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Synthesis and characterization of a series of carbazole-based monolithic photorefractive molecules
Materials Letters 59 (2005) 2199 – 2203 www.elsevier.com/locate/matlet
Synthesis and characterization of a series of carbazole-based monolithic photorefractive molecules Jun Shia, Maomao Huangb, Yuanrong Xina, Zhijian Chenb, Qihuang Gongb, Shengang Xua, Shaokui Caoa,T a
College of Materials Engineering, Zhengzhou University, Zhengzhou 450052, China b Department of Physics, Peking University, Beijing 100871, China
Received 6 November 2004; received in revised form 21 February 2005; accepted 27 February 2005 Available online 21 March 2005
Abstract A series of monolithic photorefractive molecules forming organic glasses with low glass transition temperature (2–69 8C) was synthesized starting from 9-(2-ethyl-hexyl)-carbazole. These molecules show distinct photoconductivity due to the carbazole moiety and obvious optical nonlinearity owing to the push–pull structure. Some of these molecules can form transparent films with good stability and excellent optical quality. The absorption of the molecules is more or less red-shifted depending on the strength of the donor–acceptor internal charge transfer. A molecule with a highly asymmetric structure using both a nitrobenzene and a cyano group as the electron-withdrawing components (M2) shows the best photorefractive performance, its gain coefficient was measured as high as 266 cm1 at an applied electric field of 42 V/Am without any extra sensitizer while the FWM efficiency was measured to be 5% at electric fields of 27 V/Am. D 2005 Elsevier B.V. All rights reserved. Keywords: Thin film; Photorefractive; Optical materials and properties; Carbazole; Organic molecules
1. Introduction Photorefractive (PR) materials have attracted much attention as principal candidates for the medium of realtime holographic display, high-density optical data storage, phase conjugation, optical computing, and pattern recognition [1,2], among which organic PR materials are considered to be the most promising since they possess unique advantages such as the compositional flexibility, ease of fabrication, low dielectric constant, and low cost compared to inorganic crystalline materials [3,4]. In the past few years, organic glasses were investigated as a new class of PR materials. Compared to polymeric systems, organic glasses do not need complicated chem-
T Corresponding author. Tel./fax: +86 371 7763561. E-mail address: [email protected] (S. Cao). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.02.066
ical synthesis or purification processes and also have a well-defined structure [5,6]. The high chromophore concentration is evidently an advantage for the photoconducting process, since the overlapping of wave functions in photoconductive chromophore strongly enhances the photo-conducting process. In this work, we report a series of monolithic PR organic molecules, some of which exhibited large net gain. These molecules (M1– M6, refer to Scheme 1) were considered to show photoconductivity due to the carbazole structure and also the electro-optic property due to the push–pull structure [7–9]. An ethylhexyl group was introduced to minimize the extent of crystallinity of the materials. The synthesis, thermal properties and TBC experiment of these monolithic molecules are described in this paper. The carbazole molecules containing push–pull structure are also considered to be potential candidates for electronic device, such as color display, organic light-emitting diode, organic semiconductor laser, solar cell, etc. [10,11]. Therefore, the
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J. Shi et al. / Materials Letters 59 (2005) 2199–2203
H N
r.t, 4h Cl 2 CH 2
B
4h
CN CH2 , piperidine CN
t, 2
2 NO
N
N
100 °C, 2h
A
BS aD ,N
+
DMF, POCl3
N
Br, NaH/DMF
, r.
N2
CHO
CN
C2H5OH, b.t, 6h
CH
(M5) N
(M1)
N
C
CN
N NO2
CN CH2
NO2
, piperidine
N
(M2) CN
C2H5OH, b.t, 2h
CH
C NO2
CN CH2
COOCH3 , piperidine
N
(M3) CN
C2H5OH, b.t, 6h
CH
C
COOCH3
COOH CH2 NO2
, piperidine
N (M4)
C2H5OH, b.t, 4.5h
CH
CH NO2
CN O
CN CN
, NaOH
N (M6)
C2H5OH, b.t, 6.5h
CH
CH
CN CN O
CN
Scheme 1. Synthetic route for the monolithic materials.
absorption spectra of these molecules are also discussed here.
reagents are analytical pure and used as received except that the solvents and piperidine were purified before using. Materials were conformed as the expected structure through characterizations using IR and 1H-NMR.
2. Experimental section 2.1. Measurement FTIR spectra were recorded on a Nicolet-460 FTIR spectrometer. 1H-NMR spectra were obtained from a Bruker DRX-400 NMR spectrometer using CDCl3 as solvent. UV/ vis spectra were recorded with a Shimadzu 3010 UV/vis spectrometer using THF as solvent. Differential scanning calorimetry (DSC) analysis was performed on a Netzsch 204 differential scanning calorimeter, experimental condition refers to Fig. 3.
