Poly(1,4-diketo-3,6-diphenylpyrrolo[3,4-c]pyrrole-alt-3,6-carbazole/2, 7-fluorene) as high-performance two-photon dyes

Article abstract of DOI:10.1002/pola.27074

This article reports the synthesis, one- and two-photon absorption, and excited fluorescence properties of poly(1,4-diketo-3,6-diphenylpyrrolo[3,4-c] pyrrole-alt-N-octyl-3,6-carbazole/2,7-fluorene) (PDCZ/PDFL). PDCZ and PDFL are synthesized by the Suzuki

Full text of DOI:10.1002/pola.27074

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Poly(1,4-diketo-3,6-diphenylpyrrolo[3,4-c]pyrrole-alt23,6-carbazole/
2,7-fluorene) as High-Performance Two-Photon Dyes
Chao Yang, Meng Zheng, Yiping Li, Deteng Zhang, Shanfeng Xue, Wenjun Yang
Key Laboratory of Rubber–Plastics, Ministry of Education/Shandong Provincial Key Laboratory of Rubber–Plastics, School of
Polymer Science and Engineering, Qingdao University of Science & Technology, 53 Zhengzhou Road, Qingdao 266042, China
Correspondence to: W. Yang (E-mail: ywjph2004@qust.edu.cn)
Received 30 September 2013; accepted 8 December 2013; published online 26 December 2013
DOI: 10.1002/pola.27074
ABSTRACT: This article reports the synthesis, one- and two-
photon absorption, and excited fluorescence properties of
poly(1,4-diketo-3,6-diphenylpyrrolo[3,4-c]pyrrole-alt-N-octyl-3,6-
carbazole/2,7-fluorene) (PDCZ/PDFL). PDCZ and PDFL are syn-
thesized by the Suzuki cross-coupling of 2,5-dioctyl-1,4-diketo-
3,6-bis(p-bromophenylpyrrolo[3,4-c]pyrrole and N-octyl-3,6-bis
(3,3-dimethyl-1,3,2-dioxaborolan-2-yl)carbazole or 2,7-bis(3,3-
dimethyl-1,3,2-dioxaborolan-2-yl)fluorene and have number-
average molecular weights of 8.5 3 103 and 1.14 3 104 g/mol
and polydispersities of 2.06 and 1.83, respectively. They are
highly soluble in common organic solvents and emit strong
orange one- and two-photon excited fluorescence (2PEF) in
THF solution and exhibit high light and heat stability. The max-
imal two-photon absorption cross-sections (d) measured in
THF solution by the 2PEF method using femtosecond laser
pulses are 970 and 900 GM per repeating unit for PDCZ and
PDFL, respectively. These 1,4-diketo-pyrrolo[3,4-c]pyrrole-con-
taining polymers with full aromatic structure and large d will
be promising high-performance 2PA dyes applicable in two-
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J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 944–951
KEYWORDS: conjugated polymers; synthesis; photochemistry;
two-photon absorption; high performance polymers
15 years, 2PA properties of most typical conjugated poly-
mers, such as poly(9,9-dialkylfluorene), poly(p-phenylenee-
thynylene), ladder-type poly(p-phenylene), poly[1,6-bis(3,6-
dihexadecyl-N-carbazol-yl)-2,4-hexadiyne], and poly(phenyle-
nevinylene)s have been measured using z-scan technique or
nonlinear transmission method to show d >10,000 GM.5–9
However, these d values are overestimated by roughly 2–3
orders because of the existence of excited-state absorption
and thermal lens effects,1,2,6 thus the real d values of these
traditional conjugated polymers are rather low (generally
less than 100 GM per repeating unit) in terms of two-photon
excitation fluorescence (2PEF) method. Therefore, there is
still a considerable demand for polymeric fluorophores with
fully aromatic structure and exhibiting high light and heat
stability and large d at present.
