Two-photon properties and excitation dynamics of poly(p-phenylenevinylene) derivatives carrying phenylanthracene and branched alkoxy pendents

Article abstract of DOI:10.1021/jp0141474

This paper presents a study of excitation dynamics and two-photon properties in new branched poly(p-phenylenevinylene) structures which can be viewed as composed of interlinked structural units producing localized but interacting electronic excitations. Poly(p-phenylenevinylene) (PPV) derivatives, specifically poly[2-(9-phenylanthracen-10-yl)-1,4-phenylenevinylene] (P-1) and poly[2-(2-ethylhexyloxy)-5-(9-phenylanthracen-10-yl)-1,4-phenylenevinylene] (P-2), carrying 9-phenylanthracene pendent were prepared, and their one- and two-photon absorption and emission properties were studied in solution. P-1 and P-2 differ in structure by the presence of additional 2-ethylhexyloxy pendent groups in P-2. Both polymers were prepared by direct polymerization of the α,α'-dibromo-p-xylene monomers having the pendent group(s) in the present of excess potassium tert-butoxide. For the sake of comparison, 9,10-diphenylanthracene (DPA) and poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] [Meh-PPV (P-3)] were also included in our study. From nanosecond and femtosecond nonlinear transmission measurements, their effective two-photon cross sections (σ_TPA) at ～ 800 nm were found to be 11.9 × 10^-20 for P-1, 66.6 × 10^-20 for P-2, and 44.0 × 10^-20 for P-3 in nanosecond pulses and 0.074 × 10^-20 for P-1, 0.196 × 10^-20 for P-2, and 0.168 × 10^-20 cm⁴/GW for P-3 in femtosecond pulses, respectively. Their two-photon excitation spectra were also studied. P-2 having both the phenylanthrace and alkoxy pendents exhibits the best performance as a two-photon absorber in both of nano- and femtosecond measurements at ～800 nm. Furthermore, a complete energy transfer from the pendent phenylanthracene moiety to the PPV backbone is observed.

Full text of DOI:10.1021/jp0141474

7512
J. Phys. Chem. A 2002, 106, 7512-7520
Two-Photon Properties and Excitation Dynamics of Poly(p-phenylenevinylene) Derivatives
Carrying Phenylanthracene and Branched Alkoxy Pendents†
S.-J. Chung, G. S. Maciel, H. E. Pudavar, T.-C. Lin, G. S. He, J. Swiatkiewicz, and
P. N. Prasad*
Institute for Lasers, Photonics and Biophotonics, Department of Chemistry, UniVersity at Buffalo,
The State UniVersity of New York, Buffalo, New York 14260-3000
D. W. Lee and J.-I. Jin*
DiVision of Chemistry and Molecular Engineering and The Center for Electro- and Photo-ResponsiVe
Molecules, Korea UniVersity, Seoul 136-701, Korea
ReceiVed: NoVember 13, 2001; In Final Form: May 3, 2002
This paper presents a study of excitation dynamics and two-photon properties in new branched poly(p-
phenylenevinylene) structures which can be viewed as composed of interlinked structural units producing
localized but interacting electronic excitations. Poly(p-phenylenevinylene) (PPV) derivatives, specifically poly-
[2-(9-phenylanthracen-10-yl)-1,4-phenylenevinylene] (P-1) and poly[2-(2-ethylhexyloxy)-5-(9-phenylanthracen-
10-yl)-1,4-phenylenevinylene] (P-2), carrying 9-phenylanthracene pendent were prepared, and their one- and
two-photon absorption and emission properties were studied in solution. P-1 and P-2 differ in structure by
the presence of additional 2-ethylhexyloxy pendent groups in P-2. Both polymers were prepared by direct
polymerization of the R,R′-dibromo-p-xylene monomers having the pendent group(s) in the present of excess
potassium tert-butoxide. For the sake of comparison, 9,10-diphenylanthracene (DPA) and poly[2-methoxy-
5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] [Meh-PPV (P-3)] were also included in our study. From
nanosecond and femtosecond nonlinear transmission measurements, their effective two-photon cross sections
(σTPA) at ∼ 800 nm were found to be 11.9 × 10-20 for P-1, 66.6 × 10-20 for P-2, and 44.0 × 10-20 for P-3
in nanosecond pulses and 0.074 × 10-20 for P-1, 0.196 × 10-20 for P-2, and 0.168 × 10-20 cm4/GW for P-3
in femtosecond pulses, respectively. Their two-photon excitation spectra were also studied. P-2 having both
the phenylanthrace and alkoxy pendents exhibits the best performance as a two-photon absorber in both of
nano- and femtosecond measurements at ∼800 nm. Furthermore, a complete energy transfer from the pendent
phenylanthracene moiety to the PPV backbone is observed.
