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Synthesis and properties of dicarbazolyltriphenylethylene-substituted fluorene derivatives exhibiting aggregation-induced emission enhancement

  • Qiang Zhang , Tian Huang , Xing He , Zhuo-Ran Zhao and Xin Zhao EMAIL logo
Published/Copyright: January 31, 2017
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Heterocyclic Communications
From the journal Heterocyclic Communications Volume 23 Issue 1

Abstract

Two novel dicarbazolyltriphenylethylene-substituted fluorene derivatives DctF and Dct2F were synthesized by the Wittig-Horner and Suzuki coupling reactions. It was reasoned that introducing linear alkyl chains into the fluorene structure could facilitate the deposition of high-quality films by an economical and simple process. The thin solid films of DctF and Dct2F show strongly enhanced emissions in comparison to their dilute solutions, which indicate the aggregation-induced emission enhancement (AIEE). Their decomposition temperatures (Td) exceed 400°C and the glass transition temperature (Tg) of DctF is 105°C. The maximum fluorescence emission wavelengths are in the region of 458–469 nm.

Keywords: aggregation; carbazole; emission; fluorene; Suzuki coupling

Introduction

Organic luminescent materials have attracted considerable attention in the field of science and technology due to their applications as bioimaging agents [1], chemical sensors [2], [3], and organic light-emitting diodes (OLEDs) [4], [5]. Generally, organic luminophores are fluorescent in dilute solutions. However, the light emission of many organic fluorescent molecules is partially or totally quenched in the aggregate or solid state owing to the formation of excimers through π-π interactions [6]. The aggregation-caused quenching (ACQ) effect greatly limits practical applications of the luminophores because most of them are normally used as thin solid films, especially in OLEDs [7], [8]. To overcome this intractable problem, a successful exploration was pioneered by Tang’s group in 2001 and a series of aromatic siloles were synthesized. These compounds show a unique intense emission in the aggregate state and a slight emission in the solution state [9]. Then, Park and co-workers reported aggregation-induced emission enhancement (AIEE). They found that 1-cyano-trans-1,2-bis-(4-methylbiphenyl)ethylene (CN-MBE) nanoparticles exhibit strong fluorescence emission in comparison to weak fluorescence of CN-MBE in solution [10]. Since these pioneering works, various organic molecules with AIEE properties, such as aromatic silole derivatives, tetraphenylethylene (TPE), and other aryl-substituted diene derivatives have been developed [11], [12], [13], [14], [15], [16]. However, some problems still exist. For example, silole-based compounds are complicated and difficult to prepare, and photoisomerization of TPE-based compounds often occurs [17]. Moreover, the solubility of many highly conjugated organic luminescent materials is normally quite low, which complicates their practical applications for the fabrication of electroluminescent devices through spin-coating or ink-jet printing processes [18]. Thus, it is highly desirable to develop compounds with improved AIEE properties.

The glass transition temperature (Tg) of organic materials is very important for the stability and lifetime of devices. If a device is heated above the Tg of the organic luminophore, performance degradation often occurs. Due to high Tg temperatures and AIEE effects, carbazole-based compounds have been attractive candidates for the design of OLEDs [19], [20], [21]. Fluorene has also been widely used as a building block to form many emissive materials because of its high thermal stability, high quantum yield of photoluminescence, and improved hole-injection ability, compared to other blue chromophores [22], [23].

Inspired by these attractive features, we tried to develop novel compounds incorporating the carbazole and fluorene moieties in the molecule. Herein, two new dicarbazolyltriphenylethylene-substituted fluorene derivatives were designed and synthesized in high yields via the classical Wittig-Horner and Suzuki coupling routes. To improve the solubility, linear alkyl chains were introduced into the molecular structure. This feature facilitates the deposition of high-quality films using an economical and simple solution process. The photophysical, thermal, AIEE, and electrochemical properties were evaluated. These molecules are potential materials for blue-light emitters in OLEDs.