INTRODUCTION Two-photon dyes have potential applications
in three-dimensional (3D) fluorescence imaging, optical
power limitation, up-conversion lasing, 3D optical data stor-
age, 3D microfabrication, and photodynamic therapy, which
has stimulated much interest in the design, synthesis, and
characterization of organic compounds exhibiting two-
photon absorption (2PA) properties.1,2 The most frequently
investigated structural motifs are donor–acceptor–donor,
donor–p-bridge–acceptor, donor–p-bridge–donor type mole-
cules, macrocycles, dendrimers, multibranched compounds,
and polymers. It has been revealed that 2PA cross-sections
(d) increase with the donor/acceptor strength, conjugation
length, molecular dimensionality, and the planarity of the p-
center.1,2 With the optimization of structure and composition,
a large number of conjugated molecules exhibiting large d
(>1000 GM) have been prepared. However, most of them
are C@C and CBC bond-containing conjugated organic mole-
cules, which are subject to photo- and heat-instability and
impair the efficiency and lifetime of materials since the pri-
mary causes of photochemical instability for optical dyes are
the photooxidation and photoisomerization of the C@C
bond.3,4 Moreover, compared with extensively studied small
molecular 2PA chromophores, light- and heat-stable-
conjugated polymers with large d are still scarce. In the past
1,4-Diketo-pyrrolo[3,4-c]pyrrole (DPP) derivatives represent
a
class of brilliant red and strongly fluorescent high-
performance pigments that have exceptional light, weather,
and heat stability. Although DPP-containing conjugated poly-
mers have been extensively synthesized and used as highly
luminescent, electroactive, and photoactive materials in opti-
cal and optoelectronic fields,10–15 the reports on their 2PA
properties are still rare.16,17 Recently, we and others have
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CHART 1 Structures of DPP-containing alternating copolymers (PDCZ and PDFL) and the model compound of DPP repeating unit
(DMU) studied here.
demonstrated that DPP could be used as efficient p-center to
construct the conjugated molecules exhibiting large d,16–18
and those monoalkylated derivatives also show one- and
two-photon fluorescence sensing selectively to fluoride
anion.19,20 To further develop new DPP-based 2PA polymers,
in this work, we have synthesized two simple fully aromatic
DPP-containing co-polymers, poly(1,4-diketo-3,6-diphenyl-
pyrrolo[3,4-c]pyrrole-alt-N-octyl-3,6-carbazole/2,7-fluorene)
(PDCZ/PDFL, Chart 1) to investigate their 2PA properties by
the 2PEF method using femtosecond laser pulses. It is well
known that 3,6-carbazole and 2,7-fluorene units have strong
electron-donating and conjugation-extending abilities, respec-
tively, which should permit a comparable investigation on
effect of the co-polymeric units on the optical properties. For
the sake of comparison, 2,5-dioctyl-3,6-di-p-tolyl-pyrrolo[3,4-
c]pyrrole-1,4-dione (DMU, Chart 1) is also synthesized and
used as the model compound of DPP repeating unit. It
should be noticed that the one-photon excitation and electro-
luminescence properties of PDCZ and PDFL homologs have
been investigated by Tieke and coworkers.13 We now report
that the two simple DPP-containing copolymers show high
light and heat stability, and increasing both the donor
strength and the conjugation length can greatly enhance
their d values.
temperature and stirred overnight and added 1 M HCl (50
mL). The organic phase was separated using ether, washed
with brine, and dried over MgSO4. After the solvent was
removed, the residue was dissolved into THF and then added
to hexane. The resulting white precipitate was collected by
filtration and washed with hexane twice and dried in vac-
uum. This intermediate (1.3 g) was mixed with toluene (50
mL) and 2,2-dimethylpropane-1,3-diol (1.1 g, 10.6 mmol)
and refluxed for 4 h, and the water produced was removed
using a Dean–Stark trap. The organic phase was washed
with brine and dried over MgSO4. After removal of the sol-
vent, the residue was re-crystallized from hexane to give the
product as white solids (1.34 g, 52.3% yield).
