2,2′-biphospholes: Building blocks for tuning the HOMO-LUMO gap of π-systems using covalent bonding and metal coordination

Article abstract of DOI:10.1002/anie.201105924

A new angle: The insertion of a 2,2′-biphosphole subunit into π-conjugated systems offers a new way to control the HOMO-LUMO gap. Tuning of the dihedral angle (θ) between the two phosphorous heterocycles, either by metal coordination or covalent bonding t

Full text of DOI:10.1002/anie.201105924

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DOI: 10.1002/anie.201105924
Phosphorous Heterocycles
2,2’-Biphospholes: Building Blocks for Tuning the HOMO–LUMO
Gap of p-Systems Using Covalent Bonding and Metal Coordination**
Hui Chen, Wylliam Delaunay, Liujian Yu, Damien Joly, Zuoyong Wang, Jin Li, Zisu Wang,
Christophe Lescop, Denis Tondelier, Bernard Geffroy, Zheng Duan,* Muriel Hissler,*
FranÅois Mathey,* and Rꢀgis Rꢀau*
Organic p-conjugated systems are useful lightweight semi-
conducting materials for electronics.[1] The control of their
HOMO–LUMO energy gap (Eg) is one of the key issues in
the optimization of these materials for applications (organic
light-emitting devices (OLEDs), organic field-effect transis-
tors (OFETs), photovoltaic cells, etc.).[2] The Eg of an isolated
conjugated system (A; Figure 1) is a function of several
Among all these structural factors, only the torsion angle q
can potentially be varied in a continuous way to allow
progressive switching from a highly delocalized low-gap
system (q = 08) to a non-delocalized high-gap system (q =
908). However, this structural parameter is probably the
most difficult factor to tune experimentally. In fact, the
torsion angle can be fixed to 08 (fully planar systems) by
rigidification through either the introduction of a covalent
bond between the elemental building blocks (ladder-type
molecules) or by metal coordination (for example 2,2’-
bipyridines chelated by metal ions).[3] However, to date no
general strategy that allows tuning of this torsion angle over a
large range is known.
Herein, we show that exploiting the properties of the 2,2’-
biphosphole unit B (modification of the substituents on P, or
metal coordination; Figure 2)[4–6] allows a unique combined
Figure 1. Structural factors determining the band gap of p-conjugated
systems.
structural factors including the bond-length alternation
(EBLA), aromatic character of the building blocks (Eres),
electronic nature of the lateral substituents (Esub), and the
torsion angle between consecutive subunits (Eq) (Figure 1).[2]
[*] W. Delaunay, Dr. D. Joly, Dr. C. Lescop, Prof. M. Hissler, Prof. R. Rꢀau
Sciences Chimiques de Rennes, CNRS- Universitꢀ de Rennes 1
Campus de Beaulieu, 35042 Rennes (France)
E-mail: muriel.hissler@univ-rennes1.fr
Figure 2. Covalent and metal coordination approaches to tune the
torsion angles (q) within the 2,2’-biphosphole building blocks.
regis.reau@univ-rennes1.fr
Homepage: http://pmm.uni-rennes1.fr
covalent/metal coordination approach for an on-demand
tuning of the torsion angle q between the two conjugated
P heteroles. This simple and straightforward molecular engi-
neering, which is not possible with other p-building blocks
(2,2’-bipyridine, 2,2’-bithiophene, 2,2’-bipyrrole, 2,2’-bisilole,
etc.), allows an unprecedented fine control of the energy gap,
Eg, of the p-systems A (Figure 1) through two orthogonal
approaches. Lastly, the impact of this tuning for optoelec-
tronic applications is demonstrated with the use of 2,2’-
biphosphole-based small molecules as emitting materials for
OLEDs, including white organic light-emitting devices
(WOLEDs).
The target 2,2’-biphosphole oligomers 2a–c and 3b,c
(Scheme 1) were prepared from 1-aryl-3,4-dimethylphosp-
holes 1a–c using a synthetic strategy discovered by Mathey
et al.,[5] and then later developed by Gouygou et al. for P-
bridged derivatives.[6] The newly prepared compounds 2b and
3b,c were isolated in approximately 50% overall yield. The
31P NMR spectroscopy shows that they exist as a mixture of
stereoisomers, as already observed for related biphosphole
B. Geffroy
Laboratoire de Chimie des Surfaces et Interfaces, CEA Saclay,
IRAMIS, SPCSI, 91191 Gif-sur-Yvette (France)
D. Tondelier
Laboratoire de Physique des Interfaces et Couches Minces, CNRS
UMR 7647, Ecole Polytechnique, 91128 Palaiseau (France)
Prof. F. Mathey
Nanyang Technological University, CBC-SPMS
21 Nanyang Link, Singapore 637371 (Singapore)
E-mail: fmathey@ntu.edu.sg
H. Chen, L. J. Yu, Z. Y. Wang, J. Li, Z. S. Wang, Prof. Z. Duan,
Prof. F. Mathey
International Phosphorus Laboratory, Chemistry Department
Zhengzhou University, Zhengzhou 450001 (P. R. China)
E-mail: duanzheng@zzu.edu.cn
[**] This research is supported by the Ministꢁre de la Recherche et de
l’Enseignement Supꢀrieur, the University of Rennes1, the CNRS (AIL
FOM), IUF. NSFC (No. 21072179), and the Zhengzhou University of
China. COST CM0802 (Phoscinet) is also acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201105924.
