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Intramolecular Exciplex Formation Induced by the Folding-Back Conformation of Poly(aryl ether) Dendrimers† Ming Li,‡,§ Yingying Li,‡ Yi Zeng,‡,§ Jinping Chen,‡ and Yi Li*,‡ Key Laboratory of Photochemical ConVersion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China, and Graduate UniVersity of Chinese Academy of Sciences, Beijing 100049, P. R. China ReceiVed: January 16, 2009; ReVised Manuscript ReceiVed: February 13, 2009
A series of poly(aryl ether) dendrimers Gn-NA (n ) 1-4), generations 1-4, containing a naphthyl core group were synthesized. The steady-state and time-resolved fluorescence studies reveal that Gn-NA (n ) 1, 2) take a loose structure and Gn-NA (n ) 3, 4) bear a more congested structure, which undergo the singlet electron transfer process in dichloromethane from dendritic backbone to the naphthyl group. As a consequence of the electron transfer process, an intramolecular exciplex is formed between the core naphthyl chromophore and the benzyloxy unit of dendritic backbone in G3-NA and G4-NA. Fluorescence measurements in the dichloromethane-acetonitrile binary solvents validate that the intramolecular exciplex formation can be enhanced by the folding back conformation of dendrimers. Introduction Dendrimers are regularly and hierarchically branched macromolecules with numerous chain ends all emanating from a single core.1 The chromophores can be accurately located at the core, focal point, periphery, or even at each branching point of the dendritic structure. Manipulation of dendrimer size, shape, and properties promises to provide a wide range of materials with different potential applications,2 including catalysis,3 selfassembly,4 molecular recognition and encapsulation.5 The chemical structure of dendrimers can be characterized spectroscopically; however, the detailed three-dimensional structures of dendrimers are still not well-understood. A thorough understanding of the molecular structure, including microstructure and conformation, will promote applications of dendrimers. In the early 1980s, de Gennes and Hervet proposed a molecular structure with a dense shell and a loose core,6 while later, Lescanec and Muthukumar predicted a density maximum in the center of dendrimers in 1990.7 Since then, numerous theoretical and experimental studies have been conducted to verify their findings. Computational investigations through molecular dynamics8,9 and Monte Carlo10,11 simulations indicate that a significant backfolding of the end groups into interior occurs in flexible dendritic structures throughout all stages of growth, which is more or less in line with the Lescanec-Muthukumar model, and the experimental results confirmed this theoretical prediction. Meier et al.12 attached an anthracene core to the focal point of Fre´chet-type dendrons, and irradiation of the corresponding dendrimers resulted in an intramolecular cycloaddition between the anthracene and dendrons. Recently, our studies13 on the intramolecular triplet energy transfer and electron transfer between periphery and core groups within poly(aryl ether) dendrimers (Fre´chet-type dendrimers) also demonstrate that these dendrimers take a folding back conformation,14 and an intramolecular exciplex formation between the periphery and †
Part of the “Hiroshi Masuhara Festschrift”. * Towhomcorrespondenceshouldbeaddressed.E-mail:
[email protected]. ‡ Technical Institute of Physics and Chemistry, Chinese Academy of Sciences. § Graduate University of Chinese Academy of Sciences.
the core groups gives a directly experimental observation for the folding back conformation of poly(aryl ether) dendrimers.15 The folding back phenomena can be understood from the conformational entropy of dendrimer molecules.11 The flexible dendrimers access many conformations to maximize the entropy, and the exploration of phase space results in a “dense core” average conformation. Continuing our study on the folding back conformation of poly(aryl ether) dendrimers, a series of Fre´chet-type dendrimers bearing a naphthyl core group Gn-NA (n ) 1-4), generations 1-4, were synthesized, and their photophysical properties in different solvents were investigated. An intramolecular exciplex formation between the core naphthyl group and the benzyloxy unit of dendritic backbone is used to detect the existence of folding back conformation of poly(aryl ether) dendrimers. Results and Discussion Synthesis, Characterization, and Solubility of the Dendrimers. Dendritic benzyl alcohols Gn-OH (n ) 1-4), generations 1-4, were synthesized with Fre´chet’s method.14 Dendrimers with a naphthyl group attached to the core Gn-NA (n ) 1-4), generations 1-4, were synthesized with Gn-OH and 2-naphthoic acid (NA-COOH). Gn-NA (n ) 1-4) were characterized by 1H NMR, IR, and mass spectrometry (MALDITOF). The structures of Gn-NA and the model compound, methyl 2-naphthoate (MNA), are shown in Figure 1. Gn-NA (n ) 1-4) are soluble in a variety of organic solvents such as CH2Cl2, CHCl3, THF, and toluene. Somewhat lower solubility in acetonitrile was observed for generation 1-4. In methanol, only lower generation Gn-NA (n ) 1, 2) could dissolve. A good solvent dichloromethane and a poor solvent acetonitrile were chosen in this study. Steady State Absorption and Fluorescence Spectroscopy of Gn-NA. To search for the evidence of ground-state interactions between the naphthyl group and the dendritic backbone, the absorption spectra of dendrimers Gn-NA, and the model compounds Gn-OH and methyl 2-naphthoate (MNA), were measured in dichloromethane and acetonitrile. All of the compounds show similar spectra in different solvents. Figure 2
10.1021/jp9004554 CCC: $40.75 2009 American Chemical Society Published on Web 03/17/2009
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Figure 1. Structures of the model compound MNA and the target compounds Gn-NA (n ) 1-4).
