Bicarbazole-benzophenone based twisted donor-acceptor derivatives as potential blue emitters for OLEDs

1. Introduction

In recent decades, there has been significant and swift advancement in organic light-emitting diode (OLED) technology, transforming it into a multi-billion-dollar market [1]. Its applications have expanded across various domains, encompassing high-contrast flat-panel displays, smartwatches, smartphones, and big-screen television sets. Also, solid-state lighting is attracting growing interest in both industrial and scientific domains [2,3,4,5,6,7]. OLED devices present superior features such as colour purity, reduced weight, lower power consumption, faster response and flexibility, surpassing capabilities offered by existing technologies [8,9,10,11].

Until now, the prevailing commercial OLED devices have primarily relied on phosphorescent materials containing noble metals like platinum and iridium [12]. Yet, incorporating atoms of noble metals into structures of phosphorescent materials presents a notable obstacle not only to the future manufacturing expenses of devices, but also prompts environmental concerns [13,14]. Moreover, there is a tendency for the nonradiative transition rate of phosphorescent metal complexes d-orbitals to increase when the emission peaks are shifting to the blue region of emission, posing challenges in achieving both high efficiency and stability in blue phosphorescent OLEDs [15,16,17,18,19,20]. In an effort to address these challenges, there is a renewed focus on the development of small-molecule fluorescent materials, primarily due to their high colour purity and cost-effectiveness [21]. In recent years, there has been considerable attention focused on thermally activated delayed fluorescence (TADF) materials. This is due to absence of metal atoms in their structures and their capability to employ reverse intersystem crossing (RISC), resulting in the upconversion of triplet excitons to emissive singlet excitons, leading to significantly enhanced external quantum efficiencies (EQEs) [22,23,24,25,26,27,28]. However, numerous TADF OLEDs face challenges including triplet-triplet and singlet-triplet annihilation, as well as concentration quenching. These issues can be attributed to prolonged exciton lifetimes, leading to a notable decrease in efficiency as luminance increases [29,30,31].

A crucial requirement for TADF OLED emitters to function efficiently is achieving the smallest possible singlet-triplet energy differential (ΔE_ST). This can be accomplished through molecular design strategies aimed at maximizing the separation between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). One effective approach is the incorporation of highly twisted donor and acceptor structured frameworks [30,32,33]. The distinct attributes of benzophenone and its derivatives, such as potent electron-withdrawing capabilities, effective intersystem crossing due to robust spin-orbit coupling, and a twisted configuration, hold promise for advancing the creation of efficient TADF emitters with shortened exciton lifetimes via a smart molecular design approach [34,35,36,37,38,39,40,41,42]. Carbazole, widely recognized for its electron-rich nature, has been extensively employed as an electron donor in a wide range of optoelectronic devices, serving for both host materials and emitters in various configurations. This is due to its capacity for facile functionalization across multiple sites, adjustable electronic and optical properties, robust electrochemical and thermal stability, and a high photoluminescence quantum yield [43,44,45,46,47,48,49]. Materials exhibiting both favourable film-forming properties and solubility in common organic solvents are extensively explored in scientific research and technological development. This interest stems from the fact that low molar mass organic derivatives, also called molecular glasses, demonstrate capability to form transparent, stable, and homogenous amorphous layers in devices [50,51]. Solubility in common organic solvents of new materials enables solution-based manufacturing processes, such as blade or spin coating, and inkjet printing, which are simpler, cost-effective and more scalable than usual vacuum evaporation method [52,53,54,55].

In this study, we present the synthesis, investigation and application of new electroactive compounds that feature benzophenone and 3,3’-bicarbazole fragments acting as electron acceptor and as an electron donor, respectively, within donor-acceptor type twisted molecules and explore their utility as blue emitters in organic LEDs. The selection of alkyl sidechains, including ethyl, butyl, pentyl, hexyl, 2-ethylhexyl, and octyl was aimed to optimizing their film-forming properties, solubility, and, consequently, solution process-ability [56].

2. Experimental Part

2.1. Instrumentation

The recording of ¹H and ¹³C nuclear magnetic resonance (NMR) spectra was conducted with the Bruker Avance III (400 MHz) instrument. Chemical shifts (δ, ppm) are presented relative to trimethylsilane standard. Acquisition of mass spectra were achieved using the Waters ZQ 2000 mass spectrometer. Thermogravimetric analysis (TGA) was carried out utilizing the TGAQ50 analyser, while thermograms of differential scanning calorimetry (DSC) were recorded using the Bruker Reflex II DSC apparatus. For both types of thermal analysis, a heating rate of 10 ^oC/min in a nitrogen atmosphere was selected. Ultraviolet–visible (UV-vis) spectroscopy involved an HP-8453 diode array spectrometer to measure compound absorption spectra, and the resultant absorbance wavelengths were used to generate Tauc plot. Aminco-Bowman Series 2 spectrofluorometer was used to record photoluminescence (PL) spectra. Low-temperature PL (LTPL) spectra at 77K to determine triplet energy was recorded with a Hitachi F-7000 fluorescence spectrophotometer. The CH instrument CH1604A potentiostat was used to perform cyclic voltammetry (CV), and based on these results, HOMO levels were calculated. Time-resolved photoluminescence (TRPL) experiments, aiming to determine compound decay time, were conducted with an Edinburgh instrument FLS980 spectrometer.

