Azadipyrromethene-based conjugated oligomers with Near-IR absorption and high electron affinity

Article abstract of DOI:10.1021/ol202211t

Solution-processable conjugated oligomers incorporating red-light absorbing azadipyrromethenes (aza-DIPY) within the main chain were synthesized via palladium-catalyzed Sonogashira coupling reactions. Thin films of these compounds absorbed light up to ～10

Full text of DOI:10.1021/ol202211t

ORGANIC
LETTERS
2011
Vol. 13, No. 19
5354–5357
Azadipyrromethene-Based Conjugated
Oligomers with Near-IR Absorption and
High Electron Affinity
ꢀ
Lei Gao, Wasana Senevirathna, and Genevieve Sauve*
ꢁ
Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106,
United States
gxs244@case.edu
Received August 15, 2011
ABSTRACT
Solution-processable conjugated oligomers incorporating red-light absorbing azadipyrromethenes (aza-DIPY) within the main chain were
synthesized via palladium-catalyzed Sonogashira coupling reactions. Thin films of these compounds absorbed light up to ∼1000 nm and
displayed reversible reductions as ascertained by cyclic voltammetry experiments. Reactions with trifluoroboron etherate yielded materials
displaying a unique combination of good solubility in organic solvents, low optical band gaps (∼1.3 eV), and high electron affinity (∼4.5 eV).
Conjugated molecules and polymers continue to attract
the attention of scientists due to their semiconducting
properties that make them suitable for a range of electronic
applications, including photovoltaics, field-effect transis-
tors, light emitting diodes, electrochromics, light detectors,
and sensors.1 Their main advantage over their inorganic
counterparts is that they can be solution-processed to give
flexible, lightweight functional films, allowing for low-cost
and large scale production.2 In several organic electronic
applications, both electron-donating (p-type) andelectron-
accepting (n-type) materials are critical, yet most high-
performance conjugated compounds are p-type.3 Stable
n-type conjugated compounds with high electron affinity
(EA > 4) are still relatively rare.4 Most electron-accepting
materials are either fullerene-based or with fluorocarbon,
cyano, or imide electron-withdrawing groups.3,5 Examples
incorporating electron-deficient boron in the main chain
are also emerging.6 Of the limited number of n-type con-
jugated compounds available to date, very few combine
solution processability and low band gaps <1.5 eV.7
Compounds with this unique combination of properties
would be especially desirable for organic photovoltaics
(OPV), where current best OPVs do not significantly
harvest sunlight at longer wavelengths, i.e. >750 nm, or
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Polymers, 3rd ed.; CRC Press: New York, 2007. (b) So, F. Organic
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r
10.1021/ol202211t
Published on Web 09/12/2011
2011 American Chemical Society

<1.6 eV,8 and where the dominant electron acceptors are
fullerenederivativesthatdonotabsorblightatthese longer
wavelengths.9
Scheme 1. Structure of Aza-DIPY and Synthetic Route for
Conjugated Oligomers
Here, we explore using red-light absorbing tetraphenyla-
zadipyrromethene (aza-DIPY, Scheme 1) dyes10 as building
blocks for the synthesis of n-type conjugated materials with
low band gaps and high electron affinity. Aza-DIPYs are
particularly interesting because they are easy to reduce.11 In
addition, the strong coordinating properties of the “pocket”
nitrogens in azadipyrromethene molecules allow their elec-
tronic properties to be readily tuned.12 Aza-DIPY dyes are
often chelated with BF2 (aza-BODIPY) to red-shift light
absorption by reducing rotational freedom at the core.13
Complexation with BF2 also shifts the reduction potential
positive, thus increasing electron affinity.11 We are aware of
only one study that has incorporated aza-BODIPY dyes
into conjugated polymers by polymerization of 1,4-diethy-
nyl-2,5-dialkyloxybenzene with aza-BODIPY molecules
where the reactive halogen is on either the distal or proximal
phenyl rings of aza-BODIPY.14 Due to extension of the
conjugation length, these conjugated polymers showed
near-IR photoluminescence in solution. To our knowledge,
linear conjugated polymers with aza-DIPY and aza-BOD-
IPY dyes incorporated into the polymer backbone through
the active pyrrolic positions have not yet been studied. We
synthesized alternating oligomers of aza-DIPY and pheny-
lene acetylene. The alkyl substituents on the phenyl groups
were varied to tune their solubility in organic solvents. These
compounds had strong absorption throughout the visible
and extending into the near-IR region and could be readily
reduced. The aza-DIPY moieties were further chelated with
BF2, which further increased solubility, lowered the band
gaps, and stabilized the HOMO and LUMO energy levels,
resulting in materials with a unique combination of low
band gaps and high electron affinity.
