Synthesis, characterization, and optical response of dipolar and non-dipolar poly(phenylenevinylene) dendrimers

Article abstract of DOI:10.1021/jo015764r

New dipolar and non-dipolar poly(phenylenevinylene) dendrimers bearing electron-donating and electron-withdrawing groups have been efficiently synthesized using Heck and Horner-Wadsworth-Emmons reactions. The photoluminescence of these systems may be tuned in the blue zone by choosing the appropriate peripheral groups. Despite the meta-substitution pattern, large Stokes shifts can be observed when π-donor and π-acceptor groups are connected by a m-phenylenevinylene system.

Full text of DOI:10.1021/jo015764r

5664
J . Org. Chem. 2001, 66, 5664-5670
Syn th esis, Ch a r a cter iza tion , a n d Op tica l Resp on se of Dip ola r a n d
Non -Dip ola r P oly(p h en ylen evin ylen e) Den d r im er s
Enrique D´ıez-Barra,* J oaqu´ın C. Garc´ıa-Mart´ınez, Sonia Merino, Ria´nsares del Rey,
J ulia´n Rodr´ıguez-Lo´pez,* Prado Sa´nchez-Verdu´, and J uan Tejeda
Facultad de Quı´mica, Universidad de Castilla-La Mancha, 13071-Ciudad Real, Spain
ediez@qino-cr.uclm.es
Received May 16, 2001
New dipolar and non-dipolar poly(phenylenevinylene) dendrimers bearing electron-donating and
electron-withdrawing groups have been efficiently synthesized using Heck and Horner-Wadsworth-
Emmons reactions. The photoluminescence of these systems may be tuned in the blue zone by
choosing the appropriate peripheral groups. Despite the meta-substitution pattern, large Stokes
shifts can be observed when π-donor and π-acceptor groups are connected by a m-phenylenevinylene
system.
In tr od u ction
aimed at improving the solubility of these systems. Non-
dipolar dendrimeric PPV’s, also called stilbenoid den-
drimers, bearing either alkyl or alkoxy peripheral groups
have been reported previously.12 The new dipolar den-
drimeric PPV’s described here are comparable to other
dendrimeric systems that contain π-donor and π-acceptor
groups and have been claimed as interesting and attrac-
The synthesis of new dendrimeric structures and the
study of such systems in the development of new ap-
plications for dendrimers have been the focus of several
research teams, particularly in the past decade. Synthetic
methodologies, involving convergent1 and divergent2
methods, and applications are well established and have
been reviewed.3 Other strategies have also been described
and these include “hypermonomers”,4 exponential5 and
orthogonal6 growth, and “activated” monomers.7
(9) a) Morgenroth, F.; Mu¨llen, K. Tetrahedron 1997, 53, 15439, and
Angew. Chem. Int. Ed. Engl. 1997, 36, 631. (b) Wang, F.; Rauh, R. D.;
Rose, T. L. J . Am. Chem. Soc. 1997, 119, 9, 11106. (c) Moore, J . S. Acc.
Chem. Res. 1997, 30, 402. (d) Kaneko, T.; Horie, T.; Asano, M.; Aoki,
T.; Oikawa, E. Macromolecules 1997, 30, 3118. (e) Stocker, W.;
Karakaya, B.; Schu¨rmann, B. L.; Rabe, J . P.; Schlu¨ter, A. D. J . Am.
Chem. Soc. 1998, 120, 7691. (f) Karakaya, B.; Clausen, W.; Gessler,
K.; Saenger, W.; Schlu¨ter, A. D. J . Am. Chem. Soc. 1997, 119, 3296.
(g) Klopsch, R.; Franke, P.; Schlu¨ter, A. D. Chem. Eur. J . 1996, 2, 1330.
(h) Stocker, W.; Schu¨rmann, B. L.; Rabe, J . P.; Fo¨rster, S.; Lindner,
P.; Neubert, I.; Schlu¨ter, A. D. Adv. Mater. 1998, 10, 793. (i) Halim,
M.; Pillow, J . N. G.; Samuel, I. D. W.; Burn, P. L. Adv. Mater. 1999,
11, 371. (j) Halim, M.; Pillow, J . N. G.; Samuel, I. D. F.; Burn, P. L.
Synth. Met. 1999, 102, 922. (k) Halim, M.; Adronov, A.; Fre´chet, J . M.
J . Chem. Commun. 2000, 1701. (l) Schenning, A. P. H. J .; Peeters, E.;
Meijer, E. W. J . Am. Chem. Soc. 2000, 122, 4489. (m) Peng, Z.; Pan,
Y.; Xu, B.; Zhang, J . J . Am. Chem. Soc. 2000, 122, 6619. (n) Robinson,
M. R.; Wang, S.; Bazan, G. C.; Cao, Y. Adv. Mater. 2000, 12, 1701.o)
Lee, Y.-Z.; Chen, X.; Chen S.-A.; Wei, P.-K.; Fann, W.-S. J . Am. Chem.
Soc. 2001, 123, 2296.
Evolution from the synthesis to the application of these
materials has been systematically evaluated by Vo¨gtle.8
Electrical, optical, nonlinear optical, electroluminescent,
and photophysical properties of polyconjugated dendrim-
ers, mainly polyphenylenes, have been discussed in
recent papers.9 The search for these kinds of properties
in dendrimeric structures is based on the general proper-
ties exhibited by polyphenylvinylene, polyphenylene,
polyfluorene, and other π-conjugated polymers.10
We report here a full study11 regarding the synthesis
of new dipolar and non-dipolar dendrimeric polyphenylene-
vinylenes (PPV’s) bearing electron-donating and electron-
withdrawing groups as well as long-chain substituents
(10) For recent reviews see: (a) Hide, F.; D´ıaz-Garc´ıa, M. A.;
Schwartz, B. J .; Heeger, A. J . Acc. Chem. Res. 1997, 30, 430. (b) Martin,
R. E.; Diederich, F. Angew. Chem., Int. Ed. 1999, 38, 1350. (c) Segura,
J . L.; Mart´ın, N. J . Mater. Chem. 2000, 10, 2403. For recent reports
see: (d) Kallinger, C.; Hilmer, M.; Haugeneder, A.; Perner, M.; Spirkl,
W.; Lemmer, U.; Feldmann, J .; Scherf, U.; Mullen, K.; Gombert, A.;
Wittwer, V. Adv. Mater. 1998, 10, 920. (e) Wong, C. T.; Chan, W. K.
