Aromatic polyimides containing main-chain diphenylaminofluorene- benzothiazole motif: Fluorescence quenching, two-photon properties, and exciplex formation in a solid state

Article abstract of DOI:10.1021/ma201407g

A new bis(4-aminophenoxy) monomer containing a two-photon absorbing (2PA) and fluorescent diphenylaminodiethylfluorene-benzothiazole chromophore (AF240) was synthesized and used as a comonomer in preparing a series of heat-resistant, 2PA-active polyimides. Highly organo-soluble, these polymers easily formed optically clear, but nonfluorescent, films that contained covalently bound, AF240-like dye in concentrations up to ～1.0 M. For comparison purposes, a model compound (AF349) with phthalimido end-caps was also prepared. From the fluorescence data, the presence of phthalimido moieties in both the model compound and the polymers drastically quenched the fluorescence emission from the 2PA moieties of these materials in solution as well as in solid state. Their intrinsic (at 780 nm and with 160 fs pulses) and effective (at 800 nm and 8 ns pulses) two-photon properties in THF solutions (0.02 M), and film samples (40-65 μm thick) were determined by a direct nonlinear transmission technique. Thus, their intrinsic (σ⁽²⁾_fs) and effective cross-section (σ⁽²⁾_ns) values in solution are 7.7-37 and 1070-6000 GM per repeat unit, respectively. Surprisingly for the film samples, while the σ⁽²⁾_fs values agree with those determined in solutions, the σ⁽²⁾_ns values are 7-9 times larger, depending on the dye content. On the basis of the results of photophysical characterization of these polymers, we propose that on a nanosecond or slower time scale a stabilized/confined excited state complex is formed in an intrachain mode between an AF240-like moiety and a phthalimido unit to reasonably account for the fluorescence quenching and enhancement in the effective two-photon responses in film form.

Full text of DOI:10.1021/ma201407g

ARTICLE
pubs.acs.org/Macromolecules
Aromatic Polyimides Containing Main-Chain
DiphenylaminofluoreneꢀBenzothiazole Motif: Fluorescence
Quenching, Two-Photon Properties, and Exciplex Formation
in a Solid State
Matthew J. Dalton,†,‡ Ramamurthi Kannan,†,§ Joy E. Haley,† Guang S. He,|| Daniel G. McLean,†,^
Thomas M. Cooper,† Paras N. Prasad,|| and Loon-Seng Tan*,†
†Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/RX, Wright-Patterson AFB, Ohio 45433-7750,
United States
‡General Dynamics Information Technology, 5100 Springfield Pike, Dayton, Ohio 45431, United States
§AT&T Government Solutions, Inc., 2940 Presidential Drive, Suite 390, Fairborn, Ohio 45324, United States
Institute for Lasers, Photonics and Biophotonics, State University of New York at Buffalo, Buffalo, New York 14260-3000, United States
^Science Applications International Corporation, 4031 Colonel Glenn Highway, Beavercreek, Ohio 45431, United States
S Supporting Information
b
ABSTRACT:
A new bis(4-aminophenoxy) monomer containing a two-photon absorbing (2PA) and fluorescent diphenylaminodiethylfluoreneꢀ
benzothiazole chromophore (AF240) was synthesized and used as a comonomer in preparing a series of heat-resistant, 2PA-
active polyimides. Highly organo-soluble, these polymers easily formed optically clear, but nonfluorescent, films that contained
covalently bound, AF240-like dye in concentrations up to ∼1.0 M. For comparison purposes, a model compound (AF349) with
phthalimido end-caps was also prepared. From the fluorescence data, the presence of phthalimido moieties in both the model
compound and the polymers drastically quenched the fluorescence emission from the 2PA moieties of these materials in solution as
well as in solid state. Their intrinsic (at 780 nm and with 160 fs pulses) and effective (at 800 nm and 8 ns pulses) two-photon
properties in THF solutions (0.02 M), and film samples (40ꢀ65 μm thick) were determined by a direct nonlinear transmission
technique. Thus, their intrinsic (σ(2)fs) and effective cross-section (σ(2)ns) values in solution are 7.7ꢀ37 and 1070ꢀ6000 GM per
repeat unit, respectively. Surprisingly for the film samples, while the σ(2)fs values agree with those determined in solutions, the σ(2)
ns
values are 7ꢀ9 times larger, depending on the dye content. On the basis of the results of photophysical characterization of these
polymers, we propose that on a nanosecond or slower time scale a stabilized/confined excited state complex is formed in an
intrachain mode between an AF240-like moiety and a phthalimido unit to reasonably account for the fluorescence quenching and
enhancement in the effective two-photon responses in film form.
’ INTRODUCTION
that witnessed the development of χ(3) conjugated polymers and
organic materials for all-optical computing and switching appli-
cations, 2PA and related higher-order absorptive processes
Simultaneous two-photon absorption (2PA) is a well-known
phenomenon in nonlinear optics (NLO) since G€oppert-Mayer’s
ground-breaking theoretical work 80 years ago,1 followed by
experimental verification by Kaiser and Garrett 30 years later.2 It
is related to the imaginary component of third-order suscept-
ibility, χ(3), of a nonlinear optical medium. During the early years
Received: June 21, 2011
Revised:
August 1, 2011
Published: August 25, 2011
r
2011 American Chemical Society
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(generically known as multiphoton absorption) along with
scattering processes were considered to be problematic because
of the potential loss of optical information during transmission
over distance.3 However, in comparison with one-photon absorp-
tion (1PA) and based on the fact that 2PA scales nonlinearly with
the squared intensity of the incident laser beam, also recognized
were several useful features: (a) near-infrared-to-visible upcon-
verted emission; (b) deeper penetration of incident NIR light
(into tissue samples, for example); (c) highly localized excitation
allowing for precise spatial control of in situ photochemical or
photophysical events; (d) amplified fluorescence by surface
plasmonic resonance process.4,5 Ostensibly, efficient two-photon
materials are pivotal to the realization of many practical applica-
tions that capitalize on these features. Thus, during the past two
decades, advances have been made by leaps and bounds in the
design and synthesis of two-photon absorbers with very large
cross-section (σ(2)) values6 that assist in expanding the range of
2PA-based application areas in photonics, nanophotonics, and
biophotonics, including but not limited to bioimaging diagnos-
tics, chemical sensing, high-density data storage, microfabrication,7
nonlinear optical materials, photodynamic therapy, photoacti-
vated release of NO,8 etc.
Although two-photon dye synthesis has been the main focus
during this developmental period, the synthesis of two-photon
active polymers based on specifically designed monomers, with
the exception of dendritic polymers,9,10 has been receiving much
less attention.11 Among high performance heat-resistant polymers,12
aromatic polyimides clearly stand out with a balance of polymer
properties that are structurally tailored with relative ease via
specific monomer designs. In addition, large-area processability
via the poly(amic acid) precursor route is a unique and attractive
feature. With such advantages that are important to scalability
and cost considerations, polyimides are frequently found in a
broad spectrum of utility,13 ranging from lightweight, heat-resis-
tant structural composites, adhesives, and antenna membranes
for the aerospace and space sectors to gas separation membranes14
and dielectric films and laminates, printed circuit boards,
photoresists,15 and orientation layers for electronic and photonic
applications.16 However, in the field of nonlinear optics, poly-
imides have been largely investigated with respect to their
second-order NLO properties for electro-optical applica-
tions17,18 and, to a lesser extent, their third-order NLO properties
for optical data storage.19ꢀ21 Thus, from the standpoint of devel-
oping multifunctional materials, aromatic polyimides are an
attractive polymer platform to impart two-photon functionality.