Figure 2. Absorption spectra for compounds G2-NA, G2-OH, and MNA in CH2Cl2. [G2-NA] ) [G2-OH] ) [MNA] ) 5.0 × 10-5 M.
illustrates the absorption spectra of G2-NA, G2-OH, and MNA in CH2Cl2. The spectrum of Gn-NA closely matches the sum of the absorption of MNA and Gn-OH, indicating the absence of measurable interaction between the naphthyl chromophore and the dendritic backbone of Gn-NA in the ground state.
Significantly, there are only naphthyl chromophore absorption bands above 300 nm. This fact permits selective excitation of the naphthyl moiety in Gn-NA. The fluorescence spectra of Gn-NA with different concentrations were measured in dichloromethane by using the 320 nm excitation light. The emission spectra of Gn-NA (n ) 1-4) are given in Figure 3a, and the fluorescence spectrum of the model compound MNA is also presented in the same figure. The fluorescence characteristic of naphthalene with maxima at 345 and 360 nm and shoulders at 380 and 400 nm was detected for Gn-NA (n ) 1-4) and MNA. In addition, the fluorescence spectra of G3-NA and G4-NA show an obvious long tail extending to the lower energy. By using PeakFit software, the fluorescence spectra of MNA, G1-NA, and G2-NA can be fit well with four bands. For G3-NA and G4-NA, in addition to the four bands that also appear in MNA, a structureless emission appears at lower energy with maximum at 430 nm. Typical spectrum for G3-NA is presented in Figure 3b. The excitation spectra for Gn-NA (n ) 1-4) by monitoring 345 and 430 nm correspond to the UV absorption of Gn-NA. These observations suggest that the structureless emission species at 430 nm is
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Figure 3. (a) Fluorescence spectra for Gn-NA and the model compound MNA in CH2Cl2. (b) Fluorescence spectrum for G3-NA in CH2Cl2 with fitting bands, dot: experiment data; blue line: sum of fitting bands; red and black line: fitting bands. λex ) 320 nm, [Gn-NA] ) [MNA] ) 5.0 × 10-5 M.
formed at the excited state, which agrees with the absorption experiment result. Measurements at different concentrations (from 1 × 10-5 M to 5 × 10-4 M) reveal that the broadband species is formed intramolecularly, which means that the excimer formation between the naphthyl chromophores in different dendrimer molecules can be excluded. Therefore, the structureless band can be assigned to the exciplex emission between the core naphthyl chromophore and the benzyloxy group of dendron.17 With the exception of the fluorescence shape difference, the fluorescence efficiency of the naphthyl chromophore in Gn-NA decreases with the generation increase. This finding indicates that the fluorescence of naphthyl group is quenched by the intramolecular dendritic backbone. Since the possibility of singlet-singlet energy transfer being responsible for the intramolecular naphthyl fluorescence quenching by the dendritic backbone in Gn-NA is excluded from thermodynamic grounds, as mentioned above, we analyzed the reality of electron transfer between the naphthyl chromophore and the dendritic backbone as the cause of the fluorescence quenching. The free energy change involved in an electron transfer process can be estimated by the Rehm-Weller equation.10
∆G(kcal/mol) ) 23.06[E(D•+ /D) - E(A/A•-) e2 /rε] - E00(kcal/mol)
(1)