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Scheme 1. Synthesis of the two series of 2,2’-biphospholes containing p-conjugated systems. THT=tetrahydrothiophene. Experimental conditions
are given in the Supporting Information.
derivatives.[5,6] This synthetic approach allows the preparation part of the absorption spectrum that is assigned to p–p*
of two series of biphosphole derivatives; one in which the two transitions of the extended p-conjugated system.[8] Three
À
P rings are free to rotate about the inter-ring C C bond (2a–c, important trends are noteworthy. Firstly, as observed for the
Scheme 1), and one in which the two P rings have a restricted corresponding monophosphole-based p-conjugated system-
rotation because of their covalent bonding by a C3 chain (3b– s,[8a–c] a noticeable red-shift of the absorption maximum (lmax
)
c, Scheme 1). These compounds were first transformed into is observed upon replacing the phenyl groups of 2a by either
the corresponding dithioxophospholes 4a–c/5b,c (Scheme 1) 2-thienyl rings (2b; Dlmax = 22 nm) or 5-(2,2’-bithienyl) sub-
to obtain air-stable and easy to handle derivatives. The stituents (2c; Dlmax = 61 nm; see Table S1 in the Supporting
sulfurization process is not diastereoselective but the major Information). Secondly, the oxidation with elemental sulphur
diastereoisomer was isolated by flash chromatography (ca. of the l3,s3-biphosphole units to give compounds
4
60% yield). They were characterized by high-resolution mass (Scheme 1) results in a blue-shift of the lmax (Dlmax ꢀ 16–
spectrometry, elemental analysis, and multinuclear NMR 20 nm, Table S1). In the solid state, this effect is less
spectroscopy.
pronounced and the l3,s3-biphosphole and l4,s4-thioxobi-
Biphospholes exhibit a rich coordination chemistry, acting phosphole derivatives exhibit similar absorption spectra.
either as 1kP,2kP donors or as P,P-chelates.[4–6] We exploited Overall, this behavior is very surprising since, in the case of
this property, which is unique for p-conjugated ligands, to all monophosphole-based p-conjugated systems, the reverse
rigidify the biphosphole moiety using the coordination trend is observed (i.e. a red-shift of ca. 20 nm upon sulfuriza-
chemistry of different metal centers. Firstly, the novel tion of the P atom both in solution and solid state).[8,3e,f]
AuI complexes 6a–c and 7b,c, in which the two P centers act Thirdly, the 2,2’-biphosphole/AuI complexes 6a–c
independently as two terminal donors, were prepared (Scheme 1) display red-shifted UV/vis spectra (Dlmax ꢀ 10–
(Scheme 1). We expected that depending on the biphosphole 40 nm), and therefore smaller HOMO–LUMO gaps, than the
series (free or restricted rotation), establishment of intra- corresponding thioxo derivatives 4a–c. This result is unex-
molecular AuI–AuI interactions[7] could offer a way to lock the pected, because for all p systems based on monophospholes
torsion angle between the phosphorous heterocycles known to date,[8,3ef] these two types of phosphole derivatives
(Figure 2). Secondly, the PdII complexes 8a-c and 9b, in (AuI complexes and thioxo derivatives) exhibit similar optical
which the biphosphole unit acts as a bidentate donor, were HOMO–LUMO gaps. These last two features demonstrate
synthesized (Scheme 1). It was also expected that, depending that phosphole and biphosphole building blocks behave very
on the biphosphole series, different torsion angles could be differently in the molecular engineering of p-conjugated
obtained since the P donor can adopt a distorted geometry as systems, and are therefore a distinct series of building blocks.
a result of the high s character of its lone pair of electrons.[4]
The origin of these different behaviors of the biphosphole
Note that all the AuI and PdII complexes exhibit a single building blocks (electronic/steric) is not straightforward,[9]
31P{1H} NMR resonance, thus showing that the coordination and to gain insight into these intriguing observations the
processes are highly diastereoselective.[5,6] The newly pre- solid-state structures of the thioxo derivatives 4a–c and AuI
pared complexes were fully characterized by high-resolution complexes 6a–c were investigated. These X-ray diffraction
mass spectrometry and elemental analysis. Their multinuclear studies show that in all cases racemic mixtures are obtained.
