Phosphinine lipids: A successful marriage between electron-acceptor and self-assembly features

Article abstract of DOI:10.1002/anie.201303729

A push in the right direction: An electron-accepting organophosphorus system has been combined with self-assembly features to create a strongly electron-accepting liquid-crystalline material (see picture). The stability and behavior of the self-assembled liquid crystal could be controlled by adjusting weak intermolecular forces, such as hydrogen bonding and π-π interactions. Copyright

Full text of DOI:10.1002/anie.201303729

Angewandte
Chemie
Communications
Phosphinine Lipids
A push in the right direction: An electron-
accepting organophosphorus system has
been combined with self-assembly fea-
tures to create a strongly electron-
accepting liquid-crystalline material (see
picture). The stability and behavior of the
self-assembled liquid crystal could be
controlled by adjusting weak intermolec-
ular forces, such as hydrogen bonding
and p–p interactions.
X. M. He, J. B. Lin, W. H. Kan,
T. Baumgartner*
&&&& — &&&&
Phosphinine Lipids: A Successful
Marriage between Electron-Acceptor and
Self-Assembly Features
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DOI: 10.1002/anie.201303729
Phosphinine Lipids
Phosphinine Lipids: A Successful Marriage between Electron-
Acceptor and Self-Assembly Features**
Xiaoming He, Jian-Bin Lin, Wang Hay Kan, and Thomas Baumgartner*
Organophosphorus chemistry has made a great impact on the
advances in inorganic/organometallic chemistry and life
sciences in the last century.[1] Moreover, as a result of their
unique optical and electronic features, organophosphorus
species have very recently also attracted much attention in the
field of organic electronics, as well as sensory and self-
assembled materials.[1] Phosphole-based conjugated systems,
in particular, exhibit extraordinary photophysical properties
that can be easily tuned through the versatile reactivity of
phosphorus. In addition, a peculiar s*–p* orbital coupling,
made possible through the pyramidal geometry of the
P center in these materials, stabilizes the LUMO energies
and thus provides a very promising alternative approach
toward electron-acceptor (n-type) materials.[1d]
In the field of organic electronics, the number of suitable
n-type materials is actually quite limited compared to that of
p-type materials, and thus impedes any significant improve-
ment in the performance of organic electronics; conventional
strategies toward n-type materials usually rely on the
introduction of electron-withdrawing moieties, such as car-
bonyl, cyano, C6F5, CF3, or imide groups into an organic
framework.[2] In this context, we have recently reported
a novel electron-accepting building block, dithieno[3,2-c:2’,3’-
e]-2,7-diketophosphepin (DTDKP), by the introduction of
two electron-withdrawing carbonyl groups into the dithieno-
phosphole framework (Scheme 1).[3] DTDKP exhibits
improved electron-accepting character over its nitrogen
analogue, thus highlighting the unique contributions phos-
phorus can provide for organic electronics.[4]
Combining organic electronic species with self-assembly
features that generate well-ordered micro-/nanostructures
has been recognized as highly desirable for the development
of efficient optoelectronic devices, such as solar cells and
field-effect transistors, but also electroactive liquid crystals.[5]
Most of these materials are based on neutral polycyclic
aromatic hydrocarbons (PAHs)[5] or cationic imidazole-based
materials.[6] However, the combination of systems based on
inorganic main group elements (B, Si, P) with self-assembly
properties remains fairly underexplored to date, mainly
because of the synthetic challenges involved. In this regard,
organophosphorus materials offer excellent advantages over
other main-group elements. Inspired by the amphiphilic
phospholipid cell membranes with unique self-assembled
bilayer structures, the research groups of Weiss and Kato
pioneered the study of the self-assembly of ionic phosphoni-
um compounds, by focusing on nonconjugated systems.[7,8]
Very recently, we reported a family of conjugated “phos-
phole-lipid” systems with p-conjugated head groups that
exhibited self-assembly features as well as intriguing stimuli-
responsive behavior. These systems combined the amphi-
philic features of lipids and the valuable photophysical
features of conjugated phospholes.[9]
Herein, we report on a novel electron-accepting organo-
phosphorus system, dithieno[2,3-b;3’,2’-e]-4-keto-1,4-dihy-
drophosphinine, obtained by introducing an electron-with-
drawing carbonyl group at the para position of the six-
membered phosphinine ring. This design is not expected to
affect the desirable reactivity of the phosphorus center, such
as oxidation or quaternization, which has limited the versa-
tility of the DTDKP system to some extent.[4] Further
functionalization of this building block with self-assembly
groups provides novel amphiphilic phosphonium materials
that combine the electronic character of the p-conjugated
phosphinine core with the amphiphilic character of phospho-
lipids, similar to our earlier studies on the phosphole-lipid
system,[9] and for a deeper understanding of conjugated
organophosphorus lipid systems in general.
