Donor-acceptor-donor-based π-conjugated oligomers for nonlinear optics and near-IR emission

Article abstract of DOI:10.1021/cm201424a

A family of multi-heterocycle donor-acceptor-donor (DAD) telechelic conjugated oligomers designed for two-photon absorption (2PA) and emission in the near-infrared (near-IR) were prepared, and the relationship between their spectral, structural, and elect

Full text of DOI:10.1021/cm201424a

ARTICLE
pubs.acs.org/cm
DonorꢀAcceptorꢀDonor-based π-Conjugated Oligomers for
Nonlinear Optics and Near-IR Emission
Stefan Ellinger,† Kenneth R. Graham,† Pengjie Shi,† Richard T. Farley,† Timothy T. Steckler,†
Robert N. Brookins,† Prasad Taranekar,† Jianguo Mei,† Lazaro A. Padilha,‡ Trenton R. Ensley,‡
Honghua Hu,‡ Scott Webster,‡ David J. Hagan,‡,§ Eric W. Van Stryland,‡,§ Kirk S. Schanze,† and
John R. Reynolds*,†
†Department of Chemistry, Center for Macromolecular Science and Engineering, University of Florida, Gainesville, Florida 32611,
United States
‡CREOL, The College of Optics and Photonics, §Department of Physics, University of Central Florida, 4000 Central Florida Boulevard,
Orlando, Florida 32816, United States
S Supporting Information
b
ABSTRACT: A family of multi-heterocycle donorꢀacceptorꢀdonor
(DAD) telechelic conjugated oligomers designed for two-photon absorp-
tion (2PA) and emission in the near-infrared (near-IR) were prepared, and
the relationship between their spectral, structural, and electrochemical
properties were investigated. These oligomers, based on electron-rich
thiophene, phenylene, and 3,4-ethylenedioxythiophene (EDOT) units as
donors along with electron-deficient benzothiadiazole or its derivative units
as acceptors, have been characterized through linear absorbance and
fluorescence measurements, nonlinear absorbance, cyclic voltammetry,
and differential pulse voltammetry to demonstrate the evolution of narrow
HOMOꢀLUMO gaps ranging from 1.05 to 1.95 eV, with the oligomers
composed of EDOT and benzo[1,2-c,3,4-c0]bis[1,2,5]thiadiazole (BBT) exhibiting the narrowest gap. The absorption maxima
ranges from 517 to 846 nm and the fluorescence maxima ranges from 651 to 1088 nm for the different oligomers. Z-scan and two-
photon fluorescence were used to measure the frequency degenerate 2PA of the different oligomers. The oligomer’s 2PA cross
sections ranged from 900ꢀ3500 GM, with the oligomer containing EDOT donor units and a BBT acceptor unit exhibiting the
largest 2PA cross section. The use of these oligomers in red to near-IR emitting polymer light-emitting diodes (PLEDs) was
demonstrated by blending the soluble emitting oligomers into a suitable host matrix. Energy transfer from the matrix to the emitting
oligomer can be achieved, resulting in PLEDs with pure oligomer emission.
KEYWORDS: near-infrared emission, PLED, two-photon absorption, donorꢀacceptor oligomers, controlled HOMOꢀLUMO gap
’ INTRODUCTION
In the work reported here, a family of donorꢀacceptorꢀ
donor (DAD) oligomers has been synthesized in an effort to
further our understanding of how donor and acceptor moieties
and their relative locations effect the linear absorbance and
fluorescence properties, the HOMOꢀLUMO gap, electroche-
mical properties, and two-photon absorbance (2PA) cross sec-
tions. Introducing a functional end group onto these oligomers
also provides an opportunity for incorporating the π-conjugated
system covalently into a more complex system similar to our
previous approach.4 For many applications, such as OLEDs9 and
photovoltaics,3 the value of the HOMOꢀLUMO gap is critical to
an optimized device because it determines the color of the
emitted light in the LED or the effectiveness with which solar
radiation is absorbed in photovoltaic devices. In the DAD
approach employed, the combination of an electron-rich donor
Conjugated polymers have received a great deal of attention
because of their applications in photovoltaic devices, light-
emitting diodes (LEDs), field-effect transistors (FET), and
electrochromic devices.1ꢀ5 Similarly, conjugated oligomers6
share many of the properties of conjugated polymers with certain
advantages including their well-defined structure, easier purifica-
tion, fewer defects, and the possibility to introduce functional-
ities. As such, the fabrication of solution-processable polymer
light-emitting diodes (PLEDs), vapor-deposited organic light-
emitting diodes (OLEDs), and FETs constructed using discrete
oligomers is not far from commercial realization. Through cross
linking, insoluble films of light-emitting and/or charge-transport-
ing polymer networks can be obtained.7,8 In one approach, we
utilized acrylate functionalized discrete π-conjugated oligomers
to create photopatterned films,4 where the charge-transporting
conjugated units could be used to fabricate solution-processable
electrochromic devices and organic field-effect transistors (OFETs).
Received: February 4, 2011
Revised:
July 18, 2011
Published: August 04, 2011
r
2011 American Chemical Society
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with a DAD core to create a structure consisting of two solu-
bilizing terminal alkyl groups and a π-conjugated DAD core
segment. By introducing a protected alcohol at the end of the
alkyl spacers, we are able to easily functionalize the π-conjugated
oligomers for the possibility of further reactivity.
