Introduction

Biscarbazoles as luminescent materials which include two aromatic heterocyclic organic compounds have been designed and synthesized for exploring some applications. These include organic light-emitting diodes (OLEDs) [1, 2, 3], organic photovoltaic and electronic devices [4, 5, 6, 7, 8, 9, 10], studying biochemical activities [11, 12, 13, 14, 15, 16, 17, 18] and fundamental points of view [19,20]. Such molecules are especially suited to implementation in OLED technologies because of the electron donation of nitrogen on the carbazole ring. The efficient charge transfer from host carbazoles to connected molecules is provided by a strong π-electron conjugation. Carbazoles have been used as host matrices in highly efficient blue, green, or red electro-phosphorescent devices [6, 7, 8,16,21]. They also have good thermal properties and structural stability, allowing them to be used as a hole transport layer in OLED technology [22, 23, 24, 25, 26, 27, 28, 29, 30]. Carbazole/thioxanthene-S, S’-dioxide (EBCz-ThX) bipolar molecules synthesized by electron accepting and electron donating groups with a solvent-free green chemistry method were presented as blue phosphorescent light emitting devices [31]. A green light with a peak maximum at 550 nm under an applied external voltage was reported from a diode based on 2,4-dicarbazolylquinoline [32]. Multi-carbazole derivatives with twisted and zigzag-shape structures were synthesized and used as sensitizers for dye-sensitized solar cells [33].

To identify the potential use of newly synthesized molecules in various applications, their photophysical and electrochemical properties need to be investigated. As an example, Slodek et al. reported a strong dependence of optical properties on the number of carbazole units and length of alkyl chain on said carbazole units in the molecule, as well as the position of substitution of carbazole for a donor-acceptor (D-A) system based on 2,4-dicarbazolyl-substituted quinolines. The low-temperature PL spectra were characterized by the spin-allowed fluorescence (400 nm) and spin-forbidden phosphorescence (490–527 nm) bands [34]. The synthesis and optical characterization of a salicylaldimine difluoroboron complex with tert-butyl group was carried out by Zhang and co-workers. The maximal emission peak of the synthesized compound in THF at 514 nm was ascribed to intramolecular charge transfer (ICT) emission. The peak positions of fluorescence emissions were blue and red-shifted to 506 and 522 nm for crystal structure and ground powder respectively [32]. Complex yellow (centered at ~574 nm) and red (centered at ~704 nm) colors were observed, with the relative intensities dependent on functional groups in PL spectra, for novel carbazole derivatives synthesized using a condensation reaction between carbazole amines and aromatic aldehydes [35].

Although carbazoles have been synthesized with substitutions in all positions (on benzene rings and nitrogen) [36], substitutions on 3- and 6- positions are very common [30, 32,33,34,35]. Various methods with several steps have been used for substitution on the aromatic rings [36, 39]. The most common and successfully used method is the Suzuki-Miyaura Cross Coupling Reaction [42, 43, 44, 45, 46, 47, 48]. The selection of groups expected to substitute on the desired positions is very important due to their crucial effect on the optical properties of the final product.

This study includes the synthesis and characterization of novel biscarbazole derivatives which were obtained by connection of two carbazole molecules through their nitrogen positions and substitution of phenyl, α-naphthyl and β-naphthyl on their 3- and 6- positions using the Suzuki-Miyaura Cross Coupling Reaction. The formation of synthesized molecules was determined by infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy (NMR), mass spectrometry (MS) and microanalysis methods. Their optical properties were studied using UV-Vis spectroscopy and temperature/excitation power density dependent photoluminescence (PL).

Materials and methods

General Procedure-I for synthesis of bromine substituted biscarbazoles

3,6-dibromocarbazole (B) (2.0 equivalent), 1,2-bis(2-chloroethoxy)ethane (1.0 equivalent), TBAI and 50% NaOH solution (10 ml) were added in 100 ml flask and refluxed while it was stirring at 78 ^°C for 48 hours [49]. After the reaction was complete, the mixture was cooled down to room temperature. The mixture was then filtered with dichloromethane (3x30 ml) and washed in water (200 ml). Organic phase was obtained after separating the layers and drying over Na₂SO₄. The solution was evaporated and the oily solid product was obtained by recrystallizing from a mixture of chloroform/n-hexane (1:1) (see Figure 1).