positions was obtained in good yields by reacting the
aza-DIPYs with N-idodosuccinimide (NIS). The synthesis
of the oligomers was effected using the Sonogashiraꢀ
Hagihara coupling reactions of 2aꢀc with 1,4-bis-
(dodecyloxy)-2,5-diethynylbenzene in the presence of Pd-
(PPh3)4 (10 mol %) and CuI (10 mol %) in a mixed solvent
of chlorobenzene/Et3N (v/v = 3:1) at 70 °C for 48 h.
Conjugated oligomers3aꢀc wereobtainedasmetallicdark
1
solids. H NMR and MALDI TOF MS confirmed the
expected structure (see Supporting Information). Chela-
tion of aza-DIPY moieties with BF2 were performed by
reacting 3aꢀc with excess trifluoroboron etherate in chlor-
obenzene at 50ꢀ70 °C under N2 for 24 h. 4aꢀc were
isolated as black solids. The reaction did not occur at room
temperature whereas, at higher temperatures, unwanted
side reactions and insoluble materials resulted. Successful
complexation of aza-DIPY moieties with BF2 was con-
firmed by 19F NMR, 11B NMR, and MALDI-TOF MS.
The chemical shifts in 19F NMR (∼ ꢀ131 ppm) and 11B
NMR (0.9ꢀ2.0 ppm) confirmed the tetracoordination
state of the boron atom. Attempts of using 2aꢀc chelated
Azadipyrromethenes 1aꢀc (Scheme 1) were synthesized
following the method developed by O’Shea and co-
workers.15 Iodination of aza-DIPYs at the pyrrolic
(8) Liang, Y.; Yu, L. Acc. Chem. Res. 2010, 43, 1227–1236.
(9) (a) Zhan, X.; Tan, Z. a.; Domercq, B.; An, Z.; Zhang, X.; Barlow,
S.; Li, Y.; Zhu, D.; Kippelen, B.; Marder, S. J. Am. Chem. Soc. 2007, 129,
7246–7247. (b) Brabec, C. J.; Gowrisanker, S.; Halls, J. J. M.; Laird, D.;
Jia, S.; Williams, S. P. Adv. Mater. (Weinheim, Ger.) 2010, 22, 3839–
3856. (c) Moore, J. R.; Albert-Seifried, S.; Rao, A.; Massip, S.; Watts, B.;
Morgan, D. J.; Friend, R. H.; McNeill, C. R.; Sirringhaus, H. Adv.
Energy Mater. 2011, 1, 230–240. (d) Brabec, C.; Dyakonov, V.; Scherf,
U. Organic Photovoltaics; Wiley-VCH: Weinheim, 2008.
(10) Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891–4932.
(11) (a) Partyka, D. V.; Deligonul, N.; Washington, M. P.; Gray,
T. G. Organometallics 2009, 28, 5837–5840. (b) Nepomnyashchii, A. B.;
16
with BF2 as the reacting monomers led to the loss of BF2
€
Broring, M.; Ahrens, J.; Bard, A. J. J. Am. Chem. Soc. 2011, 133, 8633–
and isolation of only a small amount of p-phenylene
ethynylene homopolymer.