This strategy has resulted in the design of new DAD-based
oligomeric systems with absorption and emission wavelengths
extending over a wide range. Through control of the donor and
acceptor strength, the linear absorption maxima ranges from
517 to 846 nm and the emission wavelength maxima from 651 to
1088 nm. On the basis of this technique, the DAD oligomers
present readily tunable absorbance and emission properties that
allow for their potential use in 2PA applications and red to near-
IR emitting PLEDs.
Figure 1. Schematic illustration of the DAD oligomers and the syn-
thetic strategy used to obtain them, where D, A, and F represent donors,
acceptors, and functional/protective groups, respectively.
and an electron-deficient acceptor results in a conjugated
material with a decreased HOMOꢀLUMO gap.10,11 By carefully
selecting the appropriate structures of the donor and acceptor, it
is possible to tailor the gap and the solubility of the resulting
oligomer. One specific donorꢀacceptor approach to small gap
oligomers and polymers is the use of nitrogen-containing hetero-
cycles as effective electron acceptors. For example, small band
gaps are reported for electropolymerized donorꢀacceptor poly-
mers with thiophene or 3,4-ethylendioxythiophene (EDOT) as
donors and benzothiadiazole and its derivative units as acceptors.12ꢀ14
In addition to our approach, recent literature describes the use of
several different classes of donorꢀacceptor type molecules such
as those consisting of shorter π-conjugated systems containing
the same units as reported herein,15 molecules with triarylamines
as electron donors,16,17 as well as molecules absorbing further
into the near-IR such as bis(pyrrolopyrrole) cyanines,18 and
benzene-fused hexaazatriphenylene derivatives19 among others.
On the basis of similar molecular systems that incorporate π-
conjugation in DAD structures and exhibit increased third-order
optical nonlinearities,20ꢀ22 the family of oligomers reported here
were predicted to display similar and potentially useful nonlinear
properties. Materials displaying nonlinear absorbance are ex-
pected to have significant impact in optical communications,
optical switching, and data storage applications. Furthermore,
this DAD oligomer concept lends itselfto readily tunable HOMOꢀ
LUMO levels, making them potential candidates for materials
with controlled 2PA absorbance20ꢀ22 and also controlled emis-
sion wavelengths for use in OLEDs or PLEDs.15ꢀ17
We have focused on developing a series of discrete DAD π-
conjugated oligomers6,12,13 to obtain materials with large 2PA
and near-infrared (near-IR) emission.11,23 Although 2PA in-
creases with increasing conjugation length, molecules with linear
absorptions that extend into the near-IR tend to be less photo-
chemically stable. The use of DADs with various strengths of
donors and acceptors that extend the linear and therefore 2PA in
the near-IR region that are photochemically stable are of interest
for many nonlinear applications. In particular, extending photo-
molecular responses that extend into the telecommunications
bands between 1260 and 1675 nm will allow for new uses of
molecular systems in optical switching. Furthermore, the use of
donorꢀacceptor-based oligomers in PLEDs and OLEDs has
previously been demonstrated and provides many advantages
over traditional polymer emitters including more readily tunable
emission wavelengths, more facile purification, no end-group
effects, and the ability for device fabrication through either
solution processing or thermal vapor deposition.15ꢀ17
’ RESULTS AND DISCUSSION
Synthesis of Telechelic Donors. In order to establish the
fundamental properties of the π-conjugated segments, we syn-
thesized the four donor synthons 2, 3, 7, and 11 as synthetic
starting points as depicted in Scheme 1.
The tetrahydropyran (THP)-protected hexanebromide14
1
was converted to the alkylated donors 2 and 3,4 whereby thio-
phene and 2,3-dihydrothieno[3,4-b][1,4]dioxine (EDOT) were
reacted with butyllithium followed by addition of the protected
hexanebromide. These alkylated donors 2 and 3 were converted
to stannylated compounds 4 and 5 by reaction with n-BuLi and
quenching with a halogenated organotin compound in high
yields. 1,4-Dibromo-2,5-bis(heptyloxy)benzene was converted
to the 1-aryl-4-bromo-2,5-bis(heptyloxy)benzene 6 via Stille cou-
pling24ꢀ26 with the stannylated alkyl-thiophene 4 using stoichio-
metric imbalance to ensure dominant monoreaction. Subsequent
Stille coupling with 2-tributylstannyl-thiophene afforded the
monoalkylated bisthieno-bis(heptyloxy)benzene 7 with a large
yield. By reaction with n-BuLi and addition of trimethyltinchlor-
ide, the stannylated compound 8 was obtained. The donor
synthon 11 was synthesized by reacting 1-bromo-3,5dimethox-
ybenzene with tribromoborane to obtain 5-brombenzen-1,3-diol
9, which was further converted to compound 10 by reacting with
THP protected hexanebromide. Stille coupling with 2-tributyl-
stannyl-thiophene afforded the synthon 11 in high yields. The
conversion into the stannylated compound 12 was accomplished
using the same method as described for 8.
Synthesis of Telechelic DonorꢀAcceptorꢀDonor Oligo-
mers. The synthesis of telechelic oligomers with 2,1,3-bezothia-
diazole (BTD) as the acceptor is shown in Scheme 2aꢀd.
