Dendritic rods with a poly(triacetylene) backbone: Insulated molecular wires

Article abstract of DOI:10.1039/a801269e

Multinanometer long phenylacetylene-end-capped poly(triacetylene) (PTA) oligomers with dendritic side chains of generation one to three have been prepared; UV-VIS measurements indicate that there is no loss of π-electron conjugation along the PTA backbone

Full text of DOI:10.1039/a801269e

View Article Online / Journal Homepage / Table of Contents for this issue
Dendritic rods with a poly(triacetylene) backbone: insulated molecular wires
Albertus P. H. J. Schenning,a Rainer E. Martin,a Masato Ito,a François Diederich,*a† Corinne Boudon,b
Jean-Paul Gisselbrechtb and Maurice Grossb
a Laboratorium fu¨r Organische Chemie, ETH-Zentrum, Universita¨tstrasse 16, CH-8092 Zu¨rich, Switzerland
b Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide, U.R.A. au C.N.R.S. no 405, Faculte´ de Chimie, Universite´
Louis Pasteur, 1 et 4, rue Blaise Pascal, F-67008 Strasbourg Cedex, France
Multinanometer long phenylacetylene-end-capped poly-
(triacetylene) (PTA) oligomers with dendritic side chains of
generation one to three have been prepared; UV–VIS
measurements indicate that there is no loss of p-electron
conjugation along the PTA backbone in the higher genera-
tion compounds despite distortion from planarity due to the
bulky dendritic wedges.
generation dimer 10b showed clearly that the conformation of
the PTA backbone, including the two end-capping phenyl
groups, is not planar due to the steric hindrance of the bulky
dendritic wedges. On the other hand, the first generation
derivatives 8a–e, similar to previous PTA oligomers,1 should
have a planar conjugated backbone. Such nonplanarity may be
achieved by rotating around C(sp)–C(sp2) and C(sp)–C(sp)
single bonds in the backbone. This, in return, suggested that a
decrease or even complete loss of the p-electron conjugation
along the PTA backbone could occur in the higher generation
compounds, which should be observable by means of spectro-
scopic measurements.8
Rigid, linearly-conjugated rod-like oligomers and polymers
with the poly(triacetylene) (PTA) backbone feature interesting
electronic, nonlinear optical and mesomorphic properties.1,2 We
now describe the merger of dendrimer chemistry3 with our
ongoing development of functionalized PTAs to generate
insulated molecular wires. Encapsulation of the linear
p-conjugated backbone by laterally attached, sterically shield-
ing dendritic wedges should enhance the processability and
stability of PTA oligomers and polymers. At the same time,
steric hindrance between adjacent dendritic wedges of higher
generation could possibly cause nonplanarity and deconjuga-
tion of the backbone. Here, we report the synthesis of
monodisperse multinanometer long dendritic PTA rods of first,
second and third generation and show that p-electron conjuga-
tion in these tubular macromolecules is fully maintained at all
generation levels. Previously, Schlu¨ter and co-workers had
described the preparation of cylindrical dendrimers with a
poly(p-phenylene) backbone as the core.4
In the electronic absorption spectra of all three oligomeric
series (CHCl3, Fig. 1), the longest-wavelength absorption
maximum (lmax) is bathochromically shifted with increasing
rod length and evidently no saturation in either case was
observed, confirming recent findings.1b,2 A comparison of the
spectra of dimers 8b, 9b and 10b revealed that, independent of
the dendritic generation number, the longer-wavelength absorp-
tions, which originate from electronic transitions within the
conjugated PTA backbone, appear at almost the same positions
(around l = 400 nm) with nearly identical fine structure and
molar extinction coefficients [Fig. 1(a)]. Similarly, position,