Results and discussion

Synthesis

The synthetic strategy is shown in Scheme 1. In the first step, bis(4-(9H-carbazol-9-yl)phenyl)methanone (C2M), was obtained from carbazole and bis(4-fluorophenyl)methanone. Then, the key intermediate compound C2Br was synthesized by the Wittig-Horner reaction of C2M with diethyl 4-bromobenzylphosphonate. In a parallel step, the alkyl-modified fluorenes 2a,b were prepared by treatment of bromofluorenes 1a,b with 1-bromohexane in the presence of a base. Then, the borate esters 3a,b were obtained by a palladium-catalyzed borylation of 2a,b. Finally, the desired compounds DctF and Dct2F were synthesized by the Suzuki coupling reaction of compounds 3a,b with C2Br. The structures of the target products DctF and Dct2F were characterized and confirmed by 1H NMR, 13C NMR, MS and elemental analysis.

Scheme 1
Scheme 1

Thermal properties

The thermal properties of the compounds were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The results are shown in Figure 1. The decomposition temperatures (Td) with 5% weight loss of DctF and Dct2F are 409°C and 418°C, respectively. Interestingly, the thermal degradation of these compounds is a two-step process. It can be suggested that breakage of the C-C bonds of the alkyl chains and the C-N bonds of the carbazole moieties mainly occur during the first step of thermal degradation, followed by decomposition of the aryl rings. The Tg of DctF was found to be 105°C, while the Tg of Dct2F could not detected. Melting could not be detected even up to the Td, presumably due to the presence of long alkyl chains and supramolecular structure, which could influence molecular tight packing in the solid state to form a highly amorphous structure. Therefore, based on the results of TGA and the DSC analysis, these compounds could form a homogeneous and stable amorphous emissive layer in OLED devices and, as such, could be useful in fabricating photoelectric devices.

Figure 1 TGA curves of the compounds.
Figure 1

TGA curves of the compounds.

The inset depicts the DSC curve of DctF upon the second heating run.

Photophysical properties

The optical properties of the compounds were investigated by ultraviolet-visible (UV-vis) absorption spectroscopy (UV) and photoluminescence spectroscopy (PL) in dichloromethane (DCM) and in solid film. The results are shown in Table 1. The films of DctF and Dct2F were conveniently prepared by a solution-processed method, and their good solubility could be ascribed to the supramolecular steric hindrance caused by the long alkyl chains. As shown in Figure 2, the UV and PL spectra of DctF and Dct2F are similar. The UV spectra show two main absorption bands. The strong absorption band in the region of 250–300 nm may correspond to the π-π* local electron transition of the individual aromatic units, and the absorption band at longer wavelengths of 300–425 nm may be attributed to the characteristic π-π* electron transition of the whole conjugation system. The maximum absorption wavelengths λmaxabs appear at 343 nm for DctF and 342 nm for Dct2F. The emission wavelengths λmaxabs are at 458 nm for DctF and at 462 nm for Dct2F in 1 μm DCM (Table 1). The values of λmaxem for DctF and Dct2F in DCM and in solid film exhibit a 9-nm and 7-nm redshift, respectively. The Stokes shifts of the emission bands of these compounds are similar, approximately 117 nm [24].

Table 1

Thermal and optical properties of the compounds.

CompoundTg (°C)Td (°C)λmaxabs (nm)aλmaxem (nm)Stokes shift (nm)cΦFd DCMΦFd THFΦFd PE
DctF105409343458a/467b1150.130.120.41
Dct2F–418342462a/469b1200.140.090.27

aDetermined in DCM. bDetermined in solid film. cCalculated from the difference of λmaxabs and λmaxem.dFluorescence quantum yields were measured in solution using quinine sulfate (QS) as standard (ΦF=0.55).

Figure 2 UV and PL spectra: DctF (A) and Dct2F (B) in DCM (1 μm) and in solid film.
Figure 2

UV and PL spectra: DctF (A) and Dct2F (B) in DCM (1 μm) and in solid film.

