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Controlling the Optical Properties of Hyperbranched Conjugated Polyazomethines through Terminal-Backbone Interactionsa Liang Song, Chunlai Tu, Yunfeng Shi, Feng Qiu, Lin He,* Yi Jiang, Qi Zhu, Bangshang Zhu, Deyue Yan, Xinyuan Zhu*

A simple approach to tune the optical properties of the hyperbranched conjugated polymers by only adjusting the terminal-backbone interactions has been reported in this article. Hyperbranched conjugated polyazomethines have been successfully prepared by the reaction of tetramine and dialdehyde. Not only varying the monomer feed ratio to change the quantity of terminal amino groups, but also adopting protonation or complexion with proper dopants (SnCl2 and b-cyclodextrin), can alter the interactions between amino terminals and imine bonds in the backbone. Correspondingly, the optical properties of the resulting hyperbranched polymers are controlled.

Introduction Conjugated polymers have received tremendous attention during the past decades because of their excellent optical and electrical properties.[1] Compared with linear conjugated polymers, the highly branched architecture of hyperbranched conjugated polymers helps to suppress aggregation and excimer formation. On the other hand, the L. Song, C. Tu, Y. Shi, F. Qiu, Y. Jiang, Q. Zhu, D. Yan, X. Zhu School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, P. R. China Fax: þ86 21 3420 5722; E-mail: [email protected] L. He, B. Zhu, X. Zhu Instrumental Analysis Center, Shanghai Jiao Tong University, Shanghai 200240, P. R. China Fax: þ86 21 3420 5722; E-mail: [email protected] a

: Supporting information for this article is available at the bottom of the article’s abstract page, which can be accessed from the journal’s homepage at http://www.mrc-journal.de, or from the author.

Macromol. Rapid Commun. 2010, 31, 443–448 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

existence of many functional terminals strengthens the intra- and inter-molecular interactions of hyperbranched conjugated polymers, which affects optical properties greatly.[2,3] However, to the best of our knowledge, controlling the optical properties of hyperbranched conjugated polymers through terminal–backbone interactions has not yet been reported. Among the different kinds of polymers, polyazomethines, which are also known as polyimines or polymeric schiff bases, are an interesting group of polymers, because of their good thermal, mechanical, electronic, and optical properties. The nitrogen atom of the imine group (C¼N) with a lone pair electrons in the backbone can form intra- or inter-molecular interactions such as hydrogen bonding with other groups. Furthermore, it also has capabilities of protonation and complexation with metal ions.[4] In the present work, a series of hyperbranched conjugated polyazomethines have been successfully prepared by changing the feed ratio of tetramine to dialdehyde. Benefiting from the highly branched architecture, the numerous amino terminals and the imine linkages in the backbone form intra- or inter-molecular interactions. By

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changing these terminal–backbone interactions, the optical properties of the hyperbranched conjugated polymers have been well controlled.

Experimental Part Materials 3,30 -Diaminobenzidine (DAB, Acros Organics), terephthalaldehyde (TPA, Alfa Aesar), trifluoroacetic acid (TFA, Sinopharm), bcyclodextrin (b-CD, Sinopharm), SnCl2 (Sinopharm) were all used as received without any purification. N,N-Dimethylformamide (DMF) was purchased from Sinopharm reagent Co. Ltd., Shanghai. DMF was used after purification by distillation under vacuum and dried with anhydrous calcium chloride.

