Thieno[3,4-c]phosphole-4,6-dione: A Versatile Building Block for Phosphorus-Containing Functional π-Conjugated Systems

Article abstract of DOI:10.1002/chem.201602392

A versatile phosphorus-containing π-conjugated building block, thieno[3,4-c]phosphole-4,6-dione (TPHODO), has been developed. The utility of this simple but hitherto unknown building block has been demonstrated by preparing novel functional organophosphorus compounds and bandgap-tunable conjugated polymers.

Full text of DOI:10.1002/chem.201602392

DOI: 10.1002/chem.201602392
Communication
&
Photophysics
Thieno[3,4-c]phosphole-4,6-dione: A Versatile Building Block for
Phosphorus-Containing Functional p-Conjugated Systems
Youhei Takeda,* Kota Hatanaka, Takuya Nishida, and Satoshi Minakata*[a]
Abstract: A versatile phosphorus-containing p-conjugated
building
block,
thieno[3,4-c]phosphole-4,6-dione
(TPHODO), has been developed. The utility of this simple
but hitherto unknown building block has been demon-
strated by preparing novel functional organophosphorus
compounds and bandgap-tunable conjugated polymers.
The incorporation of main-group elements into the main
frameworks or the peripherals of p-electron conjugated sys-
tems has been emerging as a powerful strategy for the figura-
tion of functional p-conjugated molecules and polymers.[1] In
this sense, there has been growing interest in phosphorus-con-
taining p-conjugated compounds and polymers.[2] Particularly,
outstanding progress has been made in the field of phosp-
hole-based functional materials over the last decade:[3] the ap-
plications to OLED emitters[4] and bioimaging probes[5] repre-
sent remarkable highlights. Yet, for bringing further flexibility
and diversity to design principles for organophosphorus p-con-
jugated functional materials, novel promising P-containing
building blocks and synthetic methodologies thereof should
be explored.[6] In association with such synthetic challenges,
the existing phosphorus-incorporated p-conjugated polymers
have been limited to poly(phosphole)s,[7] poly(p-arylene/vinyl-
ene phosphane)s,[8] poly(p-phenylene phosphaalkene/diphos-
phene)s,[9] and poly(p-phenylenediethynylene phosphane)s.[10]
To tackle the challenges, we have recently developed new P-
containing electron-acceptors, DPITOs (Figure 1a),[11a] and revis-
ited related aromatic-fused diketophosphanyl compounds.[11b]
In addition, the Baumgartner group has reported seven-mem-
bered diketophosphanyl compounds, DTDKPs (Figure 1a), as
a unique building block for functional organophosphorus mol-
ecules.[12] Nevertheless, from the viewpoint of structural diversi-
ty of diketophosphanyl compounds, only 5–6–5,[11a] 5–7–5,[12]
6–7–6,[13b,d] 6–12–6,[13e] 5–6,[13a,b,c] and 6–6–6-fused[13d] ring sys-
tems have been reported to date. Moreover, diketophosphan-
yl-containing conjugated polymers are unknown.
Figure 1. Diketophosphanyl-based functional molecules and building blocks.
ture of their N-surrogates, TPYRDOs (Figure 1b).[14] The
TPYRDO skeleton has been emerging as a promising electron-
acceptor (A) unit for constructing donor–acceptor (D–A) conju-
gated low-bandgap polymeric materials for photovoltaics,[15]
due to its synthetic modulability, good electron-accepting abili-
ty, high planarity, and quinoidal character for effective exten-
sion of p-systems along the polymer backbone. Additionally,
such low-bandgap materials that absorb and/or emit near-IR
(NIR) light can find numerous applications like photodetectors
and security information displays.[16] Based on this background,
we envisioned that TPHODOs serve as a unique acceptor build-
ing block for assembling P-containing D–A p-conjugated sys-
tems spanning from small molecules to polymers, beyond the
precedent nitrogen-based systems. For example, TPHODO-
cored D–A conjugated molecules and polymers can have
lower bandgaps (Eg) than those of the corresponding nitrogen
surrogates, given that the (CꢀP)s*-p* orbital coupling operates
effectively. Moreover, the versatile reactivity of s3,l3-P-cen-
ters[17] would allow for fine-tuning the optoelectronic proper-
ties of the polymers through post-polymerization modifica-
tion.[7d,10]
Initially, we established preparative methods for the simplest
TPHODOs 1 and 2 (Scheme 1a).[18] The condensation of S1
with [(Tipp)P(SiMe3)2] (Tipp=2,4,6-triisopropylphenyl), which
was generated in situ through deprotonation/silylation of
(Tipp)PH2, afforded 1 in 44% yield (Scheme 1a). Importantly,
yellowish compound 1 was stable enough to be isolated by
column chromatography on silica gel, probably due to the
presence of two electron-withdrawing carbonyl groups.[11] The
structure of 1 was fully characterized by spectroscopic meth-
ods such as 1H, 13C, and 31P NMR (d= +7.54 ppm).[18] In a similar
Herein, we disclose a novel diketophosphanyl building
block, thieno[3,4-c]phosphole-4,6-diones (TPHODOs, Fig-
ure 1b). A blueprint was drawn up by focusing on the struc-
[a] Prof. Dr. Y. Takeda, K. Hatanaka, T. Nishida, Prof. Dr. S. Minakata
Department of Applied Chemistry, Graduate School of Engineering
Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871 (Japan)
E-mail: takeda@chem.eng.osaka-u.ac.jp
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201602392.
