2,6-diphospha-s-indacene-1,3,5,7(2H,6H)-tetraone: A phosphorus analogue of aromatic diimides with the minimal core exhibiting high electron-accepting ability

Article abstract of DOI:10.1002/chem.201403735

Phosphorus analogues of pyrromellitic diimides (PyDIs), which represent a family of privileged electron-accepting organic compounds, have been successfully synthesized as novel electron-accepting π-conjugated molecules. Investigation into their physicochemical properties uncovered their prominent electron-accepting abilities over the corresponding PyDI. Furthermore, theoretical studies revealed the significant contribution of σ*-π* hyperconjugation in stabilizing the LUMO+1.

Full text of DOI:10.1002/chem.201403735

DOI: 10.1002/chem.201403735
Communication
&
Organic Materials
2,6-Diphospha-s-indacene-1,3,5,7(2H,6H)-tetraone: A Phosphorus
Analogue of Aromatic Diimides with the Minimal Core Exhibiting
High Electron-Accepting Ability
Youhei Takeda,*[a, b] Takuya Nishida,[b] and Satoshi Minakata*[b]
injection. Meanwhile, it would be intriguing to explore to what
Abstract: Phosphorus analogues of pyrromellitic diimides
(PyDIs), which represent a family of privileged electron-ac-
cepting organic compounds, have been successfully syn-
thesized as novel electron-accepting p-conjugated mole-
cules. Investigation into their physicochemical properties
uncovered their prominent electron-accepting abilities
over the corresponding PyDI. Furthermore, theoretical
studies revealed the significant contribution of s*–p* hy-
perconjugation in stabilizing the LUMO+1.
extent the essential core of functional p-conjugated materials
can be downsized without impairing their intrinsic functions.
In this regard, Katz and co-workers have successfully demon-
strated that pyromellitic diimides (PyDIs, Figure 1), which have
the minimal rylene core, serve as semiconducting materials for
n-channel OFET devices.[6]
Organophosphorus p-conjugated compounds have very re-
cently been emerging as fascinating molecular frameworks for
developing organic functional materials owing to their unique
physicochemical properties.[1] For example, phosphole-based
conjugated compounds represent promising candidates for
electron-accepting or luminescent materials,[2] ascribed to s*–
p* hyperconjugation, which lowers the LUMO energy levels,
and the versatile reactivity of the PIII center,[3] which allows
fine-tuning of their properties. Therefore, the exploration of
novel phosphorus-containing p-conjugated systems is a valua-
ble challenge to open up a new stream in the field of materials
science.
Figure 1. Molecular design of bis(diketophosphanyl) compounds 1–3.
In a related move, organic electron-accepting p-conjugated
compounds have been drawing much more attention than
ever before because they would offer great opportunities for
significant progress in organic electronics applications exempli-
fied with organic field-effect transistors (OFETs), complementa-
ry logic circuits, organic photovoltaics (OPVs), and organic
light-emitting diodes (OLEDs).[4] Above all, rylene and related
aromatic diimides constitute privileged electron-accepting
building blocks for functional organic materials[5] because they
possess stabilized LUMO orbital energies suitable for electron
Because phosphorus is a congener of nitrogen, we designed
phosphorus analogues of PyDIs (1 and 2, Figure 1) and a P,N
analogue, 3 (Figure 1), as novel P-containing electron-accept-
ing p-conjugated frameworks possessing the minimal conju-
gated core. The molecular design can offer an advantageous
option to allow for stabilizing the energies of the LUMO and
upper vacant orbitals through twofold s*–p* hyperconjugation
involving the exocyclic s*(CꢀP) and the p* moiety with contri-
butions from the two carbonyls and the benzene unit
(Figure 1). By association, Baumgartner and co-workers have
recently demonstrated that such s*(CꢀP)–p*(carbonyl) orbital
coupling effectively lowers the LUMOs of bithiophene-fused
seven-membered diketophosphanyl systems.[7] In addition to
the scarcity of reported examples of such diketophosphanyl
compounds in the literature,[8] there are only two studies that
shed light on their physicochemical properties, both of which
have been performed by the Baumgartner group.[7] Herein we
disclose the synthesis, characterization, and physicochemical
properties of five-membered diketophosphanyl p-conjugated
[a] Dr. Y. Takeda
Frontier Research Base for Global Young Researchers
Graduate School of Engineering, Osaka University
Yamadaoka 2-1, Suita, Osaka 565-0871 (Japan)
E-mail: takeda@chem.eng.osaka-u.ac.jp
[b] Dr. Y. Takeda, T. Nishida, Prof. Dr. S. Minakata
Department of Applied Chemistry
Graduate School of Engineering, Osaka University
Yamadaoka 2-1, Suita, Osaka 565-0871 (Japan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201403735.
