J. Phys. Chem. B 2001, 105, 8729-8731
8729
Excimer Formation in a Naphthalene-Labeled Dendrimer† Tarek H. Ghaddar, James K. Whitesell,* and Marye Anne Fox* Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695 ReceiVed: March 12, 2001; In Final Form: May 26, 2001
Steady-state and time-resolved fluorescence measurements were conducted on newly synthesized Frechettype dendrimers with naphthalene peripheral groups in homogeneous and sodium dodecyl sulfate (SDS) micellar solutions. Intense excimer emission was observed for the third-generation dendrimer 3 but not for the firstgeneration dendrimer 1a. This different spectroscopic behavior was attributed to the geometry of dendrimer 3, where excimeric interactions between adjacent π-stacked naphthyl groups are present.
Introduction
SCHEME 1
Polymers containing high concentrations of redox chromophores have been extensively studied for their long-range charge transport properties, in part because of their potential to create a long-lived charge-separated state.1-8 However, the flexible nature of many polymer scaffolds limited their applications, and excimers and exciplexes were found to act as energy traps.1,5,8,9 In contrast, dendritic polymers possess an ideal framework for the study of chromophore-labeled polymers because of their unique molecular architectures.10 First, their core-shell structure lessens any entanglement of the polymer chains that can lead to excimer formation and, hence, to an energy trap for the harvested light. Second, their preparation and solubility properties are far superior to those of extended linear polymers. Results and Discussion In our quest for a better understanding of the photophysical and charge transport properties of chromophore-labeled polymers, we synthesized Frechet-type dendrimers11 (1a, 2a and 3 Scheme 1) bearing naphthyl peripheral groups and studied their photophysical properties in dichloromethane (CH2Cl2), tetrahydrofuran (THF), and sodium dodecyl sulfate (SDS) micellar solution. Dendrimer 3 showed interesting steady-state fluorescence spectra in THF and CH2Cl2, where a shoulder at 400 nm was shown to be concentration independent in the range 10-4 to 10-9 M (Figures 1 and 2). This concentration independence suggests that excimer fluorescence may not be due to groundstate aggregation. However, the intensity of the 400 nm band was weaker in THF than in CH2Cl2, probably because of a difference in conformation and dynamics of the dendrimer in different solvents. This 400 nm band was not present in the monomer 1a, even at 10-3 M concentration. In the secondgeneration dendrimer 2a, this band was present but with a lower intensity than that in 3 (Table 1). To investigate this excimer emission, the excitation spectrum of dendrimer 3 was monitored at 333 and 400 nm. No difference in the two excitation spectra could be observed: this rules out any significant charge-transfer processes between the naphthyl and the phenoxy groups within dendrimer 3. Consistent with this explanation, the fluorescence spectrum of monomer 1a lacked the 400 nm band. †
Part of the special issue “Royce W. Murray Festschrift”. * Corresponding author.
To understand better the excimer fluorescence at 400 nm, we measured the fluorescence spectrum of dendrimer 3 in aqueous SDS micellar solution. In micellar solution with a fixed ratio of solute to micelle ([S]/[micelle] < 0.05), one dendrimer molecule can be trapped at a time inside the micelle.12 In this situation, the fluorescence of dendrimer 3 can reveal whether the 400 nm band is or is not caused by excimer aggregation. A very different fluorescence profile for dendrimer 3 was found in the micelle, where a much more intense band at 380 nm was observed compared to that at 333 nm (Figure 1). The fluorescence spectrum of 3 is an overlay of two fluorescence spectra; one is very similar to that observed in CH2Cl2 or THF, and the other is red-shifted. This spectral shift can be the result of two different environments where the dendrimer resides; one where it is accessible to the solvent near the micelle surface or even outside the micelle, and the other likely within the micellar pocket. This phenomenon was further investigated by fluorescencelifetime measurements in CH2Cl2, THF, and SDS solutions.
10.1021/jp010933x CCC: $20.00 © 2001 American Chemical Society Published on Web 07/10/2001
8730 J. Phys. Chem. B, Vol. 105, No. 37, 2001
Ghaddar et al.
Figure 1. Steady-state fluorescence spectra of dendrimer 3 in: (solid line) CH2Cl2, (dotted line) THF, and (dotted-dashed line) 2 mM SDS; λex ) 265 nm.
Figure 3. Decay trace of dendrimer 3 in THF; λex ) 265 nm and λem ) 333 nm.
