The effect of macromolecular architecture in nanomaterials: A comparison of site isolation in porphyrin core dendrimers and their isomeric linear analogues

Article abstract of DOI:10.1021/ja025536u

The influence of macromolecular architecture on the physical properties of polymeric materials has been studied by comparing poly(benzyl ether) dendrons with their exact linear analogues. The results clearly confirm the anticipation that dendrimers are un

Full text of DOI:10.1021/ja025536u

Published on Web 03/23/2002
The Effect of Macromolecular Architecture in Nanomaterials:
A Comparison of Site Isolation in Porphyrin Core Dendrimers
and Their Isomeric Linear Analogues
Eva M. Harth,† Stefan Hecht,‡,§ Brett Helms,†,‡ Eva E. Malmstrom,†
Jean M. J. Fre´chet,*,‡ and Craig J. Hawker*,†
Contribution from the IBM Almaden Research Center, 650 Harry Road,
San Jose, California 95120, and Department of Chemistry, UniVersity of California,
Berkeley, California 94720-1460
Received January 7, 2002
Abstract: The influence of macromolecular architecture on the physical properties of polymeric materials
has been studied by comparing poly(benzyl ether) dendrons with their exact linear analogues. The results
clearly confirm the anticipation that dendrimers are unique when compared to other architectures. Physical
properties, from hydrodynamic volume to crystallinity, were shown to be different, and in a comparative
study of core encapsulation in macromolecules of different architecture, energy transduction from the polymer
backbone to a porphyrin core was shown to be different for dendrimers as compared to that of isomeric
four- or eight-arm star polymers. Fluorescence excitation revealed strong, morphology dependent
intramolecular energy transfer in the three macromolecular isomers investigated. Even at high generations,
the dendrimers exhibited the most efficient energy transfer, thereby indicating that the dendritic architecture
affords superior site isolation to the central porphyrin it surrounds.
Introduction
following: (i) their regularly layered and symmetrically branched
three-dimensional architecture, (ii) their near-perfect mono-
The controlled preparation of functionalized nanoscale ma-
terials has rapidly become a vigorous research topic due to their
application as active components in advanced materials.1-4 In
surveying materials for the growing needs of nanotechnology,
a range of different macromolecular architectures has been
developed in recent years to meet this demand. These include
shell cross-linked nanoparticles,5 hyperbranched macromol-
ecules,6 dendrimers,7 etc. The latter have received particular
attention due to their unique structural features including the
disperse nature, and (iii) their accurately controlled placement
of functionalities.8 Such features, not found in other synthetic
polymers, have spurred rapid growth of the field of dendrimer
research, largely based on the assumption that inherent differ-
ences in architecture can lead to a range of new and improved
properties for dendrimers when compared to more traditional
polymers.
While a number of studies have addressed the effect of
macromolecular architecture on physical and chemical proper-
ties,9 comparative studies involving different macromolecular
* To whom correspondence should be addressed. E-mail: hawker@
almaden.ibm.com.
† IBM Almaden Research Center.
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‡ University of California, Berkeley.
§ Present address: Freie Universitat Berlin, Institute of Organic Chem-
istry.
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J. AM. CHEM. SOC. 2002, 124, 3926-3938
10.1021/ja025536u CCC: $22.00 © 2002 American Chemical Society

Macromolecular Architecture in Nanomaterials
A R T I C L E S
architectures10,11 have seldom been performed, mainly due to
the synthetic difficulties intrinsic to the preparation of molecular
structures sufficiently similar to enable comparisons between
dendrimers and their architectural isomers.11-13 In this paper,
we report on the synthesis and characterization of monodisperse
architectural isomers of poly(benzyl ethers) dendrons and
dendrimers with a porphyrin core, ranging from exact linear
analogues to four- and eight-arm star polymers also containing
a central porphyrin. In an effort to fully understand the steric
environment and encapsulation behavior of these architectural
isomers, a detailed investigation into intramolecular energy
transfer involving the polymer backbone is presented and the
results related to previously reported morphology and antenna
effects in porphyrin-core dendrimers of the poly(benzyl ether)
type.14
Fluorolog 3.22 equipped with a 450 W Xe lamp, double excitation
and double emission monochromators, and a digital photon-counting
photomultiplier. Excitation spectra of compounds 19b-d, 20b-d, and
21b-d were acquired at λem ) 653 nm with slit widths set to 2 nm
band-pass for both excitation and emission. Emission spectra of model
compounds 22 and 23 were measured at λexc ) 280 nm and λexc ) 258
nm, respectively, and slit widths were set to 2 nm band-pass for
excitation as well as 4 nm band-pass for emission. Correction for
variations in lamp intensity over time and wavelength was achieved
with a solid-state silicon photodiode as the reference. The spectra were
further corrected for variations in photomultiplier response over
wavelength and for the path difference between the sample and the
reference by multiplication with emission correction curves generated
on the instrument. The energy transfer efficiencies for compounds 19b-
d, 20b-d, and 21b-d were calculated from the ratio of the integrated
donor excitation and absorption spectra normalized at the acceptor Soret
band.
Experimental Section
Nomenclature. The nomenclature used for the linear macromol-
ecules is as follows: X-[n]-Y, where X describes the functional group
at the one chain end, either P for phenacyl ether or HO for phenol; n
is the number of repeat units; and Y describes the functional group at
the other chain end, either hydroxymethyl, OH, or bromomethyl, Br.
Dendritic porphyrins are denoted as D-[G-n]8Por, where D indicates a
dendritic framework, n gives its generation (n ) 1-4), while the
particular substituent number identifies the porphyrin core moiety (Por).
Similarly, for the linear and branched analogues, L-[G-n]4Por and L-[G-
n]8Por will denote linear and branched architectures about their
porphyrin cores (n ) 2-5 and 1-4, respectively). Note that the first
generation linear and dendritic substituents are identical. For this reason,
no linear analogue for D-[G-1]8Por was prepared.
General Methods. Tetrakis(3,5-dimethoxyphenyl)porphyrin, 24,15
and tetrakis(4-methoxyphenyl)porphyrin, 25,16 were prepared from
pyrrole and the respective aromatic aldehydes using Adler-Longo
condensation conditions.17 Tetrakis(4-hydroxy-phenyl)porphyrin (THPP)
and tetrakis(3,5-dihydroxyphenyl)porphyrin18 (TDHPP) were prepared
by boron tribromide deprotection19 of 25 and 24, respectively. 3,5-
Dimethoxybenzyl methyl ether 22 was prepared as described in the
literature.20 Column chromatography was carried out with Merck silica
gel for flash columns, 230-400 mesh. NMR spectra were recorded on
a Bruker AM 200 (200 MHz) spectrometer with the residual protonated
solvent peak as internal standard. GPC was carried out on a Waters
chromatograph connected to a Waters 410 differential refractometer
with THF as the carrier solvent. Absorption spectra were recorded in
degassed THF solution (containing no stabilizers) on a Cary 50 UV-
visible spectrophotometer. Fluorescence spectra were measured of
degassed solutions (1 cm cells, ODmax < 0.2) using an ISA/SPEX
General Procedure for Bromination. 3-Benzyloxy-5-phenacyl-
oxybenzyl Bromide, 4. To a solution of the alcohol 313 (32.0 g, 132
mmol) in tetrahydrofuran (80 mL) was added carbon tetrabromide (54.9
g, 165 mmol) followed by the portion-wise addition of triphenylphos-
phine (43.3 g, 165 mmol). The reaction was quenched immediately
with 40 mL of water after the slightly yellow clear solution changed
to a deep yellow-green suspension in ca. 5 min. THF was evaporated,
and dichloromethane (400 mL) and water (150 mL) were added. The
organic layer was dried with MgSO4 and evaporated to dryness. The
crude product was purified by flash chromatography eluting with 8:1
dichloromethane/hexane to give the bromide, 4, as a white solid in
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Murphy, C. J.; Ploehn, H. J. Chem. Mater. 1999, 11, 3595. (g) Adronov,
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39, 1366. (h) Merino, S.; Brauge, L.; Caminade, A. M.; Majoral, J. P.;
Taton, D.; Gnanou, Y. Chem.-Eur. J. 2001, 7, 3095. (i) Trollsas, M.;
Claesson, H.; Atthoff, B.; Hedrick, J. L. Angew. Chem., Int. Ed. 1998, 37,
3132. (j) Trollsas, M.; Kelly, M.; Claesson, H.; Siemens, R.; Hedrick, J.
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T.; Schaefer, J.; Wooley, K. L. Polym. Prepr. 1998, 39(2), 1169. (c) Fields,
H. R.; Kowalewski, T.; Schaefer, J.; Wooley, K. L. Polym. Prepr. 1998,
39(1), 333. (d) Hawker, C. J.; Fre´chet, J. M. J. ACS Symposium Series
624; American Chemical Society: Washington, DC, 1996. (e) Hawker, C.
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Soc. 1997, 119, 9903.