1H NMR (500 MHz, CDCl3, d): 8.65 (s, 2H), 7.89 (d, J 5 8.0 Hz,
2H), 7.37 (d, J 5 8.5 Hz, 2H), 4.29 (t, J 5 6.0 Hz, 2H), 3.83 (s,
8H), 1.85 (m, 2H), 1.26 (m, 10H), 1.06 (s, 12H), 0.95 (t, J 5 6.0
Hz, 3H); anal. calcd (%) for C30H43B2NO4: C, 71.59; H, 8.61; B,
4.30; N, 2.78; O, 12.72; found: C, 71.68; H, 8.63; N, 2.73.
2,7-Bis(5,5-dimethyl-1,3,2-dioxaborinan-2-yl)-9,9-
dioctylfluorene
n-Butyllithium (7.0 mL of a 1.6 M solution in hexane, 11.2
mmol) was added dropwise into a solution of 9,9-dioctyl-
2,7-dibromofluorene (2.75 g, 5.0 mmol) in THF (60 mL) at
278 ꢀC under dry nitrogen. After stirring for 30 min, the
mixture was added promptly trimethylborꢀate (5.2 g, 50
mmol) and stirred for additional 1 h at 278 C. The reaction
mixture was allowed to warm to room temperature and
stirred overnight and then added 1 M HCl (80 mL). The
organic phase was separated using ether, washed with brine,
and dried over MgSO4. After the solvent was removed, the
residue, without further purification, was refluxed with 2,2-
dimethylpropane-1,3-diol (1.2 g, 11.5 mmol) in toluene for 6
h, meanwhile, the water produced was removed using a
Dean–Stark trap. After cooling to room temperature, the mix-
ture was washed with brine and dried over MgSO4. The sol-
vent was removed by rotary evaporator, and the residue was
passed through a short silica gel column using ethyl acetate/
petroleum ether (4/1) as the eluent and re-crystallized from
THF/hexane (1/5) to give the product as white solids (1.12
g, 36.4% yield).
EXPERIMENTAL
Materials
2,5-Dioctyl-1,4-diketo-3,6-bis(bromophenyl)pyrrolo[3,4-c]pyrrole
(DPP-Br),18 N-octyl-3,6-dibromocarbazole,21 and 9,9-dioctyl
-2,7-dibromofluorene22 were from our previous works.
Pd(PPh3)4, (C4H9)4NBr, n-butyllithium, trimethylborate, and
iodobenzene were purchased from Energy Chemical, China,
and used without further purification.
Other chemicals were of analytical grade and were obtained
commercially from available resources. Tetrahydrofuran
(THF) was distillated over metallic sodium before use.
Synthesis of 3,6-Bis(5,5-dimethyl-1,3,2-dioxaborinan-2-yl)-
N-octylcarbazole
A solution of N-octyl-3,6-dibromocarbazole (2.21 g, 5.1
mmol) in THF (50 mL) was added dropwise 1.6 M n-butyl-
lithium in hexane (7.0 mL, 11.2 mmol) over 10 min at 278
ꢀC under dry nitrogen. After stirring for 30 min at that tem-
perature, trimethylborate (3.13 g, 30.1 mmol) was added
into the mixture and then stirred for additional 30 min at
1H NMR (500 MHz, CDCl3, d): 7.79 (d, J 5 7.0 Hz, 2H), 7.75
(s, 2H), 7.71 (d, J 5 7.5 Hz, 2H), 3.82 (s, 8H), 2.00 (m, 4H),
1.02–1.27 (m, 36H), 1.06 (s, 12H), 0.83 (t, J 5 6.0 Hz, 6H);
anal. calcd (%) for C39H60B2O4: C, 76.23; H, 9.84; B, 3.52; O,
10.41; found: C, 76.31; H, 9.79.