Scheme 1. Design strategy for electron-accepting organophosphorus
materials by introducing carbonyl groups.
The synthetic route is shown in Scheme 2 and the
procedures are described in detail in the Supporting Infor-
mation. Notably, the trivalent phosphorus center in 1 is easily
modified by oxidation and quaternization to form 2 and 3,
respectively. Transformation of the 1,3-dioxolane-protected 3
to ketone 4 was carried out with HBr to retain the BrÀ
counterion in the final product. The identity and purity of
all new compounds was confirmed by conventional spectro-
scopic methods. The thermal stability of the phosphonium
salts 3–5 was evaluated by thermogravimetric analysis (TGA)
and all showed decomposition temperatures between 225–
2358C, which indicate good thermal stability.
[*] Dr. X. M. He, Dr. J. B. Lin, W. H. Kan, Prof. Dr. T. Baumgartner
Department of Chemistry & Centre for Advanced Solar Materials
University of Calgary
2500 University Drive NW, Calgary, AB T2N 1N4 (Canada)
E-mail: thomas.baumgartner@ucalgary.ca
Homepage: http://www.ucalgary.ca/chem/pages/Baumgartner
[**] Financial support from the NSERC of Canada and the Canada
Foundation for Innovation (CFI) is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201303729.
2
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observed, which result in a stable diamond motif
À
(Figure 1b) with two C H···Br distances of 3.95
and 3.68 ꢀ. The bromide ion acts a bridge that
connects two individual molecules to stabilize
the layer. Moreover, two adjacent conjugated
phosphinine cores also exhibit partial p–p
interactions with a distance of 3.46 ꢀ (Fig-
ure 1c).
The optical and electrochemical features
were investigated by UV/Vis spectroscopy and
cyclic voltammetry, respectively, and the data
are summarized in Table 1. Keto-protected
compounds 1 and 3 only exhibit a high-energy
absorption band at 262–270 nm, while conver-
sion into their corresponding keto-containing
counterparts 2 and 4 is accompanied by a large
red-shift in the absorption, thus indicating the
off-on switching of the electronic communica-
tion through the p orbital of the carbonyl group
(Figure 2a). Compared to the phosphonium
model compound with a benzyl group (4m;
R = H), 4 exhibits a low-energy tail to 500 nm,
which is assigned to an intramolecular charge
transfer from the electron-donating trialkoxy-
Scheme 2. Synthesis and functionalization of dithieno[2,3-b;3’,2’-e]-4-keto-1,4-dihydro-
phosphinines. LDA=lithium diisopropylamide, PCC=pyridinium chlorochromate.
Importantly, the molecular structure of phosphonium salt
Table 1: Optical and electrochemical data for 1–4.