2.2. Synthesis and Structural Analysis

Carbazole (1), 1-bromooctane, 2-ethylhexylbromide, 1-bromohexane, 1-bromopentane, 1-bromobutane, bromoethane, FeCl₃, KOH, K₂CO₃, Na₂SO₄, 4-fluorobenzophenone, chloroform, dimethyl sulfoxide (DMSO) and tetrahydrofuran (THF) were bought from Aldrich and used without further purification.

9H,9‘H-3,3’-Bicarbazole (2) was synthesized using 9H-carbazole as a starting material and FeCl₃ as an oxidising agent, as it was described earlier [57].

9-Ethyl-9‘H-3,3’-bicarbazole (3) was synthesized by partially alkylating 9H,9‘H-3,3’-bicarbazole (2) as it was described previously [58].

9-Butyl-9‘H-3,3’-bicarbazole (4) was also synthesized by partially alkylating 9H,9‘H-3,3’-bicarbazole (2) as it was described previously [58].

9-Pentyl-9‘H-3,3’-bicarbazole (5). 9H,9‘H-3,3’-bicarbazole (2) (2.00 g, 6.02 mmol) was dissolved in 50 ml of tetrahydrofuran, and 1-bromopentane (0.91 g, 6.02 mmol) was subsequently added. Potassium carbonate (1.66 g, 12.04 mmol) and powdered potassium hydroxide (2.02 g, 36.12 mmol) were gradually introduced while the solution was stirred continuously and heated to boiling temperature. After 4 hours, TLC analysis was conducted, and the solution was filtered using a paper filter. The pure product was then isolated through column chromatography using tetrahydrofuran/hexane (volume ratio 1:5) as the mobile phase and silica gel as the stationary phase. The yield obtained was 1.06 g (44%) of pale-yellow material. ¹H NMR (400 MHz, CDCl₃, δ, m.d.): 8.47 (d, 2H, J = 10 Hz), 8.26 (d, 1H, 7.6 Hz), 8.23 (d, 1H, 8.0 Hz), 7.96 (s, 1H), 7.88 (d, 1H, J = 8.4 Hz), 7.84 (dd, 1H, J₁ = 8.4 Hz, J₂ = 1.6 Hz), 7.58-7.53 (m, 2H), 7.50-7.47 (m, 3H), 7.43 (d, 1H, J = 8 Hz), 7.33 (t, 2H, J = 7.2 Hz), 4.37 (t, 2H, J = 7,0 Hz), 1.96 (qu, 2H, J = 7.2 Hz), 1.49-1.41 (m, 4H), 0.96 (t, 3H, J = 7.2 Hz). ¹³C NMR (101 MHz, CDCl₃, δ, m.d.): 140.98, 140.04, 139.65, 138.56, 133.33, 125.95, 125.87, 125.75, 125.60, 124.00, 123.61, 123.44, 123.10, 120.53, 120.47, 119.52, 119.02, 118.91, 118.83, 110.86, 110.79, 108.97, 108.87, 43.23, 29.50, 28.81, 22.58, 14.05.

9-Hexyl-9‘H-3,3’-bicarbazole (6). 9H,9‘H-3,3’-bicarbazole (2) (2.00 g, 6.02 mmol) was dissolved in 50 ml of tetrahydrofuran, and 1-bromohexane (0.99 g, 6.02 mmol) was subsequently added. Potassium carbonate (1.66 g, 12.04 mmol) and powdered potassium hydroxide (2.02 g, 36.12 mmol) were gradually introduced while the solution was stirred continuously and heated to boiling temperature. After 4 hours, TLC analysis was conducted, and the solution was filtered using a paper filter. The pure product was then isolated through column chromatography using tetrahydrofuran/hexane (volume ratio 1:7) as the mobile phase and silica gel as the stationary phase. The yield obtained was 1.00 g (40%) of pale-yellow material. ¹H NMR (400 MHz, CDCl₃, δ, m.d.): 8.46 (d, 2H, J = 9.8 Hz), 8.25 (d, 1H, J = 8.0 Hz), 8.22 (d, 1H, J = 7.6 Hz), 7.99 (s, 1H), 7.88 (dd, 1H, J₁ = 8.4 Hz, J₂ = 1.6 Hz), 7.83 (dd, 1H, J₁ = 8.4 Hz, J₂ = 2.0 Hz), 7.57-7.43 (m, 6H), 7.32 (t, 2H, J = 7.4 Hz), 4.37 (t, 2H, J = 7.2 Hz), 1.95 (qu, 2H, J = 7.4 Hz), 1.50-1.33 (m, 6H), 0.94 (t, 3H, J = 7.2 Hz). ¹³C NMR (101 MHz, CDCl₃, δ, m.d.): 140.97, 140.03, 139.64, 138.55, 134.13, 133.32, 125.95, 125.87, 125.73, 125.58, 124.00, 123.61, 123.44, 123.10, 120.51, 120.46, 119.51, 119.01, 118.91, 118.81, 110.83, 110.76, 108.95, 108.85, 43.26, 31.67, 29.05, 27.06, 22.62, 14.10.