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compounds 1–3 (Figure 1). Intriguingly, comparative studies
with the corresponding diimide counterpart (PyDI-Mes,
Figure 1) revealed the prominent electron-accepting abilities of
1–3.
Synthetic routes of diketophosphanyl compounds 1–3 and
reference compound PyDI-Mes are outlined in Scheme 1.[9] Ac-
cording to a similar method reported by Cowley[8a] and Baum-
suggesting the existence of two stereoisomers for 1 depending
on the two P geometries: anti and syn isomers as argued for
other bis[phosphorus(III)] compounds.[14] Notably, the structure
of a stereoisomer (anti-1) in the solid state was unambiguously
determined by X-ray diffraction analysis of a single crystal
grown from a CH2Cl2/n-hexane solution under a N2 atmosphere
(Figure 2).[15] Both P centers (P1 and P2) have pyramidal geom-
Figure 2. Molecular structure of anti-1 in the solid state. Thermal ellipsoids
are set at the 50% probability level; hydrogen atoms are omitted for clarity.
etry as anticipated, with two mesityl groups pointing in oppo-
site directions. The bond lengths of the exocyclic (ca. 1.82 ꢁ)
and endocyclic PꢀC bonds (ca. 1.84–1.85 ꢁ) are similar to
values reported in literature.[7a,8b,c]
Scheme 1. Synthesis of 1–3 and PyDI-Mes.
gartner,[7] bis(diketophosphanyl) compound 1 was successfully
synthesized in 54% yield through the condensation of pyro-
mellitic acid tetrachloride, which was prepared in situ from tet-
racarboxylic acid S1, with 2 equivalents of bis(trimethylsilyl)me-
sitylphosphine[10] in diethyl ether at ꢀ788C, while warming to
room temperature.[11] Notably, trivalent phosphorus compound
1 was tolerant to isolation by silica gel column chromatogra-
phy to give a pale-orange solid, which was stable enough to
store for a few months under an inert atmosphere without de-
composition. More importantly, bis(phosphanyl) compound
1 was found to exist as an inseparable equimolar mixture of
anti and syn conformers in solution owing to rapid intercon-
version (see below), while the mixture of anti/syn-1 was fully
characterized by conventional spectroscopic methods such as
multinuclear NMR (1H, 13C, and 31P) and IR spectroscopy and
mass spectrometry.[9] Taking advantage of the multivalency of
phosphorus, we then transformed 1 into dioxide 2 by oxida-
tion with mCPBA, albeit in a low yield. At present, it seems
rather difficult to clearly state the stereochemistry of 2, except
that the 31P resonance solution of 2 in CDCl3 appeared at
ꢀ3.23 ppm as a sharp singlet peak both at room temperature
and ꢀ508C.[12] The failure to chemically modify P centers by
other methods might reflect the electron-deficient nature of
trivalent P centers directly connected to diketo groups.[7,13] To
investigate the effect of P substitution, P,N analogue 3 was
also synthesized by a similar way to 1 from phthalic acid S2.
For comparison, reference diimide compound PyDI-Mes was
prepared by classical condensation of dicarboxylic acid anhy-
dride S3 with mesitylamine.
The rapid interconversion between anti and syn conformers
in solution was clearly observed by 31P{1H} NMR analysis of
a CDCl3 solution of 1 at various temperatures (Figure 3).
Figure 3. a) Variable-temperature 31P{1H} NMR spectra of 1 in CDCl3 and b)
line-fitting simulations with the rate constants (kAB) for the interconversion
of A and B.
