Synthesis and properties of a dicationic π-extended dithieno[3,2- c:2′,3′- e ]-2,7-diketophosphepin

Article abstract of DOI:10.1021/om400991a

Methylation of the triazole units of a π-extended, fluorescent dithieno[3,2-c:2′,3′-e]-2,7-diketophosphepin gives rise to a N,N-dimethylated species that shows considerably blue-shifted absorption and emission wavelengths, as well as two reversible and remarkably low reduction steps at E_red = -1.12 and -1.39 V that underline its improved electron-acceptor features. The observed properties are the result of considerably altered electronic as well as structural features triggered by simple methylation of the scaffold.

Full text of DOI:10.1021/om400991a

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Synthesis and Properties of a Dicationic π‑Extended
Dithieno[3,2‑c:2′,3′‑e]‑2,7-diketophosphepin
Xiaoming He and Thomas Baumgartner*
Department of Chemistry & Centre for Advanced Solar Materials, University of Calgary, 2500 University Drive, NW, Calgary, Alberta
T2N 1N4, Canada
S
* Supporting Information
ABSTRACT: Methylation of the triazole units of a π-extended, fluorescent
dithieno[3,2-c:2′,3′-e]-2,7-diketophosphepin gives rise to a N,N-dimethylated
species that shows considerably blue-shifted absorption and emission
wavelengths, as well as two reversible and remarkably low reduction steps at
Ered = −1.12 and −1.39 V that underline its improved electron-acceptor
features. The observed properties are the result of considerably altered
electronic as well as structural features triggered by simple methylation of the
scaffold.
he exploration of π-conjugated cyclic organophosphorus
species as electron-accepting materials is a promising field
conjugated materials can easily undergo methylation at the 2-
position. With the aforementioned facts in mind, we envisioned
the triazole-extended DTDKP system B to be a suitable scaffold
for accessing enhanced electron-acceptor features via N-
methylation. For this model study, we have thus synthesized
compound 1 with two dodecyl chains (for improved solubility),
as well as its N,N-dimethylated relative 2. The neutral species 1
was prepared according to our previously reported procedure,2
and methylation toward 2 was easily achieved by reaction with
an excess of methyl triflate (Scheme 2).
T
of research, due to the fact that the LUMO energy in these
systems is commonly stabilized via σ*−π* orbital coupling.1
Very recently, we reported a new series of multifunctional
conjugated materials (B) with the novel dithieno[3,2-c:2′,3′-e]-
2,7-diketophosphepin core (DTDKP; A) that were accessible
via simple Huisgen click reactions with azides (Scheme 1).2
Scheme 1. Synthesis of Conjugated Materials B from the
DTDKP Building Block A
Scheme 2. Synthesis of 2 from 1
The species showed intriguing electron-acceptor and photo-
physical properties, as well as promising self-assembly behavior.
Moreover, in a couple of related studies we have revealed that
methylation of the nitrogen centers in diazadibenzophosphole
oxide3 and P-triazole phosphole oxides4 enhances the electron-
acceptor character of these systems and also provides an
opportunity for the tuning of their optical properties and self-
assembly behavior.4 This report now presents the synthesis of a
dicationic N,N-dimethylated π-extended DTDKP, as well as the
effect of the methylation on the optical and electronic
properties of the scaffold.
In our previous study, the trivalent phosphorus center of the
DTDKP building block (A) was found to be inert toward P-
methylation, because of the electron-accepting properties of the
scaffold that considerably reduce the Lewis basicity of the P
center.2 On the other hand, previous research by others5 and
us4 also showed that a triazole unit in corresponding
The identity and purity of 1 were confirmed by conventional
spectroscopic methods, whereas the NMR characterization of 2
in CD2Cl2 posed some problems. Most of the 1H NMR signals
and the 31P resonance of 2 were found to be very broad (see
the Supporting Information), likely as a result of the ionic
nature of the species and/or the presence of several conformers
(vide infra). This also precluded the collection of satisfactory
13C NMR data. However, using d6-DMSO as solvent provided
sharp 1H and 31P NMR signals, allowing for a clear
identification of 2. In addition, the identity of 2 was
unequivocally confirmed by high-resolution MALDI-TOF
mass spectroscopy, with the isotope pattern of the expanded
Received: October 9, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/om400991a | Organometallics XXXX, XXX, XXX−XXX

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Note
[2 − OTf]+ ion fully agreeing with its simulated pattern (Figure
1).
