Photoinduced electron transfer and nonlinear absorption in poly(carbazole-alt-2,7-fluorene)s bearing perylene diimides as pendant acceptors

Article abstract of DOI:10.1021/jp3006712

This paper reports the synthesis, photophysical behavior, and use in nanosecond optical-pulse suppression of a poly(2,7-carbazole-alt-2,7-fluorene) and a poly(3,6-carbazole-alt-2,7-fluorene) in which the carbazole N-positions are linked by an alkyl chain to one of the nitrogen atoms of a perylene-3,4,9,10-tetracarboxylic diimide (PDI) acceptor. It was found that the PDI pendants on the polymer side chain aggregated even in dilute solution, which extended the onset of PDI absorption into the near-infrared (NIR). Transient-absorption spectra of these polymers provide evidence for efficient electron transfer following either donor or acceptor photoexcitation to form long-lived charge-separated species, which exhibit strong absorption in the NIR. The spectral overlap between the transient species and the long-wavelength absorption edge of the aggregated PDI leads to reverse saturable absorption at 680 nm that can be used for optical-pulse suppression. Additionally, at high input energies, two-photon absorption mechanisms may also contribute to the suppression. PDI-grafted polymers exhibit enhanced optical-pulse suppression compared with blends of model materials composed of unfunctionalized poly(carbazole-alt-2,7-fluorene)s and PDI small molecules.

Full text of DOI:10.1021/jp3006712

Article
pubs.acs.org/JPCA
Photoinduced Electron Transfer and Nonlinear Absorption in
Poly(carbazole-alt-2,7-fluorene)s Bearing Perylene Diimides as
Pendant Acceptors
Chun Huang, Matthew M. Sartin, Matteo Cozzuol, Nisan Siegel, Stephen Barlow, Joseph W. Perry,*
and Seth R. Marder*
School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta,
Georgia 30332-0400, United States
ABSTRACT: This paper reports the synthesis, photophysical
behavior, and use in nanosecond optical-pulse suppression of a
poly(2,7-carbazole-alt-2,7-fluorene) and a poly(3,6-carbazole-
alt-2,7-fluorene) in which the carbazole N-positions are linked
by an alkyl chain to one of the nitrogen atoms of a perylene-
3,4,9,10-tetracarboxylic diimide (PDI) acceptor. It was found
that the PDI pendants on the polymer side chain aggregated
even in dilute solution, which extended the onset of PDI
absorption into the near-infrared (NIR). Transient-absorption
spectra of these polymers provide evidence for efficient
electron transfer following either donor or acceptor photo-
excitation to form long-lived charge-separated species, which
exhibit strong absorption in the NIR. The spectral overlap
between the transient species and the long-wavelength absorption edge of the aggregated PDI leads to reverse saturable
absorption at 680 nm that can be used for optical-pulse suppression. Additionally, at high input energies, two-photon absorption
mechanisms may also contribute to the suppression. PDI-grafted polymers exhibit enhanced optical-pulse suppression compared
with blends of model materials composed of unfunctionalized poly(carbazole-alt-2,7-fluorene)s and PDI small molecules.
pulse suppression is to exploit long-lived triplet excited-state
absorptions; this is feasible in cases where rapid intersystem
crossing leads to high triplet populations and where the triplet
cross-section is high compared to the ground-state cross-
section at the wavelength of interest. For example, heavy-metal-
containing phthalocyanines6,7 or of heavy-metallopolyynes and
their oligomers, especially by Wong et al.,8−12 have proven
effective for use in the 450−600 nm range.
Another approach to RSA, especially effective for use in the
NIR, has recently been developed based on the formation of
strongly absorbing radical ions following photoinduced intra- or
intermolecular electron transfer.13−15 Blends of polythiophene
and fullerene derivatives were among the earliest reported
effective optical-pulse suppression systems relying on this
mechanism; the strong optical suppression observed at 700 and
760 nm was attributed to excitation of the weak long-
wavelength tail of the polythiophene absorption followed by
excited-state electron transfer from the polythiophene to the
C60 derivative to give the strongly absorbing polythiophene
positive polaron (radical cation).14 Similar phenomena have
been utilized in other polymer blends15 and in molecular dyads
to achieve good optical-pulse suppression at various NIR
INTRODUCTION
■
Because of their potential use in protecting eyes and optical
sensors, systems able to suppress strong optical pulses in
various wavelength regions have attracted significant research
interest over the past two decades.1,2 Effective optical-pulse
suppression systems require materials that maintain high
transparency under low-intensity irradiation, but can respond
rapidly (on the time scale of the optical pulse) to attenuate a
high-intensity laser pulse. Depending on the duration and
intensity of the incident pulses, different nonlinear transmission
mechanisms may be useful.1−3 Two widely used approaches are
(i) to use chromophores that exhibit efficient two-photon
absorption (2PA) and subsequent strong excited-state
absorption (ESA) in the same spectral region,2−5 and (ii) to
use materials that exhibit reverse saturable absorption (RSA),
that is, materials for which the ESA cross section (σe) exceeds
that of the ground state (σg) at the wavelength of interest.1 The
effectiveness of using RSA for optical-pulse suppression is
determined by the cross sections of the relevant states, with
larger σe/σg generally leading to more efficient optical-pulse
suppression where there is sufficiently large σg to allow a sizable
excited-state population to be built up, and the temporal
evolution of their populations during the pulse, which in turn,
depends upon the relative magnitudes of the excited-state
lifetime and pulse duration and on the rate of formation of the
relevant excited states.1 One approach to RSA-based optical-
Received: January 19, 2012
Revised: April 10, 2012
Published: April 25, 2012
© 2012 American Chemical Society
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Figure 1. Two PDI-based “double-cable” polycarbazole copolymers.19,20
.
Figure 2. Chemical structures of PDI grafted polymers and the model compounds.
wavelengths.13,16 In principle, donor−acceptor (D-A) type
“double-cable” conjugated polymers,17−21 in which electron-
accepting moieties are covalently linked to an electron-donating
conjugated polymer backbone can also be candidates for the
application of optical-pulse suppression using the same
mechanism. Rapid photoinduced charge separation and long-
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Scheme 1. Synthetic Scheme for the PDI-Grafted Polymers and Respective Model Materials
lived charge-separated states have been previously demon-
strated in such materials, particularly in research on such
materials for organic photovoltaics.17−20 Moreover, because of
their homogeneous molecular distribution and the relatively
well-controlled distance between the donor and acceptor
building blocks in these systems (at least compared to that in
simple donor/acceptor material blends), “double-cable” poly-
mers of this type could help overcome the phase-separation
issues commonly encountered for physical blends, and
maximize the donor−acceptor interaction in the solid state.
This could potentially result in higher-quality films, because of
better control of film morphology, as well as the possibility of
creating a larger population of radical ions following the
photoexcitation.