fine structure and molar extinction coefficients of the longer
wavelength absorption bands in the spectra of trimers 8c (first
generation) and 9c (second generation) are nearly identical
[Fig. 1(b)]. A precise determination of lmax required deconvolu-
tion of the UV–VIS spectra.¶ The obtained values for the dimers
[Fig. 1(a)] showed a minimal bathochromic shift in changing
from generation one to three: lmax = 428.0 ± 0.2 (8b), 430.0 ±
0.3 (9b) and 431.1 ± 0.2 nm (10b). For all dendritic rods, the
lmax values were converted into energies (Emax/eV) which were
then plotted against the reciprocal number of monomer units
(1/n). These plots revealed for all three oligomeric series
straight lines intersecting the ordinate at nearly identical Emax
(2.57 ± 0.06 eV).1b,2 All these data provide impressive support
that p-electron delocalisation and effective conjugation length1b
of the PTA backbone are not affected by distortions out of
planarity due to steric compression of the bulky dendritic
wedges at higher generations. Apparently, p-electron conjuga-
tion involving the acetylenic fragments in the PTA backbone is
best described as being cylindrical rather than resulting from
orbital overlap within a distinct plane and is therefore fully
maintained upon rotation about C(sp)–C(sp2) and C(sp)–C(sp)
single bonds.8–10
The convergent synthesis of the different dendritic rods is
described in Scheme 1. Different generations (G1, G2 and G3)
of Fre´chet-type dendrons5 were attached to (E)-2,3-bis[(tri-
isopropylsilyl)ethynyl]but-2-ene-1,4-diol 11a using the Mitsu-
nobu reaction6 to give the dendritic silyl-protected monomers
2–4. The yield of the third generation compound 4 was very low
(4%), possibly due to steric shielding of the carboxylic acid
reaction centres by the bulky dendritic wedges in the molecule.
After deprotection with TBAF in wet THF, the free trans-
enediynes 5–7 were obtained. The dendritic wedges sub-
stantially stabilise the usually rather unstable free trans-
enediynes, and compounds 5–7 can be stored in the air at
ambient temperature for months without decomposition. Oxida-
tive Hay coupling7 of 5–7 in the presence of phenylacetylene as
an end-capping reagent1a provided the oligomeric PTAs as
solids. The first generation compound 5 afforded separable
oligomers up to the pentamer (8a–e) which were isolated by
size-exclusion chromatography (SEC), whereas, for steric
reasons, the second generation derivative 6 only yielded
isolable oligomers up to the trimer (9a–c).‡ Finally, due to
severe steric overcrowding, conversion of the third generation
enediyne 7 only gave end-capped monomer and dimer (10a,b)
in pure form and sufficient yields.§
The electrochemical properties of the dendritic rods 8a–e and
9a–c were studied by steady-state voltammetry and cyclic
voltammetry in CH2Cl2 (+0.1 m Bu4NPF6).∑ All oligomers
could not be oxidised in the accessible potential range but were
reduced in several irreversible steps, with the electrons being
transferred to the conjugated PTA backbone.1a,b The irreversi-
bility increases with the dendritic generation due probably to
steric hindrance.11 As the oligomeric length increased, the first
reduction step occurred at increasingly less negative potential.
Plots of E1/2 vs. 1/n (n = oligomeric length) gave a straight line
in both series.
In the 1H NMR spectra of the different generation oligomers,
with averaged C2h-symmetry, the number of tert-butyl reso-
nances is equal to the number of monomeric sub-units. The 13
C
NMR spectra did not display a significant difference in
chemical shift between the clearly discernible acetylenic
C-atom resonances in oligomers of same length but different
dendritic generation. Force-field calculations of the third
Chem. Commun., 1998
1013