2.2.1. Compound A (Eh-Cz) A solution of 10 g (0.06 mol) of carbazole, 4 g (60%) NaH and 13.52 g(0.07 mol) 2-ethylhexyl bromide in 40 ml of DMF was stirred under argon atmosphere at room temperature for 4 h. This mixture was then poured into a large excess of water, extracted with hexane and dried over Na2SO4. Solvent was evaporated under reduced pressure. Crude product was then purified by column chromagraphy on silica gel using hexane as eluent. 8.2 g of product (49% yield) was obtained as a colorless viscous fluid. 1H-NMR, y (CDCl3), 0.83(d, 3H), 0.89(d, 3H), 1.2–1.4(m, 8H), 2.06(t, 1H), 4.15(m, 2H), 7.19–7.47(m, 6H), 8.10(d, 2H).
2.2. Synthesis The overall synthetic route and structure of the carbazole-based compounds are illustrated in Scheme 1. All
2.2.2. Compound B A total of 3.38 g (0.022 mol) of POCl3 was slowly added to a stirred solution of 2.8 g (0.01 mol) compound A in 10
J. Shi et al. / Materials Letters 59 (2005) 2199–2203
2201
4H), 8.1(d, 1H), 8.2(m, 2H). IR (KBr): 1522 and 1341 cm1(NO2). M6, dark purple powder, yield: 75%. 1H-NMR: y (in CDCl3), 0.86(d, 3H), 0.93(d, 3H), 1.2–1.4(m, 8H), 1.8(s, 6H), 2.1(m, 1H), 4.2(d, 2H), 7.1(d, 1H), 7.3(t, 1H), 7.4(d, 2H), 7.5(t, 1H), 7.7(d, 1H), 7.8(d, 1H), 8.1(d, 1H), 8.3(s, 1H). IR (KBr): 2200 cm1(CN).
ml of DMF. Subsequently the solution was stirred at 100 8C for 2 h, and then poured into ice water. The resulting mixture was extracted with chloroform. After being dried over MgSO4, solvent was removed by evaporation under reduced pressure. Product was obtained in 1.48 g (48% yield) as a light yellow viscous fluid through purification with column chromatography over silica gel using chloroform as eluent. 1H-NMR: y (in CDCl3), 0.83(d, 3H), 0.89(d, 3H), 1.2–1.4(m, 8H), 2.05(t, 1H), 4.18(m, 2H), 7.25–8.6(m, 7H), 10.08(s, 1H). 2.2.3. Compound M1 Glacial acetic acid containing piperidine (piperidine: glacial acetic acid=1:3) was slowly added to a stirred solution of 0.28 ml compound B and 2.3 mmol malononitrile in 8 ml of dry ethanol. Subsequently the solution was refluxed for 6 h and then cooled down to room temperature. A pale yellow solid was isolated through filtration. Product was obtained through recrystallization from ethanol. Yield: 66%. 1H-NMR: y (in CDCl3), 0.85(d, 3H), 0.92(d, 3H), 1.2– 1.4(m, 8H), 2.05(m, 1H), 4.2(d, 2H), 7.3(t, 1H), 7.4(d, 2H), 7.5(t, 1H), 7.8(s, 1H), 8.0(t, 1H), 8.1(t, 1H), 8.6(t, 1H). IR (KBr): 2200 cm1(CN). Compounds M2, M3, M4 and M6 were prepared in a similar manner with M1 using 4-nitrophenylacetonitrile for M2, methyl cyanoacetate for M3, 4-nitrophenylacetic acid for M4 and 2-dicyanomethylene-3-cyano-5,5-dimethyl-2,5dihydro furan for M6, respectively. M2, orange powder, yield: 60%. 1H-NMR: y (in CDCl3), 0.86(d, 3H), 0.93(d, 3H), 1.2–1.4(m, 8H), 2.07(m, 1H), 4.2(d, 2H), 7.3(t, 1H), 7.4(m, 3H), 7.8(d, 3H), 8.1(t, 2H), 8.3(d, 2H), 8.7(s, 1H). IR (KBr): 2200 cm1 (CN), 1520 and 1340 cm1 (NO2). M3, yellow powder, yield: 78%. 1H-NMR: y (in CDCl3), 0.85(d, 3H), 0.92(d, 3H), 1.2–1.4(m, 8H), 2.05(m, 1H), 3.9(s, 3H), 4.2(d, 2H), 7.3(m, 1H), 7.4(m, 2H), 7.5(t, 1H), 8.1(d, 1H), 8.2(d, 1H), 8.4(s, 1H), 8.7(s, 1H). IR (KBr): 2200 cm1(CN), 1720 cm1(COO). M4, orange powder, yield: 40%. 1H-NMR: y (in CDCl3), 0.86(d, 3H), 0.9(d, 3H), 1.2–1.5(m, 8H), 2.1(m, 1H), 4.1(d, 2H), 7.1(s, 1H), 7.2(s, 1H), 7.4(m, 2H), 7.4(m, 2H), 7.6(d,
2.3. Sample preparation The sample for photorefractive measurement was prepared by putting the compounds onto a indium tin oxide (ITO) glass plate and heating the ITO glass plate to the Melting temperature (M p) of the compounds. Then another ITO glass plate was covered. Sample thickness was controlled to be 80 Am through a Teflon spacer.