Perylene-3,4,9,10-tetracarboxylic acid diimides (perylene
diimides, PDIs) are a group of acceptors extensively used for
organic optical and electronic applications.22−29 An interesting
property of PDI-based materials is their tendency to aggregate
with one another in solution or in the solid state because of
their strong π−π stacking interactions; these aggregates can
extend the low-energy absorption edge of PDI materials
beyond 600 nm.23 In addition, the existence of long-lived
charge-separated states has been demonstrated in many donor-
PDI systems, and PDI radical anions exhibit several strong
absorptions in the visible and NIR (ca. 600 to 1000 nm).29−35
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Hence, the spectral overlap between the weak low-energy tail of
the PDI-aggregate ground-state absorption and the fairly strong
absorption from PDI radical anions suggests the possibility of
RSA-type charge-transfer optical-pulse suppression from PDI
aggregates attached to suitable donor polymers.
dride-9,10-dicarboximide and 4-(2,7-dibromo-9H-carbazol-9-
yl)aniline in molten imidazole at 180 °C. M2 was prepared
by the same method using 4-(3,6-dibromo-9H-carbazol-9-
yl)aniline instead of 4-(2,7-dibromo-9H-carbazol-9-yl)aniline.
The yields for these condensation reactions are in the range of
85−100%, and silica-gel column chromatography was used to
purify the products. The incorporation of long “swallow-tail”
chains in the PDI N-terminus provides the PDI-grafted
monomers with excellent solubility for use in polymerizations.
The chemical structures and the purity of these monomers
were confirmed by 1H and 13C NMR spectroscopy, mass
spectroscopy, and elemental analysis. High-molecular-weight
(>15 kDa, see Table 1) alternating poly(carbazole-alt-2,7-
Carbazole-containing conjugated polymers or oligomers are
typically electron donors, exhibiting high hole mobility,
relatively low ionization potentials, and stable radical cations
(polarons). Over the past decade, carbazole-based conjugated
materials have been extensively studied for organic electronic
applications, resulting in high-performance devices.36−47
Recently, efficient photoinduced electron-transfer processes
have been demonstrated in PDI/polycarbazole blends or PDI-
grafted D−A “double-cable” polycarbazole copolymers (Figure
1), and promising devices have been achieved using these
materials as active layers in solution-processed solar
cells.19−21,48 However, to the best of our knowledge, there
are no reports on using photogenerated radical-ion absorption
in PDI/donor polymer blends or PDI-based D−A “double-
cable” polymers for optical-pulse suppression applications.
This paper reports, the syntheses of two new poly(carbazole-
alt-2,7-fluorene)s (PCFs) bearing PDIs on the side chains as
pendant acceptors, along with the characterization and optical-
pulse suppression behavior of these materials. As shown in
Figure 2, the PDI moieties are covalently bound to the
electron-donating poly(2,7-carbazole-alt-2,7-fluorene) and
poly(3,6-carbazole-alt-2,7-fluorene) polymeric backbones in
the D−A type “double-cable” polymers, P1 and P2,
respectively. The polymeric architecture is expected to enhance
the π−π stacking interaction21 between the PDI moieties,
providing sufficient ground-state from aggregated PDIs
((PDI)n) for achieving optical-pulse suppression at wavelengths
where the PDI radical anion absorbs strongly. In addition, 2PA
absorption by the PCF backbones could also contribute to the
optical suppression. In this work, a phenylene linker was chosen
to covalently bind the PDI moieties on the polymer side-chain,
based on previous observations of a long-lived charge-separated
state in a PDI-phenylene-donor dyad.16 PDI-grafted poly(2,7-
carbazole-alt-2,7-fluorene), poly(3,6-carbazole-alt-2,7-fluorene),
and model compounds for the donors (P1′ and P2′) and
acceptors (PDI1) were synthesized to investigate effects of the
donor polymer architecture on the optical-pulse suppression
performance of these materials.
Table 1. Polymerization Results and Thermal Properties of
the Polymers
polymer
yield
Mn (kDa) Mw/Mn Td (°C) Tg (°C) Tm (°C)
P1
P2
P1′
P2′
83%
26%
80%
68%
15.4
18.1
61.1
24.5
3.68
1.91
4.00
3.55
388
390
409
412
277
210
160
95
N.A.
N.A.
ca. 280
N.A.
fluorene)s with grafted PDI pendants (P1 and P2) were then
obtained through palladium-catalyzed Suzuki coupling between
2,2′-(9,9-dioctyl-9H-fluorene-2,7-diyl)bis(1,3,2-dioxaborinane)
and M1 or M2 using Pd(PPh3)4 (1 mol % with respect to
monomer) as the source of catalyst.51,55 Both PDI-grafted
polymers show good solubility in toluene, dichloromethane,
chloroform, and tetrahydrofuran (THF), which allows optical-
pulse suppression measurements to be performed in high-
concentration solutions and is useful for fabricating high-quality
films for solid-state measurements. The much lower yield in the
synthesis of P2 (Table 1) is associated with a large quantity of
low-molecular-weight materials formed during the Suzuki
polycondensations; these were removed in the subsequent
purification through Soxhlet extraction with methanol and
acetone. It is worth noting that it was found that Ni(COD)2-
catalyzed Yamamoto polycondensations of M1 and M2 only
lead to oligomers, with only 2 or 3 repeat units, rather than
high-molecular-weight polymers, despite the use of polymer-
ization conditions similar to those reported by Mullen and co-
̈
workers to afford high-molecular-weight poly(N-alkyl-2,7-
carbazole)s.48 The model polymers were synthesized using
similar procedures and were found to have higher molecular
weights and larger polydispersities (see Table 1). These model
polymers, especially P2′, also show high solubility in common
organic solvents such as toluene, chlorobenzene, THF, and
chloroform (>10 mg/mL) at ambient temperature; however,
P1′ exhibits limited solubility in dichloromethane (<1 mg/mL).
Thermal Properties. As shown in Figure 3 and Table 1, all
polymers exhibited good thermal stability, with the decom-
position temperatures (Td), defined as that at which 5% weight
loss is observed using thermogravimetric analysis, exceding 385
°C under nitrogen, the PDI-containing polymers showing
values of Td about 20 °C lower than those for the model
polymers, presumably because of either degradation of the PDI
aromatic core or its “swallow-tail” substituent. The thermal
behavior of these polymers was also investigated by differential
scanning calorimetry (DSC) from −25 to 300 °C under
nitrogen atmosphere. Glass-transition temperatures (Tg) were
in excess of 200 °C for the PDI-functionalized polymers,
considerably higher than those for P1′ and P2′ (Table 1),
RESULTS AND DISCUSSION
■
Synthesis. Many carbazole- and fluorene-containing con-
jugated polymers such as poly(carbazole)s,48,49 poly(2,7-
fluorene)s,50 and poly(3,6-carbazole-alt-2,7-fluorene)s51 have
been synthesized for organic electronic applications, and the
methods developed in these previous reports guided our
synthesis of the PDI-grafted PCFs. Two new monomers
containing dibromo-carbazole and PDI moieties with phenyl-
ene linkers were designed, and the desired PDI-grafted
polymers were prepared following typical Suzuki polymer-
ization procedures.50,51 Scheme 1 outlines the synthesis of the
new monomers and respective copolymers. N-(1-Undecyl-
dodecyl)-perylene-3,4-dicarboxyanhydride-9,10-dicarboxi-
mide,52 4-(2,7-dibromo-9H-carbazol-9-yl)aniline,53 4-(3,6-di-
bromo-9H-carbazol-9-yl)aniline,53 2,7-dibromo-9-dodecyl-9H-
carbazole,49 and 3,6-dibromo-9-dodecyl-9H-carbazole54 were
prepared as described in the literature. M1 was readily
accessible through a Zn(OAc)2-catalyzed condensation reaction
between N-(1-undecyl-dodecyl)-perylene-3,4-dicarboxyanhy-
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Figure 3. TGA traces (left) and DSC traces (right) of the polymers under nitrogen.
perhaps because of strong π−π interactions between the side-
chain PDI groups in the solid state. P1′ melts at about 280 °C
under nitrogen, while no obvious melting transition was
observed for the other three polymers. The good thermal
stability and high glass-transition temperatures are attractive
attributes from the point of view of using these polymers as
active materials for organic optical and electronic applications.