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M.; Fre´chet, J. M. J. Chem. Mater. 1998, 10, 286. (c) Sato, T.; Jiang, D.-
L.; Aida, T. J. Am. Chem. Soc. 1999, 121, 10658. (d) Wakabayashi, Y.;
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Chem. 1991, 28, 635.
88% yield. mp 101-102 °C. IR: 1685, 1605, 1375, and 1170 cm-1
.
1H NMR (CDCl3): δ 4.36 (s, 2H, CH2Br), 5.04 (s, 2H, CH2O), 5.23
(s, 2H, CH2O), 6.55, 6.68, 6.72 (each t, 3H, ArH), 7.20-7.35 (m, 8H,
PhH), and 8.07 (A of AB2, J ) 9 Hz, 2H, ArH). 13C NMR (CDCl3):
δ 33.34, 70.76, 114.87, 115.49, 122.32, 128.14, 128.90, 129.99, 133.98,
134.50, 139.35, 158.20, 194.10. Anal. Calcd for C22H19BrO3: C, 64.25;
H, 4.66. Found: C, 64.4; H, 4.93.
General Procedure for Alkylation. P-[2]-OH, 5. To a solution of
the bromide 4 (12.5 g, 20 mmol) and 3-benzyloxy-5-hydroxybenzyl
alcohol,13 1 (8.64 g, 19.5 mmol), in acetone (200 mL) were added
potassium carbonate (7.9 g) and 18-crown-6 (70 mg). The reaction
mixture was then heated at reflux under nitrogen for 16 h, filtered, and
evaporated to dryness. The crude product was partitioned between water
(200 mL) and dichloromethane (200 mL), the aqueous layer was
extracted with dichloromethane (2 × 100 mL), and the combined
extracts were dried and evaporated to dryness. Purification by flash
chromatography eluting with dichloromethane, gradually increasing to
1:4 ether/dichloromethane gave the alcohol, 5, as a colorless oil (yield
1
87%). IR: 3400-3100, 1690, 1600, 1380, and 1165 cm-1. H NMR
(17) (a) Kim, J. B.; Adler, A. D.; Longo, F. R. In The Porphyrins; Dolphin, D.,
Ed.; Academic Press: New York, 1978; Vol. 1, Part A, p 85. (b) Rocha
Gonsalves, A. M. d’A.; Varejao, J. M. T. B.; Pereira, M. M. J. Heterocycl.
Chem. 1991, 28, 635.
(CDCl3): δ 1.80 (br s, 1H, OH), 4.59 (s, 2H, CH2OH), 5.04, 5.08 (each
s, 6H, CH2O), 5.27 (s, 2H, COCH2), 6.47-6.68 (complex m, 6H, ArH),
7.29-7.60 (complex m, 13H, PhH), and 7.94 (A of AB2, J ) 7 Hz,
2H, PhHCO). 13C NMR (CDCl3): δ 53.45, 65.27, 69.81, 70.07, 70.18,
70.74, 101.35, 101.64, 105.71, 105.87, 106.08, 106.97, 125.19, 127.54,
127.62, 128.01, 128.11, 128.61, 128.87, 133.93, 134.50, 136.66, 136.84,
(18) James, D. A.; Arnold, D. P.; Parsons, P. G. Photochem. Photobiol. 1994,
59, 441.
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9
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A R T I C L E S
Harth et al.
139.52, 143.51, 159.33, 159.97, 160.15, 160.21, 194.21. Anal. Calcd
for C36H32O6: C, 77.1; H, 5.76. Found: C, 76.8; H, 5.80.
HO-[4]-OH, 14. This compound was prepared from 8 according to
the general procedure for deprotection with Zn dust and acetic acid in
THF. Because the remaining product is very crystalline, the Zn was
filtered off and washed with hot THF. The filtrate was evaporated to
dryness with the acetic acid being removed by several azeotropes with
toluene. The crude product was precipitated out of ether to give a white
crystalline product, 14, in 90% yield. mp 122-124 °C. IR: 3500-
General Procedure for Deprotection of the Phenacyl Protecting
Group. HO-[2]-OH, 6. The phenacyl ether, 5 (42.0 g, 75 mmol), was
dissolved in tetrahydrofuran (70 mL) and acetic acid (200 mL). To
this solution was added zinc dust (40 g, 640 mmol) portion-wise, and
the solution was stirred vigorously overnight under argon. The reaction
mixture was filtered and evaporated to dryness with the acetic acid
being removed by several azeotropes with toluene. The crude product
was purified by flash chromatography eluting with dichloromethane,
gradually increasing to 1:3 ether/dichloromethane to obtain the depro-
tected alcohol, 6, as a white solid in 88% yield. IR: 3400-3100, 1690,
1600, 1380, and 1165 cm-1. 1H NMR (CDCl3): δ 4.55 (s, 2H, CH2OH),
4.95, 4.99, 5.01 (each s, 2H, CH2O), 6.70-6.90, 7.12-7.24 (each m,
16H, ArH). 13C NMR (CDCl3): δ 65.07, 65.97, 69.66, 113.42, 114.33,
114.38, 115.07, 119.41, 119.66, 129.68, 138.63, 142.05, 156.09, 158.85.
Anal. Calcd for C28H26O5: C, 76.0; H, 5.92. Found: C, 76.1; H, 5.67.
1
3100, 1600, 1375, and 1175 cm-1. H NMR (CDCl3): δ 4.55 (s, 2H,
CH2OH), 4.69-4.98 (complex m, 14H, CH2O), 6.37-6.66 (complex
m, 12H, ArH), 7.24-7.41 (complex m, 20H, PhH). 13C NMR
(CDCl3): δ 65.21, 69.76, 69.91, 70.04, 70.06, 70.11, 101.46, 101.85,
105.81, 105.93, 106.05, 106.31, 106.40, 106.47, 107.02, 126.96, 127.63,
128.04, 128.61, 136.75, 139.31, 139.47, 143.15, 157.21, 159.99, 160.12,
160.28, 160.12. Anal. Calcd for C56H50O9: C, 73.6; H, 5.81. Found:
C, 73.5; H, 5.72.
L-[G-2]-OH, 12. This compound was prepared from 6 and 11
according to the general procedure for alkylation with potassium
carbonate and 18-crown-6 in acetone. The crude product was purified
by flash chromatography eluting with dichloromethane, gradually
increasing to 5% diethyl ether/dichloromethane to give the exact linear
analogue of the dendrimer, L-[G-2]-OH, 12, as a colorless solid in 91%
P-[2]-Br, 7. This compound was prepared from the alcohol 5
according to the general procedure with carbon tetrabromide and
triphenylphosphine in THF. The crude product was purified by flash
chromatography eluting with 5:1 dichloromethane/hexane to give the
bromide, 7, as a white solid in 84% yield. mp 105-106 °C. IR: 1680,
1605, 1380, and 1180 cm-1. 1H NMR (CDCl3): δ 4.39 (s, 2H, CH2Br),
4.94, 5.00, 5.01 (each s, 6H, CH2O), 5.24 (s, 2H, COCH2), 6.50-6.63
(complex m, ArH), 7.33-7.51 (complex m, 13H, PhH), 7.96 (A of
AB2, J ) 10 Hz, 2H, PhHCO). 13C NMR (CDCl3): δ 33.59, 69.91,
70.16, 101.72, 102.20, 106.13, 107.01, 108.14, 108.29, 127.60, 128.11,
128.63, 128.87, 133.93, 134.52, 136.61, 139.26, 139.79, 159.37, 159.89,
160.07, 160.23, 194.09. Anal. Calcd for C36H31BrO5: C, 69.3; H, 5.01.
Found: C, 69.1; H, 5.22.
yield. mp 126-127 °C. IR: 3300-3100, 1600, 1380, and 1170 cm-1
.
1H NMR (CDCl3): δ 1.73 (br s, 1H, CH2OH), 4.60 (s, 2H, CH2OH),
4.96 (s, 4H, ArCH2OAr), 5.02 (s, 8H, PhCH2O), 6.53-6.68 (complex
m, 9H, ArH), 7.31-7.43 (complex m, 20H, PhH). 13C NMR (CDCl3):
δ 65.29, 69.95, 70.02, 70.10, 70.14, 101.34, 101.61, 105.73, 105.8,
106.41, 127.56, 127.61, 128.05, 128.63, 136.80, 136.86, 130.28, 139.35,
143.15, 168.09, 160.18. Anal. Calcd for C49H44O7: C, 79.0; H, 5.95.
Found: C, 79.2; H, 6.17.