The physical properties of these materials are summarized
in Table 1. 3aꢀc were cleaned by performing a series of
Soxhlet extractions with methanol, acetone, hexanes, and
8645.
(12) (a) Teets, T. S.; Partyka, D. V.; Esswein, A. J.; Updegraff, J. B.;
Zeller, M.; Hunter, A. D.; Gray, T. G. Inorg. Chem. 2007, 46, 6218–6220.
(b) Teets, T. S.; Partyka, D. V.; Updegraff, J. B.; Gray, T. G. Inorg.
Chem. 2008, 47, 2338–2346. (c) Teets, T. S.; Updegraff, J. B., III;
Esswein, A. J.; Gray, T. G. Inorg. Chem. 2009, 48, 8134–8144.
(13) Killoran, J.; Allen, L.; Gallagher, J. F.; Gallagher, W. M.;
O’Shea, D. F. Chem. Commun. 2002, 1862–1863.
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Chem. 2010, 48, 5348–5356.
(15) Gorman, A.; Killoran, J.; O’Shea, C.; Kenna, T.; Gallagher, W.;
O’Shea, D. J. Am. Chem. Soc. 2004, 126, 10619–10631.
(16) Bellier, Q.; Pegaz, S.; Aronica, C.; Le Guennic, B.; Andraud, C.;
Maury, O. Org. Lett. 2011, 13, 22–25.
(17) (a) Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan,
G. C. Adv. Mater. (Weinheim, Ger.) 2011, 23, 2367–2371. (b) Pavlishchuk,
V. V.; Addison, A., W. Inorg. Chim. Acta 2000, 298, 97–102. (c) Trasatti, S.
Pure Appl. Chem. 1986, 58, 955–66.
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2106.
Org. Lett., Vol. 13, No. 19, 2011
5355

Table 1. Physical Properties of the Oligomers
GPCa
c
Mn
Mw
PDI
solubilityb
(mg/mL)
Td
(kDa)
(kDa)
(Mw/Mn)
(°C)
3a
3b
3c
4a
4b
4c
1.6
3.4
10
2.7
6.2
27
1.7
1.8
2.7
1.8
1.7
2.1
2
9
266
334
281
260
311
287
14.5
2.5
22
25
2.2
3.5
11
4.1
6.0
24
a Determined by GPC in chloroform using polystyrene standards.
b Measured in chlorobenzene at room temperature. c 5% weight loss
determined by TGA under N2.
chloroform. Desired products were recovered from the
chloroform fractions. Molecular weights estimated by gel
permeation chromatography (GPC) were modest and in-
creased with the aza-DIPY phenyl substituent in the order
of H < hexyl < tert-butyl. Polydispersity indices (1.7ꢀ2.1)
were as expected for this type of step polymerization. We
hypothesize that molecular weights were limited by solubi-
lity. Conducting the reactions in THF or lowering the
temperature to 50 °C yielded products with lower molecular
weights. The chloroform fractions of 3aꢀc were soluble in
chlorinated organic solvents, such as chloroform and chlor-
obenzene, and their solubility increased as expected in the
order of 3a < 3b < 3c. In addition, although 3c was
thoroughly extracted with chloroform, there was still a fair
amount of 3a or 3b left after their chloroform extraction.
The leftover solids were further extracted with chloroben-
zene, but the products recovered from the chlorobenzene
fraction could not be fully characterized due to their limited
solubility. Compared to their precursors, compounds 4aꢀc
had slightly higher molecular weights and higher solubility.
All oligomers were thermally stable with 5% weight loss
onsets occurring in the range of 260 to 334 °C based on
thermogravimetry analysis (TGA). Differential scanning
calorimetry (DSC) analysis between ꢀ90 °C and 250ꢀ
300 °C showed no phase transitions, suggesting that these
oligomers have very rigid chains.