It is well-known that excited-state absorption (ESA) plays a
critical role in enhancing the properties of two-photon materials
in the nanosecond regime.22,23 However, a self-quenching pro-
cess within a system that depletes the excited state population
could have a negative effect on the enhanced optical nonlinearity
of the material. On the other hand, if the self-quenching process
leads to a new long-lived species, then an enhancement in the
nonlinearity may be observed. Interestingly, while the low fluo-
rescence and self-quenching properties of aromatic polyimides24
are well established, the excited-state quenching phenomenon of
aromatic imide moieties on the optical properties of 2PA-active
polymers, to our knowledge, has not been reported.
polyimides are unique in the sense that their films are both 2PA-
active and nonfluorescent. Since the studies of organic and poly-
meric two-photon chromophores in a solid-state environment
are rather sporadic, a highly organo-soluble 2PA-active polymer
allows an investigation to compare and understand the funda-
mental relationships between polymer structureꢀproperty and
two-photon processes in solution and solid state as well as the
correlations between one-photon and two-photon properties
influenced by these environments.
’ EXPERIMENTAL SECTION
Materials. All chemicals were reagent grade, purchased from
Aldrich, and used as received unless otherwise noted. 7-(Benzothiazol-
2-yl)-9,9-diethyl-2-bromofluorene was synthesized according to the
reported procedure.25 2,2-Bis(phthalic anhydride)-1,1,1,3,3,3-hexa-
fluoroisopropane (6-FDA; 99%) was purified by sublimation and stored
in a desiccator before use; 1,3-bis(3-aminophenoxy)benzene (APB) was
(99% min) was used as received. Both monomers were purchased from
Chriskev Co., Inc.
Instrumentation. NMR spectra were obtained using a Bruker
Avance 400 MHz spectrometer, and chemical shifts were referenced to
the solvent residual peak. Elemental analyses and mass spectral analyses
were performed at the Systems Support Branch, Materials & Manufacturing
Directorate, Air Force Research Lab, Dayton, OH. Melting points were
obtained on either a Buchi-B545 melting point apparatus or a MelTemp
apparatus. Polymer molecular weights were determined by GPC using a
PL-Gel mixed-C column and RI detector with THF at a flow rate of
1.0 mL/min. Conventional calibration against polystyrene standards was
used. Intrinsic viscosities [η] were measured using a Cannon-Ubbelohde
dilution viscometer with an initial concentration of ∼0.5 g/dL in
NMP (1 wt % LiBr) at 30 ( 0.1 ꢀC. Differential scanning calorimetry
(DSC) analyses were performedin nitrogen at a heating rate of 10 ꢀC/min
using a TA Instruments Q1000 differential scanning calorimeter.
Thermogravimetric analyses (TGA) were obtained in nitrogen and air
atmospheres at a heating rate of 10 ꢀC/min using a TA Hi-Res TGA
2950 thermogravimetric analyzer.
N,N-Di(3-methoxyphenyl)-7-(benzothiazol-2-yl)-9,9-di-
ethylfluoren-2-amine (2). A mixture of 7-(benzothiazol-2-yl)-
9,9-diethyl-2-bromofluorene (10.85 g, 25 mmol), 3,30-dimethoxydiphe-
nylamine (6.87 g, 30 mmol), and toluene (100 mL) was azeotroped dry
under nitrogen and cooled. Bis(dibenzylidene acetone)palladium(0)
(0.28 g, 0.49 mmol), bis(diphenylphosphino)ferrocene (0.25 g, 0.45 mmol),
and sodium tert-butoxide (3.5 g, 36.4 mmol) were then added, and the
mixture was heated to 100 ꢀC. After 24 h at 100 ꢀC, the mixture was
cooled, diluted with toluene, and filtered. The filtrate was washed with
water, dried, and concentrated on a rotary evaporator. The residue was
chromatographed over silica gel. Elution with tolueneꢀheptane (3:1)
mixture gave the product, which was recrystallized from a mixture of
1
tolueneꢀheptane; mp 178ꢀ179.5 ꢀC, 11.13 g (76% yield). H NMR
(CDCl3) δ ppm: 0.35ꢀ0.41 (t, 6H), 1.91ꢀ2.14 (m, 4H), 3.69 (s, 6H),
6.54ꢀ6.74, 7.05ꢀ7.68, 7.84ꢀ8.10 (m, 18H). 13C NMR (CDCl3)
δ ppm: 8.61, 32.66, 55.18, 56.44 (sp3 C), 108.62, 109.77, 116.66,
119.16, 119.42, 121.00, 121.44, 121.52, 122.94, 123.77, 124.95, 126.28,
127.28, 129.82, 131.55, 134.91, 135.61, 144.48, 147.84, 148.94, 150.67,
151.99, 154.24, 160.46, 168.81 (sp2C). Anal. Calcd for C38H34N2O2S:
C, 78.33%; H, 5.88%; N, 4.81%; S, 5.49%. Found: C, 78.26%; H, 5.96%;
N, 4.68%; S 5.47%.
N,N-Di(3-hydroxyphenyl)-7-(benzothiazol-2-yl)-9,9-di-
ethylfluoren-2-amine (3) [AF240-(OH)2]. A mixture of N,N-
di(3-methoxyphenyl)-7-(benzothiazol-2-yl)-9,9-diethylfluoren-2-amine
(1 g) and pyridine hydrochloride (10 g) was heated at 200 ꢀC under a
nitrogen atmosphere in an oil bath for 10 h. The reaction mixture was
allowed to cool to room temperature and slurried in water, and the red
Here, we report the results on the synthesis and optical
characterization of a diamino-monomer based on the structural
motif of a diphenylaminofluorene-benzothiazole-based chromo-
phore (AF240)25 and the resulting aromatic polyimides that are
related to a well-known low-color polyimide (CP2).26ꢀ28 These
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solids were collected on a fritted filter funnel. The crude (protonated)
red product was subsequently slurried in dilute ammonium hydroxide to
get the greenish-yellow solid product; 1.13 g, mp 314ꢀ316 ꢀC. EIMS:
m/z 554 (M+). Anal. Calcd for C36H30N2O2S: C, 77.95%; H, 5.45%; N,
5.05%; S, 5.78%. Found: C, 77.74%; H, 5.39%; N, 4.83%; S, 5.78%.
7.48ꢀ7.55 (m, 1H), 7.35ꢀ7.42 (m, 1H), 7.05ꢀ7.18 (m, 4H),
6.52ꢀ6.59 (m, 2H), 3.5ꢀ3.6 (br. s, 4H), 1.95ꢀ2.15 (m, 4H),
0.34 (t, J = 7 Hz, 6H). 13C NMR (100 MHz, CDCl3) δ 168.97,
159.73, 154.38, 152.23, 150.91, 149.04, 148.61, 147.69, 144.56,
142.72, 135.98, 135.07, 131.71, 129.98, 127.39, 126.42, 125.11,
124.19, 123.08, 121.68, 121.59, 121.19, 121.00, 119.60, 119.54,
117.90, 116.31, 113.33, 111.70, 56.57, 32.77, 8.73; MS (m/z):
736 (M+). Anal. Calcd for C48H40N4O2S: C, 78.23; H, 5.47; N,
7.60; S, 4.35. Found: C, 78.01; H, 5.60; N, 7.62; S, 4.31.
7-(Benzo[d]thiazol-2-yl)-9,9-diethyl-N,N-bis(3-(4-nitro-
phenoxy)phenyl)-9H-fluoren-2-amine (4a) [AF240(PhNO2)2].