Starting with a palladium-catalyzed Stille coupling of the stanny-
lated compound 4 and bis-brominated-thiophene-benzothiadi-
azole10,11,27 (T-BTD-T) 13 or the bis-brominated-EDOT-ben-
zothiadiazole23,27 (E-BTD-E) 14 DAD core, afforded the difunc-
tionalized THP-protected thiophene oligomers TT-BTD-TT
(15) and TE-BTD-ET (16) in high yields (Scheme 2a). Stille
coupling of compound 5 with 13 and 14 resulted in the corre-
sponding difunctionalized THP-protected EDOT oligomers
ET-BTD-TE (17) and EE-BTD-EE (18) in reasonable yields
(Scheme 2b). Deprotection of 15, 16, 17, and 18 were easily
achieved under mildly acidic conditions to give the functional
oligomers TT-BTD-TT-diol, TE-BTD-ET-diol, ET-BTD-TE-
diol, and EE-BTD-EE-diol in excellent yields. After deprotecting
the terminal hydroxyl group, the solubility of the oligomers in
common organic solvents decreased dramatically. To obtain more
soluble alcohols for a later functionalization, TPT-BTD-TPT
As illustrated in Figure 1, the DAD oligomer synthetic strategy
involves coupling a donor with a protective or functional end
group (F) attached to an alkyl chain, followed by further coupling
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Scheme 1. Synthesis of Telechelic Donors
(19) was synthesized via the palladium-catalyzed cross coupling
of stannyl 8 (TPT) and 4,7-dibromobenzo-[c][1,2,5]thiadiazole
(Scheme 2c). Dialkyloxybenzene-thiophene 12 was coupled via
Stille coupling to 13 to obtain compound 20 (Scheme 2d). TPT-
BTD-TPT (19) showed a much higher solubility for the alcohol,
introduced by the alkoxy-benzene, which was accomplished
through mild acidic conditions. The functionalization of the
terminal alcoholgroup, forexampleintoacrylateorestermoieties, is
significantly easier according to the increasing solubility of these
compounds.
Two thiadiazolo-quinoxaline (TQ) acceptor oligomers were
synthesized as described in Scheme 3. Dihexyl-4,9-di(thiophen-
2yl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline11 21 was dibromi-
nated by 22 and then successfully converted to the TT-TQ-
TT (23) by reaction with the THP-protected stannylated com-
pound 4. By Stille coupling with the THP-protected 2,3-
dihydrothieno[3,4-b][1,4] dioxin-5-yl)-stannane 5, ET-TQ-TE
(24) could be obtained in good yields. Deprotection under mild
acidic conditions yielded TT-TQ-TT-diol and ET-TQ-TE-diol.
Oligomers with benzo[1,2-c;4,5-c0]bis[1,2,5]thiadiazole (BBT)
as the acceptor28 were synthesized as described in Scheme 4.
Therein, 4,8-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)benzo-
[1,2-c;4,5-c0]bis[1,2,5]-thiadiazole 25 was brominated with NBS
to obtain dibrominated compound 26. Stille coupling with either
4 or 5 afforded the difunctionalized THP-protected oligomers
TE-BBT-ET (27) and EE-BBT-EE (28). Deprotection of 26
and 27 was achieved under mildly acidic conditions to give the
functional oligomers TE-BBT-ET-diol and EE-BBT-EE-diol in
high yields.
All oligomers were characterized by 1H and 13C NMR
spectroscopy and showed narrow and well-defined signals as
expected for monodisperse samples; identity and purity were
confirmed by EA and HRMS (see Supporting Information for
details). Furthermore, thecompoundsshowrelativelygoodthermal
stability; only 5% degradation is observed in all cases up to
330 °C. A summary of the synthesized oligomers for ease of
reference is presented in Scheme 5.
Optical Spectroscopy. The difference in the donorꢀacceptor
properties of the BTD oligomers is reflected in their photo-
physical properties as they show two absorbance bands in solution:
a high energy band between 320 and 360 nm (3.88ꢀ3.44 eV)
followed by a low energy band between 517 and 585 nm
(2.40ꢀ2.12 eV) likely arising from the πꢀπ* transition and
chargeꢀtransfer transition, respectively. All of the oligomers
show extinction coefficients between 2 ꢁ 104 and 4 ꢁ 104
M
ꢀ1cmꢀ1 (Table 1). Importantly, the relative strengths of the
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Scheme 2. (A) Synthesis of BTD-Containing Oligomers (TT-BTD-TT and TE-BTD-ET). (B) Synthesis of BTD-Containing
Oligomers (ET-BTD-TE and EE-BTD-EE). (C) Synthesis of Extended BTD Oligomer (TPT-BTD-TPT). (D) Synthesis of
Extended BTD Oligomer (PTT-BTD-TTP).
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Scheme 3. Synthesis of ThiadiazoloꢀQuinoxaline-Containing Oligomers (TT-TQ-TT) and (ET-TQ-TE)
Scheme 4. Synthesis of Benzo[1,2-c;4,5-c0]bis[1,2,5]thiadiazole-Containing Oligomers (TE-BBT-ET and EE-BBT-EE)
electron donor and acceptor moieties, as well as the length of the
conjugated system, are observed to have a large influence on the
spectral characteristics of the oligomers and allow us to effec-
tively tune the absorption and emission wavelengths of the
systems. In the DAD structures studied here, their linear
absorption and emission spectra are characterized by broad
spectra with large Stokes shifts in the emission spectra of
∼2000ꢀ4500 cmꢀ1. The relatively large Stokes shifts exhibited
are common to π-conjugated donorꢀacceptor compounds and
are attributed to a degree of chargeꢀtransfer character.29ꢀ31
Because of the stronger π-donor properties of the EDOT (E)
unit in comparison with thiophene (T), panels (a) and (b) of
Figure 2 show that changing the outer donor heterocycle in TT-
BTD-TT into an EDOT donor in ET-BTD-TE leads to an
absorbance maxima red shift of 30ꢀ546 nm, and the solution
color changes from red to purple.10,11 A similar effect is obtained
when the position of the thiophene and EDOT units are swapped
in TE-BTD-ET, as illustrated by the results in panel (c) of
Figure 2, where there is a slightly larger DA interaction as the
λmax moves to 564 nm. When the stronger EDOT donor is
exclusively used in EE-BTD-EE (Figure 2d) there is a further
bathochromic shift of ∼20 nm, and the solution color changes
from dark violet to blue. The incorporation of a conjugation
extending phenyl unit to the TT-BTD-TT oligomer results in a
further 11 and 16 nm red shift for the PTT-BTD-TTP and TPT-
BTD-TPT oligomers, respectively (Supporting Information). In
agreement with the absorbance data, the corresponding fluores-
cence maxima displayed in Figure 2 show similar spectral shifts
with emission maxima ranging from 651 nm for TT-BTD-TT up
to 725 nm for EE-BTD-EE.