L-[G-2]-Br, 13. This compound was prepared from 12 according
to the general procedure for bromination with 3 equiv of carbon
tetrabromide and 3 equiv of triphenylphosphine. The crude product
was purified by flash chromatography eluting with 9:1 dichloromethane/
hexane, gradually increasing to dichloromethane to give the bromide,
13, as a colorless solid in 88% yield. mp 102-103 °C. IR: 1600, 1375,
and 1175 cm-1. 1H NMR (CDCl3): δ 4.42 (s, 2H, CH2Br), 4.98, 5.00
(each s, 4H, ArCH2OAr), 5.04, 5.06 (each s, 8H, PhCH2O), (each s,
8H, PhCH2O), 6.57-6.73 (complex m, 9H, ArH), 7.11-7.47 (complex
m, 20H, PhH). 13C NMR (CDCl3): δ 33.68, 70.07, 70.18, 101.67,
101.71, 102.27, 106.45, 106.53, 108.21, 108.31, 127.64, 128.09, 128.14,
128.32, 128.67, 136.86, 139.16, 139.31, 139.86, 160.04, 160.14, 160.25.
Anal. Calcd for C49H44BrO6: C, 72.9; H, 5.37. Found: C, 73.2; H,
5.43.
P-[4]-OH, 8. This compound was prepared from the bromide 7 and
the monophenol 6, according to the general procedure for alkylation
with potassium carbonate and 18-crown-6 in acetone. The reaction
mixture was evaporated to dryness, and the crude product was
partitioned between water (200 mL) and dichloromethane (200 mL);
the organic layer was dried with MgSO4 and evaporated to dryness.
The crude product was purified by flash chromatography eluting with
dichloromethane to give 8 as colorless crystals (96% yield). IR: 3300-
1
3100, 1710, 1685, 1605, 1380, and 1165 cm-1. H NMR (CDCl3): δ
1.69 (br s, 1H, CH2OH), 4.51 (s, 2H, CH2OH), 4.86 (s, 6H, ArCH2-
OAr), 4.93 (s, 8H, PhCH2O), 5.14 (s, 2H, COCH2O), 6.44-6.58
(complex m, 12H, ArH), 7.22-7.42 (complex m, 23H, PhH), 7.86 (A
of AB2, J ) 10 Hz, 2H, PhHCO). 13C NMR (CDCl3): δ 65.27, 69.93,
70.00, 70.07, 70.13, 70.18, 70.74, 101.31, 101.62, 101.68, 105.73,
105.80, 106.15, 106.32, 106.43, 106.51, 107.01, 127.53, 127.60, 128.03,
128.11, 128.61, 128.86, 133.91, 134.52, 136.67, 136.78, 136.85, 139.29,
139.35, 139.44, 143.52, 159.34, 160.01, 160.07, 160.17, 160.21, 160.33,
194.13. Anal. Calcd for C64H56O10: C, 78.0; H, 5.73. Found: C, 77.8;
H, 5.89.
L-[G-3]-OH, 15. This compound was prepared from 13 and 14
according to the general procedure for alkylation with potassium
carbonate and 18-crown-6 in acetone. The crude product was purified
by flash chromatography eluting with dichloromethane, gradually
increasing to 5% diethyl ether/dichloromethane to give the alcohol,
15, in 93% yield as a colorless solid. mp 128-129 °C. IR: 3300-
1
3100, 1605, 1375, and 1170 cm-1. H NMR (CDCl3): δ 1.70 (t, 1H,
P-[4]-Br, 26. This compound was prepared from the alcohol 8
according to the general procedure for bromination with 2.5 equiv of
carbon tetrabromide and 2.5 equiv of triphenylphosphine. The crude
product was purified by flash chromatography eluting with 5:1
dichloromethane/hexane, gradually increasing to dichloromethane to
give the bromide, 26, as a colorless solid in 91% yield. mp 110-112
°C. IR: 1680, 1605, 1380, and 1180 cm-1. 1H NMR (CDCl3): δ 4.39
(s, 2H, CH2Br), 4.51 (s, 2H, CH2OH), 4.95 (s, 6H, ArCH2OAr), 5.00
(s, 8H, PhCH2O), 5.28 (s, 2H, COCH2OH), 6.53-6.66 (complex m,
12H, ArH), 7.32-7.47 (complex m, 23H, PhH), 7.96 (A of AB2, J )
10 Hz, 2H, PhHCO). 13C NMR (CDCl3): δ 33.63, 69.90, 70.03, 70.15,
70.76, 101.67, 102.22, 106.15, 106.37, 106.51, 107.01, 108.16, 108.26,
127.59, 128.05, 128.11, 128.62, 128.87, 133.90, 134.53, 136.62, 136.67,
136.78, 139.10, 139.26, 139.45, 139.80, 159.36, 159.98, 160.02, 160.09,
160.19, 194.09. Anal. Calcd for C64H55BrO9: C, 73.3; H, 5.29. Found:
C, 73.1; H, 5.15.
CH2OH), 4.42 (d, J ) 5.7 Hz, 2H, CH2OH), 4.82 (s, 12H, ArCH2-
OAr), 4.88 (s, 16H, PhCH2O), 6.40-6.56 (complex m, 21H, ArH),
7.16-7.35 (complex m, 40H, PhH). 13C NMR (CDCl3): δ 65.24, 70.04,
70.14, 101.33, 101.66, 105.71, 105.79, 106.44, 106.53, 127.64, 128.65,
136.83, 136.90, 139.34, 139.40, 143.59, 160.12, 160.21. Anal. Calcd
for C105H92O15: C, 79.1; H, 5.82. Found: C, 78.9; H, 6.05.
P-[8]-OH, 9. This compound was prepared from 14 and 26 according
to the general procedure for alkylation with potassium carbonate and
18-crown-6 in acetone. Because the product is moderately insoluble in
acetone, the addition of more solvent was required during the reaction
as necessary. The reaction mixture was evaporated to dryness and the
crude product partitioned between dichloromethane and water. The
organic phase was dried with MgSO4, and the protected alcohol, 9,
was purified by precipitation out of diethyl ether (yield: 96%). mp
1
132-134 °C. IR: 3300-3100, 1680, 1605, 1380, and 1165 cm-1. H
NMR (CDCl3): δ 1.69 (br s, 1H, CH2OH), 4.51 (s, 2H, CH2OH), 4.85
9
3928 J. AM. CHEM. SOC. VOL. 124, NO. 15, 2002

Macromolecular Architecture in Nanomaterials
A R T I C L E S
(s, 12H, ArCH2OAr), 4.92 (s, 16H, PhCH2O), 5.18 (s, 2H, COCH2O),
6.40-6.56 (complex m, 24H, ArH), 7.25-7.42 (complex m, 48H, PhH),
7.86 (A of AB2, J ) 10 Hz, 2H, PhCO). 13C NMR (CDCl3): δ 65.25,
70.00, 70.11, 101.29, 101.62, 105.69, 105.77, 106.13, 106.36, 106.48,
107.01, 127.59, 128.03, 128.10, 128.60, 128.85, 133.89, 134.50, 136.67,
136.78, 139.27, 139.43, 143.53, 159.34, 160.01, 160.17, 194.08. Anal.
Calcd for C120H104O18: C, 78.6; H, 5.72. Found: C, 78.9; H, 5.55.
P-[8]-Br, 28. This compound was prepared from 9 according to the
general procedure for bromination with 1.5 equiv of carbon tetrabromide
and 1.5 equiv of triphenylphosphine. The crude product was purified
by flash chromatography eluting with hexane, gradually increasing to
pure 2.5% diethyl ether/dichloromethane. The bromide, 28, was
obtained as a colorless solid in 91% yield. mp 135-136 °C. IR: 1685,
1605, 1380, and 1180 cm-1. 1H NMR (CDCl3): δ 4.38 (s, 2H, CH2Br),
4.86 (s, 12H, ArCH2OAr), 5.00 (s, 16H, PhCH2O), 5.21 (s, 2H,
COCH2O), 6.52-6.67 (complex m, 24H, ArH), 7.29-7.46 (complex
m, 48H, PhH), 7.96 (A of AB2, J ) 10 Hz, 2H, PhHCO). 13C NMR
(CDCl3): δ 33.64, 69.89, 70.02, 70.13, 70.73, 101.64, 102.22, 106.15,
106.38, 106.49, 107.02, 108.16, 108.26, 127.60, 128.04, 128.18, 128.61,
128.86, 128.90, 134.53, 136.63, 136.68, 136.79, 139.11, 139.27, 139.45,
139.81, 159.93, 160.02, 160.10, 106.18, 194.08. Anal. Calcd for
by flash chromatography eluting with dichloromethane, gradually
increasing to 5% diethyl ether/dichloromethane to give the alcohol,
30, as a colorless solid in 81% yield. mp 137-138 °C. IR: 3300-
1
3100, 1690, 1600, 1380, and 1170 cm-1. H NMR (CDCl3): δ 4.57
(d, J ) 6 Hz, 2H, CH2OH), 4.94 (s, 33H, ArCH2OAr), 5.08 (s, 32H,
PhCH2O), 5.20 (s, 2H, COCH2O), 6.54-6.68 (complex m, 48H, ArH),
7.24-7.59 (complex m, 83H, PhH), 7.95 (A of AB2, J ) 10 Hz, 2H,
PhHCO). 13C NMR (CDCl3): δ 53.51, 65.22, 70.01, 70.12, 101.31,
101.64, 105.69, 105.78, 106.18, 106.40, 106.52, 107.04, 127.12, 127.62,
128.33, 128.63, 128.88, 133.91, 134.53, 136.72, 136.91, 139.32, 139.40,
139.48, 143.63, 159.38, 160.11, 160.20, 194.08. Anal. Calcd for
C232H200O34: C, 78.9; H, 5.71. Found: C, 79.2; H, 5.88.