NMR spectroscopy data compare well with those of related Considering the mean plane between the two phosphole rings
phosphole complexes[4–6,8] and support the proposed struc- in 4b, one can consider the sulfur atoms to occupy “axial”
tures.
positions and the P-methyl substituents “equatorial” positions
To evaluate the impact of this molecular engineering on (Figure 3). The fact that the two P rings are free to rotate
À
the HOMO–LUMO gap of the corresponding p systems, the about the inter-ring C C bond within the thiooxobiphosphole
optical properties of all derivatives were investigated. All units is clearly shown by the different torsion angles between
derivatives display an intense absorption band in the visible the two central P rings (4a: 81.08; 4b: 82.18; 4c: 51.08). These
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the high torsion angle within the
biphosphole unit, the p–p* transi-
tions observed in the UV/Vis spec-
tra of the PdII complexes 8a–c are
considerably blue-shifted com-
pared to those of the AuI com-
plexes 6a–c (for example: Dlmax
=
95 nm for 6c versus 8c; see
Table S1 in the Supporting Inform-
tion). These results clearly show
how it is possible to fully exploit
the biphosphole coordination
chemistry to control their torsion
Figure 3. Views of the X-ray crystal structures of a 2,2’-biphosphole derivative series (see the
Supporting Information for the other series).[11]
torsion angles are very high because of the tetrasubstituted
P atoms which impose a twisted conformation to minimize
steric congestion. These twisted conformations probably also
exist in solution because of an additional steric repulsion of
the methyl groups at the 3,4-positions of the P rings (4b,
Figure 3). This structural feature can account for the rather
high HOMO–LUMO gap of the thiooxobiphosphole deriv-
atives 4a–c. Complexes 6a–c are also isolated as a racemic
mixture, but the P-methyl substituents now occupy the axial
positions (6b, Figure 3). This conformation, which contrasts
to that of the thiooxophosphole derivative 4b is due to the
fact that two coordinated AuI ions are engaged in aurophilic
interactions[7] (6a: 2.9889(2) ꢀ, 6b: 3.050(4) ꢀ; 6c:
3.077(3) ꢀ). This metallophilic interaction results in the
formation of a C2P2Au2 six-membered rings, which display a
highly distorted chair conformation as a result of the long Au–
Au distance and the presence of bulky methyl substituents
(Figure 3). It is very likely that these six-membered rings exist
in solution since 1) the energy associated with aurophilic
interaction is about 10 kcalmolÀ1,[7] and 2) this conformation
minimizes the steric repulsion between the ring substituents.
Therefore, the metallophilic interactions lock the 2,2’-
diphosphole subunits in place and the torsion angles between
the two P rings are now fixed (6a: 51.38, 6b: 55.08; 6c: 54.28).
These torsion angles are lower than those recorded for their
thioxophosphole analogues and can explain why these two
series of derivatives exhibit different HOMO–LUMO gaps.
This use of metal coordination to control the torsion angle of
conjugated building blocks through metallophilic interactions
nicely illustrates the impact of the biphosphole moiety for the
molecular engineering of p-conjugated systems.
To further tune the torsion angle between the two central
P rings of the biphosphole subunit, its bidentate behavior
towards a PdII metal ion was exploited (Scheme 1, 8a–c). The
X-ray diffraction study of the complex 8a reveals a square-
planar metal center as part of a five-membered ring
(Figure 3). Note that here the racemic mixture is also
obtained and, as a result of the chelating behavior of the
biphosphole fragment, the methyl groups occupy the axial
positions. The formation of the five-membered ring upon
coordination on the PdII ion imposes a large torsion angle
within the biphosphole unit (87.78). Furthermore, the phosp-
holes rings are bent with respect to each other to help the
phosphorus atoms to accommodate the square-planar coor-
dination sphere of the PdII center (see Figure S2 in the
Supporting Information). Interestingly, in accordance with
angle using transition-metal atoms with specific properties,
and to impact the HOMO–LUMO gap of the corresponding
p-conjugated systems.
