Poly(p-phenylenediethynylene phosphane): A Phosphorus-Containing Macromolecule that Displays Blue Fluorescence Upon Oxidation

Article abstract of DOI:10.1002/anie.201504464

Despite the challenges associated with their synthesis, hybrid inorganic-organic polymers featuring heavier main-group elements spaced by π-conjugated organic functionalities have garnered considerable recent attention due to their chemical functionality and novel photophysical properties. We have succeeded in the preparation of an unprecedented organophosphorus polymer possessing functional phosphane-di-yne moieties in the main chain. Namely, poly(p-phenylenediethynylene phosphane) (PPYP) is prepared using a nickel(II)-catalyzed P-C bond-forming reaction. The hexyl-substituted PPYPs are solution processible and have been thoroughly characterized (molecular weight, M_w, ca. 10⁴Da vs. polystyrene; degree of polymerization, DP, ca. 10). Remarkably, although PPYP shows very weak emission upon irradiation with UV light, its oxide shows blue "turn-on" fluorescence. The present discovery bridges the areas of main-group and polymer science and opens the door to a new class of σ-π-conjugated macromolecules with unique chemical functionality.

Full text of DOI:10.1002/anie.201504464

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International Edition: DOI: 10.1002/anie.201504464
German Edition: DOI: 10.1002/ange.201504464
Hybrid Polymers
Hot Paper
Poly(p-phenylenediethynylene phosphane): A Phosphorus-Containing
Macromolecule that Displays Blue Fluorescence Upon Oxidation**
Benjamin W. Rawe and Derek P. Gates*
Dedicated to Dr. Howard C. Clark
Abstract: Despite the challenges associated with their syn-
thesis, hybrid inorganic–organic polymers featuring heavier
main-group elements spaced by p-conjugated organic func-
tionalities have garnered considerable recent attention due to
their chemical functionality and novel photophysical proper-
ties. We have succeeded in the preparation of an unprecedented
organophosphorus polymer possessing functional phosphane-
di-yne moieties in the main chain. Namely, poly(p-phenyl-
enediethynylene phosphane) (PPYP) is prepared using
À
coordination of the phosphorus moiety has the potential to
finely tune or modulate the electronic properties.
Poly(p-phenyleneethynylene)s represent a widely studied
organic p-conjugated polymer possessing useful photophys-
ical properties and potential applications as sensors.[8] There
has been growing interest in combining alkyne functionalities
with phosphane or phosphaalkene moieties as building blocks
in molecular chemistry.[9] Although researchers have pro-
posed more extended structures, polymers possessing P-
alkyne functionalities in the main chain have not been
realized until now.
Herein, we provide an efficient synthetic route to
a fascinating class of “turn-on” luminescent and chemically
functional phosphorus-containing macromolecule. Namely,
we report the synthesis and characterization of poly(p-
phenylenediethynylene phosphane) (PPYP, 1) the first di-
yne-substituted phosphane macromolecule (Scheme 1).
A modified nickel(II)-catalyzed coupling procedure[10]
was adopted for the synthesis of the title poly(p-phenyl-
enediethynylene phosphane)s (PPYPs) (1a and 1b) and
model compounds 2 and 3. To a solution of 1,4-diethynylben-
zene and PhPCl2 (1:1) in toluene was added Ni(acac)2
(5 mol%; acac = acetylacetonato) in the presence of excess
triethylamine (6 equiv). The reaction mixture was heated to
858C for several hours whereupon the initially colorless
solution turns dark red-brown and a precipitate is formed.
Analysis of the soluble fraction by 31P NMR spectroscopy
revealed signals assigned to monomer PhPCl2 (d = 161.2) and
to oligomer/polymer 1a (d = À60.6; cf. 2: d = À60.4).
Unfortunately, the majority of the macromolecular product
a
nickel(II)-catalyzed P C bond-forming reaction. The
hexyl-substituted PPYPs are solution processible and have
been thoroughly characterized (molecular weight, Mw, ca.