A mixture of AF240-(OH)2 (2.94 g, 5.3 mmol), 4-nitrofluorobenzene
(2.56 g, 20 mmol), potassium carbonate (2.29 g, 16.6 mmol), and N,N-
dimethylacetamide (DMAc; 27 mL) was heated at 97 ꢀC for 5 h, allowed
to cooled to room temperature, and poured into water. The separated
solids (4.38 g) were transferred to a column of silica gel and eluted with
toluene to get the product; 4.12 g (97% yield), mp 200ꢀ201 ꢀC.
Recrystallization from tolueneꢀheptane provided a purer sample with
mp 202ꢀ203 ꢀC. 1H NMR (400 MHz, CDCl3) δ 8.15ꢀ8.22 (m, 4H),
8.07ꢀ8.12 (m, 2H), 8.02 (dd, J1 = 7.9 Hz, J2 = 1.6 Hz, 1H), 7.91 (d, J =
7.9 Hz, 1H), 7.72 (d, J = 7.9 Hz, 1H), 7.68 (d, J = 8.1 Hz, 1H), 7.45ꢀ7.55
(m, 1H), 7.36ꢀ7.42 (m, 1H), 7.31 (t, J = 8.1 Hz, 2H), 7.12ꢀ7.2
(m, 2H), 6.98ꢀ7.05 (m, 6H), 6.89 (t, J = 2.2 Hz, 2H), 6.72ꢀ6.77
(m, 2H), 1.88ꢀ2.15 (m, 4H), 0.31 (t, J = 7 Hz, 6H). 13C NMR (100
MHz, CDCl3) δ 168.52, 162.96, 155.53, 154.15, 152.45, 150.66, 149.23,
146.49, 143.77, 142.63, 137.09, 134.90, 132.07, 130.83, 127.32, 126.31,
125.90, 125.05, 124.54, 122.97, 121.55, 121.47, 121.40, 120.19, 120.09,
119.73, 117.00, 115.47, 114.66, 56.49, 32.61, 8.55. MS (m/z): 796 (M+).
Anal. Calcd for C48H36N4O6S: C, 72.35; H, 4.55; N, 7.03; S, 4.02.
Found: C, 72.15; H, 4.87; N, 6.87; S, 3.95.
Model Compound 5 (AF349); 2,20-(((((7-(Benzo[d]thiazol-
2-yl)-9,9-diethyl-9H-fluoren-2-yl)azanediyl)bis(3,1-phenylene))-
bis(oxy)bis(4,1-phenylene))diisoindoline-1,3-dione. To a
25 mL flame-dried vial equipped with a magnetic stir bar was added 4b
(0.425 g, 0.577 mmol), and the vial was sealed with a Teflon-coated
septum cap. Anhydrous DMAc (5 mL) was added by a hypodermic
syringe to give a fluorescent solution. Phthalic anhydride (0.180 g, 1.21
mmol) was added under a stream of argon, and the vial was sealed. After
40 h, acetic anhydride (0.22 mL) and pyridine (0.19 mL) were added,
and the mixture was stirred for another 24 h. The product was
precipitated upon adding the reaction mixture to 50/50 methanol/water
(50 mL) and collected after filtration. The product was then slurried in
isopropanol/methanol, filtered, and finally dried in vacuo at 120 ꢀC
1
overnight to give 0.53 g (92%) of the product. H NMR (400 MHz,
CDCl3) δ 8.13ꢀ8.04 (m, 2H), 8.00 (dd, J = 7.9, 1.3 Hz, 1H), 7.98ꢀ7.86
(m, 5H), 7.82ꢀ7.74 (m, 4H), 7.72(d, J = 7.9 Hz, 1H), 7.67 (d, J = 8.1 Hz,
1H), 7.50 (dd, J = 11.3, 4.0 Hz, 1H), 7.46ꢀ7.34 (m, 5H), 7.34ꢀ7.22
(m, 2H), 7.22ꢀ7.06 (m, 6H), 7.01ꢀ6.87 (m, 4H), 6.72 (dd, J =
8.1, 1.7 Hz, 2H), 2.11 (dq, J = 14.1, 7.1 Hz, 2H), 1.97 (dq, J = 14.5,
7.2 Hz, 2H), 0.35 (t, J = 7.3 Hz, 6H). 13C NMR (100 MHz, CDCl3)
δ 168.93, 167.46, 157.38, 157.07, 154.35, 152.42, 150.94, 149.27, 147.30,
144.40, 136.48, 135.06, 134.50, 131.84, 130.49, 128.09, 127.38, 126.47,
126.40, 125.10, 124.32, 123.86, 123.07, 121.68, 121.59, 121.42, 119.82,
119.75, 119.41, 118.78, 115.32, 113.98, 56.63, 32.80, 8.74. EI-MS (m/z):
997 (M+). Anal. Calcd for C64H44N4O6S: C, 77.09; H, 4.45; N, 5.62; S,
3.22; Found: C, 76.83; H, 4.47; N, 5.47; S, 3.14.
Homopolyimide Synthesis (7e). To a 25 mL flame-dried vial
equipped with a magnetic stir bar was added 4b (1.4738 g, 2.00 mmol),
and the vial was sealed with a Teflon-coated septum cap. Anhydrous
DMAc (15 mL) was added by a hypodermic syringe to give a fluorescent
solution. 6-FDA (0.8885 g, 2.00 mmol) was then added under a stream
of argon, and the vial was sealed under argon. After 40 h, acetic anhydride
(0.75 mL) and pyridine (0.65 mL) were added, and the mixture was
stirred for 24 h. The viscous solution was precipitated into 50/50
methanol/water (200 mL), and the yellow, fibrous solid was filtered and
dried at 120 ꢀC overnight in a vacuum oven. The solid was dissolved in
chloroform (25 mL) and filtered through a 0.45 μm PTFE membrane
directly into methanol (200 mL). The purified polymer was dried in
vacuo at 120 ꢀC overnight and further at 250 ꢀC for 2 h to yield 1.99 g
(87%). 1H NMR (400 MHz, CDCl3) δ 7.8ꢀ8.1 (m, 10H), 7.7 (d, J =
8 Hz, 1H), 7.65 (d, J = 8.1 Hz, 1H), 7.45ꢀ7.52 (m, 1H), 7.32ꢀ7.40
(m, 5H), 7.22ꢀ7.30 (m, 2H), 7.08ꢀ7.20 (m, 6H), 6.88ꢀ6.98 (m, 4H),
6.68ꢀ6.74 (m, 2H), 1.90ꢀ2.20 (m, 4H), 0.34 (t, J = 7 Hz, 6H).
13C NMR (100 MHz, CDCl3) δ 168.75, 166.17, 166.01, 157.35, 157.04,
154.19, 152.32, 150.79, 149.17, 147.08, 144.20, 139.10, 136.45, 135.90,
134.90, 132.62, 132.33, 131.74, 130.40, 127.93, 127.27, 126.29, 125.80,
125.29, 125.00, 124.22, 124.11, 122.93, 121.54, 121.46, 121.31, 119.73,
119.63, 119.40, 118.62, 115.29, 113.97, 65.50, 56.50, 32.66, 8.60.
Representative Copolyimide Synthesis (7d). To a 25 mL
flame-dried vial equipped with a magnetic stir bar was added 4b (0.7368 g,
1.00 mmol) and 1,3-bis(3-aminophenoxy)benzene (0.2923 g, 1.00 mmol),
and the vial was sealed with a Teflon-coated septum cap. Anhydrous
DMAc (13 mL) was added by syringe to give a fluorescent solution.