Whereas at 29.98C,
a sharp singlet was observed at
ꢀ15.2 ppm, the singlet split into two (designated as A and B)
below ꢀ108C (Figure 3a). Line-shape simulation of the dynam-
ic NMR experiments (Figure 3b) gave the rate constants (kAB)
for the interconversion (A!B),[16] and the activation energy (Ea)
for this process derived from the Arrhenius plot (see the Sup-
Limited precedents of crystallographic analysis of diketo-
phosphanyl compounds show that trivalent P centers have
pyramidal geometry like typical phosphine compounds,[7a,8b,c]
porting Information, Figure S7) was as low as 13.9 kcalmolꢀ1
Furthermore, the Eyring plot (see the Supporting Information,
.
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Figure S8) provided thermodynamic parameters (DHꢀ =
13.3 kcalmolꢀ1 and DSꢀ =5.2 calmolꢀ1), which correspond to
DGꢀ25 =11.7 kcalmolꢀ1. The DFT calculation at the B3LYP/6-
31G(d) level of theory strongly supports that the observed dy-
namic phenomena correspond to anti–syn interconversion
through pyramidal inversion of the P center.[9] The calculated
energy difference between anti- and syn-1 was negligible
(0.04 kcalmolꢀ1; see the Supporting Information, Figure S9), ex-
plaining the equally populated resonances of A and B (Fig-
ure 3a). The calculated inversion barrier (calcEa) and calcDG2ꢀ5
were 11.5 kcalmolꢀ1 and 12.9 kcalmolꢀ1, respectively, in good
accordance with the experimental values. It would be intrigu-
ing to compare the inversion barrier for 1 with other related
pyramidal compounds:[17] much lower than those of typical
acyclic trialkyl- and dialkylarylphosphines (29–36 kcalmolꢀ1),[18]
even lower than phospholes (15–16 kcalmolꢀ1),[19] and slightly
greater than amines (4–8 kcalmolꢀ1).[17] This extraordinarily low
P-inversion barrier for 1 could be attributed to 3p–2p p conju-
gation in the transition state as proposed for phospholes[14b,19]
and mono-acyl phosphines.[20] Although there have been
a number of studies on the inversion barrier of phosphines in
literature,[17b] to the best our knowledge, this is the first experi-
Table 1. Summary of optical and electrochemical data of 1–3 and
PyDI-Mes.
1
2
Compd
lmax1 [nm][a] (e/104)[b] E1/2, E1/2 [V][c] ELUMO [eV][d] DE [eV][e]
1
2
3
260 (5.79)
262 (6.47)
251 (6.48)
ꢀ1.16, ꢀ1.68
ꢀ3.64
3.22
2.72
3.42
3.44
ꢀ0.76, ꢀ1.26[f] ꢀ4.04
ꢀ1.21, ꢀ1.77
ꢀ3.59
PyDI-Mes 231 (7.24)
ꢀ1.27, ꢀ1.92
ꢀ3.53
[a] Absorption maxima of CH2Cl2 solutions (10ꢀ5 m order) observed in the
higher energy region (200–300 nm). [b] Mꢀ1 cmꢀ1. [c] Half-potentials of
the first and second reduction processes versus Fc/Fc+ redox couple
measured with CH2Cl2 solutions using 0.1m nBu4N·PF6 as the supporting
electrolyte. [d] Estimated from the following equation: ELUMO =ꢀ(4.8+
1E1/2) eV. [e] Estimated from the onset wavelength (lonset) of absorption
spectra. [f] Observed as a quasireversible wave.
Dlmax2 =7 nm; Dlmax3 =21 nm for 1). These redshifts in the
low-energy region are probably ascribed to enhanced charge
transfer brought about by P incorporation, as supported by
the TD-DFT calculation;[9] the absorptions at lmax3 for 3 and
1 are assigned to a transfer of charge from the P center to the
benzene p* (HOMO-4!LUMO+1, HOMO-5!LUMO+1, re-
spectively). Furthermore, the redshift in the onsets of absorp-
tions (Dlonset: 19 nm for 3; 41 nm for 1) indicates the narrower
HOMO–LUMO band gaps (DE in Table 1). Notably, the oxida-
tion of 1 resulted in a drastic decrease of the HOMO–LUMO
bandgap (DE of 2: 2.72 eV), while the maximum absorption
lmax being almost the same as that of 1. These trends in
HOMO–LUMO band gaps are in agreement with the results of
TD-DFT calculations (see below). No photoluminescence was
observed with all phosphorus compounds 1–3 as well as PyDI-
Mes upon excitation with ultraviolet light.