E00 is the excited-state energy, and which represents the singlet excited-state energy of the naphthyl group here (83 kcal/mol, which is estimated from the 0-0 emission band of the naphthyl fluorescence spectra). The redox potentials of MNA, E(MNA/ MNA•-), and G1-OH, E(G1-OH•+/G1-OH), were determined to be -1.58 and +1.14 V in CH2Cl2, respectively, with respect to Ag/Ag+. The e2/rε represents the Coulombic energy associated with bringing separated radical ions at a distance r in a solvent of dielectric constant ε. Analysis of eq (1) shows that the change of r will result in only a minor impact on ∆G intramolecularly. Estimation according to eq (1) reveals that the electron transfer from the dendrimer backbone to the naphthyl group in CH2Cl2 (ε ) 9.08D) is exothermic within dendrimer molecule for generation 1-4. Therefore, the fluorescence quenching of the naphthyl chromophore in G3-NA and G4-NA is caused by the intramolecular electron transfer between the naphthyl group and the dendritic backbone. Generally, the electron transfer process requires a strong donor-acceptor orbital overlap, while G1-NA and G2-NA do not meet this
requirement due to their loose structures. Therefore, no fluorescence quenching of the naphthyl chromophore is observed in G1-NA and G2-NA, even the electron transfer from the dendrimer backbone to the naphthyl group is thermodynamically possible. The highly folding back conformation of G3-NA and G4NA can be inferred as the cause of intramolecular electron transfer and exciplex formation, which is confirmed by the further fluorescence studies in dichloromethane-acetonitrile binary solvents. Dichloromethane and acetonitrile are good and poor solvents for poly(aryl ether) dendrimers, respectively. Dendrimers will take an extended conformation in good solvent. The poor solvent, with a poor capability to solvate the dendrimers, leads to a decrease in hydrodynamical volume indicative of increased intramolecular π-π interactions.16 Therefore, the intramolecular exciplex formation, which requires a direct orbital overlap between the two chromophores, should increase with the poor solvent portion in the CH2Cl2-CH3CN binary solvents. Figure 4a shows the fluorescence spectra of G4-NA and MNA (5 × 10-5 M) in CH2Cl2-CH3CN at ambient conditions. In the mixed solvents with various ratios of CH2Cl2 to CH3CN, G1-NA, G2-NA, and MNA only exhibit fluorescence characteristic of the naphthyl monomer. Less solvent effect on the conformation of lower generation dendrimers leads to no intramolecular exciplex formation within G1-NA and G2-NA. The behavior of G3-NA and G4-NA is quite different from that of MNA and lower generations Gn-NA (n ) 1-2). The fluorescence emission of naphthyl monomer decreases and the structureless exciplex emission increases with increasing the portion of acetonitrile in the mixed solvents. For clear observation, Figure 4b illustrates that the ratio of the fluorescence intensity of exciplex to monomer, I430/I345 changes with the composition of the mixed solvents. At concentrations of lower than 5 × 10-5 M, I430/I345 is essentially independent of concentration, indicating that the exciplex is intramolecular. These experiment results validate that the intramolecular electron transfer and exciplex formation within G3-NA and G4-NA are induced by the folding of dendritic backbone. Time-Resolved Fluorescence Spectra of Gn-NA. To further investigate the intramolecular interaction of Gn-NA, the fluorescence lifetimes for Gn-NA (n ) 1-4, 5 × 10-6 M) were determined in CH2Cl2-CH3CN binary solvent with ratios 1:0, 1:1, and 1:9 at ambient conditions. The excitation wavelength was 280 nm, and the fluorescence decays were monitored at 345 and 430 nm, which correspond to the naphthyl monomer and the exciplex emission maxima, respectively. The steady state
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Figure 4. (a) Fluorescence spectra of G4-NA and MNA (5 × 10-5 M) in the CH2Cl2-CH3CN mixed solvents with various ratios of CH2Cl2 to CH3CN at ambient conditions. (b) I430/I345 changes with the ratio of CH2Cl2 to CH3CN. λex ) 320 nm.