The UVꢀvis absorption spectra for the series with
unsubstituted aza-DIPY moieties (1a, 3a, 4a) are shown
in Figure 1. Aza-DIPY ligands in chloroform generally
had astrongnarrowabsorptionat∼600 nm corresponding
to a πfπ* transition (1a: λmax = 596 nm, 1b: 614 nm, 1c:
607 nm). Incorporating aza-DIPY ligands into oligomers
red-shifted the absorption maxima from 596 nm for 1a to
624 nm for 3a and significantly broadened the absorption
spectra. This is attributed to the increased conjugation
length with the phenylene ethynylene moieties. Chelation
with BF2 further red-shifted the absorption spectra to 729
nm for 4a. All optical properties are summarized in Table
2. A deep blue thin film with a metallic tint was easily
formedby drop-casting a 1 mg/mL chlorobenzenesolution
of 3a onto a piece of glass. An optical band gap of 1.45 eV
Figure 1. Absorption spectra of 1a, 3a, and 4a in chloroform and
3a/4a in drop-casted films. The straight lines shown were used to
estimate the optical band gaps.
was calculated based on the absorption onset. The absorp-
tion spectrum of 4a further red-shifted to the near-IR
region, with an optical band gap estimated at 1.26 eV
(982 nm) and absorption tailing up to 1100 nm (1.13 eV).
In films, the compounds with tert-butyl substituents
(3c and 4c) showed the most red-shifted maximum absorp-
tion peaks, perhaps due to their longer conjugation length.
The substituents on the phenyl rings, on the other hand,
had little effect on optical band gaps. DFT calculations of
model compounds (shown in Supporting Information)
show that hexyl and tert-butyl substituents have little effect
onHOMOꢀLUMO energygaps. Inaddition, the presence
of these substituents did not affect the planarity of the aza-
DIPY-phenylene acetylene conjugated units.
Solutions of 3aꢀc had negligible emission around 800
nm under ambient conditions, and complexation with BF2
did not enhance emission intensity. This is in contrast to
the report by Chujo and co-workers, where the conjuga-
tion was extended through the phenyl groups of aza-
BODIPY instead of the pyrrolic positions.14 We hypothe-
size that our materials, being linear, can better π-stack and
form aggregates, which would quench luminescence. Con-
sistent with this hypothesis, no emission could be detected
from films of these compounds.
The electrochemical properties of the conjugated oligo-
mers are summarized in Table 2, and cyclic voltammo-
grams of 3a and 4a are shown in Figure 2. All oligomers
showed two consecutive reversible reduction peaks and an
irreversible oxidation peak. Chelation with electron-with-
drawing BF2 shifted the reduction and oxidation peaks
positively, with a larger shift for the reduction waves. The
first reduction of 4a had an E1/2 of ꢀ0.64 V vs Ag/Agþ,
corresponding to ∼ꢀ0.34 V vs SCE. This value is more
positive than E1/2 for the first reduction of both aza-
BODIPY and the aza-BODIPY dimer,11b consistent with
5356
Org. Lett., Vol. 13, No. 19, 2011

Table 2. Optical and Electrochemical Properties
LUMOd
(ꢀEA)
(eV)
Eg, optc
E1/2,red
(V) vs Ag/Agþ
Epa
(V) vs Ag/Agþ
HOMOd
(eV)
Eg, elec.
a
b
λabs
λabs
(nm)
(nm)
(eV) (nm)
(eV)
3a
3b
3c
4a
4b
4c
624
686
678
729
716
813
625
686
712
746
738
828
1.45 (856)
1.43 (869)
1.46 (852)
1.26 (982)
1.25 (990)
1.33 (935)
ꢀ1.08, ꢀ1.60
ꢀ1.15, ꢀ1.62
ꢀ1.04, ꢀ1.67
ꢀ0.64, ꢀ1.42
ꢀ0.77, ꢀ1.55
ꢀ0.63, ꢀ1.44
0.76
0.79
0.97
0.96
1.11
1.26
ꢀ5.28
ꢀ5.30
ꢀ5.39
ꢀ5.47
ꢀ5.45
ꢀ5.67
ꢀ4.06
ꢀ3.97
ꢀ4.08
ꢀ4.51
ꢀ4.40
ꢀ4.50
1.22
1.33
1.31
0.96
1.05
1.17
a Maxima in chloroform. b Maxima in films. c Estimated optical band gaps from the spectra in films. d HOMO and LUMO energy levels were
estimated from the onset of the oxidation and reduction waves, respectively, using the value of 5.1 for Fc/Fcþ vs vacuum.17 The E1/2 of Fc/Fcþ was
observed at þ0.081 V vs Ag/Agþ. Electron affinity (EA) was estimated as the absolute value of the LUMO energy level.
