oligomers that contain phenylene repeats (as opposed to the
biphenyl and bipyridyl repeats present in 1–4), in which the
bandgap continues to decrease for 10 or more repeat units.2b,d,f
This suggests that the poor electronic coupling between the non-
coplanar phenyl (and pyridyl) rings in the biphenyl and
bipyridyl units in 2–4 restricts the conjugation length.
Comparison of the spectra of Re-1–Re-4 with those of the
corresponding free oligomers demonstrates that metallation
induces a substantial red-shift of the lowest p,p* absorption.
The oscillator strength of the low energy band is large, but
relatively constant throughout the series Re-1–Re-4. The low-
energy band likely arises from a long-axis polarized p,p*
transition localized predominantly on the chromophore defined
by the bis(dioctadecyloxyphenylethynyl)-capped bipyridine
segment. The transition is red-shifted because metallation
forces the bipyridyl unit into a planar conformation, thereby
effectively increasing the conjugation length.4d In addition to
this effect, the electrophilic metal center likely decreases the
LUMO energy and consequently decreases the HOMO–LUMO
gap. An important point is that the dp (Re) ? p* (bpy-
oligomer) MLCT transition, which is expected to arise in the
400–500 nm region (with e ≈ 103–104 M21 cm21), is likely
buried under the considerably more intense ligand-centered
p,p* transition.
Scheme 2 Reagents and conditions: i, HC·CCMe2OH (1 equiv.), Pd/Cu
(cat.); ii, 5,5A-diethynyl-2,2A-bipyridine (0.5 equiv.), Pd/Cu (cat.); iii, KOH,
toluene, reflux; iv, 4-bromobiphenyl, Pd/Cu (cat.).
Once 6 was available in multigram quantities, synthesis of the
desired oligomers proceeded rapidly. Thus, reaction of 2 equiv.
of 6 with 5,5A-diethynyl-2,2A-bipyridine7 produces 7a, which is
subsequently deprotected by KOH, toluene and heat to afford
7b. Endcapping of 7b with 2-iodo-1,4-dimethoxybenzene
affords 3, and subsequent metallation of 3 with Re(CO)5Cl in
toluene or Ru(bpy)2(CF3SO3)2 in THF–acetone yields Re-3 and
Ru-3, respectively. A further iterative sequence beginning with
coupling of 7b with 2 equiv. of 6 followed by deprotection and
endcapping affords oligomer 4.
As shown in Scheme 2, 2 was synthesized by a different
approach that also relies upon the use of the 2-HP protecting
group. First, 1,4-diiodo-2,5-dioctadecyloxybenzene was cou-
pled with 1 equiv. of 2-methylbut-3-yn-2-ol to produce 9. This
compound was readily separated from unreacted starting
material owing to the polar 2-HP function. Reaction of 2 equiv.
of 9 with 5,5A-diethynyl-2,2A-bipyridine afforded 10a, which
was deprotected to 10b by KOH, toluene and heat. Finally, 10b
was coupled with 4-bromobiphenyl to produce 2 which is
subsequently metallated to afford Re-2 or Ru-2.
The spectra of the RuII complexes Ru-1–Ru-4 are also
dominated by the p,p* transitions of the ligand; in fact, these
bands have approximately the same lmax and e in the RuII and
ReI complexes. A distinct shoulder is observed (l ≈ 485 nm, e
≈ 8000 M21 cm21) in the spectrum of Ru-1 which is very
likely the dp (Ru) ? p* (1) MLCT transition. A similar MLCT
band is not observed in the spectra of Ru-2–Ru-4 because it is
obscured by the more intense p,p* transition that occurs at a
lower energy in these oligomers.
In conclusion, we have applied an iterative method to
synthesize a series of mono-disperse PPE-based metal–organic
oligomers. Ongoing spectroscopic and photophysical studies
demonstrate that the oligomers are excellent models for
structurally related bipyridine-containing poly(p-phenylenee-
thynlene) polymers.
Fig. 1 compares the UV-visible absorption spectra of the free
oligomers with those of the corresponding (L)Re(CO)3Cl and
2+
(L)Ru(bpy)2 complexes. The free oligomers all feature two
We are grateful to the National Science Foundation (Grant
No. CHE 99-01862) for support of this work.
absorption bands in the 300–500 nm region. The lowest energy
band is assigned to the long-axis polarized p,p* (HOMO ?
LUMO) transition, while the second band is assigned to the
short-axis polarized p,p* transition. The low energy band red-
shifts considerably from 1 to 2, but the position and bandshape
of the transition remains relatively constant in 2–4, indicating
that the bandgap of the oligomers is defined early in the series.
This observation contrasts with observations made on PPE
Notes and references
‡ Godt and co-workers recently reported the use of the hydroxymethyl
(HM) moiety as a protecting group for terminal acetylenes in the synthesis
of PPE oligomers [ref. 2(f)].
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Fig. 1 Absorption spectra of oligomers: (a) 1–4 in THF solution; (b) Re-
1–Re-4 in THF solution; (c) Ru-1–Ru-4 in CH2Cl2 solution.
Communication 9/03476E
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Chem. Commun., 1999, 1749–1750