9-(2-Ethylhexyl)-9‘H-3,3’-bicarbazole (7) was synthesized by partially alkylating 9H,9‘H-3,3’-bicarbazole (2) as it was described previously [58].

9-Octyl-9‘H-3,3’-bicarbazole (8) was synthesized by partially alkylating 9H,9‘H-3,3’-bicarbazole (2) as it was described previously [59].

4-(9’-Ethyl-[3,3’]-bicarbazol-9-yl)benzophenone (DB41) was synthesized by stirring 9-ethyl-9‘H-3,3’-bicarbazole (3) (0.50 g, 1.39 mmol) with 4-fluorobenzophenone (0.28 g, 1.39 mmol) in 10 ml of DMSO at 150 °C under an inert nitrogen atmosphere with potassium carbonate (1.92 g, 13.90 mmol) present. After 4 hours, TLC was used to confirm the completion of the reaction, following which the reaction mixture was slowly added to ice water. Chloroform was employed to extract the organic phase, and any remaining water traces in the organic phase were removed by adding anhydrous Na₂SO₄, which was filtered off later. The desired product was purified via column chromatography using tetrahydrofuran/hexane (volume ratio 1:3) as the mobile phase and silica gel as the stationary phase, resulting in yellow amorphous material with a yield of 0.62 g (82%). T_g = 102 °C (DSC). ¹H NMR (400 MHz, CDCl₃, δ, m.d.): 8.48 (dd, 2H, J₁ = 11.2 Hz, J₂ = 1.6 Hz), 8.28 (d, 1H, J = 7.6 Hz), 8.24 (d, 1H, J = 7.6 Hz), 8.14 (d, 2H, J = 8.4 Hz), 7.97-7.95 (m, 1H), 7.89-7.81 (m, 4H), 7.69-7.65 (m, 2H), 7.61 (dd, 2H, J₁ = 8.0 Hz, J₂ = 1.6 Hz), 7.58-7.45 (m, 6H), 7.40 (t, 1H, J = 7.4 Hz), 7.33-7.30 (m, 1H), 4.46 (q, 2H, J = 7.2 Hz), 1.52 (t, 3H, J = 7.2 Hz). ¹³C NMR (101 MHz, CDCl₃, δ, m.d.): 195.66, 141.81, 140.73, 140.46, 139.31, 139.21, 137.51, 135.97, 135.32, 132.95, 132.65, 131.95, 130.07, 128.49, 126.33, 126.21, 126.11, 125.81, 125.52, 124.51, 124.13, 123.59, 123.15, 120.69, 120.61, 120.56, 119.09, 119.00, 118.89, 110.07, 109.94, 108.73, 108.62, 37.69, 13.89. MS (APCI⁺, 20 V): 540.26 ([M+H], 100%).

4-(9’-Butyl-[3,3’]-bicarbazol-9-yl)benzophenone (DB44) was synthesized by stirring 9-butyl-9‘H-3,3’-bicarbazole (3) (0.50 g, 1.29 mmol) with 4-fluorobenzophenone (0.26 g, 1.29 mmol) in 10 ml of DMSO at 150 °C under an inert nitrogen atmosphere with potassium carbonate (1.78 g, 12.90 mmol) present. After 4 hours, TLC was used to confirm the completion of the reaction, following which the reaction mixture was slowly added to ice water. Chloroform was employed to extract the organic phase, and any remaining water traces in the organic phase were removed by adding anhydrous Na₂SO₄, which was filtered off later. The desired product was purified via column chromatography using tetrahydrofuran/hexane (volume ratio 1:5) as the mobile phase and silica gel as the stationary phase, resulting in yellow amorphous material with a yield of 0.66 g (90%). T_g = 82 °C (DSC). ¹H NMR (400 MHz, CDCl₃, δ, m.d.): 8.49 (dd, 2H, J₁ = 8.4 Hz, J₂ = 1.6 Hz), 8.29 (d, 1H, J = 7.6 Hz), 8.25 (d, 1H, J = 7.6 Hz), 8.14 (d, 2H, J = 7.2 Hz), 7.97 (d, 2H, J = 7.2 Hz), 7.89-7.84 (m, 2H), 7.82 (d, 2H, 8.4 Hz), 7.69-7.65 (m, 2H), 7.63-7.58 (m, 3H), 7.56-7.47 (m, 4H), 7.40 (t, 1H, J = 7.4 Hz), 7.31 (t, 1H, J = 7.6 Hz), 4.39 (t, 2H, J = 7.2 Hz), 1.95 (qu, 2H, J = 7.4 Hz), 1.48 (sext, 2H, J = 7.6 Hz), 1.02 (t, 3H, J = 7.4 Hz). ¹³C NMR (101 MHz, CDCl₃, δ, m.d.): 196.66, 141.81, 140.98, 140.73, 139.74, 139.31, 137.53, 135.96, 135.33, 132.89, 132.66, 131.96, 130.08, 128.50, 126.34, 126.21, 126.11, 125.78, 125.49, 124.52, 124.15, 123.48, 123.04, 120.71, 120.62, 120.50, 119.01, 118.86, 110.08, 109.96, 108.99, 108.87, 43.00, 31.23, 20.64, 13.96. MS (APCI⁺, 20 V): 568.39 ([M+H], 100%).