Synthesis of Hyperbranched Conjugated Polyazomethines A typical polymerization procedure (where the feeding ratio of DAB to TPA is 1.5: 1) is as follows: DAB (160.5 mg, 0.75 mmol) was dissolved in anhydrous DMF (7.5 mL), and an anhydrous DMF (5 mL) solution of TPA (67 mg, 0.5 mmol) was added dropwise. The reaction vessel was alternately evacuated and flushed with dry nitrogen five times to surround the mixture with a nitrogen atmosphere. After stirring continuously at 60 8C for 48 h, the mixture was cooled gradually to room temperature and poured into methanol. The precipitate was washed repeatedly with abundant methanol and extracted with refluxing methanol in a Soxhlet apparatus for one night, and finally dried under vacuum at 50 8C for 24 h to give a red powder (Yield: 35%). IR (KBr): 3 358 (NH2), 3 182 (NH), 3 022, 2 924, 2 856 (CH), 1 652 (C¼N), 1 608, 1 478, 1 444 (phenyl), 1 284 (PhN), 1 190 (PhN¼), 965, 813 (CH), 698, 528 (Ph) cm1. 1 H NMR (DMSO-d6): d ¼ 8.7–8.8 (m, CH¼N), 8.4–6.5 (m, phenyl), 5.4 (br, NH2). The same polymerization procedure was used to synthesize samples P2 (DAB/TPA ¼ 1: 1) and P3 (DAB/TPA ¼ 1: 3). P2: an orange powder, yield ¼ 59%; IR (KBr): 3 354 (NH2), 3 180 (NH), 3 021, 2 925, 2 853 (CH), 1 688 (C¼O), 1 650 (C¼N), 1 606, 1 477, 1 442 (phenyl), 1 286 (PhN), 1 185 (PhN¼), 961, 812 (CH), 694, 525 (Ph) cm1. 1 H NMR (DMSO-d6): d ¼ 10.2 (s, CH¼O), 8.7– 8.8 (m, CH¼N), 8.4–6.5 (m, phenyl), 5.6 (br, NH2). P3: a yellow powder, yield ¼ 51%; IR (KBr): 3 357 (NH2), 3 178 (NH), 3 020, 2 921, 2 852 (CH), 1 692 (C¼O), 1 653 (C¼N), 1 603, 1 475, 1 441 (phenyl), 1 286 (PhN), 1 188 (PhN¼), 962, 811 (CH), 695, 526 (Ph) cm1. 1 H NMR (DMSO-d6): d ¼ 10.1 (s, CH¼O), 8.7– 8.8 (d, CH¼N), 8.4–6.5 (m, phenyl), 5.6 (br, NH2).

dimethyl sulfoxide-d6 (DMSO-d6) as solvent at 298 K. FT-IR spectra were measured as KBr pellets on a Perkin Elmer Paragon 1000 spectrophotometer in the range of 4 000–400 cm1. Gel permeation chromatography (GPC) measurements were carried out on a Perkin–Elmer Series 200 system (100 mL injection column, PL gel 10 mm 300 7.5 mm mixed-B columns, polystyrene calibration; DMF containing 0.05 mol L1 LiBr was utilized as the mobile phase at a flow rate of 1.0 mL min1) to determine the molecular weight and molecular weight distribution. UV-vis measurements were performed on a Perkin Elmer Lambda 20 UV-vis spectrometer in the range of 300–650 nm. The fluorescence spectra were measured on a Perkin Elmer LS 50B fluorescence spectrometer in the range of 370–700 nm. Excitation wavelength lex ¼ 365 nm. The thermogravimetric analysis (TGA) was recorded on a Perkin–Elmer TGA-7 thermogravimetric analyzer in nitrogen atmosphere at a heating rate of 20 8C min1 from 40 to 800 8C. The calorimetric measurements were carried out on a TA Q2000 series modulated differential scanning calorimeter under a flowing nitrogen atmosphere at a heating rate of 20 8C min1 from 20 to 450 8C, calibrated with In and Zn standards respectively.

Results and Discussion The synthetic route of hyperbranched conjugated polyazomethines is shown in Scheme 1. The polymerization was accomplished by the condensation of DAB with TPA under a N2 atmosphere in anhydrous DMF solution at 60 8C for 48 h. Three samples with different DAB-to-TPA molar feeding ratios of 1.5:1, 1:1, and 1: 3 were prepared, and the correspondent 1H NMR spectra are given in the Supporting Information (Figure S1). The signal around 8.7 ppm, which corresponds to the proton resonance of the CH¼N group, confirms the presence of the imine structure

Characterization 1

H NMR spectra were recorded using a Varian MERCURY plus-400 spectrometer with

444


Scheme 1. Synthetic route of the hyperbranched conjugated polyazomethines.

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Controlling the Optical Properties of Hyperbranched Conjugated . . .

Table 1. Monomer molar ratio, molecular weight, degree of branching, and thermal data of the synthesized polyazomethines.

Sample

DBA: TPAa)

Tgc)

Td-10%d)

We)

-C

-C

%

0.40

391

618

49.5

2.56

0.47

387

605

44.2

2.14

0.51

353

512

28.1

PDI

Mn

Mw

g mol1

g mol1

1.5:1

1.96 104

3.27 104

1.88

P2

1:1

4

1.68 10

4

4.31 10

P3

1:3

1.71 104

3.66 104

P1

DBb)

a) Molar ratio; b)Determined by 1H NMR spectroscopy; c)The glass transition temperature; d)The temperature at 10% weight loss; e)The residual weight percent at 800 8C.