Linear Absorption and Emission Properties. As shown
in Figure 4, the solution UV−vis absorption spectra of P1 and
P2 in toluene (ca. 10−5 M, based on the polymer repeat unit)
are essentially superpositions of the characteristic absorption
bands of the polymer models and of aggregated PDIs.23,56 The
absorption maxima of P1′ and P2′ are slightly bathochromically
shifted relative to the corresponding bands from the analogous
PDI-grafted polymers; this may be tentatively ascribed to a
shorter conjugated polymer backbone because of the relatively
lower molecular weight of PDI-grafted polymers and/or the
PDI pendants leading to less intermolecular packing between
the polymer backbones. In the polymers with PDI pendants,
the ratio of the absorbance of the (0,0) vibronic sub-band of the
PDI absorption to that of the (0,1) sub-band was found to be
significantly smaller than the value of 1.66 typical for
nonaggregated PDIs (1.23 for P2 and 1.13 for P1), suggesting
substantial aggregation of the PDI side-chains, even in dilute
toluene solution (ca. 10−5 M).21,23,56 In more concentrated
toluene solution (ca. 10−3 M), significant absorption can be
detected in the 600−800 nm range for P1 and P2, as illustrated
in Figure 5. The molar absorption coefficients for both P1 and
P2 are more than 200 M−1 cm−1 at 700 nm and larger than 500
M−1 cm−1 at 650 nm, respectively, in toluene, higher than the
corresponding values of PDI1 in the same spectral region at
similar concentration (ε700 < 10 M−1 cm−1, concentration-
independent, even up to 10 mM).16 This significant enhanced
NIR absorption might originate from (PDI)n absorption, while
the weak absorption band seen for concentrated solutions of P2
at about 920 nm might be attributable to absorption from an
Figure 4. Linear absorption spectra of the polymers in dilute toluene
solution.
Figure 5. (A) Absorption spectra of the PDI-grafted polymers in highly concentrated toluene solutions. The band peaked at 920 nm for P2 is
tentatively attributed to ground-state intermolecular or intramolecular charge-transfer complexes; (B) molar absorptivities of the PDI-grafted
polymers in highly concentrated toluene solution at different wavelengths.
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intra- or intermolecular ground-state charge-transfer complex
between the PCF backbone and PDI pendants, similar to that
seen in some previously reported donor-PDI systems.16,57 The
molar absorptivities in the NIR range for P1 and P2 are not
significantly dependent on the PDI concentration in toluene
solution (at least in the concentration range of 0.1 to 3 mM),
perhaps indicating saturated aggregation in the investigated
concentration range for the grafted PDI moieties on the
polymer side-chains.
absorption. The relatively weak contribution of the structured
PDI feature to the excitation spectra suggest that the
observation of emission from both donor and acceptor species
following excitation at 375 nm is primarily a result of energy
transfer from the photoexcited donor polymer backbone. The
fluorescence quantum yields observed for the PDI-grafted
polymers (0.5% and 0.7% for P1 and P2, respectively, when
excited at 375 nm) are much lower than that for PDI1 (83% in
toluene, excited at 490 nm) and aggregated PDIs (generally
between 20% and 50%),59,60 or the polymer models (65% and
56% for P1′ and P2′ in toluene, respectively). The reduced
emission from the PDI-grafted polymers relative to the
respective model compounds indicates quenching of the
excited states, consistent with photoinduced electron transfer.
Radical-Ion Absorption Spectra. Chemical oxidation and
reduction of model donors and acceptor, respectively, was
carried out to establish the wavelengths at which charge-transfer
optical pulse suppression might be feasible and to aid with the
interpretation of transient absorption spectra (see below).
Positive polarons were generated in the model polymers in
dichloromethane by addition of a large excess of P1′ or P2′ to a
dilute solution of tris(4-bromophenyl)aminium hexachloroan-
timonate.61 The radical anion of the PDI1 was prepared in
THF by using cobaltocene under nitrogen.62 The resulting
radical-ion absorption spectra are shown in Figure 8. The
The emission spectra of the PDI-grafted polymers and the
model compounds in toluene are shown in Figure 6. The
Figure 6. Emission spectra of the PDI grafted polymers and respective
model compounds in toluene. P1 and P2, as well as the respective
model polymers, were excited at 375 nm. The PDI1 emission
spectrum was collected using excitation at 490 nm.
emission spectra of the polymer polymers show characteristic
features of both the respective donor polymer, nonaggregated
PDI molecules (peaked at ca. 535 nm), and PDI aggregates
(peaked at ca. 650 nm).58 The observation of PDI-aggregate-
type emission is consistent with the PDI-aggregate-type UV−
vis absorption spectra (see above).21,58 The PDI-based
emission observed upon 375 nm excitation of the PDI-grafted
polymers could result either from energy transfer from the
excited donor polymer backbone to the PDI units or from
direct excitation of the weak UV absorption of the PDI units
(<10% of the total absorption at 375 nm). The excitation
spectra of P1 and P2 obtained while monitoring the emission at
620 nm are shown in Figure 7. These spectra show the
characteristic absorption bands of the donor polymer backbone,
a vibrationally structured PDI absorption, and PDI aggregate
Figure 8. Polaron absorption of the P1′ and P2′, in dichloromethane
and the PDI1 radical-anion absorption in THF.
absorption maximum of the polaron from P1′ in the UV−vis
range is around 560 nm. This shows a bathochromic shift of
about 50 nm compared with the polaron generated from P2′,
presumably because of the cross-conjugation in the polymer
backbone of P2′. The polarons of P1′ and P2′ also exhibit
strong, but broad, absorption bands in the NIR region. The
absorption spectrum of the PDI1 radical anion (Figure 6) is
similar to those of other PDI radical anions with similar
chemical structures shown in literature, exhibiting peaks at 698,
798, and 958 nm.22,29,63
Redox Properties. The electrochemical behavior of P1, P2
and respective model polymers was investigated by cyclic
voltammetry (CV). The measurement was performed under
nitrogen, in a solution of tetrabutylammonium hexafluorophos-
phate (0.10 M) in anhydrous acetonitrile at a scan rate of 50
mV/s using the polymer films drop-cast onto a platinum
working electrode from 2 mg/mL polymer solutions in
chloroform. Separate oxidative and reductive scans were carried
out for the film samples. The results are summarized in Table 2,
Figure 7. Excitation spectra of P1 and P2 in toluene acquired using a
fixed emission wavelength of 620 nm.