M6
M5 M4
Absorbance
Fig. 1. Experimental setup for two-beam coupling and four-wave-mixing.
2.2.4. Compound M5 4-Nitroaniline (5 mmol) was dissolved in a solution of concentrated HCl (2.5 ml) in water (75 ml). The mixture was cooled to lower than 4 8C with an ice bath, then an aqueous solution containing sodium nitrite (0.06 g) was slowly added, the mixture was kept stirring for further 30 min. Sodium dodecyl benzenesulfonate (NaDBS) (1 g) and a solution of compound B (1.4 g) in dichloromethane (50 ml) were added dropwise. Resultant mixture was vigorously stirred at room temperature for 24 h. Ethanol (200 ml) was added and the mixture was heated to remove dichloromethane. The red precipitate was filtered, washed with water, and air-dried. The solid purified by column chromatography over silica gel using chloroform as the eluent, yielding 1.48 g (40%) of dark red solid. 1H-NMR: y (in CDCl3): 0.83(d, 3H), 0.89(d, 3H), 1.2–1.4(m, 8H), 2.06(t, 1H), 4.15(m, 2H), 7.19–7.47(m, 6H), 8.10–8.70(m, 5H). IR (KBr): 1521 and 1342 cm1(NO2), 1596 cm1(NMN).
M3
M2
M1
Eh-Cz 300
400
500
600
Wavelength (nm) Fig. 2. UV/vis spectra of the monolithic molecules and Eh-Cz in 0.1 Amol/L THF solution.
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J. Shi et al. / Materials Letters 59 (2005) 2199–2203
2.4. PR measurement
Table 1 Thermal and optical properties of monolithic molecules
PR performance was characterized by two-beam coupling (TBC) and four-wave-mixing (FWM) experiments measurement. The experimental setup is depicted in literature [12] and also described in Fig. 1. In TBC experiment, the coherent polarized He–Ne laser beams (k = 632.8 nm) overlapped on the sample. The power of beam 1 and beam 2 was measured to be 15 mw and 10 mw, respectively. The TBC coefficient C can be calculated by the following expression:
Sample
k max (nm)
T g (8C)
M p (8C)
T rec (8C)
C 633 (cm1)
M1 M2 M3 M4 M5 M6
406 410 396 409 437 500
2 23 0 2 2 69
81 151 77 141 79 182
– 51.94 – – – 130
8 266 11 10 8 –
C¼
1 ðlnðc0 bÞ lnðb þ 1 c0 ÞÞ d
Where d is the optical path length, b is the ratio of the writing beam intensity, and c 0 = I/I 0 is the beam-coupling ratio where I 0 is the signal intensity without the pump beam and I is the signal intensity with the pump beam.
3. Results and discussion Absorption spectra of the six compounds are presented in Fig. 2 together with the starting material Eh-Cz. Pure Eh-Cz didn’t show any characteristic absorption beyond 350 nm. All the other samples exhibit a relatively broad absorption peaking at 410 nm, 422 nm, 400 nm, 417 nm, 439 nm and 510 nm, respectively for compounds M1–M6. From M1 to M6, the absorption maximum is gradually red-shifted, which is derived from the internal charge transfer between the electron-donating carbazole nucleus and the electronwithdrawing methine or azo groups. The dipole strength between the Eh-Cz and methine or azo groups reveals in fact the electron-withdrawing ability of the functional groups in the chromophore. Among the six molecules, the absorption of compound M6 is most red-shifted, which is due to the
69°C
strong electron-withdrawing ability of the three cyano groups. Such dipolar features have already been exploited in nonlinear optics (NLO) [13] and photorefractive [14] experiments. Fig. 3 shows the DSC curves of the compounds from M1 to M6. All the six compounds studied can form a glassy state upon quenching from their melt. Glass transition temperature (T g ) of these samples varies from 2 8C to 69 8C. All the thermal transition temperatures measured by DSC are summarized in Table 1. M p was recorded from the first heating scan of the initial powdered samples. T g was measured from the quenched samples from their melt. For compounds M2 and M6, re-crystallization can be observed from the second heating scan of the quenched samples, the corresponding transition is reported as Trec in Table 1. A melting endotherm in accordance with the re-crystallization was also observed in the DSC measurement of compounds M2 and M6. Amongst the six compounds studied, compounds M2, M3 and M5 can form stable films with excellent optical quality that did not crystallize over the time frame of two months so far. Compound M1 can also form an optical film, but eventually crystallized over several days. Compounds M4 and M6 can not form a film with optical quality. The PR performance was characterized by TBC and FWM experiments. TBC measurements revealed that the compounds we synthesized have PR effect in some extent. Therefore, these compounds can be regarded as monolithic PR materials. The best PR performance was found for
M6
23°C
1.8
M2
Beam 1 1.6
M1
-2°C
M5
2°C
M4
0°C M3
1.4
Intensity (I/I0)
Exo
-2°C
1.2 1.0 0.8
-30
0
30
60
90
120
150
180
Temperature (°C) Fig. 3. DSC curves of quenched M1–M6 samples (T g and T rec are determined by heating samples at 10 8C/min that had been previously melted and then quenched to glasses).