A similar red shift in both the absorbance and fluorescence
spectra is observed when the strength of the electron acceptor is
increased. Substitution of BTD with the stronger electron
accepting thiadiazolo-quinoxaline (TQ) shifts the DA absorption
peak into the near-IR with a maximum of 708 nm for TT-TQ-TT
and 755 nm for ET-TQ-TE asshowninFigure3. Thisbathochromic
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Scheme 5. Chemical Structures and Acronyms for the Complete Family of DAD Oligomers
Table 1. Linear and Nonlinear Optical Properties for THP-Protected Oligomer Derivatives
a,b,c
λmax ab
(nm)a,b
εmax (Mꢀ1cmꢀ1
( ꢁ 104)
)
λmax em
(nm)a
μge
(D)a
δmax
(GM)f,
photodecomposition
threshold (nJ)a,g
Stokes shift
(cmꢀ1 a
)
c
ΦF
TT-BTD-TT
ET-BTD-TE
TE-BTD-ET
EE-BTD-EE
TPT-BTD-TPT
PTT-BTD-TTP
TT-TQ-TT
364, 517
377, 546
388, 564
396, 585
389, 533
390, 528
389, 708
404, 755
391, 786
359, 404, 846
3.0
2.7
3.0
2.2
3.3
4.2
2.1
2.4
2.1
2.7
651
686
692
725
690
676
900
974
1020
1088
0.85 d
0.33d
0.24d
0.07d
0.40d
0.44d
0.03e
0.01e
0.01e
0.005e
6.7
6.8
6.9
6.4
7.4
8.3
6.7
7.1
6.3
7.3
2500
_
>100
10
3981
3738
3279
3300
4269
4146
3073
2978
2919
2629
_
10
_
10
2500
2500
900
_
>100
>100
>100
40
ET-TQ-TE
TE-BBT-ET
1600
>100
EE-BBT-EE
3500
>100
a Measurements performed in CHCl3. b All measurements performed with a Varian Cary 500 Scan UVꢀvis-near-IR spectrophotometer at the University
of Florida. c Estimated error in εmax and ΦF values is (10%; ΦF values measured in CHCl3 or toluene as noted. d Standard: Rhodamine B in EtOH (ΦF =
0.65),38 excitation wavelength 510 nm, oligomer fluorescence measured in CHCl3. e Standard: H2TPTBP in toluene (ΦF = 0.03),39 excitation wavelength
430 nm, oligomer fluorescence measured in toluene due to poor stability of H2TPTBP standard in CHCl3. f ET-BTD-TE, TE-BTD-ET, EE-BTD-EE, and
ET-TQ-TE have low optical damage thresholds, which do not enable 2PA measurement. g Damage threshold energies correspond to 140 fs (fwhm) pulse
widths focused to a spot size of ∼20 μm (HW/e2) in CHCl3. Error bars are typically 15ꢀ20%. Of the oligomers that showed photodecomposition thresholds
<100 nJ, the thresholds are reported at wavelengths of 820 nm for TE-BTD-ET and ET-BTD-TE, 740 nm for EE-BTD-EE, and 1140 nm for ET-TQ-TE.
shift of the low energy peak, with little change in the high energy
peak, leads to a green color of the solution as a window of
transmission opens up in the middle of the visible spectrum at
532 nm, which may be useful for excited state absorption appli-
cations.27,32ꢀ34 The fluorescence of the two TQ compounds are
completely shifted to the near-IR region, with emission maxima
of 900 nm for TT-TQ-TT and 974 nm for ET-TQ-TE. By
replacing the TQ acceptor with bisbenzothiadiazole (BBT), the
strongest acceptor of the BTD derivatives, an additional bath-
ochromic shift to the near-IR is observed due to the more intense
donorꢀacceptor interaction as illustrated by the spectral data for
TE-BBT-ET and EE-BBT-EE in Figure 4.10,11 When the external
thiophene donor (4) is coupled to the E-BBT-E core, an
absorption maximum of 786 nm is observed, while the stronger
donor EDOT (5) yields a further bathochromic shift to 847 nm.
In spite of the large difference in the absorption maxima, both
solutions have nearly the same color as the window of transmission
in the center of the visible spectrum dominates. As expected from
the absorbance data, the fluorescence of these compounds is
shifted further into the near-IR with fluorescence maxima at 1020
and 1088 nm for TE-BBT-ET and EE-BBT-EE, respectively.