Adv. Mater. 1999, 11, 455. (f) Eichen, Y.; Nakhmanovich, G.; Gorelik,
V.; Epshtein, O.; Poplawski, J . M.; Ehrenfreund, E. J . Am. Chem. Soc.
1998, 120, 10463. (g) Chung, S. J .; Kwon, K. Y.; Lee, S. W.; J in, J . I.;
Lee, C. H.; Lee, C. E.; Park, Y. Adv. Mater. 1998, 10, 1112. (h)
Nieregarten, J . F.; Eckert, J . F.; Nicoud, J . F.; Ouali, L.; Krasnikov,
V.; Hadziioannou, G. Chem. Commun. 1999, 617. (i) Mart´ınez-Ruiz,
P.; Behnisch, B.; Schweikart, K.-H.; Hanack, M.; Lu¨er, L.; Oelkrug,
D. Chem. Eur. J . 2000, 6, 1294. (j) Xiao, Y.; Yu, W.-L.; Chua, S.-J .;
Huang, W. Chem. Eur. J . 2000, 6, 1318. (k) Apperloo, J . J .; J anssen,
R. A. J .; Malenfant, P. R. L.; Groenendaal, L.; Fre´chet, J . M. J . J . Am.
Chem. Soc. 2000, 122, 7042.
(1) Hawker, C. J .; Fre´chet, J . M. J . J . Am. Chem. Soc. 1990, 112,
7638.
(2) a) Newkome, G. R.; Yao, Z.; Baker, G. R.; Gupta, V. K. J . Org.
Chem. 1985, 50, 2004. (b) Tomalia, D. A.; Baker, H.; Dewald, J .; Hall,
M.; Kallos, G.; Martin, S.; Roeck, J .; Ryder, J .; Smith, P. Polym. J .
1985, 17, 117.
(3) a) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendritic
Molecules; VCH: Weinheim, 1996. (b) Hawker, C. J .; Wooley, K. L.
Advances in Dendritic Macromolecules; Newkome, G. R., Ed.; J ai Press
Inc.: Greenwich, CT, 1995; vol 2. (c) Top. Curr. Chem. 1998, 197.
Vo¨gtle, F., Ed. (d) Bosman, A. W.; J anssen, H. M.; Meijer, E. W. Chem.
Rev. 1999, 99, 1665. (e) Majoral, J . P.; Caminade, A. M. Chem. Rev.
1999, 99, 845. (f) Zeng, F.; Zimmerman, S. C. Chem. Rev. 1997, 97,
1681. (g) Smith, D. K.; Diederich, F. Chem. Eur. J . 1998, 4, 1353.
(4) a) L′abbe´, G.; Forier, B.; Dehaen, W. J . Chem. Soc., Chem.
Commun. 1996, 1262. (b) Gilat, S. L.; Adronov, A.; Fre´chet, J . M. J . J .
Org. Chem. 1999, 64, 7474.
(5) Kawaguchi, T.; Walker, K. L.; Wilkins, C. L.; Moore, J . S. J . Am.
Chem. Soc. 1995, 117, 2159.
(6) Zeng, F.; Zimmerman, S. C. J . Am. Chem. Soc. 1996, 118, 5326.
(7) Freeman, A. W.; Fre´chet, J . M. J . Org. Lett. 1999, 1, 685.
(8) a) Fischer, M.; Vo¨gtle, F. Angew. Chem., Int. Ed. 1999, 38, 884.
(b) Issberner, J .; Moors, R.; Vo¨gtle, F. Angew. Chem., Int. Ed. Engl.
1994, 33, 2413.
(11) D´ıez-Barra, E.; Garc´ıa-Mart´ınez, J . C.; Rodr´ıguez-Lo´pez, J .
Tetrahedron Lett. 1999, 40, 8181.
(12) (a) Maddux, T. M.; Yu, L. J . Am. Chem. Soc. 1997, 119, 9079.
(b) Meier, H.; Lehmann, M. Angew. Chem., Int. Ed. Engl. 1998, 37,
643. (c) Meier, H.; Lehmann, M.; Kolb, U. Chem. Eur. J . 2000, 6, 2462.
(d) Pilow, J . N. G.; Burn, P. L.; Samuel, I. D. W.; Halim, M. Synth.
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10.1021/jo015764r CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/26/2001

Poly(phenylenevinylene) Dendrimers
J . Org. Chem., Vol. 66, No. 17, 2001 5665
Ch a r t 1. Ben zyl a n d Ar yl Br om id es Alon g w ith Der ived P h osp h on a tes
tive targets.13 It is well established that the presence of
electron-donating and electron-withdrawing groups can
modify the HOMO and LUMO energies differently, thus
allowing modulation of the molecular “band gap” and
alteration of the redox and optical properties of the
molecule.14 The ability of conjugated dendrimers to act
as light-emitting diodes has also been reported.9i,j Den-
drimer growth has been claimed as the cause of interest-
ing modifications in fluorescence spectra,15 although not
much work has been done in this respect in the area of
cross-conjugated systems. In the work described here, we
also present a study of the UV and photoluminescence
(PL) properties of most of the dendrons and dendrimers
synthesized.
in 97% yield from the corresponding triol by reaction with
an excess of a 35% HBr solution in acetic acid.
On the other hand, dipolar dendrimers needed to be
prepared from other basic starting units. 3,5-Bis(bromo-
methyl)-1-bromobenzene (3) and 5-bromomethyl-1,3-di-
bromobenzene (4) were obtained by reaction of 1-bromo-
3,5-dimethylbenzene and 1,3-dibromo-5-methylbenzene
with 2 and 1 equiv of NBS, respectively, in refluxing
acetonitrile and a catalytic amount of AIBN. Although
several mono- and polybrominated products appeared in
the crude mixtures, the desired compounds could be
easily separated and purified by column chromatography
to give moderate yields (48% for 3, and 56% for 4).
Undesired polybromination was a more significant prob-
lem due to the relatively large amount of NBS added to
the reaction mixtures. Thus, a larger amount of byprod-
ucts was detected in the preparation of 3 than in the
synthesis of 4, where only 1 equiv of NBS was required.