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Scheme 1. a,b) Synthetic routes to 1–4; c) P-functionalization of 1, 3, and 4.
Figure 2. ORTEP drawings of a) Mes-2 and c,d) 3 (H atoms are omitted for
clarity); dimeric pairs of b) Mes-2 and e) 3 (iPr groups and H atoms are omit-
ted for clarity).
manner, 1,3-dibrominated TPHODO 2 was prepared in a moder-
ate yield (Scheme 1a). Through this method, a variety of analo-
gous TPHODOs were prepared from S2 and [RP(SiMe3)2] (R=
Cy (Cy-2), Ph (Ph-2), Mes (Mes-2; Mes=2,4,6-trimethylphe-
nyl)).[18] As the result of screening of coupling conditions, the
Pd-catalyzed Migita–Kosugi–Stille coupling, applying 2 as an
acceptor and mono- and bithienyl tributylstannanes as donors,
was found to be effective for assembling D–A–D compounds 3
and 4 in high yields (Scheme 1b). Notably, the presence of
a sterically demanding group (Tipp) on the P-center was key to
the successful cross-coupling: phosphorus compounds bearing
a less-bulkier groups on the P-centers (Cy-2 and Ph-2) did not
give coupled products, even though they were consumed
(Table S1, Supporting Information). On the other hand, Mes-2
gave the corresponding doubly coupled product Mes-3 in
a moderate yield.[18] The nitrogen surrogates TPYRDOs (N-1–N-
4) were also prepared as reference compounds.[18] Most impor-
tantly, the developed s3,l3-phosphorus compounds are func-
tionalizable on the P-centers (Scheme 1c). Treatment of 1 with
[Au(tht)Cl] (tht=tetrahydrothiophene) provided s4-phosphorus
complex 1-AuCl in 90% yield, whereas 4-AuCl was obtained
from 4 in 82% yield (Scheme 1c). The coordination of Au to
the P centers caused significant upfield shifts of 31P NMR reso-
nances (Dd (1-AuCl)=ꢀ2.50 ppm; Dd (4-AuCl)=ꢀ2.20 ppm).
The oxidation of 3 and 4 with meta-chloroperoxybenzoic acid
(mCPBA) gave the corresponding oxides 3-O and 4-O, respec-
tively (Scheme 1c).[19]
the dimeric pairs that are apparently connected through a pair
of self-complementary two-point halogen bonds (XBs)[22] (Cꢀ
Br···O=C, Figure 2b), as evidenced by the linearity (<CꢀBr···O=
1728) and the shorter distance [d(Br···O)=3.08 ꢁ] than the sum
of van der Waals radii (3.37 ꢁ).[23] The nitrogen surrogate of 2
(N-2) also adopts self-complementary XB motifs to form a 1D-
sheet in the solid state (Figure S15a, Supporting Information).
Since such self-complementary multipoint XB motifs are very
rare in the literature,[24] but should serve as a mimic of hydro-
gen-bonding counterparts,[25] these findings would offer an op-
portunity for utilizing the 1,3-dibromo-TPHODO and related
structures as fundamental motifs in crystal engineering. The
terthiophene unit of 3 lies coplanar (Figure 2c) and perpendic-
ular to the Tipp ring (Figure 2d). In both cases, the lengths of
the endo- (1.85–1.87 ꢁ) and exocyclic PꢀC bonds (ca. 1.82 ꢁ)
are almost the same as those of reported diketophosphanyl
compounds.[11,12a,13e,21] The terthiophene units of 3 are stacked
in a face-to-face manner so that the two diketophosphanyl
units point to opposite sides, with the interplane distance of
3.55 ꢁ (Figure 2e).