View Article Online
SiPri3
O
SiPri3
O
H
H
O
(a)
(b)
200
150
100
50
(iii)
HO
Gm
O
Gm
O
(i)
i
ii
OH
O
Gm
O
O
Gm
Pri3Si
Pri3Si
(i)
1
2 m = 1
3 m = 2
4 m = 3
5 m = 1
6 m = 2
7 m = 3
iii
O
(ii)
(ii)
O
Gm
0
250 350
450
550
650
250 350
450
550
650
l / nm
l / nm
Gm
O
Fig. 1 Electronic absorption spectra of (a) (i) 8b, (ii) 9b and (iii) 10b and (b)
(i) 8c and (ii) 9c in CHCl3 at 298 K
O
n
m = 1
m = 2
m = 3
[–(C·C–C·C–CRNCR–C·C–C·C)n–] which, thus far, has
proven quite unstable in the absence of dendritic encapsula-
tion.12
Support from the ETH Research Council (TEMA grant) and
a JSPS fellowship (M. I.) is gratefully acknowledged.
8a n = 1 9a n = 1 10a n = 1
b n = 2
c n = 3
d n = 4
e n = 5
b n = 2
c n = 3
b n = 2
But
But
Notes and References
O
O
† E-mail: diederich@org.chem.ethz.ch
‡ The dendritic rods extend in length from 19.4 (monomer) to 49.4 Å
(pentamer).1a
CO2H
§ The separation of the oligomers was achieved by preparative size-
exclusion chromatography (SEC) on Bio-Beads S-X1 (Bio-Rad) with
CH2Cl2 as eluent, and the purity of the fractions was checked by analytical
SEC. All new compounds were fully characterised by 1H and 13C NMR, FT-
IR, UV–VIS and elemental analyses (except 10b) and MALDI-TOF-MS
(matrix: 9-nitroanthracene), which depicted either the [M + Na]+ or [M +
K]+ ions as base peaks.
¶ The UV–VIS spectra were deconvoluted using software programme PRO
FIT, ver. 5.0.0 for Power Macintosh, Quantum Soft, Zu¨rich, 1990–1996,
with a self-written plug-in module.
G1 (G1–CO2H)
But
But
O
O
But
But
O
O
∑ Electrochemical data in CH2Cl2 (+0.1 m Bu4NPF6, glassy carbon
electrode). Half-wave potentials E1/2 (V vs. Fc/Fc+) and slopes (mV) in
parenthesis: 8a: 21.79 (98), 22.30 (120). 8b: 21.60 (90), 21.76 (90). 8c:
21.54 (140), 21.76 (110), 22.21 (100). 9a: 21.84 (135), 22.35 (114). 9b:
21.62 (105), 21.82 (110), 22.30 (75). 9c: 21.56 (145), 21.78 (100),
22.22 (105).
O
O
CO2H
G2 (G2–CO2H)
But
But
But
But
1 (a) J. Anthony, C. Boudon, F. Diederich, J.-P. Gisselbrecht, V.
Gramlich, M. Gross, M. Hobi and P. Seiler, Angew. Chem., Int. Ed.
Engl., 1994, 33, 763; (b) R. E. Martin, U. Gubler, C. Boudon,
V. Gramlich, C. Bosshard, J.-P. Gisselbrecht, P. Gu¨nter, M. Gross and
F. Diederich, Chem. Eur. J., 1997, 3, 1505.
But
But
O
O
O
O
2 J.-F. Nierengarten, D. Guillon, B. Heinrich and J.-F. Nicoud, Chem.
Commun., 1997, 1233.
3 G. R. Newkome, C. N. Moorefield and F. Vo¨gtle, Dendritic Molecules:
Concepts, Synthesis, Perspectives, VCH, Weinheim, New York, 1996;
D. A. Tomalia, A. M. Naylor and W. A. Goddard, Angew. Chem., Int.
Ed. Engl., 1990, 29, 138; J. M. J. Fre´chet, Science, 1994, 263, 1710.
4 R. Klopsch, P. Franke and A.-D. Schlu¨ter, Chem. Eur. J., 1996, 2, 1330;
B. Karakaya, W. Claussen, K. Gessler, W. Saenger and A.-D. Schlu¨ter,
J. Am. Chem. Soc., 1997, 119, 3296.
5 C. J. Hawker and J. M. J. Fre´chet, J. Chem. Soc., Chem. Commun., 1990,
1010; C. J. Hawker and J. M. J. Fre´chet, J. Am. Chem. Soc., 1990, 112,
7638.
O
O
O
O
O
O
O
O
O
O
But
But
CO2H
G3 (G3–CO2H)
6 T. Tsunoda, Y. Yamamiya, Y. Kawamura and S. Ito, Tetrahedron Lett.,
1995, 36, 2529; O. Mitsunobu, Synthesis, 1981, 1.
7 A. S. Hay, J. Org. Chem., 1962, 27, 3320.
8 H. Tanaka, M. Thakur, M. A. Gomez and A. E. Tonelli, Polymer, 1991,
32, 1834.
9 K. P. C. Vollhardt, Organic Chemistry, Freeman, New York, 1987,
p. 519.
10 For a discussion of the bonding in buta-1,3-diyne, see: D. A. Plattner,
Y. Li and K. N. Houk, in Modern Acetylene Chemistry, ed. P. J. Stang
and F. Diederich, VCH, Weinheim, 1995, p. 1.
Scheme 1 Reagents and conditions: i, G1, G2 or G3, N,N,NA,NA-
tetramethylazodicarboxamide (TMAD), PBu3, THF, 60 °C, 24 h, 81% (2),
61% (3), 4% (4); ii, TBAF, wet THF, 1 h, 99% (5), 99% (6), 99% (7); iii,
5, 6 or 7, CuCl, TMEDA, PhC·CH, CH2Cl2, air, 25 °C, 2 h, 17% (8a,
MW = 1193 D), 6% (8b, MW = 2185 D), 3% (8c, MW = 3176 D), 1%
(8d, MW = 4167 D), 0.4% (8e, MW = 5159 D), or 9% (9a, MW = 2267
D), 6% (9b, MW = 4332 D), 2% (9c, MW = 6396 D), or 13% (10a,
MW = 4414 D), 3% (10b, MW = 8626 D)
11 C. B. Gorman, Adv. Mater., 1997, 9, 1117; P. J. Dandliker, F. Diederich,
A. Zingg, J.-P. Gisselbrecht, M. Gross, A. Louati and E. Sanford, Helv.
Chim. Acta, 1997, 80, 1773.
In the dendritic PTA rods described here, the insulating layer
created by the dendritic wedges protects and stabilises the
central conjugated backbone but does not alter its electronic
characteristics. Efforts are now under way to prepare insulated
molecular wires with the novel poly(pentaacetylene) backbone
12 M. Schreiber, Doctoral Dissertation, ETH Zu¨rich, No. 11904, 1996.
Received in Cambridge, UK, 13th February 1998; 8/01269E
1014
Chem. Commun., 1998