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Effect of the substituting groups and their positions on the optical properties of phenylacetylenic compounds
Materials Chemistry and Physics 96 (2006) 283–288
Effect of the substituting groups and their positions on the optical properties of phenylacetylenic compounds Wei Zhang, Peng Cheng Huang ∗ Department of Polymer Materials and Composites, School of Materials Science and Engineering, Beihang University, Beijing 100083, PR China Received 28 March 2005; received in revised form 10 June 2005; accepted 14 July 2005
Abstract A series of phenylacetylenic compounds were synthesized. Some substituting groups with different electronic effects were introduced into the phenylene core of the m-bis(phenylethynylene) benzene (3a) at different positions. The introduction of Ph-C C, Br-, NO2 - or NH2 - into the 5-(meta-) position of 3a do not significantly increase the positive value of Qd , the charge density of the carbon atom which connects the phenylene core and the phenylethynylene branch in 3a, and the conjugation is still interrupted by the phenylene core and their UV absorptions are similar to that of 3a. Introducing NH2 - into the 4-position of 3a decreases the Qd from positive to negative value, which makes the UV absorption blue-shifting. However, introducing NO2 - into 4-positon of 3a increases the positive value of Qd much significantly, which makes the conjugation extending to the whole molecule and the UV absorption red-shifting obviously. The compound 4a with a NH2 - substituent in the 4-position of 3a can be used as a blue-light-emitting material. © 2005 Elsevier B.V. All rights reserved. Keywords: Blue-light-emitting material; Optical property; Synthesis; Phenylacetylenic compounds
1. Introduction Recently, the phenylacetylenic compounds have attracted much attention due to their special electronic and optical properties. For para-connected phenylacetylenic compounds, the fluorescence wavelength progressively redshifted to the region of visible light with the elongation of the molecular chain [1]. But their solubility decreases sharply even when the molecular chain only increases one unit. This makes them hard to be processed. One way to solve this problem is the use of meta-connected phenylacetylenic compounds. The meta-connected phenylacetylenic compounds have less symmetry and better solubility, so they can be processed much more easily. But their optical properties are limited because the meta-connections interrupt the conjugation [2–7]. Therefore, if a way can be found to eliminate the interruption, it is possible to get a soluble phenylacetylenic compound, which can emit visible lights. ∗
Corresponding author. Tel.: +86 10 82317123; fax: +86 10 82317127. E-mail address: [email protected] (P.C. Huang).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.07.014
In this paper, a series of phenylacetylenic compounds (Plate 1Chart 1) with different electron-donor and acceptor groups at different positions of m-bis(phenylethynyl)benzene were synthesized. The effects of the substituting groups and their positions on the UV absorption and fluorescence (FL) properties of the phenylacetylenic compounds were studied. And the effects of Qd (the charge density of the carbon atoms that connect the phenylene core and the phenylethynylene branch) on the conjugation systems in the phenylacetylenic compounds were discussed.
2. Experimental 2.1. General methods 1H
NMR spectra were measured by a JNM-AL300FT NMR SYSTEM in CDCl3 . IR spectra were measured by a NEXUS-470 FTIR (Nicolet). UV–vis spectra were measured by a Cintra 10e UV–vis Spettrumeter in cyclohexane or chloroform. Fluorescence spectra were measured by a RF-530
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2.4. 1,4-Bis(phenylethynyl)benzene (2) 2 was synthesized following the general procedure with triethylamine (50 ml), 1,4-dibromobenzene (2.359 g, 10 mmol), Pd2 (dba)3 (184 mg, 0.20 mmol), CuI (77 mg, 0.40 mmol), PPh3 (534 mg, 2.0 mmol) and phenylacetylene (2 ml, 18 mmol). 1.7 g white powders (yield 61%) were obtained by the recrystallization of the crude product from toluene. m.p. 183.5–185.5 ◦ C. 1 H NMR: δ7.56–7.51 (m, 8 H), 7.36–7.26 (m, 6 H). 2.5. 1,3,5-Tris(phenylethynyl)benzene (3b) 3b was synthesized following the general procedure with triethylamine (15 ml), 1,3,5-tribromobenzene (1.664 g, 5.3 mmol), pyridine (23 ml), Pd(PPh3 )2 Cl2 (112 mg, 0.16 mmol), PPh3 (222 mg, 0.85 mmol), CuI (102 mg, 0.54 mmol) and phenylacetylene (2.00 g, 19.6 mmol). The crude product was recrystallized from isopropanol to afford white needles (811mg, yield 41%). m.p. 146.8–148 ◦ C. 1 H NMR: δ7.64 (s, 3H), 7.52 (d, J = 6.6 Hz, 6H), 7.368 (m, 9H). Chart 1. This is a sample for exhibit.
PC Spectroflorophotometer in cyclohexane or chloroform. The ground-state configurations of the phenylacetylenic compounds were optimized with the semi-empirical AM1 method and the frontier orbital diagrams and their energies were calculated with ZINDO/S method using Hperchem7.
2.2. Materials Pyridine and triethylamine were freshly distilled before use. 1,4-Dibromobenzene, 1,3,5-tribromobenzene, 2,6dibromo-4-nitro-aniline and 2,4-dibromoaniline are all commercially available and used as received.
2.3. General procedure for synthesizing phenylacetylenic compounds Nitrogen bubbled through the pyridine and/or triethylamine solution for about 1 h, then bromobenzene, Pd catalyst, PPh3 and CuI were added. The solution was bubbled for another 30 min, and phenylacetylene was added to the solution. Then, the reaction was carried out at 85 ◦ C and monitored by TLC. After the reaction, mixture was cooled down to room temperature. The solid in the mixture was removed by suction filtration and washed thoroughly with CH2 Cl2 . Then, the combined filtrate was washed with water and dried with anhydrous MgSO4 . After the solvent was removed, the product was purified by column chromatography on silica gel.