Figure 2. Concentration dependence of the steady-state fluorescence of dendrimer 3 in CH2Cl2. The solid circles are the fluorescence intensity of the 333 nm band. The open circles are the excimer fluorescence intensity at 400 nm.
TABLE 1: Absorption and Emission Data of the Different Dendrons Excited at 265 nm compound
(cm-1 M-1)a
λem(nm)a
I400/I333a,c
I400/I333b,c
1a 2a 3
14000 32000 130000
333 333, 400 333, 400
0.053 0.222
0.033 0.154
a
b
c
In CH2Cl2. In THF. I400/I333 is the fluorescence intensity ratio of the 400 nm to the 333 nm bands.
TABLE 2: Fluorescence Lifetimes of Dendrimer 3 in Different Solvents Excited at 265 nm solvent
τ (ns) at 333 nm
τ (ns) at 400 nm
THF CH2Cl2 SDS
9 (0.93), 53 (0.07) 7 (0.95), 53 (0.05) 6 (0.98), 59 (0.02)
11 8 13 (0.50), 46 (0.50)
Upon excitation at 333 nm, dendrimer 3 showed biexponential decay in all three solvents (Table 2). The major component (more than 90%) exhibits a lifetime of 6-9 ns (Figure 3), which is very similar to that of dendrimer 1a (6 ns). The minor component’s lifetime is in the range of 53 ns to 59 ns. Upon excitation at 400 nm, dendrimer 3 shows a monoexponential decay in CH2Cl2 and THF, with lifetimes of 8 and 11 ns, respectively. Although this lifetime is longer than that observed at 333 nm, it is much lower in intensity. However, in the SDS
Figure 4. Decay trace of dendrimer 3 in SDS; λex ) 265 nm and λem ) 380 nm.
solution, the fluorescence decay at 400 nm is biexponential (13 and 46 ns) with approximately the same intensity (Figure 4). This much longer component is not present at 400 nm in CH2Cl2 or THF. Again, this may reflect dendrimer 3 experiencing two different environments, one in direct contact with the solvent which gives rise to the fast component at 333 nm and another where the dendrimer is buried inside the micellar cavity and does not experience a large solvent effect. The SDS micelle has a hydrophobic core where water molecules can barely penetrate.13 If the dendrimer resides in that hydrophobic core, then little effect of the solvent on the fluorescence lifetime can be seen, giving rise to a much longer lifetime. Such longeremission lifetime of a fluorophore has been observed before in a micellar solution,14 but no significant red shift was reported for unsubstituted naphthalene in SDS.15 Molecular modeling studies for dendrimers 1a, 2a, and 3 showed that in dendrimers 2a and 3, there are one and four pairs of naphthyl groups found within 3.5 Å of each other, respectively. Figure 5 shows a representation of the van der Waals structure of the energy-minimized dendrimer obtained
Naphthalene-Labeled Dendrimer
Figure 5. Energy-minimized structure of dendrimer 3.
through molecular mechanics calculations where π-stacked naphthyl groups can be seen. This kind of π-stacking is likely responsible for the observed excimer fluorescence. Conclusions The photophysical properties of first-, second-, and thirdgeneration dendrimers bearing peripheral naphthyl groups have been studied in different solvents. An intense excimer emission was observed in CH2Cl2 and THF. To understand the structural feature producing this excimer emission, we conducted steady state and fluorescence lifetime measurements in homogeneous solution and in micellar solution. All of these measurements, clarified through molecular modeling, suggest that excimer emission derives from neighboring naphthyl groups within the dendrimer itself, rather than from intermolecular aggregation of the dendrimer. In a dendrimer, with a framework in which the end-bound chromophores are less flexible than those in an extended polymer, and the peripheral groups have more freedom of motion, which may produce noncovalent association between groups, resulting in excimer formation as in dendrimers 2a and 3. Therefore, geometry and conformation are very important qualities of a desired dendrimer for the study of the photophysical properties of attached chromophores. Experimental Section Instrumentation. The NMR spectra were recorded with a Varian Gemini 300 MHz. Absorption spectra were recorded on a Shimadzu PC-3101PC instrument. Fluorescence spectra were measured on a PTI QuantaMaster model C-60/2000 spectrofluorometer. Fluorescence-lifetime measurements were made on a PTI StrobeMaster Fluorescence spectrometer with a photon counting stroboscopic detector. Molecular Mechanics Calculations. All calculations were carried out using a Molecular Mechanics program (MM2),16 a part of the CAChe WorkSystem (v. 3.8), from CAChe Scientific Co. The initial structure of the dendrimer molecule was constructed from individual energy-minimized monomers. Local minima were avoided by choosing the lowest-energy configuration from a sequential minimization by varying all the dihedral angles of the inter-ring bonds in 12° steps. The selected structure was further optimized to within 0.0003 kcal/mol.