Beam 2
0.6 0
200
400
600
800
1000
Time (s) Fig. 4. Energy transfer in two-beam coupling experiment of compound M2.
J. Shi et al. / Materials Letters 59 (2005) 2199–2203
compound M2, which exhibits gain efficient as high as 266 cm1 at electric fields of 42 V/Am without any extra photosensitizer while the FWM efficiency was measured to be 5% at electric fields of 27 V/Am. Fig. 4 shows the TBC energy transfer of M2. While for compounds M1, M3, M4 and M5, their gain coefficients are only around 10 cm1, even with 1% or 2% TNF was added as a photo-sensitizer. We originally expected that the compound M6 would show a good PR performance at near infrared region. While, for the poor film forming ability and no suitable laser instrument available, we did not measure the PR performance for this molecule. Results for PR measurements of these monolithic materials are also summarized in Table 1. For the excellent PR performance of compound M2, we considered that there may be two factors. Firstly, the introduction of a cyano group in the chromophore of M2 may break the molecular symmetry, which may impart the monolithic molecule with a good glassy property [15]. Secondly, in many organic glasses, the best temperature region to exhibit good PR performance was found to be from T g to T g +(2–3) 8C, in which the steady-state properties are still good, and the dynamic properties are significantly improved [16]. It means that T g of the glass, which is around room temperature, is suitable for a PR material. T g of compound M2 is 23 8C, which is very close to room temperature. While, for the rest molecules, their T g is far below the room temperature. The TBC coefficient and diffraction efficiency perhaps decreased as the temperature increase above T g. The decrease was attributed to thermal disruption of chromophore alignment, significantly increased dark conductivity, and reduced trap densities at elevated temperature.
4. Conclusions We have synthesized a series of 9-(2-ethyl-hexyl)carbazole(Eh-Cz)-based molecules (M1–M6) as monolithic photorefractive materials forming low T g (ranging from 2 8C to 69 8C)organic glasses via azo-coupling-reaction or Knovenagel condensation reaction. The compounds showed distinct photoconductivity due to the carbazole moity and the optical nonlinearity by the push–pull structure, among which a molecule with a highly asymmetric structure using both a nitrobenzene and a cyano group as the electron-
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withdrawing components (M2) shows the best photorefractive performance, its gain coefficient was measured as high as 266 cm1 at an applied electric field of 42 V/Am without any extra sensitizer while the FWM efficiency was measured to be 5% at electric fields of 27 V/Am. UV/vis spectra showed that depending on the strength of the donor– acceptor internal charge transfer the absorption spectra are more or less red-shifted.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China, project 20274042.
