D. Zhao et al.
ring, the torsion strain caused by unfavourable eclipsing or
gauche interactions of CH bonds[12] is absent in M2 and M4
but present in M3 and M5. This must have greatly stabilised
the hydrogen-bonding motif in the former group.
Additionally, the hydrogen bonds in M3 and M5 were
found to be more sensitive to steric interference. If the ter-
minal acetylene group was attached to a trimethylsilyl
(TMS) group, as in M5-TMS (Figure 1), the resonance of
the NH protons was further upfield shifted by approximate-
ly 0.5 ppm relative to that of M5, from 8.7 to 8.2 ppm. A
similar chemical shift change was recorded between M3 and
M3-TMS (spectrum not shown). On the other hand, upon
TMS substitution, a much smaller extent of upfield shifting
(<0.15 ppm) was displayed by M4-TMS (Figure 1) and M2-
TMS (spectrum not shown). Apparently, such upfield shift-
ing evidenced weakening of the intramolecular hydrogen
bonds in these monomers. This was considered to be a result
of steric repulsion caused by the bulky TMS group. One
possible explanation for the larger susceptibility of the hy-
drogen bonds in M3 and M5 to steric interference could be
that the six-membered cyclic hydrogen-bonding motif was
intrinsically labile and easier to disrupt (see above). An ad-
ditional reason might be that, due to the larger ring size,
part of the side chain in M3-TMS or M5-TMS was forced to
be in closer proximity to the TMS moiety, and this imposed
greater steric strain that resulted in distortion and weaken-
ing of the hydrogen bonds. The latter explanation was sup-
ported by theoretical calculations simulating the energy-
minimised molecular conformation of M5-TMS (Figure S1
in the Supporting Information).
Scheme 2. Intramolecular hydrogen-bonding motifs in M2 and M3 involv-
ing the back-folded OEG side chains forming five- or six-membered
cyclic structures (left and middle, respectively). The bifurcated hydrogen-
bonding motif (right) was evidenced to be unimportant for the current
system.
the Ha protons. This result confirmed our prediction that the
OEG side chains would fold back and form intramolecular
hydrogen bonds with the contained ether oxygen atom(s)
(Scheme 2). To further elucidate the properties of these hy-
1
drogen bonds, the H NMR spectrum of M3 was then exam-
ined. Interestingly, the amido NH groups in M3 displayed a
chemical shift of approximately 8.7 ppm, an intermediate
value between those of corresponding protons in M1 and
M2. The extent of downfield shift of hydrogen-bonded pro-
tons semi-quantitatively reflects the bond strength, so it was
concluded that intramolecular hydrogen bonds were formed
by the Ha protons in M3, but their strength was weaker than
that of the hydrogen bonds in M2. Subsequently, the chemi-
cal shifts of the amido protons in M4 and M5 were com-
pared. The NMR spectra showed that the Ha protons in M4
exhibited a similar chemical shift to that of M2, and M5 pre-
sented a similar spectrum to that of M3.
The information provided by 1H NMR spectra of the
monomers can be summarised as follows. First, because M2
and M4 exhibited similar chemical shifts, it was indicated
that a single oxygen atom (the one nearest to the amido car-
bonyl group) played a dominant role in forming the hydro-
gen bond and driving the back-folding of the OEG chain.
Any effect of the additional oxygen atoms was not detected.
The similar chemical shifts displayed by the amido NH
groups in M3 and M5 was also informative, in that no pro-
nounced improvement in bond stability was manifested by
the presence of additional hydrogen-bond acceptors, even in
an hydrogen bond of attenuated strength. Essentially, these
results suggested that the bifurcated hydrogen-bond motif
was unimportant in these monomers (Scheme 2). This obser-
vation was different from that made by Meijer and co-work-
ers with a benzene tricarboxamide, in which the presence of
the second hydrogen-bond acceptor was critical for back-
folding of the OEG side chain.[13] These results combined
show that subtle changes in chemical structure strongly in-
fluence the hydrogen-bonding motif and its properties.
Furthermore, by correlating the amido NH chemical shifts
with the structure differences between M2/M4 and M3/M5,
it was evident that the five-membered ring formed in M2/
M4 was more stable than the six-membered ring in M3/M5.
The reason for this could be a smaller entropy cost for fold-
ing a shorter segment of aliphatic spacer. Additionally, be-
cause there is only one methylene unit in the five-membered
Spectroscopic study of the OPEs in non-polar solvent: In
spite of the different chemical shifts observed for the Ha
protons in the monomers, all of the amido NH protons in
the various oligomers exhibited a similar chemical shift of
approximately 9.3 ppm. Such a value suggested that relative-
ly stable hydrogen bonds were formed in all of these oligo-
mers by the amido NH proton. In 1, unambiguously, the in-
tramolecular hydrogen bonds were formed between the
amido NH proton and the ester carbonyl oxygen atom on
adjacent phenylACTHNUTRGNEUNG(ene) rings. However, due to the presence of
competing hydrogen-bond acceptors (namely the ether
oxygen atoms from the side chains), the structure of the hy-
drogen-bonding motif became equivocal in oligomer series
2–5. That is, although 1H NMR spectroscopy gave explicit
evidence for hydrogen-bond formation, it was incapable of
identifying the hydrogen-bond acceptor.
Uniquely, UV/Vis absorption spectra provided critical in-
formation for the hydrogen-bond structure in the oligomers.
In the previous study,[18] we showed that the effective conju-
gation length of the oligomers was extended by restraining
the rotational motion of the phenylene units in the OPE
backbone and confining them into a co-planar conformation
by virtue of intramolecular hydrogen bonds. This was evi-
denced by a bathochromic shift of the absorption band, rela-
tive to that of analogous OPEs without such intramolecular
hydrogen bonds. Shown in Figure 2 are the absorptions of
7090
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 7087 – 7094
Hu, Wei