fluorescence experiments indicate that only naphthyl chromophore emits at 345 nm and the emission at 430 nm is basically from the exciplex. G1-NA and G2-NA mainly exhibit monoexponential profile at 345 nm except G2-NA in the CH2Cl2-CH3CN solvent with ratio of 1:9, but the decay curves obtained at 430 nm can not be fitted well to give quality lifetime data due to the very weak emission. Neither of G3-NA and G4NA shows monoexponential decay at 345 nm. The acquired data could be well fitted by double-exponentials with acceptable χ2 value. However, it should be noted that the number of exponentials used is not intended to signify the exact number of distinct processes being observed and is merely a qualitative indication of the degree of inhomogeneity in the system. This result is quite compatible with the data reported for laser-dyelabeled poly(aryl ether) dendrimers by Fre´chet and co-workers.18 The flexible nature of dendrimers is proposed to be responsible for the nonmonoexponential behavior. The conformational freedom of the dendritic backbone creates a variety of local microenvironments for the individual naphthyl chromophore. Since the fluorescence lifetime of naphthyl chromophore is known to be sensitive to the environments, such as solvent polarity (the fluorescence lifetimes of MNA were measured to be 7.7, 5.8, and 8.5 ns in toluene, dichloromethane, and acetonitrile, respectively), the changes of the local microenvironment induced by different dendrimer conformations would affect the lifetime of the individual naphthyl chromophore dramatically. This phenomenon would be expected to become more significant at higher generations, and this is observed in the nonexponential decays for G3-NA and G4-NA in CH2Cl2. The decay curves obtained at 430 nm for G3-NA and G4-NA in CH2Cl2 could also be well fitted by double-exponentials with a shorter and a longer lifetimes. Combining the lifetime and the steady-state fluorescence experiment results the longer lifetime species could be assigned to the exciplex, and the minor shorter lifetime species could be assigned to the naphthyl group. No exciplex formation process was detected in our exciplex kinetics studies. The rapidly occurred electron transfer process between the naphthyl group and the dendritic backbone might be responsible for this because the naphthyl group was quite close to the dendritic backbone within the densely packed G3NA and G4-NA molecules. All of the lifetime data are summarized in Table 1. Dendrimers, especially for the higher generation molecules, prefer to take more congested conformation in poor solvent. With increasing the portion of the poor solvent acetonitrile in the CH2Cl2-CH3CN mixed solvent the shorter fluorescence lifetime at 345 nm for the naphthyl chromophore of G3-NA
TABLE 1: Fluorescence Lifetime for Dendrimers Gn-NA (n ) 1-4) and MNA in CH2Cl2-CH3CN Mixed Solvents, λex ) 280 nm CH2Cl2/CH3CN λem ) 345 nm MNA 1:0 G1-NA 1:0 5:5 1:9 G2-NA 1:0 5:5 1:9 G3-NA 1:0 5:5 1:9 G4-NA 1:0 5:5 1:9 λem ) 430 nm G3-NA 1:0 5:5 1:9 G4-NA 1:0 5:5 1:9
a1
τ1 (ns)
a2
τ2 (ns)
average
5.8
5.8
5.0 5.4 5.9
5.0 5.4 5.9
0.45
5.3 5.6 2.1
0.55
5.6
5.3 5.6 4.0
0.48 0.75 0.83
4.0 2.1 1.7
0.52 0.25 0.17
7.7 10.0 10.3
5.9 4.0 3.2
0.57 0.72 0.88
2.2 1.7 1.5
0.43 0.28 0.12
8.8 10.3 11.0
5.0 4.1 2.6
0.20 0.06 0.08
6.0 3.5 3.2
0.80 0.94 0.92
8.1 11.8 10.7
0.16 0.06 0.10
5.1 4.1 2.6
0.84 0.94 0.90
10.7 10.7 12.6
and G4-NA manifests the enhancement of the electron transfer between the naphthyl chromophore and the dendritic backbone. The increase of the exciplex formation with adding the poor solvent acetonitrile to dichloromethane is also observed in the increasing amplitudes of the longer lifetime component in the fluorescence decays at 430 nm. The exciplex lifetime of G4NA is longer than that of G3-NA, which can be attributed to the increasing steric protection for the core. Conclusions In conclusion, we have prepared a series of poly(aryl ether) dendrimers containing a naphthyl chromophore at their focal point. Steady-state and time-resolved fluorescence measurements in CH2Cl2 at ambient conditions reveal that Gn-NA (n ) 1, 2)