2.6. 3,5-Bis(phenylethynyl)-1-bromobenzene (3c) 3c was synthesized following the general procedure with Triethylamine (30 ml), pyridine (15 ml), 1,3,5tribromobenzene (1.891 g, 6.0 mmol), PPh3 (253mg, 0.97 mmol), Pd(PPh3 )2 Cl2 (126 mg, 0.18 mmol), CuI (115 mg, 0.60 mmol) and phenylacetylene (1.227 g, 12 mmol). The crude product was purified by column chromatography with petroleum ether as eluent to afford a white needles of 3c (1.084g, yield 51%). m.p. 88.9–90.4 ◦ C. 1 H NMR: δ7.63 (s, 3H), 7.54–7.51 (m, 4H), 7.38–7.36 (m, 6H). 2.7. 3,5-Bis(phenylethynyl)nitrobenzene (3e) A suspension of 2,6-dibromo-4-nitroaniline (25.08 g, 85 mmol) in glacial acetic acid (325 ml) was added slowly at a temperature not above 20 ◦ C to a solution of sodium nitrite (7.64 g, 110 mmol) in concentrated sulfuric acid (90 ml). After stirring for 80 min, the reaction mixture was then slowly added to a suspension of cuprous oxide (15.76 g, 110 mmol) in 95% ethanol (250 ml). After stirring overnight, the reaction was quenched with water. The organic layer was separated, washed with saturated NaHCO3 , dried with MgSO4 , and concentrated to afford a green solid. The crude product was recrystallized from 95% ethanol to produce a pure green crystals of 3,5-dibromo-nitrobenzene (16.08 g, yield 68%). m.p. 106.9–108.4 ◦ C. 3e was synthesized following the general procedure with triethylamine (12 ml), pyridine (8 ml), 3,5-dibromonitrobenzene (1.126 g, 4.0 mmol), CuI (53 mg, 0.28 mmol), PPh3 (118 mg, 0.44 mmol), Pd(PPh3 )2 Cl2 (58 mg, 0.08 mmol) and phenylacetylene (996 mg, 9.76 mmol). The crude product was purified by column chromatography
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with petroleum ether/CH2 Cl2 (8:1) as eluent followed by recrystallization from 95% ethanol to afford yellow needles of 3e (895 mg, yield 69%). m.p. 139.5–140.0 ◦ C; 1 H NMR: δ8.30–8.30 (d, J = 0.7 Hz, 2H), 7.97–7.96 (m, 1H), 7.58–7.55 (m, 4H), 7.41–7.37 (m, 6H); IR: 3081, 2213, 1535, 1355, 1596–1442, 810, 757, 687 and 665 cm−1 . 2.8. 3,5-Bis(phenylethynyl)aniline (3d) A solution of 3,5-bis(phenylethynyl)nitrobenzene (648 mg, 2 mmol), stannous chloride dihydrate (2.268 g, 10 mmol) in 95% ethanol was heated at 75 ◦ C for 100 min under nitrogen. Upon cooling, 20% aqueous NaOH was added to the solution. The aqueous layer was extracted with 150 ml ether three times. The combined organic extracts were dried with MgSO4 , filtered, and concentrated under reduced pressure followed by recrystallization from 95% ethanol to afford pure yellow crystals (540 mg, yield 92%). m.p. 132.1–132.5 ◦ C; 1 HNMR: δ7.54–7.51 (m, 4H), 7.36–7.34 (m, 6H), 7.195 (s, 1H), 6.897 (s, 2H), 1.550 (s, 2H); IR: 3472, 3203, 3056, 2210, 1586, 1625–1489, 1235, 852, 757 and 689 cm−1 . 2.9. 3,5-Bis(phenylethynyl)benzene (3a) The procedure was the same as that used for preparing 3,5dibromo-nitrobenzene and 90 mg (1.3 mmol) sodium nitrite, 2 ml concentrated sulfuric acid, 293 mg (1.0 mmol) 3,5bis(phenylethynyl)aniline, 4 ml glacial acetic acid, 188 mg (1.3 mmol) cuprous oxide and 5 ml 95% ethanol were used. The crude product was purified by column chromatography with petroleum ether as the eluent to afford 58 mg white needles (yield 21%). m.p. 110–112 ◦ C; 1 H NMR: δ7.72 (s, 1H), 7.56–7.48 (m, 6H), 7.37–7.35 (m, 7H). 2.10. 2,4-Bis(phenylethynyl)-nitrobenzene (4b) To a solution of 85% H2 O2 (1.08 g, 25 mmol) in CH2 Cl2 (17.5 ml) was added the maleic anhydride (3.06 g, 31 mmol) powder. The mixture was stirred for 50 min until most of the solids were disappeared. Then, it was heated at 45 ◦ C and the 2,4-dibromoaniline was added. After 90 min, the mixture was cooled to room temperature, washed with saturated NaHCO3 and NaCl aqueous solution, dried with MgSO4 , filtered and concentrated under reduced pressure. The obtained crude product was separated by column chromatography with petroleum ether to afford yellow crystals of 2,4-dibromonitrobenzene (1.014 g, yield 72%). m.p. 59.7–61.9 ◦ C. 4b was synthesized following the general procedure with triethylamine (9 ml), pyridine (7 ml), 2,4-dibromonitrobenzene (874 mg, 3.0 mmol), CuI (41 mg, 0.21 mmol), PPh3 (87 mg, 0.33 mmol), Pd(PPh3 )2 Cl2 (42 mg, 0.06 mmol) and phenylacetylene (746 mg, 7.2 mmol). The crude product was purified by column chromatography with petroleum ether/CH2 Cl2 (6:1) to afford orange crystals (614 mg, yield 63%). m.p. 102.2–103.8 ◦ C; 1 H NMR: δ8.10 (d, J = 4.2 Hz,
285
1H), 7.87 (d, J = 0.9 Hz, 1H), 7.63–7.55 (m, 5H), 7.41–7.38 (m, 6H); IR: 3060, 2209, 1573, 1334, 903, 839, 756 and 688 cm−1 . 2.11. 2,4-Bis(phenylethynyl)aniline (4a) The procedure was the same as that used for preparing 1,3-bis(phenylethynyl)aniline, and 8 ml 95% ethanol, 420 mg (1.3 mmol) 4b and 1.469 g (6.5 mmol) stannous chloride dihydrate were used. The crude product was recrystallized from 95% ethanol to afford 312 mg yellow crystals (82%). m.p. 147.1–149.2 ◦ C; 1 HNMR: δ8.09 (d, J = 0.9 Hz, 1H), 7.86–7.83 (m, 1H), 7.56–7.52 (m, 2H), 7.38–7.24 (m, 6H), 6.71 (d, J = 4.2 Hz, 1H), 4.75 (s, 1H), 4.20 (s, 1H); IR: 3450, 3307, 3057, 2206, 1664, 1617, 1579, 1502, 1347, 746 and 685 cm−1 .