4-(9’-Pentyl-[3,3’]-bicarbazol-9-yl)benzophenone (DB37) was synthesized by stirring 9-pentyl-9‘H-3,3’-bicarbazole (3) (0.50 g, 1.24 mmol) with 4-fluorobenzophenone (0.25 g, 1.24 mmol) in 10 ml of DMSO at 150 °C under an inert nitrogen atmosphere with potassium carbonate (1.72 g, 12.40 mmol) present. After 4 hours, TLC was used to confirm the completion of the reaction, following which the reaction mixture was slowly added to ice water. Chloroform was employed to extract the organic phase, and any remaining water traces in the organic phase were removed by adding anhydrous Na₂SO₄, which was filtered off later. The desired product was purified via column chromatography using tetrahydrofuran/hexane (volume ratio 1:5) as the mobile phase and silica gel as the stationary phase, resulting in yellow amorphous material with a yield of 0.66 g (91%). T_g = 77 °C (DSC). ¹H NMR (400 MHz, CDCl₃, δ, ppm): 8.47 (dd, 2H, J₁ = 13.4 Hz, J₂ = 1.4 Hz), 8.28 (d, 1H, J = 7.6 Hz), 8.23 (d, 1H, J = 7.6 Hz), 8.14 (d, 2H, J = 8.4 Hz), 7.97-7.94 (m, 2H), 7.88-7.81 (m, 4H), 7.69-7.65 (m, 2H), 7.62-7.57 (m, 3H), 7.55-7.46 (m, 4H), 7.39 (t, 1H, J = 7.4 Hz), 7.30 (t, 1H, J = 7.4 Hz), 4.38 (t, 2H, J = 7.2 Hz), 1.96 (qu, 2H, J = 7.2 Hz), 1.47-1.39 (m, 4H), 0.94 (t, 3H, J = 7.0 Hz). ¹³C NMR (101 MHz, CDCl₃, δ, m.d.): 195.65, 141.80, 140.95, 140.71, 139.72, 139.29, 137.51, 135.95, 135.31, 132.86, 132.66, 131.96, 130.07, 128.49, 126.33, 126.20, 126.10, 125.78, 125.48, 124.50, 124.13, 123.46, 123.02, 120.69, 120.61, 120.48, 118.99, 118.84, 110.07, 109.94, 108.97, 108.85, 43.23, 29.49, 28.78, 22.54, 14.01. MS (APCI⁺, 20 V): 582.34 ([M+H], 100%).

4-(9’-Hexyl-[3,3’]-bicarbazol-9-yl)benzophenone (DB38) was synthesized by stirring 9-hexyl-9‘H-3,3’-bicarbazole (3) (0.50 g, 1.20 mmol) with 4-fluorobenzophenone (0.24 g, 1.20 mmol) in 10 ml of DMSO at 150 °C under an inert nitrogen atmosphere with potassium carbonate (1.66 g, 12.00 mmol) present. After 4 hours, TLC was used to confirm the completion of the reaction, following which the reaction mixture was slowly added to ice water. Chloroform was employed to extract the organic phase, and any remaining water traces in the organic phase were removed by adding anhydrous Na₂SO₄, which was filtered off later. The desired product was purified via column chromatography using tetrahydrofuran/hexane (volume ratio 1:7) as the mobile phase and silica gel as the stationary phase, resulting in yellow amorphous material with a yield of 0.67 g (94%). T_g = 68 °C (DSC). ¹H NMR (400 MHz, CDCl₃, δ, ppm): 8.48 (dd, 2H, J₁ = 12.4 Hz, J₂ = 1.6 Hz), 8.28 (d, 1H, 7.6 Hz), 8.23 (d, 1H, J = 8.0 Hz), 8.14 (d, 2H, J = 8.8 Hz), 7.97-7.95 (m, 2H), 7.88-7.81 (m, 4H), 7.69-7.65 (m, 2H), 7.62-7.58 (m, 3H), 7.56-7.45 (m, 4H), 7.39 (t, 1H, J = 7.2 Hz), 7.30 (t, 1H, J = 7.2 Hz), 4.38 (t, 2H, J = 7.2 Hz), 1.95 (qu, 2H, J = 7.4 Hz), 1.49-1.33 (m, 6H), 0.92 (t, 3H, J = 7.0 Hz). ¹³C NMR (101 MHz, CDCl₃-d₆, δ, m.d.): 196.04, 141.80, 140.95, 140.70, 140.22, 139.01, 137.50, 135.95, 135.32, 132.86, 132.66, 131.97, 130.07, 128.55, 128.48, 127.24, 126.33, 126.21, 126.10, 125.77, 125.48, 124.49, 123.45, 123.01, 120.69, 120.62, 120.48, 119.00, 118.83, 110.07, 109.94, 108.97, 108.84, 43.26, 31.63, 29.03, 27.02, 22.60, 14.06. MS (APCI⁺, 20 V): 596.36 ([M+H], 100%).