4 could be successfully determined by single-crystal X-ray
crystallography (Figure 1). As a result of its high flexibility,
literature reports on the structure of species containing the
3,4,5-tris(dodecyl-1-oxy)benzyl group are few and far
between.[10] Selected bond lengths and angles are summarized
in Table S1 in the Supporting Information. The conjugated
tricyclic core is highly planar and the phosphorus center
adopts a typical pyramidal geometry. The solid-state packing
clearly reveals a lamellar bilayer architecture, where the
lipophilic layers created by three long alkyl chains surround
the ionic layers. This solid packing not only supports our
previously proposed structures of the phosphole-lipid liquid
crystals,[9] but also predicts that 4 should form a similar
smectic liquid-crystal phase. In the ionic layer, inter- and
intramolecular hydrogen-bonding interactions between the
two benzylic hydrogen atoms and two bromide anions are
Compd
labs[a] [nm]
Ered[b] [V]
(e [dm3 molÀ1 cmÀ1])
1
2
3
4
262 (17250)
272 (12090), 314 (7320), 345 (8520)
270 (23200)
NA
À1.53
NA
272 (17820), 345 (8690)
À1.12, À1.75
[a] In CH2Cl2 solution at 298 K. [b] Half-potentials versus Fc/Fc+, 0.1m
nBu4NPF6 as the supporting electrolyte, in CH2Cl2. NA: not available.
phenyl ring to the electron-accepting phosphinine backbone
(Figure 2b). In addition, a small blue shift in the absorption
was observed when the solvent was changed from CH2Cl2 to
methanol (see Figure S1 in the Supporting Information),
which suggests the suppression of the hydrogen-bonding
interactions. A slight blue
shift is also observed in 5
with the weakly coordinat-
ing BF4À ion (see Figure S2
in the Supporting Informa-
tion). In contrast to the
established dithienophosp-
hole and the recently
reported DTDKP, how-
ever, the new phosphinines
are not fluorescent.
As expected, the intro-
duction of the carbonyl
group endows this new
class of phosphinines with
excellent electron-accept-
ing properties. As shown
Figure 1. a) Layer structure of 4 in the solid state, b) inter- and intramolecular hydrogen-bonding interactions,
and c) p–p interactions (for the crystal data, molecular structure, bond lengths, and angles, see the Supporting
Information).
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À2.66 eV),[3]
2 shows an even lower
LUMO energy level at ELUMO = À2.99 eV,
thus supporting its improved electron-
accepting capability, which is in good agree-
ment with the electrochemical studies. The
frontier orbitals (LUMO + 1, LUMO,
HOMO, and HOMO-1) are depicted in
Figure S5 in the Supporting Information.
Interestingly, the LUMO and LUMO + 1
orbitals mainly represent the p and p* orbi-
tals of the phosphinine core in 2 and 4’, and
not the HOMO and LUMO orbitals, which
is typically observed in dithienophosphole
Figure 2. a) UV/Vis spectra of 1 and 2 in CH2Cl2 at a concentration of 5ꢀ10À5 m.
b) Normalized UV/Vis spectra of 4 and a phosphonium model (4M; R=H) in CH2Cl2.
in Figure 3, 2 exhibits one reversible reduction at E1/2
=
Table 2: Frontier orbital energies [eV] for 2 and 4’.
À1.53 V. Interestingly, the phosphonium salt 4 shows two
reversible reduction processes at E1/2 = À1.12 V and À1.75 V;
its cationic nature can likely help stabilize the negative charge
formed during the process. By using ferrocene as an internal
reference, the LUMO energies (ELUMO) of 2 and 4 were
Compd[a]
LUMO+1
LUMO
HOMO
HOMO-1
2
À1.34
À4.43
À2.99
À5.77
À6.98
À8.29
À7.01
À8.40
4’[b]
[a] Calculated at the Gaussian03; B3LYP/6-31G(d) level of theory.[14]
[b] The cationic portion was calculated using PCM solvation (CH2Cl2).
and DTDKP systems. The HOMO level of 2 consists of the
p system of the exocyclic phenyl ring, a lone pair of electrons
on the oxygen atom, as well as some minor contribution from
the phosphinine core. By contrast, the HOMO of 4’ shows no
contribution from the phosphinine core, but from the
p orbital from the trialkoxyphenyl ring, thereby supporting
the possibility of a charge-transfer process from this group to
the electron-deficient phosphinine core, which is consistent
with the low-energy tail in the UV/Vis spectrum (Figure 2b).
The self-assembly of 4 was studied by differential scanning
calorimetry (DSC) and powder X-ray diffraction (PXRD).
