Photoluminescent energy transfer from poly(phenyleneethynylene)s to near-infrared emitting fluorophores

Article abstract of DOI:10.1002/pola.24123

Photoluminescent energy transfer was investigated in conjugated polymer-fluorophore blended thin films. A pentiptycene-containing poly(phenyleneethynylene) was used as the energy donor, and 13 fluorophores were used as energy acceptors. The efficiency of

Full text of DOI:10.1002/pola.24123

Photoluminescent Energy Transfer from Poly(phenyleneethynylene)s
to Near-Infrared Emitting Fluorophores
MINDY LEVINE,1 INJA SONG,1 TRISHA L. ANDREW,1 STEVEN E. KOOI,2 TIMOTHY M. SWAGER1,2
1Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, Massachusetts 02139
2Institute for Soldier Nanotechnologies, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge,
Massachusetts 02139
Received 28 March 2010; accepted 7 May 2010
DOI: 10.1002/pola.24123
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Photoluminescent energy transfer was investigated
in conjugated polymer-fluorophore blended thin films. A pen-
tiptycene-containing poly(phenyleneethynylene) was used as
the energy donor, and 13 fluorophores were used as energy
acceptors. The efficiency of energy transfer was measured by
monitoring both the quenching of the polymer emission and
the enhancement of the fluorophore emission. Near-infrared
emitting squaraines and terrylenes were identified as excellent
energy acceptors. These results, where a new fluorescent
signal occurs in the near-infrared region on a completely dark
background, offer substantial possibilities for designing highly
C
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sensitive turn-on sensors.
2010 Wiley Periodicals, Inc.
J Polym Sci Part A: Polym Chem 48: 3382–3391, 2010
KEYWORDS: conjugated polymers; fluorescence; sensors; thin
films
INTRODUCTION The highly efficient energy transfer1 and exci-
ton migration processes2 in conjugated polymers can be
exploited in various electronic applications3–11 and in amplify-
ing sensor responses.12–19 Highly sensitive, amplified quench-
ing of polymer emission has been accomplished with various
quenchers in solution as well as in the solid state.20–25 Appli-
cations of this amplified quenching include the detection of
chemical and biological analytes,26–38 and explosives.39
the emission of the acceptor occurs on a dark background,
without interfering donor emission.
Additionally, turn-on sensors that display a new fluorescence
emission in the near-infrared (NIR) region (650–900 nm)
are highly desirable for biological applications.47,48 Biological
chromophores exhibit low absorption and auto-fluorescence
in this spectral region, which allows photons to penetrate
biological tissue.49 Some applications of NIR fluorophores in
biological imaging have been reported50–55; however, the use
of conjugated polymers as energy donors in combination
with NIR energy acceptors allows for highly amplified fluo-
rescence emission in a spectral region that is free of interfer-
ing signals (neither the polymer donor nor biological analy-
tes fluoresce in this region).
In contrast to turn-off sensors based on amplified polymer
quenching, turn-on sensors have the advantage of potentially
being even more sensitive and selective,40,41 especially if the
new signal can be generated on a completely dark back-
ground. Some examples of turn-on sensors have been devel-
oped previously.42–45 In many of these sensors, the emission
spectrum of the donor overlaps with the emission spectrum
of the acceptor. This overlap leads to decreased sensitivity in
turn-on sensory applications, as even in the absence of the
acceptor there is background donor emission in the same
spectral region, and hence not the desired completely dark
background.
We report herein a thorough investigation of the energy
transfer between a conjugated PPE and 13 commercially
available and readily-synthesized fluorophores. These com-
pounds have absorption maxima ranging from 537 to 686
nm, with many of the compounds absorbing and fluorescing
in the NIR region. We show highly efficient energy transfer
from the PPE to the fluorophores, with nearly 100-fold fluo-
rescence amplification in the NIR region from exciting the
PPE compared to exciting a squaraine chromophore directly.
Recent results from our group have demonstrated superior
energy transfer with reduced spectral overlap between the
absorption spectra of the streptavidin-functionalized fluoro-
phore acceptors and the emission spectrum of the biotin-
functionalized poly(phenyleneethynylene) (PPE) donor.46
Sensors with completely separated donor and acceptor emis-
sions, which rely on electronic exchange mechanisms, have
the potential to exhibit significantly enhanced sensitivity, as
EXPERIMENTAL
Spectral grade chloroform and n-butanol were obtained from
Mallinckrodt Company. Anhydrous toluene, benzene, and
Additional Supporting Information may be found in the online version of this article. Correspondence to: T. M. Swager (E-mail: tswager@mit.edu)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 3382–3391 (2010) 2010 Wiley Periodicals, Inc.
