Extended 2,5-diazaphosphole oxides: Promising electron-acceptor building blocks for π-conjugated organic materials

Article abstract of DOI:10.1002/chem.201000382

(Chemical equation presented) BTD makeover-phosphorus edition: Replacing the sulfur atom in πextended 2,1,3-benzo[c]thiadiazoles (BTD) by a phosphoryl group affords the materials with improved electronacceptor properties. The significantly lower reduction potentials and competitive electron-transfer rates make the new diazaphosphole oxides excellent candidates for application in π-conjugated organic materials (see figure).

Full text of DOI:10.1002/chem.201000382

COMMUNICATION
DOI: 10.1002/chem.201000382
Extended 2,5-Diazaphosphole Oxides: Promising Electron-Acceptor
Building Blocks for p-Conjugated Organic Materials
Thomas Linder, Todd C. Sutherland,* and Thomas Baumgartner*[a]
The growing need for energy-efficient, low-cost electron-
ics has led to the development of a variety of conceptually
new building blocks. A large number of organic p-conjugat-
ed materials have now successfully proven their utility for
practical applications in organic light-emitting diodes
(OLEDs),[1] organic field-effect transistors (OFETs),[2] and
organic photovoltaics (OPVs).[3] The significant benefits of
using organic components to access semiconducting materi-
als lie in the countless opportunities for fine-tuning the
bandgap of the materials through variation of their building-
block components.[4] One of the most common electron-ac-
cepting building-block components in this context is the
2,1,3-benzo[c]thiadiazole unit that is utilized in a plethora of
molecular and polymeric materials.[1,3–5] By using this unit,
p-conjugated materials can be formed with low-energy
LUMOs that improve the n-type character of materials, or
help significantly reduce the bandgap in conjunction with
suitable donor building blocks.
thene and phenanthrene backbones and will address general
accessibility, as well as their photophysical and electronic
properties, including theoretical calculations.
Remarkably, the number of known 2,5-diazaphospholes to
date is fairly limited, due to rearrangement reactions of the
heterocyclic ring system. Most known systems either exist as
W(CO)5-complexes, or exhibit donor substituents at the 4,5-
position of the ring;[8] Dyer and co-workers very recently re-
ported fused diazaphospholes in the context of p-conjugated
materials,[9] and Rꢀhlmann and co-workers reported the syn-
thesis of two diphenyl-substituted derivatives several de-
cades ago, but no properties were reported.[10] Using a simi-
lar strategy, we synthesized the p-extended 2,5-diazaphos-
AHCTUNGTREGpNNUN hole oxides, shown in Scheme 1, from the corresponding
In the context of studies on organophosphorus p-conju-
gated materials, some of us have established the dithieno-
ACHTUNGTRENNUNG
[3,2-b:2’,3’-d]phosphole system over the past seven years.[6]
Our studies, and those of others,[6,7] have revealed that the
incorporation of phosphole units in ring-fused systems
afford p-conjugated materials with a variety of beneficial
features, one of which is low-lying LUMO energy levels that
emphasize their electron-acceptor characteristics. In this
communication, we report a new molecular building block
that combines the beneficial features of benzothiadiazoles
and phospholes to access improved electron-acceptor char-
acteristics. For synthetic reasons, our study focuses on p-ex-
tended 2,5-diazaphosphole oxide systems based on acenaph-
Scheme 1. Synthesis of the 2,5-diazaphosphole oxide derivatives.
[a] Dr. T. Linder, Prof. Dr. T. C. Sutherland, Prof. Dr. T. Baumgartner
Department of Chemistry, University of Calgary
2500 University Drive NW, Calgary
AB T2N 1N4 (Canada)
phenanthrene and acenaphthene diimines 1a and 2. The re-
actions proceed directly by mixing both reactants without
the use of solvent to provide products 3a and 4 as orange
powders in good yields (see the Supporting Information).[11]
All substances were characterized by NMR spectroscopy,
HRMS, and elemental analysis. Interestingly, the 31P NMR
Fax : (+1)403-289-9488
E-mail: todd.sutherland@ucalgary.ca
thomas.baumgartner@ucalgary.ca
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201000382.
Chem. Eur. J. 2010, 16, 7101 – 7105
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7101

