Y. Morisaki, Y. Chujo et al.
styrene matrices are attributed mainly to the triplet excited
state.
On the basis of extinction coefficient calculated from the
spectrum fitting and calculated oscillator strength,[33,34] the
triplet yields of 10 and 12 in polystyrene matrices generated
upon exposure to a 355 nm nanosecond laser were estimated
to be 6.1ꢁ10ꢀ3 and 8.7ꢁ10ꢀ3, respectively.[33] This suggests
that the photo charge carrier generation yields (f) of 10 and
12 are on the same order and much smaller than triplet
yield. This presumption is corroborated by the direct-photo-
current measurements of pristine 10 and 12 polymers using
a comb-type electrode,[30] where the integrated photocur-
rents at fixed applied voltage (e.g., 2ꢁ104 Vcmꢀ1) also indi-
cated the values on the same order (3.6 pC for 10 and
4.8 pC for 12).[35] From the FP-TRMC, TAS, and photocur-
rent measurements, it is of interest to note again that 10
(through-space conjugated PAE) and 12 (twisted PAE) pos-
sesses comparable charge carrier transport property. On
a closer look at the triplet yields and direct-photocurrent
measurements, these values of 10 tend to be smaller than
12, suggesting that intramolecular charge carrier mobility of
10 is a few tens of percentage larger than 12.
Figure 6. Time-resolved fluorescence emission spectra of polymer 9 excit-
ed at 337 nm (N2 laser) in CHCl3 (1.0ꢁ10ꢀ6 m).
10ꢁ106; 100 mg) film without any dopants.[31] The cast film
on quartz was excited by a laser pulse at 355 nm, and kinetic
traces of microwave conductivity transients were obtained
as shown in Figure 8A. The product of the photo carrier
generation yield (f) and the sum of charge mobilities (Sm)
was estimated to be 1.9ꢁ10ꢀ5 cm2 Vꢀ1 sꢀ1 at the peak. In
some cases, a f value can be determined by transient ab-
sorption spectroscopy (TAS) of photo-generated charged
species. We therefore conducted TAS of polystyrene-bound
10 film and found the transient absorption spectrum with
a peak at around 570 nm (Figure 8B). This absorption is,
however, attributed to a triplet excited state of 10 (vide
infra).
The similar measurement was carried out for polymer 12
(Mn =6800) in which compound 1 was linearly connected
and twisted (Figure 10). The photoconductivity kinetics and
the fSm maximum value (1.9ꢁ10ꢀ5 cm2 Vꢀ1 sꢀ1) of 12 were
the same as those of 10 (Figure 9), suggesting that the
stacked and linear polymers have comparable charge carrier
transport property. The TAS intensity of 12 was about three
times higher than 10, while the peak and the shape were
almost identical (centered at 560 nm).
In order to assign the transient absorption observed for 10
and 12 diluted in a polystyrene matrix, we performed TAS
on solutions of 10 and 12 in Ar- or air-bubbled toluene . The
almost identical absorption spectra with those in polystyrene
matrices were obtained in the Ar-bubbled solutions, for
which the absorption peaks were located at 580 and 550 nm
for 10 and 12, respectively.[32] These absorptions were signifi-
cantly suppressed by air bubbling, due to the quenching by
O2. Time-dependent density functional theory (TD-DFT)
calculation of the dimers showed that the photoabsorptions
of triplet excited states and their oscillator strengths (f) of
10 and 12 were 622 (f=1.56) and 550 nm (f=2.38), respec-
tively,[33] which are in good coincidence with the experi-
ments. Considering the observed lifetime (ca. 10–20 ms),
quenching by O2, and TD-DFT results, we concluded that
the transient absorptions in both toluene solutions and poly-
Figure 11 presents the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital
(LUMO) of neutral state of 10 and 12 dimers obtained by
the DFT calculations. Both of the molecules show the delo-
calized HOMO and LUMO over the two repeating units.
The singly-occupied molecular orbital (SOMO) of radical
cations are also delocalized in the similar manner; however,
the dihedral angle of 12 at the twisted point (indicated by
an arrow in the figure) was reduced from 88.98 for the neu-
tral state to 56.98 for radical cation. Such a planarization of
polymer backbone has been reported as well for the neutral
and radical cationic states of polyfluorene,[30,36] facilitating
the delocalization of the charges. In contrast, the structure
of 10 did not change significantly between neutral and cat-
ionic states, while its SOMO is evidently delocalized
through the space at not only the center cyclophane unit but
also the end cyclophane units. The rigidity of the main chain
of 10 can decrease the reorganization energy for the charge
transport,[37] which contributes to the increase of intramolec-
ular charge carrier mobility. We are currently investigating
the dependence of the conductivity in a single polymer
chain of the through-space conjugated polymer on the chain
length by employing a well-defined oligomer with electron-
acceptor units such as fullerene[38] and/or perylene[39] deriva-
tives at the polymer chain ends. Considering the recent
report on the molecular junction behavior of [2.2]paracyclo-
phane-based
oligophenylenevinylens,[40]
[2.2]paracyclo-
phane-based through-space conjugated oligomers and poly-
mers can be potentially applied in the molecular wire trans-
porting charges and energy in one direction.
4220
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 4216 – 4224