3. Results and discussion The UV and FL spectra of 2 and 3a are given in Fig. 1. 2 and 3a are p- and m-bis(phenylethynyl)benzenes, respectively. As shown in Fig. 1a, the maximum absorbing wavelength, λmax of 2 is red-shifted by about 40 nm from 3a. The UV absorption of o-bis(phenylethynyl)benzene (1) is also red-shifted by 28 nm from 3a [2]. Moreover, it can be seen from Table 1 that the energy gap between HOMO and LUMO (E) of 1 and 2 are 6.874 and 7.043 eV, respectively, which are much lower than that of 3a (7.338 eV). The HOMO and LUMO orbitals of 3a are both interrupted by C2 and C5 carbon atoms (Chart 1) in the phenylene core (Table 1), and the electrons are localized on each phenylethynyl branches. But in 1 and 2, the electrons are delocalized on the whole molecule. The conjugation can be extended to the whole molecule when the two phenylethynyl branches are para- or ortho-connected to the phenylene core (as in 1 and 2), but is interrupted by the phenylene core and restricted in each phenylethynyl branches when the two phenylethylnyl branches are meta-connected to the phenylene core (as in 3a). The reason resulting this has not been discussed in the literature. One possible explanation is that the conjugation being extended to the whole molecule or being interrupted by the phenylene core is related to the charge density Qd of the carbon atom in the phenylene core, which connects the phenylene core with the phenylethynyl branches. Herein, Qd is the charge density of C1 and C3 in 3a–3e, 4a and 4b, and the charge density of C1 and C4 in 2, etc. (see Chart 1). The Qd of 3a, 2 and 1 are +0.029, +0.038 and +0.054, respectively (see Table 1). Increasing the positive value of Qd is in favor of withdrawing the electrons from the phenylethynyl branches to the phenylene core, and consequently in favor of extending the conjugation to the whole molecule. On the other hand, the configuration of the molecule also influences its electronic energy level. When the benzene rings on the end of the molecule rotate to the different angels with the phenylene core, its molecular orbitals and the conjugation
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286
Fig. 1. UV absorption (a) and FL emission (b) spectra of 3a and 2 (in chloroform, 10−5 M).
system in the molecule may be changed. This will also affect the optical properties of the phenylacetylenic compounds. However, the above calculations on Qd are based on the most stable configuration of the molecule, that is, the two benzene rings on the end are in the same plane with the phenylene core. So, the influence of the molecular configurations is not considered, and the molecules are regarded as totally planar structure in the dilute solution. From the above, it can be speculated that if an appropriate substituent is introduced into the phenylene core of 3a to increase the positive value of Qd , it is possible to make the conjugation in the substituted 3a extending to the whole molecule and finally obtain a meta-connected phenylacetylenic compound emitting visible light. 3.1. Substituting groups with different electronic effect on the meta-position 3b–3e are the derivatives of 3a with a phenyethynyl-, bromo-, amino- or a nitro-group attached on the 5-(meta-)
positon of 3a, respectively. Their UV absorption and FL emission spectra are shown in Fig. 2. The shapes and peak positions of the UV spectra of 3b–3e are all similar to 3a except that 3d and 3e both have another red-shifted weak absorption bands. However, their FL spectra are red-shifted from 3a according to the order: 3e, 3c, 3b and 3d. In 3b, the third phenylethynyl branch attached on the metaposition of 3a does not affect the UV absorption wavelength. The Qd of 3b is the same as 3a, and the electrons are still localized on each phenylethynyl branches. But its FL spectrum is red-shifted from 3a by 23 nm because the additional phenylethynyl branch enhances the coupling between the phenylethynyl branches, allowing the orbital energy to be further decreased [3,4]. The electron-withdrawn effect of the nitro-group increases the positive value of Qd from +0.029 of 3a to +0.037 of 3e, but this does not allow the conjugation to extend from one phenylethynyl branch to another. The p– conjugation between the nitro-group and the phenylene core results a weak absorption band in 320–360 nm. Therefore, 3e has an
Table 1 Frontier orbitals of 2, 3a and 5* E (eV)
Qd
2
6.874
+0.038
3a
7.338
+0.029
5
7.043
+0.054
HOMO
LUMO
* Ground-state configurations of the phenylacetylenic compounds are optimized by the semi-empirical AM1 method. HOMO and LUMO orbital diagrams and Es are calculated by the semi-empirical ZINDO/S method. And Qd are calculated by the AM1 method.