104 Da vs. polystyrene; degree of polymerization, DP, ca. 10).
Remarkably, although PPYP shows very weak emission upon
irradiation with UV light, its oxide shows blue “turn-on”
fluorescence. The present discovery bridges the areas of main-
group and polymer science and opens the door to a new class of
s-p-conjugated macromolecules with unique chemical func-
tionality.
T
he development of phosphorus-containing macromolecules
is a highly active research area lying at the interface of main-
group chemistry and polymer science. Inspired by the
prospect of finding materials with novel chemical function-
ality and properties, researchers are attracted by the wide
range of coordination numbers, oxidation states and bonding
environments that are uniquely provided by phosphorus.[1]
Perhaps resulting from the synthetic difficulty to incorporate
phosphorus atoms into long chains, the major classes of
organophosphorus polymers remain limited to polyphospha-
zenes,[1f] poly(phosphole)s,[2] poly(p-phenylene phosphaal-
kene/diphosphene)s,[3] poly(arylene/vinylene phosphane)s,[4]
poly(ferrocenylphosphane)s,[5] polyphosphinoboranes,[6] and
poly(methylenephosphane)s.[7] A major thrust in this area has
involved incorporating phosphorus moieties within extended
p-conjugated organic frameworks. The ready oxidation or
[*] B. W. Rawe, Prof. Dr. D. P. Gates
Department of Chemistry, University of British Columbia
2036 Main Mall, Vancouver, BC, V6T 1Z1 (Canada)
E-mail: dgates@chem.ubc.ca
[**] The authors are grateful to the Natural Sciences and Engineering
Research Council (NSERC) of Canada for Discovery and Research
Tools grants to D.G. in support of this work. We thank Dr. S. Kamal
(UBC-LASIR) for assistance with excited-state lifetime measure-
ments, and Mr. S. C. Serin and Dr. B. O. Patrick for assistance with
crystal structure determinations.
Scheme 1. Synthetic procedure to poly(p-phenylenediethynylene phos-
phane)s (PPYPs) 1a and 1b. Analogous procedures are used to
prepare model compounds 2 and 3. For each model and polymer, the
corresponding phosphane oxide has also been prepared and fully
characterized (e.g. 1b·O, 2·O and 3·O).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201504464.
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was insoluble in common organic solvents (such as toluene,
CH2Cl2, THF = tetrahydrofuran, diethyl ether, DMSO =
dimethyl sulfoxide, hexanes). MALDI-TOF MS (DHB
matrix) of the solid provided strong evidence for its formu-
lation as PPYP 1a. Importantly, a series of ions were observed
spaced by the mass of the monomer unit (233 Da) in 1a that
could be assigned to fragments of the protonated molecular
ion. Due to its poor solubility, we are unable to obtain
additional information about this new material at the present
time.
A common strategy to solubilize p-conjugated polymers
involves attaching long chain alkyl-substituents in the 2- and/
or 5-positions of the arylene spacer. Thus, the di-n-hexyl-
substituted polymer 1b was prepared from 1,4-diethynyl-2,5-
di-n-hexylbenzene[11] and PhPCl2 (1:1) under otherwise iden-
tical catalytic conditions to those described above. Two trials
were conducted with the only difference being the monomer
concentrations in the toluene reaction mixture (trial 1: 0.11m;
trial 2: 0.24m). For trial 1, the reaction progress was
monitored by 31P NMR spectroscopy (Figure 1a). Within
the first hour, several new major signals (d = À61) and two
new minor signals (d = 51.2 and À0.5) were observed along
with residual monomer PhPCl2 (d = 161.2). The overlapping
signals at À61 ppm, assigned to PPYP (1b), continued to grow
and broaden over the next several hours as PhPCl2 was slowly
consumed. In contrast to that observed for a typical step
polymerization, where the monomer is entirely consumed at
early stages of the polymerization, PhPCl2 remains until late
in the reaction. Moreover, the intensity of the signal
attributed to -C ꢀ C-P(Ph)Cl end groups (d = 51.2)[12] appears
to reach a steady state and is only depleted at very late stages
of the polymerization. These observations are consistent with
a polymerization where one end group (-P(Ph)Cl) has
a significantly higher reactivity than the other (-C ꢀ CH).