L-[G-4]-Br, 31. This compound was prepared from the alcohol 17
according to the general procedure for bromination with 4.0 equiv of
carbon tetrabromide and 4.0 equiv of triphenylphosphine. The crude
product was purified by flash chromatography eluting with hexane,
gradually increasing to 5% diethyl ether/dichloromethane to give the
bromide, 31, as a colorless solid in 89% yield. mp 139-141 °C. IR:
1600, 1370, and 1175 cm-1. 1H NMR (CDCl3): δ 4.37 (s, 2H, CH2Br),
4.93 (s, 28H, ArCH2OAr), 5.00 (s, 32H, PhCH2O), 6.50-6.66 (complex
m, 45H, ArH), 7.26-7.41 (complex m, 80H, PhH). 13C NMR
(CDCl3): δ 33.64, 70.00, 70.11, 101.62, 102.21, 106.49, 108.15, 108.26,
127.59, 128.03, 128.61, 136.63, 136.79, 139.10, 139.80, 159.97, 160.09,
160.18. Anal. Calcd for C217H187BrO30: C, 77.7; H, 5.62. Found: C,
78.0; H, 5.61.
C120H103BrO17: C, 76.0; H, 5.47. Found: C, 76.2; H, 5.45.
HO-[8]-OH, 16. This compound was prepared from 9 according to
the general procedure for deprotection with Zn dust and acetic acid in
THF. Because the material is very crystalline, the Zn was filtered off
and washed with hot THF. The filtrate was evaporated to dryness with
the acetic acid being removed by several azeotropes with toluene. The
crude product was precipitated out of ether to give 16 as a white
crystalline product in 90% yield. mp 136-138 °C. IR: 3500-3100,
1605, 1370, and 1175 cm-1. 1H NMR (CDCl3): δ 4.56 (s, 2H, CH2OH),
4.88-5.00 (complex m, 30H, CH2O), 5.7 (br s, 1H, ArOH), 6.37, 6.41
(s, 2H, ArHOH), 6.52-6.66 (complex m, 22H, ArH), 7.27-7.42
(complex m, 40H, PhH). 13C NMR (CDCl3): δ 69.74, 70.02, 70.13,
101.38, 101.67, 101.83, 105.74, 105.85, 106.05, 106.52, 106.99, 127.56,
127.62, 128.05, 128.61, 136.78, 139.50, 143.37, 157.19, 160.06, 160.16,
160.30. Anal. Calcd for C112H98O17: C, 78.4; H, 5.76. Found: C, 78.4;
H, 5.93.
HO-[16]-OH, 32. This compound was prepared from the alcohol
30 according to the general procedure for deprotection with Zn dust
and acetic acid in THF. The crude product was purified by flash
chromatography eluting with dichloromethane, gradually increasing to
10% diethyl ether/dichloromethane to give the phenol, 32, as a colorless
solid in 83% yield. mp 140-142 °C. IR: 3400-3100, 1600, 1370,
and 1170 cm-1. 1H NMR (CDCl3): δ 4.57 (d, J ) 6 Hz, 2H, CH2OH),
4.88 (s, 30H, ArCH2OAr), 5.00 (s, 32H, PhCH2O), 5.52 (s, 1H, ArOH),
6.36, 6.37 (each s, 2H, ArHOH), 6.52-6.72 (complex m, 46H, ArH),
7.28-7.49 (complex m, 80H, PhH). 13C NMR (CDCl3): δ 33.62, 69.73,
70.02, 70.13, 101.35, 101.66, 105.73, 105.83, 106.12, 106.42, 106.54,
106.99, 127.63, 128.06, 128.63, 136.81, 139.31, 139.54, 143.49, 157.13,
160.10, 160.19. Anal. Calcd for C224H194O33: C, 78.8; H, 5.73. Found:
C, 78.7; H, 5.49.
L-[G-3]-Br, 29. This compound was prepared from 15 according
to the general procedure for bromination employing 2 × 1.5 equiv of
carbon tetrabromide and 2 × 1.5 equiv of triphenylphosphine. The crude
product was purified by flash chromatography eluting with hexane,
gradually increasing to dichloromethane to give the bromomethyl
derivative, 29, as a colorless solid (yield: 84%). mp 127-129 °C. IR:
1605, 1380, and 1175 cm-1. 1H NMR (CDCl3): δ 4.41 (s, 2H, CH2Br),
4.91-5.04 (complex m, 28H, CH2O), 6.56-6.72 (complex m, 21H,
ArH), 7.32-7.51 (complex m, 40H, PhH). 13C NMR (CDCl3): δ 33.71,
70.06, 101.69, 105.27, 106.45, 108.21, 108.32, 127.65, 127.91, 128.67,
136.70, 136.86, 139.17, 139.34, 139.86, 160.03, 160.15, 160.24. Anal.
Calcd for C105H91BrO15: C, 75.4; H, 5.48. Found: C, 75.2; H, 5.40.
L-[G-4]-OH, 17. This compound was prepared from 15 and 16
according to the general procedure for alkylation with potassium
carbonate and 18-crown-6, except that a 4:1 mixture of THF/acetone
was used as the solvent mixture. The product was purified by flash
chromatography eluting with dichloromethane, gradually increasing to
5% diethyl ether/dichloromethane to give the fourth generation linear
analogue, 17, as a colorless solid in 81% yield. mp 142-144 °C. IR:
P-[16]-Br, 33. This compound was prepared from 30 according to
the general procedure from bromination with 5.0 equiv of carbon
tetrabromide and 5.0 equiv of triphenylphosphine in THF. The crude
product was purified by flash chromatography eluting with 5:1
dichloromethane/hexane, gradually increasing to dichloromethane to
give the bromide, 33, as a colorless solid in 86% yield. mp 143-144
°C. IR: 1680, 1605, 1375, and 1180 cm-1. 1H NMR (CDCl3): δ 4.39
(s, 2H, CH2Br), 4.87 (s, 24H, ArCH2OAr), 5.02 (s, 32H, PhCH2O),
5.21 (s, 2H, COCH2O), 6.50-6.68 (complex m, 24H, ArH), 7.25-
7.45 (complex m, 48H, PhH), 7.94 (A of AB2, J ) 10 Hz, 2H, PhHCO).
13C NMR (CDCl3): δ 33.62, 69.90, 70.02, 70.08, 70.15, 70.70, 101.68,
102.25, 106.10, 106.35, 106.55, 107.08, 108.27, 127.62, 128.04, 128.10,
128.63, 128.90, 134.52, 136.68, 136.82, 139.16, 139.42, 139.85, 159.44,
159.95, 160.04, 160.19, 194.10. Anal. Calcd for C232H199BrO33: C, 77.5;
H, 5.58. Found: C, 77.7; H, 5.33.
L-[G-5]-OH, 34. This compound was prepared from 31 and 32
according to the general procedure for alkylation with potassium
carbonate and 18-crown-6 in 4:1 THF/acetone. The crude product was
purified by flash chromatography eluting with dichloromethane, gradu-
ally increasing to 5% diethyl ether/dichloromethane to give the alcohol,
34, as a colorless solid in 76% yield. mp 148-150 °C. IR: 3300-
1
3300-3100, 1605, 1380, and 1175 cm-1. H NMR (CDCl3): δ 4.59
(s, 2H, CH2OH), 4.97 (s, 28H, ArCH2OAr), 5.03 (s, 32H, PhCH2O),
6.58-6.70 (complex m, 45H, ArH), 7.32-7.51 (complex m, 80H, PhH).
13C NMR (CDCl3): δ 53.57, 65.23, 70.05, 70.15, 101.34, 101.68,
105.71, 105.80, 106.45, 106.56, 127.67, 128.67, 136.87, 139.37, 143.68,
160.15, 160.24. Anal. Calcd for C217H188O31: C, 79.2; H, 5.76. Found:
C, 79.0; H, 5.59.
P-[16]-OH, 30. This compound was prepared from 16 and 28
according to the general procedure for alkylation with potassium
carbonate and 18-crown-6 in acetone. The crude product was purified
1
3100, 1600, 1375, and 1175 cm-1. H NMR (CDCl3): δ 4.61 (s, 2H,
CH2OH), 4.97 (s, 56H, ArCH2OAr), 5.04 (s, 66H, PhCH2O), 6.55-
6.70 (complex m, 93H, ArH), 7.25-7.53 (complex m, 160H, PhH).