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take a loose structure and Gn-NA (n ) 3, 4) bear a more congested structure, which undergo the intromolecuar electron transfer process between dendritic backbone and the core naphthyl chromophore. As a consequence of the electron transfer process, an intramolecular exciplex between the core naphthyl chromophore and the benzyloxy unit of dendritic backbone is formed. Photophysical studies in the CH2Cl2-CH3CN binary solvents with various ratios of CH2Cl2 to CH3CN manifest that the intramolecular electron transfer and the exciplex formation are induced by the folding back conformation of dendrimers. These findings provide a substantial insight into the folding back conformation of poly(aryl ether) dendrimers and are of great benefit to their application. Experimental Section Materials. Reagents which were purchased from Aldrich, Acros, or Beijing Chemical Work were used without further purification, unless otherwise noted. Spectral-grade acetonitrile (CH3CN) and dichloromethane (CH2Cl2) were used for steadystate and time-resolved photophysical measurements. Instrumentation. 1H NMR spectra were recorded on a Bruker 400 MHz spectrometer. IR spectra were run on a Bio-Rad Win IR spectrometer. MALDI-TOF mass spectrometry was performed on a Bruker MicroFlex MALDI MS. Melting point was determined on a XT4A apparatus and was uncorrected. Steadystate absorption spectra and fluorescence spectra were measured by a Shimadzu UV-1601PC spectrometer and a Hitachi F-4500 spectrometer, respectively. Fluorescence decay processes were recorded with single photon counting technique on an Edinburgh FLS920 fluorescence lifetime system, and the equipment resolution is ∼0.1 ns. Redox Potentials of G1-OH and MNA. The redox potentials of G1-OH and methyl 2-naphthoate (MNA) were determined by cyclic voltammetry in dichloromethane, using a 10 µm platinum microelectrode and a Ag/Ag+ (the concentration of Ag+ is 0.01 M) reference electrode in the presence of 0.1 M tetrabutylammonium perchlorate as the supporting electrolyte. General Procedure for the Synthesis of Gn-NA. These reactions were carried out on scales of about 100 mg. A mixture of the appropriate dendritic benzyl alcohol (Gn-OH, 1.00 equiv.), 2-naphthoic acid (NA-COOH, 3.00 equiv.), 1,3-dicyclohexylcarbodiimide (DCC, 3.00 equiv.), and 4-dimethylaminopyridine (0.1 equiv.) in dry CH2Cl2 was stirred under nitrogen for 24 h. For generations 3 and 4, larger excess amounts of NA-COOH and DCC were required to force the reaction completion. After the reaction, the reaction mixture was diluted with CH2Cl2 and partitioned into the water and the CH2Cl2 phases, and the aqueous layer was extracted with CH2Cl2 (3×). The combined organic layers were dried with MgSO4 and evaporated to dryness. The crude product was purified as outlining in the following text. G1-NA (1) was prepared from G1-OH and NA-COOH, purified by column chromatography eluting with 1/3 ethyl ether/ CH2Cl2 to give (1) as white solid: yield 88%; mp 116-117 °C. 1 H NMR (400 MHz, CDCl3) δ 5.06 (s, 4H, PhCH2-), 5.36 (s, 2H, -COOCH2-), 6.62 (s, 1H, ArH), 6.73 (s, 2H,ArH), 7.30-7.33 (t, 2H, ArH), 7.37-7.39 (t, 4H, ArH), 7.42-7.44 (d, 4H, ArH), 7.53-7.62 (m, 2H, naphthalene ring H), 7.88-7.90 (d, 2H, naphthalene ring H), 7.95-7.97 (d, 2H, naphthalene ring H), 8.07-8.09 (m, 1H, naphthalene ring H), 8.64 (s, 1H, naphthalene ring H). IR (KBr) ν (cm-1) 1716, 1596, 1452, 1375, 1283, 1159, 1056. MS (MALDI-TOF) for C32H26O4: m/z 497 [M+Na+], calcd m/z 474.55.