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287
Fig. 2. UV absorption (a) and FL emission (b) spectra of 3a–3e (in cyclohexane, 10−5 M).
absorption band in 250–310 nm similar to 3a and a weak absorption band in 320–360 nm. Because the nitro-group makes the electrons on the phenylene core flowing to the meta-position, which disturbs the coupling between two phenylethynyl branches in the excited state, its FL spectra is only red-shifted from 3a by 6 nm. The UV absorption of bromo-substituted 3c was the same as 3a because of the balance between the electron-withdrawn effect of Br- and the electron-donor effect caused by the p– conjugation between Br and the benzene ring. And the electron-withdrawn effect of Br- is weaker than nitro-group, so the red-shift of its FL spectra is a little greater than 3e. For 3d, the amino group increases the positive value of Qd from +0.029 of 3a to +0.064 of 3d. But this still does not fundamentally alter the UV absorption of 3d, except for resulting a red-shifted weak absorption band in 325–370 nm by the p– conjugation between the amino-group and the phenylene core. The FL spectrum of 3d is red-shifted from 3a by 38 nm, because the amino-group allows the electrons on the phenylene core flowing away from the meta-position,
which will be in favor of the electron coupling between the phenylethynyl branches in the excited state. 3.2. Substituting groups with different electronic effect on the 4-position 4a and 4b are the 4-substituted m-bis(phenylethynyl)benzenes (see Chart 1). Their UV absorption and FL emission spectra are shown in Fig. 3. As can be seen in Fig. 3, both of their UV absorptions are changed a lot as compared with 3a. The amino-group in 4a greatly increases the negative value of Qd (−0.029 for the carbon atom C1 in the para-position of -NH2 and −0.054 for the carbon atom C3 in the ortho-position of -NH2 , see Chart 1), so the main absorption band of 4a is blue-shifted to about 276 nm. At the mean time, the p– conjugation between the amino-group and the phenylene core results in a absorption band greater than 300 nm. The nitro-group in 4b increases the positive value of Qd significantly from +0.029 of 3a to +0.086 (for the carbon atom C1 in the para-position of -NO2 )
Fig. 3. UV absorption (a) and FL emission (b) spectra of 3a, 4a and 4b (in cyclohexane, 10−5 M).
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and +0.121 (for the carbon atom C3 in the ortho-position of NO2 ) of 4b, the large positive charge withdraw the electrons flowing from phenylethynyl branches to the penylene core and makes the conjugation extending to the whole molecule. Therefore, the maximum absorption peak of 4b is red-shifted from 3a by about 20 nm, and a broad peak appears at 328 nm. The FL spectra of 4a and 4b are both red-shifted in comparison with 3a obviously. This indicates that introducing amino-group into the 4-positon of 3a can significantly increase the coupling between the phenylethynyl branches in the excited state, and introducing nitro-group into the 4-position of 3a can extend the conjugation to the whole molecule. Both decrease the orbital energy and make the FL emission of 4a and 4b red-shifted from 3a. It is particularly interesting that the FL emission of 4a has red-shifted to the region of visible light, which will allow it to be used as a fluorescence material emitting blue light. In the experiments, it can be found that the solubility of the p-phenylacetylenic compound is much lower than that of the m-phenylacetylenic compounds. 3 and 4 are soluble in most of the organic solvent, such as petroleum ether, benzene, chloroform, ethanol and so on, while 2 is partly soluble in non-polar solvent, such as petroleum ether. If another phenylacetylenic unit is introduced into 2 and 3a, that is 1,4-bis[4-(phenylethynyl)phenyl]acetylene and 1,3,5tris(phenylethynyl)benzene(3b), the former one is insoluble in non-polar solvent, such as petroleum ether, and partly soluble in chloroform, while the latter one can still be soluble in most of the organic solvent. In addition, the melting points of the m-phenylacetylenic compounds are lower than the p-phenylacetylenic compounds. So the m-phenylacetylenic compounds are obviously in great advantages of good solubility and processibility.
4. Conclusion The reason that the conjugation is interrupted by the phenylene core in the m-bis(phenylethynyl)benzene (3a) but can be extended to the whole molecule in the p- or obis(phenylethynyl)benzene may relate to the charge density
Qd (the charge density of the carbon atom which connects the phenylene core and the phenylethynyl branch in the bis(phenylethynyl)benzenes). Increasing the positive value of Qd can make the electron more easily flowing from phenylethynyl branch to the phenylene core and the conjugation more easily being extended to the whole molecule. The substitution of hydrogen in 5-position (meta-position to the phenylacetylenic branch) of 3a by Ph-C C-, Br-, NO2 - or NH2 - do not increase the positive value of Qd significantly, the conjugation is still interrupted by the phenylene core and their UV absorptions are similar to that of 3a. Introducing NH2 into the 4-position (ortho-position to the phenylacetylenic branch) of 3a decreases the charge density Qd from positive value to the negative value, which makes the UV absorption of 4a blue-shifting from that of 3a. However, introducing NO2 - into the 4-position of 3a increases the positive value of Qd much significantly, which makes the conjugation in 4b extending to the whole molecule and the UV absorption redshifting from 3a obviously. The compound 4a can be used as a light-emitting material.
Acknowledgement We thank the National Nature Science Foundation of China (NSFC, No. 29974002) for support of this research.