Figure 3a illustrates the UV/Vis and PL spectra of 1, 3, 4, and
4-O, and the properties of TPHODOs are summarized in
Table 1.[18] The CH2Cl2 solution of 1 showed a strong absorption
at around 250 nm that corresponds to a H—5!L transition,
which was assigned by TDDFT calculation (Table S7, Supporting
Information), and a weak n–p* absorption (H—1!L transition)
at around 320 nm (Figure 3a). As the p-system was extended,
the peaks in the lower-energy regime (300–560 nm) assignable
to p–p* (H!L) transitions significantly redshifted (1: 329 nm,
3: 400 nm, 4: 484 nm). In line with the increased overlap be-
tween the HOMOs and LUMOs (1<3<4, Figure S17, Support-
ing Information), the molar absorption coefficient (e) drastically
increased in the same order. The absorptions in the higher
The single crystals of Mes-2, 3 and N-2 suitable for the X-ray
crystallographic analysis were grown from CH2Cl2/hexane solu-
tions.[20] It should be noted that this is the first report to eluci-
date the molecular geometry of 5–5-fused diketophosphanyl
compounds (Figure 2). As expected, the P-centers of Mes-2
and 3 adopt trigonal pyramidal structures (Figure 2a and c)
that are deviated from the mean plane of the thiophene-car-
bonyls with distances of 0.49 and 0.67 ꢁ, respectively. An intri-
guing feature about the packing structures of Mes-2 involves
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Table 1. Summary of properties of TPHODOs.
ox
red
Compd
labs [nm][a] (e [mꢀ1 cmꢀ1])
lem [nm][a] (FPL)[b]
E
/
E
[V vs. Fc/Fc+][c]
1/2
EHOMO/ELUMO [eV][e]
onset
1
329 (3,700)
380 (450)
ND (ꢀ)
ND/ꢀ2.00[d]
0.99/ꢀ1.53[d]
ND/ꢀ1.84
ꢀ6.58[f]/ꢀ3.10
ꢀ6.09/ꢀ3.57
ꢀ5.91[f]/ꢀ3.26
ꢀ6.04[f]/ꢀ3.68
ꢀ5.22/ꢀ3.37
ꢀ5.28/ꢀ3.73
ꢀ5.83/ꢀ3.66
1-AuCl
3
3-O
4
4-O
4-AuCl
ND (ꢀ)
400 (15,000)
448 (7,600)
484 (34,000)
540 (21,000)
515 (10,000)
492 (0.01)
561 (<0.01)
573 (0.07)
650 (0.02)
591 (0.01)
ND/ꢀ1.42
0.12/ꢀ1.72
0.18/ꢀ1.37
0.73/ꢀ1.44[d]
[a] Measured with CH2Cl2 solutions (10ꢀ5 m). [b] Determined with an integrating sphere. [c] Measured by CV using CH2Cl2 solutions (10ꢀ3 m) containing
nBu4N·PF6 as an electrolyte. [d] The onset potential. [e] EHOMO =-(5.1+
ox
red
E
) [eV]; ELUMO =ꢀ(5.1+
E
) [eV]. [f] EHOMO =ELUMOꢀ1240/onsetlabs [eV].
onset
1/2
yields (FPL), phosphorus compounds showed lower FPL than
those of the corresponding nitrogen surrogates,[18] probably
due to the existence of extra nonradiative decay channels,
such as dynamic pyramidal inversion[11a] and internal conver-
sion when compared with rigid and planar imide compounds.
CV revealed the energy profiles of the TPHODOs (Table 1
and Figure S7–S9, Supporting Information). Compared with the
N-surrogates, the LUMO energies of 3 and 4 are lower than
those of N-3 and N-4 by more than 0.1 eV, indicating the oper-
ation of s*–p* electronic coupling in these phosphorous com-
pounds. It is noted that the Au-complexation of 1 and 4 result-
ed in complex electrochemical profiles (Figure S8, Supporting
Information), which made it difficult to precisely determine
their redox potentials. Nevertheless, from the tentatively identi-
fied onset reductive potentials (redEonset ꢀ1.53 and ꢀ1.44 eV for
1-AuCl and 4-AuCl, respectively), the ELUMO of the gold com-
plexes were lower than those of 1 and 4 (Table 1), probably
due to the decreased electron densities of the diketophos-
phanyl rings through P-coordination to Au atoms. In contrast,
P-oxidation (3-O and 4-O) gave clear CV profiles, and distinc-
tive reversible reduction waves were observed (Figure S8b,
Supporting Information) in the more positive potential regime
when compared with 3 and 4 (Table 1).