Compounds 3 and 4 are benzyl bromides as well as aryl
bromides and, therefore, could be used as starting
materials in both HWE and Heck reactions. The synthe-
sis of 1-acetyloxymethyl-3,5-bis(bromomethyl)benzene (5)
and 1,3-bis(acetyloxymethyl)-5-bromomethylbenzene (6)
was carried out by treatment of 2 with 1.5 equiv of
sodium acetate in DMF at 100 °C to give 33% and 29%
yields, respectively (62% overall). Although the presence
of starting material 2 (11%) and the corresponding
triacetate (27%) were also observed in the crude mixture,
all the products could be easily separated by column
chromatography.
Some of the dendrons reported here have been suc-
cessfully connected to binaphthyl16 and [60]-fullerene17
cores, and these routes demonstrate some of the pos-
sibilities for these systems in the field of new organic
materials.
Resu lts a n d Discu ssion
Syn th esis of Cor es. The convergent synthesis of
dipolar and non-dipolar dendrimers bearing a 1,3,5-
tris(phenylenevinylene)benzene core made use of the
Horner-Wadsworth-Emmons (HWE) and Heck reac-
tions, which required different benzyl and aryl bromides
as starting materials (Chart 1). Non-dipolar dendrimers
were constructed from either 1,3,5-trisbromobenzene (1)
or 1,3,5-tris(bromomethyl)benzene (2). While compound
1 is commercially available, intermediate 2 was obtained
The Arbuzov reaction of bromides 2-6 with triethyl
phosphite at 140 °C gave the corresponding phosphonates
7-11 in quantitative yield (Chart 1).
(13) (a) Bryce, M. R.; de Miguel, P.; Devonport, W. Chem. Commun.
1998, 2565. (b) Wooley, K. L.; Hawker, C. J .; Fre´chet, J . M. J . J . Am.
Chem. Soc. 1993, 115, 11496.
(14) Renak, M. L.; Bartholomew, G. P.; Wang, S.; Ricatto, P. J .;
Lachicotte, R. J .; Bazan, G. C. J . Am. Chem. Soc. 1999, 121, 7787.
(15) Devadoss, C.; Bharati, P.; Moore, J . S. Angew. Chem., Int. Ed.
Engl. 1997, 36, 1633.
(16) D´ıez-Barra, E.; Garc´ıa-Mart´ınez, J . C.; Rodr´ıguez Lo´pez, J .;
Go´mez, R.; Segura, J . L.; Mart´ın, N. Org. Lett. 2000, 2, 3651.
(17) (a) Langa, F.; Go´mez-Escalonilla, M. J .; D´ıez-Barra, E.; Garc´ıa-
Mart´ınez, J . C.; de la Hoz, A.; Rodr´ıguez Lo´pez, J .; Gonza´lez-Corte´s,
A.; Lo´pez-Arza, V. Tetrahedron Lett. 2001, 42, 3435. (b) Segura, J . L.;
Go´mez, R.; Mart´ın, N.; Luo, C.; Swartz, A.; Guldi, M. Chem. Commun.
2001, 707.
It has recently been demonstrated that compound 12
is also a useful core for the preparation of different
structures related to that described in this work.12b,c An
alternative method for the synthesis of acetal 12 was also
developed as part of this study and is outlined in Scheme
1. Compound 12 was used in this work as a core to
prepare some dendrons that are difficult to obtain by the
methodologies reported here (vide infra).
Syn th esis of Non -Dip ola r (sym m etr ic) F ir st Gen -
er a t ion Den d r im er s (R )3G1. Two strategies were

5666 J . Org. Chem., Vol. 66, No. 17, 2001
D´ıez-Barra et al.
Sch em e 1. P r ep a r a tion of th e Cor e 12
and 9 the dendrons (R)2G1-Br and (CN)G1-Br 2 (E-
isomers) were prepared as solids in moderate to good
yields (Table 1).
Compound (CN)2G1-Br , taken as a model, was coupled
by a Heck reaction with p-methoxystyrene to afford the
dipolar first generation dendrimer (CN)2(OMe)G1 in
62% isolated yield. This system contains two electron-
withdrawing groups and one electron-donating group at
the periphery. The dendrimer with one cyano group and
two methoxy groups at the periphery, (OMe)2(CN)G1,
was prepared in the same way from (CN)G1-Br 2 and 2
equiv of p-methoxystyrene. However, after column chro-
matography only an impure sample of this compound
could be isolated, and even this was obtained in only 12%
yield. Analogously, starting from (OMe)2G1-Br , (CN)G1-
Br 2, and p-methylstyrene, compounds (OMe)2(Me)G1
and (Me)2(CN)G1 were synthesized (Table 1). In all
cases the crude products were difficult to purify, yields
were not good and reaction times were too long (2 days
minimum). These drawbacks, together with the small
number of commercially available styrenes, caused us to
abandon this route and focus our efforts on the synthesis
of dendrimers using exclusively the HWE methodology.
This reaction had proven to afford cleaner crude products,
which were correspondingly easier to purify and gave
better yields.
employed for the preparation of these compounds (Scheme
2, routes a and b). The first route started with the
palladium-catalyzed coupling of three molecules of para-
substituted styrene derivatives with 1,3,5-tribromoben-
zene (1) according to the method described by de Meijere
et al.18 This method uses potassium carbonate and tetra-
n-butylammonium bromide (TBAB) as a solid-liquid-
phase transfer agent. We found that when p-methyl-
styrene was used, the triple Heck reaction was successful
and led to (Me)3G1 as the major product. However, a
tedious and difficult isolation and purification procedure
was required, and (Me)3G1 was obtained in only 39%
yield. When p-methoxystyrene was used, only the re-
quired disubstituted product could be isolated (in 23%
yield). Both purified compounds did not contain any
double bonds with the Z-configuration. Overall, however,
the Heck reaction proved to be unsatisfactory in terms
of obtaining first generation dendrimers in good yields.
The second method involved formation of the required
E-configured double bonds by a HWE reaction. The
readily available 3-fold tris(phosphonate) 7 was used as
a basic unit to form the core. When this compound was
coupled with a set of para-substituted benzaldehydes,
using KButO as the base and THF as the solvent, only
the all-trans isomers (R)3G1 were observed within the
limits of NMR detection (Table 1). 1H and 13C NMR
spectroscopy confirmed the 3-fold symmetry of (R)3G1.
Compound [CH(OMe)2]3G1 was not isolated but hydro-
lyzed in situ to give (CHO)3G1 under mild acid condi-
tions.