shift of the phenanthrene-derived compound 3a (d31P=
76.9 ppm) differs significantly from that of the acenaphthene
derivative 4 (d31P=103.0 ppm).
C N bonds (128.7(7) and 128.4(6) pm) are clearly in the
ꢀ
ꢀ
range of double bonds, whereas the P N bonds (171.9(4)
ꢀ
and 170.8(4) pm) and the C C bond (151.8(7) pm) in the
Addition of water to the NMR spectroscopy samples nei-
ther causes shifting of the 31P resonances nor the appearance
of new signals from decomposition products, which suggests
environmental stability. Compounds 3a and 4 also show
good thermal stability; upon heating to 2508C, neither could
mass loss be detected by thermogravimetric analysis (TGA)
nor were phase transitions observed with differential scan-
ning calorimetry (DSC). In addition, we have investigated
the photophysical properties of the substances by means of
fluorescence spectroscopy; both 3a and 4 are fluorescent in
dilute dichloromethane solutions. Compound 4 shows a
maximum emission at lem =495 nm, which gives a blue–
green fluorescence, whereas 3a reveals an emission maxi-
mum of lem =550 nm, which results in a yellow fluorescence.
The fluorescence spectra of the compounds are shown in the
Supporting Information. For both substances, the maximum
absorption is found in the UV region at lmax =350 nm for 3a
and lmax =355 nm for 4.
five-membered ring possess bond lengths typical for single
bonds involving pentavalent P-species.[8a,c]
The molecular packing of 3a in the solid state supports
stacks of molecules that are stabilized by p–p interactions
(Figure 1). The nonplanar parts of the molecule arrange in
an alternating manner. The stacking distance between two
molecules is found to be 338.7 pm, which is significantly
shorter than the parent 9,10-phenanthrenequinone
(351.5 pm),[13] or phenanthro
(349.5 pm).[14]
ACHTUNGTRNE[NUNG 9,10-c]-1,2,5-thiadiazole oxide
We also succeeded in accessing the brominated species 3b
(d31P=76.7 ppm) through a similar protocol by using the di-
brominated diimine 1b (Scheme 1), however, the fluores-
cence of 3b was found to be negligible, which is likely to be
due to the heavy atom effect of bromine. The synthesis of
3b nevertheless proves that access to a functional compo-
nent is possible and that it could be employed in organic
materials through, for example, cross-coupling procedures.
Efforts to reduce the pentavalent phosphorus center in 3a
and 4 for further manipulation of the electronic properties
of the scaffold, however, were found to be challenging.
Even with a range of reductants (BH3/NMe3, HSiCl3, PBu3)
that have successfully been used for this purpose in related
systems,[6c,7] clean and selective P-reduction could not be
achieved.
Moreover, single crystals of 3a suitable for X-ray struc-
ture determination were obtained from a concentrated tolu-
ene solution.[12] As expected, the analysis shows a fused tet-
racyclic compound, with the phenanthrene backbone and
the five membered diazaphosphole unit forming a planar p-
conjugated molecular scaffold (Figure 1). The lengths of the
To better understand the observed features for the ex-
tended 2,5-diazaphospholes 3a and 4, we have performed
DFT calculations (B3LYP/6-31G(d+) level of theory)[15] on
the neutral species, as well as their radical anions. For com-
parison, we have also included the corresponding sulfur ana-
logue 5,[10a] as well as the parent phosphorus and sulfur
benzo-systems 6 and 7 (Scheme 2).
Scheme 2. Additional compounds used for comparison in the DFT calcu-
lations.
The calculations on the parent systems reveal that the re-
placement of the sulfur in benzothiadiazole (7) by a phos-
phoryl group (6) has a significant effect on the energy level
of the LUMO. The LUMO of 6 is more than 1 eV lower
than the LUMO of the sulfur analogue 7 (6: E=ꢀ3.87 eV,
7: E=ꢀ2.67 eV), but the HOMO energy levels of 6 and 7
are similar (6: E=ꢀ7.04 eV, 7: E=ꢀ6.89 eV). The same is
true for the extended systems (see the Supporting Informa-
tion). The low-lying LUMO energy levels in 3a (E=
ꢀ3.03 eV; cf., 5: E=ꢀ2.01 eV), 4 (E=ꢀ2.60 eV), and 6 cor-
relate with those of native phosphole derivatives and can be
Figure 1. Molecular structure and packing of 3a in the solid state (50%
probability level). The hydrogen atoms are omitted for clarity. Selected
ꢀ
ꢀ
ꢀ
bond lengths [pm] and angles [8]: P1 N1 171.9(4), P1 N2 170.8(4), P1
ꢀ
ꢀ
ꢀ
ꢀ
O1 147.3(4), P1 C15 179.0(5), N1 C1 128.7(7), N2 C2 128.4(6), C1 C2
151.8(7), N(2)–P(1)–N(1) 98.74(8), C(1)–N(1)–P(1) 105.92(13), C(2)–
N(2)–P(1) 106.24(13), N(1)–C(1)–C(2) 114.55(16), N(2)–C(2)–C(1)
114.53(16).
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Chem. Eur. J. 2010, 16, 7101 – 7105