The quantum yields (ΦF) of the compounds in good solvents DCM and tetrahydrofuran (THF) and poor solvent petroleum ether (PE) were determined by the standard method using quinine sulfate as the reference [25]. As can be seen from Table 1, these compounds show low fluorescence efficiency in good solvents DCM and THF, with the ΦF values ranging from 0.09 to 0.14. By contrast, these compounds exhibit higher ΦF values in the poor solvent PE, with the ΦF values of DctF and Dct2F of 0.41 and 0.27, respectively. The poor solvents may cause the solute molecules to form nanoaggregates [19].

Electrochemical properties

The electrochemical properties of the compounds were investigated by cyclic voltammetry (CV) in DCM containing 0.1 m tetrabutylammonium perchlorate (n-Bu4NClO4) as a supporting electrolyte, at a scan rate of 50 mV s−1 (Figure 3). The energy level parameters of DctF and Dct2F are listed in Table 2. The highest occupied molecular orbital (HOMO) levels of DctF and Dct2F are −6.18 and −5.97 eV, respectively. The HOMO level of Dct2F with two dicarbazolyltriphenylethylene building blocks is higher than that of DctF. It can be suggested that the electron-donating substituents reduce oxidation potentials of the molecule and raise the HOMO energy level. The band gap energies of DctF and Dct2F are 2.97 and 2.90 eV, respectively. Compound Dct2F has a lower band gap energy because both its dicarbazolyltriphenylethylene building blocks are more conjugated. Therefore, both compounds might be used as hole-transporting and emissive materials in OLEDs.

Figure 3 CV curves of the compounds in DCM.
Figure 3

CV curves of the compounds in DCM.

Table 2

Electrochemical properties of the compounds.

Compdλonset (nm)aEonset (V)bEg (eV)cEHOMO (eV)dELUMO (eV)e
DctF4171.442.97−6.18−3.21
Dct2F4271.232.90−5.97−3.07

aThe onset absorption wavelengths of the UV absorption spectra. bThe onset oxidation potentials from the CV curves. cEstimated from Eg=1240 λonset−1. dCalculated using the empirical equation EHOMO=−(4.74+Eonset). eCalculated from ELUMO=EHOMO+Eg.

AIEE properties

The compounds are soluble in common organic solvents such as THF and DCM but insoluble in water, so increasing the water fraction in the mixed solvent can change their forms from a solution in pure THF to the aggregated particles in water-THF mixtures. As shown in Figure 2, the emission of the solid film is much stronger than that in the DCM solution. The emission images of DctF in THF (1 μm) with different water fractions under 365 nm UV illumination at room temperature are shown in Figure 4, and the compound in high water fractions of water-THF mixtures exhibits stronger emission than that in pure THF solution. To further study the AIEE properties of the compounds, detailed PL emission behaviors of their dilute mixed solutions were evaluated.

Figure 4 The emission images of DctF in THF (1 μm) with different water fractions under 365 nm UV illumination at room temperature.
Figure 4

The emission images of DctF in THF (1 μm) with different water fractions under 365 nm UV illumination at room temperature.

The PL spectra of 1 μmDctF and Dct2F in the water-THF mixtures with different volume fractions of water are shown in Figure 5. DctF in THF exhibits weak PL intensity (~60 a.u.). However, the fluorescent intensity is significantly enhanced when the water fraction exceeds 50%. The enhancement of fluorescent intensity from 60 to 230 a.u. is observed as the volume fraction of water increases from 50% to 90%. Similar phenomena were found for Dct2F in water-THF mixtures. The fluorescent intensity is also enhanced when the water fraction exceeds 50% and the enhancement of fluorescent intensity from 45 to 130 a.u. is observed as the volume fraction of water increases from 50% to 90%. As DctF and Dct2F undergo aggregation in the system with higher water fractions, the fluorescence enhancement phenomena can be attributed to the aggregate state. Therefore, the prepared compounds were characterized as AIEE-active. The ΦF of the compounds in the water-THF mixtures were also evaluated using quinine sulfate as the reference. The highest ΦF of DctF is 0.48 when the water fraction is 90% in the mixture, which has the similar change trend with the PL intensity with a different water fraction. However, the highest ΦF of Dct2F is 0.30 when the water fraction is 70% in the mixture, and a further increase of the water fraction results in decrease of photoluminescent quantum yield. This phenomenon has been observed in some AIEE compounds [19], [26], and the reasons remain unclear as yet.