Thus, polymer (1b) is expected to consist primarily of alkyne
end-groups (i.e. X = H-C ꢀ C-C6H2R2-; Y=-C ꢀ C-H) and all
characterization data obtained for the polymer suggests this is
the case (vide infra).
Soluble PPYP (1b) may be isolated as a dark red gum
after precipitation (” 3) of a THF solution of the crude
polymer with degased methanol. The isolated material slowly
oxidizes in air or can be oxidized with hydrogen peroxide to
afford an air-stable film-forming phosphane oxide 1b·O (this
has also been fully characterized—for details, see the
Supporting Information). Unoxidized 1b was characterized
1
by using 31P, H and 13C{1H} NMR spectroscopy, MALDI-
TOF MS, GPC, UV/Vis spectroscopy (absorption and emis-
sion), and elemental analysis. The 31P NMR spectrum of
precipitated 1b (CDCl3) reveals a broad resonance at
À61 ppm with no detectable signals attributed to P-Cl end
groups [Figure 1(b)]. Importantly, the 13C{1H} NMR spec-
trum of 1b (CDCl3) shows broad resonances assigned to
alkyne carbon atoms of a P-CA ꢀ CB- functionality (d = 87.6
and 104.9). For comparison, model 2 shows similar spectral
features for the P-CA ꢀ CB- moiety [d = 82.8 (1JPC = 3 Hz),
106.3 (2JPC = 7 Hz)]. In addition to showing the expected
signals with the proper integration for the aliphatic and
aromatic protons within 1b, the 1H NMR spectrum shows
a resonance attributed to C ꢀ C-H end groups (d = 3.3).
Integration of this signal against those assigned to the n-hexyl
substituents allows for the estimation of the degree of
polymerization (trial 1: DPn = 7; trial 2: DPn = 10; DP =
1
degree of polymerization). 13C APT, 13C DEPT135 and H-
13C HSQC NMR experiments permitted the assignment of
the -CA ꢀ CB-H end groups of polymer 1b (CA: d = 82.2, CB:
d = 81.8; cf. 1,4-diethynyl-2,5-di-n-hexylbenzene: CA: d =
82.3, CB: d = 81.5).
PPYP (1b) and the MeOH soluble fraction containing
oligomers were further characterized by using gel permeation
chromatography (GPC, see Figure 2). As expected for a step
Figure 1. a) Selected 31P NMR spectra (121.5 MHz, toluene, 358 K)
showing: the progress of the polymerization of PhPCl2 and 1,4-
diethynyl-2,5-n-dihexylbenzene using Ni(acac)2 (5 mol%) as catalyst
and NEt3 (6 equiv) as base (trial 1). The measurement times are given
in hours. † indicates signals assigned to -P(Ph)-Cl moieties. We
speculate that the signals marked with * result from hydrolysis of
PhPCl2 by traces of water found in the nickel(II) catalyst. b) 31P NMR
spectrum (121.5 MHz, CDCl3, 358 K) of isolated polymer 1b after
precipitation.
Figure 2. Gel permeation chromatography (GPC) traces of PPYP (1b)
[trial 1: Mw =10,000 Da; PDI=2.0, vs. polystyrene standards] and the
methanol-soluble fraction containing oligomers of 1b (n=1–5 are
resolved; X=H-CꢀC-; Y=-C6H2R2-CꢀC-H). The assignments are
tentative. [°] assigned to monomer 1,4-diethynyl-2,5-di-n-hexylbenzene
by comparison to the GPC trace of an authentic sample.