Extended 2,5-Diazaphosphole Oxides
COMMUNICATION
attributed to an interaction of the s* orbital of the exocyclic
substituents at phosphorus with the p* orbital of the conju-
gated main scaffold.[7] Despite the different energy levels,
the shape of the LUMOs (and other relevant frontier orbi-
tals) is almost identical in both the phosphorus and sulfur
species, which supports the strong correlation between these
systems. Importantly, upon reduction to the radical anions,
the frontier orbitals retain their shape in both families; this
indicates that the radical is easily delocalized within the p
system without affecting the molecular integrity of the scaf-
fold. It is interesting to note, however, that the spin densities
of the phosphorus-based systems are significantly different
to both of the sulfur species 5 and 7. In 3a and 4, a large
portion of the spin density is located at the carbon atoms of
the diazaphosphole subunit, but only little spin density is
found on the phosphorus center, which supports the pres-
ence of a diazabutadiene radical anion fragment (see the
Supporting Information). In stark contrast, the spin density
of the sulfur analogue 5 is mostly located on the SN2 frag-
ment. This difference in spin density distribution can also
explain the observed variances in the electrochemical reduc-
tion of 3a, 4, and 7 (see below), which supports a much
more delocalized and stable radical anion for the P-systems.
To verify the electron-acceptor characteristics of the di-
ported electron mobility of 8ꢂ10ꢀ5 cm2 Vꢀ1 sꢀ1 (at E=
106 Vcmꢀ1) for phosphole P-oxides, which demonstrates the
prospects for 3 and 4 in similar applications.
Figure 2b demonstrates that compound 3a operates under
diffusion control and is not absorbing to the electrode sur-
face and the linear fit is used to extract the diffusion coeffi-
cient of 4.4ꢂ10ꢀ7 cm2 sꢀ1, according to the Randles–Sevcik
equation.[17] Note that compounds 4 and 7 also show a linear
relationship between the square root of scan rate and peak
current. By using Nicholsonꢃs method,[18] the anodic (Epa)
and cathodicACTHNUTRGNE(UNG Epc) peak separation is correlated with the ap-
parent electron-transfer rate constant, kapp, resulting in a
moderate rate of 9.6ꢂ10ꢀ4 cmsꢀ1 for 3a. To compare, com-
pounds 4 and 7 have kapp values of 4.4ꢂ10ꢀ4 cmsꢀ1 and 3.1ꢂ
10ꢀ4 cmsꢀ1, respectively. The electrochemical results show
that both 3a and 4 are easier to reduce than benzothiadi-
AHCTUNGTREGaNNUN zole 7 and possess competitive electron-transfer character-
istics, which could lead to improved n-type materials.
ACHTUNGTRENNUNGazaphosphole oxides, we have performed cyclic voltammetry
using 3a and 4 and compared the results with the perfor-
mance of 2,1,3-benzo[c]thiadiazole 7. Compound 3a shows
quasireversible redox behavior at various scan rates, which
indicates the occurrence of stable redox switching (Fig-
ure 2a). The average of the anodic and cathodic peak poten-
Figure 3. UV/Vis spectroelectrochemistry of 3a at various time periods of
ꢀ0.7 V applied potential. Top: UV/Vis absorption profile of 3a at 0
(c), 5 (b), 10 (a), and 15 (d) minute intervals after an applied
ꢀ0.7 V potential. The arrows indicate the direction of peak movement.
Bottom: The absorption difference spectra, initial spectra were subtract-
ed from each of the spectra at the indicated time intervals, which clearly
show the radical anion spectral evolution and isosbestic points.
Figure 3 is the UV/Vis spectroelectrochemical result of re-
ꢀC
ducing 3a to the radical anion 3a . The initial UV/Vis ab-
sorption spectrum of 3a consists of two main features: a
strong p–p* transition at 275 nm and a weaker n–p* transi-
tion at 330 nm. A negative potential of ꢀ0.7 V was applied
to the spectroelectrochemical cell and UV/Vis spectra were
recorded every 5 min. The absorption difference spectra,
shown at the bottom of Figure 3, illustrate clear isosbestic
points that support a clean conversion to the radical anion.
In addition, the negative growing peaks of Figure 3 can be
clearly correlated with neutral 3a, whereas the new spectral
features appearing at 260 nm and 310 nm can be assigned to
the p–p* and n–p* transitions of the radical anion. The
blueshift in the radical anion structure is consistent with the
Figure 2. Cyclic voltammogram of 3a (3.1 mm) in DMF containing
Bu4NPF6 (0.1m) obtained by using a glass carbon working electrode, a
AgjAgCljKCl3m reference electrode, and a Pt wire counter electrode.
tials provides an estimate of the E1/2 value of ꢀ949 mV (vs.
Fc/Fc+). For comparison, under the same conditions 4 and 7
have significantly lower E1/2 values of ꢀ1224 and ꢀ1257 mV,
respectively. Matano and co-workers[16] recently reported an
acenaphthoACHTUNGTRENNUNG[1,2-c]phosphole P-oxide with a reduction poten-
tial of ꢀ1820 mV (vs. Fc/Fc+) that displayed the highest re-
ꢀC
DFT-calculated HOMO–LUMO energy gaps of 3a .
Chem. Eur. J. 2010, 16, 7101 – 7105
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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7103