ꢀ
278 C. The reaction mixture was allowed to warm to room
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Poly(2,5-dioctyl-1,4-diketo-3,6-di(phenyl)
pyrrolo[3,4-c]pyrrole-alt-N-octyl-3,6-carbazole)
(U) was determined in THF at room temperature by the
dilution method using rhodamine B in methanol as the refer-
ence.23 Optically dilute THF solutions of DPP derivatives, and
MeOH solution of the rhodamine B standard were prepared
in 1 cm path length quartz cuvettes with absorbances <0.1.
Polymer film for absorption and emission spectra was pre-
pared by spin-coating the polymer solution in THF (15 mg/
mL) on a quartz glass plate. The DSC analysis and the TGA
were performed under N2 atmosphere (50 mL/min) on a
NETZSCH DSC 204F1 and a NETZSCH 209F1 thermogravi-
metric analyzer at heating rates of 10 ꢀC/min (DSC) and 20
ꢀC/min (TGA), respectively.
DPP-Br (0.35 g, 0.522 mmol), 3,6-bis(5,5-dimethyl-1,3,2-
dioxaborinan-2-yl)-N-octylcarbazole (CZB) (0.263 g, 0.522
mmol), Pd(PPh3)4 (0.013 g, 0.011 mmol), and K2CO3 (1.22 g,
8.83 mmol) were added into the mixture of toluene (7 mL)
and water (7 mL) under nitrogen. The resulting mixture was
refluxed under stirring and nitrogen protection for 48 h.
Then 0.025 g of CZB (0.05 mmol) in toluene (0.5 mL) was
added by a syringe, and reaction was continued for 6 h; sub-
sequently, 0.15 g of iodobenzene (0.735 mmol) in toluene
(0.5 mL) was added, and the reaction mixture was refluxed
for additional 6 h. After cooling, 100 mL of chloroform was
added, and the organic phase was washed with water and
dried over MgSO4. The filtrate was passed through a Celite
pad to remove the residual catalysts. The solution obtained
was concentrated to 20 mL and then poured into 100 mL of
ethanol and stirred for 30 min. The solid obtained by filtra-
tion was extracted with acetone in a Soxhlet apparatus for
10 h. After drying, 0.22 g of dark red solids was obtained
(53.5% yield).
2PA cross-sections (d) were measured with the two-photon-
induced fluorescence (TPEF) method using femtosecond
laser pulses as described before by us and others.16,18,24,25
The excitation light source was a mode-locked Ti:sapphire
femtosecond laser (Spectra-Physics, Tsunami 3941, 700–910
nm, 80 MHz, <120 fs) pumped by a compact cw prolite
diode laser (Spectra-Physics, Millennia Pro 5S). The fluores-
cence signal was recorded by spectrofluorometer (Ocean
Optics, USB2000). Samples were dissolved in THF at a con-
centration of 1.0 3 1025 M in repeating unit, and the TPEF
intensity was measured at 710–870 nm and compared under
the same measurement conditions using Rhodamine B in
methanol (1.0 3 1025 M) as the reference.26 The d of sam-
Poly(2,5-dioctyl-1,4-diketo-3,6-di(phenyl)pyrrolo
[3,4-c]pyrrole-alt-9,9-dioctyl-2,7-fluorene)
This polymer was prepared as described for PDCZ except
that 2,7-bis(5,5-dimethyl-1,3,2-dioxaborinan-2-yl)-9,9-dioctyl-
fluorene (FB) was used instead of CZB. After drying, 0.28 g
of dark red solids was obtained (59.7% yield).
2
ple (ds) was calculated using the equation: ds 5 [(SsUrnr cr)/
2
(SrUsns cs)]dr. The subscripts “s” and “r” indicate the sample
and reference molecules, respectively. S is the integral area
of the TPEF; U is the fluorescence quantum yield; n is the
refractive indices of the solvents for the sample and refer-
ence; and c is the number density of the molecules in solu-
tion. dr is the d of the reference molecule.