References [1] U.H.F. Bunz, Chem. Rev. 100 (2000) 1605–1644. [2] J.S. Melinger, Y. Pan, V.D. Kleiman, Z. Peng, B.L. Davis, D. McMorrow, M. Lu, J. Am. Chem. Soc. 124 (2002) 12002– 12012. [3] K.M. Gaab, A.L. Thompson, J. Xu, T.J. Martinez, C.J. Bardeen, J. Am. Chem. Soc. 125 (31) (2003) 9288–9289. [4] A.L. Thompson, K.M. Gabb, J. Xu, C.J. Bardeen, T.J. Martinez, J. Phys. Chem. A 108 (2004) 671–682. [5] S. Tretiak, V. Chernyak, S. Mukamel, J. Phys. Chem. B 102 (1998) 3310–3315. [6] R. Kopelman, M. Shortreed, Z. Shi, W. Tan, Z. Xu, J.S. Moore, Phys. Rev. Lett. 78 (1997) 1239–1242. [7] K. Harigaya, Phys. Chem. Chem. Phys. 1 (1999) 1687–1689.
Effect of the substituting groups and their positions on the optical properties of phenylacetylenic compounds Wei Zhang, Peng Cheng Huang ∗ Department of Polymer Materials and Composites, School of Materials Science and Engineering, Beihang University, Beijing 100083, PR China Received 28 March 2005; received in revised form 10 June 2005; accepted 14 July 2005
Abstract A series of phenylacetylenic compounds were synthesized. Some substituting groups with different electronic effects were introduced into the phenylene core of the m-bis(phenylethynylene) benzene (3a) at different positions. The introduction of Ph-C C, Br-, NO2 - or NH2 - into the 5-(meta-) position of 3a do not significantly increase the positive value of Qd , the charge density of the carbon atom which connects the phenylene core and the phenylethynylene branch in 3a, and the conjugation is still interrupted by the phenylene core and their UV absorptions are similar to that of 3a. Introducing NH2 - into the 4-position of 3a decreases the Qd from positive to negative value, which makes the UV absorption blue-shifting. However, introducing NO2 - into 4-positon of 3a increases the positive value of Qd much significantly, which makes the conjugation extending to the whole molecule and the UV absorption red-shifting obviously. The compound 4a with a NH2 - substituent in the 4-position of 3a can be used as a blue-light-emitting material. © 2005 Elsevier B.V. All rights reserved. Keywords: Blue-light-emitting material; Optical property; Synthesis; Phenylacetylenic compounds
1. Introduction Recently, the phenylacetylenic compounds have attracted much attention due to their special electronic and optical properties. For para-connected phenylacetylenic compounds, the fluorescence wavelength progressively redshifted to the region of visible light with the elongation of the molecular chain [1]. But their solubility decreases sharply even when the molecular chain only increases one unit. This makes them hard to be processed. One way to solve this problem is the use of meta-connected phenylacetylenic compounds. The meta-connected phenylacetylenic compounds have less symmetry and better solubility, so they can be processed much more easily. But their optical properties are limited because the meta-connections interrupt the conjugation [2–7]. Therefore, if a way can be found to eliminate the interruption, it is possible to get a soluble phenylacetylenic compound, which can emit visible lights. ∗
Corresponding author. Tel.: +86 10 82317123; fax: +86 10 82317127. E-mail address: [email protected] (P.C. Huang).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.07.014
In this paper, a series of phenylacetylenic compounds (Plate 1Chart 1) with different electron-donor and acceptor groups at different positions of m-bis(phenylethynyl)benzene were synthesized. The effects of the substituting groups and their positions on the UV absorption and fluorescence (FL) properties of the phenylacetylenic compounds were studied. And the effects of Qd (the charge density of the carbon atoms that connect the phenylene core and the phenylethynylene branch) on the conjugation systems in the phenylacetylenic compounds were discussed.
2. Experimental 2.1. General methods 1H
NMR spectra were measured by a JNM-AL300FT NMR SYSTEM in CDCl3 . IR spectra were measured by a NEXUS-470 FTIR (Nicolet). UV–vis spectra were measured by a Cintra 10e UV–vis Spettrumeter in cyclohexane or chloroform. Fluorescence spectra were measured by a RF-530
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2.4. 1,4-Bis(phenylethynyl)benzene (2) 2 was synthesized following the general procedure with triethylamine (50 ml), 1,4-dibromobenzene (2.359 g, 10 mmol), Pd2 (dba)3 (184 mg, 0.20 mmol), CuI (77 mg, 0.40 mmol), PPh3 (534 mg, 2.0 mmol) and phenylacetylene (2 ml, 18 mmol). 1.7 g white powders (yield 61%) were obtained by the recrystallization of the crude product from toluene. m.p. 183.5–185.5 ◦ C. 1 H NMR: δ7.56–7.51 (m, 8 H), 7.36–7.26 (m, 6 H). 2.5. 1,3,5-Tris(phenylethynyl)benzene (3b) 3b was synthesized following the general procedure with triethylamine (15 ml), 1,3,5-tribromobenzene (1.664 g, 5.3 mmol), pyridine (23 ml), Pd(PPh3 )2 Cl2 (112 mg, 0.16 mmol), PPh3 (222 mg, 0.85 mmol), CuI (102 mg, 0.54 mmol) and phenylacetylene (2.00 g, 19.6 mmol). The crude product was recrystallized from isopropanol to afford white needles (811mg, yield 41%). m.p. 146.8–148 ◦ C. 1 H NMR: δ7.64 (s, 3H), 7.52 (d, J = 6.6 Hz, 6H), 7.368 (m, 9H). Chart 1. This is a sample for exhibit.
PC Spectroflorophotometer in cyclohexane or chloroform. The ground-state configurations of the phenylacetylenic compounds were optimized with the semi-empirical AM1 method and the frontier orbital diagrams and their energies were calculated with ZINDO/S method using Hperchem7.
2.2. Materials Pyridine and triethylamine were freshly distilled before use. 1,4-Dibromobenzene, 1,3,5-tribromobenzene, 2,6dibromo-4-nitro-aniline and 2,4-dibromoaniline are all commercially available and used as received.