Introduction
absorbed by a chromophore that would normally be excited by
a single photon with a shorter wavelength. As a result, whereas
the rate of single-photon absorption scales linearly with the
intensity of incident radiation, the rate of TPA scales quadrati-
cally. Since pioneering work by Rentzepis in optical data
storage18,19 and by Webb in optical microscopy20,21 that
demonstrated early potential application of two-photon pro-
cesses, there has been considerable interest in the design,
synthesis and characterization of new organic chromophores and
polymers with potentially large two-photon absorption cross
sections. Potential photonic applications for two-photon chro-
mophores and polymers include two-photon upconversion
lasing,22,23 two-photon power limiting,24,25 three-dimensional
optical data storage,26,27 two-photon microfabrication,28,29 and
two-photon photodynamic cancer therapy.30,31
A wide variety of conjugated polymers have been reported
in the past decades as electroactive materials for diverse
applications such as batteries, molecular electronic devices,
nonlinear optical switches, and light-emitting diodes (LEDs),
etc.1-5 Among them, PPV and its derivatives have received
considerable attention because of their feasibilities in tuning
electrical and optical properties.6-8 Processable PPV derivatives
and PPV itself have been used to form an active layer in thin-
film light emitting diodes (LEDs) with promising efficiencies.9-11
By tuning the morphologies, band gaps, and charge-transport
properties of these polymers, device stabilities have been
increased, and emitters at a wide variety of wavelengths have
been fabricated.12-14 Recently, superradiance in the PPV and
its derivatives have been achieved,15 suggesting the prospect
of polymer semiconductor lasers. PPV formed in pores of a
glass have shown tunable lasing. PPV also exhibits a strong
two-photon pumped up-conversion emission when excited by
near-IR laser pulses of 800 nm.16,17 This opens another prospect,
that of up-conversion lasing.
Recently, our work has focused on the designs and studies
of branched conjugated structures which can produce one and
two-photon excitations localized in various segments of their
molecular structures.32 These types of structures provide two-
photon absorption over a broad spectral range and also produce
efficient excitation transfer from one segment to another.
Two-photon absorption (TPA) is a nonlinear optical process
in which two photons of longer wavelength are simultaneously
We reported earlier photoluminescence (PL) and electrolu-
minescence (EL) characteristics in the thin films of PPV with
its derivatives carrying 9-phenylanthracene pendent, poly[2-(9-
† Part of the special issue “G. Wilse Robinson Festschrift”.
10.1021/jp0141474 CCC: $22.00 © 2002 American Chemical Society
Published on Web 07/24/2002

Branched Poly(p-phenylenevinylene) Structures
J. Phys. Chem. A, Vol. 106, No. 33, 2002 7513
SCHEME 1: Synthetic Route to P-1
phenylanthracen-10-yl)-1,4-phenylenevinylene] (P-1), and both
the phenylanthracene and 2-ethylhexyloxy substituent, poly[2-
(2-ethylhexyloxy)-5-(9-phenylanthracen-10-yl)-1,4-phenylene-
vinylene] (P-2). These polymers can be viewed as composed
of the interlinked structures. These two PPV derivatives are
soluble in organic solvents and P-2 revealed a greatly enhanced
power efficiency in EL devices when compared with PPV and
P-1.33,34
In this article, we report the synthesis, structural characteriza-
tions and optical properties including two-photon absorption
(TPA) of these two PPV derivatives containing phenyl-
anthracene moiety and also the 2-ethylhexyloxy substituent as
an electron donating group. For the sake of comparison, Meh-
PPV (P-3)9,12 and diphenylanthracene (DPA) are also included
in the present work. Their TPA properties were investigated
by the nonlinear transmission method utilizing nanosecond
transmission and the femtosecond Z-scan techniques to obtain
the effective two-photon absorption cross section and to examine
the effect of pulse-width on the effective cross section.
monomer bears the 2-ethylhexyloxy group on the phenyl ring.
Compound 3 was prepared from 9-bromo-10-phenylanthracene
and 2,5-dimethyl-methoxyphenylmagnesium bromide in the
presence of NiCl2‚2PPh3 as catalysts.36 The methoxy group in
the compound 3 was deprotected to the corresponding phenolic
compound 4 by acidic cleavage of the ether linkage, which then
was reacted in ethanol with 2-ethylhexylbromide in the presence
of KOH to produce compound 5. Finally, monomer 6 was
synthesized via benzylic bromination of compound 5 using
N-bromosuccinimide (NBS) and benzoyl peroxide (BPO) as
utilized in the synthesis of monomer 6. The structures of all of
the intermediates and the final monomers were confirmed by
FT-IR and 1H NMR spectroscopy and also by elemental
analysis.