2,5-Dioctyl-1,4-diketo-3,6-di(p-tolyl)pyrrolo [3,4-c]pyrrole
To a stirring suspension of 3,6-bis(4-methyl-phenyl)pyr-
rolo[3,4-c]pyrrole-1,4(2H,5H)-dione (0.82 g, 2.6 mmol) in
DMF (30 mL), potassium tert-butoxide (1.2 g, 10.6 mmol)
was added at room temperature under N2. The resulting
mixture was stirred for 30 min and then 1-bromooctane
(1.64 g, 8.6 mmol) was added slowly. After stirring for addi-
tional 1 h at room temperature, the mixture was heated to
RESULTS AND DISCUSSION
Synthesis
ꢀ
60 C and stirred overnight. The cooled mixture was poured
The synthetic route for PDCZ, PDFL, and DMU is shown in
Scheme 1. The monomers carbazole-diboronic ester (CZB),
fluorene-diboronic ester (FB), and 2,5-dioctyl-1,4-diketo-3,6-
bis(bromo-phenylpyrrolo[3,4-c]-pyrrole (DPP-Br) were pre-
pared according to the known procedures. The Suzuki cou-
plings of DPP-Br and CZB or FB in a mixture of 2 M K2CO3
and toluene with Pd(PPh3)4 as catalyst afforded the corre-
sponding full aromatic alternating copolymers PDCZ and
PDFL. The model compound of DPP repeating unit (DMU)
was synthesized by N-alkylation of commercially available
3,6-di(p-methyl-phenyl)pyrrolo[3,4-c]-pyrrole-1,4(2H,5H)dione
with 1-bromooctane in DMF using t-BuOK as catalyst. These
compounds are all highly soluble in common organic solvents
such as dichloromethane, toluene tetrahydrofuran (THF), and
chloroform. The number-average molecular weight (Mn) esti-
mated by gel permeation chromatography (GPC) against poly-
styrene standard with THF as eluent is 8.5 3 103 g/mol with a
polydispersity of 2.06 for PDCZ and 1.14 3 104 g/mol with a
polydispersity of 1.83 for PDFL, respectively (Table 1). It has
been shown that, with regard to PPV type polymers, the
changes in electronic properties caused by increasing the
into 400 mL of water and stirred for 1 h. The solid obtained
by filtration was purified by flash column chromatography
using DCM/ethyl acetate 5 20/1 as the eluent. An orange
solid was obtained (0.88 g, 62.5% yield).
1H NMR (500 MHz, CDCl3, d): 7.71 (d, J 5 8.0 Hz, 4H), 7.32
(d, J 5 8.0 Hz, 4H), 3.73 (t, J 5 7.5 Hz, 4H), 2.45 (s, 6H), 1.58
(m, 4H), 1.23 (m, 20H), 0.84 (t, J 5 7.0 Hz, 6H); anal. calcd
(%) for C36H48N2O2: C, 79.96; H, 8.95; N, 5.18; O, 5.92;
found: C, 79.88; H, 8.97; N, 5.21.
Measurements
1H NMR spectra were recorded on a Bruker-AC500 (500
MHz) spectrometer in CDCl3. The ELEM. ANAL was performed
on Perkin–Elmer 2400. Molecular weights were determined
using GPC (Waters 1515 HPLC) equipped with calibrated
styragel columns versus commercially available polystyrene
standards with THF as an eluent. UV–visible absorption spec-
tra were obtained on a Hitachi U-4100 spectrophotometer.
Fluorescence emission spectra were performed on a Hitachi
F-4600 spectrophotometer. The fluorescence quantum yield
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nitrogen or air) and differential scanning calorimetry (DSC,
under nitrogen) and the corresponding data are summarized
in Table 1. As shown in Figure 1 and Table 1, PDCZ and
PDFL solidꢀs have similar but unobvious glass transitions at
about 105 C (as indicated by arrows in the inset of Fig. 1).