Figure 5 PL spectra of DctF (A) and Dct2F (B) in THF (1 μm) with different water fractions. The inset depicts the changes in PL peak intensity.
Figure 5

PL spectra of DctF (A) and Dct2F (B) in THF (1 μm) with different water fractions. The inset depicts the changes in PL peak intensity.

Conclusions

Two novel dicarbazolyltriphenylethylene-substituted fluorene derivatives with AIEE properties were synthesized and characterized. Introducing linear alkyl chains into the fluorene structure highly improves the solubility in common organic solvents, which facilitates the deposition of high-quality films by a simple solution-processed method. Their Td exceed 400°C and the Tg of DctF is 105°C. The excellent solubility, high thermal stability, and AIEE property of these compounds make them potential materials for the fabrication of photoelectric devices by the solution process.

Experimental

1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were recorded in CDCl3 on a Bruker Avance III spectrometer with tetramethylsilane as the internal reference. Mass spectra were obtained using a Finnigan TSQ Advantage spectrometer. Elemental analyses (C, H, N) were measured on an Elemental Vario EL III elemental analyzer. Ultraviolet and visible (UV-vis) spectra were recorded on a Beijing Puxi TU-1901 UV-vis spectrometer. PL spectra were recorded on a Perkin Elmer LS55 spectrometer with a slit width of 4 nm for emission. DSC analysis and TGA were performed on a Perkin Elmer Pyris Diamond DSC thermal analyzer and a Beijing Henven HCT-2 thermal analyzer, respectively, with heating rate of 10 min−1 under nitrogen. CV measurement was carried out on a Shanghai Chenhua electrochemical workstation CHI660C with a three-electrode cell (a platinum button working electrode, an SCE reference electrode, and a platinum wire counter electrode). THF was distilled from sodium-benzophenone. All other chemicals were commercially purchased and used without further purification.

2-Bromofluorene (1a) and 2,7-dibromofluorene (1b) were prepared as previously described [27]. 2-Bromo-9,9-dihexylfluorene (2a), 2,7-dibromo-9,9-dihexylfluorene (2b), 2-(4,4,5,5-tetramethyl[1.3.2]dioxaborolan-2-yl)-9,9-dihexylfluorene (3a), and 2,7-bis(4,4,5,5-tetramethyl[1.3.2]dioxaborolan-2-yl)-9,9-dihexylfluorene (3b) were prepared as previously described [28]. Bis(4-(9H-carbazol-9-yl)phenyl)methanone (C2M) and 9,9′-(4,4′-(2-(4-bromophenyl)ethene-1,1-diyl)bis(1,4-phenylene)) bis(9H-carbazole) (C2Br) were prepared as previously described [29].

Synthesis of DctF

A mixture of 3a (0.92 g, 2.0 mmol) and C2Br (2.00 g, 3.0 mmol) in toluene (20 mL) and 2M aqueous Na2CO3 solution (10 mL) was degassed and treated with Pd(PPh3)4 (0.17 g, 0.15 mmol) under argon atmosphere. The mixture was heated under reflux with vigorous stirring for 30 h, then cooled to room temperature and treated with DCM and water. The organic layer was separated, washed with diluted HCl and brine, dried over MgSO4, and concentrated. The solvent was removed under reduced pressure. The residue was purified by silica gel column chromatography eluting with petroleum ether:DCM (6:1). Compound DctF is a yellow solid; yield 1.14 g (62%); 1H NMR: δ 0.65–0.81 (m, 10H), 1.03–1.21 (m, 12H), 1.99 (t, J=8.2 Hz, 4H), 7.17–7.21 (m, 1H), 7.27–7.35 (m, 9H), 7.43–7.76 (m, 22H), 8.17 (dd, J=7.5, 3.8 Hz, 4H); 13C NMR: δ 151.5, 151.0, 142.0, 140.9, 140.8, 140.7, 140.7, 140.7, 140.4, 139.3, 139.2, 137.2, 135.9, 132.0, 130.2, 129.2, 129.0, 127.4, 127.1, 126.8, 126.8, 126.0, 125.7, 123.6, 123.5, 122.9, 121.1, 120.4, 120.4, 120.1, 120.1, 120.0, 119.8, 109.9, 109.8, 55.2, 40.4, 31.5, 29.7, 23.8, 22.6, 14.0; ESI-MS: m/z 919.4 [M+H]+. Anal. Calcd for C69H62N2: C, 90.15; H, 6.80; N, 3.05; Found: C, 90.01; H, 6.91; N, 3.02.

