Synthesis and aggregation-induced emission properties of tetraphenylethylene-based oligomers containing triphenylethylene moiety

Tetrahedron Letters 53 (2012) 6838–6842

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Synthesis and aggregation-induced emission properties of tetraphenylethylene-based oligomers containing triphenylethylene moiety Debabrata Jana, Binay K. Ghorai ⇑ Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711 103, India

a r t i c l e

i n f o

Article history: Received 24 August 2012 Revised 3 October 2012 Accepted 5 October 2012 Available online 12 October 2012

a b s t r a c t Three new tetraphenylethylene based oligomers bearing triphenylethylene moiety were synthesized utilizing Suzuki coupling reaction and their aggregation-induced emission properties were studied. They show excellent solubility in common organic solvents and emit light in blue region (440504 nm). AIE effect is predominant for tetra-substituted TPE (20.1-fold) than mono-substituted one (4.5-fold). Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Tetraphenylethylene Triphenylethylene Suzuki coupling Aggregation-induced emission

Most conjugated organic molecules have very high luminescence efficiency in dilute solutions, but they exhibit relatively weak or no emissions when fabricated into thin films due to intra- and intermolecular interactions, such as p–p interactions, quench the emission process.1 It is difficult to obtain highly emissive materials in the solid state and also to make optoelectronic devices because the organic emissive materials in the devices are normally used as thin solid films, in which aggregation is inherently accompanied with film formation.2 It is thus highly desirable to develop electroluminescent materials that can emit intense light in the solid state, which is expected to overcome the aggregation quenching problem. Tang et al.3 and Park et al.4 have reported several molecules which show significant enhancement in their light emission upon aggregation or in the solid state. This intriguing phenomenon was named aggregation-induced emission (AIE)5 or aggregationinduced emission enhancement (AIEE).4 Recently, molecules with AIE properties have drawn more attention, because of their enhanced emission in the aggregates. The search for molecules endowed with significant light emission in the solid state has mainly focused on silole-based compounds, which are complicated and difficult to prepare. Among the AIE molecules, tetraphenylethene (TPE) enjoys the advantages of facile synthesis and efficient solid-state emission. TPE and its derivatives have been widely used as a functional building block in the fabrication of the electroluminescence materials for organic light-emitting diodes (OLEDs),6 bioprobes,7 chem-sensors,8 explosive detection9 and latent finger print10 due to their AIE and electrochemical properties. A wide ⇑ Corresponding author. Tel.: +91 33 26684561; fax: +91 33 26682916. E-mail address: [email protected] (B.K. Ghorai). 0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tetlet.2012.10.023

variety of substituents has been attached to phenyl blades of tetraphenylethylene moiety to enhance its electronic and optical properties. Xu et al.11 reported that triphenylethylene derivatives also show remarkable aggregation-induced emission properties. In this Letter, we report the synthesis and their aggregation-induced emission properties of three new oligomers, in which triphenylethylene units directly connected to tetraphenylethene moiety. These compounds exhibit remarkable aggregation-induced emission properties, which make them promising candidates as luminescent materials for electroluminescence applications. Our strategy for the synthesis of tetraphenylethylene based core units and triphenylethylene boronic acid derivatives is outlined in Scheme 1. The 1-(4-bromophenyl)-1,2,2-triphenylethylene (1)12 was synthesized via the reaction of 4-bromobenzophenone with diphenylmethyllithium at a lower temperature (0 °C) followed by acid catalysed dehydration of the resulting alcohol in 85% yield. The dimethoxy-substituted TPE 2 was obtained from 4,40 -dimethoxybenzophenone and 4-bromobenzophenone using conventional McMurry reaction in 40% yield. Low yield of 2 is due to the formation of dibromo-tetraphenylethylene and tetra-methoxytetraphenylethylene derivatives. Tetrakis(4-bromophenyl)ethylene (3)13 was prepared from 4,40 -dibromobenzophenone through a titanium(0)-catalysed McMurry reaction in 76% yield. Benzophenone (4) was converted into 1-bromo-4-(2,2-diphenylvinyl)benzene (6)14 using diethyl 4-bromobenzylphosphonate in the presence of potassium tert-butoxide as a base in anhydrous THF at 0 °C temperature. The related methoxy-substituted triphenylethylene derivative 7 was obtained in a similar manner starting from 4,40 -dimethoxybenzophenone (5) in 82% yield. The desired boronic acid derivatives 8/9 were prepared from 6/7 as described