Figure 1

Synthesis of the original bipod carbazole derivatives (C-1, C-2, C-3, C-4)

Results

In this work 1,2-bis(2-(3,6-diphenyl-9H-carbazole-9-yl) ethoxy)ethane (C-1), bis[2-(2-(3,6- diphenyl-9H-carbazole-9-yl) ethoxy)etyl]ether (C-2), bis[2-(2-(3,6-di(naphthalene-1-yl)-9H-carbazol-9-yl)ethoxy)etyl]ether (C-3) and bis[2-(2-(3,6-di(naphthalene-2-yl)-9H-carbazol-9-yl) ethoxy)ethyl]ether (C-4) were synthesized. The syntheses of these compounds were started by brominating the 3-and 6- position of the carbazole ring. During this process, SiO₂ was used as an efficient-reusable catalyst [42]. Then, two molar equivalents of bromo carbazole were led to react with dichloroether derivatives via S_N2 reaction. The oily solid product was obtained by recrystallizing the mixture of chloroform/n-hexane (1:1). Finally, the bromines were replaced by phenyl boronic acid, α-naphthyl boronic acid and β-naphthyl boronic acid moieties via the Suzuki-Miyaura Cross Coupling Reaction. The oily solid products were re-crystallized from ethanol. The yields of C-1, C-2, C-3 and C-4 bipod carbazole derivatives were obtained as 13.5, 80.3, 61.3 and 81.6%, respectively.

In FT-IR spectra, the N-H stretch peak of secondary amines which is only seen in carbazole and 3,6-dibromocarbazole, disappeared due to the reaction between 3,6-dibromocarbazole and 1,2-bis(2-chloroethoxy)ethane. Therefore, the N-H stretch peak was not observed in FT-IR spectra of C-a1, a2, 1, 2, 3, and 4. The C-Br stretch peak was observed in 3,6-dibromocarbazole, C-a1 and C-a2 but not in C-1, 2, 3, and 4. These results indicate that no Br atoms attached on carbazole were left. Br atoms were replaced by substitution of boron acid molecules via the Suzuki-Miyaura Cross Coupling reaction. Symmetric and asymmetric peaks of aliphatic C-H and C-O-C were observed in C-a1, a2, 1, 2, 3 and 4, but not in carbazole or 3,6-dibromocarbazole. This indicates the presence of ether and aliphatic groups. As a result, the syntheses of intermediate and final products were confirmed with FT-IR data.

The ¹H-NMR and ¹³C-NMR spectra data also support successful synthesis of C-1, C-2, C-3 and C-4 (supplementary materials). For example, in ¹H-NMR spectra of C-3 molecule, ether peaks labeled as 1, 2, and 3 were observed at δ 3.472-4.291 ppm and aromatic peaks at δ 7.619-8.106 ppm (supplementary materials). In ¹³C-NMR spectra of the same molecule, ether peaks were observed at δ 60.644-70.901 ppm, and aromatic peaks at δ 110.831143.323 ppm (supplementary materials).

The molecular ion peaks of C-1, C-2, C-3, and C-4 were obtained from LC-MS spectra taken in dichloromethane. The spectra contain peaks of molecular ions and other possible ions (supplementary materials). The results of microanalysis also support successful synthesis of compounds.

In UV-vis spectra, the absorption bands were observed in the range of 225-380 nm for all samples. This range is suitable for our PL measurement using 349 nm laser.

The PL spectra are shown in Figure 2 together with reference sample containing only carbazole for comparison. The reference sample has several relatively narrow peaks over the spectral range between 370 and 500 nm with the most intense peak situated at 420 nm.

