showed the presence of both cis (867 cm⁻¹) and trans (971 cm⁻¹) vinylic absorptions²⁰. In fact, according to the chemical structures, the yield used in this work can be classified as semi-stabilized yield, and the normal Wittig reaction with aldehyde produces mixtures of Z- and E- configurations nonstereospecifically ^21,22. The polymer number-average weights were estimated by comparing the ¹H-NMR signal integration of terminal groups (Ar-CH₃ in Poly-BPAn and CH₂-CN in Poly-BPAn-CN) to that of the OCH₂. The calculated values were 11,397 g·mol⁻¹ and 15,273 g·mol⁻¹ for Poly-BPAn and Poly-BPAn-CN respectively. SEC analysis showed a polydispersity index (Ip) around 1.1; nevertheless, the polymer weights were underestimated in comparison with the RMN calculated values. In fact, contrary to NMR method which gives absolute exact weights, the SEC analysis is related to the polymer hydrodynamic volume and whose results depend on the nature of the polymer used as reference. Indeed, the conjugated polymers present a rigid molecular structure which makes using the flexible polystyrene as reference unreliable. The thermal properties of Poly-BPAn and Poly-BPAn-CN were evaluated by thermogravimetric analysis (TGA) under nitrogen atmosphere and by differential scanning calorimetry (DSC), with a heating rate of 10 °C·min⁻¹. DSC analysis confirmed the amorphous morphology of both polymers, as indicated by the absence of endothermic melting. A glass transition temperature (Tg) of 75 °C was observed for the free-cyanated polymer (Poly-BPAn). As shown in Fig. 2, the onset of thermal degradation occurs at approximately 245 °C for Poly-BPAn and 175 °C for its cyanated analogue (Poly-BPAn-CN). The enhanced thermal stability of the cyanated polymer can be attributed to increased molecular rigidity resulting from the incorporation of the cyano group into the conjugated system, compared to its free-cyanated analogue.

Fig. 2.

ATG thermogram of polymers of Poly-BPAn and Poly-BPAn-CN.

Photophysical properties

The optical behavior of polymers was examined by UV–visible and fluorescence spectroscopies in dilute solution and as thin films. Their fluorescence quantum yields were determined in solution.

Optical properties in dilute solution

The UV–visible absorption spectra of Poly-BPAn and Poly-BPAn-CN were carried out at room temperature in dilute solution in CHCl₃ with a concentration of 5·10^–5 mol·L⁻¹ (Fig. 3). PL measurements were achieved at room temperature in dilute solution in CHCl₃ with a concentration of 2·10^–7 mol·L⁻¹ (Fig. 4). The obtained spectral data were assembled in Table 1.

Fig. 3.

Left: UV–vis absorption spectra of polymers in chloroform (5·10^–5 mol·L⁻¹); right: optimized molecular structure of two polymers by Chem3D software.

Fig. 4.

Left: PL spectra of polymers in chloroform dilute solution (2·10^–7 mol·L⁻¹) and thin film; right: Color coordinate of the studied material in solution states in the CIE 1931 diagram.

Table 1.

Optical properties of Poly-BPAn and Poly-BPAn-CN polymers in dilute solution.

Absorption
	λmax (nm)	εmax (104 M−1 cm−1)	FWHMb (nm)	λonset (nm)
Poly-BPAn	366a; 385; 408	0.09; 0.14; 0.15	64	442
Poly-BPAn-CN	360a; 381; 403	0.17; 0.20; 0.21	104	442
Fluorescence
	λmax (nm)	FWHMb (nm)	ds(c) (nm)	ϕfl
Poly-BPAn	439; 460a	65	54	0.76
Poly-BPAn-CN	453; 495; 570	116	72	0.06