6-FDA (0.8885 g, 2.00 mmol) was added under a stream of argon, and
the vial was sealed. After 27 h, acetic anhydride (0.75 mL) and pyridine
N,N-Bis(3-(4-aminophenoxy)phenyl)-7-(benzo[d]thiazol-
2-yl)-9,9-diethyl-9H-fluoren-2-amine (4b) [AF240(PhNH2)2].
To a solution of AF240(PhNO2)2 (13.68 g, 17.2 mmol) in 300 mL of 1/
1 tetrahydrofuran/ethanol under argon was added 10% palladium/carbon
(500 mg). The mixture was heated to 60 ꢀC, and hydrazine hydrate
(12.7 mL, ∼206 mmol) was added slowly over 40 min via an addition
funnel. The reaction mixture became dark, fluorescent green and, after
8 h, was cooled to room temperature and poured into 1.5 L of distilled
water. After a few hours of stirring to evaporate some THF in a well-
ventilated hood, a fine yellow solid was filtered and purified by column
chromatography eluting with 20ꢀ50% ethyl acetate/toluene. Three
byproducts generated in the reaction and separated by column chroma-
tography were identified by mass spectrometry as the following
compounds:
It is not clear how the byproduct m/z 645 was produced in the
reaction, and it was also particularly difficult to remove by column
chromatography. The product was further slurried in 100 mL of
hot EtOH, filtered at room temperature, and dried in vacuo at
70 ꢀC overnight to give 8.4 g (65% yield) of the desired diamine
product (1 spot on TLC); mp 170ꢀ171 ꢀC (in a separate DSC
experiment, a sharp melting endotherm was observed at 170 ꢀC,
which was preceded by a somewhat broad endotherm staring at
∼137 ꢀC when a sample was scanned under N2 and at 20 ꢀC/min).
1H NMR (400 MHz, CDCl3) δ 8.08 (m, 2H), 8.01 (dd, J1 = 8 Hz,
J2 = 1.6 Hz, 1H), 7.91 (d, J = 8 Hz, 1H), 7.70 (d, J = 8 Hz, 1H),
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Scheme 1. Syntheses of Diamine Monomer (4) and Model Compound (5)a
a Reagents and conditions: (i) Pd catalyst, tert-BuONa, PhMe; (ii) pyridine hydrochloride, 200 ꢀC; (iii) K2CO3/DMAc; (iv) NH2NH2.H2O, Pd/C,
EtOH/THF; (v) DMAc; Ac2O/pyridine. Inset shows chemical structure for model compound 6.
(0.65 mL) were added, and the mixture was stirred for 24 h. The viscous
solution was precipitated into 50/50 methanol/water (200 mL), and the
yellow, fibrous solid was filtered and dried at 120 ꢀC overnight in a
vacuum oven. The solid was dissolved in chloroform (25 mL) and
filtered through a 0.45 μm PTFE membrane directly into methanol
(200 mL). The purified polymer was dried in vacuo at 120 ꢀC overnight
and further at 250 ꢀC for 2 h to yield 1.60 g (87%). Anal. Calcd for
C52H30F6N3O6S0.5(repeat unit): C, 67.68; H, 3.28; N, 4.55; S, 1.74;
Found: C, 67.56; H, 3.31; N, 4.29; S, 1.68.
CP2 Polyimide Film Density Determination and Dye Con-
centration Calculation. Small pieces of CP2 polyimide films were
suspended individually in a mixture of carbon tetrachloride and ethanol
in a 10 mL graduated cylinder which had previously been tarred. The
total solvent volume was between 9.4 and 10 mL, and the films were
suspended around the 5 mL mark when the solvent was weighed. The
mass of the solution and the total volume were used to calculate a
density. The films did not swell in the solvent mixture.
One-Photon Absorption and Emission Characterization.
A Cary 500 spectrophotometer was used to obtain the ground-state
UV/vis absorption spectra. These spectra were then used to obtain molar
absorption coefficientsbyapplying Beer’s lawto particular wavelengthsfor
comparison across the samples. Ground-state spectra were run twice in
which each run had a new solution prepared. All techniques mentioned
herein were performed under air-saturated conditions.
The emission spectra were obtained from a Perkin-Elmer model LS
50B fluorometer. Each solution sample was excited at 355 nm with a
matched OD of 0.1 with a 1 cm path length. Fluorescence quantum
yields were determined by using the relative actinometry method.29 The
actinometer used is quinine sulfate, which has a known fluorescence
quantum yield of 0.546 in 1.0 N H2SO4.30
Emission measurements of the film samples were done at 400 nm using a
front-face geometry to eliminate any inner filter effects that might be caused
bythe high absorbance atthe excitingwavelength. Despite the large emission
observed in solution, weak emission was detected for these samples.
Time-correlated single photon counting (Edinburgh Instruments
OB920 spectrometer) was used to determine singlet excited-state life-
times. The fluorescence was excited using a 70 ps laser diode at 375 nm.
The emission was detected using a cooled microchannel plate PMT.
Data were analyzed using a deconvolution software package provided by
Edinburgh Instruments.
Two-Photon Cross-Section Determination. Since the linear
absorption peaks of all the samples are located below the 450 nm (see
Figure 4) spectral range, and the residual linear absorption influences are
entirely negligible around the 800 nm range, any measured intensity-
dependent nonlinear absorption in the latter wavelength range should be
Dye concentrations were calculated based on the following equation,
assuming the model compound (AF-349) structure as the “dye” frag-
ment (FW = 995 g/mol):
conc ½Mꢁ
!
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
g
1000 mL
L
mol
995 g=mol
¼ density
ð% dyeÞ
mL
repeat unit mol wt
1145 g=mol
The resulting density values and corresponding 2PA dye concentrations
for polyimide films 7aꢀ7e are given in Table S-1 of the Supporting
Information.
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Scheme 2. Poly(amic acid) Formation Followed by Chemical Imidization To Give Polymers 7aꢀea
a General scheme for the preparation of two-photon active polyimides and their poly(amic acid) precursors.
mainly ascribed to a two-photon absorption (2PA) process. In this work,
the 2PA coefficient (dependent on the concentration of absorbing
center) was directly measured using a nonlinear transmission (NLT)
method. Knowing the 2PA coefficient (β) and sample’s concentration,
the 2PA cross-section value of each absorbing center (molecule or repeat
unit) was finally extracted. For the 2PA measurement performed in the
femtosecond regime, the input pulsed Ti:sapphire laser beam of
∼780 nm wavelength, ∼160 fs pulse duration, and 1 kHz repetition
rate was focused into the center of the sample cuvette via an f = 20 cm
lens.31 In order to avoid the possible thermal-lensing effect or boiling of
the solvent due to light-absorption-induced heating, the laser beam was
passed through a beam chopper with an opening/blocking ratio of 1:10
before entering the focusing lens, so that the effective pulse repetition rate
(as well as the heating effect) was reduced by 10 times. For a fixed sample
position, the nonlinear transmission was measured as a function of laser
pulse intensity(orenergy) thatwas controlledbya variable attenuatorand
was varied from 0.1 to 1.1 μJ. On the basis of this type of nonlinear
transmission measurement, the 2PA coefficient was readily determined.