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diisopropylamine were purchased from Aldrich Chemical
Company, and were used as received. Ethanol was obtained
from Pharmo-Aaper Company. 1,4-diethynylbenzene pur-
chased from Alfa Aesar was purified by chromatography on
silica gel before polymerization. Rhodamine 6G (2a),
1,10,3,3,30,30-hexamethylindodicarbocyanine iodide (3b), and
Nile Red (4) were purchased from Aldrich. Sulforhodamine
101 (2b) and rhodamine 800 (2c) were obtained from
Molecular ProbesTM. 1,10-Diethyl-3,3,30,30-tetramethylindocar-
bocyanine iodide (3a) was obtained from Fluka. Oxazine
170 (5) was obtained from Acros. 3-Benzyloxyaniline and
1,8-bis(dimethylamino)naphthalene were obtained from
Sigma-Aldrich. tert-Butyl bromoacetate, cyclohexene, and 3,4-
dihydroxycyclobut-3-ene-1,2-dione were obtained from Alfa
Aesar. 10% palladium on carbon was obtained from Acros.
Trifluoroacetic acid was obtained from Oakwood Products.
Polymer 1 (Mn ¼ 88,400, PDI ¼ 2.3)56 and polymer 957 were
synthesized according to previously reported procedures.
The synthesis of two squaraine fluorophores, 1,3-bis[4-(dime-
thylamino)phenyl]-2,4-dihydroxycyclobutenediylium dihydr-
oxide (6a) and 1,3-bis[4-[bis(2-methylpropyl) amino]-2,6-
dihydroxyphenyl]-2,4-dihydroxy cyclobutenediylium (6b) was
adapted from literature procedures,58 and purified further by
column separation after recrystallization. Compound 6c was
synthesized according to literature procedures.59,60
The synthesis of terrylene compounds 7 was accomplished
according to literature procedures.61 Known compounds 7b
and 7c were synthesized from compound 7a using proce-
dures that were developed by Mu¨llen and coworkers.62
FIGURE 1 The absorption (solid line) and emission (dashed
line) spectra of polymer 1a in spin-cast films (Thin films: kmax
abs. ¼ 446 nm, kmax em. ¼ 460 nm, UF ¼ 0.33, n (633 nm) ¼
1.66; chloroform solution: kmax abs. ¼ 427 nm, kmax em. ¼ 453
nm, UF ¼ 0.50).
RESULTS AND DISCUSSION
Materials
The photophysical spectra and properties of polymer 1 are
summarized in Figure 1. All of the properties and responses
of polymers 1a and 1b are identical (absorption and emis-
sion spectra, quantum yields, lifetimes) and these polymers
are therefore utilized interchangeably. Interestingly, the
results described in this article are not dependent upon the
minor spatial perturbations that will exist between these
polymers.
chromic shift in its absorption and emission maxima com-
pared to its lower homologues, naphthalene and
perylene.77 This bathochromic shift has been investigated
theoretically.78 Terrylene diimides show remarkable pho-
tostability,79 and have been utilized for a variety of bio-
logical applications.80–82
Energy Transfer Studies
Blended thin films of conjugated polymer 1 and the fluoro-
phores were fabricated at a variety of fluorophore concentra-
tions. Efficient energy transfer from the polymer to the fluo-
rophores was observed for NIR-emitting fluorophores, that
is, exciting the film at the absorption maximum of the poly-
mer resulted in a new fluorescence emission in the NIR
region. The efficiency of the energy transfer from the conju-
gated polymer to the fluorophores in thin film blends was
quantified by two methods. First, the comparison between
the fluorophore emission intensity obtained by the excitation
of the polymer (IAD) to the emission intensity observed by
direct excitation of the fluorophore (IA) demonstrates photo-
luminescent energy transfer from the polymer donor to the
fluorophore acceptors.