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Table 2. Summary of the Optical and Electrochemical Properties of the Polymers
a
b
b
c
c
abs
em
onset
onset
polymer
λmax (nm)
λmax (nm)
ηem (%)
Egopt (eV)
Eox
(V)
Ered
(V)
ΔGCSD* (eV)
ΔGCSA* (eV)
e
f
P1
381, 491, 528
340, 491, 527
386
408, 539, 661
ca. 0.5
2.1
2.1
2.9
3.1
2.2
+0.44
+0.55
+0.40
+0.54
N.A.
−1.04
−1.06
N.A.
−1.4
−1.5
−0.7
−0.5
e
f
P2
538, 667
ca. 0.7
f
P1′
P2′
PDI1
417, 441
65
f
349
396, 418
56
N.A.
g
d
459, 490, 527
541, 582, 630
83
−1.01
a
b
Eg (optical gap) was estimated from the absorption on-set of UV absorption of respective materials at low concentration (ca. 10−5 M). Versus
ferrocenium/ferrocene. Free-energy changes for excited-state donor-to-PDI electron transfer from donor or acceptor-based excited states (D* and
c
onset
onset
A* respectively), estimated according to ΔGCS = eEox
− eEred
− Eg(optical), where the value of Eg(optical) used is that of the appropriate
d
0/−
model polymer and of the model PDI for ΔGCSD* and ΔGCSA* respectively. E1/2 obtained from solution measurement in dichloromethane.
e
f
λmaxem values for the low-energy emissions are approximate and were estimated after smoothing the emission spectra shown in Figure 6. Excitation
g
at 375 nm. Excitation at 490 nm.
and the cyclic voltammograms of the polymers and PDI1 are
shown in Figure 9. The reduction potentials for P1 and P2 are
Figure 10. Femtosecond laser pulse transient absorption at 5 ps after
excitation of PDI1 (excited at 425 nm), P2′ (excited at 350 nm), and
P1′ model (excited at 350 nm) in toluene (absorbances at the
excitation wavelengths were ca. 0.3).
Figure 9. Cyclic voltammograms of polymer films drop-cast from ∼2
mg/mL CHCl3 solution onto a Pt working electrode in acetonitrile
with 0.1 M tetra-n-butylammonium hexafluorophosphate, except for
PDI1, for which the CV was performed in dichloromethane solution.
Potential was scanned at a rate of 50 mV/s.
with femtosecond laser pulses, are shown in Figure 11. Shortly
after the photoexcitation of the PDI moieties of P1, a feature
peaked at 680 nm and attributable to PDI excited-state
absorption (Figure 10) is observed. This is followed by the
rapid (<5 ps) growth of absorption bands with peaks at about
580 and about 710 nm. Comparison with chemically oxidized
and reduced species above suggests these features can be
assigned to the P1′ postive polaron and to the PDI radical
anion, respectively, that is, there is efficient electron transfer
from the polymer backbone to the PDI excited state to form an
ion pair. Excitation of P1 at 350 nm, that is primarily exciting
the polymer backbone, with femtosecond laser pulses, leads
initially (0.3 ps) to spectra essentially identical to those seen for
model polymer P1′ (Figure 10,11), followed by rapid growth of
the 580 and 700 nm absorptions (<1.5 ps) consistent with
formation of the ion pair. The transient absorption-spectra
obtained from P2 by pumping of either donor or acceptor were
similar to those of P1 although no donor polymer radical-cation
absorption was observed in the transient absorption spectra of
P2, presumably because of the spectral overlap between the
polaron absorption, peaked at about 500 nm, and the PDI
ground-state bleach. The spectrum of P2 obtained 1.5 ps after
excitation at 350 nm shows a small dip at 575 nm absent from
the 0.3 ps spectrum that may correspond to the stimulated
emission peak observed from direct excitation of the PDI
moiety. Its late appearance suggests energy transfer may play a
role in the overall mechanism of charge-separation from the
polymer-based excitation, consistent with fluorescence data.
The lifetimes of the charge-separated states for both PDI
grafted polymers are longer than 3 ns, which is the maximum
time delay achievable using our instrumentation. Excitation of
similar to that of the PDI1 (for which a CV measurement was
performed using a dichloromethane solution). The similarity of
the potentials for the PDI-grafted polymers to those for the
model compounds suggests weak ground-state electronic-
coupling between the PDI pendants and donor polymer
backbones, consistent with the UV−vis absorption spectra (see
above). The free-energy changes for electron transfer from
either a donor-based excited-state to the pendant PDI or from
D
the donor polymer backbone to the PDI excited-state, ΔGCS
*
and ΔGCSA*, respectively, estimated using the electrochemical
potentials and the absorption onsets64 from the appropriate
model compounds (see Table 2) are highly exergonic, even
when Coulombic stabilization of the resultant ion pair is
neglected, confirming the energetic feasibility of obtaining the
excited-state electron-transfer in these systems.
Transient Absorption. Femtosecond (fs) laser pulse
transient-absorption spectra of the model materials are shown
in Figure 10. PDI1 shows excited-state absorption peaked at
about 680 nm, with a lifetime that exceeds the temporal range
of our experimental setup, and P1′ and P2′ exhibit broad
excited-state absorption between 400 to 800 nm with lifetimes
shorter than 1.5 ns.
Transient-absorption spectra of the PDI grafted PCFs, P1
and P2, in toluene, generated by pumping the PDI moiety at
530 nm or primarily the donor polymer backbone at 350 nm
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Figure 11. Femtosecond transient-absorption spectra of P1 (A) and P2 (B) in toluene excited at 530 nm. Transient-absorption spectra of P1 (C)
and P2 (D) in toluene following excitation at 350 nm. All samples were prepared to be ca. 30 μM in toluene in a 2 mm cuvette to yield an
absorbance of ca. 0.3 at the excitation wavelength.
Figure 12. Proposed energy level diagram showing the lowest excited singlet states of PCF and nonaggregated PDI, of the aggregates (PDI)n, and
that of the charge-transfer state. The arrows show the processes that might occur upon photoexcitation of PCF or PDI.
PDI aggregates at 680 nm in highly concentrated polymer
solution (ca. 4 mg/mL in toluene) also results in long-lived
charge-separated states and a fairly strong transient-absorption
band between 600−800 nm, which demonstrates the possibility
of optical-pulse suppression at these wavelengths.
The results of the transient absorption measurements
together with the emission study in solution can be summarized
́
in a Jabłonski diagram (Figure 12). After photoexcitation of the
donor polymer backbone (PCF), the excited state is largely
quenched by energy and electron transfer from the PCF
excited-states to PDI or (PDI)n, leading to low fluorescence
quantum yields. The newly generated PDI- or (PDI)n-based
excitons are quenched predominantly via electron-transfer by
accepting an electron from PCF. If PDI or (PDI)n are
photoexcited directly, these PDI-based excited-states could be
quenched in the same way, that is, by electron-transfer from
PCF, forming the PDI radical anion and donor polymer
polaron (or by other radiative and nonradiative decay
mechanisms).
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Nonlinear Absorption and Optical Pulse Suppression.