The same experimental setup and procedure were applied to perform
2PA measurements in the nanoseconds regime, where the input pulsed
laser beam of ∼800 nm wavelength, ∼8 ns pulse duration, and 10 Hz
repetition rate was from a tunable-dye laser pumped by a frequency-
doubled and Q-switched Nd:YAG laser device.32 However, in this case
the pulse energy was much greater (from 0.1 to 1.3 mJ).
has a relatively high effective (nanosecond) two-photon cross
section, its multistep synthesis25 has been optimized, it has been
subject to several theoretical and modeling studies,23,34,35 and it
should serve well to probe the subtle effects of polymer archi-
tecture on the 2PA responses. Thus, to incorporate it into an
aromatic diamine that could be conveniently synthesized and
polymerized, we chose to functionalize it at the 3,30-positions of
the diphenylamino donor, partly because of the commercial
availability of 3,30-dimethoxydiphenylamine. In addition, ether
spacers at the meta positions of the phenyl rings would provide
additional structural flexibility for improved solubility.
Thus, the 3,30-dihydroxy derivative of AF240, i.e., AF240-
(OH)2 (3 in Scheme 1), was prepared from a standard Pd-
catalyzed N-arylation of 3,30-dimethoxydiphenylamine with a pre-
viously reported intermediate, benzothiazoleꢀdiethylfluorenyl
bromide (1),25 followed by a double demethylation through the
agency of pyridinium chloride. The dinitro precursor, AF240-
(PhNO2)2, to the requisite diamine monomer was subsequently
synthesized from AF240(OH)2 and 4-nitrofluorobenzene via a
nucleophilic aromatic substitution in excellent yield (97%).
However, a subsequent catalytic reduction of the dinitro com-
pound to give the desired monomer AF240(PhNH2)2, 4b, using
standard Parr hydrogenator conditions (H2ꢀ10% Pd/C) proved
to be problematic as the solubility of AF240(PhNO2)2 in alcoholic
solvents was poor. The reaction proceeded very sluggishly in
THF/ethanol solvent to give a mixture of AF240(PhNH2)2,
monoreduced product, and an unidentified byproduct seen by
TLC analysis. Thus, we turned to transfer hydrogenation with
hydrazine hydrate, which proceeded more smoothly to generate
AF240(PhNH2)2, that was obtained after column chromatogra-
phy as a yellow solid in reasonable yield (62%). 1H and 13C NMR
spectroscopy, electron-impact mass spectroscopy, and elemental
AF350 was used as a calibration standard (0.01 M THF solution in
10 mm cuvette) for all NLT measurements. In this work, femtosecond
β value of 0.0306 cm/GW and nanosecond β value of 5.16 cm/GW were
obtained, which agree well with those reported previously.33
’ RESULTS AND DISCUSSION
Monomer and Model Compound Syntheses. Since a pre-
viously reported two-photon chromophore designated as AF240
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Figure 1. NMR spectra taken before (bottom) and after (top) chemical imidization of 90/10 copolymer (7b), with inset showing amic acid proton
ratios. The peaks due to the proton residues of pyridine-d5 solvent are indicated by X’s.
analysis were consistent with the expected structure for the
diamino monomer.
against the polystyrene standards. GPC results (Table 1) indi-
cated that the number-averaged MW (Mn) values vary between
99 and 40 kDa with polydispersity (PDI) range of 1.87ꢀ2.36,
corresponding to the degrees of polymerization (DP) range of
35ꢀ141. Polymers 7a, 7b, and 7c formed tough, creasable, and
optically transparent films from their chloroform or DMAc
solutions. The films of 7d and 7e were optically transparent,
but somewhat brittle due to the lower molecular weights. The
GPC results indicate that as the content of chromophore is
increased, the polymer molecular weight decreases. This is most
likelydue to thesensitivityofpolymer-growingprocesstothe mass
imbalance as the monomer 4b is an amorphous solid, which was
found to be very difficult to obtain in ultrahigh purity necessary for
exact stoichiometry in the polycondensation reaction.
The molecular weights of the polymer samples were further
corroborated using viscometry in NMP at 30 ( 0.1 ꢀC with 1
wt % LiBr. The intrinsicviscosity ([η]) values ranged from 1.31 to
0.70 dL/g (Table 2). We observed that some samples displayed
polyelectrolyte effects at dilute concentrations even with 1 wt %
LiBr. Therefore, [η] was estimated using the viable data points at
higher concentrations. The linear relationship between log([η])
and log(M) (based on MarkꢀHouwinkꢀSakurada (MHK)
equation, [η] = KMa; see Figure S-1) still holds, even though
the polymer composition is changing, which suggests that the
solution properties for the copolymers are very similar. The plot
gives an a value of 0.53, which corresponds to a flexible polymer
in a theta solvent.
In order to evaluate the contribution of polymer architecture as
well as the influence of phthalimide functionality on the one- and
two-photon properties of the subject polyimides, a model com-
pound (5 and designated as AF349 in Scheme 1) was synthesized
by end-capping AF240(PhNH2)2 with phthalic anhydride.
Polymer Synthesis. The subject CP2-based polyimide (AF349P)
and copolymers (AF349P-XXCP2) were prepared as depicted in
Scheme 2.36 Thus, the poly(amic acid) precursors were formed by
stirring the appropriate monomers 1,3-bis(3-aminophenoxy)benzene
(APB), and 6FDA in anhydrous DMAc for 24ꢀ36 h, followed by
chemical imidization with a mixture of acetic anhydride and pyridine
(4 equiv) at room temperature for 24 h. The AF240(NH2)2
comonomer (4b) content was varied between 0 and 100% to give
polyimides 7aꢀe in purified yields of 80ꢀ90%.
An attractive feature of these polyimides is their excellent
solubility in common organic solvents such as THF, DMAc,
DMF, NMP, and DMSO. Therefore, NMR spectroscopy is a
convenient method to follow the course of the imidization
reaction, i.e., the cyclo-dehydration of the amic acid group.
For example, in the 1H NMR spectrum as shown in Figure 1 for
AF349-10CP2 (7b), the amic acid (NꢀH) proton signals at
∼10.0 ppm are completely gone after chemical imidization. The
amic acid proton adjacent to Ar1, i.e., in the 2PA-active segment,
at 9.97 ppm was slightly upfield from TMS standard but clearly
separated from the amic acid proton near Ar2, i.e., in the “CP2”
segment, at 10.10 ppm. The integrations of these two protons
compare well with the molar feed ratio (4b:APB); see inset in
Figure 1. This result, along with the elemental analysis data
shown in Table 1, is a good indicator that the feed ratio is very
close to the final copolymer composition.
Thermal Properties. The polymers were characterized by
differential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA) in both nitrogen and air atmospheres, and their
glass transition (Tg) and decomposition temperatures are noted
in Table 1. Experimentally, the samples were heated to 300 ꢀC in
the DSC chamber in the first run and then cooled to ambient
temperature at 10 ꢀC/min under a steady nitrogen purge. Then
The molecular weights (MW) of resulting polymers were
characterized by size-exclusion chromatography (GPC) in THF
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Table 1. Elemental Analysis and Thermal Properties for 7aꢀe
elemental analyses
polymer
mol formula (mol wt)
37H18F6N2O6
C
H
N
S
Tga (ꢀC)
Tdb (5%) N2/air (ꢀC)
dyec conc (M)
0.00
7a
C
calc
63.44
63.12
64.49
64.38
66.25
65.85
67.68
67.56
70.27
70.48
2.59
2.74
2.76
2.80
3.05
3.19
3.28
3.31
3.70
3.89
4.00
3.87
4.14
3.94
4.37
4.22
4.55
4.29
4.89
4.74
N/A
208
527
516
522
500
513
502
506
498
500
500
700.57
found
calc
7b
7c
7d
7e
C40H20.4F6N2.2O6S0.1
745.03
0.43
0.60
1.15
1.19
1.74
1.68
2.80
2.80
211
217
223
241
0.17
0.44
0.63
0.99
found
calc
C46H25.2F6N2.6O6S0.3
833.95
found
calc
C52H30F6N3O6S0.5
922.87
found
calc
C67H42F6N4O6S
1145.17
found
a Inflection point in baseline of DSC at heating rate of 10 ꢀC/min in N2. b Decomposition temperature at 5 wt % loss from TGA (10 ꢀC/min) in N2
and air. c Dye (AF349) concentrations in polymer samples are calculated as described in the Experimental Section.