2.3. General procedure for synthesizing phenylacetylenic compounds Nitrogen bubbled through the pyridine and/or triethylamine solution for about 1 h, then bromobenzene, Pd catalyst, PPh3 and CuI were added. The solution was bubbled for another 30 min, and phenylacetylene was added to the solution. Then, the reaction was carried out at 85 ◦ C and monitored by TLC. After the reaction, mixture was cooled down to room temperature. The solid in the mixture was removed by suction filtration and washed thoroughly with CH2 Cl2 . Then, the combined filtrate was washed with water and dried with anhydrous MgSO4 . After the solvent was removed, the product was purified by column chromatography on silica gel.
2.6. 3,5-Bis(phenylethynyl)-1-bromobenzene (3c) 3c was synthesized following the general procedure with Triethylamine (30 ml), pyridine (15 ml), 1,3,5tribromobenzene (1.891 g, 6.0 mmol), PPh3 (253mg, 0.97 mmol), Pd(PPh3 )2 Cl2 (126 mg, 0.18 mmol), CuI (115 mg, 0.60 mmol) and phenylacetylene (1.227 g, 12 mmol). The crude product was purified by column chromatography with petroleum ether as eluent to afford a white needles of 3c (1.084g, yield 51%). m.p. 88.9–90.4 ◦ C. 1 H NMR: δ7.63 (s, 3H), 7.54–7.51 (m, 4H), 7.38–7.36 (m, 6H). 2.7. 3,5-Bis(phenylethynyl)nitrobenzene (3e) A suspension of 2,6-dibromo-4-nitroaniline (25.08 g, 85 mmol) in glacial acetic acid (325 ml) was added slowly at a temperature not above 20 ◦ C to a solution of sodium nitrite (7.64 g, 110 mmol) in concentrated sulfuric acid (90 ml). After stirring for 80 min, the reaction mixture was then slowly added to a suspension of cuprous oxide (15.76 g, 110 mmol) in 95% ethanol (250 ml). After stirring overnight, the reaction was quenched with water. The organic layer was separated, washed with saturated NaHCO3 , dried with MgSO4 , and concentrated to afford a green solid. The crude product was recrystallized from 95% ethanol to produce a pure green crystals of 3,5-dibromo-nitrobenzene (16.08 g, yield 68%). m.p. 106.9–108.4 ◦ C. 3e was synthesized following the general procedure with triethylamine (12 ml), pyridine (8 ml), 3,5-dibromonitrobenzene (1.126 g, 4.0 mmol), CuI (53 mg, 0.28 mmol), PPh3 (118 mg, 0.44 mmol), Pd(PPh3 )2 Cl2 (58 mg, 0.08 mmol) and phenylacetylene (996 mg, 9.76 mmol). The crude product was purified by column chromatography
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with petroleum ether/CH2 Cl2 (8:1) as eluent followed by recrystallization from 95% ethanol to afford yellow needles of 3e (895 mg, yield 69%). m.p. 139.5–140.0 ◦ C; 1 H NMR: δ8.30–8.30 (d, J = 0.7 Hz, 2H), 7.97–7.96 (m, 1H), 7.58–7.55 (m, 4H), 7.41–7.37 (m, 6H); IR: 3081, 2213, 1535, 1355, 1596–1442, 810, 757, 687 and 665 cm−1 . 2.8. 3,5-Bis(phenylethynyl)aniline (3d) A solution of 3,5-bis(phenylethynyl)nitrobenzene (648 mg, 2 mmol), stannous chloride dihydrate (2.268 g, 10 mmol) in 95% ethanol was heated at 75 ◦ C for 100 min under nitrogen. Upon cooling, 20% aqueous NaOH was added to the solution. The aqueous layer was extracted with 150 ml ether three times. The combined organic extracts were dried with MgSO4 , filtered, and concentrated under reduced pressure followed by recrystallization from 95% ethanol to afford pure yellow crystals (540 mg, yield 92%). m.p. 132.1–132.5 ◦ C; 1 HNMR: δ7.54–7.51 (m, 4H), 7.36–7.34 (m, 6H), 7.195 (s, 1H), 6.897 (s, 2H), 1.550 (s, 2H); IR: 3472, 3203, 3056, 2210, 1586, 1625–1489, 1235, 852, 757 and 689 cm−1 . 2.9. 3,5-Bis(phenylethynyl)benzene (3a) The procedure was the same as that used for preparing 3,5dibromo-nitrobenzene and 90 mg (1.3 mmol) sodium nitrite, 2 ml concentrated sulfuric acid, 293 mg (1.0 mmol) 3,5bis(phenylethynyl)aniline, 4 ml glacial acetic acid, 188 mg (1.3 mmol) cuprous oxide and 5 ml 95% ethanol were used. The crude product was purified by column chromatography with petroleum ether as the eluent to afford 58 mg white needles (yield 21%). m.p. 110–112 ◦ C; 1 H NMR: δ7.72 (s, 1H), 7.56–7.48 (m, 6H), 7.37–7.35 (m, 7H). 2.10. 2,4-Bis(phenylethynyl)-nitrobenzene (4b) To a solution of 85% H2 O2 (1.08 g, 25 mmol) in CH2 Cl2 (17.5 ml) was added the maleic anhydride (3.06 g, 31 mmol) powder. The mixture was stirred for 50 min until most of the solids were disappeared. Then, it was heated at 45 ◦ C and the 2,4-dibromoaniline was added. After 90 min, the mixture was cooled to room temperature, washed with saturated NaHCO3 and NaCl aqueous solution, dried with MgSO4 , filtered and concentrated under reduced pressure. The obtained crude product was separated by column chromatography with petroleum ether to afford yellow crystals of 2,4-dibromonitrobenzene (1.014 g, yield 72%). m.p. 59.7–61.9 ◦ C. 4b was synthesized following the general procedure with triethylamine (9 ml), pyridine (7 ml), 2,4-dibromonitrobenzene (874 mg, 3.0 mmol), CuI (41 mg, 0.21 mmol), PPh3 (87 mg, 0.33 mmol), Pd(PPh3 )2 Cl2 (42 mg, 0.06 mmol) and phenylacetylene (746 mg, 7.2 mmol). The crude product was purified by column chromatography with petroleum ether/CH2 Cl2 (6:1) to afford orange crystals (614 mg, yield 63%). m.p. 102.2–103.8 ◦ C; 1 H NMR: δ8.10 (d, J = 4.2 Hz,
285