The optical-pulse suppression behavior of P1 and P2 upon
photoexcitation with a 680 nm laser pulse (6 ns pulse width) is
shown in Figure 13, along with those of the blend of P1′/PDI1
at 680 nm, although differences in excited-state cross-section
are also likely to be important. A measure of the strength of the
nonlinear attenuation of the laser pulses is the optical
suppression (defined here as S = To/TF, where To is the linear
transmittance, and TF is the transmittance just below the
damage fluence of the cell). The S values of the P1 and P2
solution here are 5.3 and 3.3, and using 680 nm excitation,
respectively (Table 3). These values of S compare favorably
with those obtained from porphyrin-viologen dyads13 (S = 2.4
at 600 nm) for which a one-photon-induced charge separation
mechanism provided the nonlinear absorption, as well as with
those for triphenylamine-PDI based materials67 that combined
2PA with ESA (S = 3.5 at 660 nm using picosecond pulse
excitation), but are lower than those for our previous reported
PDI-based molecular dyads16 (S = 17 at 700 nm) that operated
primarily via 2PA-induced ion-pair absorption. The enhanced
optical-pulse suppression behavior of the PDI-grafted polymers
at high pulse energy relative to that seen in the model blends
may be attributed to one-photon-induced charge separation
through excitation of the PDI aggregates and/or ground-state
charge-transfer complexes (S values of P1′/PDI1 or P2′/PDI1
are 2.7 and 2.0, at 680 nm respectively). Additionally, the
threshold energies, defined as the energy at which the
transmittance is To/2, show stronger optical-pulse suppression
at lower energies for the PDI-functionalized polymers than for
the corresponding model material blends. At high pulse energy,
two-photon excitations of the donor polymer backbones are
also likely to contribute to the pulse suppression in P1 and P2.
At lower energies, slightly stronger optical-pulse suppression
can be observed for P2 than for P1 at 680 nm. However, at
higher pulse energies (ca. E > 10−4 J), a steeper slope can be
observed for P1 over P2, indicating stronger optical-pulse
suppression at these pulse energy for P1 against 680 nm laser
pulses. This could be attributed to the much larger 2PA cross-
section at 680 nm of the 2,7-carbazole-based backbone than its
3,6-carbazole analogue, which leads to higher excited-state and
PDI-radical-ion populations that could contribute to the
nonlinear absorption for P1 at energies where two-photon
excitation is more significant.
Figure 13. Optical pulse suppression of 680 nm, 6 ns pulses focused in
an f/5 geometry into the center of a 2 mm, N2-purged cell containing
P1 and P2 (2.5 and 3.1 mM/subunit in toluene respectively) and the
respective polymer model compounds mixed with PDI1, each at ca.
3.5 mM, in toluene.
or P2′/PDI1 (∼3.5 mM: 3.5 mM in toluene) solution in a 2
mm cuvette. P1 and P2 samples were prepared at high
concentration (2.5 mM for P1 and 3.1 mM for P2 in toluene,
based on the repeat unit concentration) to ensure about 70%
linear transmittance of the samples. Both PDI-grafted polymers
showed noticeable optical-pulse suppression starting at lower
input energy as compared with the blend samples, even when
the model blend systems had higher material concentration.
The optical-pulse suppression phenomena of the model
materials blends at high energy is presumably attributable to
2PA, followed by excited-state absorption or possibly
absorption by some intermolecularly charge-separated species.
The 2PA spectra of the two model polymers in toluene
acquired using the two-photon-excited fluorescence meth-
od65,66 (Figure 14) show 2PA cross sections (based on
polymer repeating units) at 680 nm of 350 and 50 GM for P1′
and P2′, respectively, while both polymers show excited-state
absorption at the same wavelength (Figure 10). The much
larger 2PA cross section for P1′ is presumably at least partly
responsible for its stronger optical-pulse suppression response
CONCLUSION
■
We have prepared two new polymers with conjugated
poly(carbazole-alt-2,7-fluorene)s as the electron-donating poly-
mer backbones and PDI moieties incorporated into the side
chains as pendant electron acceptors. Efficient photoinduced
charge transfer in these copolymers in generating the respective
ion-pairs was confirmed by the transient-absorption spectra.
Long-lived charge-separated states with strong absorption
between 600−800 nm were observed following photoexcitation
of either the PDI or the donor polymer absorptions. Moreover,
the PDI-grafted copolymers exhibited stronger enhancement of
the nonlinear optical absorption compared with blends of the
model compounds at 680 nm. This is attributed to the
combination of one-photon absorption from the aggregated
PDI and possible 2PA from the polymer backbones. The
poly(2,7-carbazole-alt-2,7-fluorene) derivative, P1, shows stron-
ger optical-pulse suppression at 680 nm than its poly(3,6-
carbazole-alt-2,7-fluorene) derivative P2, especially of higher
energy laser pulses, probably because of the larger 2PA cross
section at 680 nm for P1.
Figure 14. 2PA spectra of P1′ and P2′ in 100 μM solution in toluene.
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Table 3. Summary of the Optical Pulse Suppression Performance of the PDI Grafted Polymers and Respective Material-Blends
P1 (2.5 mM)
P2 (3.1 mM)
P1′/PDI1 (3.5 mM)/(3.5 mM)
P2′/PDI1 (3.5 mM)/(3.5 mM)
linear transmittance
threshold energy (μJ)
To/TF
0.68
200
5.3
0.68
440
3.3
0.96
520
2.7
0.97
910
2.0
grade) in aqueous NaOH solution (pH 11) were used as
references for the 630−680 nm and 690−1040 nm ranges,
respectively. The 2PA cross-section values of 1,4-bis(2-
methylstyryl)benzene reported by Kennedy and Lytle69 were
reduced in scale by a factor of 10, as described by Fisher et al.65
The uncertainties in the measured cross sections are
approximately 15%.
EXPERIMENTAL SECTION
■
General Procedures. Most organic and inorganic chem-
icals were obtained from Aldrich and Alfa Aesar. Palladium-
based catalysts were purchased from Strem Chemicals and used
1
without further purification. H and 13C{1H} NMR spectra
were collected on Bruker 400 or 500 MHz spectrometers using
tetramethylsilane (TMS; δ = 0 ppm) as an internal standard.
Mass spectra were measured on an Applied Biosystems 4700
Proteomics Analyzer using MALDI mode. Elemental analyses
were carried out by Atlantic Microlabs using a LECO 932
CHNS elemental analyzer. Solution UV−vis, absorption spectra
were recorded on a UV3101PC (Shimadzu, Kyoto, Japan)
absorbance spectrophotometer. The solution emission spectra
and excitation spectra of toluene solutions were recorded using
a Shimadzu FP-5301PC spectrofluorometer. For electro-
chemical measurements, the polymer films were drop-cast
onto a platinum disk working electrode from a 2 mg/mL
polymer solution in chloroform. A platinum wire served as the
auxiliary electrode, and an Ag wire anodized with AgCl served
as a pseudoreference electrode. The experiments were
performed on deoxygenated 0.1 M solutions of tetra-n-
butylammonium hexafluorophosphate in anhydrous acetonitrile
at a scan rate of 50 mV s−1, using a computer-controlled BAS
100B electrochemical analyzer. Potentials were referenced to
the ferrocenium/ferrocene (FeCp2+/0) couple by using
ferrocene as an external standard. Thermogravimetric analysis
measurements were performed on an NETZSCH STA 449C
analyzer under 40 mL/min N2 flow with a heating rate of 5 °C/
min. Differential scanning calorimetry measurements were
performed on a TA Instruments DSC Q200 analyzer under 50
mL/min N2 flow with heating rate of 5 °C/min.