Thus, the HWE reaction of phosphonates 10 and 11
with the appropriate p-substituted benzaldehydes gave
alcohols (R)2G1-CH2OH and (R)G1-(CH2OH)2, respec-
tively, in good overall yields after in situ hydrolysis of
the ester moiety with NaOH/H2O (Table 1).
Some of these alcohols served as building blocks for
the first generation dipolar dendrimers. Treatment of the
alcohol with CBr4/PPh3 in acetonitrile gave the corre-
sponding benzyl bromides. The isolation of dibromide
(OMe)G1-(CH2Br )2 in only 25% yield prompted us to
concentrate our efforts on dendrons bearing only one focal
point. The use of solvents other than acetonitrile, includ-
ing the use of THF, led to lower yields or intractable
crude products. Bromination of derivatives (Me2N)2G1-
CH2OH and (Me2N)G1-(CH2OH)2 proved fruitless. Al-
though the desired products should have been generated
in the reaction, attack at the dimethylamino group by
the bromomethyl group of a second product molecule
precludes the isolation of the target compounds. Route e
was successfully completed by reaction of phosphonate
(MeO)2G1-CH2P (O)(OEt)2 with the appropriate alde-
hydes under the same experimental conditions as de-
scribed above. Dendrimers (MeO)2(CN)G1 and (MeO)2-
[CH(OMe)2]G1 [not isolated, hydrolyzed in situ to
(MeO)2(CHO)G1] could be prepared by this method
(Table 1). Once again, only the all-E isomers were
1
Syn th esis of Dip ola r F ir st Gen er a tion Den d r im -
er s (R)2(R′)G1. To obtain the first generation of dipolar
dendrimers we also conceived two different synthetic
strategies: the linking of one of the branches to the core
followed by the other two (identical) branches (Scheme
2, route d) or, alternatively, the coupling of two identical
branches to the core and the subsequent linking of the
third branch in the final reaction step (Scheme 2, routes
c and e).
detected by H NMR spectroscopy. Dendritic structures
that possess a C2v symmetry are excellent candidates to
show NLO properties19 as well as good transparencies.20
Iter a tive Syn th esis of Den d r on s. Alcohols (R)2n Gn -
CH2OH are also precursors of the next generation
dendrons, which were synthesized by a two-step iterative
procedure in which the dendron is doubled in size in each
iteration. The synthesis was based around the oxidation
The initial idea was to use a combination of Heck and
HWE reactions. Thus, starting from the phosphonates 8
(19) (a) Oudar, J . L.; Chemla, S. J . Chem. Phys. 1997, 66, 1664. (b)
Oudar, J . L. J . Chem. Phys. 1997, 67, 446.
(20) Wolff, J . J .; La¨ngle, D.; Hillenbrand, D.; Wortmann, R.;
Matschiner, R.; Glania, C.; Kra¨mer, P. Adv. Mater. 1997, 9, 138.
(18) Lansky, A.; Reiser, O.; de Meijere, A. Synlett 1990, 405.

Poly(phenylenevinylene) Dendrimers
J . Org. Chem., Vol. 66, No. 17, 2001 5667
Sch em e 2. Syn th esis of F ir st Gen er a tion Den d r im er sa
a
Reagents and conditions: (i) 3 p-R-C6H4-CHdCH2, Pd(AcO)2, TBAB, K2CO3; (ii) 3 p-R-C6H4-CHO, KButO, THF; (iii) 2 p-R-C6H4-
CHO, KButO, THF; (iv) p-R′-C6H4-CHdCH2, Pd(AcO)2, TBAB, K2CO3; (v) p-R′-C6H4-CHO, KButO, THF; (vi) 2 p-R-C6H4-CHdCH2, Pd(AcO)2,
TBAB, K2CO3; (vii) 2 p-R-C6H4-CHO, KButO, THF; then NaOH/H2O; (viii) CBr/PPh3; (ix) P(OEt)3; (x) p-R′-C6H4-CHO, KButO, THF; then
NaOH/H2O.
Sch em e 3. Iter a tive Syn th esis of Den d r on sa
Ta ble 1. P r ep a r a tive Rou tes a n d Isola ted Yield s of
Com p ou n d s P r esen ted in Sch em e 2
compound
route
yield (%)
(Me)3G1
(OMe)3G1
(NMe2)3G1
(CN)3G1
[CH(OMe)2]3G1
(CHO)3G1
(OMe)2G1-Br
(NMe2)2G1-Br
(CN)2G1-Br
(a)
(b)
(b)
(b)
(b)
(b)
(c)
(c)
(c)
(c)
(d)
(e)
(e)
(e)
(e)
-
39
85
60
92
-
86
78
42
81
40
89
96
100
80
85
82
52
25
79
61
a
(NO2)2G1-Br
(CN)G1-Br 2
(OMe)2G1-CH2OH
(OC12H25)2G1-CH2OH
(NMe2)2G1-CH2OH
(CN)2G1-CH2OH
(OMe)G1-(CH2OH)2
(NMe2)G1-(CH2OH)2
(OMe)G1-(CH2Br )2
(OMe)2G1-CH2Br
(CN)2G1-CH2Br
(OMe)2G1-CH2P (O)(OEt)2
(CN)2G1-CH2P (O)(OEt)2
(OMe)2(Me)G1
(CN)2(OMe)G1
(Me)2(CN)G1
-
-
(e)
(e)
(e)
(e)
(c)
(c)
(d)
(d),(e)
(e)
(e)
100
100
34
62
25
(OMe)2(CN)G1
(OMe)2[CH(OMe)2]G1
(OMe)2(CHO)G1
12,b 75
a
-
a
Reagents and conditions: (i) PCC or MnO2; (ii) 10, KButO,
25
THF; then NaOH/H2O; (iii) p-R-C6H4-CHO, KButO, THF; then
H3O+; (iv) 12, KButO, THF; then H3O+.
a
b
Not isolated, hydrolyzed in situ with H3O+. Not pure.
of the alcohol group with either PCC or MnO2 to give the
corresponding aldehyde-containing dendrons (R )2n Gn -
CHO. This step was followed by coupling with the
diphosphonate 10 (Scheme 3, route a). However, the
oxidation reaction failed when side chains in the periph-
ery were dialkylamino groups. Thus, we were obliged to
prepare this kind of aldehyde by applying the methodol-
ogy previously described by Meier et al.,12b,c which makes
use of compound 12 as a core. This alternative was used
to obtain dendrons (Bu 2N)2G1-CHO and (Bu 2N)4G2-
CHO, as well as (CF 3)2G1-CHO and (CF 3)2G2-CHO
(Scheme 3, route b). Therefore, diphosphonates 10 and
12 served as the extension and branching units for the
next, higher generation of dendron (Table 2). The prepa-
ration of third generation dendrons required longer
reaction times; however, the molecular peaks correspond-
ing to the desired product and the carboxylic acid derived
from the oxidation of starting material were always
detected in different relative intensity by mass spectrom-
etry. These results may be explained considering the
higher steric hindrance around the reactive site.