[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) is
ꢀ4.2 eV.18 These oligomers therefore have great potential
as electron acceptors in organic electronics.
In conclusion, we have demonstrated the first example of
incorporating aza-DIPY into the backbone of conjugated
oligomers/polymers through the active pyrrolic positions.
The synthesized compounds have broad absorption in the
red and near-IR region. Furthermore, they can be readily
and reversibly reduced, pointing to their potential as n-type
conjugated materials for organic field-effect transistors and
organic solar cells. Complexation of azadipyrromethene
moieties with BF2 further stabilized both HOMO and
LUMO energy levels, lowered the band gaps, and increased
solubility, giving a class of materials with a unique combi-
nation of low band gaps and high electron affinity. Varying
substituents on the phenyl rings influenced solubility and
the chain length but had little effect on the band gaps. This
is consistent with the DFT calculations of model com-
pounds. The readily tunable properties of the aza-DIPY
moieties by substitution on the phenyl rings and complexa-
tion with different metals in the nitrogen “pocket” offer the
Figure 2. Cyclic voltammetry (CV) plots of 3a and 4a film drop-
versatility of synthesizing variable aza-DIPY-based oligo-
casted on a glassy carbon working electrode and measured in 0.1
mers and polymers with interesting electronic properties.
M Bu4NPF6 acetonitrile solution using a Ag/AgNO3 nonaqu-
Charge-transport properties and evaluation of these com-
eous reference electrode. Straight lines illustrate how the onsets
were estimated.
pounds in organic solar cells are underway.
Acknowledgment. We are grateful to Case Western
Reserve University (CWRU) for providing start-up funds.
We thank Prof. Thomas Gray (CWRU) for initial DFT
calculations that guided this work, Prof. Daniel A.
Scherson (CWRU) and Adriel Jebin Jacob Jebaraj
(CWRU) for their help with cyclic voltammetry, Prof.
Lei Zhu (CWRU) and Saide Tang (CWRU) for help with
DSC, and Dr. Dale Ray (CWRU) for help with NMR.
This material is based upon work supported by the Na-
tional Science Foundation under Grant No. MRI-0821515
(for the purchase of the MALDI-TOF/TOF).
stabilization of radical anions through either increased
conjugation length or πꢀπ stacking. HOMO and LUMO
energy levels were estimated from the onsets of the oxida-
tion and reduction waves, respectively. The electrochemi-
cal band gaps estimated were low, e.g. 1.2 eV for 3a, and
even lower for the 4aꢀc series. The values were lower than
the estimated optical band gaps, perhaps due to the
estimation method (see Figure 1 and 2). All compounds
had a HOMO energy level deeper than the air stability
threshold of about ꢀ5.2 eV,18 indicating that these com-
pounds should be oxidatively stable in air. LUMO energy
levels for the 3aꢀc series were ꢀ4.0 to ꢀ4.1 eV. Chelation
with BF2 stabilized the LUMO energy levels by about
0.4 eV to give compounds with high electron affinity. For
comparison, the LUMO energy level of electron acceptor
Supporting Information Available. Detailed experimen-
tal procedures of synthesis, characterization, and DFT
calculations. This material is available free of charge via
the Internet at http://pubs.acs.org.
Org. Lett., Vol. 13, No. 19, 2011
5357