Generation of Radical Ions. The radical anion of the
PDI1 was generated in anhydrous THF solution by reduction
with cobaltocene in a nitrogen atmosphere glovebox. The
radical cation of P1′ or P2′ was generated in anhydrous
dichloromethane after oxidation with tris(p-bromophenyl)-
aminium hexachloroantimonate. The spectra of the radical
ions were recorded with a Varian Cary 5E UV−vis−NIR
spectrophotometer using 1 cm path length cells under nitrogen.
Two-Photon Absorption Spectroscopy. 2PA spectra
were obtained using the reference-based two-photon-excited
fluorescence (2PEF) method.66,68 The excitation source was an
optical parametric oscillator (Quanta-Ray MOPO 730)
pumped by 6 ns pulses from the third harmonic of a Q-
switched Nd:YAG laser (Quanta-Ray PRO250). The 2PEF
method determines the 2PA spectra of unknowns by measuring
the fluorescence emitted by the unknowns under two-photon
excitation conditions and comparing it to the fluorescence
emitted by a known reference compound under the same
conditions. The 2PEF measurements of the model compounds
were carried out in toluene (Sigma-Aldrich spectroscopic
grade) solution at chromophore concentrations of 80−110
μM. The data shown here comprise several collections of over
200 pulses at each wavelength. 1,4-bis(2-methylstyryl)-
benzene65,69 (Sigma-Aldrich, 99%) in cyclohexane (Sigma-
Aldrich, spectroscopic grade) and fluorescein66 (Acros, laser
Femtosecond Transient-Absorption Measurements.
The excitation source for femtosecond transient-absorption
measurements was generated by an optical parametric amplifier
(TOPAS, Newport) pumped by a Ti:Sapphire regenerative
amplifier (Spitfire, Newport), operating at 1 kHz repetition
rate. The 800 nm Spitfire output could be varied by the TOPAS
over 465−2900 nm. Approximately 5% of the 800 nm Spitfire
output was used to generate the white light continuum probe
beam (420−950 nm) in a sapphire plate. Transient data were
collected using a commercially available Helios spectrometer
(Ultrafast Systems, Sarasota, FL). The time resolution for this
system is pulse-width limited at 200 fs, and the maximum time
delay was 3.2 ns. At each temporal delay, the signal was
averaged for 1 s. The pump beam was chopped at 500 Hz to
alternate between signal and reference data. A correction factor
for the chirp of the white light was generated using the ultrafast
response of CCl4. All samples were prepared in 2 mm cuvettes
and deaerated with N2. The pump wavelengths were 530 nm
for the PDI-grafted polymers. The pump energy for all samples
was about 3.3 μJ/pulse.
Nanosecond Optical-Pulse Suppression Measure-
ments. The excitation source for optical-pulse suppression
measurements was the same as for the ns TA measurements. A
mechanical shutter reduced the pulse repetition rate to 1 Hz to
minimize damage to the sample. The samples were prepared as
deaerated solutions in 2 mm cuvettes, with transmittance of
about 0.7 at the excitation wavelength of 680 nm. The laser was
focused into the center of the cuvette using an f/5 geometry,
and the transmitted light was detected by a New Focus
photoreceiver (San Jose, California), sampled using a Stanford
Research Systems boxcar average (Sunnyvale, CA), and
recorded on a 300 MHz Tektronix oscilloscope (Richardson,
Texas). A beam splitter placed before the sample redirected
part of each pulse to a reference photoreceiver to determine
fluctuations in the input energy of each pulse. The focus of the
laser was positioned in the center of the 2 mm cell by
translating the cell through the focus along the axis of the beam,
while exciting the sample with 1 μJ pulses (z-scan). Reverse
saturable absorption reduced the transmitted signal while the
sample was passing through the focus, so the position where
the focus was in the middle of the cuvette could be determined
from the middle of the region of reduced transmittance.
M1. N-(1-Undecyl-dodecyl)-perylene-3,4-dicarboxyanhy-
dride-9,10-dicarboximide52 (0.356 g, 0.50 mmol), 4-(2,7-
dibromo-9H-carbazol-9-yl)aniline53 (0.413 g, 0.99 mmol),
anhydrous zinc acetate (80 mg, 0.44 mmol), and imidazole
(3.0 g) were heated under N2 at 180 °C overnight. The
reaction mixture was then allowed to cool to about 130 °C
before being poured into a 4 N aqueous HCl solution (200
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mL). The resultant red precipitate was filtered and washed
sequentially with 2 N aqueous HCl (3 × 10 mL), water (3 × 10
mL), and MeOH (3 × 10 mL). The solid was then dissolved in
CHCl3 (5 mL), and a minimum amount of silica gel was added
to adsorb the material. After the solvent was removed under
reduced pressure, the dried silica gel was added to the top of a
hexane-packed silica gel column, and the column was eluted
P1. 2,2′-(9,9-Dioctyl-9H-fluorene-2,7-diyl)bis(1,3,2-dioxa-
borinane) (0.234 g, 0.418 mmol), M1 (0.465 g, 0.418
mmol), Aliquat 336 (40 mg), and Pd(PPh3)4 (4.8 mg, 0.0040
mmol) were charged to a 25 mL two-neck round-bottomed
flask with a condenser. The system was then evacuated and
refilled with N2 four times. Toluene (5.0 mL) and 2 N aqueous
K2CO3 (3.0 mL) were added before the mixture was heated to
90 °C and kept at that temperature for 3 d. Then, 2,2′-(9,9-
dioctyl-9H-fluorene-2,7-diyl)bis(1,3,2-dioxaborinane) (100 mg,
0.17 mmol) in toluene (1.0 mL) was added, and the mixture
was stirred for another 12 h. Iodobenzene (0.3 mL) was then
added to end-cap the polymer, and the mixture was kept
stirring for another 12 h. After the mixture was cooled to room
temperature, it was added dropwise to 125 mL of MeOH. The
resultant solid was filtered and washed with water and MeOH
before drying under vacuum. The solid was washed sequentially
with hot MeOH and hot acetone using a Soxhlet apparatus.