4-(9’-{2-Ethylhexyl}-[3,3’]-bicarbazol-9-yl)benzophenone (DB39) was synthesized by stirring 9-{2-ethylhexyl}-9‘H-3,3’-bicarbazole (3) (0.50 g, 1.13 mmol) with 4-fluorobenzophenone (0.23 g, 1.13 mmol) in 10 ml of DMSO at 150 °C under an inert nitrogen atmosphere with potassium carbonate (1.56 g, 11.30 mmol) present. After 4 hours, TLC was used to confirm the completion of the reaction, following which the reaction mixture was slowly added to ice water. Chloroform was employed to extract the organic phase, and any remaining water traces in the organic phase were removed by adding anhydrous Na₂SO₄, which was filtered off later. The desired product was purified via column chromatography using tetrahydrofuran/hexane (volume ratio 1:7) as the mobile phase and silica gel as the stationary phase, resulting in pale-yellow amorphous material with a yield of 0.56 g (79%). T_g = 64 °C (DSC). ¹H NMR (400 MHz, CDCl₃, δ, ppm): 8.48 (dd, 2H, J₁ = 15.2 Hz, J₂ = 1.6 Hz), 8.28 (d, 1H, J = 7.6 Hz), 8.24 (d, 1H, J = 7.6 Hz), 8.14 (d, 2H, J = 8.8 Hz), 7.97-7.95 (m, 2H), 7.86-7.81 (m, 4H), 7.69-7.65 (m, 2H), 7.62-7.58 (m, 3H), 7.55-7.45 (m, 4H), 7.40 (t, 1H, J = 7.2 Hz), 7.30 (t, 1H, J = 8.0 Hz), 4.26-4.23 (m, 2H), 2.21-2.13 (m, 1H), 1.50-1.31 (m, 8H), 0.98 (t, 3H, J = 7.4 Hz), 0.93 (t, 3H, J = 7.2 Hz). ¹³C NMR (101MHz, CDCl₃, δ, m.d.): 195.66, 141.81, 141.43, 140.71, 140.21, 139.29, 137.51, 135.94, 135.30, 132.82, 132.66, 131.97, 130.08, 128.50, 126.33, 126.20, 126.09, 125.75, 125.46, 124.51, 124.15, 123.42, 122.99, 120.70, 120.61, 120.43, 118.99, 118.92, 118.82, 110.08, 109.95, 109.27, 109.14, 47.57, 39.50, 31.07, 28.90, 24.46, 23.12, 14.11, 10.97. MS (APCI⁺, 20 V): 624.44 ([M+H], 100%).

4-(9’-Octyl-[3,3’]-bicarbazol-9-yl)benzophenone (DB40) was synthesized by stirring 9-octyl-9‘H-3,3’-bicarbazole (3) (0.50 g, 1.13 mmol) with 4-fluorobenzophenone (0.23 g, 1.13 mmol) in 10 ml of DMSO at 150 °C under an inert nitrogen atmosphere with potassium carbonate (1.56 g, 11.30 mmol) present. After 4 hours, TLC was used to confirm the completion of the reaction, following which the reaction mixture was slowly added to ice water. Chloroform was employed to extract the organic phase, and any remaining water traces in the organic phase were removed by adding anhydrous Na₂SO₄, which was filtered off later. The desired product was purified via column chromatography using tetrahydrofuran/hexane (volume ratio 1:7) as the mobile phase and silica gel as the stationary phase, resulting in pale-yellow amorphous material with a yield of 0.67 g (95%). T_g = 57 °C (DSC). ¹H NMR (400 MHz, CDCl₃, δ, ppm): 8.48 (dd, 2H, J₁ = 13.8 Hz, J₂ = 1.4 Hz), 8.28 (d, 1H, J = 7.6 Hz), 8.24 (d, 1H, J = 7.6 Hz), 8.14 (d, 2H, J = 8.4 Hz), 7.97-7.95 (m, 2H), 7.86-7.81 (m, 4H), 7.70-7.65 (m, 2H), 7.62-7.58 (m, 3H), 7.56-7.46 (m, 4H), 7.39 (t, 1H, J = 7.4 Hz), 7.30 (t, 1H, J = 7.6 Hz), 4.38 (t, 2H, J = 7.6 Hz), 1.95 (pent, 2H, J = 7.6 Hz), 1.48-1.29 (m, 10H), 0.91 (t, 3H, J = 7.4 Hz). ¹³C NMR (101 MHz, CDCl₃, δ, m.d.): 195.66, 141.81, 140.96, 140.72, 139.72, 139.30, 137.51, 135.96, 135.33, 132.87, 132.65, 131.95, 130.07, 128.49, 126.32, 126.21, 126.10, 125.77, 125.48, 124.50, 124.14, 123.46, 123.02, 120.69, 120.60, 120.48, 119.00, 118.83, 110.06, 109.94, 108.97, 108.85, 43.26, 31.83, 29.43, 29.21, 29.06, 27.37, 22.63, 14.09. MS (APCI⁺, 20 V): 624.44 ([M+H], 100%).