TGA curves indicate that the two polymers have the onset
decomposition temperatures around 310 ꢀC in nitrogen
(based on 5% weight loss). Noticeably, the onset decomposi-
tion temperatures of PDCZ and PDFL in air are still up to
279 and 290 ꢀC, respectively, and there are hardly weight
losses before 200 ꢀC (Fig. 1). These results indicate that
PDCZ and PDFL exhibit excellent light and heat stability in
both inert and air atmospheres, which are fairly valuable for
2PA dyes.
Linear Absorption and One-Photon Excited Fluorescence
Properties
The one-photon absorption and emission spectra of PDCZ
and PDFL are investigated in both THF solutions and solid
films, and the absorption and emission spectra of their DPP
repeating unit analog DMU in THF solution are also recorded
for comparison. Related spectroscopic data are summarized
in Table 1. Figure 2 depicts the absorption and emission
spectra of PDCZ, PDFL, and DMU in THF solutions. Although
the absorption maxima of copolymers are red-shifted only
by 7–9 nm related to that of DMU (Table 1), the long-
wavelength absorption bands of PDCZ and PDFL are signifi-
cantly broadened toward long-wavelength regions [Fig. 2(a)].
This indicates that the electronic states of DPP and carbazole
or fluorene units are mixed to a large extent to render a
strong intramolecular charge transfer (ICT). It is observed
that the long-wavelength absorption bands of PDCZ and
PDFL are very similar, implying that the ICT effect induced
by strong electron-donating 3,6-carbazole is similar to that
by strong conjugation-extending 2,7-fluorene in this system.
The strong and similar ICT effect in the two copolymers is
further evidenced by the fluorescence emission spectra [Fig.
2(b)]. The emission spectra of PDCZ and PDFL in THF solu-
tions are very same in both shape and position but red-
shifted significantly by about 55 nm compared to that of
DMU (Table 1). The solution fluorescence quantum yields
(U) of PDCZ, PDFL, and DMU in THF measured by dilute
solution method using Rhodamine B as the reference are
28.6, 43.4, and 61.1%, respectively. Figure 3 shows the
SCHEME 1 Synthetic route of PDCZ, PDFL, and DMU.
conjugated length saturate at five to seven repeating units.27
The average numbers of repeating units of the as-synthesized
polymers are estimated to be approximately 10–13. Therefore,
the chain lengths of the as-synthesized polymers are sufficient
to represent the optoelectronic properties of extended p-
systems even if their Mn values might be overestimated, as
observed in other conjugated polymers.28
Light- and Heat-Resistant Properties
Unlike arylenevinylene derivatives, DPP derivatives have
exceptional light, weather, and heat stability. To test the light
and heat stability of full aromatic copolymers PDCZ and
PDFL, the polymer solutions are put into glass tubes and
exposed to sunlight or irradiated with an ordinary light bulb
(100 W) in air for 3 days. No changes are found in both
absorption and emission spectra. The thermal properties are
investigated with thermogravimetric analysis (TGA, under
TABLE 1 Molecular Weight, Optical Properties, and Thermal Analysis Data of PDCZ and PDFL
Mn/PDa
kmax Soln/Film kem Soln/Film Stokes-Shiftd Soln/Film U (%)e dmax (GM) Tg/ꢀCg Td/ꢀCh in N2/Air
b
c
f
Cpds
PDCZ 8,540/2.06
502/506
11,400/1.83 500/505
540/–
493/ndi
583/631
579/600
526/nd
81/125
79/95
33/-
28.6
43.4
61.1
970
899
107
103
105
nd
307/279
312/290
nd
PDFL
DMU
a
f
Number-average molecular weight (Mn) and polydispersity (PD) meas-
Maximal two-photon absorption cross section in 10250 cm4 s/photon
ured by GPC using THF as the eluent.
(GM).
b
g
Peak wavelength (nm) of the lowest energy one-photon absorption
band in THF solutions (soln) and solid films (film).