Figure 3. a) UV/Vis and PL spectra of 1, 3, 4, and 4-O (10ꢀ5 m CH2Cl2 solu-
tions); b) frontier orbitals and energy diagrams of 4, 4-O, and N-4 calculated
TGA profiles of TPHODOs revealed their high stabilities for
trivalent phosphorus compounds (Figure S10–S13, Supporting
Information), probably because of the existence of two elec-
tron-withdrawing carbonyl groups. Such high thermostability
would be a favorable feature for practical use as organic mate-
rials.
by the DFT method.
energy regime (240–360 nm) were redshifted, and e was de-
creased, in the order of 1, 3, and 4 (Figure 3a), which was
probably caused by the change in the transitions from p–p* to
mixed n–p* and p–p* character. Diketophosphanyl com-
pounds 3 and 4 are emissive in solution under the UV irradia-
tion (3: 492 nm, 4: 573 nm) with subnanosecond order life-
times (Table S3, Supporting Information). Notably, the P-func-
tionalization of 3 and 4 resulted in a significant redshift in
both of the absorption (e.g., 4: 484 nm, 4-AuCl: 514 nm, 4-O:
540 nm) and the emission (e.g., 4: 573 nm, 4-AuCl: 591 nm, 4-
O: 650 nm). To investigate the effects of the P-incorporation,
the properties were compared with those of N-1, N-3, and N-
4.[18] Taking 4 and N-4 as an example, both of labs and lem of 4
locate in the lower-energy region compared with those of the
For further understanding the impact of P-incorporation and
P-functionalization, theoretical calculations using the DFT
method were performed (Figure 3b).[18] When compared with
N-4, the EHOMO of 4 (ꢀ5.77 eV) is almost the same as that of N-
4 (ꢀ5.75 eV), while a significant stabilization of the ELUMO was
gained by more than 0.1 eV, in agreement with the results of
CV experiments. This would be caused by the operation of s*–
p* orbital coupling, which is supported by the largely devel-
oped LUMO orbitals around the P-center (Figure 3b). Notably,
the P-oxidation of 4 resulted in a drastic stabilization of the
ELUMO by 0.3 eV and a slight drop of EHOMO, leading to the nar-
rower HOMO–LUMO band gap (2.79 eV) than those of 4
(3.00 eV) and N-4 (3.09 eV). This is again consistent with the CV
experiments and the redshift trend in absorption/PL spectra in
the order of N-4<4<4-O.
imide counterpart (4: labs =484 nm, lem =573 nm; N-4: labs
=
450 nm, lem =530 nm), indicating the validity of the P incorpo-
ration to narrow the HOMO–LUMO band gaps as well as to
tune emission color to longer wavelength. Regarding quantum
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To demonstrate the versatility of the TPHODO
building block 2, P-containing conjugated polymers
P1 and P2, along with the reference N-P1, were syn-
thesized through a Pd-catalyzed cross-coupling poly-
merization (Scheme 2).[18] The presence of the
TPHODO unit was confirmed by 13C and 31P NMR
(e.g., the carbonyl 13C resonance of P1: d=
204.4 ppm, the 31P resonances were found to range
from d= +8.42 to +9.25 ppm with the largest peak
at d=9.24 ppm).[18]
Properties of synthesized polymers are summarized
in Table 2. The CH2Cl2 solutions of P1 and P2 absorb
almost all the visible light as well as NIR light up to
800 nm (Figure 4a) and are emissive in the NIR
region (P1: 753 nm, P2: 712 nm) under UV irradiation
(Figure 4b). When compared with the nitrogen ana-
logue of P1 (N-P1), the optical bandgaps (optEg) of P1
(1.53 eV) and P2 (1.47 eV) were found to be much
narrower than that of N-P1 (1.63 eV), in line with the
similar trend with small molecules. The electrochemi-
cal bandgap (elcEg) reasonably agreed with optEg. Nota-
bly, as P1 was titrated with an aliquot of [Au(tht)Cl] in
CH2Cl2,[18] the absorbance in the NIR region from 650 to
800 nm gradually increased, while the absorption in the region
Scheme 2. Synthesis of low-bandgap polymers a) P1 and P2; b) N-P1.
of 500–650 nm decreased, as a function of the amount of Au
added (Figure 4c). After the titration, the 31P resonances of P1
were shifted to the upper-filed (i.e., the main resonance
Table 2. Properties of polymers.