J. Phys. Chem. B, Vol. 105, No. 37, 2001 8731 Chemicals. All chemicals were purchased from Aldrich except for sodium dodecyl sulfate SDS (Sigma). Potassium carbonate was dried in an oven at 130 °C prior to use. 3,5Dihydroxybenzyl alcohol was recrystallized from ethyl acetate prior to use. Methylene chloride and tetrahydrofuran were distilled from calcium hydride and sodium/benzophenone, respectively. Samples for the Fluorescence Measurements. All timeresolved and steady-state fluorescence spectra were conducted on a PTI fluorescence spectrometer, with an excitation wavelength 265 nm and concentrations of around 10-6 M. The micellar solutions were prepared by the addition of 1 µL of the fluorophores (10-2 M) in THF to 2 mM SDS (10 mL), followed by sonication for 1 h and being left in the dark overnight prior to use. Preparation of Dendrimers 1a, 1b, 2a and 2b. These compounds were prepared using the same procedures as those in ref 6. Preparation of Dendrimer 3. 3,5-Bis[3,5-bis(2-naphthylmethyloxy) benzyloxy]-benzyl bromide (2b) (220 mg, 0.218 mmol), pentaerythritol (6.5 mg, 0.0478 mmol), TBAB (20 mg, 0.063 mmol), and excess sodium hydride (200 mg, 8.33 mmol) in 10 mL anhydrous DMF were stirred under argon for 18 h, after which 20 mL of water was added slowly. The gummy solid was collected by filtration and purified by column chromatography (silica gel, 17:8 CH2Cl2:Hexane). The major fraction was collected and the volatiles were removed under vacuum to afford a white crystalline powder (30 mg, 15% yield), mp 80-84 °C. H1 NMR 300 MHz (CDCl3): 3.55 (s, 8 H), 4.38 (s, 8 H), 4.73 (s, 16 H), 4.96 (s, 32 H), 6.41-6.50 (m, 12 H), 6.52-6.59 (m, 24 H), 7.34-7.39 (m, 48 H), 7.69-7.74 (m, 64 H). Anal. Calcd for C256H212O28: C, 82.8; H, 5.6; O, 11.6. Found: C, 82.7; H, 5.7; O, 11.5. Acknowledgment. This work was supported by the National Science Foundation. The authors would like to thank Dr. Kevin Kittredge for his assistance with the fluorescence measurements. References and Notes (1) Webber, S. E. Chem. ReV. 1990, 90, 1469. (2) Spies, C.; Gehrke, R. Macromolecules 1997, 30, 1701-1710. (3) Fox, M. A.; Watkins, D. M.; Jones, W. E. Chem. Eng. News 1993, 38, 8. (4) Fox, M. A. Acc. Chem. Res. 1992, 25, 569. (5) Fox, M. A.; Thompson, H. W. Macromolecules 1997, 30, 73917396. (6) Stewart, G. M.; Fox, M. A. J. Am. Chem. Soc. 1996, 118, 43544360. (7) Stewart, G. M.; Fox, M. A. Chem. Mater. 1998, 10, 860-863. (8) Watkins, D. M.; Fox, M. A. J. Am. Chem. Soc. 1994, 116, 64416442. (9) Swallen, S. F.; Zhu, Z.; Moore, J. S.; Kopelman, R. J. Phys. Chem. B 2000, 104, 3988-3995. (10) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. ReV. 1999, 99, 1665-1688. (11) Hawker, C. J.; Frechet, J. M. J. J. Am. Chem. Soc. 1990, 112, 76387647. (12) Alargova, R. G.; Kochijashky, I. I.; Sierra, M. L.; Zana, R. Langmuir 1998, 14, 5412-5418. (13) Thomas, J. K. Chem. ReV. 1980, 80, 283-299. (14) Huang, H.; Verrall, R. E. Langmuir 1997, 13, 4821-4828. (15) Almgren, A.; Grieser, F.; Thomas, J. K. J. Am. Chem. Soc. 1979, 101, 279-291. (16) Allinger, N. L. J. Am. Chem. Soc. 1977, 99, 8127.
