12832
J. Am. Chem. Soc. 2001, 123, 12832-12836
Pulse Radiolysis Studies of Dendritic Macromolecules with Biphenyl
Peripheral Groups and a Ruthenium Tris-bipyridine Core
Tarek H. Ghaddar,§ James F. Wishart,† James P. Kirby,† James K. Whitesell,§ and
Marye Anne Fox*,§
Contribution from the Department of Chemistry, North Carolina State UniVersity,
Raleigh, North Carolina 27695, and Chemistry Department, BrookhaVen National Laboratory,
Upton, New York 11973
ReceiVed July 2, 2001
Abstract: Electron-transfer reactions in Fre´chet-type dendrimers with biphenyl peripheral groups and a ruthenium
core were investigated by pulse radiolysis techniques. Fast electron-transfer rates found in the two ruthenium
dendrimers suggest a very efficient electronic coupling between the peripheral donor groups and the core
acceptor.
chloride (CH2Cl2) and tetrahydrofuran (THF) were distilled from CaH2
and sodium/benzophenone, respectively. All chemicals were dried under
vacuum for at least 24 h prior to use.
Introduction
Long-range electron-transfer reactions have been studied
extensively because of their importance in redox enzymatic
processes1 and photosynthesis.2 Recently, the use of dendrimers3
as light-harvesting antennae,4-9 fluorescent chemosensors,10 and
organic light-emitting diodes (OLEDs)11 has been a focus of
many groups because of the dendrimers’ three-dimensional
hyper-branched structure. Even though energy transfer in
dendrimers has been studied by many groups, very little has
been reported on the study of electron-transfer reactions within
a dendrimer framework.8,12,13 Here we report for the first time
a pulse radiolysis study of electron-transfer reactions within a
Fre´chet-type dendrimer (Chart 1) bearing biphenyl peripheral
groups and a ruthenium tris-bipyridine core, and compare their
transient absorption spectra with nonmetalated model com-
pounds.
Pulse Radiolysis. Samples for pulse radiolysis were prepared under
Ar in a glovebox (Vacuum Atmospheres). The dendrimer or compound
of interest was weighed and dissolved in THF, then placed in cylindrical
Suprasil cells of 5, 10, and 20 mm path lengths. Pulse radiolysis
experiments were performed at the BNL Laser-Electron Accelerator
Facility (LEAF). The electron pulse parameters were the following:
duration ∼30 ps, charge 2-4 nC, and energy 8.5 MeV. Transient
absoption data were collected by using a pulsed xenon arc lamp, a
selection of 10 and 40 nm wide band-pass filters, silicon (FND-100)
or germanium photodiodes, and a Tektronix TDS-680C oscilloscope.
Kinetic traces were averaged over 2-8 shots.
Preparation of the Benzyl Alcohol Dendrons [Gn-OH] (n ) 1,
2, 3), Scheme 1. The respective benzyl bromide dendron (2.1 equiv),
3,5-dihydroxy benzyl alcohol (1.0 equiv), K2CO3 (2.5 equiv), and 18-
crown-6 (0.2 equiv) were heated to reflux in a minimum amount of
acetone for 48 h. The solvent was removed under vacuum, dissolved
in CH2Cl2, and washed with water (3×), then dried over anhydrous
MgSO4. The solution was then concentrated under vacuum and applied
on a silica gel column, and the product was eluted with CH2Cl2. Yield
70-80%.
Experimental Section
NMR spectra were recorded with a Varian Gemini 300 MHz
instrument. All melting points (uncorrected) were determined with a
Melt-Temp apparatus. All chemicals (Aldrich) were used as received.
K2CO3 was dried in an oven at 130 °C before use. 3,5-Dihydroxybenzyl
alcohol was recrystallized from ethyl acetate before use. Methylene
1
[G1-OH]: Mp 122-125 °C. H NMR (CDCl3) δ 4.67 (d, J ) 4.4
Hz, 2 H), 5.11 (s, 4 H), 6.61 (d, J ) 1.9 Hz, 1 H), 6.68 (d, J ) 1.9 Hz,
2 H), 7.27-7.52 (m, 10 H), 7.59-7.64 (m, 8 H). Calcd. for C33H28O3:
C, 83.8; H, 6.0; O, 10.2. Found: C, 83.6; H, 5.9; O, 10.0.