Syntheses and Properties of Polymers P-1 and P-2. Both
polymers were prepared by polymerization of respective mono-
mers at room temperature in THF in the presence of a strong
base, potassium tert-butoxide. This synthetic method was
originally described by Gilch and Wheelwright.37 This method
requires the use of excess alkali to ensure the formation of the
fully eliminated structure as shown in Schemes 1 and 2. The
mechanism of the polymerization process is two steps: forma-
tion of the benzylic anion which links the monomers via an SN
2 reaction, followed by dehydrohalogenation. Both polymers
were purified by reprecipitation using methanol. Their structures
Results and Discussion
Syntheses of Monomers 2 and 6. P-1 and P-2 were
synthesized by polymerizing bis(bromophenyl)monomers, 2-(9-
phenylanthracen-10-yl)-1,4-bis(bromomethyl)benzene (2) and
2-ethylhexyloxy-5-(9-phenylanthracen-10-yl)-1,4-bis(bromo-
methyl)benzene (6), which were prepared via the multistep route
shown in Schemes 1 and 2, respectively. For the synthesis of
monomer 2, we had to synthesize 2-(9-phenylanthracen-10-yl)-
p-xylene (1) by the Suzuki reaction35 with 9-bromo-10-
phenylanthracene and 2,5-dimethylphenyl boronic acid in the
presence of the tetrakis(triphenylphosphine) palladium (0)
catalyst. Finally, compound 1 was treated with N-bromosuc-
cinimide (NBS) using benzoyl peroxide (BPO) as the initiator,
to produce the benzylic bromides in 42% yield. The second
monomer, 2-ethylhexyloxy-5-(9-phenylanthracen-10-yl)-1,4-bis-
(bromomethyl) benzene (6), was synthesized in a similar fashion,
but its synthesis is a little bit more complicated because the
1
were confirmed by FT-IR, H NMR, and elemental analysis.
The polymers are soluble in organic solvents such as toluene,
xylene, chloroform, and 1,1,2,2-tetrachloroethane. Their average
molar mass measured by gel permeation chromatography (GPC)
was Mn ) 44000 for P-1 and 34000 for P-2, respectively.
Toluene was employed as an eluent and polystyrene as standard.
The polydispersity index for P-1 and 2 were 2.42 and 2.27,
respectively.
The glass transition temperatures (Tg) of polymer samples
cannot be detected up to 300 °C by differential scanning calo-
rimetry (DSC). In TGA analyses, both polymers did not lose
any weight up to 300 °C and started to under go fast weight

7514 J. Phys. Chem. A, Vol. 106, No. 33, 2002
Chung et al.
SCHEME 2: Synthetic Route to P-2
loss from about 370 °C for P-1 and 350 °C for P-2, respectively.
The weight loss at high temperature is ascribed to thermal
degradation. The presence of the alkoxy substituent appears to
reduce the thermal stability of P-2 when compared with that of
P-1.
Linear Optical Properties of Polymer Solutions. Figure 2a
compares the UV-vis absorption spectra of the PPV derivatives
along with that of DPA, obtained in 1,1,2,2-tetrachloroethane
solutions. The spectra can be analyzed by considering the
polymers as composed of two types of interlinked structure: a
diphenylanthracene moiety and a derivatized PPV type main
backbone. The polymers carrying phenylanthracene commonly
exhibit strong four vibronic absorption bands at about 325-
410 nm overlapped with a weaker and broader absorptions at a
longer wavelength region that arise from the electronics
transition of the main chainssystem. The four vibronic absorp-
tion peaks at the shorter wavelength region are caused by the
9,10-diphenylanthracene moieties.38-40 The peak maxima posi-
tions for the 9,10-diphenylanthracene units appear at 340, 360,
378, and 400 nm both for films and solutions regardless of the
polymer structures.40 Essentially, there are no changes in
absorption maxima of the 9,10-diphenylanthracene (DPA) units
for P-1 and P-2. The peak positions for 9,10-diphenylanthracene
(DPA) in 1,1,2,2-tetrachloroethane solution appear at 337, 355,
373, and 394, respectively. The absorption band maxima for
this DPA moiety in the polymers are slightly red shifted
compared with the absorption maxima of the corresponding
DPA molecule by itself. This suggests that there is some
interaction between the π-electrons in the main chain and the
pendent groups. This interaction is also evidenced by a change
in the extinction coefficient of the DPA band as exhibited by
Figure 2a.
Figure 2b shows the one-photon excitation spectra obtained
by monitoring the fluorescence at 500 nm for P-1, at 550 nm
for P-2, and at 600 nm for P-3, respectively. For P-1 and P-2,
there are significant differences between the absorption and the
excitation spectra. While in absorption, the peaks in the shorter
Figure 1. Chemical structures of P-1,P-2, and Meh-PPV (P-3).

Branched Poly(p-phenylenevinylene) Structures
J. Phys. Chem. A, Vol. 106, No. 33, 2002 7515
Figure 3. a) One-photon excited emission spectra of P-1, P-2, P-3,
and 9,10-diphenylanthracene (DPA) (path length ) 1 cm and OD <
0.01). and b) two-photon induced emission spectra of P-1, P-2, and
P-3 (path length of the cell is 0.1 cm) in 1,1,2,2-tetrachloroethane.
Concentration ) 10-4 mol/L in repeating unit.