The DSC experiments revealed several thermal phase tran-
sitions (Figure 4). Heating 4 from À508C resulted in two
endothermic transitions at 308C and 358C, which are assigned
Figure 3. Cyclic voltammograms of 2 (2 mm) and 4 (2 mm) in CH2Cl2
solution with 0.1m nBu4NPF6 as a supporting electrolyte. Scan rate:
100 mVsÀ1. Ferrocene was added as an internal standard and refer-
enced to 0 V.
determined to be E = À3.27 and À3.68 eV, respectively
[ELUMO = À(Ered1 +4.8) eV].[11] The diffusion coefficient of 4
is calculated to be 3.0 ꢁ 10À7 cm2 sÀ1, according to the Ran-
dles–Sevcik equation.[12] In addition, two similar reduction
processes were also observed in the thin film of 4 (see
Figure S4 in the Supporting Information) which reflect the
retention of its electron-acceptor features in the self-assem-
bled structures. Both 2 and 4 show better electron-accepting
properties than DTDKP (ELUMO = À3.14 eV), thus suggesting
their potential application as n-type materials for optoelec-
tronics. In this context, Matano and co-workers recently
demonstrated that fused phospholes with a reduction poten-
tial in the range of À1.82 to À1.40 V (versus the ferrocene/
ferrocenium (Fc/Fc+) couple), indeed display promising
electron mobilities up to 2.4 ꢁ 10À3 cm2 VÀ1 sÀ1.[13]
To better understand the optical and electronic character-
istics, DFT calculations (B3LYP/6-31G(d) level of theory)[14]
were performed on 2 and 4’ (dodecyl replaced by Me). Table 2
shows the corresponding frontier orbital energies. Compared
to the calculated LUMO energies of a related dithienophos-
Figure 4. DSC traces of 4 (a) and 5 (b) with heating and cooling rates
at 58CminÀ1 (negative values for the heat flow indicate an endother-
mic process); and polarized optical micrographs of 4 (c) at 1808C and
5 (d) at 1008C.
phole oxide (ELUMO = À1.86 eV)[15] and DTDKP (ELUMO
=
4
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to a crystal to soft crystal, and soft crystal to liquid crystal
transition, respectively. As the temperature increased to
2158C, compound 4 started to exhibit phase transitions to an
isotropic liquid, where a dark homeotropic area was observed.
As anticipated from the lamellar layer structure in the single
crystal of 4, smectic liquid-crystal properties are observed
within a very broad temperature range (30–2008C), which was
further confirmed by PXRD analysis (see Figure S7 in the
Supporting Information). At 2008C, 4 gives an intense sharp,
low-angle peak at 2q = 3.168, which corresponds to a d spacing
of 27.9 ꢀ for a smectic phase indexed to the (100) plane, and is
consistent with the calculated reflection from the single-
crystal X-ray data. In addition, a broad peak at 2q = 208
indicates the liquidlike state of the long alkyl chains. At 308C,
the PXRD pattern of 4 exhibits a more crystalline structure,
with several new reflection peaks in both the low and high
angle regions observed. The thickness of the layers tends to
decrease at low temperature, thus indicating closer packing.
The formation of a liquid-crystal phase of 4 is also
confirmed by polarized optical microscopy (POM). However,
the optical texture of 4 observed by POM is not recovered
after several scans, probably because of its very close melting
(Tm) and the decomposition (Td) points (Tm = 2158C, Td =
2278C). To improve the stability, we assumed that weakening
the intermolecular hydrogen bonding or p–p interactions
would be helpful in decreasing the melting point of the
material while maintaining the high decomposition temper-
ature. To approach the first goal of decreasing the strength of
the hydrogen bonding, we designed 5 by exchanging the
could be controlled by adjusting weak intermolecular forces,
such as hydrogen bonding and p–p interactions. Future
studies will involve the extension of this novel electron-
accepting building block with various electron-donating
moieties for donor–acceptor conjugated oligomers and poly-
mers for sustainable energy applications and will be reported
in due course.
Received: May 1, 2013
Published online: && &&, &&&&
Keywords: electron acceptors · liquid crystals ·
.
phosphorus heterocycles · self-assembly
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225
185
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215
235
146
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