mental report on the
compounds.[21]
P inversion of diketophosphanyl
Absorption spectra of dilute CH2Cl2 solutions of synthesized
compounds are shown in Figure 4, and the absorption maxima
To assess the electron-accepting abilities of 1–3 in compari-
son to diimide PyDI-Mes, the electrochemical properties of
dilute CH2Cl2 solutions of 1–3 were investigated by cyclic vol-
tammetry (CV, Figure 5). With all compounds, noticeable oxida-
tion peaks were not detected. In contrast, reference compound
PyDI-Mes showed two sets of reversible electrochemical re-
duction peaks at 1E1/2 =ꢀ1.27 and 2E1/2 =ꢀ1.92 V against the
Figure 4. UV/Vis absorption spectra of dilute CH2Cl2 solutions (1ꢂ10ꢀ5 m) of
1–3 and PyDI-Mes.
(lmax1) in the high-energy region (200–300 nm) are listed in
Table 1. Diimide PyDI-Mes exhibited a strong absorption
(lmax1 =231 nm) that corresponds to the p–p* transition of the
benzene unit (HOMO-4!LUMO+1, assigned by TD-DFT[9]) and
two weak and broad absorption maxima in the low-energy
region (300–400 nm) at around lmax2 =308, which is assigned
to a transfer of charge from the mesityl units to the benzene
p* moiety (HOMO-2!LUMO+1 transition), and lmax3
=
319 nm, which is assigned to the carbonyl n–p* (HOMO-7!
LUMO) transition. Replacement of the two nitrogen atoms of
PyDI-Mes with one and two phosphorus atoms (3 and 1, re-
spectively) led to a distinct redshift of the lmax value in the
high-energy region (Dlmax1: 20 nm for 3; 29 nm for 1) and in
the low-energy region (Dlmax2 =12 nm; Dlmax3 =36 nm for 3;
Figure 5. Cyclic voltammograms of CH2Cl2 solutions of 1–3 and PyDI-Mes
(10ꢀ5 m order) with 0.1m nBu4N·PF6 as the supporting electrolyte measured
at a scanning rate of 100 mVsꢀ1. Potentials are indicated against the half-po-
tential of the Fc/Fc+ redox couple.
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Fc/Fc+ redox potential, in line with the reported values of re-
1
lated PyDIs (e.g., N,N’-bis(CH2C6H4-p-CF3)-PyDI, E1/2 =ꢀ1.30 and
2E1/2 =ꢀ1.90 V).[6a] Likewise, two sets of reversible redox events
were clearly observed for phosphorus surrogates 1 and 3,
while quasireversible events were observed for dioxide 2
(Figure 5). Most importantly, compared with PyDI-Mes, distinct
2
positive shifts in reduction potentials (1E1/2 and E1/2) were ob-
served for 1–3, indicating stronger electron-accepting features
of these compounds. The LUMO energy levels (ELUMO) of 1–3
derived from the CV experiments were determined to be
ꢀ3.64, ꢀ4.04, and ꢀ3.59 eV, respectively (Table 1). Notably, the
ELUMO of trivalent compounds 1 and 3 are comparable to that
of representative electron-accepting material PC61BM
(ꢀ3.7 eV)[22] and the ELUMO of dioxide 2 is far more stabilized,
suggesting that these compounds can serve as promising can-
didates for electron-accepting materials. Intriguingly, the
degree of potential shift [DnE1/2(Compd)=nE1/2(Compd)ꢀ E1/2
-
n
Figure 6. Energy diagram and molecular orbitals of PyDI-Mes, anti-1, anti-2,
and 3 calculated by DFT method at the B3LYP/6-31+G(d)/PCM (CH2Cl2)
level.
(PyDI-Mes)] in the second reduction process (D2E1/2) was much
greater than that in the first potential (D1E1/2) (1: D1E1/2 =0.11 V
versus D2E1/2 =0.24 V; 2: D1E1/2 =0.51 V versus D2E1/2 =0.66 V;
3: D1E1/2 =0.06 V versus D2E1/2 =0.15 V), presumably reflecting
the marked stabilized LUMO+1 orbital energies of the newly
designed phosphorus compounds through effective s*–p*
conjugation, as verified by theoretical calculation (see below).