13C NMR (CDCl3): δ 65.34, 70.02, 70.14, 101.38, 101.70, 105.78,
105.86, 106.54, 127.70, 128.14, 128.73, 136.82, 139.33, 143.64, 160.15,
9
J. AM. CHEM. SOC. VOL. 124, NO. 15, 2002 3929

A R T I C L E S
Harth et al.
160.15, 160.25. Anal. Calcd for C441H380O63: C, 79.2; H, 5.73. Found:
C, 79.4; H, 5.87.
concentrated, was employed to increase the rate of reaction and
dramatically improve yields. In addition, the products were purified
by fractional precipitation rather than column chromatography. This
was accomplished by dissolving the filtered reaction mixture in a
minimal amount of dichloromethane and adding ethyl ether slowly until
the product starts to precipitate. The supernatant was poured off, and
the process is repeated until no more precipitates. Analysis of the
different fractions by GPC permitted the pure fractions to be identified
and combined.
L-[G-5]-Br, 36. This compound was prepared from 34, according
to the general procedure for bromination with 5.0 equiv of carbon
tetrabromide and 5.0 equiv of triphenylphosphine. The crude product
was purified by flash chromatography eluting with 9:1 dichloromethane/
hexane, gradually increasing to dichloromethane to give the bromide,
36, as a colorless solid in 81% yield. mp 145-147 °C. IR: 1605, 1375,
1
and 1180 cm-1. H NMR (CDCl3): δ 4.39 (s, 2H, CH2Br), 4.94 (s,
D-[G-1]8Por, 19a. The first generation benzylic bromide, 11 (1.0
g, 2.6 mmol), was dissolved in acetone (20 mL), followed by the
addition of tetrakis(3,5-dihydroxyphenyl)porphyrin, 37 (232 mg, 31
mmol), K2CO3 (3.0 g, 22 mmol), and 18-crown-6 (50 mg). The mixture
was heated at reflux under N2 for 5 days in the dark and purified by
fractional precipitation from ethyl ether. This afforded purple crystals
60H, ArCH2OAr), 5.05 (s, 64H, PhCH2O), 6.50-6.71 (complex m,
93H, ArH), 7.25-7.45 (complex m, 160H, PhH). 13C NMR (CDCl3):
δ 33.7, 69.82, 70.14, 101.56, 102.20, 106.43, 108.20, 127.44, 128.09,
128.55, 136.69, 139.13, 139,91, 160.00, 160.25. Anal. Calcd for
C441H379BrO62: C, 78.5; H, 5.66. Found: C, 78.7; H, 5.54.
General Procedure for the Preparation of Porphyrin Cored
1
of 19a in 90% yield. H NMR (250 MHz, CDCl3): δ 4.88 (s, 32H,
Dendrimers. The preparation of these materials was accomplished
using the conventional alkylation procedure described above, with two
notable exceptions. At higher generations, alkylation in acetone proved
to be extremely slow, and so the procedure of Weintraub and Parquette38
using a mixture of THF and DMF, which is repeatedly evaporated/
Ar-CH2-O), 5.22 (s, 16H, PhCH2O), 6.36 (t, 8H, J ) 2 Hz, Ar-H),
6.66 (t, 16H, J ) 2 Hz, ArH), 6.97 (t, 4H, J ) 2 Hz, ArH), 7.10-7.30
(m, 48H, PhH), 7.38 (d, 12H, ArH), 8.78 (s, 8H, Pyrrol).
D-[G-2]8Por, 19b. The second generation benzylic bromide, 38 (2.0
g, 2.5 mmol),13 was dissolved in acetone (40 mL), followed by the
addition of tetrakis(3,5-dihydroxyphenyl)porphyrin, 37 (297 mg, 0.40
mmol), K2CO3 (4.0 g, 29 mmol), and 18-crown-6 (50 mg). The mixture
was heated at reflux under N2 for 6 days in the dark and purified by
fractional precipitation from ethyl ether. This afforded the dendritic
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Hawker, C. J.; Fre´chet, J. M. J. J. Chem. Soc., Chem. Commun. 1990, 1010.
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1
porphyrin, 19b, as a purple solid in 61% yield. H NMR (250 MHz,
CDCl3): δ 4.81 (s, 96H, Ar-CH2-O), 5.01 (s, 16H, PhCH2O), 6.32 (t,
16H, J ) 2 Hz, ArH), 6.52 (t, 8H, J ) 2 Hz, ArH), 6.53 (d, 32H, J )
2 Hz, ArH), 6.66 (d, 16H, J ) 4 Hz, ArH), 7.00 (t, 4H, J ) 2H, ArH),
7.14-7.24 (m, 160H, PhH), 7.42 (d, 8H, J ) 4 Hz, ArH), 8.85 (s, 8H,
Pyrrol).
D-[G-3]8Por, 19c. The third generation benzylic bromide, 39 (2.0
g, 1.08 mmol),13 was dissolved in a 4:1 mixture of THF/DMF (40 mL),
followed by the addition of tetrakis(3,5-dihydroxyphenyl)porphyrin, 37
(75 mg, 0.1 mmol), K2CO3 (3.0 g, 22 mmol), and 18-crown-6 (50 mg).
The mixture was heated at reflux under N2 for 14 h with solvent cycling
through an addition funnel in the dark and purified by fractional
precipitation from diethyl ether. This afforded the dendritic porphyrin,
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1
19c, as a dark purple/brown solid in 57% yield. H NMR (250 MHz,
CDCl3): δ 4.66 (s, 224H, Ar-CH2-O), 4.73 (s, 16H, PhCH2O), 6.37
(m, 96H, ArH), 6.46 (m, 40H, ArH), 6.63 (m, 32H, ArH), 7.12-7.24
(m, 320H, PhH), 8.88 (s, 8H, Pyrrol).
D-[G-4]8Por, 19d. The fourth generation benzylic bromide, 40 (2.0
g, 0.597 mmol),13 was dissolved in a 4:1 mixture of THF/DMF (50
mL), followed by the addition of tetrakis(3,5-dihydroxyphenyl)-
porphyrin, 37 (35 mg, 0.05 mmol), K2CO3 (3.0 g, 22 mmol), and 18-
crown-6 (50 mg). The mixture was heated at reflux under N2 for 12 h
with solvent cycling through an addition funnel in the dark and purified
by fractional precipitation from diethyl ether. This afforded the dendritic
1
porphyrin, 19d, as a dark purple/brown solid in 57% yield. H NMR
(250 MHz, CDCl3): δ 4.90 (s, 480H, Ar-CH2-O), 4.99 (s, 16H, CH2O),
6.37 (m, 96H, ArH), 6.42 (m, 40H, ArH), 6.60 (m, 32H, ArH), 7.12-
7.33 (m, 640H, PhH), 8.87 (s, 8H, Pyrrol).
(33) (a) Hecht, S.; Ihre, H.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1999, 121,
9239. (b) Hecht, S.; Vladimirov, N.; Fre´chet, J. M. J. J. Am. Chem. Soc.
2001, 123, 18.
L-[G-2]8Por, 20b. The linear benzylic bromide, 13 (1.50 g, 1.86
mmol), was dissolved in acetone (40 mL), followed by the addition of
tetrakis(3,5-dihydroxyphenyl)porphyrin, 37 (113 mg, 0.153 mmol),
K2CO3 (3.5 g, 25 mmol), and 18-crown-6 (50 mg). The mixture was
heated at reflux under N2 for 6 days in the dark and purified by
fractional precipitation from diethyl ether. This afforded purple crystals
(34) (a) Weber, G.; Teale, F. W. J. Trans. Faraday Soc. 1958, 54, 640. (b)
Stryer, L.; Haugland, R. P. Proc. Natl. Acad. Sci. U.S.A. 1967, 58, 719. (c)
Mugnier, J.; Pouget, J.; Bourson, J.; Valeur, B. J. Lumin. 1985, 33, 273.
(35) Consult standard photochemistry textbooks: (a) Turro, N. J. Modern
Molecular Photochemistry; University Science Books: Sausalito, 1991. (b)
Gilbert, A.; Baggott, J. Essentials of Molecular Photochemistry; CRC
Press: Boca Raton, 1991. (c) Klessinger, M.; Michl, J. Excited States and
Photochemistry of Organic Molecules; VCH: Weinheim, 1995.
(36) (a) Fo¨rster, T. Fluoreszenz Organischer Verbindungen; Vandenhoech and
Ruprech: Go¨ttingen, 1951. (b) Van der Meer, W. B.; Coker, G., III; Chen,
S.-Y. Resonance Energy Tranfer, Theory, and Data; VCH: Weinheim,
1994.
1
of 20b in 51% yield. H NMR (250 MHz, CDCl3): δ 4.79 (m, 32H,
ArCH2O), 4.86 (s, 32H, PhCH2O), 5.02 (s, 16H, ArCH2O), 6.44-6.66
(complex m, 72H, ArH), 6.98 (s, 4H, ArH), 7.17-7.41 (complex m,
160H, PhH), 7.41 (d, J ) 4 Hz, ArH), 8.84 (s, 8H, Pyrrol).