Glass transition temperature at a heating rate of 10 ꢀC/min.
h
Thermal decomposition (5% weight losses) temperature under nitro-
c
Peak wavelength (nm) of one-photon emission spectra.
gen or air atmosphere at a heating rate of 20 ꢀC/min.
i
d
Stokes shift in nanometer.
Not determined.
e
Fluorescence quantum yield in THF.
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FIGURE 1 TGA thermogram (heating rate 20 ꢀC/min) and DSC traces (inset, heating rate 10 ꢀC/min) of PDCZ and PDFL. [Color fig-
ure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
absorption and emission spectra of PDCZ and PDFL in solid
films. The absorption spectra of PDCZ and PDFL in solid
films are similar to those in solutions [Fig. 2(a)], whereas
the emission spectra in solid films are much broader and
red-shifted significantly by 48 and 21 nm for PDCZ and
PDFL, respectively, relative to those in solutions [Fig. 2(a)
and Table 1]. This suggests the presence of aggregation of
the polymer main chains in the solid states arising from the
inherent rigid nature of their backbones and the higher pla-
narity and polarity of heterocyclic DPP moieties. The broader
emission spectrum and larger Stokes shift observed for
PDCZ than PDFL imply that there are stronger interactions
between transition moments of nearest neighbor PDCZ mol-
ecules in the solid sates. The similar phenomenon has been
observed for other narrow-band-gap heterocycle-containing
copolymers.
could give reliable 2PA cross-sections (d) and avoid possible
complications due to the excited-state absorption and ther-
mal lens effects.
To confirm the occurrence of nonlinear absorption, the two-
photon excited fluorescence (2PEF) of PDCZ and PDFL is
recorded under 820 nm excitation with changed input laser
powers (60–180 mW). It has been confirmed that the
copolymers have strong linear absorption before 600 nm,
but no linear absorption in the spectral range of 600–1100
nm. Thus, if there undoubtedly is any fluorescence emission
emerged from the polymer solutions under the irradiation
with an 820 nm of laser pulses; it must not be from one-
photon excitation. We have observed the fluorescence emis-
sion from the polymer solutions under different laser inten-
sities, moreover, the fluorescence intensities are gradually
increased with the input laser power (Fig. 4). To confirm the
occurrence of nonlinear absorption, the relation plot
between logarithmic output fluorescence intensity and loga-
rithmic input laser power is drawn (Fig. 5). The slopes
obtained by the linear fitting of the experimental data are
1.93 for PDCZ and 1.96 for PDFL, respectively, which are
very close to power-law dependence and indicative of a two-
Two-Photon Absorption and Excited Fluorescence
We used two-photon excited fluorescence measurement tech-
nique to investigate the two-photon properties of the two
copolymers and their DPP repeating unit analog in THF solu-
tions. The femtosecond laser pulses (mode-locked Ti:sap-
phire femtosecond laser, 120 fs, 80 MHz) were used, which
FIGURE 2 UV–visible absorption (a) and one-photon excited fluorescence (b) spectra of PDCZ, PDFL, and DMU in THF solutions at
1.0 3 1025 M in repeating unit.
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photon excitation process (Fig. 5). Figure 6(a) shows the
one- and two-photon excitation fluorescence spectra. There
is a good spectral overlap between the respective one- and
two-photon excitation fluorescence whose peak emission
wavelengths are the same as each other, indicating that the
fluorescence emission occurs from the same excited states,
regardless of the mode of excitation. Meanwhile, an obvious
difference between one- and two-photon excited emissions is
observed for both PDCZ and PDFL [Fig. 6(a)]. The 2PEF
spectra are obviously narrower than the one-photon ones.
However, it is not clear whether the two copolymers have
up-conversion lasing properties, and further investigations
are needed.