The molecular structures of the fluorophores used are
shown in Chart 1, and their photophysical properties are
summarized in Table 1. The wide variety of fluorophores
investigated includes near-infrared emitting squaraine com-
pounds 6 and terrylene compounds 7. Squaraine compounds
have characteristic narrow absorption and emission bands
with high extinction coefficients.63,64 They have been utilized
in a variety of applications, including as chemosensors for
metal ions65–68 and thiols,69 as indicators of membrane po-
larity,70 and as guests encapsulated in aromatic macro-
cycles.71–73 Squaraines have also been utilized extensively
for biological imaging.74,75
The synthesis of terrylenediimide fluorophores
7 is
shown in Scheme 1. Terrylene, which was first crystal-
lized by Clar in 1958,76 demonstrates a substantial batho-
Second, the energy transfer between the conjugated poly-
mer and the fluorophores was quantified by measuring
PHOTOLUMINESCENT ENERGY TRANSFER, LEVINE ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
CHART 1 Organic fluorophores used in the energy transfer studies.
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TABLE 1 The Absorption and Fluorescence Maxima of
Fluorophores in Spin-Cast PMMA Films and Their Extinction
Coefficientsa at Their Absorption Maxima
where FDA and FD are the integrated emission of the donor
in the presence and absence of acceptors, respectively.
We investigated all of the acceptor fluorophores at 5 and
0.5 wt % (Table 2). For some of the more interesting NIR
acceptors, we examined films with extended compositional
ranges (Table 3).
Extinction
kmax abs.
kmax
Coefficient
e (cmꢀ1Mꢀ1) ꢁ 104
Fluorophore
(nm)
em. (nm)
The fluorophore emission observed for rhodamine com-
pounds 2 decreases as the absorption maximum of the com-
pounds increases [Fig. 2(A–C)]. Compound 2a, which has the
shortest wavelength absorption maximum, displays the
greatest fluorescence enhancement. This result is in accord
with Fo¨rster energy transfer theory,83 which dictates that the
efficiency of energy transfer depends on the spectral overlap
between the donor emission and acceptor absorption
spectra.
2a
2b
2c
3a
3b
4
552
575
592
707
577
680
612
638
647
667
645
675
687
760
8.75
6.07
579
686
6.02
556
14.00
20.90
4.76
652
537
5
620
2.94
6a
6b
6c
7a
7b
7c
a
635
12.30
27.30
3.27
The cyanine compounds 3 show reduced sensitized fluoro-
phore emissions, especially for compound 3b [Fig. 2(D,E)].
Although thin films with Nile Red 4 show efficient energy
transfer [Fig. 2(F)], its cationic analogue oxazine 5 does not
function as an efficient energy acceptor [Fig. 2(G)]. The
charged nature of compound 5, as well as its decreased
spectral overlap, likely affect the efficiency of energy transfer.
648
638
671
1.22
630, 674
630, 679
0.98
2.89
The extinction coefficients were measured in chloroform.
Remarkably, squaraine compounds 6a and 6b display a sig-
nificant amplification of fluorescence from polymer excitation
compared to direct fluorophore excitation, with 99-fold and
the fractional quenching of donor emission (Eexp) as in
Equation 1:
Eexp ¼ 1 ꢀ FDA=FD
(1)
SCHEME 1 Synthesis of terrylene fluorophores 7.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 2 Energy Transfer Efficiencies Measured by the
Decrease in Polymer Emission (Eexp) and the Increase of
Fluorophore Emission (IAD/IA) in Thin Films of Polymer 1 with
the Fluorophores at 0.5 and 5 wt % Doping Concentration
TABLE 3 Energy Transfer Efficiencies Measured by the
Decrease in Polymer Emission (Eexp) and the Increase of
Fluorophore Emission (IAD/IA) in Thin Films of Polymer 1 with
Fluorophores 6c–7c at Various Concentrations
5 wt %
0.5 wt %
1 wt %
0.1 wt %
Fluorophore
Eexp
I
AD/IA
Eexp
I
AD/IA
Fluorophore
Eexp
IAD/IA
Eexp
IAD/IA
b
a
2a
2b
2c
3a
3b
4
0.91
0.82
0.97
0.99
0.95
0.99
0.85
0.98
0.99
0.40
76.0
64.2
10.1
6.0
0.99
0.77
0.78
0.89
0.62
0.95
0.66
0.87
0.81
0.09
118.0
84.5
61.7
88.3
66.8
6c
7a
7b
7c
a
0.19
46.8
35.8
47.2
31.7
0.79
b
0.21
b
46.3
26.0
33.8
b
0.16
5.4
Fluorophore is not emissive at this concentration.
b
Polymer emission is not quenched at this concentration.