L-[G-3]8Por, 20c. The linear third generation benzylic bromide, 29
(0.75 g, 0.45 mmol), was dissolved in a 4:1 THF:DMF solution (20
mL), followed by the addition of tetrakis(3,5-dihydroxyphenyl)-
(37) Jeong, M.; Mackay, M. E.; Vestberg, R.; Hawker, C. J. Macromolecules
2001, 34, 4927.
(38) Weintraub, J. G.; Parquette, J. R. J. Org Chem. 1999, 64, 3796.
9
3930 J. AM. CHEM. SOC. VOL. 124, NO. 15, 2002

Macromolecular Architecture in Nanomaterials
A R T I C L E S
Scheme 1. Synthesis of Oligomeric Benzyl Ethers
porphyrin, 37 (20 mg, 27.0 µmol), K2CO3 (1.5 g, 11 mmol), and 18-
crown-6 (25 mg). The mixture was heated and cycled at reflux under
N2 for 6 h in the dark and purified by fractional precipitation from
diethyl ether. This afforded 20c as a dark purple/brown solid in 67%
yield. 1H NMR (250 MHz, CDCl3): δ 4.76-4.92 (complex m, 224H,
ArCH2O, PhCH2O), 6.42-6.53 (complex m, 168H, ArH), 7.12-7.25
(complex m, 320H, PhH), 8.85 (s, 8H, Pyrrol).
L-[G-4]8Por, 20d. The linear fourth generation benzylic bromide,
31 (0.75 g, 0.224 mmol), was dissolved in a 4:1 THF:DMF solution
(20 mL), followed by the addition of tetrakis(3,5-dihydroxyphenyl)-
porphyrin, 37 (9.8 mg, 14 µmol), K2CO3 (2.5 g, 18 mmol), and 18-
crown-6 (40 mg). The mixture was heated and cycled at reflux under
N2 for 6 h in the dark and purified by fractional precipitation from
diethyl ether. This afforded 21a as a dark purple/brown solid in 71%
1
yield. H NMR (250 MHz, CDCl3): δ 4.82-489 (complex m, 494H,
ArCH2O, PhCH2O), 6.43-6.54 (complex m, 360H, ArH), 7.19-7.25
(complex m, 640H, PhH), and 8.83 (s, 8H, pyrrol).
L-[G-2]4Por, 21a. The linear second generation benzylic bromide,
13 (2.06 g, 2.52 mmol), was dissolved in acetone (40 mL), followed
by the addition of tetrakis(4-hydroxyphenyl)porphyrin, 38 (279 mg,
0.40 mmol), K2CO3 (4.0 g, 29 mmol), and 18-crown-6 (52 mg). The
mixture was heated at reflux under N2 for 6 days in the dark and purified
by fractional precipitation from diethyl ether. This afforded 21a as
purple crystals in 71% yield. 1H NMR (250 MHz, CDCl3): δ 4.96 (s,
16H, ArCH2O), 5.25-5.28 (m, 32H, PhCH2O), 6.52-6.84 (complex
m, 36H, ArH), 7.24-7.47 (complex m, 80H, PhH), 8.07 (d, 8H, J )
8 Hz), 8.83 (s, 8H, pyrrol).
L-[G-3]4Por, 21b. The linear third generation benzylic bromide, 29
(0.500 g, 0.285 mmol), was dissolved in a 4:1 THF:DMF solution (40
mL), followed by the addition of tetrakis(4-hydroxyphenyl)porphyrin,
38 (24 mg, 36 µmol), K2CO3 (1.5 g, 11 mmol), and 18-crown-6 (25
mg). The mixture was heated and cycled at reflux under N2 for 6 h in
the dark and purified by fractional precipitation from diethyl ether. This
afforded 21b as a purple solid in 61% yield. 1H NMR (250 MHz,
CDCl3): δ 4.88 (complex m, 96H, CH2O), 5.06 (s, 8H, CH2O), 5.21
(s, 8H, CH2O), 6.49-6.85 (complex m, 76H, ArH), 6.85 (d, 8H, J )
4 Hz, ArH), 7.24-7.42 (complex m, 160H, PhH), 8.05 (d, 8H, J ) 8
Hz, ArH), 8.82 (s, 8H, Pyrrol).
L-[G-4]4Por, 21c. The linear fourth generation benzylic bromide,
31 (980 mg, 0.293 mmol), was dissolved in a 4:1 THF:DMF solution
(40 mL), followed by the addition of tetrakis(4-hydroxyphenyl)-
porphyrin, 38 (25 mg, 36 µmol), K2CO3 (1.7 g, 12 mmol), and 18-
crown-6 (25 mg). The mixture was heated and cycled at reflux under
N2 for 6 h in the dark and purified by fractional precipitation from
diethyl ether. This afforded 21c as a dark purple/brown solid in 37%
yield. 1H NMR (250 MHz, CDCl3): δ 4.81 (complex m, 216H, CH2O),
4.89 (s, 8H, CH2O), 6.54 (complex m, 180H, ArH), 7.18-7.25 (complex
m, 328H, PhH), 8.01 (s, 8H, ArH), 8.85 (s, 8H, Pyrrol).
L-[G-5]4Por, 21d. The linear fifth generation benzylic bromide, 36
(1.05 g, 0.157 mmol), was dissolved in a 4:1 THF:DMF solution (50
mL), followed by the addition of tetrakis(4-hydroxyphenyl)porphyrin,
38 (8.9 mg, 13 µmol), K2CO3 (1.7 g, 12 mmol), and 18-crown-6 (25
mg). The mixture was heated and cycled at reflux under N2 for 12 h in
the dark and purified by fractional precipitation from diethyl ether. This
number of monomer units. For example, a generation four
dendron derived from 3,5-dihydroxybenzyl alcohol, D-[G-4]-
OH, has an odd number, 15 or (24 - 1), of repeat units and an
even number, 16 or (24), of chain end benzyl groups. Such
features preclude a traditional exponential growth strategy21,22
involving the synthesis of well-defined oligomers with an even
number of repeat units, that is, 2, 4, 8 (2n). A combination of
two growth strategies was therefore adopted to overcome this
problem and allow the preparation of a linear series of oligomers
with (2n - 1) repeat units.
1
afforded 21d as a wine red solid in 42% yield. H NMR (250 MHz,
CDCl3): δ 4.82-4.88 (complex m, 240H, CH2O), 4.90 (complex m,
256H, CH2O), 6.56 (complex m, 372H, ArH), 7.12-7.27 (complex m,
640H, PhH), 8.85 (s, 8H, Pyrrol).
Results and Discussion
Synthesis and Characterization. To preserve the composi-
tional analogy of the various architectural isomers, new synthetic
procedures had to be developed to mimic the special features
of the dendritic architecture with its incremental growth in the
Initially, a series of oligomers with an even number of repeat
units was constructed using a standard exponential growth
strategy from 5-benzyloxy-3-hydroxybenzyl alcohol, 1. The two
9
J. AM. CHEM. SOC. VOL. 124, NO. 15, 2002 3931

A R T I C L E S
Harth et al.
Scheme 2. Synthesis of Exact Linear Analogues
dormant functionalities in this strategy were a phenacyl protected
phenol group and a hydroxymethyl group. The latter could be
activated by reaction with CBr4/PPh3 to give a bromomethyl
group, while the phenacyl group could be deprotected with zinc
in acetic acid to give the desired phenol. By using these
activation reactions and a coupling step based on Williamson
ether chemistry, oligomeric linear poly(benzyl ethers) could be
prepared in high yield (Scheme 1). Reaction of 1 with
phenylacyl bromide, 2, gave the alcohol, 3, which is activated
with CBr4/PPh3 to afford the protected bromide, 4. Coupling
of 1 with 4 then gives the dimer, 5, which like 3 is either
deprotected with Zn/HOAc to give the monophenol, 6, or
brominated to give the bromomethyl derivative, 7. Coupling of
6 and 7 resulted in the linear tetramer, 8, which can be
subsequently carried forward to the octamer, 9, hexadecamer,
10, etc., using the same series of reactions. To achieve the
necessary number of repeat units matching that found in regular
dendrimers, the monophenolic derivatives from the above
synthetic sequence were coupled with a series of bromomethyl
substituted oligomers containing an odd number of repeat units.
In this way, linear oligomers could be built up with the correct
number sequence of repeat units, 2n - 1. Therefore, 3,5-bis-
(benzyloxy)benzyl bromide, 11, which is actually the first
generation dendritic bromide, is coupled with the monophenolic
dimer, 6, in the presence of potassium carbonate to give the
trimer, 12. Activation of the hydroxymethyl group with CBr4/
PPh3 gives the bromomethyl derivative, 13, which in turn can
be coupled with the tetramer, 14, to afford the septamer, 15.
Repetition of this activation/coupling strategy with the mono-
phenolic octamer, 16, finally gives the pentadecamer, 17, the
exact linear analogue of the fourth generation dendrimer,
D-[G-4]-OH, 18, described above.
