Preparation of a redox-gradient dendrimer. Polyamines designed for one- way electron transfer and charge capture [16]

Full text of DOI:10.1021/ja9821091

J. Am. Chem. Soc. 1998, 120, 12155-12156
Preparation of a Redox-Gradient Dendrimer.
12155
Polyamines Designed for One-Way Electron Transfer
and Charge Capture
Trent D. Selby and Silas C. Blackstock*
Department of Chemistry, The UniVersity of Alabama
Tuscaloosa, Alabama 35487-0336
ReceiVed June 18, 1998
Directional control of electron transfer (ET) within redox-active
macromolecules is an important design feature of new molecule-
based charge storage materials or charge-transfer devices. Of
recent interest in this field have been redox-active dendrimers,1,2
some of which have been designed for directed energy transfer3
and for electrical conduction.4 Here, we present the synthesis
and electron-transport properties of a new redox-active polyary-
lamine dendrimer 1 which possesses a radial redox-gradient.
Dendrimer 1 (MW ) 2625) contains a benzene core, interior
Figure 1. Cyclic voltammogram of 1 in CH2Cl2 (0.1 M Bu4NBF4) at
200 mV s-1 scan at 25 °C.
included hexaaryl and tetraaryl triaminobenzenes which were
separated from the mixture for use in other experiments. Treat-
ment of B with excess diiodobenzene yielded C, which upon
Ullmann-coupling with A gave dendrimer 1 in 70% purified yield.
Compound 1 is a monodisperse, colorless amorphous solid that
is soluble in organic solvents.8
Electrochemical oxidation of dendrimer 1 by cyclic voltam-
metry (CV) reveals multiple oxidations as displayed in Figure 1.
The first three oxidation peaks are chemically reversible and are
assigned as one-, two- and three-electron processes, respectively.9
The corresponding oxidation potentials are E1°′ 0.48, E2°′ ≈ E3°′
0.63, and E4°′ ≈ E5°′ ≈ E6°′ 0.88 V vs SCE in CH2Cl2.
Coulometric analysis confirms that oxidation of 1 at 0.53 V is a
one-electron process and of 1+ at 0.75 V is a two-electron event.
The CV data of related structures allows the assignment of the
first three oxidations as electron removal from interior PD groups.
(For example, the anisyl-substituted tris(phenylenediamino)-
benzene 2 has E1°′, E2°′, E3°′, and E4°′ values of 0.41, 0.54, 0.61,
and 0.97 V vs SCE in CH2Cl2.)10 The fourth, fifth, and sixth
oxidations of 1 at 0.88 V are assigned as electron loss from remote
peripheral AA groups. This assignment is consistent with the
E°′ values found for 1,3,5-tris(di-p-anisylamino)benzene11 which
are 0.65, 0.87, and 0.98 V vs SCE in CH2Cl2 for mono-, di-, and
trication formation, respectively. As expected, oxidation of the
uncoupled peripheral AA groups of 13+ occurs at a single potential
because these groups are essentially uncoupled to each other.12
Subsequent oxidation waves beyond the sixth-electron oxidation
are also observed for 1 as chemically quasi-reversible or irrevers-
ible waves at room temperature.
p-phenylenediamine groups, and perimeter diarylamino groups.
It has nominal C3 symmetry with nine distinct, meta-linked redox
functions. The more difficult to oxidize peripheral arylamino
(AA) groups form a partial shell around the more easily oxidized
interior phenylenediamino (PD) groups within the dendrimer
architecture to afford a radial redox-gradient in the molecule. The
surface-to-core potential gradient is hypothesized to support 1’s
core charging (oxidation) but inhibit core discharging (reduction
of core cation). Such structures, of which 1 is a first-generation
prototype, might serve as three-dimensional charge funnels and
as charge storage reservoirs in which core charge is gradient
“protected” from neutralization.
Chemical oxidation of 1 with NOPF6 provides isolable 1+, 12+
,
and 13+ PF6 salts in high yield.13 Monocation 1+ gives a broad
single-line ESR spectrum at 25 °C in CH2Cl2. At lower
temperatures (<-20 °C) this spectrum begins to reveal a 5-line
pattern with a splitting of roughly 5 G that we assign as a(2N).14
Dendrimer 1 was prepared via sequential Ullmann reactions
as outlined in Scheme 1.5 Anisidine condensation with phloro-
glucinol according to the procedure of Buu-Ho¨ı6,7 gave in good
yield tris(arylamino)benzene A which was then diarylated under
stoichiometric Ullmann conditions to form intermediate B in 28%
yield after chromatographic purification. Byproducts of this step
(7) (a) Ishikawa, W.; Inada, H.; Nakano, H.; Shirota, Y. Mol. Cryst., Liq.
Cryst. 1992, 211, 431. (b) Stickley, K. R.; Blackstock, S. C. Tetrahedron
Lett. 1995, 36, 1585.
