Conjugated Polymers Featuring Heavier Main Group Element Multiple Bonds: A Diphosphene-PPV

Article abstract of DOI:10.1021/ja0394683

A new sterically encumbered bifunctional ligand has been developed, and its ability to provide simultaneous stabilization for two low-coordinate phosphorus centers has allowed the synthesis of new hybrid inorganic-organic conjugated materials. The new pho

Full text of DOI:10.1021/ja0394683

Published on Web 02/04/2004
Conjugated Polymers Featuring Heavier Main Group Element Multiple Bonds:
A Diphosphene-PPV
Rhett C. Smith and John D. Protasiewicz*
Department of Chemistry, Case Western ReserVe UniVersity, CleVeland, Ohio, 44106-7078
Received November 6, 2003; E-mail: protasiewicz@case.edu
Chart 1
Organic π-conjugated polymers are of much current academic
and significant technological interest due to their optical and
electronic properties. Of these materials, poly(phenylenevinylene)s
(PPVs) have attracted the most attention and found greatest utility
in devices.1 This particular architectural framework thus serves as
an attractive platform upon which one may consider systematic
isolobal replacements of the constituent carbene (CR2) fragments
for other main group fragments having available a p orbital capable
of participating in p-p π conjugation with carbon-carbon double
bonded systems. Phosphinidenes (PR) are fragments capable of
replacing carbene units within molecules and providing a wide array
of molecules containing PdC and PdP functional groups.2 While
phosphorus has long been an important component of polymers,3
materials engaging the phosphorus atom in extended linear p-p π
conjugation have only recently been realized.4,5 Hybrid inorganic-
organic conjugated systems promise novel electronic properties and
the processability and tunability typical of many organic polymers.
A key challenge to the incorporation of heavier elements into
extended conjugated systems is the need for steric protection to
promote stability for more reactive multiple bonds.6 A general
bifunctional sterically demanding platform that would stabilize
conjugated systems for a variety of EE multiple bonds is lacking.
Scheme 1
In this report we present a new type of bifunctional sterically
demanding ligand and demonstrate its general applicability for
synthesis of polymers having phosphorus centers engaged in pπ-
pπ conjugation along the main chain.
As a class of sterically demanding ligands, m-terphenyl ligands
(Chart 1, A) have proven highly successful for stabilizing a plethora
of compounds featuring multiply bonded main group elements.7
Our initial strategy using 2,3,5,6-tetrarylbenzenes (Chart 1, red
portion of B) as bifunctional mimics of m-terphenyls to confer
simultaneous steric protection to two low-coordinate phosphorus
centers in conjugated polymers met with two major difficulties.
First, tetrarylbenzenes having steric protection comparable to the
bulky aryl Dmp (Dmp ) 2,6-Mes2C6H3) were not readily accessible,8b
Preparation of the organophosphorus derivatives 1b and 1c followed
and thus the possibilities for isolating polymers having the broadest
range of element-element π-bonds were limited to materials such
as poly(phosphaalkenes) (B).8a Second, such polymers were rela-
tively insoluble unless solubilizing groups such as 2,5-dihexyloxy-
benzene were introduced. These findings led to development of
the second-generation bifunctional bulky ligand 1 (Chart 1). It was
anticipated that this ligand would finally allow access to polymers
containing a variety of EdE units, as the steric bulk afforded at
each end of 1 is identical to that of Dmp, a ligand that has been
previously demonstrated to support EdE bonds all the way from
E ) P through As, Sb, to Bi.9 In addition, the polymers should be
soluble due to the 2,5-dihexyloxybenzene core that has been
integrated into the bulky ligand.
reported procedures.8a,c,10 The greater steric protection conferred
by 1 over B allows successful isolation of the stable (in the absence
of air and water) diphospha-Wittig reagent 1c and a diphosphene-
containing polymer. Thus, deployment of 1c for synthesis of
phosphaalkene polymers 2a-c (Scheme 1) was easily achieved
upon addition of 1.005 equiv of various conjugated dialdehydes
(excess of dialdehyde provides stable end groups). This variation
of the phospha-Wittig reaction proceeds in essentially quantitative
yields at room temperature, forming phosphaalkenes exclusively
in the E conformation.12 Polymers 2a-c were thus produced in