bands and the second band at high wavelength gradually weakens from P1 to P3. The lmax of P1, P2, and P3 exhibits a hypsochromic shift from 466 to 378, and then 366 nm, and the solution color changes from red, orange, to yellow gradually. The fluorescence emission spectrum of P3 in Figure 1b exhibits an intense maximum at about 522 nm and a shoulder at 416 nm under an excitation wavelength of 365 nm, while P2 has two weak peaks at 543 and 425 nm and P1 displays only one weak emission peak at 418 nm, respectively. The optical properties of the three hyperbranched polyazomethines are quite different, which could be tentatively attributed to the effect of molecular interactions originating from the numerous functional terminals. It has been well reported that the imine linkage is not coplanar with the neighboring phenylene ring in schiff base molecules, because of the conjugation between the imine nitrogen lone pair electrons and the p-electron on the Nphenyl ring. However, the intra- or inter-molecular hydrogen bonding or donor–acceptor interchain charge transfer can influence the conformation planarity.[5] Benefiting from the highly branched architecture, hyperbranched polyazomethine P1 has many amino terminals. The hydrogen atoms of the amino terminals can form interand intra-molecular hydrogen bonds with nitrogen atoms of the imine linkages, which results in the long molecular conjugated length and the red-shift of the maximum UV-vis absorption. However, the replacement of amino terminals with aldehyde groups weakens these interactions greatly, so the lmax of the hyperbranched polyazomethines shifts towards low wavelengths. Increasing the chain planarity and efficient conjugated length induces a high wavelength absorption, but easily generates excimers by the p–p stacking interaction among the chromophores, which results in luminescence quenching.[3] So, compared with P3, samples P2 and P1 have higher absorption but Figure 1. UV-vis spectra (a), and fluorescence emission spectra (b) of three samples in DMF solution, lex ¼ 365 nm. Solution concentration 105 M. lower emission intensity. On the other

in each polymer. The signals at 6.5–8.4 ppm are ascribed to the protons on the aromatic rings. The terminal aldehyde signal is detected at about 10.2 ppm in P3 and P2. However, for sample P1, the aldehyde signal disappears while a broad terminal amine peak appears at 5.4 ppm. 1H NMR results indicate that the terminal units of P1 are amino groups, whereas the terminal aldehydes are dominant in P3. It should be pointed out that even after adding excessive TPA to the reaction mixtures, the amino terminals can not completely react with aldehyde because of steric hindrance. The degree of branching (DB) of the three samples was calculated by 1H NMR data, and the details can be found in the Supporting Information. The molecular weights and molecular weight distributions were determined by GPC, and the GPC data together with DB values are listed in Table 1. The proposed structure of the three hyperbranched polyazomethines has been further confirmed by the FT-IR spectra in Figure S4. Table 1 displays the thermal properties of the hyperbranched polyazomethines. For all samples, the glass transition temperature is above 350 8C and the decomposition temperature of 10% weight loss (Td-10%) is higher than 500 8C, which indicates the excellent thermal stability of the hyperbranched conjugated polyazomethines. Figure 1 presents the UV-vis and fluorescence emission spectra of three samples. The UV-vis spectra in Figure 1a show that each sample (P1, P2, and P3) has two absorption


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the loose packing of highly branched polymers would facilitate the fluorescence enhancement. Considering that the molecular interactions affect the polymer’s conformation and p-electron properties dramatically, we further studied the optical properties of three hyperbranched polyazomethines by protonation or complexation with proper dopants, such as SnCl2 and b-CD (see Scheme S1 in the Supporting Information). Figure 2 gives the real time UV-vis and fluorescence emission spectra of P1 and P3 after adding TFA. By the addition of TFA, the high wavelength peaks at 465 nm for P1 and at 456 nm for P3 gradually diminish, while the other peak at around 368 nm enhances with time. In the meantime, the fluorescence emission Figure 2. Real time UV-vis spectra (a), and fluorescence emission spectra (b) of P1 after intensity enhances with time. A similar behavior has also been observed for P2 adding 1 mL of TFA to the colorimetric dish. Real time UV-vis spectra (c), and fluorescence emission spectra (d) of P3 after adding 1 mL of TFA to the colorimetric dish. lex ¼ 365 nm. (Figure S5). This phenomenon can be explained by the variation of terminal– backbone interactions. Because of the protonation of nitrogen atoms, the interactions between amino terminals and imine bonds in the backbone are destroyed. The emergence of isosbestic points in the UV-vis spectra confirms the gradual destruction of molecular interactions. The optical properties of hyperbranched polyazomethines can also be changed by coordination with metal ions.[6] After the addition of SnCl2, the molecular interactions between amino terminals and imine bonds in the backbone are replaced by the coordination of Sn2þ ions with nitrogen atoms, which results in the fluorescence enhancement and the second band intensity decrease in UV absorption (Figure 3 and S6). Finally, host–guest interactions have also been used to adjust the terminal– backbone interactions by the introducFigure 3. UV-vis spectra (a), and fluorescence emission spectra (b) of P1 and P1/SnCl2. UVvis spectra (c), and fluorescence emission spectra (d) of P3 and P3/SnCl2. lex ¼ 365 nm. tion of b-CD. b-CD can encapsulate the amino terminal of the hyperbranched polyazomethines into its internal cavity so as to act as an insulating layer,[7] which weakens the hand, the DB variation might play an important role on affecting the optical properties of these three molecular interactions between amino terminals and hyperbranched polyazomethines. With the increase of imine bonds in the backbone. Therefore, both molecular branching, the steric hindrance on backbone planarity conjugated length and p–p stacking interactions among could lead to the blue-shift of UV-vis spectra and the chromophores are reduced. As a result, a low

