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Single-component films of different generations of dendrimers bearing a diphenylanthracene core
EUROPEAN POLYMER JOURNAL
European Polymer Journal 41 (2005) 1219–1224
www.elsevier.com/locate/europolj
Single-component films of different generations of dendrimers bearing a diphenylanthracene core Jing Sun, Liyan Wang *, Xi Yu, Xi Zhang
*
Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, China Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China Received 3 November 2004; received in revised form 20 December 2004; accepted 20 December 2004 Available online 29 January 2005
Abstract Different generations of carboxyl-terminated poly(aryl ether) dendrimers bearing a diphenylanthracene core were designed and synthesized. It is interesting to see that not only two-generation dendrimers but also one-generation dendrimers can be fabricated into thin films by self-deposition. Fluorescence spectra indicate that increasing the generation number of a dendrimer can effectively control the quenching of the fluorescence. Moreover, the fluorescence property of the diphenylanthracene core of the dendrimers in a solid film is quite similar to that of one in a solution, which is important for designing light-emitting materials. 2005 Elsevier Ltd. All rights reserved. Keywords: Dendrimer; Self-deposition; Fluorescence; Single-component film
1. Introduction In 1991, Decher and Hong first reported that a polyelectrolyte multilayer film was constructed by alternating deposition of a polycation and a polyanion on a charged surface [1]. This method has attracted much attention because it is a convenient and effective way to produce complex layered structures with precise control of the composition and thickness [2–5]. In addition, the driving force for multilayer assembly has been extended from electrostatic force [6] to other interactions
*
Corresponding authors. Fax: +86 431 5193421 (L. Wang), fax: +86 10 62771149 (X. Zhang). E-mail addresses: [email protected] (L. Wang), xi@ tsinghua.edu.cn (X. Zhang).
such as charge-transfer interaction [7], hydrogen bonding [8,9] and stereocomplex formation [10]. Recently, we have reported that carboxyl-terminated poly(aryl ether) dendrimers can form a single-component film by self-deposition, since it is a hydrogen-bonding acceptor as well as a hydrogen-bonding donor [11]. This research is significant because we can apply it to construct a single-component thin film with controllable thickness, which is different from conventional layer-by-layer assembly. Dendrimers have a number of advantages over linear polymers: controllable exterior groups, identical size, perfectly branched three-dimensional structure, etc. As a result, dendrimer films may significantly enrich the application of thin solid films [12,13]. Herein, we designed and synthesized two generations of carboxyl-terminated dendrimers with a diphenylanthracene core. On the one hand, we would like to know
0014-3057/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.12.014
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if low-generation dendrimers could also form a singlecomponent film by self-deposition. On the other hand, a chromophore encapsulated by dendrons will allow us to fabricate a functional thin film, since the isolation effect of dendrimers can improve the fluorescence properties of dye molecules in the solid state.
2. Experimental 2.1. Synthesis and characterization of dendrimers Two dendrimers were synthesized according to the literature [14], and named An(G1)4 and An(G2)4 (Scheme 1). An(G2)4: The brief route of synthesis is shown in Scheme 2. A mixture of (MeO2C)4-[G2]-Br(5.0 equiv), 9,10-Bis(3,5-Dihydroxyphenyl)anthracene (1.0 equiv), anhydrous K2CO3 (10.0 equiv), 18-crown-6 (0.1 equiv) and acetone was heated to reflux and stirred vigorously under nitrogen in darkness for 144 h. Then the acetone was evaporated, and the residue was put in a mixture of water and CH2Cl2 for partitioning. The organic layer was dried with anhydrous Na2SO4 and then evaporated.