^a Shoulder

^b Spectrum full width at half maximum

^c stokes shift

The cyano-free polymer (Poly-BPAn) displays a structured absorption spectrum featuring a shoulder at 366 nm and two maxima at 385 nm and 408 nm. These are attributed to π → π* transitions within the 9,10-bis(phenylvinyl)anthracene chromophore. Notably, both Poly-BPAn and the polymer containing an isolated distyrylanthracene-type chromophore (P1), described in our previous work²³, exhibit the same absorption onset wavelength (443 nm). This similarity is explained by the interruption of conjugation along the macromolecular backbone of Poly-BPAn, resulting from steric distortion around the biphenyl linkages which reduces the overall planarity of the structure. Comparison of the absorption spectra for Poly-BPAn and Poly-BPAn-CN (Fig. 3) reveals that the cyano group has no significant impact on the ground-state absorption, as both polymers exhibit nearly identical profiles. However, the spectrum of Poly-BPAn-CN in solution appears less structured and slightly broader in the high-energy region.

Moreover, both spectra display nearly identical absorption onsets at 443 nm, indicating comparable optical gap values (2.8 eV). Furthermore, several studies have established that cyano group incorporation enhances the Effective Conjugation Length (ECL) through mesomeric effects. We can attribute the similar ECL values observed for Free-CN polymer (Poly-BPAn) and its cyano analogue (Poly-BPAn-CN) to torsional distortion at the anthracene-cyanovinyl linkage, caused by steric repulsion between the anthracene unit and the α-position nitrile group²⁴. A density functional theory (DFT) study was performed to elucidate the absorption behavior of both polymers. In fact, the quantum chemical calculations on the studied chemical structure were performed using DFT implemented in Gaussian 09 software²⁵. The ground state was simulated using the Becke-3-Lee–Yang–Parr (B3LYP) exchange–correlation functional²⁶ combined with the 6-31 g (d,p) basis set²⁵. The optimized chemical structure of the synthesized materials Poly-BPAn and its analogue Poly-BPAn-CN, obtained with the DFT/B3LYP/6-31G(d,p) level of theory was presented in Fig. 5.

Fig. 5.

DFT/B3LYP/6-31G(d,p) optimized chemical structure of the Poly-BPAn and Poly-BPAn-CN polymers.

To evaluate the planarity of the optimized chemical structures of the synthesized materials, the dihedral angles φ_i (i = 1–4) were calculated using GaussView software and are summarized in Table 2. The optimized structure of Poly-BPAn exhibits a non-planar conformation, as evidenced by the values of φ₂ and φ₃ (~ 52°). In contrast, φ₁ approaches 180° and φ₄ is near 0°, suggesting partial planarity in specific regions of the structure. The introduction of the cyano group, as described, leads to an increase in the measured dihedral angles, resulting in a more pronounced non-planar geometry. This structural distortion reduces the effective conjugation length (ECL) of the system; which could explain the absorption data obtained.

Table 2.

Collected dihedral angle φ_i (i = 1–4) of the Poly-BPAn and Poly-BPAn-CN optimized chemical structure.

	φ1	φ2	φ3	φ4
Poly-BPAn	180	52	52	− 0.38
Poly-BPAn-CN	− 22	− 90	− 89	− 161

As shown in Fig. 4, the Poly-BPAn exhibits a relatively narrow Pl spectrum featuring a maximum at 439 nm and a shoulder at 460 nm with a spectrum full width at half maximum of 65 nm. The introduction of the nitrile group causes an increase in spectrum full width at half maximum of 50 nm; indicating enhanced vibrational states upon nitrile functionalization in the case of Poly-BPAn-CN. The chromaticity coordinates of the emitted fluorescence were illustrated on the 1931 CIE diagram in Fig. 4, indicating a blue emission for Poly-BPAn and cyan-blue fluorescence for Poly-BPAn-CN.