2.3. Fabrication and Characterization of Devices

A glass substrate with a pre-patterned indium tin oxide (ITO) layer was utilized in the production process of OLEDs. The mentioned substrate underwent a cleaning procedure firstly with acetone for 30 minutes at 50°C followed by a cleaning with isopropyl alcohol for 30 minutes at 60°C. Subsequently, the cleaned substrates were exposed to UV radiation for 10 minutes in a preheated UV chamber. The layer deposition process took place within a glove box under an inert atmosphere. For the subsequent layer, the hole injecting material poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was employed. It was spin-coated onto the substrate at 4000 rpm for 20 seconds, followed by heating the substrates for 10 minutes at 130°C. After cooling the substrates, the emissive layers, composed of host material and emitter, were formed on top of the hole injection layer. For deposition, spin-coating was also employed by spinning substrates for 20 seconds at 2500 rpm. Subsequently, the electron transporting layer, consisting of 1,3,5-tris(N-phenyl-benzimidazol-2-yl)benzene (TPBi), a LiF electron injecting layer, and aluminum cathode, were formed in a thermal evaporation chamber under a vacuum of 10⁻⁶ torr. Following this, the resulting devices, with an area of 0.09cm², were placed in a mini chamber within the glove box under vacuum until further tests were conducted. These tests were performed under normal atmospheric conditions in complete darkness. The CS-100A luminance and color meter was utilized to record voltage-current density and voltage-luminance characteristics. Additionally, the SpectraScan® spectroradiometer PR-655 was used to create luminance-power efficacy and luminance-current graphs. The measurements of current-voltage characteristics were conducted using a Keithley voltmeter. EQE was calculated using the method outlined in the literature [60].

3. Results and Discussions

3.1. Synthesis

Novel electroactive bicarbazole-based derivatives were synthesized via a three-step procedure illustrated in Scheme 1. Initially, 9H-carbazole underwent oxidation with iron(III) chloride to yield 9H,9’H-3,3’-bicarbazole (2). Subsequently, various alkyl bromides were utilized for the partial N-alkylation of 9H,9’H-3,3’-bicarbazole (2) in THF solution, resulting in the production of 9-alkyl-9‘H-3,3’-bicarbazoles (3-8) in the presence of potassium hydroxide and potassium carbonate. Finally, the obtained bicarbazole derivatives (3-8) underwent nucleophilic substitution reactions with 4-fluorobenzophenone in DMSO in the presence of potassium carbonate, leading to the formation of the target derivatives DB37, DB38, DB39, DB40, DB41, and DB44. The chemical structures of these new electroactive compounds were confirmed using mass spectrometry and NMR spectroscopy, demonstrating excellent alignment with the theoretical structures. The aliphatic chains present in the synthesized target compounds contributed to increased solubility in commonly used organic solvents, consistent with the findings of Inoue et al. regarding the relationship between alkyl chain length and the solubility of organic materials [56]. The solubility of a material in an appropriate solvent is enhanced by extending the length of the alkyl chain. While the thermal evaporation method is suitable for producing thin layers for devices using these electroactive compounds, the good solubility of the new materials suggests a cost-effective alternative method for forming thin films through spin coating from a solution.

3.2. Thermal and Morphological Properties

The response of the synthesized materials DB37-DB41 and DB44 to heating was investigated using DSC and TGA methods, heating the samples under an inert nitrogen atmosphere. Following TGA experiments conducted at a heating rate of 10 °C/min, it was observed that the target compounds exhibit remarkable stability under heating. As depicted in Figure 1, the TGA curve of compound DB37 illustrates a temperature of 5% weight loss (T_d) at 406 °C. Similarly, derivatives DB41 and DB44 demonstrated stability under heating, with respective T_d values of 374 °C and 389 °C. Materials DB38, DB39, and DB40, which feature longer aliphatic groups, exhibited comparable thermal stability, reaching T_d values of 398 °C, 383 °C, and 397 °C, respectively. The TGA curves for all investigated derivatives are provided in Figure S1 of the supplementary material for the publication.

Figure 2 displays the thermograms from DSC experiments conducted on all compounds DB37-DB41 and DB44. Upon examining the curves from the second heating, it becomes evident that the glass transition temperatures (Tg) are influenced by the length of their alkyl sidechains. For instance, material DB41, containing an ethyl group, exhibited a notably high Tg of 102 °C. Conversely, compounds DB44 and DB37, which are substituted with butyl and pentyl groups respectively, displayed slightly lower Tg values of 80 °C and 77 °C. This trend persists for materials featuring even longer alkyl groups: derivatives DB38, DB39, and DB40, substituted with hexyl, 2-ethylhexyl, and octyl groups respectively, exhibited glass transition temperatures of 68 °C, 64 °C, and 57 °C. In summary, the findings from TGA and DSC analyses affirm the suitability of these materials for use in the amorphous electroactive layers of OLED devices. Thermal characteristics are presented in Table 1 as well.