References [1] D.D. Nolte, Photorefractive Effect and Materials, Kluwer Academic, Boston, MA, 1995. [2] Y. Zhang, T. Wada, H. Sasabe, J. Mater. Chem. 8 (1998) 809. [3] W.E. Moerner, A. Grunnet-Jepsen, C.L. Thompson, Annu. Rev. Mater. Sci. 27 (1997) 585. [4] S. Ducharme, J.C. Scott, R.J. Twieg, W.E. Moerner, Phys. Rev. Lett. 66 (1991) 1846. [5] L.M. Wang, M.-K. Ng, L.P. Yu, Appl. Phys. Lett. 78 (2001) 700. [6] J. Sohn, J. Hwang, S.Y. Park, J.-K. Lee, J.-H. Lee, J.-S. Chang, G.J. Lee, B. Zhang, Q.H. Gong, Appl. Phys. Lett. 77 (2000) 1422. [7] R. Forster, Molecular Association Including Molecular Complexes, vol. 2, Academic, New York, 1979. [8] M.C. Castex, C. Olivero, G. Pichler, D. Ades, E. Cloutet, A. Siove, Synth. Met. 122 (2001) 59. [9] D. Ades, V. Boucard, E. Cloutet, A. Siove, C. Olivero, M.C. Castex, G. Pichler, J. Appl. Phys. 87 (2000) 7290. [10] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Mark, C. Taliani, D.D.C. Bradley, D.A. Dos Santos, J.L. Bredas, M. Logdlund, W.R. Salaneck, Nature 397 (1999) 121. [11] D. Romero, N. Nvesch, T. Benazzi, D. Ades, A. Siove, L. Zuppiroli, Adv. Mater. 9 (1997) 1158. [12] J. Shi, M.M. Huang, Z.J. Chen, Q.H. Gong, S.K. Cao, J. Mater. Sci. 39 (2004) 3783. [13] Y. Zhang, L. Wang, T. Wada, H. Sasaba, Macromolecules 29 (1996) 1569. [14] Y. Zhang, L. Wang, T. Wada, H. Sasaba, Appl. Phys. Lett. 70 (1997) 2949. [15] O. Ostroverkhova, D. Wright, U. Gubler, W.E. Moerner, M. He, A. SSantos, R.J. Twieg, Adv. Funct. Mater. 12 (2002) 621. [16] O. Ostroverkhova, M. He, R.J. Twieg, W.E. Moerner, ChemPhysChem 4 (2003) 732.
Synthesis and characterization of a series of carbazole-based monolithic photorefractive molecules Jun Shia, Maomao Huangb, Yuanrong Xina, Zhijian Chenb, Qihuang Gongb, Shengang Xua, Shaokui Caoa,T a
College of Materials Engineering, Zhengzhou University, Zhengzhou 450052, China b Department of Physics, Peking University, Beijing 100871, China
Received 6 November 2004; received in revised form 21 February 2005; accepted 27 February 2005 Available online 21 March 2005
Abstract A series of monolithic photorefractive molecules forming organic glasses with low glass transition temperature (2–69 8C) was synthesized starting from 9-(2-ethyl-hexyl)-carbazole. These molecules show distinct photoconductivity due to the carbazole moiety and obvious optical nonlinearity owing to the push–pull structure. Some of these molecules can form transparent films with good stability and excellent optical quality. The absorption of the molecules is more or less red-shifted depending on the strength of the donor–acceptor internal charge transfer. A molecule with a highly asymmetric structure using both a nitrobenzene and a cyano group as the electron-withdrawing components (M2) shows the best photorefractive performance, its gain coefficient was measured as high as 266 cm1 at an applied electric field of 42 V/Am without any extra sensitizer while the FWM efficiency was measured to be 5% at electric fields of 27 V/Am. D 2005 Elsevier B.V. All rights reserved. Keywords: Thin film; Photorefractive; Optical materials and properties; Carbazole; Organic molecules
1. Introduction Photorefractive (PR) materials have attracted much attention as principal candidates for the medium of realtime holographic display, high-density optical data storage, phase conjugation, optical computing, and pattern recognition [1,2], among which organic PR materials are considered to be the most promising since they possess unique advantages such as the compositional flexibility, ease of fabrication, low dielectric constant, and low cost compared to inorganic crystalline materials [3,4]. In the past few years, organic glasses were investigated as a new class of PR materials. Compared to polymeric systems, organic glasses do not need complicated chem-
T Corresponding author. Tel./fax: +86 371 7763561. E-mail address: [email protected] (S. Cao). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.02.066
ical synthesis or purification processes and also have a well-defined structure [5,6]. The high chromophore concentration is evidently an advantage for the photoconducting process, since the overlapping of wave functions in photoconductive chromophore strongly enhances the photo-conducting process. In this work, we report a series of monolithic PR organic molecules, some of which exhibited large net gain. These molecules (M1– M6, refer to Scheme 1) were considered to show photoconductivity due to the carbazole structure and also the electro-optic property due to the push–pull structure [7–9]. An ethylhexyl group was introduced to minimize the extent of crystallinity of the materials. The synthesis, thermal properties and TBC experiment of these monolithic molecules are described in this paper. The carbazole molecules containing push–pull structure are also considered to be potential candidates for electronic device, such as color display, organic light-emitting diode, organic semiconductor laser, solar cell, etc. [10,11]. Therefore, the
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J. Shi et al. / Materials Letters 59 (2005) 2199–2203
H N
r.t, 4h Cl 2 CH 2
B
4h
CN CH2 , piperidine CN
t, 2
2 NO
N
N
100 °C, 2h
A
BS aD ,N
+
DMF, POCl3
N
Br, NaH/DMF
, r.