The crude product was purified by silica gel column chromatography with mixture of ether and CH2Cl2 as the eluent. Poly(aryl ether) dendrimers with a diphenylanthracene core and carboxyl methyl esters at the periphery were successfully synthesized (yield 75%). Hydrolysis of the methyl ester was performed in a solution of water (60 ml), ethanol (120 ml), and potassium hydroxide (0.6 g). It was then heated at reflux and vigorously stirred under nitrogen in darkness for 8 h. Next, the ethanol was evaporated and the residue was acidified with dilute hydrochloric acid to form a precipitate. The precipitate was filtered and washed with water to remove all potassium salts. The carboxyl methyl esters were successfully changed to carboxyl groups, i.e. An(G2)4 was synthesized. An(G1)4 was synthesized with the same route. An(G1)4: 1H NMR (500 MHz, DMSO-d6): d 7.93 (16H, d, ArH), d 7.57 (4H, m, core ArH), d 7.51(16H, d, ArH), d 7.32 (4H, m, core ArH), d 6.88(2H, s, ArH), d 6.75 (8H, bs ArH), d 6.68 (4H, bs, ArH), d 6.66 (4H, s, ArH), d 5.15 (16H, s, OCH2), d 5.10 (8H, s, OCH2), MALDI-TOFMS: Calc. for C118H90O28 [M + H]+: m/z 1956.96 Found [M + H]+: m/z 1956.4. An(G2)4: 1H NMR (500 MHz, DMSO-d6): d 7.90(32H, d, ArH), d 7.58 (4H, m, core ArH), d 7.47 (32H, d, ArH), d 7.32 (4H, m, core ArH), d 6.88 (2H, s, ArH), d 6.68 (24H, bs, ArH), d 6.66 (4H, s, ArH), d 6.60 (8H, s, ArH), d 6.58 (4H, s, ArH), d 5.10(32H, s, OCH2), d 5.06 (8H, s, OCH2), d 4.96 (16H, s, OCH2), MALDI-TOFMS: Calc. for C238H186O60 [M + Na]+: m/z 4028.99 Found [M + Na]+: m/z 4028.7. 2.2. Instruments 1 H NMR spectra were recorded on a Bruker Avance500 NMR Spectrometer (500 MHz) using tetramethylsilane as an internal standard. UV–Vis spectra were collected on a Perkin–Elmer Lambda 800 UV–Vis spectrometer. Fluorescence spectra were measured on a Shimadzu RF-5301PC spectrometer. Atomic force microscopy (AFM) images were taken with an atomic force microscope (DimensionTM 3100, Digital Instrument).
2.3. Fabrication of self-deposition films
Scheme 1. The molecular structures of dendrimers.
A self-deposition film of An(G1)4 was fabricated as follows. A quartz substrate was modified with 4-aminobutyldimethylmethoxysilane in advance. The modified substrate was immersed into methanolic solution of An(G1)4 (7.5 · 10 6 mol/l) for 2 min to adsorb the first layer. Then, it was rinsed in methanol and dried in the ambient atmosphere. Simply repeating the above process leads to the formation of a self-deposition film of An(G1)4. We fabricated a self-deposition film of An(G2)4 in the same way.
J. Sun et al. / European Polymer Journal 41 (2005) 1219–1224
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Scheme 2. The brief synthesis route of An(G2)4.
3. Results and discussion UV–Vis absorption spectroscopy was used to monitor the assembling process of the self-deposition films. Fig. 1 shows the UV–Vis absorption spectra of selfdeposition films of An(G2)4 on a quartz substrate. Characteristic absorption bands of diphenylanthracene appear at 256 nm and in the range of 350 410 nm [15], as indicated in inset (b). The absorption bands in the range of 230 300 nm are ascribed to phenyl groups in the branches of dendrimer. The linear increase of absorbance with the number of layers is indicative of a progressive and uniform deposition of An(G2)4, as shown in inset (a). Moreover, UV–Vis spectroscopy indicated that dendrimers adsorbed on the quartz were full-covered and homogeneous. The size of An(G2)4 is similar to the dendrimer in Ref. [11]; both have 16 carboxyl groups at the periphery.
In contrast, An(G1)4 has only eight carboxyl groups at the periphery. Therefore, we are wondering if we can construct a single-component film of An(G1)4 using the self-deposition method. Fig. 2 shows the UV–Vis absorption spectra of self-deposition films of An(G1)4. The linear increase of absorbance with the number of layers is also indicative of a progressive and uniform deposition of An(G1)4. These results suggest that the self-deposition method is suitable for different generations of dendrimers. With this method, it is easy to obtain a single-component film with a controllable thickness at the nanoscale. In order to further confirm the homogeneity of the self-deposition film of dendrimers, AFM was used to observe the morphology of the two self-deposition films. Fig. 3 shows the AFM images of nine-layer films of (a) An(G1)4 and (b) An(G2)4 on quartz slides. Both images exhibit granular morphology with a full and
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Fig. 1. UV–Vis absorption spectra of self-deposition film of An(G2)4. The insets show (a) the growth of absorbance at 230 nm as a function of the number of layers, and (b) the characteristic absorption of diphenylanthracene in a 16-layer film.