Figure 2. a) Absorption spectra of P-1, P-2, P-3, and 9,10-
diphenylanthracene (DPA); b) one-photon excitation spectra of P-1,
P-2, and P-3 (fluorescence monitored at 500 nm, 550 nm, and 600 nm
for P-1, P-2, and P-3, respectively) in 1,1,2,2-tetrachloroethane.
Concentration ) 10-4 mol/L in repeating unit.
compared to P-1. The electron donating alkoxy substituents in
P-2 and Meh-PPV (P-3) produce this red shift as has been
reported earlier for Meh-PPV.42
wavelength region derived from the pendent side chain domi-
nate, the excitation spectra show that a more efficient excitation
for emission is provided by absorption in the longer wavelength
region derived from the main chain. For P-3, which does have
a pendent chromophore, the absorption and the excitation peaks
are closely matched.
Nonlinear Optical Properties of Polymer Solutions
Two-Photon Absorption Measurement. These PPV deriva-
tives have strong linear absorption around 400-500 nm, but
no absorption in the spectra ranging from 600 to 1100 nm. The
feature at 825 nm is due to the exchange of the lamps in the
spectrophotometer. Thus, upon irradiation with a laser at 800
nm, there should not be any linear (one-photon) absorption
induced fluorescence emission. The energy of two infrared
photons at 800 nm falls within the strong one-photon absorption
band of these polymer solutions. Experimental measurements
have shown that a strong frequency-upconverted fluorescence
emission can be easily observed from the polymer solutions
with an unfocused Q-switched near-IR laser beam at the
wavelength range mentioned above. This indicates that a quite
strong two-photon absorption process is occurring within the
polymer samples. Here, we investigated the nonlinear (two-
photon) absorption properties of the PPV derivatives using two
different laser systems: a 8 ns, 810 nm dye laser pumped by a
frequency-doubled Nd:YAG laser operating at 10 Hz repetition
rate and a 135 fs, 796 nm, Ti:sapphire oscillator/amplifier laser
operating at a 1 kHz repetition rate. The nonlinear absorption
behavior of the chromophores P-1, P-2, P-3, and DPA were
studied by measuring the transmission changes with the incident
laser intensity. This was achieved by focusing the excitation
beam in the sample cell using a ∼20 cm focal length lens. The
The photoluminescence (PL) behaviors of these PPV deriva-
tives were studied in 1,1,2,2-tetrachloroethane solution using
the excitation wavelength of 405 nm (Figure 3a). 9,10-
Diphenylanthracene (DPA) in 1,1,2,2- tetrachloroethane solution
emits light in the spectral range 400-500 nm, with the peak of
emission at 405 nm and a strong vibronic band at 428 nm.
However, emissions of P-1 and 2 start from about 450 nm. This
emission has characteristics of PPV emission. This suggests
complete energy transfer from the pendent phenylanthracene
group to the PPV backbone from where the emission originates.
Jin and co-workers41 reported the steady-state and time-resolved
PL spectra for the film of PPV derivatives using the excitation
wavelength of 300-325 nm to photoexcite the side groups,
phenylanthracene, to investigate the side-chain effect on the PL
emission preferentially. According to their results, the PL spectra
of PPV derivatives (P-1) carrying phenylanthracene as a pendent
group are composed of well-resolved vibronic bands, being
similar to the spectral features of PPV emission without
interference by the pendent group and the energy-transfer
process may occur from the pendent group to the PPV main
chain. The PL spectrum of P-2 shows red-shifted emission

7516 J. Phys. Chem. A, Vol. 106, No. 33, 2002
Chung et al.
Figure 4. Nonlinear transmission data using a nanosecond laser (8
ns, 10 Hz, 810 nm) as the excitation source.
sample cell was a 1 cm long quartz cuvette filled with 0.02
mol/L of the PPV derivatives in 1,1,2,2-tetrachloroethane
solutions. Then, the sample cell position was smoothly varied
along the laser beam direction, so that the local incident intensity
could be changed under a fixed incident pulse energy level (Z-
scan technique). The transmitted light changes were monitored
using a large area pyroelectric detector (30 nm diameter) placed
at the far-field region (∼30 cm far from the focusing lens).
The nonlinear (two-photon) absorption coefficient can be
obtained by the analysis of the transmission changes with the
incident irradiance using the propagation equation of light
through a nonlinear optical material that presents two-photon
absorption:
Figure 5. The Z-scan traces for P-1, P-2, and P-3 of the equal
concentration solutions in 1,1,2,2-tetrachloroethane (0.02 mol/L in
repeating unit). The average pulse length is 135 fs, and the peak
irradiance at the focal point is 50 GW/cm2 at a wavelength of 796 nm.