8.4
64.4
a
5
18.7
20.5
6.3
6a
6b
6c
7a
7b
7c
a
98.6
69.5
50.9
41.2
40.6
38.3
mer excitation, with up to a 54-fold increase in the fluoro-
phore’s emission [Fig. 2(J)]. This NIR-emitting compound is
particularly intriguing, as the ester moiety can be readily
functionalized84,85 and utilized for biological applications
such as protein labeling.75
53.8
20.6
46.6
15.0
0.86
b
0.70
b
b
Similarly, NIR terrylene fluorophores function as efficient
energy acceptors in thin film blends with polymer
0.62
1
Fluorophore is not emissive at this concentration.
[Fig. 2(K–M)]. Unsubstituted terrylene diimide 7a displays a
46-fold amplification of fluorescence from polymer excitation
compared to direct fluorophore excitation, which is similar
to the amplification observed for the substituted compounds
7b and 7c.
b
Polymer emission is not quenched at this concentration.
70-fold increases in the fluorophore emission (IAD/IA) for
compounds 6a and 6b, respectively, [Fig. 2(H,I)]. This highly
efficient energy transfer occurs despite the limited spectral
overlap between the polymer emission and the fluorophore
absorption spectra. The functionalized squaraine compound
6c also exhibits substantially enhanced emission from poly-
The degree of spectral overlap between the polymer’s emis-
sion spectrum and the fluorophores’ absorption spectra is
illustrated in Figure 3. This figure illustrates that neither
FIGURE 2 Fluorescence spectra
of polymer-fluorophore blends in
thin films with polymer excita-
tion at 410 nm. (Y-axis expanded
spectra of direct excitation of
fluorophores is provided in the
Supporting Information).
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FIGURE 3 Overlaid spectra of the
emission of polymer 1 with the
absorbance of the fluorophores.
The hatched area represents the
emission band of the pristine
polymer.
the squaraines (compounds 6) nor the terrylenes (com-
pounds 7) exhibit appreciable spectral overlap between
their absorption spectra and the polymer’s emission spec-
trum, which is in stark contrast to several other fluoro-
phores (compounds 2a, 2b, 3a, 4, and 5) which exhibit
substantial overlap. Nonetheless, highly efficient energy
transfer is observed for all the NIR fluorophores
investigated.
indicates that aggregation has occurred (this band is absent
in the emission spectra of the polymer in chloroform
solution).
These results reveal that both squaraine and terrylene com-
pounds are excellent candidates as energy acceptors with
PPE types of donors. The rigidity and compact size of the
squaraines allow them to interact well with the planar poly-
mer backbones. The fluorophores can stack close to the poly-
mers, causing more efficient energy transfer through orbital
overlap. Moreover, the internal free volumes associated with
the pentiptycene in polymer 1 may work as a guiding frame
to locate fluorophores close to the polymer backbone. Squar-
aines have already been shown to interact efficiently with p
systems, as well as with iptycene-containing frameworks.86
The ability of terrylene fluorophores to function as excellent
energy acceptors is likely a result of their ability to p-stack
efficiently with the conjugated polymer backbone.81 The fact
that both squaraines and terrylenes display highly amplified
NIR emission opens the possibility of using these systems
for turn-on biological sensors.
Comparison to a Simple PPE
The performance of pentiptycene-containing polymer 1 was
compared to that of a simple PPE, compound 9.57 The pho-
tophysical spectra and properties of polymer 9 are sum-
marized in Figure 4. The broad excimer emission (510 nm)
in the fluorescence spectrum of polymer 9 in thin films
FIGURE 4 The absorption (solid line) and emission (dashed
line) spectra of polymer
9 in spin-cast films (Thin films:
kmax abs. ¼ 449 nm, kmax em. ¼ 463 nm, 511 nm, UF ¼ 0.04,
n (633 nm) ¼ 1.57; chloroform solution: kmax abs. ¼ 413 nm,
kmax em. ¼ 447 nm, UF ¼ 0.41).
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 4 Energy Transfer Efficiencies Between Polymer 9 and
Fluorophores 2-6b Measured by the Decrease in Polymer
Emission (Eexp) and the Increase in Fluorophore Emission
(IAD/IA)
polymer chains in 9 likely makes it hard to incorporate fluo-
rophores into the polymer matrix. Hence, the fluorophores
tend to self-aggregate, and the incorporated fluorophore con-
tent decreases. On the other hand, polymer 1, with the po-
rous internal structure derived from the pentiptycene moi-
eties, prevents phase separation during the spin-casting
process and facilitates the entrapment of the fluorophores in
close proximity to the polymer backbone.