Fig. 2. UV–Vis absorption spectra of self-deposition film of An(G1)4. The insets show: (a) the growth of absorbance at 230 nm as a function of the number of layers, and (b) the characteristic absorption of diphenylanthracene in an eightlayer film.
homogeneous coverage. Similar morphology appeared on different places of each film and the experimental results were reproducible. The roughness of the two images was (a) 0.254 nm and (b) 0.457 nm. Differences between the two films are most likely related to the different molecular structures of dendrimers. We also attempted to prepare thin films using the spin-coating and simple casting. Based on UV–Vis spectroscopy, we found it was difficult to control the thickness of the thin film at the nanoscale through
spin-coating and simple casting. Because An(G1)4 has poor solubility (less than 1 · 10 5 mol/l) in methanol and is insoluble in many other solvents, it is difficult to prepare a ‘‘thicker’’ An(G1)4 thin solid film using spincoating or simple casting. We also found that it is difficult to obtain a homogeneous spread of dendrimers on the whole film in cases of spin-coating and simple casting. The above results suggest that self-deposition is a better way to fabricate a homogeneous and thicknesscontrollable single-component film. We further studied the fluorescence property of the dendrimer self-deposition film. For comparison, we initially studied the fluorescence of dilute methanolic solutions of An(G1)4 and An(G2)4. As shown in Fig. 4, the intensity of fluorescence is approximately equal for solutions of An(G1)4 and An(G2)4 with the same molar concentration. This indicates that the fluorescence intensity in dilute solutions is mainly determined by the amount of fluorophore. Although the fluorescence intensity increases with increasing concentration, neither a shift of the maximum emission nor a broadening of the spectra was observed. In order to study the effect of generation number on fluorescence of the dendrimer self-deposition film, we prepared An(G1)4 and An(G2)4 films with approximately equal UV–Vis absorptions in the range of 350 410 nm, the characteristic absorption of a diphenylanthracene core, as shown in Fig. 5 and its inset. Fig. 6 shows fluorescence spectra (excitation at 376 nm) of the self-deposition films of An(G1)4 and An(G2)4. The fluorescence spectrum of each film was obtained from the average intensity at nine different positions. The differences of fluorescence intensities at the nine positions are small, which further confirmed that the films are homogeneous. The fluorescence of the An(G2)4 self-deposition film is 2.4 times as strong as that of the An(G1)4 film at the maximum emission. Considering that the size of An(G2)4 is greater than that of An(G1)4, the density of fluorophore is lower in the An(G2)4 film than in the An(G1)4 film. The diphenylanthracene core is better isolated in a two-generation dendrimer (An(G2)4) than in a one-generation dendrimer (An(G1)4) [16–18]. Therefore, self-quenching in high generation dendrimers can be effectively depressed. The maximum emissions of An(G1)4 and An(G2)4 in solutions and those in self-deposition films are shown in Table 1. Based on these data, the spectra shift (Dk) was calculated. We found the red shift of An(G2)4 is much smaller than that of An(G1)4. This phenomenon further indicates that high-generation dendrimers have a stronger site-isolation effect on their cores than low-generation ones. Many research groups have reported that the photoluminescent property of dye molecules in solid films showed considerable shift from those in solution. In contrast, the difference is quite slight for our dye-con-
J. Sun et al. / European Polymer Journal 41 (2005) 1219–1224
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Fig. 3. AFM height images (1.5 lm · 1.5 lm) of nine-layer films of (a) An(G1)4, and (b) An(G2)4 on quartz slides.
Fig. 5. UV–Vis absorption spectra of the self-deposition films of An(G1)4 and An(G2)4 with approximately equal characteristic absorption of diphenylanthracene in the range of 350 410 nm.
Fig. 4. Fluorescence spectra of: (a) An(G1)4 and (b) An(G2)4 in methanol solution with different concentrations.
taining dendrimers. Therefore, we inferred that this molecular design offers the possibility of predicting dye properties in the solid state according to their properties in a solution, which is important for designing lightemitting materials.
Fig. 6. Fluorescence spectra (excitation at 376 nm) of the selfdeposition films of An(G1)4 and An(G2)4.