N2
CHO
CN
C2H5OH, b.t, 6h
CH
(M5) N
(M1)
N
C
CN
N NO2
CN CH2
NO2
, piperidine
N
(M2) CN
C2H5OH, b.t, 2h
CH
C NO2
CN CH2
COOCH3 , piperidine
N
(M3) CN
C2H5OH, b.t, 6h
CH
C
COOCH3
COOH CH2 NO2
, piperidine
N (M4)
C2H5OH, b.t, 4.5h
CH
CH NO2
CN O
CN CN
, NaOH
N (M6)
C2H5OH, b.t, 6.5h
CH
CH
CN CN O
CN
Scheme 1. Synthetic route for the monolithic materials.
absorption spectra of these molecules are also discussed here.
reagents are analytical pure and used as received except that the solvents and piperidine were purified before using. Materials were conformed as the expected structure through characterizations using IR and 1H-NMR.
2. Experimental section 2.1. Measurement FTIR spectra were recorded on a Nicolet-460 FTIR spectrometer. 1H-NMR spectra were obtained from a Bruker DRX-400 NMR spectrometer using CDCl3 as solvent. UV/ vis spectra were recorded with a Shimadzu 3010 UV/vis spectrometer using THF as solvent. Differential scanning calorimetry (DSC) analysis was performed on a Netzsch 204 differential scanning calorimeter, experimental condition refers to Fig. 3.
2.2.1. Compound A (Eh-Cz) A solution of 10 g (0.06 mol) of carbazole, 4 g (60%) NaH and 13.52 g(0.07 mol) 2-ethylhexyl bromide in 40 ml of DMF was stirred under argon atmosphere at room temperature for 4 h. This mixture was then poured into a large excess of water, extracted with hexane and dried over Na2SO4. Solvent was evaporated under reduced pressure. Crude product was then purified by column chromagraphy on silica gel using hexane as eluent. 8.2 g of product (49% yield) was obtained as a colorless viscous fluid. 1H-NMR, y (CDCl3), 0.83(d, 3H), 0.89(d, 3H), 1.2–1.4(m, 8H), 2.06(t, 1H), 4.15(m, 2H), 7.19–7.47(m, 6H), 8.10(d, 2H).
2.2. Synthesis The overall synthetic route and structure of the carbazole-based compounds are illustrated in Scheme 1. All
2.2.2. Compound B A total of 3.38 g (0.022 mol) of POCl3 was slowly added to a stirred solution of 2.8 g (0.01 mol) compound A in 10
J. Shi et al. / Materials Letters 59 (2005) 2199–2203
2201
4H), 8.1(d, 1H), 8.2(m, 2H). IR (KBr): 1522 and 1341 cm1(NO2). M6, dark purple powder, yield: 75%. 1H-NMR: y (in CDCl3), 0.86(d, 3H), 0.93(d, 3H), 1.2–1.4(m, 8H), 1.8(s, 6H), 2.1(m, 1H), 4.2(d, 2H), 7.1(d, 1H), 7.3(t, 1H), 7.4(d, 2H), 7.5(t, 1H), 7.7(d, 1H), 7.8(d, 1H), 8.1(d, 1H), 8.3(s, 1H). IR (KBr): 2200 cm1(CN).
ml of DMF. Subsequently the solution was stirred at 100 8C for 2 h, and then poured into ice water. The resulting mixture was extracted with chloroform. After being dried over MgSO4, solvent was removed by evaporation under reduced pressure. Product was obtained in 1.48 g (48% yield) as a light yellow viscous fluid through purification with column chromatography over silica gel using chloroform as eluent. 1H-NMR: y (in CDCl3), 0.83(d, 3H), 0.89(d, 3H), 1.2–1.4(m, 8H), 2.05(t, 1H), 4.18(m, 2H), 7.25–8.6(m, 7H), 10.08(s, 1H). 2.2.3. Compound M1 Glacial acetic acid containing piperidine (piperidine: glacial acetic acid=1:3) was slowly added to a stirred solution of 0.28 ml compound B and 2.3 mmol malononitrile in 8 ml of dry ethanol. Subsequently the solution was refluxed for 6 h and then cooled down to room temperature. A pale yellow solid was isolated through filtration. Product was obtained through recrystallization from ethanol. Yield: 66%. 