TABLE 1: TPA Cross-Section Values Measured for P-1,
P-2, and P-3 under Nanosecond Pulses at 810 nm and
Femtosecond Pulses at 796 nm in 1,1,2,2-Tetrachloroethane
Solutiona
P-1
TCE
concentration (mol/L) 0.02
P-2
P-3
solvent
TCE
0.02
810 (796)
1.665
TCE
0.02
810 (796)
1.101
wavelength (nm)
NLO absorption
coefficient
810 (796)
0.297
dI(z)
) - RI(z) - â(I(z))2
(1)
â (cm/GW)
σ
TPA (10-20 cm4/GW)
11.9 (0.074) 66.6 (0.196) 44.0 (0.168)
dz
a Values are femtosecond data.
where I(z) is the laser irradiance (intensity) along the position
z, the propagation direction. The parameters R and â are the
linear and two-photon absorption (TPA) coefficients, respec-
tively. The parameter R is obtained from the linear absorption
spectra of the samples and â is obtained by fitting the solution
of eq 1 with the experimental data. The values of the TPA
coefficients for the polymers studied here were calculated from
the nonlinear transmission data obtained using nanosecond
pulses and the Z-scan data obtained using femtosecond pulses.
The TPA cross section is estimated from â using the following
equation:
strongest nonlinear absorption at the wavelength of ∼800 nm.
The σTPA values measured in nanoseconds and femtoseconds
are given Table 1. The σTPA values measured by using
femtosecond laser pulses are roughly 2-3 orders of magnitude
smaller than those measured by using nanosecond laser pulses.43,44
Predominant excited state absorption in nanosecond excitation
is cited as the reason for this difference in nanosecond and
femtosecond measurements. A similar trend was found in a
recent studies of new two-photon organic molecules.45 Never-
theless, we can observe that P-2 exhibits the best performance
as a two-photon absorber in both nano- and femtosecond
measurements. The two-photon cross section obtained from the
σTPA ) â/(NAd10-3)
(2)
46
sum-over-states approach is given by
where NA is the Avogadro number (6.02 × 1023), d is the
repeating unit concentration (mol/L), and σTPA is the two-photon
absorption cross section. Figure 4 shows the nonlinear absorption
results for P-1, P-2, and P-3 using a 8 ns, 810 nm dye laser
pumped by a frequency-doubled Nd:YAG laser operating at 10
Hz repetition rate as the source for nanosecond pulses. The leaser
beam was focused with a f ) 20 cm lens and passed through a
1 cm path quartz cuvette filled with the solution sample. DPA
did not exhibit any significant two-photon absorption at this
wavelength. Note that P-2 presents a stronger nonlinear absorp-
tion. The same trend was observed in femtosecond Z-scan
measurements, shown in Figure 5. Again, no significant two-
photon absorption was observed for DPA at this wavelength.
The depth of valley in the Z-scan is proportional to the nonlinear
absorption coefficient of the respective samples and, for their
equal concentration, they are proportional to the effective two-
photon cross section. P-2 is the material that presents the
2
2
M01 M12
(E1 - ηu`)2A˜20
o´02
(3)
where subscripts 0, 1, and 2 refer to levels S0, S1, and S2. Mij
is the transition dipole moment from level i to level j, Γ02 is
the damping factor, E1 is the energy of S1, and hω is the
excitation photon energy. Therefore, according to eq 3, there
are basically two ways of increasing the TPA cross section:
by increasing the transition dipole moments and by reducing
the detuning between the first singlet excited state and the laser
energy. Our results showed that P-2 has the largest TPA cross
section. However, this result reflects the TPA cross-section
measurements only at one wavelength (∼800 nm). To get more
insight into the structure and two-photon absorption property
relation, a measurement of the two-photon absorption spectrum

Branched Poly(p-phenylenevinylene) Structures
J. Phys. Chem. A, Vol. 106, No. 33, 2002 7517
needs to be obtained. This was accomplished by studying the
two-photon excitation spectra as described below after a
description of the two-photon excited fluorescence.
Two-Photon Excited Fluorescence Emission. Figure 3b
shows the two-photon induced emission spectra of 1 mm path
PPV derivative samples excited with the 800 nm laser beam.
DPA exhibited only weak two-photon excited emission. Com-
paring Figure 3b to Figure 3a, one can see that the spectral
profile of the emission bands of P-1, P-2, and P-3 samples are
basically the same for both one-photon and two-photon excita-
tions. This indicates that the emissions in these polymers
originate from the same singlet (S1) in both one- and two-photon
excitations.
Two-Photon Excitation Spectra. To obtain information on
the two-photon absorption as a function of wavelength, the two-
photon excitation spectra were obtained monitoring the two-
photon upconverted fluorescence. A mode-locked, femtosecond,
Ti:sapphire laser (Spectra-Physics, Tsunami) pumped by a DPSS
laser (Spectra Physics, Millenia) was used for pulsed excitation.
The pulse width was continuously monitored using an autocor-
relator (NT&C, Germany). A spectrum analyzer (IST-rees) was
used to monitor the wavelengths of excitation. The laser beam
was focused onto the sample using a 5 cm focal length lens
and the fluorescence spectra were collected using a fiber coupled
monochromator (Holospeec from Kaiser Institute) and a CCD
camera system (Princeton Instruments). The total fluorescence
intensity collected for each sample at each wavelength was
calculated as the area under the collected fluorescence spectra.