Significantly, 17 contains 15 repeat units, 16 benzyl “chain
ends”, and a single hydroxymethyl functional group (Scheme
2). MALDI-TOF analysis of the linear analogue, 17, and the
corresponding dendrimer, D-[G-4]-OH, 18, showed that both
are essentially monodisperse and have a molecular ion at 3288
amu (Figure 1). This is in total agreement with the synthetic
strategy and the observation that 17 and 18 are architectural
isomers that differ only in the linear versus dendritic placement
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Macromolecular Architecture in Nanomaterials
A R T I C L E S
Figure 1. MALDI-TOF analysis for the linear analogue, L-[G-4]-OH 17, and the corresponding dendrimer, D-[G-4]-OH 18.
Figure 2. DSC analysis of the linear analogue, L-[G-5]-OH 34, and the corresponding dendrimer, D-[G-5]-OH 35.
of their repeat units. The preparation of the exact linear
analogues of poly(benzyl ether) dendrimers from generation one
to generation six now permits a direct comparison of the physical
properties of these two series of macromolecular isomers. From
the synthetic work, it was immediately apparent that the linear
analogues were highly crystalline at both low and high genera-
tion numbers.
This is in direct contrast to the corresponding dendrimers,
which are crystalline at generation one and two but become
totally amorphous at generation four and above. As can be seen
in Figure 2, DSC analysis shows that the dendrimer, D-[G-5]-
OH, 35, has a glass transition temperature of 43 °C as compared
with that of the exact linear analogue, 34, which shows a
prominent melting transition at 153 °C. This high level of
crystallinity was also apparent for the third, fourth, and sixth
generation exact linear analogues and impacted the synthetic
efforts through their decreased solubility. This change in
solubility has also been observed in vapor-liquid equilibrium
experiments.23
Another difference in physical characteristics between the
linear and dendritic poly(benzyl ethers) was in the hydrodynamic
volume of the macromolecules.13 Comparison of the two
isomeric series by gel permeation chromatography showed a
marked nonlinear change in hydrodynamic volume on going
from the linear molecule to the corresponding dendrimer. At
low generation numbers, G ) 1 to 4, the linear macromolecules
are only marginally larger than the corresponding dendrimers.
However, this difference increases dramatically upon reaching
generations five and six with both linear molecules having a
hydrodynamic volume approximately 40-50% larger than that
of the corresponding dendrimers (Figure 3).
This result is fully consistent with the dendrimers having a
more compact and globular structure than do their linear archi-
tectural isomers, which would be expected to assume a more
random coil structure in THF solution.24 It is of interest to note
that similar generation dependent discontinuities in physical
properties are observed between generation four and five for a
number of other dendritic systems.25 In this case, however, the
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A R T I C L E S
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Table 1. Chemical Composition of Isomeric Porphyrin Core
Poly(benzyl ether) Macromolecules 19a-d, 20b-d, and 21a-d
(20a Has the Same Structure as 19a)
internal terminal
compound
series
generation
formula
units
units
19a
19b
19c
19d
20a
20b
20c
20d
21a
21b
21c
21d
D-[G-n]8P
n ) 1
n ) 2
n ) 3
n ) 4
n ) 1
n ) 2
n ) 3
n ) 4
n ) 2
n ) 3
n ) 4
n ) 5
C212H174N4O24
8
24
56
120
8
24
56
120
12
28
60
16
32
64
128
16
32
64
128
16
32
64
C436H366N4O56
C884H750N4O120
C1780H1518N4O248
L-[G-n]8P
L-[G-n]4P
C212H174N4O24
C436H366N4O56
C884H750N4O120
C1780H1518N4O248
C240H198N4O28
C464H390N4O60
C912H774N4O124
C1808H1542N4O252
124
128
favored, and the predominant product was the fully alkylated
derivative for all the structures studied. Table 1 detailing the
chemical composition of the various macromolecules demon-
strates their isomeric relationship. Of particular note is the fact
that the dendritic and eight-arm series of macromolecules have
the same molecular formulas and are exact structural isomers.
In contrast, the series of four-arm star polymers is based on a
central tetraphenyl porphyrin core and, therefore, contains four
extra phenyl rings, which leads to a slight increase in molecular
weight as compared to the other two series.
Figure 3. Difference in GPC retention volume for the exact linear analogues
versus the corresponding dendrimers (Rtlin - Rtden) as a function of
generation number.
comparison with linear isomers having exactly the same
molecular weight and composition permits the difference to be
directly attributed to the architectural change in the polymer
and suggests that these architectural changes only manifest
themselves as actual changes in the physical properties at higher
generations, G ) 5 or above, where the dendrimer adopts a
compact globular architecture.
To obtain the desired structural isomers for evaluation of the
effect of architecture on photophysical properties, a range of
porphyrin core poly(benzyl ether) macromolecular architectures
was also prepared. These isomeric architectures ranged from
four- or eight-arm stars to dendrimers. Synthetically, the latter
two were obtained by coupling an octaphenolic porphyrin core
with the appropriate linear or isomeric dendritic bromides of
the same generation, while the former was obtained by coupling
a tetraphenolic core to the linear analogues of the next higher
generation. In this way, three different porphyrin core poly-
(benzyl ether) architectures 19a-d, 20b-d, 21a-d were
prepared (Scheme 3). In analogy with previous work using
highly functionalized cores,26 the formation of partially func-
tional porphyrin cores, that is, di-, tri-alkylated, etc., was not
For illustration purposes, the actual chemical structures of
the three third generation isomers 19c, 20c, and 21c are shown
in Scheme 4 (structures drawn to scale in their extended
conformations).
Characterization of the different architectural isomers by gel
permeation chromatography (GPC) gave evidence for the
influence of macromolecular configuration on hydrodynamic
volume. As can be seen in Figure 4, the dendritic D-[G-4]8-Por
derivative, 19d, has a substantially smaller hydrodynamic
volume than does the isomeric eight-arm star, 20d, which in
turn is substantially smaller than the four-arm star, 21d.
Interestingly, calibration of the GPC using linear polystyrene
standards led to surprisingly accurate molecular weight values
for the linear four-arm series 21a-d. Hence, the linear portion
of the macromolecule seems to dominate the hydrodynamic
Scheme 3. Structure of Porphyrin Derivatives
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Macromolecular Architecture in Nanomaterials
A R T I C L E S
Scheme 4. Chemical Structures of Third Generation Isomers 19c, 20c, and 21c
Table 2. GPC Characterization of the Isomeric Porphyrin Core
Poly(benzyl ether) Macromolecules 19a-d, 20a-d, and 21a-d
compound
tR/min
M /D
n
MWtheory/D
PD
19a
19b
19c
19d
20a
20b
20c
20d
21a
21b
21c
21d
40.4
38.8
37.3
36.4
40.4
38.6
36.6
33.4
39.4
37.6
34.3
29.0
3261
5496
8857
13 347
3261
5828
12 030
22 500
4515
8183
15 000
28 000
3162
6558
13 350
26 933
3162
1.005
1.009
1.026
1.050
1.005
1.008
1.024
1.020
1.007
1.014
1.010
1.010
6558
13 350
26 933
3586
6982
13 774
27 358
Figure 4. GPC traces for the fourth generation isomeric porphyrin core
poly(benzyl ether) macromolecules, dendritic 19d, eight-arm star 20d, four-
arm star 21d.
The more globular, three-dimensional shape in solution for
the dendrimer may also result in greater shielding of the
porphyrin core when compared to the isomeric eight- and four-
arm stars. To evaluate the effect of architecture on core
shielding, the photophysical properties for these isomeric series
of porphyrin core poly(benzyl ether) macromolecules were
properties of the four-arm star leading to a less globular
conformation in solution (Table 2). In the case of the eight-
arm series 20a-d and the true dendrimers 19a-d, the increased
branching causes a progressively larger deviation when com-
pared to linear polystyrene standards (Figure 5).
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J. AM. CHEM. SOC. VOL. 124, NO. 15, 2002 3935

A R T I C L E S
Harth et al.
Figure 6. Absorption spectra of isomer series 19b-d (____), 20b-d
(‚‚‚‚‚‚‚‚), and 21b-d (- - -) in THF (25 °C).
Figure 5. Correlation of molecular weight by GPC and actual molecular
weight for the dendritic series 19a-d (b), the eight-arm linear series 20b-d
(2), and the four-arm linear series 21a-d (9). Linear polystyrene standard
calibration (‚‚‚‚‚‚‚‚) is shown.
structural isomer series 19b-d, 20b-d, and 21b-d. Because
of the negligible absorption of the dendritic backbone in low
generations, compounds 19a and 21a could not be investigated.