1H-NMR: y (in CDCl3), 0.85(d, 3H), 0.92(d, 3H), 1.2– 1.4(m, 8H), 2.05(m, 1H), 4.2(d, 2H), 7.3(t, 1H), 7.4(d, 2H), 7.5(t, 1H), 7.8(s, 1H), 8.0(t, 1H), 8.1(t, 1H), 8.6(t, 1H). IR (KBr): 2200 cm1(CN). Compounds M2, M3, M4 and M6 were prepared in a similar manner with M1 using 4-nitrophenylacetonitrile for M2, methyl cyanoacetate for M3, 4-nitrophenylacetic acid for M4 and 2-dicyanomethylene-3-cyano-5,5-dimethyl-2,5dihydro furan for M6, respectively. M2, orange powder, yield: 60%. 1H-NMR: y (in CDCl3), 0.86(d, 3H), 0.93(d, 3H), 1.2–1.4(m, 8H), 2.07(m, 1H), 4.2(d, 2H), 7.3(t, 1H), 7.4(m, 3H), 7.8(d, 3H), 8.1(t, 2H), 8.3(d, 2H), 8.7(s, 1H). IR (KBr): 2200 cm1 (CN), 1520 and 1340 cm1 (NO2). M3, yellow powder, yield: 78%. 1H-NMR: y (in CDCl3), 0.85(d, 3H), 0.92(d, 3H), 1.2–1.4(m, 8H), 2.05(m, 1H), 3.9(s, 3H), 4.2(d, 2H), 7.3(m, 1H), 7.4(m, 2H), 7.5(t, 1H), 8.1(d, 1H), 8.2(d, 1H), 8.4(s, 1H), 8.7(s, 1H). IR (KBr): 2200 cm1(CN), 1720 cm1(COO). M4, orange powder, yield: 40%. 1H-NMR: y (in CDCl3), 0.86(d, 3H), 0.9(d, 3H), 1.2–1.5(m, 8H), 2.1(m, 1H), 4.1(d, 2H), 7.1(s, 1H), 7.2(s, 1H), 7.4(m, 2H), 7.4(m, 2H), 7.6(d,
2.3. Sample preparation The sample for photorefractive measurement was prepared by putting the compounds onto a indium tin oxide (ITO) glass plate and heating the ITO glass plate to the Melting temperature (M p) of the compounds. Then another ITO glass plate was covered. Sample thickness was controlled to be 80 Am through a Teflon spacer.
M6
M5 M4
Absorbance
Fig. 1. Experimental setup for two-beam coupling and four-wave-mixing.
2.2.4. Compound M5 4-Nitroaniline (5 mmol) was dissolved in a solution of concentrated HCl (2.5 ml) in water (75 ml). The mixture was cooled to lower than 4 8C with an ice bath, then an aqueous solution containing sodium nitrite (0.06 g) was slowly added, the mixture was kept stirring for further 30 min. Sodium dodecyl benzenesulfonate (NaDBS) (1 g) and a solution of compound B (1.4 g) in dichloromethane (50 ml) were added dropwise. Resultant mixture was vigorously stirred at room temperature for 24 h. Ethanol (200 ml) was added and the mixture was heated to remove dichloromethane. The red precipitate was filtered, washed with water, and air-dried. The solid purified by column chromatography over silica gel using chloroform as the eluent, yielding 1.48 g (40%) of dark red solid. 1H-NMR: y (in CDCl3): 0.83(d, 3H), 0.89(d, 3H), 1.2–1.4(m, 8H), 2.06(t, 1H), 4.15(m, 2H), 7.19–7.47(m, 6H), 8.10–8.70(m, 5H). IR (KBr): 1521 and 1342 cm1(NO2), 1596 cm1(NMN).
M3
M2
M1
Eh-Cz 300
400
500
600
Wavelength (nm) Fig. 2. UV/vis spectra of the monolithic molecules and Eh-Cz in 0.1 Amol/L THF solution.
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J. Shi et al. / Materials Letters 59 (2005) 2199–2203
2.4. PR measurement
Table 1 Thermal and optical properties of monolithic molecules
PR performance was characterized by two-beam coupling (TBC) and four-wave-mixing (FWM) experiments measurement. The experimental setup is depicted in literature [12] and also described in Fig. 1. In TBC experiment, the coherent polarized He–Ne laser beams (k = 632.8 nm) overlapped on the sample. The power of beam 1 and beam 2 was measured to be 15 mw and 10 mw, respectively. The TBC coefficient C can be calculated by the following expression:
Sample
k max (nm)
T g (8C)
M p (8C)
T rec (8C)
C 633 (cm1)
M1 M2 M3 M4 M5 M6
406 410 396 409 437 500
2 23 0 2 2 69
81 151 77 141 79 182
– 51.94 – – – 130
8 266 11 10 8 –
C¼
1 ðlnðc0 bÞ lnðb þ 1 c0 ÞÞ d
Where d is the optical path length, b is the ratio of the writing beam intensity, and c 0 = I/I 0 is the beam-coupling ratio where I 0 is the signal intensity without the pump beam and I is the signal intensity with the pump beam.