3.3. Electrochemical and Photophysical Properties

The compounds DB37, DB38, DB39, DB40, DB41, and DB44 demonstrate elevated photoluminescence quantum yields (PLQY) of 65.5%, 45.3%, 75.5%, 52.5%, 62.5%, and 68.5%, respectively. Summarized values of PLQY can be found in Table 1. Figure 3 illustrates the UV-absorption bands of compound DB37 as an example. All the UV-absorption bands and Tauc plots for all the compounds are illustrated in Figure S2 of supplementary material of this article. The derivatives were examined in THF solvent under standard conditions using a quartz cuvette.

Notably, each of the derivatives consistently displayed two absorption peaks around 380 and 410 nm, attributed to the presence of identical chromophores within their structures. Tauc plots for objective compounds were generated by employing The UV absorption wavelength and intensity, using equations (α × hν)^1/2 and hν for the x-axis and y-axis, respectively, where α denotes intensity and hν stands for energy (hν = 1240/wavelength). The Tauc plots unveiled bandgaps for the studied derivatives: DB37, DB38, and DB41 had a bandgap of 3.09 eV, DB40 had a bandgap of 3.10 eV, DB39 had a bandgap of 3.08 eV, and DB44 had a bandgap of 3.07 eV (see Table 1).

Figure 4 illustrates the evaluation of electrochemical characteristics of DB37, DB38, DB39, DB40, DB41, and DB44 through CV measurements. Obtained oxidation onset values were used for calculations of HOMO levels, employing equation E_HOMO = -[4.4 + $E_{onset}^{ox}$], while the determination of LUMO levels was accomplished using equation E_LUMO = E_HOMO + E_g following the methodology described in the literature [18,61,62]. The determined HOMO levels for DB37, DB38, DB39, DB40, DB41, and DB44 were -5.67, -5.70, -5.68, -5.69, -5.73, and -5.69, respectively. Meanwhile, LUMO levels were, in the same order, -2.58, -2.61, -2.60, -2.59, -2.64, and -2.62. These values along with E_g levels are outlined in Table 1. HOMO and LUMO levels of the compounds are appropriate to forming blue-emitting layers in tandem with the commercial host material CBP.

In Figure 5 (left) as an example, the PL spectrum of DB37 compound is presented, displaying emission wavelength maximum at about 510 nm with cyan blue emission. Singlet state energies of the potential emitters were calculated by utilizing the crossing point of PL and absorbance charts, resulting in values of 3.04 eV for DB37, 2.94 eV for DB38, 3.10 eV for DB39, 3.06 eV for DB40, 3.22 eV for DB41, and 3.18 eV for DB44 (see Table 1).

Additionally, low-temperature photoluminescence (LTPL) spectra were registered to ascertain the triplet energy levels. The spectrum for DB37 is depicted in Figure 5 (right) as an example. The compounds DB37, DB38, DB39, DB40, DB41, and DB44 demonstrate elevated levels of triplet energy at 2.76, 2.89, 2.81, 2.80, 2.80, and 2.82 eV, respectively, suggesting their potential suitability as blue emitters. The LTPL spectra of all the objective derivatives are presented in Figure S3 of supplementary material, and the triplet state energy values are listed in Table 1.

Figure 6 displays the results of the time-resolved photoluminescence (TRPL) experiments illustrating decay times of photoluminescence for the new emitters. The determined values of the time for DB37, DB38, DB39, DB40, DB41, and DB44 was 5.53, 1.88, 4.27, 2.41, 2.24, and 6.28 ns, respectively. Typically, the decay lifetime of fluorescent emitters falls within the picosecond range. However, the presented materials exhibit decay on the nanosecond scale, suggesting potential utilization of triplet excited states. The photoluminescence decay times are detailed in Table 1. In the graphs, IRF denotes the instrument response function, which was measured both prior to and following each measurement as a control parameter.

3.4. Electroluminescent Properties

The OLED devices architecture utilized in this study is represented by the energy level diagram depicted in Figure 7. These devices incorporate emitters DB37, DB38, DB39, DB40, DB41, and DB44 doped in a CBP host material. The straightforward device structures consisted of a 125 nm ITO anode layer, followed by a 35 nm PEDOT:PSS hole injection layer (HIL), and subsequently a 30 nm emissive layer (EML) comprising a CBP host with dopants DB37, DB38, DB39, DB40, DB41, or DB44 (at concentrations of 5%, 10%, 15%, and 100% by weight). As for the electron transporting layer (ETL), 1,3,5-Tris(N-phenyl-benzimidazol-2-yl)benzene (TPBi, 32 nm) was employed, while lithium fluoride (LiF, 0.8 nm) served as the electron injecting layer (EIL), and aluminum (Al, 150 nm) was used as the cathode layer.