T. Baumgartner, T. C. Sutherland and T. Linder
Furthermore, compound 3a was reduced chemically with
Keywords: conjugation · density functional calculations ·
Na in THF at room temperature, yielding a purple solution
of the stable radical anion 3a that was subjected to EPR
electron acceptor
heterocycles
· organic electronics · phosphorus
ꢀC
spectroscopy (Figure 4). The observed giso value of 2.0049 is
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used as starting values for the simulation (for details, see the
Supporting Information).
ꢀC
In conclusion, we have demonstrated the convenient syn-
thesis of p-extended 2,5-diazaphosphole oxides that can be
considered as phosphorus analogues of the very popular
2,1,3-benzo[c]thiadiazole-based materials. Theoretical calcu-
lations, as well as electrochemical studies have revealed that
the phosphorus-based systems show favorable reduction fea-
tures (reduction potential and electron-transfer rates) over
those of the ubiquitous benzothiadiazole, even in their p-ex-
tended forms. The DFT calculations further support the
strong correlation between the two classes of compounds,
but also indicated a different spin density distribution in the
radical anion that is very likely to be the reason for the
greater stability of the P-based radical anions. Our studies
have revealed that the replacement of the sulfur by a phos-
phoryl group affords materials that have dramatically im-
proved electron-acceptor characteristics that are highly de-
sirable for the next generation of organic electronic materi-
als. We are currently exploring the scope of potential appli-
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Acknowledgements
[11] Only with the pentavalent phosphorus species does the reaction
lead to the desired diazaphosphole framework. The use of trivalent
phosphorus synthons affords an inseparable mixture of compounds,
none of which could be identified as diazaphosphole species.
[12] Crystal data for 3a: (C20H13 N2OP): Mr =328.29; T=173(2) K;
Financial support by NSERC of Canada, the Canada Foundation of Inno-
vation, and the Institute of Sustainable Energy, Environment and Econo-
my (ISEEE) is gratefully acknowledged. T.B. thanks Alberta Ingenuity
for a New Faculty Award. We also thank T. K. Wood for helpful discus-
sions regarding the EPR spectroscopy.
mono
ACHTUNGTRENcNGNU linic, space group P21/c; a=7.3950(10), b=9.6610(10), c=
22.339(2) ꢉ; b=96.695(2)8; V=1585.1(3) ꢉ3; Z=4; 1calcd
=
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Extended 2,5-Diazaphosphole Oxides
COMMUNICATION
1.376 mgmꢀ3
measured reflections; 3258 [RAHCNUTGTRENNUNG
;
m=0.182 mmꢀ1
(int)=0.0179] independent reflections;
;
l=0.71070 ꢉ; qmax =27.418; 5244
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O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
Received: February 11, 2010
Published online: May 12, 2010
Chem. Eur. J. 2010, 16, 7101 – 7105
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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