These stimulating early findings have encouraged us to further
investigate these properties attributed to the dendritic state. With
regard to site isolation, porphyrins and their metal complexes
have been thoroughly investigated as core moieties due to their
distinct photophysical, electrochemical, and catalytic char-
acteristics.30b,32 Recently, we have reported on approaches to
site isolation involving alternative architectures based on star-
shaped branched-linear copolymers.33 It is clear that further
insight into the role of the polymer backbone in site isolation
of core functionality could be gained by the study of isomeric
molecules in which a porphyrin probe is surrounded by building
block assemblies with different architectures. The absorption
spectra of the three different isomer series clearly show that
the absorbance of the poly(benzyl ether) backbone is essentially
the same for all isomers within a given generation, while the
absorbance doubles with each increase in generation number
(Figure 6). It should be noted that compounds 21b-d show
slightly higher absorbencies due to the incorporation of four
more phenyl rings (Table 1). The Soret bands of the porphyrin
core exhibit minor bathochromic shifts in the order of 3-4 nm
that are most pronounced in the dendrimer series 19b-d and
are attributed to increasing core encapsulation.30b
investigated. To fully understand these systems, compounds
modeling individual chromophore subunits were also prepared.
3,5-Dimethoxybenzyl methyl ether, 22, and benzyl methyl ether,
23, were chosen as model compounds for the internal and
terminal units, respectively, while tetrakis(3,5-dimethoxy-
phenyl)porphyrin, 24, and tetrakis(4-methoxyphenyl)porphyrin,
25, served as models for the core (Scheme 5).
Absorption/Emission Properties and Energy Transfer
Studies. The encapsulation of functional cores within den-
dritic27,28 shells has attracted considerable attention in recent
years.29 This approach enables the tailoring of the overall
molecular properties via peripheral modification while also
allowing the tuning of the properties of the core itself. In some
cases, there have been observations of dramatic influence of
morphology30 and focal point geometry31 on the efficiency of
energy transduction, that is, the “antenna effect”, within the
dendrimer architecture, in particular for those of the poly(benzyl
ether) type. The antenna effect, arising from indirect excitation
of a dendritic core moiety via efficient light-harvesting of the
poly(benzyl ether) dendrimer backbone and subsequent energy
transfer to the core, was found to lead to considerably enhanced
emission from the core30,31 and even multiphoton processes30a
for rigidified structures with sufficient site isolation.29 Further-
more, the focal point geometry seemed to play an important
role in energy funneling.31 While Jiang and Aida have varied
the number of dendron subunits around a free base porphyrin
core,30b a different approach was taken to investigate morphol-
ogy effects on the energy transfer process by comparing the
Emission from the porphyrin core was observed by either
direct excitation (Soret or Q-bands) or by indirect excitation
(dendrimer backbone). This can be demonstrated by obtaining
the corresponding excitation spectra showing all chromophore
subunits that are responsible for population of the emitting
Scheme 5. Structure of Model Compounds 22-25
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Macromolecular Architecture in Nanomaterials
A R T I C L E S
Figure 9. Comparison of an “ensemble” spectrum consisting of the
combined absorption spectra of all individual chromophore subunits (120
× 22 + 128 × 23 + 24, s) with compound 19d (‚‚‚‚‚‚‚‚). The inset shows
the absorption spectra of the model compounds for the branched (22) and
terminal units (23).
Figure 7. Excitation spectra (λem ) 653 nm) for isomers 19d (____), 20d
(‚‚‚‚‚‚‚‚), and 21d (- - -) in THF (25 °C).
comparison furthermore demonstrates the apparent core isolation
in the dendrimers as indicated by the relatively large shift of
the Soret band.
This finding suggests no significant electronic cooperativity
of the dendron fragments. Hence, in a first approximation, we
can assume that the observed macroscopic energy transfer arises
from a combination of individual events involving single
chromophore subunits. Therefore, the overall average distance
of the 3,5-dialkoxybenzyl donor chromophores to the central
porphyrin acceptor will determine ΦET in the system as
illustrated in Figure 10 (see also Scheme 3). Obviously, this
average distance will largely depend on the molecular archi-
tecture, that is, connectivity, and should therefore differ
significantly by comparing the isomer series as found in the
energy transfer experiments.
To further validate this assumption, a more detailed analysis
of the resonance energy transfer35 in the model systems was
carried out by calculating the Fo¨rster radius R0,36 that is, the
distance at which ΦET ) 0.5, using
Figure 8. Energy transfer efficiencies of isomer series 19b-d, 20b-d,
and 21b-d in THF (25 °C).
6
excited state of the porphyrin core. For example, one set of
isomers (19d, 20d, 21d) depicted in Figure 7 clearly shows
different contributions of the poly(benzyl ether) backbone in
the energy transfer.
Comparison of the excitation and the absorption spectra
allows quantification of the energy transfer efficiency (ΦET).34
With increasing generation, the dendrimer series 19b-d exhibits
only a slight decline in ΦET, while the branched linear series
20b-d shows a significant decrease, and the linear series 21b-d
shows the steepest decrease in ΦET (Figure 8). Clearly, there is
a pronounced morphology dependence on the energy transfer
event in the investigated molecules.
To explain these results, the individual chromophore subunits
were studied. UV/vis spectroscopy revealed a much stronger
absorbance for 22 as compared to that of 23, indicating that
the photophysical properties of the dendrimer backbone are
dominated by the interior branching units. This is further
supported by the observed absorption maximum and the
vibrational fine structure (Figure 9, inset). By adding the
absorbencies of all individual components to construct “en-
semble” spectra, a good correlation with the absorption spectra
of the actual macromolecule was found (Figure 9). This spectral
0.5291κ2J
R0 )
(1)
n4NA
x
where κ2 is the orientation factor, J is the overlap integral of
the fluorescence intensity of the donor and the molar extinction
coefficient of the acceptor normalized by the frequency ex-
pressed in wavenumbers, n is the index of refraction of the
solvent, and NA is Avogadro’s constant. The overlap integrals
were calculated to be J(22 f 24) ) 1.788 × 10-14 mol-1 cm6
and J(22 f 25) ) 1.697 × 10-14 mol-1 cm6 giving rise to
Fo¨rster radii of R0(22 f 24) ) 3.73 nm (series 19b-d and
20b-d) and R0(22 f 24) ) 3.70 nm (series 21b-d). Although
the R0 values for energy transfer from 23 f 24 and 23 f 25
were calculated to be 3.50 nm in both cases, the contribution
of the terminal groups to the overall energy transfer process is
almost negligible due to their very low absorbance. This is
supported by the few available experimental size data, since
compound 19c has a hydrodynamic radius of approximately 1.7
nm in THF, which is much smaller than the Fo¨rster radius (3.73
nm) allowing efficient energy transfer to take place (ΦET
88.4%). For the linear poly(benzyl ether) chains, on the other
)
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J. AM. CHEM. SOC. VOL. 124, NO. 15, 2002 3937

A R T I C L E S
Harth et al.
Figure 10. Illustration of morphology dependent energy transfer arising from a collective interaction of individual donor chromophores with an acceptor
core. The aVerage donor-acceptor separation increases with the linearity of the system from (a) dendritic (19c) over (b) branched linear (20c) to (c) linear
(21c) leading to a reduced energy transfer efficiency.
hand, we have to rely on molecular modeling that predicts a
0.6 nm separation of adjacent 3,5-dialkoxybenzyl chromophores.
Therefore, in a good solvent for the polymer such as THF,37
compound 21c in its fully extended conformation would have
a radius of approximately 9.0 nm (15 × 0.6 nm), giving rise to
an average donor-acceptor distance of 4.5 nm, which is in good
agreement with the observed value of 3.7 nm (ΦET ) 0.55,
which corresponds approximately to the Fo¨rster radius).
led to the observation of strongly morphology dependent
intramolecular energy transfer in the different porphyrin core
isomers series. The energy transduction from the poly(benzyl
ether) backbone to the core was found to be facilitated in the
dendritic case, whereas significant decreases in the energy
transfer efficiencies were observed in the linear cases at higher
molecular weights. Initial spectral analysis revealed the 3,5-
dialkoxybenzyl ether internal units as the donor chromophores,
whose average distance to the porphyrin core dictates the energy
transfer efficiency. The extremely efficient energy transfer of
the dendrimers, even in higher generations, derives from the
relatively short distances that are maintained between the internal
donor units and the acceptor core, clearly suggesting the superior
encapsulation properties of the dendritic architecture.
Conclusion
For the first time, a comparative study of site isolation as a
function of the architecture of the shielding polymer backbone
has been carried out. The design of exact linear analogues of
dendritic poly(benzyl ether) wedges allowed the synthesis of
three architectural isomer series having a porphyrin probe at
the core. The isomers displayed dramatically different hydro-
dynamic properties, crystallinity, and solubility characteristics
when compared to those of their dendritic analogues. Photo-
physical studies also showed significant differences based on
architecture. First static absorption and emission experiments
Acknowledgment. Financial support from NSF (DMR-
9808677, Center for Polymeric Interfaces and Macromolecular
Assemblies), IBM Corp., ARO (MURI program, No. 98-139),
and AFOSR is gratefully acknowledged.
JA025536U
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3938 J. AM. CHEM. SOC. VOL. 124, NO. 15, 2002