Dialcohols (OMe)n Gn -(CH2OH)2 (n ) 1,2) could also
be oxidized in the same way, although the yields obtained
were not as good as for the monoalcohols (see Supporting
Information). The resulting dialdehydes might be visual-
ized as dendrons for the synthesis of first and second
generation dendrimers (vide infra). However, this pos-
sibility was not considered because of the lower yields
obtained.
Syn t h esis of Secon d Gen er a t ion Den d r im er s
(R)6G2 a n d (R)4(R′)2G2. Compounds (R)2n Gn -CHO
may be also regarded as the starting materials for dipolar

5668 J . Org. Chem., Vol. 66, No. 17, 2001
D´ıez-Barra et al.
Sch em e 4. Syn th esis of Secon d Gen er a tion a
Den d r im er s
Ta ble 2. P r ep a r a tive Rou tes a n d Isola ted Yield s of
Com p ou n d s P r esen ted in Sch em e 3
compound
route
yield (%)
(OMe)2G1-CHO
(a)
(a)
(b)
(a)
(b)
(a)
(a)
(a)
(a)
(b)
(b)
(a)
(a)
85
79
91
91
93
97
87
92
82
97
53
(OC12H25)2G1-CHO
(NBu 2)2G1-CHO
(CN)2G1-CHO
(CF 3)2G1-CHO
(OMe)4G2-CH2OH
(OC12H25)4G2-CH2OH
(OMe)4G2-CHO
(OC12H25)4G2-CHO
(NBu 2)4G2-CHO
(CF 3)4G2-CHO
a
(OMe)8G3-CH2OH
(OC12H25)8G3-CH2OH
-
a
-
a
Not isolated, obtained as a mixture together with (R)4G2-
COOH.
and non-dipolar second generation dendrimers (Scheme
4). The 3-fold reaction of (OC12H25)2G1-CHO with the
core 7 gave the compound (OC12H25)6G2. The last reac-
tion step in the formation of the dipolar second generation
dendrimers (MeO)4(CN)2G2 and (OC12H25)4(CN)2G2
consisted of a HWE reaction between the phosphonates
(MeO)4G2-CH2P (O)(OEt)2 or (CN)2G1-CH2P (O)(OEt)2
and the aldehydes (CN)2G1-CHO or (OC12H25)4G2-
CHO, respectively.
All new compounds were characterized using various
analytical techniques. MS and NMR experiments proved
very useful to confirm the structures of the compounds
(see Experimental Section and Supporting Information).
The selectivity of the HWE reaction is sufficiently high
to generate all-trans isomers within the limits of NMR
detection. This stereochemistry for the double bonds was
established by the coupling constant of the vinylic protons
1
in the H NMR spectra (J ) 16-17 Hz). It is also worth
a
Reagents and conditions: (i) KButO, THF.
1
noting the line broadening observed when the H NMR
spectra were registered at room temperature for com-
pounds (OC12H25)6G2 and (OC12H25)4(CN)2G2. The sig-
nals for these compounds appeared well-resolved upon
heating the sample, a phenomenon that has been re-
ported for other π-conjugated dendrimers and is probably
due to a tendency for this kind of compound to aggre-
gate.12c The low solubility of the dendrimer (OMe)4-
(CN)2G2 precluded the acquisition of any NMR spec-
trum, and its structure could only be confirmed by
MALDI mass spectrometry. In addition to THF, chloro-
form, and dichloromethane were found to be very good
solvents for the dendrons and dendrimers. In all cases,
solubility decreased as molecular size increased. It is
worth noting that NO2, CN, and CF3 groups at the
periphery lower the solubility of the system, while
compounds bearing NBu2 and OC12H25 groups invariably
have the highest solubility.
UV a n d P L Sp ectr oscop y. The optical properties of
several of the dendrimers and dendrons synthesized were
investigated by UV/vis and PL spectroscopy of CH2Cl2
solutions and thin solid films of the compounds. The data
obtained are listed in Table 3. Due to the meta arrange-
ment of the dendrons, the observed absorption spectra
consisted of a superposition of the absorptions due to the
different stilbene chromophores. A red shift of the
absorption band is observed with increasing electron-
withdrawing or electron-donating strength of the periph-
eral groups. Only small changes are observed in the onset
of absorption in the UV/Vis spectra between the different
generations. As one would expect, the molar extinction
coefficients increase with increasing generation. The very
high intensity of absorption can be deduced from the
large extinction coefficients of these molecules at their
absorption maxima. The size of the alkoxy groups has
practically no influence on the absorption properties. The
introduction of different groups has a negligible influence
on the extinction coefficients. All of the compounds
studied are absolutely transparent above 400 nm, with
the exception of the dialkylamino derivatives. The fluo-
rescence spectra also show typical bands for stilbenoid
compounds. Although contributions of more than one
fluorescent species cannot be excluded, we assume that
in the emission spectra only the lowest energy π,π*
transition is fluorescent. The familiar crossing between
absorption and emission is far from both maxima, i.e.,
large Stokes shifts occur. This phenomenon is particu-
larly evident in compounds bearing both π-donor and
π-acceptor groups, indicating large (vibrational, elec-