The versatility of this approach can be additionally
extended to using 2,2’-biphosphole building blocks with a
restricted rotational ability that results from a C3 alkyl chain
that is covalently bonded to the P atoms (derivatives 5, 7, and
9; Scheme 1). Replacing the methyl substituent on P with the
bridging C3 alkyl chain has almost no influence on the UV/vis
spectra of the corresponding p-conjugated systems that
incorporate either the s3-biphosphole or s4-thioxobiphosp-
hole building blocks (see Table S1 in the Supporting Infor-
mation). Indeed, the solid-state structure of the dithioxo-
phosphole derivative 6c reveals a torsion angle of 678
between the two P rings, a value that is within the range of
those recorded for the nonbridged compounds 4a–c. These
data show that the C3 chain does not lock the biphosphole
unit with a precise torsion angle and that a certain rotation
À
about the inter-ring C C is possible. In contrast to what was
I
À
observed within the P CH3 series, the Au complexes 7b,c
exhibit absorption maxima that are similar to the correspond-
ing dithioxophospholes 5b,c (see Table S1 in the Supporting
Information). The origin of this difference in behavior
between the bridged and nonbridged series (Scheme 1) was
rationalized by considering the solid-state crystal structure of
complex 7b. In this racemic mixture, which contains the
bridging C3 chain, the two AuI ions occupy the axial positions
and therefore cannot be engaged in aurophilic interactions
(Figure 3). The torsion angle between the two P rings (64.98)
is controlled by the C3 chain only and the P rings are not
locked. As a consequence, the HOMO–LUMO gaps of these
AuI complexes 7b,c are similar to those of the thioxobiphosp-
holes 5b,c, as observed for monophosphole-based p-conju-
gated systems. Indeed, the presence or absence of a covalent
C3 bridge between the P rings allows to change the relative
position of the AuI metal ions in the resulting complexes in
order to control the torsion angle within the biphospholes.
The trick to obtaining a biphosphole unit doubly locked
through both covalent bonding and metal coordination, was
to prepare the PdII complex 9b using the precursor 3b
(Scheme 1). Indeed, the X-ray diffraction study of 9b
revealed a torsion angle of 25.88, by far the smallest value
observed within the biphosphole series. For example, in the
related PdII complex 8a having P-CH3 moieties, this angle
reaches 86.78 (Figure 3). The consequence of this double-lock
approach (C3 chain and metal coordination) is a large red-
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shifted absorption of the PdII complex compared to that of all
other biphosphole derivatives (lmax = 479 nm). Indeed,
biphosphole appears to be a unique building block for
tuning the HOMO–LUMO gap of p systems because their
torsion angle can be controlled through varying the nature of
the substituents on P (bridging or non-bridging) and using
different chelating modes (1kP,2kP donors or P,P-chelates) of
this subunit. Moreover, these structural modifications result
in orthogonal covalent and coordination tuning: with non-
bridging substituents, the AuI complexes have the lowest
HOMO–LUMO gap and the PdII complexes have the highest
ones, whereas with the covalent bridging substituent, the PdII
complexes display much lower HOMO–LUMO gaps than the
AuI complexes.
It was of great importance to show that these novel p-
conjugated systems based on biphosphole synthons exhibit
the required properties (high solid-state quantum yield,
thermal stability, and suitable reversible redox potential)
required for the application as materials in optoelectronic
devices. All the structural modifications (Scheme 1) also have
an impact on the emission properties of the 2,2’-biphosphole
derivatives, both in solution and in the solid state. For
example, amongst the AuI complexes, only compounds having
a C3 aliphatic bridge (7b,c) between the two phosphorus
atoms are emissive, probably because of the rotational
restriction between the ground and excited states. Derivative
5c exhibits an emission maximum in solution centered at
lmax = 585 nm (F = 2% versus fluoresceine). The same emis-
sion wavelength was observed using this compound as an
emitting material for OLEDs having a classical structure (see
the Supporting Information). Interestingly, this wavelength is
complementary to that of 4,4-bis[2,2’-di(4-trifluoromethyl-
phenyl)ethenyl]-1,1’-biphenyl (DPVBi), a communally used
blue-emitting matrix. Therefore, biphosphole 5c was used as a
dopant of this blue matrix for the development of WOLEDs.
Multilayered OLEDs were fabricated by co-evaporation of
DPVBi and 5c (2.2% doping rate; WOLED structure in
Figure S3 in the Supporting Information). The device exhibits
satisfying performances (external quantum yield: 0.5%;
brigthness: 189 cdmÀ2 at 20 mAcmÀ2). The most important
data is that the CIE (CIE = Commission International
d’Eclairage) coordinates (0.34, 0.34) are almost similar to
those of the perfect white emission (0.33, 0.33) because of a
well-balanced dual emission from the DPVBI matrix and 5c.
This is the first time that a perfect white emission has been
obtained with a phosphole-based dopant.[10]
Keywords: p conjugation · phosphorous heterocycles ·
optical properties · semiconductors · synthetic methods
.
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In conclusion, exploiting the specific properties of
biphospholes allows control of the torsion angles within p-
conjugated systems. The fact that this novel series of P-based
units within p-conjugated systems can be employed as
materials in optoelectronics opens interesting perspectives
towards the development of novel materials in plastic
electronics.
Received: August 22, 2011
Published online: November 14, 2011
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