All of the THP protected oligomers are highly soluble in a wide
range oforganicsolvents(e.g.,toluene,tetrahydrofuran,chloroform),
but unfortunately after acidic deprotection the resulting alcohols
are only slightly soluble in toluene or chloroform and only show
sufficient solubility in THF. This low solubility of the alcohols
precluded further chemistry being carried out on these deriva-
tives, and as such, the absorbance and emission spectra reported
here and compiled in Table 1 are all for the THP protected
oligomers.
Table 1 compiles the fluorescence quantum yields for the
series of THP protected oligomers. Interestingly, it is evident that
the quantum yield decreases systematically as the fluorescence
red shifts (lower energy). This general trend is likely the result of
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Figure 2. Absorption (black) and emission (red) spectra for THP protected TT-BTD-TT (a), ET-BTD-TE (b), TE-BTD-ET (c), and EE-BTD-EE
(d) in chloroform solution (C = 50 μM).
Figure 3. Absorption (black) and emission (red) spectra for THP protected TT-TQ-TT (a) and ET-TQ-TE (b) in chloroform solution (C = 50 μM).
an increase in the nonradiative decay rate with decreasing excited
state energy (energy gap law).35,36 The quantum yields are
highest for the oligomers that emit at λ < 700 nm, with the
TT-BTD-TT system having the highest yield at ∼85%. The
oligomers that feature three aryl rings flanking the acceptor unit
(TPT-BTD-TPT and PTT-BTD-TTP) have comparatively
high yields. This effect may be due to an increase in the
radiative rate resulting from the longer conjugation length in
the three ring systems. The fluorescence yields are substantially
lower for the systems that emit at λ > 700 nm, with yields ∼1%
or less for the systems that emit at 1 μm or longer. While these
yields are relatively low, they are comparable with other
organic dyes that emit at comparable wavelengths in the
near-IR region.16,37
The frequency degenerate 2PA spectroscopy for the samples
in Table 1 was performed with two different experimental techni-
ques, Z-scan40,41 and two-photon fluorescence (2PF).42 For all
these samples, the fluorescence spectra extend beyond the range
of the 2PF detection system, and thus, the 2PF method does not
provide an absolutely calibrated measurement of the 2PA cross
section. Instead, open-aperture Z-scans were used to absolutely
calibrate the values for the 2PA cross section, δ2PA. All com-
pounds were prepared in chloroform for the nonlinear optical
characterization
Figure 5 shows the linear and 2PA spectra for the measurable
BTD acceptor samples with three different donor groups; TT,
TPT, and PTT. It should be noted that the molar absorptivity
values differ slightly (e10%) from εmax as listed in Table 1. This is
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Figure 4. Absorption (black) and emission (red) spectra for THP protected TE-BBT-ET (a) and EE-BBT-EE (b) in chloroform solution (C = 50 μM).
Figure 5. Molar absorptivity (1,2,3) and 2PA measured with 2 photon
fluorescence and scaled with Z-scan (10,20,30) for BTD acceptors with
TT, TPT, and PTT donors in chloroform, respectively. Molar absorp-
tivity measurements performed with a Varian Cary 500 Scan UVꢀvis-
near-IR spectrophotometer at the University of Central Florida.
Figure 6. Molar absorptivity (1,2,3) and 2PA measured with Z-scan
(10,20,30) for the TQ acceptor and BBT acceptors with TE and EE donor
groups, respectively. Molar absorptivity measurements performed with a
Varian Cary 500 Scan UVꢀvis-near-IR spectrophotometer at the
University of Central Florida.
due to the measurements being performed at two different
universities and potential discrepancies in the calibration of the
balances used in weighing the samples and/or the calibration of
the UVꢀvis-near-IR. All measurements included in Table 1 were
measured on the same instrument, and all values reported in
Figures 5 and 6 were measured on the same model instrument at
another university, thereby allowing direct comparisons within
the respective data sets. One of many factors influencing 2PA is
the transition dipole moment from the ground to first excited
state, μge. A large μge leads to an enhancement of 2PA because the
first excited state is considered the intermediate state in this 2PA
transition.43,44 The transition dipole moment μge can be calcu-
lated (results shown in Table 1) from the integrated strength of
the lowest energy band in the linear absorption spectrum by45ꢀ47
the integration is over the main absorption band (all parameters
are in CGS units). For the BTD acceptor series (Figure 5),
transition dipole moments increase slightly with donor strength
as shown in Table 1, but no significant changes are observed in
either the magnitude or peak spectral positions of the 2PA
spectra. Because of the symmetry of the molecules, 2PA under-
neath the main absorption band are expected to be small with
contributions arising from symmetry breaking and possibly a
low-lying electronic transition. Although symmetry breaking ap-
pears to be the most reasonable explanation, quantum chemical
calculations would be necessary to provide a more direct con-
clusion. The 2PA cross sections of 2500 GM with maxima near
700 nm are comparable to other conjugated molecules with 2PA
in this wavelength range.20ꢀ22
Shown in Figure 6 are the linear and 2PA spectra for the
molecules containing the stronger acceptors, TQ and BBT
indicating a significant red shift from the BTD acceptors.
Comparing the linear absorption of TT-BTD-TT from Figure 5
and TT-TQ-TT from Figure 6 (identical donors), yields a red
shift by ∼200 nm, presumably due to the influence of the
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Z
2
1500ðpcÞ ln 10
μge
¼
εgeðνÞdν
πNAEge
where NA is Avogadro’s number (6.022ꢁ 1023), εge(ν) is the extinc-
tion coefficient in cmꢀ1 Mꢀ1 at the wavenumber υ, in cmꢀ1, and
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stronger acceptor TQ as previously discussed. Unfortunately, a
comparison can not be made between the two TQ containing
oligomers because of the photoinstability of the ET-TQ-TE
derivative. Combining the strongest acceptor in the series (BBT)
with the strongest donor group (EE) shows an additional red
shift in the linear absorption spectrum, a larger transition dipole
moment, and the largest measured 2PA cross section of all the
samples, being nearly twice as large as the TE donor group asso-
ciated with the BBT acceptor.