§ North Carolina State University.
† Brookhaven National Laboratory.
1
[G2-OH]: Mp 64-66 °C. H NMR (CDCl3) δ 4.61 (s, 2 H), 5.00
(1) Nocek, J. M.; Zhou, J. S.; DeForest, S.; Priyadarshy, S.; Beratan, D.
N.; Onuchic, J. N.; Hoffman, B. M. Chem. ReV. 1996, 96, 2459.
(2) Wasielewski, M. R. Chem. ReV. 1992, 92, 435.
(3) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. ReV. 1999,
99, 1665.
(4) (a) Neuwahl, F. V. R.; Righini, R.; Adronov, A.; Malenfant, P. R.
L.; Fre´chet, J. M. J. J. Phys. Chem. B 2001, 105, 1307. (b) Balzani, V.;
Juris, A. Coord. Chem. ReV. 2001, 211, 97.
(5) Adronov, A.; Fre´chet, J. M. J. Chem. Commun. 2000, 1701.
(6) Kawa, M.; Fre´chet, J. M. J. Chem. Mater. 1998, 10, 286.
(7) Sato, T.; Jiang, D. L.; Aida, T. J. Am. Chem. Soc. 1999, 121, 10658.
(8) Stewart, G. M.; Fox, M. A. J. Am. Chem. Soc. 1996, 118, 4354.
(9) Plevoets, M.; Vo¨gtle, F.; De Cola, L.; Balzani, V. New J. Chem.
1999, 23, 63.
(10) Balzani, V.; Ceroni, P.; Gestermann, S.; Kauffmann, C.; Gorka, M.;
Vo¨gtle, F. Chem. Commun. 2000, 853.
(11) Freeman, A. W.; Koene, S. C.; Malenfant, P. R. L.; Thompson, M.
E.; Fre´chet, J. M. J. J. Am. Chem. Soc. 2000, 122, 12385.
(12) Sadamoto, R.; Tomioka, N.; Aida, T. J. Am. Chem. Soc. 1996, 118,
3978.
(s, 4 H), 5.09 (s, 8 H), 6.61 (d, J ) 1.9 Hz, 1 H), 6.62 (m, 4 H), 6.71
(d, J ) 1.9 Hz, 4 H), 7.27-7.51 (m, 20 H), 7.58-7.62 (m, 16 H).
Calcd. for C73H60O7: C, 83.5; H, 5.8; O, 10.7. Found: C, 83.3; H, 6.1;
O, 10.1.
1
[G3-OH]: Mp 85-90 °C. H NMR (CDCl3) δ 4.51 (s, 2 H), 4.94
(s, 4 H), 4.96 (s, 8 H), 5.04 (s, 16 H), 6.52-6.65 (m, 9 H), 6.66 (d, J
) 1.8 Hz, 4 H), 6.69 (d, J ) 1.9 Hz, 8 H), 7.27-7.46 (m, 40 H),
7.55-7.58 (m, 32 H). Calcd. for C153H124O15: C, 83.4; H, 5.7; O, 10.9.
Found: C, 83.2; H, 6.0; O, 10.5.
Preparation of the Benzyl Bromide Dendrons [Gn-Br] (n ) 0,
1, 2, 3), Scheme 1. To the respective benzyl alcohol dendron (1.0 equiv)
in a minimum amount of THF was added CBr4 (1.2 equiv) and the
solution was stirred for 5 min before the addition of PPh3 (1.2 equiv).
For [G3-Br] 10.0 equiv of CBr4 and PPh3 were required to drive the
reaction to completion. The solution was stirred under Ar for 1 h, after
which the solvent was removed under vacuum and the remaining solid
was dissolved in CH2Cl2 and extracted (3×) with water. The organic
layer was dried over anhydrous MgSO4 and concentrated under vacuum,
(13) Gorman, C. B.; Smith, J. C.; Hager, M. W.; Parkhurst, B. L.;
Sierzputowska-Garcz, H.; Haney, C. A. J. Am. Chem. Soc. 1999, 121, 9958.
10.1021/ja011615e CCC: $20.00 © 2001 American Chemical Society
Published on Web 12/01/2001