When considering their potential use as organic materials,
the air- and thermal stabilities are very important factors. The
measurement of decomposition starting temperature (Td,
5 wt% loss) by thermogravimetric analysis (TGA) under the air
and nitrogen flow revealed prominent stability of 1 toward air
and heat (Td: 3098C under the air), which is comparable to
PyDI-Mes (Td: 3268C under the air).[9]
carbonyl–benzene moiety, compared to those of PyDI-Mes,
suggesting that s*–p* conjugation is effectively operating in
the LUMO+1.
In conclusion, we have designed and synthesized phospho-
rus analogues of pyromellitic diimides as novel phosphorus-
containing electron-accepting compounds and revealed their
structure, molecular dynamic processes, and unique physico-
chemical properties. This novel conjugated framework should
give us a perspective on new phosphorus-containing electron-
accepting materials. Further research on the application of
these promising conjugated compounds in the field of organic
electronics is moving on the next stage in our laboratory.
To profoundly understand the effect of P incorporation on
electronic structures and properties, theoretical calculations
were performed using DFT.[9] As results, the PCM method
(CH2Cl2 solvation) at the B3LYP/6-31+G(d) level can partly ex-
plain the experimental results (Figure 6).[23] The HOMO energy
levels (EHOMO) of trivalent compounds rise as the number of
phosphorus atoms increases (EHOMO: PyDI-Mes<3<1), while
the EHOMO of 2 is slightly lower than that of 1, reflecting the
contribution of the high-energy orbitals of the P center in the
formation of their HOMO orbitals. The LUMO orbitals of all
compounds clearly present p* symmetry derived from two car-
bonyls and the central benzene core. The LUMO energy levels
decrease in the order of PyDI-Mes>3>1@2 in line with the
Acknowledgements
This research was partly supported by the Shorai Foundation
for Science and Technology (to Y.T.). One of the authors (Y.T.)
would like to acknowledge warm support from the Frontier Re-
search Base for Global Young Researchers, Osaka University, on
the Program of MEXT, Japan.
Keywords: aromatic diimides
·
electron acceptors
·
electrochemical outcomes (1E1/2
: 2@1>3>PyDI-Mes), al-
phosphorus heterocycles · photophysics · p conjugation
though the differences in the LUMO energies among PyDI-
Mes, 3, and 1 were very small. While the p*(benzene–dicar-
bonyl) moiety in the LUMO of PyDI-Mes is developed com-
pletely in a symmetric manner, those of 1–3 are slightly un-
symmetrical and more largely developed on the benzene
planes opposite the pyramidal P centers.[9] More interestingly,
the LUMO+1 energy levels of 1–3 are found to be significantly
stabilized compared with that of PyDI-Mes, thus nicely ac-
counting for the distinct positive shift in the second reduction
half-potentials (2E1/2) of 1–3 in the CV measurements. More-
over, when focusing on the orbital distribution of the LUMO+
1, more extended p* lobes are evidently developed over the
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quest/cif.
[16] Line-shape simulations were performed using the WINDNMR-Pro pro-
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[23] As for 1 and 2, only the results of anti isomers were depicted because
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[11] At the preliminary stage of our research, we attempted the condensa-
tion using bis(trimethylsilyl)phenylphosphine, PhP(SiMe3)2, instead of
MesP(SiMe3)2. However, PhP(SiMe3)2 was highly sensitive to air and
moisture, as reported by Baumgartner (ref. 7a). Taking into account the
stability of both disilylphosphine reagents and forming products, we
chose MesP(SiMe3)2 as the phosphorus source in this study.
[12] The DFT calculation performed at the B3LYP/6–31G(d) level suggested
that the anti isomer of 2 is more thermodynamically stable than the
syn isomer in the gas phase by 0.43 kcalmolꢀ1
.
[13] Attempts to complex 1 with [Au(tht)Cl] resulted in precipitation of
Received: May 29, 2014
a yellow solid, but it was not soluble in any organic solvents.
Published online on July 17, 2014
Chem. Eur. J. 2014, 20, 10266 – 10270
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