It is well-known that increasing the length of conjugation
typically red shifts the main linear absorption band and increases
2PA in several types of conjugated systems.48,49 In acceptor
modified cyanines and thiophene-based systems, increasing the
strength of the donor and/or acceptor also enhances 2PA cross
sections.50 In accordance with these previous reports, an increase
in 2PA cross sections is observed for the stronger donors (EE vs
TE) in the BBT system; however, no increase is observed for the
derivatives based on the weaker BTD acceptor. Recently, a series
of modified donor groups has been shown to significantly red
shift the linear absorption band by extending the conjugation
further into the end groups.51 This effective lengthening of the
conjugated system may also be a possible explanation for the red
shift observed when the donor and/or acceptor strength is
increased in this oligomer family.
Z-scan measurements suggest that the presence of one or
more pair of EDOT donors on the BTD and TQ oligomers
reduces the photochemical stability of the molecule because
photoinduced decomposition was observed when ET-BTD-TE,
TE-BTD-ET, and EE-BTD-EE were pumped with as little as 10
nJ per pulse, an energy much smaller than that needed to observe
any 2PA. Photodecomposition thresholds for ET-TQ-TE were
also observed but at larger input energies (∼40 nJ) as shown in
Table 1. These energies are for 140 fs (fwhm) pulse widths
focused to a spot size of ∼20 μm (HW/e2 M).
Figure 7. Cyclic voltammetry (bottom) at 50 mV sꢀ1 and differential
pulse voltammetry (top) with a step time of 0.1 s and a step size of 2 mV
for EE-BTD-EE (4 mM) in CH2Cl2/TBAP (0.1 M) solution.
where E1/2 red and E1/2 ox are the half-wave potential measured
for the compounds in solution versus the Fc/Fc+ reference. The
electrochemically determined HOMOꢀLUMO gaps were then
compared with the optical gap.56 All optical HOMOꢀLUMO
gaps were taken as the intersection of the fluorescence and absor-
bance spectra.
The oxidation and the reduction processes recorded by DPV,
for example oligomer EE-BTD-EE as shown in Figure 7, have
sharper onsets than those associated with cyclic voltammetry
(See CV and DPV data of all oligomers in the Supporting
Information). The E1/2 of oxidation and reduction for EE-
BTD-EE are ꢀ0.08 V and ꢀ1.68 V, respectively, leading to an
estimated HOMOꢀLUMO gap of 1.60 eV. The optical HOMO
ꢀLUMO gap for the neutral oligomer determined by the
intersection of the absorbance and fluorescence spectra appeared
to be 1.89 eV, which is 0.29 eV greater than the electrochemical
gap. Using the electrochemical results, we are able to estimate the
HOMO value at ꢀ5.02 eV and the LUMO value at ꢀ3.42 eV for
EE-BTD-EE. The electrochemical results for all oligomers are
summarized in Table 2, along with their estimated HOMO and
LUMO levels. Cyclic voltammetry of ET-TQ-TE (Figure 8)
shows an E1/2 of 0.27 V for oxidation and two reductions with E1/2
at ꢀ0.94 and ꢀ1.45 V, respectively.28 To the best of our knowledge,
this is the first report of an oligomer containing the [1,2,5]
thiadiazolo[3,4-g]quinoxaline acceptor showing two reversible re-
ductions. Two reversible reductions are also obtained for the TT-
TQ-TT, TE-BBT-ET and EE-BBT-EE oligomers (Supporting
Information).
For all oligomers in this paper, there is a good correlation
between the donorꢀacceptor interaction strength and the
decreasing of the HOMOꢀLUMO gap. This may allow us to
more accurately predict HOMO and LUMO gaps for unknown
compounds using the same or similar donorꢀacceptor units.