The photoluminescence quantum yields (Φ) of the synthesized polymers were determined in dilute solution using quinine sulfate as a reference standard²⁷. The decrease in fluorescence quantum yield from 76% for Poly-BPAn to 6% for Poly-BPAn-CN upon introduction of a cyano group can be primarily attributed to the formation of intramolecular charge transfer (ICT) states that effectively quench fluorescence²⁸. The cyano group is a strongly electron-withdrawing substituent that, when incorporated into the conjugated system, creates donor–acceptor polymers exhibiting intramolecular charge transfer states. These ICT states are responsible for fluorescence quenching because the electron transfer from the donor to the acceptor competes with radiative emission. In fact, in donor–acceptor polymers, charge transfer states can deactivate through both radiative and non-radiative recombination pathways to the ground state. A clear enhancement in fluorescence quantum yield was observed for Poly-BPAn compared to reference polymer P1²⁴. This enhancement is attributed to the significantly increased rigidity of the polymer backbone, resulting from the incorporation of biphenyl units in main chain of Poly-BPAn.

Optical properties in thin film

The optical properties of the thin films were investigated at room temperature. The films were prepared using the spin-coating technique from CHCl₃ solutions (2 × 10⁻² M). Emission spectra were recorded under 397 nm excitation (Fig. 6), and the collected spectral data are summarized in Table 3.

Fig. 6.

UV–vis absorption spectra of polymers in thin films (60 nm); inset: the calculation of Gap energy using the Tauc method.

Table 3.

Optical properties of Poly-BPAn and Poly-BPAn-CN polymers in thin film.

Absorption
	λmax (nm)	FWHMb (nm)	λonset (nm)	Eg-op(eV) d
Poly-BPAn	368a; 388a; 411	84	483	2.80
Poly-BPAn-CN	363a; 385a; 406	132	471	2.77
Fluorescence
	λmax (nm)	FWHMb (nm)	ds(c) (nm)
Poly-BPAn	610	115	199
Poly-BPAn-CN	576	146	170

^a Shoulder

^b Spectrum full width at half maximum

^c stokes shift

^d Gap energy calculated using the Tauc method

When moving from solution-phase to thin-film characterization, the absorption spectra exhibited a broadening of the full width at half maximum (FWHM) by 20 nm for Poly-BPAn and 28 nm for Poly-BPAn-CN (Table 3). This broadening can be attributed to the formation of aggregates resulting from π-π interactions within the conjugated biphenylvinylanthracene systems¹³. The gap energies of polymers in solid state were determined from the Tauc relation²⁹:

where hν is the energy of the absorbed photon and α is the absorption coefficient (E_g is determined from extrapolation to α = 0 of the linear part of ((αhν) 2/h). The obtained gap energies (E_g) were below 3 eV (threshold for semiconductor materials), so we could classify these materials as organic semiconducting polymers. Fluorescence measurements revealed a pronounced bathochromic shift in the emission maxima upon transitioning from dilute solutions to the solid state for both polymers (Tables 2 and 3). As evidenced by the thin-film absorption study, this behavior stems from π–π interactions among excited conjugated systems, leading to excimer formation in the solid state. However, the shift was less prominent in Poly-BPAn-CN, likely due to sterically induced distortions in fluorophore planarity caused by CN group incorporation. As shown in Fig. 4, the Poly-BPAn show a green-yellow emission and orange fluorescence for Poly-BPAn-CN.

HOMO and LUMO energy levels

The characteristic HOMO and LUMO energy levels of the two polymers were determined by cyclic voltammetry using ferrocene as a reference.

The analyses were carried out on polymer films deposited on ITO substrate as working electrodes. We used an Ag/AgCl electrode as reference and a solution of (n-Bu)₄NBF₄ in acetonitrile as electrolyte. Experiments were carried out at room temperature, under a nitrogen atmosphere and with a scan rate of 50 mV·s⁻¹. The HOMO and LUMO levels of polymers Poly-BPAn and Poly-BPAn-CN determined from the oxidation and reduction onset potentials are illustrated in Fig. 7. Indeed, the results demonstrate that the addition of the CN group in the conjugate system led to an increase in the electron affinity and ionization potential in the case of Poly-BPAn-CN, compared to its Free-CN analogue (Poly-BPAn). In fact, this increase, particularly that of electron affinity, is due to the electron-withdrawing character of the cyano group through its mesomeric effect³⁰.