3. Results and discussion Absorption spectra of the six compounds are presented in Fig. 2 together with the starting material Eh-Cz. Pure Eh-Cz didn’t show any characteristic absorption beyond 350 nm. All the other samples exhibit a relatively broad absorption peaking at 410 nm, 422 nm, 400 nm, 417 nm, 439 nm and 510 nm, respectively for compounds M1–M6. From M1 to M6, the absorption maximum is gradually red-shifted, which is derived from the internal charge transfer between the electron-donating carbazole nucleus and the electronwithdrawing methine or azo groups. The dipole strength between the Eh-Cz and methine or azo groups reveals in fact the electron-withdrawing ability of the functional groups in the chromophore. Among the six molecules, the absorption of compound M6 is most red-shifted, which is due to the
69°C
strong electron-withdrawing ability of the three cyano groups. Such dipolar features have already been exploited in nonlinear optics (NLO) [13] and photorefractive [14] experiments. Fig. 3 shows the DSC curves of the compounds from M1 to M6. All the six compounds studied can form a glassy state upon quenching from their melt. Glass transition temperature (T g ) of these samples varies from 2 8C to 69 8C. All the thermal transition temperatures measured by DSC are summarized in Table 1. M p was recorded from the first heating scan of the initial powdered samples. T g was measured from the quenched samples from their melt. For compounds M2 and M6, re-crystallization can be observed from the second heating scan of the quenched samples, the corresponding transition is reported as Trec in Table 1. A melting endotherm in accordance with the re-crystallization was also observed in the DSC measurement of compounds M2 and M6. Amongst the six compounds studied, compounds M2, M3 and M5 can form stable films with excellent optical quality that did not crystallize over the time frame of two months so far. Compound M1 can also form an optical film, but eventually crystallized over several days. Compounds M4 and M6 can not form a film with optical quality. The PR performance was characterized by TBC and FWM experiments. TBC measurements revealed that the compounds we synthesized have PR effect in some extent. Therefore, these compounds can be regarded as monolithic PR materials. The best PR performance was found for
M6
23°C
1.8
M2
Beam 1 1.6
M1
-2°C
M5
2°C
M4
0°C M3
1.4
Intensity (I/I0)
Exo
-2°C
1.2 1.0 0.8
-30
0
30
60
90
120
150
180
Temperature (°C) Fig. 3. DSC curves of quenched M1–M6 samples (T g and T rec are determined by heating samples at 10 8C/min that had been previously melted and then quenched to glasses).
Beam 2
0.6 0
200
400
600
800
1000
Time (s) Fig. 4. Energy transfer in two-beam coupling experiment of compound M2.
J. Shi et al. / Materials Letters 59 (2005) 2199–2203
compound M2, which exhibits gain efficient as high as 266 cm1 at electric fields of 42 V/Am without any extra photosensitizer while the FWM efficiency was measured to be 5% at electric fields of 27 V/Am. Fig. 4 shows the TBC energy transfer of M2. While for compounds M1, M3, M4 and M5, their gain coefficients are only around 10 cm1, even with 1% or 2% TNF was added as a photo-sensitizer. We originally expected that the compound M6 would show a good PR performance at near infrared region. While, for the poor film forming ability and no suitable laser instrument available, we did not measure the PR performance for this molecule. Results for PR measurements of these monolithic materials are also summarized in Table 1. For the excellent PR performance of compound M2, we considered that there may be two factors. Firstly, the introduction of a cyano group in the chromophore of M2 may break the molecular symmetry, which may impart the monolithic molecule with a good glassy property [15]. Secondly, in many organic glasses, the best temperature region to exhibit good PR performance was found to be from T g to T g +(2–3) 8C, in which the steady-state properties are still good, and the dynamic properties are significantly improved [16]. It means that T g of the glass, which is around room temperature, is suitable for a PR material. T g of compound M2 is 23 8C, which is very close to room temperature. While, for the rest molecules, their T g is far below the room temperature. The TBC coefficient and diffraction efficiency perhaps decreased as the temperature increase above T g. The decrease was attributed to thermal disruption of chromophore alignment, significantly increased dark conductivity, and reduced trap densities at elevated temperature.
4. Conclusions We have synthesized a series of 9-(2-ethyl-hexyl)carbazole(Eh-Cz)-based molecules (M1–M6) as monolithic photorefractive materials forming low T g (ranging from 2 8C to 69 8C)organic glasses via azo-coupling-reaction or Knovenagel condensation reaction. The compounds showed distinct photoconductivity due to the carbazole moity and the optical nonlinearity by the push–pull structure, among which a molecule with a highly asymmetric structure using both a nitrobenzene and a cyano group as the electron-
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withdrawing components (M2) shows the best photorefractive performance, its gain coefficient was measured as high as 266 cm1 at an applied electric field of 42 V/Am without any extra sensitizer while the FWM efficiency was measured to be 5% at electric fields of 27 V/Am. UV/vis spectra showed that depending on the strength of the donor– acceptor internal charge transfer the absorption spectra are more or less red-shifted.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China, project 20274042.
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