tronic, geometric) differences between the excited state
reached immediately after absorption and the excited
state from which the emission starts. This effect also
leads to a very low degree of self-absorption of emitted
light. In this sense the UV and PL spectra of compounds
(OMe)2(CN)G1, (CN)2(OMe)G1, and (OMe)2(CHO)G1
are significant. Stokes shifts up to 10400 cm-1 are
observed. Hence, these compounds are expected to per-
form well in organic light emitting diodes and as laser
dyes. The magnitude of the Stokes shifts decreased from
G1 to G2 dendrons, indicating that charge transfer

Poly(phenylenevinylene) Dendrimers
J . Org. Chem., Vol. 66, No. 17, 2001 5669
Ta ble 3. UV/Vis a n d P h otolu m in escen ce (P L) Da ta a
UV/vis (CH2Cl2) λmax
,
PL (CH2Cl2) λmax
,
UV/vis (thin film) λmax
,
PL (thin film) λmax
,
Stokes shifts,
cm-1
compound
(Me)3G1
(OMe)3G1
(NMe2)3G1
(CN)3G1
(CHO)3G1
nm (ꢀ, M-1 cm-1
)
nm
nm
nm
319 (107300)
327 (103500)
368 (117800)
329 (130400)
341.5 (80200)
323 (103600)
327.5 (79300)
331 (83800)
320.5 (112100)
329.5 (136800)
327.5 (257000)
325.5 (178900)
326.5 (189200)
325.5 (18000)
323.5 (49600)
326.5 (56100)
383 (50400)
396, 414
403
461
399, 419
409, 425
404
451
505
428
456
420
417
424
493, 426
454
458
582
404
412
6095
5767
5482
5332
4833
6207
8361
10409
7836
8419
6724
6741
7042
10438
8885
8794
8927
6255
11126
7043
7115
4939
7441
345.5
367.5
456, 493
413, 487
(OMe)2(Me)G1
(OMe)2(CN)G1
(OMe)2(CHO)G1
(Me)2(CN)G1
(CN)2(OMe)G1
(OC12H25)6G2
(OMe)4(CN)2G2
(OC12H25)4(CN)2G2
(OMe)G1-(CHO)2
(OMe)2G1-CHO
(OC12H25)2G1-CHO
(NBu 2)2G1-CHO
(CN)2G1-CHO
(CF 3)2G1-CHO
(OMe)4G2-CHO
(OC12H25)4G2-CHO
(NBu 2)4G2-CHO
(CF 3)4G2-CHO
336.5
330.5
340.5
354
340.5
325
434.5
336.5
340.5
336.5
340.5
378
430
410, 455
453
490
448
439
648
455
412
457
445
571, 608
459
322.5 (67700)
282.5 (79000)
326.5 (179600)
327.5 (188900)
375.5 (138300)
316.5 (364000)
424, 421
427, 421
461
414, 504
330
a
All spectra were recorded at room temperature at c = 3 × 10-6 M.
distilled under reduced pressure, and stored over molecular
sieves (4 Å). Column chromatography was carried out with
Merck silica gel for flash columns (230-400 mesh). NMR
spectra were recorded, except when stated otherwise, in CDCl3
on a Varian FT-300 or Gemini FT-200 instrument with TMS
or the solvent carbon signal as the standards. IR spectra were
recorded on a Nicolet 550 spectrophotometer (FT-IR). UV/vis
spectra were recorded in CH2Cl2 on a J asco V-530 spectropho-
tometer using standard 1 cm quartz UV cells. Fluorescence
spectra were recorded on a J asco FP-750 spectrofluorimeter.
Mass spectrometry and elemental analyses were performed
at the Universidad Auto´noma de Madrid (Servicio Interde-
partamental de Investigacio´n, S. I. D. I.). Melting points were
recorded on a Gallenkamp apparatus and are uncorrected.
1-Acet yloxym et h yl-3,5-b is(br om om et h yl)b en zen e (5)
an d 1,3-Bis(acetyloxym eth yl)-5-br om om eth ylben zen e (6).
A stirred solution of 1,3,5-tris(bromomethyl)benzene (2) (7.2
g, 20 mmol) and sodium acetate (2.46 g, 30 mmol) in DMF (50
mL) was heated at 100 °C for 3 h. After evaporation of the
solvent under vacuum, EtOAc (150 mL) was added. The
solution was washed successively with water (×3) and brine
(×1) and then dried (MgSO4). After filtration and evaporation
of the solvent under reduced pressure, the crude product was
purified by column chromatography (hexanes/EtOAc, 9:1) to
give, in sequence, 830 mg (11%) of starting material 2, 2.20 g
(33%) of 5, and 1.8 g (29%) of 6. Further purification of 5 and
6 was achieved by crystallization from EtOAc/hexanes.
5. White crystals. Mp: 61.5-63 °C. 1H NMR δ: 2.13 (s, 3H),
4.47 (s, 4H), 5.09 (s, 2H); 7.31 (d, 2H, J ) 1.5 Hz), 7.39 (t, 1H,
J ) 1.7 Hz). 13C NMR and DEPT δ: 170.7 (C), 138.8 (C), 137.3
(C), 129.4 (CH), 128.6 (CH), 65.4 (CH2), 32.4 (CH2), 21.0 (CH3).
IR ν: 1738 (CdO) cm-1. Anal. Calcd for C11H12Br2O: C, 39.53;
H, 3.62. Found: C, 39.50; H, 3.71.
6. White needles. Mp: 59.5-61 °C. 1H NMR δ: 2.12 (s, 6H),
4.49 (s, 2H), 5.10 (s, 4H), 7.27 (broad s, 1H), 7.35 (broad s,
2H). 13C NMR and DEPT δ: 170.7 (C), 138.5 (C), 137.0 (C),
128.5 (CH), 127.8 (CH), 65.0 (CH2), 32.6 (CH2), 20.9 (CH3). IR
ν: 1725 (CdO) cm-1. Anal. Calcd for C13H15BrO4: C, 49.68;
H, 4.81. Found: C, 49.67; H, 4.68.
processes are only effective when π-donor and π-acceptor
groups are directly connected by the 1,3-phenylene-
vinylene system. It seems reasonable to assume that
higher generations will not afford a change in this
tendency. When recording PL spectra of different samples
at lower concentrations (10-8 M), changes in the shape
and shift of the bands were detected. For instance, a red
shift for (CF3)4G2-CHO and a blue-shift for (OC12H25)2G1-
CHO were observed. An explanation of these results is
beyond the scope of this work and will be considered soon.