Figure 3. Cyclic voltammograms of 1 and 2 in CH2Cl2 solution
containing 0.1 M tetrabutylammonium hexafluorophosphate as the
supporting electrolyte. Scan rate: 100 mV/s. Ferrocene was added as
an internal standard and referenced to 0 V.
Figure 1. High-resolution MALDI-TOF spectrum of 2. The inset
shows the expanded ion cluster of [M − OTf]+ and its simulated
isotope pattern.
The UV−vis and emission spectra of 1 and 2 are depicted in
reversible reduction peaks (Ered = −1.53 and −1.84 V) that are
quite similar to those of the related, aryl-terminated systems
(Ered ≈ −1.5, −1.8 V).2 The cyclic voltammogram of the
dicationic species 2 indicated considerably enhanced electron-
accepting properties, with the two reduction events occurring at
Ered = −1.12 and −1.39 V. Moreover, the reversibility of the
two reduction steps suggests further improved stability of the
singly and doubly reduced species, in comparison to the
reduced species of the neutral congener 1. Using ferrocene as
internal reference, the LUMO energies of 1 and 2 were
determined to be ELUMO = −3.27 and −3.68 eV, respectively
(ELUMO= −(Ered1+4.8) eV).6 The value for 2, in particular, is
comparable to that of fullerene derivatives such as phenyl-C61-
butyric acid methyl ester (PCBM; cf. ELUMO = −3.7 eV), which
are commonly employed as n-type components in organic
electronics.7 According to the Randles−Sevcik equation,8 the
diffusion coefficients for 1 and 2 were calculated to be 5.6 ×
10−7 and 5.3 × 10−7 cm2 s−1, respectively. The linear fit (see the
Supporting Information) demonstrates that the reduction of
both compounds operates under diffusion control and they
were not absorbing to the electrode surface.
Figure 2, and the photophysical data are shown in Table 1.
Figure 2. UV−vis (a) and emission (b) spectra of 1 (c = 5.0 × 10−5
M) and 2 (c = 5.0 × 10−5 M) in CH2Cl2 solution. Inset pictures show
their colors under normal light (a) and upon excitation at 365 nm (b).
Similar to its related neutral congeners (B, R = aryl; λabs ∼286,
460 nm),2 the UV−vis spectrum of the orange CH2Cl2 solution
of 1 showed two intense absorption bands at λabs 288 and 462
nm. An orange-red emission at λem 599 nm which also
correlates with that of the aryl-terminated congeners (λem ∼595
nm)2 was observed. After methylation, 2 exhibited strongly
blue-shifted UV−vis absorption (λabs 275 and 393 nm) and
emission (λem 458 nm) wavelengths. This hypochromic shift
can be attributed to the suppression of the charge transfer (CT)
from the peripheral electron-donating triazole unit to the
electron-accepting DTDKP moiety.2 In addition, N-methyl-
ation also likely disrupts the coplanarity of the conjugated
molecules, further inhibiting CT (vide infra).
DFT calculations at the B3LYP/6-31G(d) level of theory9
using simplified models (dodecyl replaced by methyl) were
performed to support the experimentally obtained optical and
electronic characteristics; to account for the omission of the
counteranions in 2′, a PCM solvation model (in CH2Cl2) was
applied. It should be noted that while the DFT calculations do
not reproduce the experimental data as well, likely due to the
use of simplified models, they nevertheless confirm the trend in
the experimental data. The HOMO and LUMO orbitals of 1′
and 2′ are shown in Figure 4, and their energy levels are given
in Table 1. For both the neutral and cationic species 1′ and 2′,
To elucidate the effect of the methylation on the electronic
properties, the electrochemical features of 1 and 2 were
determined by cyclic voltammetry (CV, Figure 3), and the data
are summarized in Table 1. Compound 1 shows two quasi-
Table 1. Photophysical, Electrochemical, and Calculation Data for 1 and 2
electrochemistry
DFT calculation/eV
HOMO
a
a
d
e
f
compd
λabs /nm (ε/dm3 mol−1 cm−1
)
λem /nm (ϕPL
)
Ered,1/2/V
LUMO/eV
LUMO
Eg
b
1
288 (20320), 462 (10650)
275 (33520), 393 (16000)
599 (0.11)
Ered1 = −1.53
Ered2 = −1.84
Ered1 = −1.12
Ered2 = −1.39
−3.27
−2.55
−5.52
2.97
3.31
c
2
458 (0.05)
−3.68
−3.81
−7.12
a
b
c
In CH2Cl2. Photoluminescence quantum yields in CH2Cl2, relative to fluorescein. Photoluminescence quantum yields in CH2Cl2, relative to
d
e
f
quinine sulfate. Potentials versus Fc/Fc+. Calculated using the ferrocene HOMO level at −(4.8 + Ered,1/2) eV. Eg = LUMO − HOMO.