The residue was then extracted with CHCl3 in a Soxhlet
apparatus. Most of the solvent was then removed, and the
residue was passed through a short silica plug, eluting with
CHCl3/Et3N (100:1). The solvent was then removed, and the
residue was dissolved in 5 mL of CHCl3 and added dropwise to
100 mL of MeOH. The resulting solid was filtered and washed
with water and MeOH before drying under vacuum. The
precipitation procedure was repeated, and the resulting solid
was then filtered and dried under vacuum to give P1 as a red
1
with CHCl3 to give M1 as a red solid (0.53 g, 89%). H NMR
(500 MHz, CDCl3): δ 8.78 (d, J = 8.0 Hz, 2H), 8.72−8.66 (m,
6H), 7.95 (d, J = 8.5 Hz, 2H), 7.75 (d, J = 8.5 Hz, 2H), 7.68 (d,
J = 1.0 Hz, 2H), 7.65 (d, J = 8.5 Hz, 2H), 7.42 (dd, J = 8.5, 1.0
Hz, 2H), 5.18 (m, 1H), 2.26 (m, 2H), 1.86 (m, 2H), 1.29−1.21
(m, 36H), 0.83 (t, J = 6.5 Hz, 6H). 13C{1H} NMR (125 MHz,
CDCl3): δ 165.0, 164.1, 163.9, 141.8, 137.2, 135.8, 134.9, 134.6,
134.5, 132.4, 132.3, 131.6, 131.2, 130.2, 129.9, 128.9, 128.0,
127.0, 126.7, 124.6, 124.4, 123.9, 123.8, 123.6, 123.5, 123.3,
122.3, 122.0, 121.8, 120.6, 116.4, 113.6, 113.5, 55.3, 32.8, 32.3,
30.2, 30.1 (3 peaks), 30.0, 29.8, 27.5, 23.2, 14.7 (The
observation of three carbonyl carbon resonances is consistent
with previous work on perylene bis(dicarboxyimide)s using
similar, swallow-tailed N-substituents, in which this has been
attributed to restricted rotation about the N−Calkyl bonds.71
Two aromatic carbon peaks and one alkyl carbon were not
observed, presumably because of overlap). HRMS (MALDI)
calcd for C65H65Br2N3O4 (M+): 1109.33, found: 1109.35. Anal.
Calcd for C65H65Br2N3O4: C, 70.20; H, 5.89; N, 3.78. Found:
C, 69.99; H, 5.92; N, 3.81. for C65H65Br2N3O4: C, 70.20; H,
5.89; N, 3.78.
1
solid (0.45 g, 83%). H NMR (500 MHz, CDCl3): δ 9.0−7.0
(m,b, 24nH), 5.18 (br s, 1nH), 2.5−0.1 (m, 80nH). Anal. Calcd
for polymer (C94H105N3O4)n: C, 84.07; H, 8.03; N, 3.13;
Found: C, 83.38; H, 7.80; N, 3.09.
M2. N-(1-Undecyl-dodecyl)-perylene-3,4-dicarboxyanhy-
dride-9,10-dicarboximide52 (0.356 g, 0.50 mmol), 4-(3,6-
dibromo-9H-carbazol-9-yl)aniline53 (0.450 g, 1.08 mmol),
anhydrous zinc acetate (80 mg, 0.44 mmol), and imidazole
(3.5 g) were heated under N2 at 180 °C overnight. The
reaction mixture was then allowed to cool to about 130 °C and
poured into 4 N aqueous HCl (160 mL). The resultant red
precipitate was filtered and washed sequentially with 2 N
aqueous HCl (3 × 10 mL), water (3 × 10 mL), and MeOH (2
× 10 mL). The solid was then dissolved in CHCl3 (5 mL) and
a minimum amount of silica gel was added to adsorb the
material. After the solvent was removed under reduced
pressure, the dried silica gel was added to the top of a
hexane-packed silica gel column, and the column was eluted
P2. 2,2′-(9,9-Dioctyl-9H-fluorene-2,7-diyl)bis(1,3,2-dioxa-
borinane) (0.234 g, 0.419 mmol), M2(0.465 g, 0.419 mmol),
Aliquat 336 (40 mg), and Pd(PPh3)4 (4.8 mg, 0.0040 mmol)
were charged to a 25 mL two-neck round-bottomed flask with a
condenser. The system was then evacuated and refilled with
nitrogen 4 times. Toluene (5.0 mL) and 2 N aqueous K2CO3
(3.0 mL) were added before the mixture was heated to 90 °C
and kept at that temperature for 3 d. Then 2,2′-(9,9-dioctyl-9H-
fluorene-2,7-diyl)bis(1,3,2-dioxaborinane) (100 mg, 0.17
mmol) in toluene (1.0 mL) was added, and the mixture was
stirred for another 12 h. Iodobenzene (0.5 mL) was then added
to end-cap the polymer, and the mixture was kept stirring for
another 12 h. After the mixture was cooled to room
temperature, it was added dropwise to 125 mL of MeOH.
The resulting solid was filtered and washed with water and
MeOH before drying under vacuum. The solid was washed
sequentially with hot MeOH and hot acetone hot acetone using
a Soxhlet apparatus. The residue was then extracted with
CHCl3 in a Soxhlet apparatus. Most of the solvent was then
removed, and the residual was run through a short silica plug,
eluting with CHCl3/Et3N (100: 1). The solvent was then
removed, and the residue was dissolved in CHCl3 (5 mL) and
added dropwise to 100 mL of MeOH. The resulting solid was
filtered and washed with water and MeOH before drying under
vacuum. The precipitation procedure was repeated, and the
resulting solid was then filtered and dried under vacuum to give
P2 as a red solid (0.14 g, 26%) 1H NMR (500 MHz, CDCl3): δ
9.0−7.5 (br m, 24nH), 5.18 (br s, 1nH), 2.3−1.0 (m, 68nH),
1.0−0.1 (m, 12nH). Anal. Calcd for polymer (C94H105N3O4)n:
C, 84.07; H, 8.03; N, 3.13; Found: C, 84.09; H, 7.98; N, 2.90.
2,7-Dibromo-9-dodecyl-9H-carbazole:49 A mixture of
2,7-dibromo-9H-carbazole53 (7.8 g, 24 mmol), 1-bromodode-
cane (12.5 g, 50 mmol), and NaOH (2.0 g, 50 mmol) in N,N-
1
with CHCl3 to give M2 as a red solid (0.58 g, 97%). H NMR
(500 MHz, CDCl3): δ 8.76 (d, J = 8.0 Hz, 2H), 8.71−8.66 (m,
6H), 8.17 (d, J = 2.0 Hz, 2H), 7.72 (d, J = 6.5 Hz, 2H), 7.61 (d,
J = 6.5 Hz, 2H), 7.53 (dd, J1 = 9.0 Hz, J2 = 2.0 Hz, 2H), 7.42
(dd, J = 9.0 Hz, 2H), 5.20 (m, 1H), 2.27 (m, 2H), 1.86 (m,
2H), 1.29−1.21 (m, 36H), 0.84 (t, J = 7.0 Hz, 6H). 13C{1H}
NMR (125 MHz, CDCl3): δ 164.7, 163.7, 163.6, 139.8, 137.2,
135.3, 135.0, 134.1, 134.0, 131.8, 131.2, 129.9, 129.7, 129.6,
127.5, 126.6, 126.3, 124.6, 124.4, 123.8, 123.7, 1231.5, 123.4,
123.1, 113.6, 112.1, 32.7, 32.4, 30.1(3 close peaks), 29.8, 27.5,
23.2, 14.4 (The observation of three carbonyl carbon
resonances and one more aromatic carbon peaks is consistent
with previous work on perylene bis(dicarboxyimide)s using
similar swallow-tailed N-substituents and has been attributed to
restricted rotation about the N−Calkyl bonds.70 Three alkyl
carbons were not observed, C65H66Br2N3O4 presumably
because of overlap). HRMS (MALDI) calcd for
C65H66Br2N3O4 (MH+): 1110.34, found: 1110.36. Anal. Calcd
for C65H65Br2N3O4: C, 70.20; H, 5.89; N, 3.78. Found: C,
70.30; H, 6.03; N, 3.76.
