The unique architecture of dendrimers provides a new
dimension for the precise placement of charge-transport units
in a single molecule. Since dendrimers are highly branched
macromolecules,13 they offer control over the placement of
functional groups in higher dimensions than that offered by
linear arrays. Dendrimers should also be advantageous for
the funneling of charges from the core to the periphery,
because the number of functional units doubles with each
layer. Several photophysical studies based on dendritic
architectures have been reported over the past few years.14
However, in most of these cases, the light harvesting
functional groups are incorporated as the core unit or as the
peripheral units, perhaps largely due to the high synthetic
facility with which these arrays can be obtained. To exploit
all the architectural features offered by dendrimers, it is
advantageous to build them where these functional groups
are incorporated as the repeating units.15 There have been a
few recent reports where triarylamine units were incorporated
as the repeating units of dendrimers.16 In all these cases, the
backbone of the dendrimer was chosen in such a way that
the triarylamines are conjugated with each other. The
presence of a conjugated backbone has its obvious merits
for charge-transport studies. However, it is interesting to
design dendrimers with nonconjugated triarylamine units, in
which facile charge transport among the repeating units is
maintained through the dendrimer’s backbone. The advantage
of such a molecular scaffold is that the frontier orbital
energies of the individual monomer units can be indepen-
dently tuned. This provides the pathway for the design of
dendrimers with energy level gradients that can direct the
vectorial transport of charges. Since the charge transporting
ability of Fre´chet-type benzyl ether backbones has been
demonstrated14d,k and since these dendrimers can be synthe-
sized with high facility using a convergent approach,17 we
targeted benzyl ether based dendrons with triarylamine
repeating units.
For the synthesis of benzyl ether based monodendrons,
the building blocks require two phenolic moieties and one
benzyl alcohol moiety. Thus, the targeted repeating monomer
is represented by structure 1 in Figure 1. The synthetic
(13) (a) Fre´chet, J. M. J. Science 1994, 263, 1710-1715. (b) Tomalia,
D. A.; Naylor, A. M.; Goddard, W. A., III. Angew. Chem., Int. Ed. Engl.
1990, 29, 138-175. (c) Newkome, G. R.; He, E.; Moorefield, C. N. Chem.
ReV. 1999, 99, 1689-1746. (d) Fischer, M.; Vo¨gtle, F. Angew. Chem., Int.
Ed. 1999, 38, 884-905. (e) Moore, J. S. Acc. Chem. Res. 1997, 30, 402-
413. (f) Zeng, F.; Zimmerman, S. C. Chem. ReV. 1997, 97, 1681-1712.
(g) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. ReV. 1999, 99,
1665-1688. (h) Matthews, O. A.; Shipway, A. N.; Stoddart, J. F. Prog.
Polym. Sci. 1998, 23, 1-56.
Figure 1. Structures of the triarylamine monomer units.
requirements could have been fulfilled with a much more
symmetrical structure in which the three functional groups
are at the para-positions. However, we envisioned that it
would be advantageous to have the current design for two
practical reasons: (i) It is desirable to leave one of the aryl
rings free from the dendrimerizable functional groups in order
to gain the ability to tune the frontier orbital energies of the
building-block units. In the present case, that ring is
substituted with a methyl group. (ii) The presence of a strong
electron-donating group para to a benzyl bromide moiety
could render the molecule highly reactive and therefore
possibly unstable.18 Therefore, the hydroxymethyl group is
placed meta to the amino moiety. Also note that in the
convergent synthesis the peripheral monomer unit should
have the two phenolic moieties in its protected form.
Therefore, structure 2 was targeted as the peripheral unit
(Figure 1).
(14) (a) Shortreed, M. R.; Swallen, S. F.; Shi, Z. Y.; Tan, W.; Xu, Z.;
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Chem. Mater. 2000, 12, 1463-1472. (f) Pollak, K. W.; Leon, J. W.; Fre´chet,
J. M. J.; Maskus, M.; Abrun˜a, H. D. Chem. Mater. 1998, 10, 30-38. (g)
Chrisstoffels, L. A. J.; Adronov, A.; Fre´chet, J. M. J. Angew. Chem., Int.
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(18) Note that the benzyl alcohol moiety will be converted to a benzyl
bromide during the dendrimer synthesis (vide infra).
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Org. Lett., Vol. 3, No. 13, 2001