(8) 1H and 13C NMR data is given in the Supporting Information. Analysis
for 1 calcd for C168H150N12O18: C, 76.88; H, 5.75; N, 6.40. Found: C, 76.74;
H, 5.73; N, 6.29.
(9) The CV analysis of isolated 13+ shows that it is electrochemically inert
at 0.8 V vs SCE but reduced at potentials below 0.7 V and oxidized at
potentials above 0.8 V, consistent with E3°′ ≈ 0.63 V but not E3°′ ≈ 0.88 V
vs SCE.
(10) Stickley, K. R.; Selby, T. D.; Blackstock, S. C. J. Org. Chem. 1997,
62, 448.
(1) For reviews of organo-metal dendrimers and their properties, see: (a)
Balzani, V.; Campagna, S.; Denti, G.; Juris, A.; Serroni, S.; Venturi, M. Acc.
Chem. Res. 1998, 31, 26. (b) Gorman, C. B. AdV. Mater. 1997, 9, 1117.
(2) Pollak, K. W.; Leon, J. W.; Fre´chet, J. M. J.; Maskus, M.; Abrun˜a, H.
D. Chem. Mater. 1998, 10, 30.
(3) Devadoss, C.; Bharathi, P.; Moore, J. S. J. Am. Chem. Soc. 1996, 118,
9635.
(4) Miller, L. L.; Duan, R. G.; Tulley, D. C.; Tomalia J. Am. Chem. Soc.
1997, 119, 1005.
(11) Stickley, K. R.; Blackstock, S. C. J. Am. Chem. Soc. 1994, 116, 11576.
(12) This behavior has been observed in other polyferrocenyl dendrimers
with remote ferrocene groups: Cuadrado, I.; Mora´n, M.; Casado, C. M.;
Alonso, B.; Lobete, F.; Garc´ıa, B.; Ibisate, M.; Losada, J. Organometallics
1996, 15, 5278.
(5) For recent reports of polyarylamine dendrimer and oligomer synthesis
by Pd-mediated C-N bond formation, see (a) Louie, J.; Hartwig, J. F. J. Am.
Chem. Soc. 1997, 119, 11695 and (b) Sadighi, J. P.; Singer, R. A.; Buchwald,
S. L. J. Am. Chem. Soc. 1998, 120, 4960.
(13) Analysis for 1+PF6- calcd for C168H150N12O18PF6: C, 72.85; H, 5.45;
N 6.07. Found: C, 72.63; H, 5.40; N 6.02. Analysis for 12+(PF6)2 calcd for
C168H150N12P2F12: C, 69.22; H, 5.18; N, 5.77. Found: C, 69.25; H, 5.19; N,
5.89. Analysis 13+(PF6)3 calcd for C168H150N12P3F18: C, 65.94; H, 4.94; N,
5.49. Found: C, 65.88; H, 4.93; N, 5.59.
(6) Buu-Ho¨ı, N. P. J. Chem. Soc. 1952, 4346.
10.1021/ja9821091 CCC: $15.00 © 1998 American Chemical Society
Published on Web 11/04/1998

12156 J. Am. Chem. Soc., Vol. 120, No. 46, 1998
Communications to the Editor
Scheme 1
Table 1. Self-Electron-Exchange Rate Constants (kET) in CDCl3
for Neutral/Cation Couples
The ESR data, along with the above CV data, suggest that the
odd electron of 1+ is localized on one of the core p-phenylene-
diamine units. For comparison, the observed E1°′ value of
N,N,N′,N′-tetra-p-anisyl-p-phenylenediamine (TAPD) is 0.46 V
vs SCE and its a(2N) splitting is 5.75 G.15
The redox gradient in dendrimer 1 is estimated to be ∼ 0.2 V,
the difference in the E1°′ values of 1,3,5-tris(di-p-anisylamino)-
benzene11 and 1. As mentioned above, this potential gradient
should provide a conduit for electron-hole transfer from surface
to core and simultaneously impart a barrier to the reverse process
to render a degree of electronic “protection” against the reverse
charge transport.
cmpda
kET (M-1 s-1
)
temp (°C) method reference
TMPD/TMPD+ 7.0 ( 0.7 × 109
TMPD/TMPD+ 4.9 ( 0.6 × 109
TAPD/TAPD+ 3.5 ( 0.3 × 108
20.0
25.5
25.5
25.5
25.5
25.5
ESR
ref 13bb
this work
this work
this work
NMRc
NMRc
NMRc
2/2+
2/2+
1/1+
2.8 ( 1.4 × 109
2.1 ( 0.3 × 109
1.8 ( 0.1 × 105
NMRd this work
NMRd this work
a All cations employed as PF6 salts. b Solvent is CHCl3. c -CH3
-
NMR signal monitored. d Core aryl CH signal monitored.