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Table 1 The maximum emission and the red shift (Dk) of fluorescence spectra of An(G1)4 and An(G2)4 Dendrimer
kmax/nm (solution)
kmax/nm (film)
Dka/nm
An(G1)4 An(G2)4
412, 430 418, 435
425, 443 423, 440
13 5
a
Dk = kmax (film)
kmax (solution).
4. Conclusions We designed and synthesized two dendrimers with the same fluorophore and different generation numbers. We found that not only two-generation dendrimers but also one-generation dendrimers can be fabricated into self-deposition films. We also found that self-deposition is a better method for fabricating a homogeneous and thickness-controllable single-component film. According to the UV–Vis spectra and fluorescence spectra of the self-deposition films, we inferred the diphenylanthracene core is better isolated in a two-generation dendrimer (An(G2)4) than in a one-generation dendrimer (An(G1)4). The fluorescence property of the dye core of dendrimers in a solid film and that of one in a solution are roughly the same, which will facilitate the design of functional materials.
Acknowledgement We would like to thank Prof. Yuguang Ma and Mr. Fengwei Huo for their help and fruitful discussions. This work is supported by the National Natural Science Foundation of China (20204003, 20334010), and the
Major State Basic Research Development Program (G2000078102).
References [1] Decher G, Hong JD. Makromol Chem Makromol Symp 1991;46:321. [2] Zhang X, Shen JC. Adv Mater 1999;11:1139. [3] Hammond PT. Curr Opin Colloid Interf Sci 2000;4:430. [4] Bertrand P, Jonas A, Laschewsky A, Legras R. Macromol Rapid Commun 2000;21:319. [5] Decher G, Schlenoff JB. Multilayer thin film—sequential assembly of nanocomposite materials. Weinheim: WileyVCH; 2003. [6] Hempenius MA, Pe´ter M, Robins NS, Kooij ES, Vancso GJ. Langmuir 2002;18:7629. [7] Shimazaki Y, Mitsuishi M, Ito S, Yamamoto M. Langmuir 1997;13:1385. [8] Stockton WB, Rubner MF. Macromolecules 1997;30: 2717. [9] Wang LY, Wang ZQ, Zhang X, Shen JC, Chi LF, Fuchs H. Macromol Rapid Commun 1997;18:509. [10] Serizawa T, Hamada K, Kitayama T, Fujimoto N, Hatada K, Akashi M. J Am Chem Soc 2000;122:1891. [11] Huo FW, Xu HP, Zhang L, Fu Y, Wang ZQ, Zhang X. Chem Commun 2003:874. [12] Tsukruk VV. Adv Mater 1998;10:253. [13] Tully DC, Fre´chet JMJ. Chem Commun 2001:1229. [14] Hawker CJ, Wooley KL, Fre´chet JMJ. J Chem Soc, Perkin Trans 1993;1:1287. [15] Jones RN. Chem Rev 1947;41:353. [16] Kawa M, Fre´chet JMJ. Chem Mater 1998;10:286. [17] Marsitzky D, Vestberg R, Blainey P, Tang BT, Hawker CJ, Carter KR. J Am Chem Soc 2001;123:6965. [18] Lee LF, Adronov A, Schaller RD, Fre´chet JMJ, Saykally RJ. J Am Chem Soc 2003;125:536.
European Polymer Journal 41 (2005) 1219–1224
www.elsevier.com/locate/europolj
Single-component films of different generations of dendrimers bearing a diphenylanthracene core Jing Sun, Liyan Wang *, Xi Yu, Xi Zhang
*
Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, China Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China Received 3 November 2004; received in revised form 20 December 2004; accepted 20 December 2004 Available online 29 January 2005
Abstract Different generations of carboxyl-terminated poly(aryl ether) dendrimers bearing a diphenylanthracene core were designed and synthesized. It is interesting to see that not only two-generation dendrimers but also one-generation dendrimers can be fabricated into thin films by self-deposition. Fluorescence spectra indicate that increasing the generation number of a dendrimer can effectively control the quenching of the fluorescence. Moreover, the fluorescence property of the diphenylanthracene core of the dendrimers in a solid film is quite similar to that of one in a solution, which is important for designing light-emitting materials. 2005 Elsevier Ltd. All rights reserved. Keywords: Dendrimer; Self-deposition; Fluorescence; Single-component film
1. Introduction In 1991, Decher and Hong first reported that a polyelectrolyte multilayer film was constructed by alternating deposition of a polycation and a polyanion on a charged surface [1]. This method has attracted much attention because it is a convenient and effective way to produce complex layered structures with precise control of the composition and thickness [2–5]. In addition, the driving force for multilayer assembly has been extended from electrostatic force [6] to other interactions
*
Corresponding authors. Fax: +86 431 5193421 (L. Wang), fax: +86 10 62771149 (X. Zhang). E-mail addresses: [email protected] (L. Wang), xi@ tsinghua.edu.cn (X. Zhang).