The wavelength of the laser was tuned from 715 to 890 nm
and the average power of excitation, the wavelength and the
pulse width were monitored and the fluorescence spectra were
recorded for each dye. By monitoring the fluorescence spectra
using a spectrometer, we were able to monitor the emission
spectra at different excitation wavelengths. In case of all the
samples prepared here we found no change in the emission
spectra with different two photon excitation wavelengths.
According to Xu and Webb,47 the total fluorescence intensity
is given by
2
g
p 8n P(t)
1
2
F(t) ) φη2Cσ
(4)
fτ
πλ
where F(t) is the total average fluorescence intensity collected,
η2 is the fluorescence quantum efficiency of the dye, and φ is
the fluorescence collection efficiency of the dye; C is the
concentration of the dye in solution and σ is the two photon
excitation cross section; gp/f is the degree of second-order
τ
coherence (gp depends only on pulse shape, which in this case
is a hyperbolic-secant function, and has value of gp ) 0.588),
n is the refractive index, and P(t) is the average excitation
power. For one dye most of the parameters can be taken as a
constant, reducing eq 4 to be
Figure 6. Normalized fluorescence efficiency as a function of
wavelength of two-photon excitation.
6 for P-1, P-2, and P-3. Comparison of the spectra shows that
P-2 has a higher two-photon absorption at ∼800 nm, resulting
from the small detuning between the two-photon excitation
wavelength and the energy of the first excited singlet, but P-3
has a higher two-photon absorption at a longer wavelength.
2
P(t)
λ
1
τ
F(t)
σ
(5)
or
1
Experimental Section
σ
2λτ F(t)
(6)
P(t)
Characterization. The structure of the intermediates and
polymers were verified by FT-IR (Bomen Michelson Series),
and 1H NMR (Varian Gemini 300) spectroscopy and elemental
analysis (Perkin-Elmer, Model 240 Elemental Analyzer). Melt-
ing points were determined using a Fisher-Johns melting point
Therefore, the collected fluorescence spectra were normalized
with the above factor to get the fluorescence excitation spectra.
The obtained two-photon excitation spectra in terms of a plot
of the normalized fluorescence efficiency are shown in Figure

7518 J. Phys. Chem. A, Vol. 106, No. 33, 2002
Chung et al.
apparatus o perating at a rate of 30 °C/min. The purity of
products was also deterimined by a combination of TLC on
silica gel plates (Merck, silica gel 60 F254) with UV lamp (254
or 365 nm) and a visualization reagent. All optical characteriza-
tions were conducted using solutions of polymers in 1,1,2,2-
tetrachoroethane. The UV-vis absorption and fluorescence
studies were conducted with 10-4 M solutions in the concentra-
tion of the repeating unit using a Shimadzu, UV-3101PC
spectrophotometer and a Shimadzu, RF5000U spectrofluorim-
eter. Relative molecular weights and molecular weight distribu-
tions of polymers were determined by gel permeation chroma-
tography (GPC) using a Waters 510 instrument equipped with
a 410 differential refractometer and photodiode array UV
detector. Toluene was employed as eluent with a flow rate of
1.0 mL/min. Styragel was the column material and a calibration
plot constructed with polystyrene standards. The thermal analysis
of the polymers was performed in N2 atm on a differential
scanning calorimeter (Metter DSC 821e) and also on a ther-
mogravimetric analyzer (Metter TGA 50) at a heating rate of
10 °C /min.
was separated and toluene was removed by distillation, leaving
behind a solid residue. The solid residue was thoroughly washed
with methanol. The product was recrystallized from a mixture
of methylene chloride and n-hexane (v/v ) 1/5). The yield was
14.5 g (90%; mp ) 176-178 °C). Anal. Calcd. (%) for C28H22:
1
C, 93.81; H, 6.19. Found: C, 93.62; H, 6.35. H NMR (300
MHz, CDCl3, ppm): 1.98 (s, 3H), 2.48 (s, 3H), 7.3-7.85 (m,
16H). FT-IR (KBr, cm-1): 3070 (aromatic C-H stretching),
2930 (aliphatic C-H stretching), 1450, 1610 (aromatic CdC
stretching).
2-(9-Phenylanthracen-10-yl)-1,4-bis(bromomethyl)benzene (2).
2-(9-Phenyl- anthracen-10-yl)-p-xylene (1) (7 g; 19.5 mmol),
N-bromosuccinimide (7.6 g; 43 mmol), and benzoyl peroxide
(0.47 g; 1.95 mmol) were dissolved in 150 mL of CCl4. The
mixture was refluxed under a N2 atmosphere for 4 h. The
succinimide formed was removed by filteration, and the CCl4
in the filtrate was evaporated using a rotatory evaporator. The
residue was recrystallized from a mixture of n-hexane and ethyl
acetate (v/v ) 5/1). The yield was 4.2 g (42%; mp ) 152 °C).