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dimethylformamide, DMF, (anhydrous, 50 mL) was stirred
overnight under N2. After the reaction, the mixture was poured
into water (200 mL). Ethyl acetate (2 × 100 mL) was used to
extract the product. The ethyl acetate solution was washed with
water (2 × 100 mL) and saturated aqueous NaCl (200 mL),
dried with MgSO4, and evaporated to dryness in vacuo. The
residue was recrystallized in 100 mL of ethanol to give 2,7-
dibromo-9-dodecyl-9H-carbazole as colorless needle crystals
9H-carbazole (0.4106 g, 0.8324 mmol), Aliquat 336 (40 mg),
and Pd(PPh3)4 (9.5 mg, 0.0080 mmol) were charged to a 25
mL two-neck round-bottomed flask with condenser. The
system was then evacuated and refilled with N2 four times.
Toluene (8.0 mL) and 2 N K2CO3 aqueous solution (4.0 mL)
were added before the mixture was heated to 90 °C and kept at
this temperature for 3 d. Then, 2,2′-(9,9-dioctyl-9H-fluorene-
2,7-diyl)bis(1,3,2-dioxaborinane) (100 mg, 0.17 mmol) in
toluene (1.0 mL) was added, and the mixture was kept stirring
for another 12 h. Iodobenzene (0.3 mL) was then added to
end-cap the polymer, and the mixture was kept stirring for
another 12 h. After the mixture was cooled to room
temperature, it was added dropwise to 125 mL of MeOH.
The formed solid was filtered and washed with water and
MeOH before drying under vacuum. The solid was washed
sequentially with hot MeOH and hot acetone using a Soxhlet
apparatus. The residue was then extracted with CHCl3 in a
Soxhlet apparatus. Most of the solvent was then removed, and
the residue was run through a short silica plug, eluting with
CHCl3 and then with THF. The solvent was removed, and the
residue was dissolved in 10 mL of CHCl3 before precipitating
in 125 mL of MeOH. The resulting solid was filtered and
washed with water and MeOH before drying under vacuum.
The precipitation procedure was repeated, and the resulting
solid was then filtered and dried to give P2′ as a white solid
1
(9.9 g, 84%). H NMR (400 MHz, CDCl3): δ 7.83 (d, J = 7.6
Hz, 2H), 7.49 (d, J = 2.0 Hz, 2H), 7.31 (dd,, J1 = 7.6 Hz, J1 =
2.0 Hz, 2H), 4.13 (t, J = 7.2 Hz, 2H), 1.81 (quintet, J = 7.2 Hz,
1
2H), 1.18−1.40 (m, 18H), 0.88 (t,, J = 7.2 Hz, 3H). The H
NMR spectrum of this compound is consistent with that
reported in the literature.
3,6-Dibromo-9-dodecyl-9H-carbazole:54 A mixture of
3,6-dibromo-9H-carbazole (7.8 g, 24 mmol), 1-bromododecane
(12.5 g, 50 mmol), and NaOH (2.0 g, 50 mmol) in DMF
(anhydrous, 50 mL) was stirred overnight under N2. After the
reaction, the mixture was poured into water (200 mL). Ethyl
acetate (2 × 100 mL) was used to extract the product. The
ethyl acetate solution was washed with water (2 × 100 mL) and
saturated aqueous NaCl (200 mL), dried with MgSO4, and
evaporated to dryness in vacuo. The residue was recrystallized
from ethanol (100 mL) to give 3,6-dibromo-9-dodecyl-9H-
1
carbazole as colorless needle crystals (10.1 g, 85%). H NMR
1
(0.34 g, 68%). H NMR (500 MHz, CDCl3): δ 8.53 (s, 2nH),
(400 MHz, CDCl3): δ 8.08 (d, J = 2.0 Hz, 2H), 7.52 (d, J = 2.0
Hz, 2H), 7.31 (d, J = 8.8 Hz, 2H), 4.18 (t, J = 7.2 Hz, 2H), 1.80
(quintet, J = 7.2 Hz, 2H), 1.18−1.44 (m, 18H), 0.86 (t,, J = 7.2
Hz, 3H). The 1H NMR spectrum of this compound is
consistent with that reported in the literature.
7.8−7.6 (br m, 8nH), 7.56 (br s, 2nH) 4.39 (br s, 2nH), 2.17
(br s, 4nH), 1.94 (br s, 2nH), 1.58 (br s, 2nH) 1.4−1.0 (br m,
40 nH), 0.9−0.2 (mb, 9nH). Anal. Calcd for P2′ (C53H71N)n:
C, 88.15; H, 9.91; N, 1.94; Found: C, 87.94; H, 9.99; N, 1.90.
P1′. 2,2′-(9,9-Dioctyl-9H-fluorene-2,7-diyl)bis(1,3,2-dioxa-
borinane) (0.4648 g, 0.8324 mmol), 2,7-dibromo-9-dodecyl-
9H-carbazole (0.4104 g, 0.8324 mmol), Aliquat 336 (40 mg),
and Pd(PPh3)4 (9.5 mg, 0.0080 mmol) were charged to a 25
mL, two-neck, round-bottomed flask with a condenser. The
system was then evacuated and refilled with N2 four times.
Toluene (8.0 mL) and 2 N aqueous K2CO3 (4.0 mL) were
added before the mixture was heated to 90 °C and kept for 3 d.
Then 2,2′-(9,9-dioctyl-9H-fluorene-2,7-diyl)bis(1,3,2-dioxabor-
inane) (100 mg, 0.17 mmol) in toluene (1.0 mL) was added,
and the mixture was kept stirring for another 12 h.
Iodobenzene (0.3 mL) was then added to end-cap the polymer,
and the reaction was kept stirring for another 6 h. After the
mixture was cooled to room temperature, it was added
dropwise to 125 mL of MeOH. A formed solid was filtered
and washed with water and MeOH before drying under
vacuum. The solid was washed sequentially with hot MeOH
and hot acetone using a Soxhlet apparatus. The residue was
then extracted with CHCl3 using a Soxhlet apparatus. Most of
the solvent was then removed, and the residual was run through
a short silica plug, eluting with CHCl3 and then with THF. The
solvent was then removed under reduced pressure; the residue
was then dissolved in 20 mL of CHCl3, and the solution was
added dropwise to 200 mL of MeOH. The resulting solid was
filtered and washed with water and MeOH before drying under
vacuum. P1′ was then obtained as a pale yellow solid (0.41 g,
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: joe.perry@chemistry.gatech.edu (J.W.P.), seth.
marder@chemistry.gatech.edu (S.R.M.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This material is based upon work supported in part by the
DARPA (through the MORPH Program, N00014-06-1-0897),
the U.S. Army Research Laboratory and the U.S. Army
Research Office (under Contract/Grant Number 50372-
CHMUR) and the National Science Foundation (through the
Science and Technology Center Program under Agreement
DMR-0120967). We thank Yan-Rong Shi for assistance with
some of the linear spectroscopy and Ya-Dong Zhang for
synthesizing some carbazole precursors.
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