1H), 7.87 (d, J = 0.9 Hz, 1H), 7.63–7.55 (m, 5H), 7.41–7.38 (m, 6H); IR: 3060, 2209, 1573, 1334, 903, 839, 756 and 688 cm−1 . 2.11. 2,4-Bis(phenylethynyl)aniline (4a) The procedure was the same as that used for preparing 1,3-bis(phenylethynyl)aniline, and 8 ml 95% ethanol, 420 mg (1.3 mmol) 4b and 1.469 g (6.5 mmol) stannous chloride dihydrate were used. The crude product was recrystallized from 95% ethanol to afford 312 mg yellow crystals (82%). m.p. 147.1–149.2 ◦ C; 1 HNMR: δ8.09 (d, J = 0.9 Hz, 1H), 7.86–7.83 (m, 1H), 7.56–7.52 (m, 2H), 7.38–7.24 (m, 6H), 6.71 (d, J = 4.2 Hz, 1H), 4.75 (s, 1H), 4.20 (s, 1H); IR: 3450, 3307, 3057, 2206, 1664, 1617, 1579, 1502, 1347, 746 and 685 cm−1 .
3. Results and discussion The UV and FL spectra of 2 and 3a are given in Fig. 1. 2 and 3a are p- and m-bis(phenylethynyl)benzenes, respectively. As shown in Fig. 1a, the maximum absorbing wavelength, λmax of 2 is red-shifted by about 40 nm from 3a. The UV absorption of o-bis(phenylethynyl)benzene (1) is also red-shifted by 28 nm from 3a [2]. Moreover, it can be seen from Table 1 that the energy gap between HOMO and LUMO (E) of 1 and 2 are 6.874 and 7.043 eV, respectively, which are much lower than that of 3a (7.338 eV). The HOMO and LUMO orbitals of 3a are both interrupted by C2 and C5 carbon atoms (Chart 1) in the phenylene core (Table 1), and the electrons are localized on each phenylethynyl branches. But in 1 and 2, the electrons are delocalized on the whole molecule. The conjugation can be extended to the whole molecule when the two phenylethynyl branches are para- or ortho-connected to the phenylene core (as in 1 and 2), but is interrupted by the phenylene core and restricted in each phenylethynyl branches when the two phenylethylnyl branches are meta-connected to the phenylene core (as in 3a). The reason resulting this has not been discussed in the literature. One possible explanation is that the conjugation being extended to the whole molecule or being interrupted by the phenylene core is related to the charge density Qd of the carbon atom in the phenylene core, which connects the phenylene core with the phenylethynyl branches. Herein, Qd is the charge density of C1 and C3 in 3a–3e, 4a and 4b, and the charge density of C1 and C4 in 2, etc. (see Chart 1). The Qd of 3a, 2 and 1 are +0.029, +0.038 and +0.054, respectively (see Table 1). Increasing the positive value of Qd is in favor of withdrawing the electrons from the phenylethynyl branches to the phenylene core, and consequently in favor of extending the conjugation to the whole molecule. On the other hand, the configuration of the molecule also influences its electronic energy level. When the benzene rings on the end of the molecule rotate to the different angels with the phenylene core, its molecular orbitals and the conjugation
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286
Fig. 1. UV absorption (a) and FL emission (b) spectra of 3a and 2 (in chloroform, 10−5 M).
system in the molecule may be changed. This will also affect the optical properties of the phenylacetylenic compounds. However, the above calculations on Qd are based on the most stable configuration of the molecule, that is, the two benzene rings on the end are in the same plane with the phenylene core. So, the influence of the molecular configurations is not considered, and the molecules are regarded as totally planar structure in the dilute solution. From the above, it can be speculated that if an appropriate substituent is introduced into the phenylene core of 3a to increase the positive value of Qd , it is possible to make the conjugation in the substituted 3a extending to the whole molecule and finally obtain a meta-connected phenylacetylenic compound emitting visible light. 3.1. Substituting groups with different electronic effect on the meta-position 3b–3e are the derivatives of 3a with a phenyethynyl-, bromo-, amino- or a nitro-group attached on the 5-(meta-)
positon of 3a, respectively. Their UV absorption and FL emission spectra are shown in Fig. 2. The shapes and peak positions of the UV spectra of 3b–3e are all similar to 3a except that 3d and 3e both have another red-shifted weak absorption bands. However, their FL spectra are red-shifted from 3a according to the order: 3e, 3c, 3b and 3d. In 3b, the third phenylethynyl branch attached on the metaposition of 3a does not affect the UV absorption wavelength. The Qd of 3b is the same as 3a, and the electrons are still localized on each phenylethynyl branches. But its FL spectrum is red-shifted from 3a by 23 nm because the additional phenylethynyl branch enhances the coupling between the phenylethynyl branches, allowing the orbital energy to be further decreased [3,4]. The electron-withdrawn effect of the nitro-group increases the positive value of Qd from +0.029 of 3a to +0.037 of 3e, but this does not allow the conjugation to extend from one phenylethynyl branch to another. The p– conjugation between the nitro-group and the phenylene core results a weak absorption band in 320–360 nm. Therefore, 3e has an
Table 1 Frontier orbitals of 2, 3a and 5* E (eV)
Qd
2
6.874
+0.038
3a
7.338
+0.029
5
7.043
+0.054
HOMO
LUMO
* Ground-state configurations of the phenylacetylenic compounds are optimized by the semi-empirical AM1 method. HOMO and LUMO orbital diagrams and Es are calculated by the semi-empirical ZINDO/S method. And Qd are calculated by the AM1 method.