Anal. Calcd. (%) for C28H20Br2: C, 65.16; H, 3.90; Br, 30.95.
Found: C, 65.62; H, 3.94. 1H NMR (300 MHz, CDCl3, ppm):
4.16 (s, 2H), 4.55 (s, 2H), 7.35-7.8 (m, 16H). FT-IR (KBr,
cm-1): 3024 (aromatic C-H stretching), 2930 (aliphatic C-H
stretching), 1450, 1610 (aromatic CdC stretching), 1162 (C-
Br bending), 704 (C-Br stretching).
2,5-Dimethyl-4-(9-phenylanthracen-10-yl)anisole (3). 2,5-
Dimethyl-4-bromo- anisole (19 g; 90 mmol) was dissolved in
150 mL of dry THF containing 2.2 g (91 mmol) of magnesium.
The reaction mixture was refluxed under nitrogen atmosphere
for 4h. The reaction mixture was cooled to 0 °C, and to this
solution was added 26.7 g (80 mmol) of 9-bromo-10-phenyl-
anthracene in 100 mL of dry THF and 0.2 g (3 mmol) of NiCl2‚
2PPh3 catalyst in ice bath. The reaction mixture was heated to
60 °C under N2 atmosphere and held at that temperature for 12
h. The mixture was cooled to room temperature and poured
into 500 mL of 1 M HCl solution. After extraction with ethyl
acetate, the solvent was removed by distillation. The residue
was purified by column (silica gel) chromatography using
n-hexane as eluent. The yield was 22 g (72%; mp ) 198-200
°C). Anal. Calcd. (%) for C29H24O: C, 89.66; H, 6.23. Found:
C, 89.72; H, 6.22. 1H NMR (300 MHz, CDCl3, ppm): 1.91 (s,
3H), 2.27 (s, 3H), 3.97(s, 3H), 6.92(s, H), 7.06 (s, H), 7.30-
7.68 (m, 13H). FT-IR (KBr, cm-1): 3061 (aromatic C-H
stretching), 2927 (aliphatic C-H stretching), 1609, 1502
(aromatic CdC stretching), 1247, 1020 (C-O-C stretching).
2,5-Dimethyl-4-(9-phenylanthracen-10-yl)phenol (4). Hydro-
bromic acid (48%, 40 mL) was added slowly with stirring to
solution of compound (3) (16 g; 40 mmol) dissolved in 300
mL of a mixture of acetic acid and trifluoroacetic acid (v/v )
5/1). The reaction mixture was refluxed for 3 days under a
nitrogen atmosphere during which period the whole mixture
became homogeneous. After the mixture was cooled to room
temperature, the precipitate formed was collected on a filter and
washed thoroughly with a mixture of distilled water and ethanol
(v/v ) 1/1). The product was recrystallized from a mixture of
n-hexane and ethyl acetate (v/v ) 9/1). The yield was 12 g
(84%; mp ) 193 °C). Anal. Calcd. (%) for C28H22O: C, 89.81;
H, 5.92. Found: C, 89.94; H, 5.92. 1H NMR (300 MHz, CDCl3,
ppm): 1.85 (s, 3H), 2.31 (s, 3H), 4.89(s, H), 6.87 (s, H), 7.04
(s, H), 7.30-7.62 (m, 13H). FT-IR (KBr, cm-1): 3700-3100
(O-H stretching), 3058 (armatic C-H stretching), 2961 (ali-
phatic C-H stretching), 1612, 1507 (aromatic CdC stretching).
Two-Photon Fluorescence Measurements. A Ti:sapphire
laser (Tsunami from Spectra Physics) operating at 84 MHz
repetition rate and 80 fs pulse duration tuned to ∼800 nm was
used as the excitation source. The laser beam at an average
power of 500 mW was focused on to the sample using an 5 cm
focal length lens. The two-photon excited emission spectra was
collected using a fiber-coupled spectrometer (Holospec from
Kaiser Optical Systems, Inc.) attached with a CCD camera
(Princeton Instruments).
Solution Characterization of Two-Photon Absorption
Properties. Two-photon absorption cross section values of the
three polymers in 2 × 10-2 M solution were measured with
both nanosecond (810 nm) and femtosecond (796 nm) pulses.
We have used direct nonlinear transmission measurement at
∼810 nm with ∼8 ns pulses from a Nd:YAG pumped dye laser
at the intensity levels of several hundreds of megawatt per square
centimeter. The experimental setup and data processing proce-
dure are basically the same as those described in previous
publications 46. Femtosecond Z-scan measurements were
performed using the same solutions as for the nanosecond
nonlinear transmission measurements. The Z-scan apparatus
utilizing femtosecond pulses and the arrangement of the pump-
probe experiment is described elsewhere (ref 43). The data
analysis follows the same method as well. The laser system
based on Ti:sapphire oscillator generated train (93 MHz) of 80
fs pulses at 796 nm central wavelength. Selected pulses were
amplified in a Ti:sapphire regenerative amplifier using a chirped
pulse amplification scheme. The final pulse energy was about
200 µJ and only a small fraction of that energy (below 1 µJ)
was used in experiments at the wavelength of 796 nm. The pulse
length of amplified pulses was checked with an autocorrelator
and typically was about 131 fs.