Figure 2

PL spectra for all samples at room temperature

In the work done by Zhang et al. [32], three absorption peaks located at 294, 354 and 379 nm were observed for salicylaldimine difluoroboron complex with tert-butyl group. They were attributed to carbazole, π-π^* and intramolecular charge transfer (ICT) transitions respectively. The emission peak of the synthesized compound in THF mixure present at 514 nm was ascribed to ICT emission. The peak positions of fluorescence emissions were blue and red-shifted to 506 and 522 nm for crystal structure and ground powder respectively. The optical properties of two 2,4-difluorenylquinoline derivatives with different lengths of alkyl chain at the fluorene unit (one with methyl and other with octyl chain) of donor-acceptor (D-A) type were reported by Slodek et al [50]. They observed bright emission in the blue spectral region at 406 nm, whereas the replacement of fluorine with carbazole unit resulted in a bathochromic shift of peak wavelength with emission bands at 425 and 530 nm.

Due to stronger π-conjugation and efficient intramolecular charge transfer from carbazole to aromatic compounds, the PL spectra of C-1, C-2, C-3 and C-4 depict completely different character compared to carbazole. As first seen, all spectra were dominated with a broad peak at about ~513, 523, 553, 565 nm for samples C-1, C-2, C-3 and C-4, respectively. However, these peaks were decomposed into two peaks using Gauss fitting as shown in the inset.

Figures 3 and 4 show temperature dependent PL peak positions (the one at high energy side of spectrum) and normalized integrated intensities deduced from the Gauss fitting to the experimental data. From figures, the peak wavelengths and integrated intensities are approximately temperature independent for C-1 and C-2. This is consistent with the spatial configuration of structures C-1 and C-2, where the most stable formation is expected at the anti-position of the carbazole and benzene rings. Within the temperature range studied in PL measurement, no changes are expected for both structures. On the other hand, for the samples (C-3 and C-4), in which the α-naphthyl and β-naphthyl are attached to carbazole, the peak wavelengths and integrated intensities increase as the temperature increases. At low temperatures, the rotation around the sigma bond is weak and prefers to be at the most stable anti-position. As the temperature increases the rotation of the sigma bond is expected to increase. A resonance occurs when the electrons of p orbital of carbazole that do not participate in hybridization and p orbital of naphthyl group come to the same parallel plane. This provides a complete resonance on the molecule that causes redshift in the peak wavelength positions and increases in integrated intensities of PL spectra, as observed. The change of integrated intensity within a temperature range of 20-300 K for C-3 is approximately three times, while for C-4 it is about 2.3 times. This is probably due to the difference in stereo-electronic effect of compounds attached to the alpha and beta positions.

Figure 3

Temperature dependent peak positions

Figure 4

The normalized integrated intensity versus temperature for all samples

Figure 5 shows the excitation power density dependent normalized integrated intensities for all samples. As seen from the figure integrated intensity increases with excitation power density as expected. At high excitation density, a small degree of saturation is realized.

Figure 5

The normalized integrated intensity versus excitation density for all samples

Conclusions

In this study, some original bipod carbazole derivatives named as C-1, C-2, C-3 and C-4 were successfully synthesized using a Suzuki-Miyaura Cross Coupling reaction between 1,2-bis-3,6-dibromo depot carbazole and aromatic boronic acids. The compounds C-3 and C-4 were obtained by substituting the α- and β-naphthalene on 3- and 6- positions of carbazole for the first time. They were characterized by FT-IR, ¹H-NMR, ¹³C-NMR, LC-MS and elemental analysis methods, confirming their formations. The optical characterization was performed by using UV visible spectroscopy and temperature and excitation power density dependent PL measurements. Compared with a reference carbazole host material, two broad peaks were resolved by applying Gaussian fitting to PL spectra in the wavelength range of 510-520 (greenish) and 620-630 (reddish) for all samples. This demonstrates an efficient charge transfer to π-conjugated systems, dependent on associated functional groups. These newly synthesized bipod carbazole derivatives could be explored for organic light emitting diodes as an active emissive or hole transport layer. The emission wavelengths can be shifted as desired by optimizing or changing the functional groups. In order to test their use in such devices, they must be produced with appropriate layer and contact configurations in a device design.