B
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promising features for the further development of cyclic
organophosphorus-based conjugated materials as powerful
electron acceptors for organic electronics.
EXPERIMENTAL SECTION
■
All chemical reagents were purchased from commercial sources
(Aldrich, Alfa Aesar, Strem) and were, unless otherwise noted, used
without further purification. Solvents were dried using an MBraun
solvent purification system prior to use. Compounds A2 and dodecyl
azide10 were prepared according to reported procedures. All reactions
and manipulations were carried out under a dry nitrogen atmosphere,
employing standard Schlenk techniques. 31P{1H} NMR, 1H NMR, and
13C{1H} NMR were recorded on Bruker Avance (-II,-III) 400 MHz
spectrometers. Chemical shifts were referenced to external 85%
H3PO4 (31P) or residual nondeuterated solvent peaks (1H, 13C). Mass
spectra were run on a Finnigan SSQ 7000 spectrometer or a Bruker
Daltonics AutoFlex III system. All photophysical experiments were
carried out on a Jasco FP-6600 spectrofluorometer and UV−vis−near-
IR Cary 5000 spectrophotometer. Cyclic voltammetry was performed
on an Autolab PGSTAT302 instrument, with a polished glassy-carbon
electrode as the working electrode, a Pt wire as the counter electrode,
and an Ag/AgCl/KCl3M reference electrode, using ferrocene/
ferrocenium as internal standard. The experiments were performed
in dichloromethane solution with tetrabutylammonium hexafluor-
ophosphate (0.1 M) as supporting electrolyte. Theoretical calculations
have been carried out at the B3LYP/6-31G(d) level using the
GAUSSIAN 09 suite of programs.9
Figure 4. Optimized structures (top) and LUMO (middle) and
HOMO (bottom) orbitals of 1′ (left) and 2′ (right), as well as their
energies calculated at the B3LYP/6-31G(d) level.
Synthesis of 1. This compound was synthesized according to a
modification of a reported procedure.2 A (40 mg, 0.11 mmol), dodecyl
azide (69 mg, 0.33 mmol), Cu(PPh3)3Br (20 mg, 20%), and DIPEA
(55 mg, 0.44 mmol) were mixed in THF (2 mL) under an N2
atmosphere, and the mixture was stirred for 48 h at room temperature.
After removal of the volatiles, the product was purified by column
chromatography using CH2Cl2 with 0−5% CH3OH to afford a red
solid. Yield: 35 mg, 40%. 31P NMR (162 MHz, CD2Cl2, δ): 65.7 ppm.
1H NMR (400 MHz, CD2Cl2, δ): 7.88 (s, 2 H; triazole), 7.68−7.58
(m, 7 H; Ar), 4.43 (t, J = 7.2 Hz, 4 H; CH2), 1.97 (m, 4 H; CH2),
1.39−1.30 (m, 36 H; CH2), 0.91 (t, J = 7.2 Hz, 6 H; CH3) ppm. 13C
NMR (100.6 MHz, CD2Cl2, δ): 200.2 (d, JCP = 36.5 Hz), 140.6 (s),
140.0 (d, JCP = 41.8 Hz), 139.0 (s), 137.9 (d, JCP = 18.8 Hz), 133.5 (s),
131.5 (s), 128.9 (d, JCP = 9.7 Hz), 126.2 (d, JCP = 8.5 Hz), 124.1 (d,
JCP = 4.0 Hz), 120.2 (s), 50.7 (s), 31.9 (s), 30.2 (s), 29.6 (s), 29.5 (s),
29.4 (s), 29.3 (s), 29.0 (s), 26.4 (s), 22.7 (s), 13.9 (s) ppm. HRMS
(MALDI-TOF): m/z 799.3940 [M]+ (calcd 799.3951), 837.3663 [M
+ K]+ (calcd 837.3516).