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Br

Br a

O

85%

b 40%

Br

MeO

OMe

1

2

Br

O

Br

c

Br

Br

76% Br

Br 3

Br

B(OH)2

O

e

d R

R

R

4, R = H 5, R = OMe

R

R

6, R = H, 72% 7, R = OMe, 82%

R

8, R = H, 60% 9,R = OMe, 65%

Scheme 1. Reagents and conditions: (a) diphenylmethane, n-BuLi, THF, 0 °C; then PTSA, toluene, reflux, 6 h; (b) Zn (powder), TiCl4, THF, reflux, 4 h; then 5, reflux, 5 h; (c) Zn (powdered), TiCl4, THF, reflux, 4 h; (d) diethyl 4-bromobenzylphosphonate, t-BuOK, THF, 12 h, rt; (e) n-BuLi, THF, 78 °C, 2 h; then B(OMe)3, 6 N aq HCl.

in the literature.15 Lithiation of bromo-derivatives 6/7 with n-BuLi in THF at 78 °C, followed by treatment with B(OMe)3 and hydrolysis with 6 N aq HCl afforded boronic acid derivatives 8/9 in good yields. Final construction of the targeted molecule is illustrated in Scheme 2. To achieve the synthesis of these oligomers Suzuki-type coupling reactions have been explored. The synthesis of the monosubstituted TPE compounds 10/11 was achieved by using Suzukitype cross-coupling reaction between boronic acid 8 and bromo

derivative 1/2. Products 10 and 11 were obtained in good yields ranging from 81% to 85%. Using the same cross coupling protocol, the tetra-substituted TPE oligomer 12 was prepared using tetrakis (4-bromophenyl)ethylene (3) with 4.5 equiv of boronic acid 9 in 76% yield. These oligomers are readily soluble in common organic solvents, such as CHCl3, CH2Cl2, THF and toluene. They were purified conveniently by flash column chromatography rather than vacuum sublimation usually used for the purification of insoluble organic semiconductors. 1H and 13C NMR spectra and MS associ-

B(OH)2 1

2

85%

81% OMe

MeO 10

11

8

MeO

OMe

MeO

OMe

MeO

OMe

MeO

OMe

B(OH)2 3 76% MeO

OMe 9

12 Scheme 2. Synthesis of TPE-based oligomers 1012. Reagents and conditions: Pd(PPh3)4, K2CO3, toluene, H2O, AliquatÒ 336, 90 °C, 16 h.