76-85% isolated yields as orange (2a, b) or violet (2c) freely
soluble materials that are fairly thermally stable in the absence of
air or water.10 Solid samples of 2a and 2b, for example, are
unaffected by heating at 140 °C for 6 h under N2. Solutions of
2a-c in C6D6 remain unchanged upon heating at 120° for 4 h.
Compound 1a was conveniently prepared in multigram quantities
from readily available starting materials in only a few steps.10
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J. AM. CHEM. SOC. 2004, 126, 2268-2269
10.1021/ja0394683 CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Properties of Select Materials
The photoluminescence (PL) intensity of these materials drops
off as phosphorus atoms are substituted for olefinic carbons. The
PL intensities of 2a-c are approximately equal to that of (E)-
stilbene, while that of B is only 8% that of (E)-stilbene, and
diphosphene-containing 3 does not exhibit appreciable PL. This
may be due to fluorescence quenching by interaction with phos-
phorus lone pairs.8a
In conclusion, a conveniently synthesized bifunctional ligand
capable of simultaneously supporting two heavier main group
multiple bonds has been reported. This ligand was employed in
the synthesis of new phospha-PPVs including the first conjugated
polymer incorporating PdP linkages along the polymer backbone.
Ligand 1 should allow access to a variety of linear conjugated
materials having multiply bonded elements previously excluded for
polymer synthesis.
PL
M
UV/vis (nm)
λπ-π*(log ꢀ)
UV/vis (nm)
n-π*(log ꢀ)
PL max intens b
(nm)
n
X
n
(g mol-1
)
λ
(%)
PDI
1a (na)
2a 4.5
1175
5000
7200
7300
5900
6500
399 (4.63) (na)
435 (4.76) (na)
427 (4.87) (na)
416 (4.65) 473 (4.54)
435 (4.63) 481 (4.42)
445 (4.03) (na)
460
481
486
481
(na)
545
620
80
130
110
∼0
8
(na)
2.3
2.2
1.9
2.1
(na)
2b
6
2c 6.5
3
B
5.8
6
a UV/vis and PL spectra in CHCl3. b Relative to (E)-stilbene.
Previous studies of DmpPdPMe3 showed that the diphosphene
DmpPdPDmp is produced in high yields by photolytic extrusion
of PMe3.13 Photolysis (C6D6, rt) of bifunctional analogue 1c does
indeed result in extrusion of two equivalents of volatile PMe3 and
formation of desired red polymer 3 in near quantitative yield
(Scheme 1).10 Thermolysis of 1c (neat, 250 °C) also effects the
same transformation and is the preferred method as it is very rapid
(ca. 2 min), is readily applied to larger quantities, and leads to fewer
side products. While reduction of ArPCl2 using magnesium or other
metals is a classic avenue to diphosphenes (ArPdPAr),14 low yields
rendered this route inappropriate for achieving polymers. To the
best of our knowledge, 3 is the first example of a polymer featuring
multiple bonds between two heavier main group elements along a
polymer backbone. More importantly, the successful stabilization
of diphosphene units by 1 suggests its viability in stabilizing other
E-E multiple bonds which may be incorporated into conjugated
polymer backbones.15
Acknowledgment. We thank Akshay Kokil, J. Benjamin Beck,
Prof. Christoph Weder, Prof. Stuart J. Rowan, and Prof. Malcolm
E. Kenney for helpful discussions and use of instrumentation, and
the National Science Foundation (CHE-0202040) for support.
Note Added after ASAP: In the version published on the Web
2/4/2004, two asterisks were missing from the column headings in
Table 1. The version posted 2/6/2004 and the print version are
correct.
Supporting Information Available: Details for synthesis and
characterization of 1-3. This material is available free of charge via
the Internet at http://pubs.acs.org.
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The degree of polymerization and molecular weights of polymers
2 and 3 were determined by end group analysis (1H NMR
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degree of polymerization, fairly high molecular weights are attained
due to the very large monomeric units. More importantly, each
repeat unit contains four phenylenevinylene-type repeat units, so
that the total number of units in a chain ranges from 18 to 26.
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Key spectroscopic data for 1-3 are also summarized in Table
1. The absorption spectra of 1-3 feature characteristic π-π* bands.
Polymer 3 also shows an n-π* band typical of diphosphenes, and
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prepared, and characterization is in progress. Smith, R. C.; Protasiewicz,
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