such as charge-transfer interaction [7], hydrogen bonding [8,9] and stereocomplex formation [10]. Recently, we have reported that carboxyl-terminated poly(aryl ether) dendrimers can form a single-component film by self-deposition, since it is a hydrogen-bonding acceptor as well as a hydrogen-bonding donor [11]. This research is significant because we can apply it to construct a single-component thin film with controllable thickness, which is different from conventional layer-by-layer assembly. Dendrimers have a number of advantages over linear polymers: controllable exterior groups, identical size, perfectly branched three-dimensional structure, etc. As a result, dendrimer films may significantly enrich the application of thin solid films [12,13]. Herein, we designed and synthesized two generations of carboxyl-terminated dendrimers with a diphenylanthracene core. On the one hand, we would like to know
0014-3057/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.12.014
1220
J. Sun et al. / European Polymer Journal 41 (2005) 1219–1224
if low-generation dendrimers could also form a singlecomponent film by self-deposition. On the other hand, a chromophore encapsulated by dendrons will allow us to fabricate a functional thin film, since the isolation effect of dendrimers can improve the fluorescence properties of dye molecules in the solid state.
2. Experimental 2.1. Synthesis and characterization of dendrimers Two dendrimers were synthesized according to the literature [14], and named An(G1)4 and An(G2)4 (Scheme 1). An(G2)4: The brief route of synthesis is shown in Scheme 2. A mixture of (MeO2C)4-[G2]-Br(5.0 equiv), 9,10-Bis(3,5-Dihydroxyphenyl)anthracene (1.0 equiv), anhydrous K2CO3 (10.0 equiv), 18-crown-6 (0.1 equiv) and acetone was heated to reflux and stirred vigorously under nitrogen in darkness for 144 h. Then the acetone was evaporated, and the residue was put in a mixture of water and CH2Cl2 for partitioning. The organic layer was dried with anhydrous Na2SO4 and then evaporated.
The crude product was purified by silica gel column chromatography with mixture of ether and CH2Cl2 as the eluent. Poly(aryl ether) dendrimers with a diphenylanthracene core and carboxyl methyl esters at the periphery were successfully synthesized (yield 75%). Hydrolysis of the methyl ester was performed in a solution of water (60 ml), ethanol (120 ml), and potassium hydroxide (0.6 g). It was then heated at reflux and vigorously stirred under nitrogen in darkness for 8 h. Next, the ethanol was evaporated and the residue was acidified with dilute hydrochloric acid to form a precipitate. The precipitate was filtered and washed with water to remove all potassium salts. The carboxyl methyl esters were successfully changed to carboxyl groups, i.e. An(G2)4 was synthesized. An(G1)4 was synthesized with the same route. An(G1)4: 1H NMR (500 MHz, DMSO-d6): d 7.93 (16H, d, ArH), d 7.57 (4H, m, core ArH), d 7.51(16H, d, ArH), d 7.32 (4H, m, core ArH), d 6.88(2H, s, ArH), d 6.75 (8H, bs ArH), d 6.68 (4H, bs, ArH), d 6.66 (4H, s, ArH), d 5.15 (16H, s, OCH2), d 5.10 (8H, s, OCH2), MALDI-TOFMS: Calc. for C118H90O28 [M + H]+: m/z 1956.96 Found [M + H]+: m/z 1956.4. An(G2)4: 1H NMR (500 MHz, DMSO-d6): d 7.90(32H, d, ArH), d 7.58 (4H, m, core ArH), d 7.47 (32H, d, ArH), d 7.32 (4H, m, core ArH), d 6.88 (2H, s, ArH), d 6.68 (24H, bs, ArH), d 6.66 (4H, s, ArH), d 6.60 (8H, s, ArH), d 6.58 (4H, s, ArH), d 5.10(32H, s, OCH2), d 5.06 (8H, s, OCH2), d 4.96 (16H, s, OCH2), MALDI-TOFMS: Calc. for C238H186O60 [M + Na]+: m/z 4028.99 Found [M + Na]+: m/z 4028.7. 2.2. Instruments 1 H NMR spectra were recorded on a Bruker Avance500 NMR Spectrometer (500 MHz) using tetramethylsilane as an internal standard. UV–Vis spectra were collected on a Perkin–Elmer Lambda 800 UV–Vis spectrometer. Fluorescence spectra were measured on a Shimadzu RF-5301PC spectrometer. Atomic force microscopy (AFM) images were taken with an atomic force microscope (DimensionTM 3100, Digital Instrument).