Table 2. Molecular Weight Characterization for Polyimides
7aꢀe
polymer
Mw (kDa)
Mn (kDa)
PDI
DP
[η] (dL/g)
7a
7b
7c
7d
7e
222
212
121
94
99
90
54
47
40
2.24
2.36
2.24
1.98
1.87
141
121
65
1.31
1.14
0.91
0.77
0.70
51
75
35
the samples were heated to 300 at 10 ꢀC/min in the second run.
The Tg’s were determined based on the midpoint of change in
slope on the second heating run. As would have been expected,
the bulky chromophoric (AF-349-like) units extending from the
polymer backbone make the segmental and long-range molecular
motions more difficult and, therefore, increase the Tg (Figure 2).
Indeed, the DSC results have confirmed that the Tg increases
monotonously from 208 to 241 ꢀC as the mole percentage of AF-
349 units varies from 0 to 100% per repeat unit in the polymer
compositions.
Figure 2. DSC thermograms for polyimides 7aꢀ7e.
According to TGA results (Figure 3), these polymers could be
considered to be very thermally and thermo-oxidatively stable
under short-term extreme conditions, as their decomposition
temperatures (5% weight loss; Td5%) are at or above 500 ꢀC in
both air and inert conditions.
Polymer Film Thickness & Density. For optical characteriza-
tion of the polymer film samples, the information on the film
thickness (equivalent to optical path length) and density is
required. The concentration of 2PA unit in a polymer film can
be calculated once the film density is determined experimentally.
Thus, the thickness of each tested film sample was measured by
using a micrometer caliper with an uncertainty of (2.5 μm. The
density of each polymer film37 was determined based on Davy’s
principle of hydrostatic suspension using a mixture of carbon
tetrachloride and ethanol as the suspension medium. The values
of 1.30ꢀ1.43 g/cm3 for 7e-7b (Table S-1, Supporting Infor-
mation) correspond to dye (structurally equivalent to AF349
model compound, 5) concentrations ranging from 0.17 to 0.99
M in the solid state. This is tantamount to 1ꢀ2 orders of
magnitude higher than what is achievable in dye-saturated
solutions or by doping an optically transparent, polymeric host
such as PMMA with AF240.
Figure 3. TGA traces of polyimides 7aꢀ7e in nitrogen and air.
One-Photon Absorption and Fluorescence Properties (Table 3).
The quantified UV/vis spectra in air-saturated THF of the model
compound 5, the homopolymer 7e, and the copolymers 7aꢀ7d are
shown in Figure 4. The molar absorption coefficient (εmax) of
the model compound 5 (AF349) was found to be 48 030 (
700 Mꢀ1 cmꢀ1 at 385 nm as opposed to 39 600 ( 300 Mꢀ1
cmꢀ1 at 384 nm for 7e (Figure 4). The UV/vis spectra for the
copolymers (7aꢀ7d) are similar to that of homopolymer 7e, except
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Table 3. Photophysical Properties of AF240, Model Compound (AF349), and Polyimides 7aꢀ7ea
b
c
c
sample
Abs (λmax), nm
ε, Mꢀ1 cmꢀ1
Fl (λmax), nm
Φfl
τs1
τs2
7a THF
7b THF
7c THF
7d THF
7e THF
AF240 THF
5 (AF349)
7a film
304
384
384
384
384
391
385
2930 ( 20
4380 ( 30
463
463
463
463
475
464
0.0719 ( 0.004
0.0683 ( 0.004
0.0712 ( 0.004
0.0660 ( 0.004
0.748 ( 0.040
0.200 ( 0.001
100 ps (25%)
125 ps (33%)
130 ps (36%)
111 ps (30%)
2.17 ns
404 ps (75%)
420 ps (67%)
415 ps (64%)
364 ps (70%)
12700 ( 160
20500 ( 300
39600 ( 300
44400 ( 2200
48030 ( 700
7b film
631
639
629
632
1.63 ns (7%)
1.63 ns (11%)
1.52 ns (13%)
1.52 ns (15%)
13.3 ns (93%)
11.9 ns (89%)
10.7 ns (87%)
10.3 ns (85%)
7c film
7d film
7e film
a Data were obtained in air-saturated THF solutions and thin film samples. b Quantum yield error is standard deviation from multiple measurements.
Measurements were done under air saturated conditions. c Lifetimes measured under air saturated conditions. Percentage values given are the
contribution of the lifetime to the overall biexponential decay.
Figure 4. UVꢀvis spectra of polyimides 7aꢀ7e, the model compound 5,
and AF240 reference compound in air-saturated THF. Data were
quantified using the Beer’s law method.
Figure 5. Linear transmission spectra of polymer 7aꢀ7e film samples.
See Table 4 also.
for the band intensity at 384 nm, which varies proportionately to
the AF240 content. The absorption band at 384 nm was found to
be slightly blue-shifted (6ꢀ7 nm) from that of the AF240
relative to that of AF240. The LUMO is most likely unchanged by
molecule (see Figure 4). The meta substitution of ether oxygen
the ether oxygen substitution at the donor end because it is
generally has an inductive electron-withdrawing effect as evi-
localized on the benzothiazole acceptor moiety.39
denced by the slightly positive Hammet parameter (σ = +0.10).38
Fluorescence quantum yields were determined using quinine
This stabilizes the HOMO relative to AF24039 and causes the
sulfate as a reference and are given in Table 3. The presence of
blue shift in absorption.
phthalimide end-groups substantially reduced the fluorescence
quantum yield of the model compound in comparison to that of
AF240 molecule. In the polymers, the collective quenching effects of
phthalimide moieties were even more severe, as indicated by more
Solid-state linear transmission spectra (Figure 5) were taken
for the polymer 7aꢀ7e films with their thickness ranging from
∼40 to 65 μm. Except for film 7a (CP2), all 2PA-active film
samples showed good optical transparency for the spectral range
than an order of magnitude reduction in the fluorescence quantum
700 ꢀ1200 nm.
yield in all the polymers. This is most likely due to the restricted
The steady-state emission of the model compoundandpolymers
mobility of the fluorophore and the quencher as well as their
confinement to the same space, increasing the probability of intra-
chain quenching. It is noteworthy that the intensity reduction was
not proportional to the AF240 content. In addition, o-benzamideꢀ
carboxylic acid moieties in the polymer backbone do not quench
fluorescence emission of AF240 fluorophore from the corresponding
poly(amic acid) precursors (Figure S-2), confirming the special
fluorescence quenching capability of the aromatic imide.24
in air-saturated THF was monitored after excitation at 355 nm
(Figure 6). The absorbance at 355 nm was nearly matched to be 0.1
at the excitation wavelength for the polymer samples. For the
model compound 5 and AF240, the data has been normalized for
comparison. All the polymers and model compound (AF349) in
THF show a peak at 463 nm, which is slightly blue-shifted from the
reference compound (AF240), which has a maximum at 475 nm.