A red-shift in the PL emission in the solid state was also
detected. This observation may be explained by the fact
that in the solid state the molecules can experience a
wider distribution of conformations, including a more
planar state that gives rise to a lower HOMO-LUMO
energy gap.9i
Con clu sion s
New poly(phenylenevinylene)dendrons and dendri-
mers bearing π-donor and/or π-acceptor groups in the
periphery were synthesized using Heck and Horner-
Wadsworth-Emmons reactions. The choice of the ap-
propriate combination of peripheral groups allows the
photoluminescence to be tuned between 400 and 580 nm.
Due to the meta-substitution pattern, all chromophores
are independent and, not only do these show good
transparency in a wide range of the visible region, but
large Stokes shifts are observed when strong electron-
donating and electron-withdrawing groups are connected
through the 1,3-phenylenevinylene system.
Exp er im en ta l Section
Gen er a l. Unless stated otherwise, all reagents were used
as received and without further purification. NBS was recrys-
tallized from water. CBr4 was purified by sublimation. MnO2
was heated under vacuum at 120 °C for at least 24 h prior to
use. The following solvents were distilled under positive
pressure of dry argon immediately prior to use: THF and Et2O
over sodium/benzophenone ketyl; CH2Cl2 from CaH2. Aceto-
nitrile was distilled under argon from CaH2 and stored over
molecular sieves (4 Å). DMF was dried over CaH2, filtered,
Gen er a l P r oced u r e for th e Heck Rea ction . All glass-
ware was oven-dried and cooled under Ar. To a stirred solution
of TBAB (1.2 mol per mol of styrene), K2CO3 (0.6 mol per mol
of styrene), and Pd(AcO)2 (10 mol %) in DMF (10 mL per mol
of TBAB), under argon, was added a solution of the corre-
sponding aryl bromide and para-substituted styrene in DMF
(10 mL per mol of aryl bromide). The mixture was immersed

5670 J . Org. Chem., Vol. 66, No. 17, 2001
D´ıez-Barra et al.
in an oil bath, which had been preheated to 140 °C, and stirred
for the indicated period of time. The mixture was then allowed
to cool, diluted with CHCl3, and successively washed with 1
N HCl (×3), H2O (×1), and brine (×1) and dried (MgSO4). The
solution was filtered and the solvent evaporated under reduced
pressure. The residues were purified by column chromatog-
raphy (hexanes/EtOAc) and/or crystallization. A representative
example for the preparation of a dendrimer is described below.
1,3-Bis(p -cya n ost yr yl)-5-p -m e t h oxyst yr ylb e n ze n e ,
(CN)2(OMe)G1. Reaction time: 48 h. Purified by column
chromatography (hexanes/EtOAc, 7:3) and crystallization from
CHCl3/EtOH. Yellow solid. Yield: 62%. Mp: 234-238 °C
(decomposition observed above 210 °C). 1H NMR δ: 3.85 (s,
3H), 6.93 (A of ABq, 2H, J ) 8.5 Hz), 7.00 (A of ABq, 1H), 7.17
(B of ABq, 1H), 7.17 (A of ABq, 2H, J ) 16.2 Hz), 7.27 (B of
ABq, 2H, J ) 16.3 Hz), 7.50 (B of ABq, 2H, J ) 8.6 Hz), 7.55
(s, 1H), 7.59 (s, 2H), 7.61 (A of ABq, 4H, J ) 8.9 Hz), 7.67 (B
of ABq, 4H, J ) 8.6 Hz). 13C NMR δ: 159.6, 141.5, 138.8, 137.1,
132.5, 131.8, 129.6, 129.5, 127.8, 127.5, 126.9, 125.5, 124.8,
124.0, 118.9, 114.2, 110.8, 55.35. HRMS, m/e calcd for
3,5-Bis(p-dodecyloxystyr yl)ben zyl alcoh ol,(OC12H25)2G1-
CH2OH. Method C. Reaction time: 1 h. The crude product
was of sufficient purity (determined by 1H NMR) and was used
as such in the next step. White solid. Quantitative yield.
Further purification could be achieved by crystallization from
CHCl3/EtOH.1H NMR δ: 0.88 (t, 6H, J ) 6.6 Hz), 1.20-1.55
(m, 36H), 1.78 (m, 4H, J ) 6.9 Hz), 3.96 (t, 6H, J ) 6.3 Hz),
4.72 (s, 2H), 6.89 (A of ABq, 4H, J ) 8.7 Hz), 6.96 (A of ABq,
2H, J ) 16.2 Hz), 7.10 (B of ABq, 2H, J ) 16.2 Hz), 7.35 (s,
2H), 7.44 (B of ABq, 4H, J ) 8.7 Hz), 7.48 (s, 1H). 13C NMR
and DEPT δ: 158.9 (C), 141.5 (C), 138.3 (C), 129.7 (C), 128.7
(CH), 127.7 (CH), 126.0 (CH), 123.7 (CH), 123.4 (CH), 114.7
(CH), 68.0 (CH2), 65.3 (CH2), 31.9 (CH2), 29.7 (CH2), 29.6 (three
signals, 3 × CH2), 29.4 (two signals, 2 × CH2), 29.3 (CH2), 26.0
(CH2), 22.7 (CH2), 14.1 (CH3). IR ν: 3547 (OH) cm-1. HRMS,
m/e calcd for C47H68O3: 680.5168. Found: 680.5153.
3,5-Bis(p-dibu tylam in ostyr yl)ben zaldeh yde, (NBu 2)4G2-
CHO. Method D. Reaction time: 2 h. Purification: column
chromatography (EtOAc/hexanes, 9:1). Yellow oil which solidi-
1
fied upon standing. Yield: 97%. H NMR δ: 0.97 (t, 24H, J )
C
33H24N2O: 464.1889. Found: 464.1898.
7.5 Hz), 1.31-1.43 (m, 16H, J ) 7.5 Hz), 1.53-1.65 (m, 16H),
3.30 (m, 16H, J ) 7.5 Hz), 6.65 (A of ABq, 8H, J ) 9.0 Hz),
6.93 (A of ABq, 4H, J ) 16.2 Hz), 7.14 (B of ABq, 4H, J ) 16.2
Hz), 7.25 (A of ABq, 2H, J ) 16.5 Hz), 7.32 (B of ABq, 2H, J )
16.5 Hz) 7.43 (B of ABq, 8H, J ) 9.0 Hz), 7.51 (s, 6H), 7.95 (s,
3H), 10.10 (s, 1H). 13C NMR and DEPT δ: 192.3 (CH), 147.9
(C), 139.0 (C), 138.8 (C), 137.2 (C), 137.1 (C), 130.9 (CH), 130.2
(CH), 129.3 (CH), 127.8 (CH), 127.0 (CH), 126.3 (CH), 124.4
(C), 123.6 (CH), 123.3 (CH), 122.6 (CH), 111.6 (CH), 50.8 (CH2),
29.5 (CH2), 20.4 (CH2), 14.0 (CH3). MS (MALDI), m/e 1227.7.