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Controlling the Optical Properties of Hyperbranched Conjugated . . .

after addition of the proper dopants is summarized in Table 2. It should be noted that the branched architecture of the hyperbranched polyazomethines does not change during the protonation and complexation with dopants. Moreover, after the addition of TFA, SnCl2, or b-CD, the UV-vis spectra become quite similar, i.e., only one obvious peak at about 369 nm appears. These results suggest that the terminal–backbone interactions are the main factor to affect the optical properties in this system. The fluorescence emission band of P1/ TFA at 470 nm (Figure 2b) shows a bathochromic shift compared with P1/ SnCl2 at 436 nm (Figure 3b) and P1/b-CD at 443 nm (Figure 4b). This might be related to the enhancement of molecular rigidity of P1 which is caused by the electrostatic repulsion among the internal and terminal ammonium cations after protonation with TFA.[8] However, for P3, the lmax of the fluorescence emission is almost unchanged after protonation or complexation with SnCl2 and b-CD (Figure 2d, 3d, and 4d). It means that, compared with P1, P3 has weaker Figure 4. UV-vis spectra (a), and fluorescence emission spectra (b) of P1 and P1/b-CD. UVintra- and inter-molecular interactions vis spectra (c), and fluorescence emission spectra (d) of P3 and P3/b-CD. lex ¼ 365 nm. because of the existence of many terminal aldehydes. Furthermore, the influence of terminal-backbone interactions on the optical Table 2. UV-vis absorption and fluorescence emission bands of properties can be confirmed by the different fluorescence the three samples P1, P2, and P3 after addition of the proper enhancement behavior from P1 and P3. After the addition of dopants. TFA, SnCl2 and b-CD, the emission intensity of P1 is enhanced about 27, 17, and 16 times, respectively, while Code Luminescence UV-vis only about 4, 1.5, and 1.4 times for P3. l Dlema) l Dlabsa)

wavelength absorption and a strong fluorescence emission are observed (Figure 4 and S7). The change of UV-vis absorption and fluorescence emission spectra of the hyperbranched polyazomethines

nm

nm

nm

nm

P1

418

–

466, 365

–

P1/TFA

470

þ52

377

þ12

P1/SnCl2

436

þ18

465, 375

1,þ10

443

þ25

465, 373

1, þ8

543, 425

–

465, 378

–

P1/b-CD P2 P2/TFA

480

þ55

378

0

P2/SnCl2

441

þ16

378

0

449

þ24

378

0

P3

522, 416

–

456, 366

–

P3/TFA

520, 415

2, 1

368

þ2

P3/SnCl2

521, 414

1, 2

458, 367

þ2, þ1

P3/b-CD

521, 413

1, 3

458, 368

þ2, þ2

P2/b-CD

a)

Stokes shift of undoped and doped compounds in DMF solution.


Conclusion In conclusion, hyperbranched conjugated polyazomethines have been successfully prepared by the polycondensation of 3,30 -diaminobenzidine with terephthalaldehyde. Because of the highly branched architecture, the terminal amino groups and the imine linkages in the backbone form intra- or inter-molecular interactions, which affect the optical properties greatly. Based on this point, a simple approach to tune the optical properties of the hyperbranched conjugated polymers has been developed by only adjusting the terminal–backbone interactions. Not only varying the monomer feed ratio to change the species and quantity of end-groups of the hyperbranched polymer, but also adopting protonation or complexion with proper dopants, can alter the terminal–backbone interactions.

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Correspondingly, the optical properties of the hyperbranched conjugated polymers can be controlled.

Acknowledgements: This work is sponsored by the National Natural Science Foundation of China (50773037, 50633010) and the National Basic Research Program 2009CB930400, the Fok Ying Tung Education Foundation (111048), NCET-06-0411, Shuguang Program (08SG14), and the Shanghai Leading Academic Discipline Project (Project Number: B202).

[3] [4]

Received: October 15, 2009; Revised: October 29, 2009; Published online: December 8, 2009; DOI: 10.1002/marc.200900747 Keywords: conjugated polymers; hyperbranched; optical properties; polyazomethines; terminal-backbone interactions

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DOI: 10.1002/marc.200900747