This blue shift is also in agreement with a stabilized HOMO level
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Figure 6. Composite fluorescence spectra of polyimides 7aꢀ7e and the
model compound 5 to show their peak positions relative to that of
AF240 reference compound. Absorbance at the exciting wavelength of
355 nm was nearly matched at 0.1 for the polymer samples 7aꢀ7e. The
data has been normalized for the model compound 5 and AF240 for
comparison. Measurements were done in air-saturated toluene.
Figure 7. Time-correlated single photon counting data for 7e in air-
saturated THF (black line) and 7e in a cast film (red line) at 463 nm.
Quenching of the emission at 463 nm for the cast film is observed. The
inset shows a long-lived emission at 630 nm for the 7e cast film.
to the nanosecond 2PA cross section. While both the effective
and intrinsic σ(2) values of the model compound 5 and the
homopolymer 7e on per repeat unit basis are comparable, the
value on per polymer chain basis42 is much larger for 7e,
with number-averaged (Mn) and weight-averaged (Mw) mo-
lecular weights as the lower and upper limits, respectively (see
Table S-2).
The steady-state emission of the film samples showed a very
weak emission (Figure S-3), and the data are cut off due to
reabsorption issues of the highly concentrated films. No fluores-
cence quantum yields of the films were obtained. There is a small
amount of emission detected at longer wavelengths around
630 nm that is not seen in the solution data.
Time-correlated single photon counting (TCSPC) measure-
ments were used to determine the singlet excited-state lifetimes
of both the solution studies as well as the film samples. Repre-
sentative data are shown in Figure 7 for polymer 7e in both THF
and as a film, and the lifetimes for all samples are given in Table 3.
For the data at the peak maximum of 463 nm, we have observed
significant quenching of the singlet excited-state lifetime in the
film to within the instrument response function of the TCSPC
(∼50 ps). The data collected at longer wavelengths (630 nm) for
the films shows a long-lived biexponential emission indicating
the formation of a new emitting species (Figure 7, inset).
Two-Photon Absorption Cross Sections. The effective
(nanosecond) and intrinsic (femtosecond) values of two-photon
Phthalimido Effect and Exiplex Formation. When the
effective and intrinsic σ(2) data obtained for dilute solutions
(0.02 M based on FW of each polymer’s repeat unit, see Table 1)
and those for the cast films are compared in Table 4, it is apparent
that whether the test sample is in solution or as a film, there is little
or no difference for the data in the femtosecond regime. However,
there is consistently a 7- to 9-fold increase in all the film samples in
thenanosecondresults. This israther surprisingwhen oneconsiders
that the extent of ESA contributions to the nanosecond cross
section is dependent on the population and lifetime of the excited
state. We have initiatially considered the possibility of a nonradia-
tive path involving a photoinduced electron-transfer mechanism,
which has been established for phthalimide-based compounds43
and related polyimides,44 especially in the presence of photo-
oxidizable molecules such as dimethylaniline.45 However, such a
mechanism would have led to chemical changes and irreversi-
bility such as photoinduced cross-links in polyimides.44 This
apparently did not occur in any of the polymer films 7aꢀ7e as
all were still still readily soluble in CHCl3 and THF after
40 min of exposure to UV light (366 nm).
cross sections (σ(2), in the unit of GM = 10ꢀ50 cm4 s photonꢀ1
)
of the CP2-polyimides (7aꢀ7e) have been determined in THF
solutions and film samples, but the model compound (5, AF349)
determined in THF solution only, by a direct nonlinear transmis-
sion (NLT) technique at ∼800 nm excitation wavelength. The
nonlinear transmission vs input pulse energy plots are shown in
Figures S-4 and S-5 of the Supporting Information.
From the results that are summarized in Table 4, we observe
that in general both effective and intrinsic σ(2) values are directly
proportional to the dye content. In other words, an increase in
the number density of 2PA chromophoric units does increase the
overall 2PA cross section for both effective and intrinsic values.
As expected, the effective σ(2)ns values are more than 2 orders of
A further probing “fluorophore quencher” experiment was
conducted that involved mixing in THF a 1:1 molar mixture of
AF240 (light yellow solid) and a faint yellow, nonfluorescent
bis(phthalimide) compound (6, Scheme 1), N,N0-{[bis(trifluoro-
methyl)methylene]di-p-phenylene}diphthalimide,46 prepared
from the condensation/imidization reaction of 6FDA and 2 equiv
of aniline. The fresh solution was light yellow and highly fluo-
rescent, but after the solvent had been slowly and completely
removed in a chemical hood overnight at room temperature, a
nonfluorescent solid residue, which had almost identical
(orange) color as polymer 7e film, remained in the vial. Upon
adding fresh THF to the resulting solid, a light yellow solution
and its “blue-green” fluorescence returned when exposed to a UV
lamp (366 nm). This simple experiment suggests that the photo-
events involving phthalimide and AF240 are reversible, and the
magnitude larger than the intrinsic σ(2) values because of the
fs
contributions from excited-state absorption.23,40,41,49
Referring to Table 4, one can see that while σ(2)fs for the model
compound 5 (AF349) is slightly larger than the mean value of
AF240s intrinsic cross sections, its effective cross section is only
ca. 50% of AF240s value. The latter result could be explained in
terms of depleting the excited state population by the phthali-
mide end-groups (quenching effect) via a nonradiative pathway
and, in turn, curtailing the excited-state absorption contributions
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Table 4. Two-Photon Absorption Properties of Polyimides 7aꢀ7e Measured as Films and THF Solutions at 780 nm (160 fs
Pulses) and 800 nm (7 ns Pulses); Experimental Uncertainty Is (15%a
sample ID
properties
mol % AF240
CP2 (7a) AF349P- 10CP2 (7b) AF349P- 30CP2 (7c) AF349P- 50CP2 (7d) AF349P (7e) AF349b (5) AF240 (2)
0
10
30
50
100
thickness
40 μm
0.0045
0.6
55 μm
0.023
3.3
65 μm
0.042
6.8
52 μm
0.052
8.8
55 μm
0.067
11
β (cm/GW; fs)
(cm/GW; ns)
2PA cross section
solution (0.02 M; THF) data
σ
σ
(2) (GM/RU) in fs regime
(2) (GM/(RU) in ns regime
1.4
7.7
16
23
3740
37
34
17,c 45d
9770
178
1070
2490
6000
5160
2PA cross section
film data
27.1
σ2 (GM/RU) in fs regime
0.62
7.7
17.4
38
σ
(2) (GM/(RU) in ns regime
2160
10100
17800
35000
56800
a Abbreviations used in the table: RU = repeat unit; 1 GM (G€oppertꢀMayer = 1 cm4 s photonꢀ1). b 2PA cross sections are in the unit of GM/molecule.
c Datum from refs 25 and 35. d Datum from ref 49.
Figure 8. Optimized molecular structure for a repeat unit of polymer 7e (AF349P) by the MM+ computation method (HyperChem, ver. 7) in stick and
space-filled renditions, where the arrows indicate the polymer-chain directions.
orange color of the 1:1 mixture hints that some intermolecular
structure similar to that of a charge-transfer molecular solid could
have formed.
distance for through-space energy (FRET range 1ꢀ10 nm)47
-
processes. Such a close packing arrangement is likely to be driven
by the strong interactions between the two planar heterocyles. In
addition to dipoleꢀdipole interactions, the fact that (benzo)thiazole
is a π-excesssystem48 and phthalimide is consideredasπ-deficient,
intermolecular donorꢀacceptor interactions are probable. An-
other similarly MM-minimized molecular structure comprised of
a copolyimidesegment, inwhicha repeat unitfor 7e (fluorophore)
is connected on either end to 3 repeat units and 4 repeat units of
CP2-polyimide (7a, quencher) seems to confirm the tendency of
these two aromatic heterocylces to be close to each other.