2.3. Fabrication of self-deposition films
Scheme 1. The molecular structures of dendrimers.
A self-deposition film of An(G1)4 was fabricated as follows. A quartz substrate was modified with 4-aminobutyldimethylmethoxysilane in advance. The modified substrate was immersed into methanolic solution of An(G1)4 (7.5 · 10 6 mol/l) for 2 min to adsorb the first layer. Then, it was rinsed in methanol and dried in the ambient atmosphere. Simply repeating the above process leads to the formation of a self-deposition film of An(G1)4. We fabricated a self-deposition film of An(G2)4 in the same way.
J. Sun et al. / European Polymer Journal 41 (2005) 1219–1224
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Scheme 2. The brief synthesis route of An(G2)4.
3. Results and discussion UV–Vis absorption spectroscopy was used to monitor the assembling process of the self-deposition films. Fig. 1 shows the UV–Vis absorption spectra of selfdeposition films of An(G2)4 on a quartz substrate. Characteristic absorption bands of diphenylanthracene appear at 256 nm and in the range of 350 410 nm [15], as indicated in inset (b). The absorption bands in the range of 230 300 nm are ascribed to phenyl groups in the branches of dendrimer. The linear increase of absorbance with the number of layers is indicative of a progressive and uniform deposition of An(G2)4, as shown in inset (a). Moreover, UV–Vis spectroscopy indicated that dendrimers adsorbed on the quartz were full-covered and homogeneous. The size of An(G2)4 is similar to the dendrimer in Ref. [11]; both have 16 carboxyl groups at the periphery.
In contrast, An(G1)4 has only eight carboxyl groups at the periphery. Therefore, we are wondering if we can construct a single-component film of An(G1)4 using the self-deposition method. Fig. 2 shows the UV–Vis absorption spectra of self-deposition films of An(G1)4. The linear increase of absorbance with the number of layers is also indicative of a progressive and uniform deposition of An(G1)4. These results suggest that the self-deposition method is suitable for different generations of dendrimers. With this method, it is easy to obtain a single-component film with a controllable thickness at the nanoscale. In order to further confirm the homogeneity of the self-deposition film of dendrimers, AFM was used to observe the morphology of the two self-deposition films. Fig. 3 shows the AFM images of nine-layer films of (a) An(G1)4 and (b) An(G2)4 on quartz slides. Both images exhibit granular morphology with a full and
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Fig. 1. UV–Vis absorption spectra of self-deposition film of An(G2)4. The insets show (a) the growth of absorbance at 230 nm as a function of the number of layers, and (b) the characteristic absorption of diphenylanthracene in a 16-layer film.
Fig. 2. UV–Vis absorption spectra of self-deposition film of An(G1)4. The insets show: (a) the growth of absorbance at 230 nm as a function of the number of layers, and (b) the characteristic absorption of diphenylanthracene in an eightlayer film.