Li et al. G2-NA (2) was prepared from G2-OH and NA-COOH, purified by column chromatography eluting with 1/5 ethyl ether/ CH2Cl2 to give (2) as a colorless glassy solid: yield 86%. 1H NMR (400 MHz, CDCl3) δ 5.00 (s, 4H, -PhCH2-), 5.02 (s, 8H, PhCH2-), 5.35 (s, 2H, -COOCH2-), 6.56-6.72 (m, 9H, ArH), 7.29-7.41 (m, 20H, ArH), 7.53-7.61 (m, 2H, naphthalene ring H), 7.85-7.88 (d, 2H, naphthalene ring H), 7.93-7.95 (d, 1H, naphthalene ring H), 8.06-8.09 (m, 1H, naphthalene ring H), 8.64 (s, 1H, naphthalene ring H). IR (KBr) ν (cm-1) 1717, 1596, 1451, 1375, 1295, 1156, 1054. MS (MALDI-TOF) for C60H50O8: m/z 921 [M+Na+], calcd m/z 899.03. G3-NA (3) was prepared from G3-OH and NA-COOH, purified by column chromatography eluting with 1/10 ethyl ether/CH2Cl2 to give (3) as a colorless glassy solid: yield 81%. 1 H NMR (400 MHz, CDCl3) δ 4.94 (s, 8H, -PhCH2-), 4.98 (s, 4H, -PhCH2-), 5.00 (s, 16H, PhCH2-), 5.33 (s, 2H, -COOCH2-), 6.52-6.72 (m, 21H, ArH), 7.26-7.40 (m, 40H, ArH), 7.51-7.55 (m, 2H, naphthalene ring H), 7.82-7.85 (m, 2H, naphthalene ring H), 7.90-7.92 (d, 1H, naphthalene ring H), 8.04-9.06 (m, 1H, naphthalene ring H), 8.63 (s, 1H, naphthalene ring H). IR (KBr) ν (cm-1) 1717, 1595, 1450, 1375, 1296, 1155, 1053. MS (MALDI-TOF) for C116H98O16: m/z 1771 [M+Na+], calcd m/z 1748.01. G4-NA (4) was prepared from G4-OH and NA-COOH, purified by column chromatography eluting with 1/20 ethyl ether/CH2Cl2 to give (4) as a colorless glassy solid: yield 89%. 1 H NMR (400 MHz, CDCl3) δ 4.90 (s, 24H, -PhCH2-), 4.92 (s, 4H, -PhCH2-), 4.96 (s, 32H, PhCH2-), 5.30 (s, 2H, -COOCH2-), 6.50-6.67 (m, 45H, ArH), 7.26-7.37 (m, 80H, ArH), 7.42-7.52 (m, 2H, naphthalene ring H), 7.76-7.80 (m, 2H, naphthalene ring H), 7.85-7.87 (d, 1H, naphthalene ring H), 8.00-8.03 (m, 1H, naphthalene ring H), 8.58 (s, 1H, naphthalene ring H). IR (KBr) ν (cm-1) 1717, 1595, 1450, 1375, 1295, 1155, 1053. MS (MALDI-TOF) for C228H194O32: m/z 3481 [M+K+], calcd m/z 3445.96. Acknowledgment. Financial support of our research by the National Natural Science Foundation of China (Nos. 20574086, 20603042, 20733007, and 20772134) and the National Basic Research Program (2007CB808004) is gratefully acknowledged. References and Notes (1) (a) Tomalia, D. A.; Durst, H. D. Top. Curr. Chem. 1993, 165, 193. (b) Ardoin, N.; Astruc, D. Bull. Soc. Chim. Fr. 1995, 132, 875. (c) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendrimers and Dendrons: Concept, Syntheses, Application; Wiley-VCH, Weinheim, Germany, 2001. (2) (a) Adronov, A.; Fre´chet, J. M. J. Chem. Commun. 2000, 1701. (b) Juris, A. Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim, 2001; Vol. 3, p 655. (c) Hecht, S.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. 2001, 40, 74. (d) Gorman, C. B.; Smith, J. C. Acc. Chem. Res. 2001, 34, 60. (e) Balzani, V.; Ceroni, P.; Juris, A.; Venturi, M.; Campagna, S.; Puntoriero, F.; Serroni, S Coord. Chem. ReV. 2001, 219221, 545. (f) Grayson, S. M.; Fre´chet, J. M. J. Chem. ReV. 2001, 101, 3819. (g) Oosterom, G. E.; Reek, J. N. H.; Kamer, P. C. J.; van Leeuwen, P. W. N. M. Angew. Chem., Int. Ed. 2001, 40, 1828. (3) Tomalia, D. A.; Dvornic, P. R. Nature 1994, 372, 617. (4) (a) Philp, D.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1996, 35, 1155. (b) Watanabe, S.; Regen, S. L. J. Am. Chem. Soc. 1994, 116, 8855. (5) Peppas, N. A.; Nagai, T.; Miyajima, M. Pharm. Techn. Jpn. 1994, 10, 611. (6) de Gennes, P. G.; Hervet, H. J. J. Phys., Lett. 1983, 44, L351. (7) Lescanec, R. L.; Muthukumar, M. Macromolecules 1990, 23, 2280. (8) Naylor, A. M.; Goddard, W. A., III.; Kiefer, G. E.; Tomalia, D. A. J. Am. Chem. Soc. 1989, 111, 2339. (9) Scherrenberg, R.; Coussens, B.; van Vliet, P.; Edouard, G.; Brackman, J.; de Brabander, E.; Mortensen, K. Macromolecules 1998, 31, 456. (10) Zhou, T.; Chen, S. B. Macromolecules 2005, 38, 8554.
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