However, presumably because of the increase in the polymer-
chain flexibility provided by the CP2 repeat units, the “inter-
plane” distance becomes larger (mean distance ∼0.58 nm), but
In the solid state, the existence of a phthalimideꢀAF240
adduct structure, either in interchain or intrachain mode, is
possible. Thus, a molecular mechanics (MM) model was utilized
to assess these possibilities. Figure 8 depicts a MM-minimized
structure that is comprised of a segment containing a repeat unit
for 7e. A striking feature of this structure is the apparent over-
lapping of the two aromatic heterocycles in an antiparallel fashion
(Figure 8a, top view). The mean distance between the phthali-
mide moiety and the benzobisthiazole end of AF240 structure is
∼0.41 nm, which is slightly larger than typical πꢀπ stacking
distance (0.30ꢀ0.35 nm), but well below the donorꢀacceptor
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Scheme 3. A Proposed Mechanism That Entails (a) Ground-State Structure Resulting from Strong DipoleꢀDipole Interaction of
Phthalimide and Benzothiazole Moieties, (b) Intramolecular Charge-Transfer Complex Formation, and (c) and Intermolecular
but Intrachain Exciplex Formation within the 2PA-Active Fragments in Polyimides 7bꢀ7ea
a The excited-state structures are shown in red. (Note that the participation of the ether oxygen in the electronic delocalization is unclear, although it is
assumed to be marginal because of a meta-substitution pattern.)
still within the range for effective charge or energy transfer
(Figure S-6).
lifetime (∼10 ns; Table 3) on the same order as the nanosecond
laser pulses (∼8 ns). Consequently, it could absorb additional
photons, resulting in an enhancement of the effective 2PA cross
section. However, in solution the lifetime (∼0.4 ns) of this
exciplex is much shorter than the laser pulse duration, and hence,
no enhancement was observed. The existence of the exciplex
appears to be supported by the appearance of a new, but weak,
fluorescence band peaked at ∼629ꢀ632 nm for all polymer films
7bꢀ7e, whichwasabsentinCP2ꢀpolyimide (7a) film (Figure S-3).
This band is red-shifted by ∼166ꢀ176 nm from the fluorescence
band (463 nm) observed for all the polymer solution samples
(see Table 3).
On the basis of these results and the fact that the degrees of
molecular freedom and mobility are much reduced in polymer
films under ambient conditions, we have tentatively concluded
that when phthalimide and AF240 moieties are restricted to the
molecular confinement defined by the 2PA-active repeat unit of
polymers 7bꢀ7e, an intrachain supramolecular structure similar
to that shown in Figure 8 is preferred. The high solubility of the
polyimides 7bꢀ7e also implicates that there are little or no
interchain interactions (physical cross-linking via interchain
charge-tranfer complexation) present. Thus, it follows that this
confined supramolecular arrangement may account for the
substantially larger enhancement in the nanosecond two-photon
response in the 7bꢀ7e polymer films than the corresponding
dilute solutions.
Thus, we propose a working mechanism as shown in Scheme 3
to visualize the photophysical events likely to occur in these
polymer films. Briefly, structure (a) represents the ground-state
molecular arrangement of the 2PA-active repeat unit. The
formation of structure (a) is driven by only strong dipoleꢀdipole
interactions, not charge-transfer-type interaction, which is ruled
out by the lack of absorption evidence (i.e., some indication of a
red-shifted charge transfer band). Upon excitation under either
one- or two-photon conditions, an intramolecular charge transfer
within the AF-240 moiety instantaneously occurs. The ben-
zothiazole-end is now more electron-rich, and with the electron-
deficient phthalimide (most likely still in ground state)
moiety nearby, they form an exciplex that quenches the fluores-
cence emission of the excited AF240 moiety. This exciplex has a
’ CONCLUSIONS
Based on the notion that a two-photon active, amorphous
polymer is a viable approach to increasing the chromophore
number density and, in turn, the overall 2PA response, a series of
organo-soluble 6FDA-based polyimides possessing composition-
variable, 2PA sensitivity upon excitation at NIR wavelength of
∼800 nm have been synthesized. Our results indicate that the
covalently bound phthalimide functionality can drastically quench
the fluorescence emission from the AF240 segments of these
polymers as well as the model compound (AF349) in solution
and film samples. Fluorescence quenching does not diminish
their two-photon sensitivities on both nanosecond and femto-
second time scales in solution. On the contrary, in the solid state,
the supramolecular interactions of the phthalimide and the
AF240 moieties in the polymer chain have actually resulted in
up to nearly an order of magnitude increase in nanosecond 2PA
response. Since this enhancement occurs only on the nanosecond
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Macromolecules
ARTICLE
time scale and excited-state absorption contributes to the effec-
tive 2PA cross-section measured, based on the evidence presented
in this work, we propose that a new excited species (an intrachain
exciplex) with a lifetime on the order of 10 ns is responsible.
Further work to define the scope and extent of solid-state
nanosecond enhancement in this polyimide and related polymer
systems is currently underway.
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’ ASSOCIATED CONTENT
S
Supporting Information. (i) Table S-1: density values
b
and corresponding 2PA dye concentrations for polyimide films;
(ii) Table S-2: two-photon absorption cross-section values (GM)
of polyimides 7aꢀ7e, model compound (AF349), and AF240,
measured at 780 nm (160 fs pulses) and 800 nm (7 ns pulses)
separately in THF (0.02 M; 1 cm cuvette); experimental
uncertainty is (15%; (iii) Figure S-1: plot of log(MW) vs log(η)
for the series of copolymers; (iv) Figure S-2: 90/10 copolymer in
amic acid precursor and imide (7b) forms in CDCl3, digital
photo (left) taken without exposure to a hand-held TLC UV-
lamp (254 nm; under room conditions) and (right) upon the
exposure to same UV-light source; (v) Figure S-3: emission
spectra of polymer films 7aꢀ7e that were excited at 400 nm using
a front-face geometry to eliminate any inner filter effects that may
be caused by high absorbance at the exciting wavelength, data are
extremely noisy but show a new emission in the 600 nm region;
(vi) Figure S-4: nonlinear transmission of polymers 7aꢀ7e and
model compound (5; AF349) measured in THF solutions and in
fs regime; (vii) Figure S-5: nonlinear transmission of polymer
7aꢀ7e and model compound (5; AF349) measured in THF
solutions and in ns-regime; (viii) Figure S-6: MM-minimized
molecular structure to represent a segment for polymers 7bꢀ7c
that is made up a repeat unit of AF-349P (polymer 7e) connected
to 3 repeat units of CP2 (polymer 7a) on one side and 4 repeat
units of CP2 on the other side; (ix) proton and C-13 NMR
spectra with tentative assignments for the compounds related to
this work. This material is available free of charge via the Internet
at http://pubs.acs.org.
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: loon-seng.tan@wpafb.af.mil; Ph 937-255-9153.
’ ACKNOWLEDGMENT
We thank Marlene Houtz (University of Dayton Research
Institute) for TGA and DSC data. Funding support was provided
by Materials & Manufacturing Directorate & Office of Scientific
Research (AFOSR), Air Force Research Laboratory.
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