All new objective compounds, owing to their solubility were suitable for layer preparation through spin-coating and were examined as emitters dispersed in a CBP host for the OLEDs. All of new emissive materials underwent concentration-dependent experiments with proportions of 5, 10, 15, or 100 wt% of each guest in the emissive layer. The electroluminescence (EL) properties, like power efficacy (PE), current efficacy (CE), EQE, maximum luminance (L_MAX) and the International Commission on Illumination (CIE) colour space coordinates of the devices utilizing the newly introduced emitting materials distributed within the CBP host, along with the respective non-doped devices, are outlined in Table 2. Furthermore, Figure 7 visually represents the EL characteristics of devices employing the most efficient emitter DB39. The Figure illustrates EL spectra of the devices, current density-voltage-luminance and power efficacy-luminance-current efficact characteristics. The same characteristics of OLEDs using other emitters DB37, DB38, DB40, DB41, and DB44 are depicted in Figures S4, S5, S6, S7, and S8 in the supplementary material of this article, respectively.

Figure 7. The EL characteristics of the device, whether with pure emitter DB39 or doped within a CBP host material at different concentrations, are depicted, illustrating EL spectra (a), current density–voltage (b), luminance–voltage (c), power efficiency–luminance (d), and current efficiency–luminance (e) characteristics.

Figure 7. The EL characteristics of the device, whether with pure emitter DB39 or doped within a CBP host material at different concentrations, are depicted, illustrating EL spectra (a), current density–voltage (b), luminance–voltage (c), power efficiency–luminance (d), and current efficiency–luminance (e) characteristics.

In Figure 7(a), the EL spectra of devices incorporating the DB39 dopant demonstrate peaks within the 460-490 nm range, indicating emission in blue region. The presence of distinct peaks implies effective energy transfer from the host to the guest. Evidently, both undoped and doped OLEDs demonstrate comparable EL emission peaks. Figures 7(b-e) illustrate the characteristics of current density-luminance-voltage and power efficiency-luminance-current efficiency. Although the undoped device exhibits a higher current density than the doped devices, it notably demonstrates lower efficiency than the doped devices, highlighting the significant influence of the host material. As depicted in Table 2, the OLED based on DB39 displays the best efficiencies out of all these devices. This enhanced performance can be ascribed to the inclusion of the elongated and branched 2-ethylhexyl sidechain in the molecule, potentially improving solubility for production of wet-processed OLEDs and contributing to favorable film-forming characteristics of the derivative. Moreover, appropriate HOMO and LUMO levels facilitate effective energy transfer from host to, while the combination of the electron-accepting benzophenone functional group with the bicarbazole donor moiety promotes balanced charge transfer, boosting performance even more. Specifically, the device containing 10 wt% of emitter DB39 demonstrates the highest PE of 4.4 lm/W, with a L_MAX reaching 3430 cd/m². However, the overall best efficiency is achieved by the device incorporating 15wt% of emitter DB39 in its emissive layer, attaining PE and CE values of 4.1 lm/W and 5.7 cd/A, respectively, shile EQE reached 2.7%, with L_MAX reaching 3581 cd/m².

4. Conclusions

We introduced novel emissive derivatives synthesized through a three-step process, utilizing bicarbazole and benzophenone as building blocks to achieve twisted donor-acceptor structures. The incorporation of alkyl sidechains of varying length was chosen to Enhance the solubility and film-forming characteristics of the materials. Newly synthesized derivatives exhibited commendable thermal and morphological stability, evidenced by elevated temperatures of 5% mass loss ranging from 374 °C to 406 °C. The manipulation of alkyl sidechain length allowed control over glass-transition temperatures that spanned from 57 °C to desirable 102 °C. Additionally, newly developed materials demonstrated short decay times, confirmed by time-resolved photoluminescence, and high photoluminescence quantum yields of up to 75.5%. The benzophenone-based derivatives exhibited favourable HOMO-LUMO levels, as well as triplet-singlet state energy values suitable for applications as blue emitters. Upon investigation electroluminescent properties of the new devices, an OLED with emissive layer comprising from 15 wt% DB39 doped in CBP host surpassed other devices in terms of efficiencies. The maximum current efficacy (CE_max) reaching 5.7 cd/A and 2.7% external quantum efficacy (EQE_max) were detected followed by maximum luminance (L_max) of 3,581 cd/m² with a turn-on voltage of 3.9 V. This study emphasized the notable influence of energy transfer from host to guest, ideal doping concentrations, also the effect of chemical structure on solubility, thereby impacting the efficiency of wet-processed devices. It's crucial to highlight that these characteristics were observed in non-optimized OLEDs using standard laboratory conditions, suggesting potential enhancements through optimization processes. Furthermore, enhancing of devices efficiency could be achieved by reducing ∆E_ST and effectively utilizing triplet-state excitons of similar materials, making them suitable for highly efficient lighting applications. We believe that our findings suggest the potential of some materials for further exploration as promising emitters.