Fig. 7.

Energy diagram of the HOMO and LUMO frontier orbitals of the elaborated polymers.

Contour plots of the HOMO and LUMO orbitals and the electrostatic MEP of both polymers were obtained on the optimized chemical structure and drawn with Gaussview software³¹. Else, the electronic structures of the investigated materials were deduced using GaussSum software and presented within the HOMO and LUMO orbitals in Fig. 8.

Fig. 8.

Electronic structure and contour plots of the HOMO and LUMO orbitals of the Poly-BPAn (left) and Poly-BPAn-CN (right) optimized chemical structures.

As shown in Fig. 8, HOMO and LUMO orbitals are distributed on all the backbone of the chemical structure, with an abundance on the anthracene group. The insertion of the cyano group induces a great change in the frontier’s orbitals HOMO and LUMO. In fact, for the optimized chemical structure of Poly-BPAn-CN, the HOMO and LUMO are localized only on anthracene conjugated systems. Added to that, the electronic structure of the molecule changes under the chemical modification. Indeed, we assist of an increase of the energy of the HOMO orbitals from − 4.87 eV for the Poly-BPAn to -5.38 e V for the cyanated polymer (Poly-BPAn-CN). The similar LUMO energies of both materials suggest that the CN group’s steric hindrance plays a more significant role than its electron-withdrawing effect.

In order to present the charge distribution on the optimized chemical structure of the modified and unmodified investigated molecules, the molecular electrostatic potential (MEP) of the optimized chemical structure of the Poly-BPAn and Poly-BPAn-CN are drawn using Gaussview software and depicted in Fig. 9.

Fig. 9.

Molecular electrostatic potential (MEP) of the optimized chemical structure of the Poly-BPAn and Poly-BPAn-CN on the ground state.

On the obtained surface, the color varies from blue to red color. region with blue color (high electrostatic potential) indicates the absence of electrons while the region with red color (low electrostatic potential) indicates an abundance of electrons. By analyzing the obtained MEP (Fig. 9), one can deduce that the electron is localized essentially on the anthracene groups, which present an attractor group for the Poly-BPAn. The insertion of the cyano group induces a great change on the MEP distribution. Indeed, red color became localised on the cyano groups, showing an abundance of the electron on the CN group for the Poly-BPAn-CN.

Electrical analysis

Organic semiconductors, which hold great promise for flexible electronics, require a fundamental understanding of charge transport mechanisms to optimize device performance.

In this study, we investigate the electrical properties of two organic thin films of Poly-BPAn and Poly-BPAn-CN organic materials, by current–voltage (I-V) characterization to elucidate the prevailing conduction mechanisms. Device structures with the configuration [ITO/conjugated polymer/Al] were elaborated by sequentially depositing the polymer layers onto ITO substrates, followed by thermal evaporation of an aluminum (Al) top electrode under vacuum. To elucidate the charge transport mechanism, the current density–voltage (J-V) characteristics were analyzed on a double-logarithmic scale (Fig. 10, right). The log(J) versus log(V) plots for both polymers reveal three distinct regions, signifying different charge conduction regimes^32,33.

Fig. 10.

The J-V characteristic for the [ITO/Polymer/Al] diode: in linear representation(left); in log–log representation (right).