Synthesis of 2. To a solution of 1 (32 mg, 0.04 mmol) in CH2Cl2
(3 mL) was added MeOTf (66 mg, 0.4 mmol). The reaction mixture
was stirred overnight, and all volatile materials were removed under
vacuum. The residue was further washed with MeOH and then diethyl
ether to give the pure product as a viscous yellow oil. Yield: 31 mg,
80%. 31P NMR (162 MHz, CD2Cl2, δ): 59.8 ppm. 31P NMR (162
MHz, d6-DMSO, δ): 62.9 ppm. 1H NMR (400 MHz, CD2Cl2, δ): 8.77
(s, br, 2 H; triazole), 7.98 (s, br, 2 H; Ar), 7.47−7.05 (m, br, 5 H; Ar),
4.45−4.40 (m, br, 10 H; CH2 and CH3), 1.95 (s, br, 4 H; CH2), 1.31
(m, 36 H; CH2), 0.92 (t, J = 6.8 Hz, 6 H; CH3) ppm. 1H NMR (400
MHz, d6-DMSO, δ): 9.42 (s, 2 H; triazole), 8.01 (s, 2 H; Th), 7.62 (m,
3 H; Ph), 6.88 (m, 2 H; Ph), 3.96 (m, 6 H; CH3), 3.83 (m, 4 H; CH3),
1.72−1.24 (m, 40 H; CH2), 0.85 (t, J = 6.8 Hz, 6 H; CH3) ppm. 19F
NMR (282 MHz, CD2Cl2, δ): −81.0 ppm. 19F NMR (282 MHz, d6-
DMSO, δ): −77.8 ppm. HRMS (MALDI-TOF): m/z 977.3843 [M −
OTf]+ (calcd 977.3863).
the HOMO and LUMO orbitals mainly consist of the π and π*
systems of the conjugated backbone, respectively.
Upon methylation two significant changes can be observed.
(1) The HOMO and LUMO energy levels of 2′ are
dramatically decreased by 1.60 and 1.26 V, respectively. In
comparison to the energy gap of 1′ (Eg = 2.97 eV), the energy
gap in 2 is consequently increased (Eg = 3.31 eV), which
supports the experimentally observed blue-shifted photophysics
of 2. (2) While the optimized structure of 1 retains coplanarity
between the triazole units and the DTDKP core, 2′ exhibits a
twisted structure with torsion angles of ∼39.5° between the
triazole units and DTDKP, due to the steric effect of the methyl
groups (note that additional conformers can be assumed to
exist in solution). These two features support the combination
of electronic and structural effects being responsible for the
observed optical and electronic features of 2. From the LUMO
in 2′ it is evident that the triazolium units show electron-
acceptor character by virtue of the extension of the π* system
(in comparison to 1′) that now also includes their ipso carbon
atoms to a greater extent. Moreover, the inductive effect of the
cationic triazolium units likely adds to the lowered energy of
the LUMO and clearly supports its considerably enhanced
electron-acceptor features, as confirmed via cyclic voltammetry.
The pronounced energy drop of the HOMO in 2′ can be
ascribed to the loss of conjugation throughout the triazole units
(as opposed to 1′) as a result of the twisted structure, which
confines the chromophore essentially to the DTDKP core.
To conclude, we have synthesized a bis(dodecyltriazole)-
terminated dithieno[3,2-c:2′,3′-e]-2,7-diketophosphepin, which
was cleanly converted into the dicationic N,N-dimethylated
congener by reaction with methyl triflate. The methylation
results in a dramatic change of the photophysical properties as a
result of a twisted backbone and the loss of donor character in
the triazole units, precluding any low-energy CT processes.
Moreover, the inherent dicationic charge also considerably
reduces the reduction potential of the material and
concomitantly stabilizes the reduced species. These are
ASSOCIATED CONTENT
■
S
* Supporting Information
Figures and tables giving a high-resolution MALDI-TOF of 1,
extensive CV data, NMR spectra of 1 and 2, and Cartesian
coordinates of DFT-optimized structures for 1′ and 2′. This
C
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material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for T.B.: thomas.baumgartner@ucalgary.ca.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support by the NSERC of Canada, as well as the
Canada Foundation for Innovation (CFI), is gratefully
acknowledged.
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