To probe the charge transport properties of 1+, we have
measured its neutral/cation self-electron-exchange reaction rate.
For “unprotected” p-phenylenediamines such as N,N,N′,N′-tet-
ramethyl-p-phenylenediamine (TMPD), kET values for neutral/
cation self-electron-exchange (eq 1) are 108 - 109 M-1 s-1
rates are near the diffusion limit value. The kinetic data is
summarized in Table 1.
From Table 1 data, we conclude that the intermolecular PD
neutral/cation electron-exchange rate for dendrimer 1 is slowed
by a factor of 103-104 relative to model (unprotected) PD neutral/
cation couples. The observed rate retardation for 1/1+ charge
transport corresponds to an increase in the effective activation
free energy for the PD neutral/cation couple of 4-6 kcal mol-1
at 25 °C and is thus consistent in magnitude with a counter-
gradient charge-transfer process for 1/1+ electron-exchange.19
Whether the 1/1+ ET actually occurs through the periphery AA
groups or whether the AA groups act more as a steric shield to
the close approach of the PD centers cannot be distinguished at
this time. It is also possible that the exterior dendrimer generation
increases the reorganization energy associated with PD redox
changes, thereby slowing ET rates. Further studies will be
required to fully elucidate the mechanistic details of 1’s and 1+’s
electron-transport dynamics. Nevertheless, the current results
demonstrate that dendrimer 1 structure imparts a significant kinetic
barrier to an otherwise extremely facile PD/PD+ intermolecular
exchange reaction. This effect is consistent with the redox-
gradient features of 1, which serves as a prototype for a new class
of organic redox systems tailored for controlled charge transport.
=
(depending on solvent) and are measurable by ESR line broaden-
ing of the TMPD+ signal upon addition of neutral TMPD.16 For
1/1+ mixtures, no effect on the ESR signal line width is observed,
suggesting that the 1/1+ ET reaction is slower than 108 M-1 s-1
.
In fact, the 1/1+ electron-exchange is slow enough that its kET
value may be measured by 1H NMR line broadening analysis in
the slow exchange limit17 to give kET of 1.8 × 105 M-1 s-1 in
CDCl3 at 25 °C. For comparison, neutral/cation kET values for
TAPD and 2 have also been measured.18 As expected, the latter
Acknowledgment. We are grateful to the National Science Foundation
and the University of Alabama for support of this research. We are also
grateful to Dr. L. Kispert and Dr. T. Konovalova (University of Alabama)
for obtaining ENDOR spectra of TAPD+ and Dr. M. Bakker and
(University Alabama) and Dr. D. Falvey (University of Maryland) for
helpful discussions.
Supporting Information Available: Synthetic procedures and 1H and
-
13C NMR structure characterization for A, B, C, 1, and 1+PF6 and
experimental details of Table 1 analyses (11 pages, print/PDF). See any
current masthead page for ordering information and Web access
instructions.
(14) The ESR data is consistent with facile intramolecular electron exchange
among core PD groups of 1+ at ambient temperature being frozen out on the
ESR time scale at lower temperatures.
(15) TAPD was prepared by Ullmann reaction of p-diiodobenzene with
di-p-anisylamine and its CV and cation-radical ESR and ENDOR properties
were measured in our laboratory.
(16) (a) Kowert, B. A.; Marcoux, L.; Bard, A. J. J. Am. Chem. Soc. 1972,
94, 5538. (b) Grampp, G.; Jaenicke, W. Ber. Bunsen-Ges. Phys. Chem. 1984,
88, 325.
(17) (a) Bruce, C. R.; Norberg, R. E.; Weissman, S. I. J. Chem. Phys. 1956,
24, 473. (b) McConnell, H. M.; Weaver, H. E. J. Chem. Phys. 1956, 25, 307.
(c) McConnell, H. M.; Berger, S. B. J. Chem. Phys. 1957, 27, 230.
(18) These rates were measured by 1H NMR line broadening in the fast
exchange limit as detailed in the Supporting Information. A complete account
of these kinetic measurements will be published elsewhere.
JA9821091
(19) The slow ET kinetics for the 1/1+ exchange are not an inherent result
of large molecular size. For spherical particles, the diffusion-limited rate
constant kd is given by kd ) 2kbT(ra + rb)2/3ηrarb, where kb is the Boltzman
constant, T the absolute temperature, ra and rb the radii of the colliding spheres,
and η the solvent viscosity. The dependence on particle size is (ra + rb)2/rarb
and when ra ) rb, as in the present case, this value is constant and equal to
4. For a detailed treatment of diffusion-limited rates see: Steinfeld, J. I.;
Francisco, J. S.; Hase, W. L. Chemical Kinetics and Dynamics; Prentice
Hall: Englewood Cliffs, New Jersey; 1989; p 162.