In Region A (low bias), the slope is approximately unity, characteristic of an ohmic conduction regime where the current is governed by thermally generated free charge carriers. As the voltage increases (Region B), the slope increases to a value greater than one, signifying a transition to a trap-limited Space-Charge-Limited Current (SCLC) regime. At higher voltages (Region C), a steep increase in the slope is observed, corresponding to the trap-filled limit (TFL), where all trap states are saturated and the current rises sharply. The effective hole mobility (μ eff) was calculated using the Mott–Gurney law for space-charge-limited current (SCLC) using the following equation³⁴:

where J is the current density (A/cm²), ε is the permittivity, V is the applied voltage (V), d is the thickness of the active layer (m), and μ eff is the effective charge carrier mobility (cm²/V·s).

The analysis of the current density–voltage (J-V) characteristics reveals distinct electrical behaviors for the two diodes. The device incorporating Poly-BPAn-CN exhibits a substantially higher current density than its analog Poly-BPAn. This enhancement is consistent with its markedly higher effective charge carrier mobility, which is greater by a factor of 35 (effective mobilities of 1.45 × 10^–4 cm²/ V·s for Poly-BPAn-CN and 4.13 × 10^–6 cm²/ V·s for Poly-BPAn). The significant improvement in charge transport is directly attributable to the introduction of the strongly electron-withdrawing cyano (CN) group into the polymer backbone. This functional group lowers the energy levels of the polymer, thereby enhancing its electron affinity and facilitating charge transport. Concurrently, it promotes favorable molecular organization, leading to enhanced π-π stacking, which reduces the inter-chain charge-hopping distance and optimizes carrier mobility. As shown in Fig. 10 and Fig. 11 (left), the Poly-BPAn-CN-based device exhibits a higher turn-on voltage (4.61 V vs. 4.20 V), suggesting a somewhat larger initial charge injection barrier, likely due to its lower-lying energy levels. Nevertheless, the exceptional bulk mobility of the Poly-BPAn-CN film effectively compensates for this injection barrier, ultimately yielding superior overall electrical performance.

Fig. 11.

Optimized chemical structure of (SWCNTs) (5.5) and (SWCNTs) (6.4) and their contour plot HOMO and LUMO.

Theoretical designing of new PLED architecture using Poly-BPAn and Poly-BPAn-CN as an emitting active layer

In this part, we are going to design new OLED architecture using our polymers as an active layer. For the first designed OLED using the synthesized Poly-BPAn as an active emitting layer with HOMO energy at around − 4.87 eV and calculated LUMO energy at around -1.9 eV. The choose as the ITO as an anode with work function 4.8 eV, can facilitate the incorporation of the positive charge from the anode to the active layer. Meanwhile, the (Ca, Mg) seems to be the better cathode with an out-work function of 2.9 eV³⁵. The choose of the (Ca, Mg) permitting to obtain the minimum barrier injection (ΔE_e) between the cathode and the active layer calculated using the following formula ΔE_e = ϕ_c—E_A, where ϕ_c is the output work of the cathode. For Poly-BPAn ΔE_e is estimated at around 1 eV.

Else, using the second active layer (Poly-BPAn-CN), it is currently to use the ITO: PEDOT-PSS as an anode with a work function Φ_A = 5.20 eV, which permitting to obtain a low value of barrier injection (ΔE_h) of the charge carrier from the anode to active layer, calculated using the following formula: ΔE_h = IP—ϕ_A, ϕ_A is the anode output work³⁵. ΔE_h is estimated at around 0.18 eV. Meanwhile, based on the electronic parameters of the conventional cathode, it is obvious the chosen of the (Ca, Mg) as a cathode permitting to obtain the minimum barrier injection of electron which is at around 1.1 eV, when using the Poly-BPAn-CN as an active layer. The OLED architectures were recently developed and the OLED efficiency is severally improved. Focused study shows that the insertion of monolayer of carbon-based molecules such as graphene or carbon nanotubes to the OLED basic architecture, permit to enhance the performance of the organic light emitting diodes based on organic active layer^36,37. In order to enhance the injection of the charge carrier (electron) from the anode to active layer, we optimize using the DFT-B3LYP-6-31G(d) level of theory, two single-walled carbon nanotube (SWCNTs) (5.5) and (6.4) having a diameter ϕ = 0.71 nm and length L = 100 nm, with. The optimized chemical structure of the investigated carbon-based material is presented in Fig. 11 with their investigated molecular orbitals HOMO and LUMO.