Near-IR PLEDs. Polymer-based light-emitting devices (PLEDs)
with near-IR emission were fabricated through doping the near-IR
emitting THP-protected oligomer, EE-BTD-EE, into a conducting
polymer matrix selected such that energy transfer from the matrix to
the oligomer occurred. The two host matrices used were MEH-PPV
andablendofPVK withPDB(6:4byweight, wherePVK =poly(N-
vinylcarbazole) and PBD = 2-(4-biphenyl)-5-(4-tert-butylphenyl)-
1,3,4-oxadiazole). Through host to oligomer energy transfer, both
host matrices resultedin nearly pureEE-BTD-EEemission. Figure 9
Electrochemistry. Electrochemical methods provide a means
of establishing the HOMO and the LUMO energies of con-
jugated systems. These values are critical for selecting the
appropriate cathode and anode materials in an LED or photo-
voltaic device, selecting an appropriate host polymer in a PLED,
and for selecting the appropriate complementary donorꢀaccep-
tor in a photovoltaic device.52 Although TT-BTD-TT53 has been
published previously, no electrochemical data have been re-
ported on the alkylated oligomers. Here, we attempted a variety
of techniques to establish the redox properties of all THP-
protected BTD, TQ, and BBT-oligomers. Cyclic voltammetry
(CV) in CH2Cl2 solution yielded reproducible redox processes
between 0.6 V and ꢀ1.9 V versus Fc/Fc+ in an oxygen and water-
free environment. Also differential pulse voltammetry (DPV)
behavior of all THP protected oligomers was obtained as DPV
offers better sensitivities than CV and leads to steeper peak
onsets due to the sharper current response occurring near the Eo
region. The peak shapes seen in DPV also allude to the redox
behavior of the system. True reversible systems exhibit sharper,
more well-defined peaks, while quasi-reversible and irreversible
systems give broad peaks.54
The two following equations are used to estimate the HOMO
and LUMO levels.52,55
EaðELUMOÞ ¼ ꢀ ðE1=2red þ 5:1ÞeV
IpðEHOMOÞ ¼ ꢀ ðE1=2ox þ 5:1ÞeV
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Table 2. Summarized Electrochemical Properties of the THP-Protected Oligomersa
oligomer
E1/2 oxb (V)
E1/2 redb (V)
HOMO (eV)
LUMO (eV)
HOMOꢀLUMO Gapec (eV)
HOMOꢀLUMO Gapopt (eV)
TT-BTD-TT
PTT-BTD-TTP
TPT-BTD-TPT
ET-BTD-TE
TE-BTD-ET
EE-BTD-EE
TT-TQ-TT
0.49
0.33
0.30
0.14
0.14
ꢀ0.08
0.23
0.27
0.29
0.09
ꢀ1.46
ꢀ1.51
ꢀ5.5
ꢀ3.64
ꢀ3.59
ꢀ3.70
ꢀ3.42
ꢀ3.49
ꢀ3.42
ꢀ3.91
ꢀ4.16
ꢀ4.17
ꢀ4.14
1.95
1.84
1.70
1.82
1.75
1.60
1.42
1.21
1.22
1.05
2.13
2.04
2.04
2.00
1.98
1.89
1.52
1.43
1.37
1.28
ꢀ5.43
ꢀ5.40
ꢀ5.24
ꢀ5.24
ꢀ5.02
ꢀ5.33
ꢀ5.37
ꢀ5.39
ꢀ5.19
ꢀ1.40
ꢀ1.68
ꢀ1.61
ꢀ1.68
ꢀ1.19, ꢀ1.69
ꢀ0.94, ꢀ1.45
ꢀ0.93, ꢀ1.59
ꢀ0.96, ꢀ1.63
ET-TQ-TE
TE-BBT-ET
EE-BBT-EE
a Note, E1/2 ox and E1/2 red were measured in CH2Cl2/TBAP, and HOMO-LUMO Gapopt was measured in CHCl3. b All measurements done in CH2Cl2
and are reported vs Fc/Fc+.
Figure 8. Cyclic voltammetry (bottom) at 50 mV sꢀ1and differential
pulse voltammetry (top) with a step time of 0.1 s and a step size of
2 mV for ET-TQ-TE (4 mM) in CH2Cl2/TBAP (0.1 M) solution at
50 mV sꢀ1
.
displays the molecules used in the devices and the HOMOꢀLUMO
levels of the two systems.
Figure 9. Structures of the molecules used in the near-IR emitting
PLEDs (a) and an energy diagram showing HOMO and LUMO levels of
PVK,57 PBD,57 MEH-PPV,52 and EE-BTD-EE. PVK, PBD, and MEH-
PPV HOMO and LUMO values were taken from the literature and
calculated with Fc/Fc+ at ꢀ5.1 eV vs vacuum.52
The energy diagram presented in Figure 9 shows the potential
of both PVK:PBD and MEH-PPV to serve as appropriate hosts
for the EE-BTD-EE oligomer based on a charge trapping mech-
anism.57,58 The good spectral overlap of the MEH-PPV emission
with the EE-BTD-EE absorption suggests that F€orster energy
transfer from MEH-PPV to EE-BTD-EE will occur in addition to
charge trapping. Efficient energy transfer from host to EE-BTD-
EE was observed to occur for both systems, resulting in primarily
EE-BTD-EE emission in the PLEDs as evident in Figures 10 and 11.
The MEH-PPV blend device shown in Figure 10 shows
incomplete energy transfer from MEH-PPV to EE-BTD-EE at
low doping levels and a significant decrease in emission intensity
with increasing EE-BTD-EE concentration. Additionally, the
EE-BTD-EE emission maximum shows a bathochromic shift with
increasing concentration. This combination of emission quench-
ing and a bathochromic shift with increasing oligomer concen-
tration are characteristics of aggregation.
observed for the MEH-PPV devices are also observed for the
PVK:PBD devices, suggesting that the EE-BTD-EE oligomer has
a strong tendency to aggregate regardless of the host matrix. The
radiant emittance of the EE-BTD-EE in MEH-PPV PLEDs was
measured and further supports the aggregation quenching evi-
dent in the spectral measurements as shown in Figure 12.
The maximum radiant emittance of the devices decreased
from 4.5 to 3.1 to 1.3 mW/cm2 as the concentration of EE-BTD-
EE in MEH-PPV was increased from 0 to 2 to 4%, respectively.
Additionally, the EQE showed a similar trend as it decreased
from 0.87 to 0.28% as the concentration of EE-BTD-EE was
increased from 0 to 4%. This decreased radiance emittance and
efficiency with increasing oligomer concentration again indicates
that aggregation is occurring and negatively affecting the device
performance.