Figure 6(b) shows the two-photon excitation spectra of
PDCZ, PDFL, and DMU in THF, and the relevant maximal d
(dmax) are compiled in Table 1. The input laser power of 100
mW for PDCZ and PDFL and 150 mW for DMU is used,
which fall within the power range of the two-photon excita-
tion mode as shown in Figure 5. The available excitation
FIGURE 3 UV–visible absorption and one-photon excited fluo-
rescence (PL) spectra of PDCZ and PDFL in film state. [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
FIGURE 4 Two-photon excited fluorescence (2PEF) spectra of PDCZ and PDFL solutions in THF at 1.0 3 1025 M in repeating unit
under the excitation wavelength of 820 nm. [Color figure can be viewed in the online issue, which is available at wileyonlineli-
brary.com.]
FIGURE 5 The dependence of output fluorescence intensity of PDCZ and PDFL solutions on the input laser power. Other condi-
tions are same as Figure 4.
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FIGURE 6 (a) Comparison between one-photon (1P) and two-photon (2P) excited fluorescence spectra. (b) Two-photon excitation
spectra of PDCZ, PDFL, and their DPP unit analog DMU under input laser power of 100 mW for PDCZ and PDFL, and 150 mW for
DMU. The concentration of 2PA dye is at 1.0 3 1025 M in repeating unit. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
wavelength is in a range of 710–870 nm. It is shown that
the 2PA band of DMU locates mainly short-wavelength
region, and the absolute d value is small although it is con-
siderable in terms of molecular skeleton size. When extend-
ing linearly the molecular conjugation length or
incorporating the strong electron-donating end-groups by
copolymerization with 2,7-fluorene or 3,6-carbazole, respec-
tively, the resulting copolymers PDCZ and PDFL exhibit
greatly broadened and enhanced 2PA bands. The dmax values
of PDCZ and PDFL are up to 970 and 899 GM per repeating
unit, respectively, which are significantly larger than that of
DPP repeating unit analog DMU and are also the largest
among those fully aromatic conjugated polymers reported so
far in terms of the TPEF measurement method. This clearly
indicates that both extending the conjugation length and
incorporating the strong electron-donating unit can signifi-
cantly enhance d of DPP-containing 2PA dyes. Moreover,
extending the conjugation length and incorporating the
strong electron-donating unit has same effectiveness in
enhancing the d of this system. The d values of these simple
fully aromatic conjugated polymers are larger than those of
both their complex analogous and CBC bond-bridged DPP–
triphenylamine hyperbranched polymers.16,17
Figure 7 shows that the two-photon-allowed states are
located at lower wavelength (higher energies) regions than
their Franck–Condon states (one-photon allowed states) for
both PDCZ and PDFL; moreover, there is no significant over-
lap between two allowed states in terms of the total absorp-
tion energy. These results imply that their one- and two-
photon allowed states (selection rules) are different.
CONCLUSIONS
We have synthesized poly(1,4-diketo-3,6-diphenylpyrrolo[3,4-c]
pyrrole-alt-N-octyl-3,6-carbazole/2,7-fluorene) (PDCZ/PDFL)
to demonstrate that simple DPP-containing conjugated copoly-
mers show large d. The dmax values of PDCZ and PDFL meas-
ured by TPEF method are up to 970 and 899 GM per
repeating unit, respectively, the largest so far among fully aro-
matic conjugated polymers. PDCZ and PDFL having the similar
large d indicate that extending the conjugation length and
incorporating the strong electron-donating unit have same
effectiveness in enhancing the d of this system. This work
demonstrates once again that DPP is a novel and efficient p-
center for the construction of high-performance 2PA dyes.
DPP-containing fully aromatic conjugated polymers with large
d and high light and heat stability may ultimately find useful
applications in two-photon science and technology.
FIGURE 7 One-photon absorption (1P) and two-photon excita-
tion (2P) spectra. The two-photon spectra are plotted against
half the wavelength (twice the photon energy). [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
ACKNOWLEDGMENTS
The authors thank the NSF of China (Nos. 51173092 and
51073083), NSF of Shandong Province (Nos. ZR2010EM023
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