Based on the calculated electronic parameters of the synthesized active layer and that of single walled carbon nanotubes, we attempt to design new OLED architecture by the incorporation of mono-layer of SWNTC between the cathode and the active layer as shown in Fig. 12. The barrier injection ΔE_e is reduced from 1 to 0.3 eV, in insertion a monolayer of SWCNTs (5.5) between the cathode and the Poly-BPAn active layer, and from 1.1 to 0.3 eV, in insertion a monolayer of SWCNTs (5.5) between the cathode (Ca, Mg) and the Poly-BpAn-CN as an active layer (Fig. 12). The low obtained value can express an enhancement of the injection of the electron charge carrier which improve the efficiency of the designed OLED.

Fig. 12.

Schematic energy-level diagram of the designed PLEDs.

Conclusion

In summary, two new π-conjugated polymers with and without CN groupe (Poly-BPAn and Poly-BPAn-CN) were elaborated and characterized. These new semiconducting materials revealed good thermal stability and good solubility in common organic solvents.

We carry out systematic studies of the photophysical properties associated by the incorporation of CN groups into the backbone structures of π-conjugated polymers. A broader absorption spectrum was obtained in the case of the Poly-BPAn-CN polymer. However, both polymers have the same onset absorption wavelength; the similarity of the LECs of Poly-BPAn and Poly-BPAn-CN may be probably due to the torsion at the anthracene-cyanovinyl bond due to the repulsion between anthracene and the α-nitrile group. The introduction of the CN group also created diminution in the HOMO level. As far as the LUMO level is concerned, the two aspects (steric and electronic) have antagonistic influences, so that equilibrium depends on the relative magnitude of the two effects. The electrical study of the different polymers confirms the semiconducting nature of the conjugated materials synthesized. In addition, the two polymers show behavior typical of a conventional diode, with relatively low voltages (4.20 V and 4.61 V for Poly-BPAn and Poly-BPAn-CN; respectively). A significantly enhanced current density was observed in the case of Poly-BPAn-CN-based device, compared to its Poly-BPAn analogue which is attributed to its markedly higher effective charge carrier mobility. (1.45 × 10⁻⁴ vs. 4.13 × 10⁻⁶ cm²/V·s). To enhance the electron injection from the cathode to the photoactive layer, two single-walled carbon nanotube (SWCNTs) (5.5) and (6.4) were theoretically optimized and incorporated into a basic OLED structure. DFT calculations revealed that introducing a SWCNTs monolayer between the cathode and the polymer layer significantly reduces the electron injection barrier. This modification improves the overall performance of the PLED devices based on Poly-BPAn and Poly-BPAn-CN.

Author contributions

Habiba Zrida: Conceptualization, Investigation, Methodology, Software, Project administration, Validation, Visualization, Writing—original draft . Khaled Hriz: Investigation, Methodology, Validation, Supervision, Writing—original draft, Software, Project administration. Khaoula Hassine: Conceptualization, Investigation, Methodology, Validation, Visualization, Writing—original draft. Mourad Chemek: DFT studies. Bilel Hadrich: Investigation, Methodology, Writing—review & editing. Zakarya Ahmed: Investigation, Validation, Supervision, Writing—review & editing. Mustapha Majdoub: Investigation, Methodology, Validation, Supervision, Project administration.

Funding

This work was supported and funded by the Deanship of Scientific Research at Imam Mohammad Ibn Saud Islamic University (IMSIU) (grant number IMSIU-DDRSP2603).

Data availability

All data generated or analyzed during this study are included in this published article.

Declarations

Competing interests

The authors declare no competing interests.