homogeneous coverage. Similar morphology appeared on different places of each film and the experimental results were reproducible. The roughness of the two images was (a) 0.254 nm and (b) 0.457 nm. Differences between the two films are most likely related to the different molecular structures of dendrimers. We also attempted to prepare thin films using the spin-coating and simple casting. Based on UV–Vis spectroscopy, we found it was difficult to control the thickness of the thin film at the nanoscale through
spin-coating and simple casting. Because An(G1)4 has poor solubility (less than 1 · 10 5 mol/l) in methanol and is insoluble in many other solvents, it is difficult to prepare a ‘‘thicker’’ An(G1)4 thin solid film using spincoating or simple casting. We also found that it is difficult to obtain a homogeneous spread of dendrimers on the whole film in cases of spin-coating and simple casting. The above results suggest that self-deposition is a better way to fabricate a homogeneous and thicknesscontrollable single-component film. We further studied the fluorescence property of the dendrimer self-deposition film. For comparison, we initially studied the fluorescence of dilute methanolic solutions of An(G1)4 and An(G2)4. As shown in Fig. 4, the intensity of fluorescence is approximately equal for solutions of An(G1)4 and An(G2)4 with the same molar concentration. This indicates that the fluorescence intensity in dilute solutions is mainly determined by the amount of fluorophore. Although the fluorescence intensity increases with increasing concentration, neither a shift of the maximum emission nor a broadening of the spectra was observed. In order to study the effect of generation number on fluorescence of the dendrimer self-deposition film, we prepared An(G1)4 and An(G2)4 films with approximately equal UV–Vis absorptions in the range of 350 410 nm, the characteristic absorption of a diphenylanthracene core, as shown in Fig. 5 and its inset. Fig. 6 shows fluorescence spectra (excitation at 376 nm) of the self-deposition films of An(G1)4 and An(G2)4. The fluorescence spectrum of each film was obtained from the average intensity at nine different positions. The differences of fluorescence intensities at the nine positions are small, which further confirmed that the films are homogeneous. The fluorescence of the An(G2)4 self-deposition film is 2.4 times as strong as that of the An(G1)4 film at the maximum emission. Considering that the size of An(G2)4 is greater than that of An(G1)4, the density of fluorophore is lower in the An(G2)4 film than in the An(G1)4 film. The diphenylanthracene core is better isolated in a two-generation dendrimer (An(G2)4) than in a one-generation dendrimer (An(G1)4) [16–18]. Therefore, self-quenching in high generation dendrimers can be effectively depressed. The maximum emissions of An(G1)4 and An(G2)4 in solutions and those in self-deposition films are shown in Table 1. Based on these data, the spectra shift (Dk) was calculated. We found the red shift of An(G2)4 is much smaller than that of An(G1)4. This phenomenon further indicates that high-generation dendrimers have a stronger site-isolation effect on their cores than low-generation ones. Many research groups have reported that the photoluminescent property of dye molecules in solid films showed considerable shift from those in solution. In contrast, the difference is quite slight for our dye-con-
J. Sun et al. / European Polymer Journal 41 (2005) 1219–1224
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Fig. 3. AFM height images (1.5 lm · 1.5 lm) of nine-layer films of (a) An(G1)4, and (b) An(G2)4 on quartz slides.
Fig. 5. UV–Vis absorption spectra of the self-deposition films of An(G1)4 and An(G2)4 with approximately equal characteristic absorption of diphenylanthracene in the range of 350 410 nm.
Fig. 4. Fluorescence spectra of: (a) An(G1)4 and (b) An(G2)4 in methanol solution with different concentrations.
taining dendrimers. Therefore, we inferred that this molecular design offers the possibility of predicting dye properties in the solid state according to their properties in a solution, which is important for designing lightemitting materials.
Fig. 6. Fluorescence spectra (excitation at 376 nm) of the selfdeposition films of An(G1)4 and An(G2)4.
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Table 1 The maximum emission and the red shift (Dk) of fluorescence spectra of An(G1)4 and An(G2)4 Dendrimer
kmax/nm (solution)
kmax/nm (film)
Dka/nm
An(G1)4 An(G2)4
412, 430 418, 435
425, 443 423, 440
13 5
a
Dk = kmax (film)
kmax (solution).
4. Conclusions We designed and synthesized two dendrimers with the same fluorophore and different generation numbers. We found that not only two-generation dendrimers but also one-generation dendrimers can be fabricated into self-deposition films. We also found that self-deposition is a better method for fabricating a homogeneous and thickness-controllable single-component film. According to the UV–Vis spectra and fluorescence spectra of the self-deposition films, we inferred the diphenylanthracene core is better isolated in a two-generation dendrimer (An(G2)4) than in a one-generation dendrimer (An(G1)4). The fluorescence property of the dye core of dendrimers in a solid film and that of one in a solution are roughly the same, which will facilitate the design of functional materials.
Acknowledgement We would like to thank Prof. Yuguang Ma and Mr. Fengwei Huo for their help and fruitful discussions. This work is supported by the National Natural Science Foundation of China (20204003, 20334010), and the
Major State Basic Research Development Program (G2000078102).
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