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ated with elemental analysis were employed to confirm the structure and purity of all new compounds. The absorption and emission spectra of the oligomers 10–12 in toluene and tetrahydrofuran solutions are shown in Figure 1 and the corresponding absorption kabs and kem maxima, as well as the fluorescence quantum yields Ufl are presented in Table 1. All compounds absorb light (kmax) in spectral region 346–357 nm, which is attributed to the triphenylethylene substituted TPE moiety and emit light in the blue region with peaks ranging from 440 to 504 nm. The absorption and emission spectra are nearly independent of solvent polarity, except for a slight, insignificant blue shift observed with increasing the solvent polarity along with a successive decrease in the fluorescence quantum yields. The quantum efficiencies (Ufl) were determined to be 13%, 4% and 14% in toluene and 6%, 7% and 9% in tetrahydrofuran for 10–12, respectively, using pyrene (Ufl = 0.32, in CH2Cl2) as a reference. Cyclic voltammetry (CV) analyses were carried out to measure the HOMO values of the synthesized compounds. On the basis of the roughly evaluated onset oxidation potentials (Eox onset ), the HOMO energy levels of 10, 11 and 12 are estimated as 5.91, 5.64 and 16 5.63 eV, respectively (HOMO = e(Eox The LUMO enonset + 4.66)). ergy levels of 10, 11 and 12 are 2.34, 2.14 and 2.16 eV, respectively, calculated from the HOMO energy level and HOMO–LUMO gap (Eg), estimated from the onset of the absorption spectra (LUMO = HOMO + Eg). The electrochemical properties as well as the energy level parameters of the oligomers are listed in Table 1. These results indicate that the HOMO energy levels of 12 and 11 are nearly same but for 10, the HOMO value is decreased. The presence of methoxy group in the oligomers 11 and 12 increases the

(a)

9.0x10 4 10 11 12

6.0x10 4

Intensity

Absorbance

0.2

0.1

0.0 300

3.0x10 4

350

400

450

500

550

0.0 650

600

Wavelength (nm)

(b)

9.0x10 4

0.3 10 12

0.2

3.0x10 4

0.1

0.0 300

6.0x10 4

350

400

450

500

550

600

Intensity

Absorbance

11

0.0 650

Wavelength (nm) Figure 1. Absorption (solid line) and photoluminescence spectra (dotted line) of 1012 (1  106 M) in toluene (a) and in THF (b).

electron donating ability of the oligomers. This results in the enhancement of the HOMO energy level and lowers the barrier to hole injection from the most widely used anode material, indium tin oxide (ITO), which has a work function of 5.0 eV. To determine whether these three new compounds are AIE active, the fluorescence behaviour of their diluted mixtures was studied in a mixture of water/THF under different water fractions. Since the compounds were insoluble in water, increasing the water fraction in the mixed solvent could change their existing forms from a solution or well-dispersed state in THF to the aggregated particles in the aq THF with high water content. Changes in the PL peak intensities versus water fraction of the mixture for compound 12 were plotted, as shown in Figure 2(a) as an example. When the water fraction was increased from 0% to 90%, the fluorescence intensity of 12 was correspondingly enhanced 20.1-fold. Similar enhancement was observed in the behaviour of the remaining two compounds. When the water fraction was increased from 0% to 90%, the fluorescence intensities of 10 and 11 were enhanced 6.5-fold and 4.5-fold, respectively. This increase in fluorescence intensity was considered to be a result of the AIE effect. As aggregates formed the restriction of intermolecular rotation increased which led to increased fluorescence emission. The results indicate that the AIE effects of the compounds increase along with an increase in the number of triphenylethylene groups that were present in the molecules. Initially PL intensity increases gradually up to 70% H2O content, then decreases at 80% H2O content, again reaches maximum intensity at 90% H2O content. This phenomenon was often observed in some compounds with AIEE properties, but the reasons remain unclear. There are two possible explanations for this phenomenon: first after the aggregation, only the surface molecule of the nanoparticles emitted light and contributed to the fluorescent intensity upon excitation, leading to a decrease in fluorescent intensity. However, the restriction of intramolecular rotations of the aromatic rings around the carbon–carbon single bonds in the aggregation state could enhance light emission. The net outcome of these antagonistic processes depends on which process plays a predominant role in affecting the fluorescent behaviour of the aggregated molecules.17 Second when water is added, the solute molecules can aggregate into two kinds of nano-particle suspensions: crystal particles and amorphous particles. The former leads to an enhancement in the PL intensity, while the latter leads to a reduction in intensity.18 Thus, the measured overall PL intensity data depend on the combined actions of the two kinds of nanoparticles. However, it is hard to control the formation of nanoparticles in high water content. Thus, the measured PL intensity often shows no regularity in high water content. The absorption spectra of the compound 12 in the THF/water mixtures (1  105 M) are shown in Figure 2(b). In the solvent mixture, the spectral profile remains almost unchanged up to addition of 50% water. The spectrum showed a decrease in absorption band at 60% water content indicating the formation of dimer or trimer.19 When the water content is 80%, the increase in absorbance (at 360 nm) suggests that the formation of nanoscopic aggregates of the compound. The nanoaggregate suspensions in the solvent mixtures effectively decrease light transmission in the mixture and cause apparent high absorbance (Mie scattering effect).20 At 90% water content again decreases in absorbance (at 363 nm) that may be due to the number of molecules in an aggregate increase.19 When the water fraction in the solvent increased from 0% to 90%, the UV absorption wavelength of 12 red-shifted from 354 to 365 nm (an 11-nm red shift), indicating the formation of J-aggregates.21 In conclusion, we have synthesized three new tetraphenylethylene based AIE compounds containing triphenylethylene moiety. The compounds exhibit a strongly enhanced emission in the aggregated state and it is predominant for tetra-substituted TPE (20.1fold) than mono-substituted one (4.5-fold). All compounds are blue