Materials. All the compounds were purchased from Aldrich,
TCI, or Fluka chemicals. Tetrahydrofuran (THF), N-bromosuc-
cinimide (NBS), benzoyl peroxide (BPO), and 1,1,2,2-tetra-
chloroethane were purified by the method in ref 48. All other
compounds were used as received.
Synthesis. 2-(9-Phenylanthracen-10-yl)-p-xylene (1). 9-Bromo-
10-phenylanthracene (15 g; 45 mmol) was dissolved in 150 mL
of toulene containing 1.73 g of the tetrakis(triphenylphosphine)-
palladium (0) catalyst. To this solution was added 7.84 g (58.5
mmol) of 2,5-dimethylphenyl boronic acid dissolved in 25 mL
of ethanol and 45 mL of 2 M Na2CO3 solution. The mixture
was refluxed for 48 h with vigorous stirring. The toluene layer
2-Ethylhexyloxy-4-(9-phenylanthracen-10-yl)-2,5-dimethyl-
benzene (5). Compound 4 (15 g; 43 mmol), 10 g (51.6 mmol)

Branched Poly(p-phenylenevinylene) Structures
J. Phys. Chem. A, Vol. 106, No. 33, 2002 7519
of 2-ethylhexyl bromide, and 2.8 g (51.6 mmol) of KOH were
dissolved in 150 mL of ethanol. The mixture was refluxed for
24 h under a nitrogen atmosphere. The insoluble was removed
by filtration. Solvent in the filtrate was removed by distillation
using a rotary evaporator. The crude product was recrystallized
from ethanol. The yield was 16 g (82%; mp ) 139 °C). Anal.
Calcd. (%) for C36H38O: C, 88.84; H, 7.87. Found: C, 88.67;
H, 7.89. 1H NMR (300 MHz, CDCl3, ppm): 0.94-1.05 (m,
6H), 1.41 (m, 4H), 1.51-1.68(m, 4H), 1.84 (m, H), 1.90 (s,
3H), 2.28 (s, 3H), 4.01 (d, 2H), 6.90 (s, H), 7.05 (s, H), 7.29-
7.71(m, 13H). FT-IR (KBr, cm-1): 3062 (aromatic C-H
stretching), 2927 (aliphatic C-H stretching), 1606, 1502
(aromatic CdC stretching), 1247, 1020 (C-O-C stretching).
2-Ethylhexyloxy-5-(9-phenylanthracen-10-yl)-1,4-bis(bromo-
methyl)benzene (6). Compound 5 (5 g; 11 mmol), N-bromo-
succinimide (4.5 g; 25 mmol), and benzoyl peroxide (0.27 g; 1
mmol) were dissolved in 150 mL of CCl4. The mixture was
refluxed under a N2 atmosphere for 4 h. The succinimide formed
was removed by filtration, and the CCl4 in the filtrate was
evaporated using a rotatory evaporator. The residue was purified
by column (silica gel) chromatography and a mixture of
n-hexane and ethyl acetate (v/v ) 5/1). The yield was 2.8 g
(42%; mp ) 129 °C). Anal. Calcd. (%) for C36H36Br2O: C,
67.27; H, 5.64. Found: C, 67.31; H, 5.59. 1H NMR (300 MHz,
CDCl3, ppm): 0.70-1.05 (m, 6H), 1.42 (m, 4H), 1.52-1.74
(m, 4H), 1.89 (m, H), 4.13 (s, 2H), 4.21 (d, 2H), 4.62 (s, 2H),
7.20-7.80 (m, 15H). FT-IR (KBr, cm-1): 3058 (aromatic C-H
stretching), 2921,2857 (aliphatic C-H stretching), 1604, 1507
(aromatic CdC stretching), 1210, 1018 (C-O-C stretching),
696 (C-Br stretching).
substituents as described by Gilch and Wheelwright.37 These
polymers were highly soluble in common organic solvents.
These PPV derivatives exhibited strong two-photon pumped
up-conversion emission when excited by both nanosecond and
femtosecond laser pulses. We observed that P-2 having both
the phenylanthracene and 2-ethylhexyloxy substiuent as pendent
groups revealed the best performance as a two-photon absorber.
Acknowledgment. J.-I. Jin wishes to express his thanks to
the Korea Science and Engineering Foundation through Center
for Electro- and Photo-Responsive Molecules, Korea University.
The work at Buffalo was supported by the Directorate of
Chemistry and Life Sciences of the Air Force Office of Scientific
Research through Grant F496200010064
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