[a,b]
ox
red
Polymer labs [nm][a] lem [nm][a] FPL
lonset [nm][c] optEg [eV][d]
E
[V vs. Fc/Fc+][e]
E
[V vs. Fc/Fc+][e] EHOMO [eV][f] ELUMO [eV][f] elcEg [eV][g]
onset
onset
P1
P2
N-P1
624
681
578
712
753
673
0.17
0.14
0.23
810
840
759
1.53
1.47
1.63
0.34
0.34
0.52
ꢀ1.27
ꢀ1.14
ꢀ1.30
ꢀ5.44
ꢀ5.44
ꢀ5.62
ꢀ3.83
ꢀ3.96
ꢀ3.80
1.61
1.48
1.82
[a] Measured with CH2Cl2 solutions (0.5 wt%). [b] Determined with an integrating sphere. [c] Measured with thin-films on a quartz plate. [d] optEg =(1240/
ox
red
lonset) [eV]. [e] Determined by CV using thin-films deposited on a Pt working electrode. [f] EHOMO =ꢀ(5.1+
[eV].
E
) [eV]; ELUMO =ꢀ(5.1+
E
) [eV]. [f] elcEg =
onset
onset
ox
red
E
ꢀ
E
onset
onset
Figure 4. a) Absorption and b) PL spectra of CH2Cl2 solutions (0.5 wt%) of the polymers. c) Gradual changes in UV spectra of P1 as a function of the amounts
of [Au(tht)Cl] added.
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B. S. Gelfand, T. Baumgartner, Angew. Chem. Int. Ed. 2016, 55, 3481–
3485; Angew. Chem. 2016, 128, 3542–3546.
moved from d= +9.24 to +7.98 ppm), which is consistent
with the behavior of 1 and 4 in Au-complexation. Since almost
no change in UV spectra was observed in the control experi-
ment using N-P1 instead of P1 (Figure S6c, Supporting Infor-
mation), the spectra change in the case of P1 would have
been caused as a result of the interaction between P and Au
moiety. In conjunction with the existence of isosbestic points
(e.g., at around 660 nm), it is speculated that this spectral
change corresponds to the coordination of the P-centers to
the Au centers in a 1:1 manner. Although this preliminary phe-
nomenon awaits further investigations, the bottom line is that
the post-polymerization functionalization of P1 successfully al-
lowed for the bandgap control ranging from 1.72 to 1.51 eV
only by regulating the amount of Au to be added.
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In summary, we have developed novel phosphorus-contain-
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was demonstrated by creating new D–A–D functional mole-
cules and low-bandgap conjugated polymers. Moreover, the
bandgap control through post-polymerization modification of
the polymer has been achieved. Further studies focused on
the utilization of developed polymers as functional materials
are ongoing in our laboratory.
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This research was supported by a Grant-in-Aid for Scientific Re-
search on Innovative Areas “p-System Figuration: Control of
Electron and Structural Dynamism for Innovative Functions”
(KAKENHI Grant Number 15H00997) from the Ministry of Edu-
cation, Culture, Science and Technology, Japan (to Y.T.) and by
the Shorai Foundation for Science and Technology (to Y.T.). The
authors gratefully acknowledge the experimental assistance of
Profs. A. Saeki, Y. Yakiyama, and H. Sakurai (Osaka University).
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[19] Although the Au-complexation of 3 also seemingly proceeded, the diffi-
culty in isolation of the resulting Au complex did not lead to the char-
acterization. Furthermore, the oxidation of 1 did not proceed under the
similar conditions using mCPBA and H2O2. Other P-functionalizations of
1, 3, and 4 with S8, Pd, and W complexes failed to give characterizable
products.
[20] For summaries of crystal data of Mes-2, N-2, and 3, see the Supporting
Information.
[21] A. Decken, E. D. Gill, F. Bottomley, Acta Crystallogr. Sect. E 2004, 60,
o1456.
Keywords: donor–acceptor
heterocycles photophysics
functionalization · p-conjugated polymers
systems
·
phosphorus
·
·
post-polymerization
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Received: May 19, 2016
Published online on && &&, 0000
Chem. Eur. J. 2016, 22, 1 – 6
www.chemeurj.org
5
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Communication
COMMUNICATION
&
Photophysics
Y. Takeda,* K. Hatanaka, T. Nishida,
S. Minakata*
&& – &&
Thieno[3,4-c]phosphole-4,6-dione: A
Versatile Building Block for
Phosphorus-Containing Functional p-
Conjugated Systems
Donor–acceptor systems: A novel class
of phosphorus-containing conjugated
building blocks, thieno[3,4-c]phosphole-
4,6-diones (TPHODOs), has been devel-
oped (see scheme). Utilizing the build-
ing block, phosphorus-containing D–A–
D-conjugated compounds and low-
bandgap conjugated polymers based
on donor–acceptor (D–A) architecture
have been synthesized.
&
&
Chem. Eur. J. 2016, 22, 1 – 6
www.chemeurj.org
6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!