1,3-Bis(p -m e t h oxyst yr yl)-5-p -cya n ost yr ylb e n ze n e ,
(OMe)2 (CN)G1. Method B. Reaction time: 2 h 30 min.
Purified by column chromatography (hexanes/EtOAc, 1:1).
Further purification could be achieved by crystallization from
Gen er al P r ocedu r es for Hor n er -Wadswor th -Em m on s
Rea ction s. Meth od A. All glassware was oven-dried and
cooled under Ar. To a stirred solution of the corresponding
phosphonate and aromatic aldehyde in THF (6-9 mL per
mmol of phosphonate), under argon, was added, in small
portions, potassium tert-butoxide. The deeply colored mixture
was stirred for the indicated period of time at room temper-
ature. After hydrolysis with water, the mixture was stirred
for a further 20 min, and the precipitated solid was filtered
off. Meth od B. All operations were identical to those described
for method A except that, after hydrolysis with water, the
mixture was extracted with CH2Cl2 (×3). The combined organic
layers were successively washed with water and brine and
then dried (MgSO4). The solution was filtered and the solvent
evaporated under reduced pressure. The crude products were
purified as indicated. Meth od C. All operations were identical
to those described for methods A and B except that hydrolysis
was carried out with 1 M NaOH (10 mL) and water (50 mL
per mmol of product), and the mixture was stirred for an
additional 2 h at room temperature in order to remove the
ester group. Meth od D. All operations were identical to those
described for method B except that hydrolysis was carried out
with 1 M HCl and the mixture was stirred for an additional 2
h at room temperature in order to remove the acetal group.
Representative examples for the preparation of different
dendrons and dendrimers are described below.
1
CHCl3/EtOH. Pale yellow solid. Yield: 75%. H NMR δ: 3.84
(s, 6H), 6.92 (A of ABq, 4H, J ) 8.7 Hz), 6.98 (A of ABq, 2H, J
) 16.2 Hz), 7.13 (B of ABq, 2H, J ) 16.2 Hz), 7.11-7.25 (m,
2H), 7.46-7.53 (m, 7H), 7.58 (A of ABq, 2H, J ) 8.7 Hz), 7.63
(B of ABq, 2H, J ) 8.7 Hz). 13C NMR and DEPT δ: 159.4 (C),
141.7 (C), 138.4 (C), 136.8 (C), 132.5 (CH), 132.2 (CH), 129.8
(C), 129.0 (CH), 127.8 (CH), 127.0 (CH), 126.8 (CH), 125.9
(CH), 124.3 (CH), 123.6 (CH), 119.0 (C), 114.2 (CH), 110.6 (C),
55.3 (2 × CH3). HRMS, m/e calcd for C33H27NO2: 469.2042.
Found: 469.2042.
(OC12H25)4(CN)2G2. Method B. Reaction time: 4 h 30 min.
Purified by crystallization from CHCl3/EtOH. Pale yellow solid.
Yield: 66%. 1H NMR (70 °C) δ: 0.89 (t, 12H, J ) 6.6 Hz), 1.10-
1.55 (m, 72H), 1.79 (m, 8H), 3.98 (t, 8H, J ) 6.6 Hz), 6.87-
7.24 (m, 26H), 7.41-7.68 (m, 28H). 13C NMR δ: 159.0, 141.4,
138.3, 138.0, 137.5, 137.1, 132.5, 131.7, 129.7, 129.1, 128.7,
128.3, 127.8, 127.4, 126.9, 126.0, 125.0, 123.8, 123.3, 118.9,
114.7, 110.8, 68.1, 31.9, 29.7 (four signals), 29.5, 29.4, 29.3,
26.1, 22.7, 14.1. IR (KBr) ν: 2224 (CN) cm-1. MS (MALDI),
m/e 1784.0.
1,3,5-Tr is(p-m eth oxystyr yl)ben zen e, (OMe)3G1. Method
A. Reaction time: 2 h 30 min. Purification: crystallized from
CHCl3/EtOH. Colorless solid. Yield: 85%. 1H NMR δ: 3.84 (s,
9H), 6.92 (A of ABq, 6H, J ) 8.8 Hz), 7.01 (A of ABq, 3H, J )
16.5 Hz), 7.16 (B of ABq, 3H, J ) 16.4 Hz), 7.49 (s, 3H), 7.49
(B of ABq, 6H, J ) 8.8 Hz). 13C NMR δ: 159.3, 138.2, 130.1,
128.5, 127.7, 126.4, 123.1, 114.1, 55.3. IR ν: 1605, 1585, 1510
cm-1. Anal. Calcd for C33H30O3: C, 83.51; H, 6.38. Found: C,
83.28; H, 6.06.
5-Br om o-1,3-bis(p-cya n ostyr yl)ben zen e, (CN)2G1-Br .
Method B. Reaction time: 1 h. Purification: washed with
EtOH. Pale yellow solid. Yield: 81%. 1H NMR δ: 7.14 (s, 4H),
7.54-7.64 (m, 3H), 7.59 (A of ABq, 4H, J ) 8.1 Hz), 7.67 (B of
ABq, 4H, J ) 8.4 Hz). 13C NMR δ: 141.0, 138.7, 132.6, 130.3,
129.2, 128.7, 127.0, 124.3, 123.5, 118.8, 111.2. MS (EI+), m/e
412.1 (100), 410.1 (97). HRMS, m/e calcd. for C24H15N279Br:
410.0419. Found: 410.0418. Anal. Calcd for C24H15N2Br: C,
70.09; H, 3.68; N, 6.81. Found: C, 70.02; H, 3.91; N, 6.97.
Ackn owledgm en t. Financial support from the Span-
ish DGES (PB97-0425 and 1FD97-0167) is gratefully
acknowledged.
Su p p or tin g In for m a tion Ava ila ble: Additional experi-
mental procedures, compound characterization data, and
copies of 1H NMR and 13C NMR spectra for all compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
J O015764R