Synthesis of Dct2F

This compound was prepared according to the procedure described above using 3b (0.59 g, 1.0 mmol), C2Br (2.00 g, 3.0 mmol), [Pd(PPh3)4] (0.17 g, 0.15 mmol) in toluene (20 mL) and 2M aqueous Na2CO3 solution (10 mL): yellow solid (0.84 g, 56%); 1H NMR: δ 0.70–0.79 (m, 10H), 1.04–1.11 (m, 12H), 2.02–2.06 (m, 4H), 7.23 (d, J=6.9 Hz, 2H), 7.28–7.34 (m, 12H), 7.43–7.78 (m, 42H), 8.16–8.19 (m, 8H); 13C NMR: δ 151.8, 151.7, 142.0, 140.9, 140.8, 140.7, 140.4, 140.2, 140.0, 139.3, 137.2, 135.9, 132.0, 130.2, 130.1, 129.0, 128.8, 128.8, 127.4, 127.2, 126.8, 126.8, 126.0, 125.9, 123.6, 123.5, 121.5, 121.1, 120.4, 120.4, 120.1, 120.1, 109.9, 109.8, 55.3, 40.5, 31.5, 29.7, 23.8, 22.6, 14.0; ESI-MS: m/z 1503.6 [M+H]+. Anal. Calcd for C113H90N4: C, 90.24; H, 6.03; N, 3.73; Found: C, 89.92; H, 6.30; N, 3.65.

Acknowledgments

This work was financially supported by Natural Science Foundation of Jiangsu Province (China) (No. BK20150282); Applied Basic Research Program of Suzhou (China) (SYG201440); National Training Program of Innovation and Entrepreneurship for Undergraduates (China) (201510332012Z); and Scientific Research Foundation of Suzhou University of Science and Technology (No. XKQ201417).

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Received: 2016-9-24
Accepted: 2017-1-8
Published Online: 2017-1-31
Published in Print: 2017-2-1

©2017 Walter de Gruyter GmbH, Berlin/Boston

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https://doi.org/10.1515/hc-2016-0159
Keywords for this article
aggregation; carbazole; emission; fluorene; Suzuki coupling
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Articles in the same Issue

  1. Frontmatter
  2. Preliminary Communications
  3. Tandem hetero-Diels–Alder-hemiacetal reaction in the synthesis of new chromeno[4′,3′:4,5]thiopyrano[2,3-d]thiazoles
  4. A new method for the reaction of cross-coupling: preparation of 5,5′-bi(1,10-phenanthroline)
  5. Design and synthesis of 4,5-diaryl/heteroarylthiophene-2-carboxylic acid derivatives and evaluation of their biological activities
  6. Research Articles
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  9. Br2- or HBr-catalyzed synthesis of asymmetric 3,3-di(indolyl)indolin-2-ones
  10. Mono- and bis-dipicolinic acid heterocyclic derivatives – thiosemicarbazides, triazoles, oxadiazoles and thiazolidinones as antifungal and antioxidant agents
  11. Synthesis and antimicrobial activity of new piperazine-based heterocyclic compounds
  12. Synthesis and evaluation of chromene-based compounds containing pyrazole moiety as antimicrobial agents
  13. Ultrasonic synthesis, characterization, and antibacterial evaluation of novel heterocycles containing hexahydroquinoline and pyrrole moieties
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