5 wt %
0.5 wt %
Eexp
IAD/IA
Eexp
IAD/IA
2a
2b
2c
3a
3b
4
0.73
0.79
0.37
14.7
13.5
21.9
9.0
0.52
42.2
0.14
b
47.3
a
Electrochemistry
Recent studies have emphasized that photoinduced electron
transfer (PET) can compete with energy transfer, and that
the relative position of the donor and acceptor frontier orbi-
tals is critical in favoring energy transfer over PET.87–89
When both the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO) energy
levels of an acceptor are located within the energy range
defined by the donor’s HOMO-LUMO gap, energy transfer
dominates. Alternatively, if the HOMO or the LUMO of the
acceptor is outside the donor’s HOMO-LUMO gap, photoin-
duced electron transfer between the donor and the acceptor
can occur followed by nonradiative relaxation.90–92 There-
fore, the HOMO-LUMO energy levels of the donor should
bracket those of the acceptor to minimize the nonradiative
loss of excited donor energy.
0.84
b
0.54
b
43.0
a
36.9
16.0
0.93
0.22
0.10
0.64
0.53
b
86.8
a
5
56.6
a
b
a
6a
6b
a
48.1
0.22
91.9
Fluorophores were not emissive at this concentration.
b
The polymer emission was not quenched at this concentration.
When compared to polymer 1, polymer 9 shows less effi-
cient energy transfer in terms of both donor quenching and
amplified acceptor emission (Table 4, Fig. 5). The only excep-
tions are compounds 4 and 6b, which accomplish compara-
ble energy transfer in blends with both 1 and 9. The quench-
ing of the emission of polymer 9 with added fluorophore is
substantially less than the quenching observed with polymer
1. One possible reason for the limited quenching in blends of
9 is that phase separation occurs between the polymer and
the fluorophores in spin-cast films. The dense packing of the
To estimate HOMO and LUMO levels, the oxidation and
reduction potential of fluorophores were obtained by cyclic
voltammetry, and the energy levels obtained from the poten-
tial differences93,94 (electrochemical band gap) were com-
pared to the onset wavelengths of the absorption spectra
(optical band gap) (Table 5). The HOMO and LUMO energies
FIGURE 5 Fluorescence spectra
of polymer 9-fluorophore blends
in thin films with polymer excita-
tion at 410 nm (solid line).
(Y-axis expanded spectra of
direct fluorophore excitation are
provided in the Supporting
Information).
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TABLE 5 HOMO and LUMO Levels of Polymer 1 and
Fluorophores 2-7 Measured from Their Oxidation (Eox) and
Reduction (Ere) Potentials, and Their Calculated Band Gapsa
mer emission does not overlap the acceptor’s emission.
Moreover, both squaraines and terrylenes fluoresce in the
NIR region, an optimal spectral area for biological imaging.47
Thus, the new emission will occur on a completely dark
background (free of both polymer emission and interfering
biological analytes), leading to even greater sensitivity in the
turn-on sensors.
HOMO
LUMO
Bandgap (eV)
Compound
Eox (V)b
Ere (V)b
Electrochemical
Optical
1
ꢀ5.74
ꢀ5.66
ꢀ5.40
ꢀ5.46
ꢀ5.47
ꢀ5.08
ꢀ5.25
ꢀ5.61
ꢀ5.03
ꢀ5.18
ꢀ5.38c
ꢀ5.17d
ꢀ3.07c
ꢀ3.47
ꢀ3.30
ꢀ3.86
ꢀ3.31
ꢀ3.53
ꢀ3.29
ꢀ3.97
ꢀ3.46
ꢀ3.51
ꢀ3.58
ꢀ3.23d
–
2.67
2.07
1.89
1.67
2.08
1.75
1.98
1.88
1.85
1.82
1.80
1.75
This work was supported by National Institute of General Medi-
cal Sciences (F32GM086044) and the U.S. Army through the
Institute for Soldier Nanotechnologies (DAAD-19-02-0002).
2a95
2b96
2c
2.19
2.08
1.60
2.16
1.54
1.96
1.64
1.57
1.68
–
3a
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