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287
Fig. 2. UV absorption (a) and FL emission (b) spectra of 3a–3e (in cyclohexane, 10−5 M).
absorption band in 250–310 nm similar to 3a and a weak absorption band in 320–360 nm. Because the nitro-group makes the electrons on the phenylene core flowing to the meta-position, which disturbs the coupling between two phenylethynyl branches in the excited state, its FL spectra is only red-shifted from 3a by 6 nm. The UV absorption of bromo-substituted 3c was the same as 3a because of the balance between the electron-withdrawn effect of Br- and the electron-donor effect caused by the p– conjugation between Br and the benzene ring. And the electron-withdrawn effect of Br- is weaker than nitro-group, so the red-shift of its FL spectra is a little greater than 3e. For 3d, the amino group increases the positive value of Qd from +0.029 of 3a to +0.064 of 3d. But this still does not fundamentally alter the UV absorption of 3d, except for resulting a red-shifted weak absorption band in 325–370 nm by the p– conjugation between the amino-group and the phenylene core. The FL spectrum of 3d is red-shifted from 3a by 38 nm, because the amino-group allows the electrons on the phenylene core flowing away from the meta-position,
which will be in favor of the electron coupling between the phenylethynyl branches in the excited state. 3.2. Substituting groups with different electronic effect on the 4-position 4a and 4b are the 4-substituted m-bis(phenylethynyl)benzenes (see Chart 1). Their UV absorption and FL emission spectra are shown in Fig. 3. As can be seen in Fig. 3, both of their UV absorptions are changed a lot as compared with 3a. The amino-group in 4a greatly increases the negative value of Qd (−0.029 for the carbon atom C1 in the para-position of -NH2 and −0.054 for the carbon atom C3 in the ortho-position of -NH2 , see Chart 1), so the main absorption band of 4a is blue-shifted to about 276 nm. At the mean time, the p– conjugation between the amino-group and the phenylene core results in a absorption band greater than 300 nm. The nitro-group in 4b increases the positive value of Qd significantly from +0.029 of 3a to +0.086 (for the carbon atom C1 in the para-position of -NO2 )
Fig. 3. UV absorption (a) and FL emission (b) spectra of 3a, 4a and 4b (in cyclohexane, 10−5 M).
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and +0.121 (for the carbon atom C3 in the ortho-position of NO2 ) of 4b, the large positive charge withdraw the electrons flowing from phenylethynyl branches to the penylene core and makes the conjugation extending to the whole molecule. Therefore, the maximum absorption peak of 4b is red-shifted from 3a by about 20 nm, and a broad peak appears at 328 nm. The FL spectra of 4a and 4b are both red-shifted in comparison with 3a obviously. This indicates that introducing amino-group into the 4-positon of 3a can significantly increase the coupling between the phenylethynyl branches in the excited state, and introducing nitro-group into the 4-position of 3a can extend the conjugation to the whole molecule. Both decrease the orbital energy and make the FL emission of 4a and 4b red-shifted from 3a. It is particularly interesting that the FL emission of 4a has red-shifted to the region of visible light, which will allow it to be used as a fluorescence material emitting blue light. In the experiments, it can be found that the solubility of the p-phenylacetylenic compound is much lower than that of the m-phenylacetylenic compounds. 3 and 4 are soluble in most of the organic solvent, such as petroleum ether, benzene, chloroform, ethanol and so on, while 2 is partly soluble in non-polar solvent, such as petroleum ether. If another phenylacetylenic unit is introduced into 2 and 3a, that is 1,4-bis[4-(phenylethynyl)phenyl]acetylene and 1,3,5tris(phenylethynyl)benzene(3b), the former one is insoluble in non-polar solvent, such as petroleum ether, and partly soluble in chloroform, while the latter one can still be soluble in most of the organic solvent. In addition, the melting points of the m-phenylacetylenic compounds are lower than the p-phenylacetylenic compounds. So the m-phenylacetylenic compounds are obviously in great advantages of good solubility and processibility.
4. Conclusion The reason that the conjugation is interrupted by the phenylene core in the m-bis(phenylethynyl)benzene (3a) but can be extended to the whole molecule in the p- or obis(phenylethynyl)benzene may relate to the charge density
Qd (the charge density of the carbon atom which connects the phenylene core and the phenylethynyl branch in the bis(phenylethynyl)benzenes). Increasing the positive value of Qd can make the electron more easily flowing from phenylethynyl branch to the phenylene core and the conjugation more easily being extended to the whole molecule. The substitution of hydrogen in 5-position (meta-position to the phenylacetylenic branch) of 3a by Ph-C C-, Br-, NO2 - or NH2 - do not increase the positive value of Qd significantly, the conjugation is still interrupted by the phenylene core and their UV absorptions are similar to that of 3a. Introducing NH2 into the 4-position (ortho-position to the phenylacetylenic branch) of 3a decreases the charge density Qd from positive value to the negative value, which makes the UV absorption of 4a blue-shifting from that of 3a. However, introducing NO2 - into the 4-position of 3a increases the positive value of Qd much significantly, which makes the conjugation in 4b extending to the whole molecule and the UV absorption redshifting from 3a obviously. The compound 4a can be used as a light-emitting material.
Acknowledgement We thank the National Nature Science Foundation of China (NSFC, No. 29974002) for support of this research.
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