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D. Jana, B. K. Ghorai / Tetrahedron Letters 53 (2012) 6838–6842 Table 1 Photophysical properties of compounds 1012 Compd

10 11 12

c d

kem max ðnmÞ

Ufla (%)

Tol/THF

Tol/THF

347/346 354/353 357/355

442/440 453/452 504/502

13/6 4/7 14/9

b Eox onset (V)

HOMO/LUMO (eV)c

E gd

1.25 0.98 0.97

5.91/2.34 5.64/2.14 5.63/2.16

3.57 3.50 3.47

Fluorescence quantum yields (U), measured in solution using pyrene (Ufl = 0.32, in CH2Cl2) as standard, excited at 275 nm. 1 Eox onset : onset oxidation potential; potentials versus Ag/Ag+, working electrode glassy carbon, 0.1 M Bu4NPF6/CH2Cl2, scan rate 100 mVs . HOMO (eV) = e(Eox + 4.66); LUMO (eV) = HOMO + E . g onset Estimated from the onset of the absorption spectra: 1240/konset.

Intensity

(a) 3.0x10

6

2.5x10

6

2.0x10

6

1.5x10

6

1.0x10

6

5.0x10

5

Water fraction / % 0 10 20 30 40 50 60 70 80 90

0.0 400

(b)

1.4

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Peak intensity / a.u.

a b

kabs max ðnmÞ Tol/THF

Supplementary data

250 200

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.10. 023.

150 100 50 0

References and notes 0

500 550 Wavelength (nm)

20 40 60 80 100 Water fraction / vol %

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Water fraction / % 0 10 20 30 40 50 60 70 80 90

1.2 1.0

Absorbance

650

0.8 0.6 0.4 0.2 0.0 300

350 400 Wavelength (nm)

450

Figure 2. (a) PL spectra of 12 (1  105 M) in THF with varying amounts of water (% fraction of volume). The inset depicts the changes of PL intensity with water fraction. (b) UV–vis spectra of 12 (1  105 M) in THF with varying amounts of water (% fraction of volume).

light emitters, and their maximum fluorescence emission wavelengths are 440–504 nm, the band gaps are 3.57, 3.50 and 3.47 eV, the HOMO energy levels 5.91, 5.64 and 5.63 eV, for oligomers 10, 11 and 12, respectively. The tetra-substituted oligomer can be viewed as a potential emitter in electroluminescence devices. Details of the optical studies of these oligomers are underway in our laboratory. Acknowledgments Financial support from UGC [No. 37–93/2009(SR)], Government of India is gratefully acknowledged. The CSIR, New Delhi, is also thanked for the award of Senior Research Fellowship to D.J.

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