The PVK:PBD blend device shown in Figure 11 shows much
more complete energy transfer to EE-BTD-EE at low concen-
trations with no PVK emission evident for the 1% device even at
larger voltages as displayed in panel (b) of Figure 11. The same
quenching effects with increasing EE-BTD-EE concentration as
AFM and TEM measurements were performed on the films to
probe their morphology. These results, as displayed in Figure 13,
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Figure 10. Electroluminescence spectra of (a) varying percent (by wt) EE-BTD-EE in MEH-PPV at 6 V and (b) electroluminescence spectra of an
optimized 2% EE-BTD-EE in MEH-PPV device as a function of applied voltage.
Figure 11. Electroluminescence spectra of varying percent (by wt) EE-BTD-EE in PVK:PBD at 26 V (a) and electroluminescence spectra of an
optimized 1% EE-BTD-EE in PVK:PBD device as a function of applied voltage (b).
Figure 12. Current density (open symbols) and radiant emittance (filled symbols) for varying percent (by wt) EE-BTD-EE in MEH-PPV (a) and
percent external quantum efficiencies (EQEs) for the same devices (b).
suggest the presence of aggregates as the concentration of EE-
BTD-EE in MEH-PPV is increased from 4% to 16%. The
evolution of aggregate formation is apparent in the AFM and
TEM images as the 4%, 8%, and 16% devices show a dramatic
increase in the number of aggregates and a changing morphol-
ogy. Additional morphological information was obtained through
AFM phase imaging, and for the 4% and 8% devices an under-
lying morphology that is not apparent in the corresponding height
images is observed. The TEM images confirm the presence of the
aggregates observed in the AFM images. Multiple TEM and
AFM images confirmed that the morphology is uniform through-
out the films in the case of the 2%, 4%, and 8% blend devices;
however, the 16% device showed evidence of two different
morphologies.
The AFM and TEM images presented in Figure 13 display the
dominant morphology of the 16% blend device. This aggregation
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Chemistry of Materials
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Figure 13. AFM height images of 4%, 8%, and 16% (aꢀc, respectively) EE-BTD-EE in MEH-PPV and the corresponding TEM images (dꢀf). AFM
images are all 5 μm ꢁ 5 μm. TEM scale bar is 1 μm.
apparent in all images is attributed to the polar nature of the DAD
oligomer and the rod-like extended π-conjugated core with
solubilizing groups present only on the ends. These oligo-
merꢀoligomer interactions in combination with the less-polar
polymer host matrix lead to phase separation and aggregation of
the oligomer.
The formation of larger aggregates visible through AFM and
TEM suggest that the EE-BTD-EE oligomer has a strong
tendency to aggregate. Given this tendency to aggregate, it is
likely that smaller aggregates not visible through TEM or AFM
are also present in the film. These smaller aggregates likely
contribute significantly to the decreased device performance
observed upon increasing oligomer concentration. This is sup-
ported by the ∼50% decrease in device performance of the 4%
EE-BTD-EE device vs the 2% EE-BTD-EE device, even though
minimal aggregates are observed in the 4% device.
Specifically, the near-IR emitting oligomer EE-BTD-EE was
blended into a charge transporting matrix, and through energy
transfer processes, PLEDs with solely oligomer emission were
created. On the basis of the oligomers presented in this paper,
PLEDs with emission maxima ranging from 651 to 1088 nm
could be fabricated; however, the EQE of the PLEDs would be
expected to decrease as the emission is further shifted to the near-
IR in accordance with the quantum yields. Additionally, the
effects of donor and acceptor interactions presented herein
provide a basis for additional wavelength tunability.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details regarding
b
the synthesis, optical spectroscopy, nonlinear optical spectros-
copy, electrochemistry, PLED fabrication and characterization,
and morphology characterization; synthetic details, mass spec-
trometric characterization, and NMR characterization; and cyclic
voltammetry and differential pulse voltammetry on all oligomers.
This material is available free of charge via the Internet at
http://pubs.acs.org.
’ CONCLUSIONS
A series of oligomers composed of aromatic donor and o-
quinoid-acceptor units was prepared. The electronic spectra and
the color of these oligomers were greatly affected by the proper-
ties of the thiophene, phenylene, or 3,4-ethylendioxythiophene
donors and by benzothiadiazole or its derivative heterocycles as
acceptors. By introducing terminal alkyl chains end capped with a
protected alcohol group, a later functionalization can be
achieved.4 The electrochemical behavior of the oligomers show
HOMOꢀLUMO gaps in the range of 1.05ꢀ1.95 eV, mainly
depending upon the properties of the o-quinoid-acceptor hetero-
cycle. Nonlinear 2PA cross sections have been determined to be
in the range of 900ꢀ3500 GM for the different oligomers and
dependent on the donor and acceptor strengths. Owing to the
solution processability, the oligomers can be easily integrated
into PLEDs as the emitting species using soluble host polymers.
’ AUTHOR INFORMATION
Corresponding Author
E-mail: reynolds@chem.ufl.edu. Phone: 352-392-9151. Fax:
352-392-9741.
’ ACKNOWLEDGMENT
We gratefully acknowledge financial support of the present
work by the support of the Office of Naval Research MORPH
N00014-06-1-0897. S.E., K.G., R.F., K.S., and J.R. also acknowl-
edge the U.S. Defense Advanced Research Projects Agency
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Chemistry of Materials
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(DARPA) and the U.S. Army Aviation and Missile Research,
Development, and Engineering Center (AMRDEC) (Project No.
W31P4Q-08-1-0003).59 L.P., T.E., H.H., S.W., D.H., and E.V.S.
also acknowledge financial support from the U.S. Army Research
Offices by both W911NF0710232 and 50372-CH-MUR.
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