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growth polymerization, PPYP 1b was of modest molecular
weight (trial 1: Mw = 10000 Da; trial 2: Mw = 12000 Da vs.
polystyrene standards) with an expectedly moderate polydis-
persity index (trial 1: PDI = 2.0; trial 2: PDI = 1.8).[13] Strik-
ingly, the GPC chromatogram shows several partially
resolved peaks at the low MW end (i.e. long retention time).
These peaks were significantly more prominent in the GPC
trace of the methanol-soluble material separated during the
precipitation of 1b (Figure 2). Speculating that these signals
were attributable to oligomers with formulation 1b bearing
alkyne end-groups [MALDI-TOF analysis shows ions for 1b
(n = 1 and 2; X = H-C ꢀ C-C6H2R2-; Y=-C ꢀ C-H)], the peak
maximum for each GPC signal was used to estimate the
molecular weight (vs. polystyrene) of each oligomer (neces-
sarily, each oligomer would be monodisperse). On that basis,
each peak in the chromatogram could be tentatively assigned
to a specific oligomer 1b (n = 1–5). Although the GPC
molecular weights (vs. polystyrene) were significantly over-
estimated [for example, n = 5: 3290 (GPC); 2297 (FW)],
a plot of formula weight vs. GPC molecular weight was linear
with a near-zero intercept (see the Supporting Information).
Importantly, such an observation not only suggests that the
assignments are correct but is consistent with the expected
overestimation of MW by GPC (vs. polystyrene) for a rigid
linear polymer such as PPYP.[14]
Figure 3. Molecular structures of 2·O (a) and 3·O (b) (50% probability
ellipsoids). Hydrogen atoms are omitted for clarity. Selected distances
(Š) and angles (8): 2·O (a): P1-O1 1.481(2), P1-C1 1.793(3), P1-C7
1.751(3), P1-C15 1.750(3), C7-C8 1.201(4), C8-C9 1.443(4), C15-C16
1.204(4), C16-C17 1.439(4); C1-P1-C7 105.57(13), C1-P1-C15 105.30-
(13), C1-P1-O1 113.22(13), C7-P1-O1 115.08(12), C15-P1-O1 112.64-
(12), P1-C7-C8 173.6(3), C7-C8-C9 178.0(3), P1-C15-C16 173.4(3), C15-
C16-C17 178.1(3); 3·O (b): P1-O1 1.480(2), P1-C1 1.802(3), P1-C7
1.797(3), P1-C13 1.746(3), C13-C14 1.196(4), C14-C15 1.441(4); C1-P1-
C7 107.3(1), C1-P1-C13 104.6(1), C1-P1-O1 112.5(1), C7-P1-O1 111.1-
(1), C13-P1-O1 116.0(1), C7-P1-C13 104.6(1), P1-C13-C14 176.5(3),
C13-C14-C15 178.7(3). Angles between planes (8): 2·O (a): C9-C14 and
C17-C22 9.1, C1-C6 and C9-C14 79.9, C1-C6 and C17-C22 88.5; 3·O
(b): C15-C17’ and C7-C12 23.6, C15-C17’ and C1-C6 82.3.
To gain insight into any possible conjugative effects within
PPYPs, the X-ray crystal structures for model compounds 2·O
and 3·O were determined. Molecular structures are shown in
Figure 3.[15] Particularly interesting is that the C ꢀ C bond
lengths [2·O: 1.203(6) Š; 3·O: 1.196(4) Š] are at the long end
of the normal range for alkynes (ca. 1.18 Š).[16] The P-Csp
bonds (2·O: 1.750(4) Š; 3·O: 1.746(3) Š] are shorter than the
P-CAr bonds [2·O: 1.793(3) Š; 3·O: 1.800(4) Š] but both are
significantly shorter than
a typical P-C single bond
(1.84 Š).[16] Similar trends have been observed for related
alkynylphosphanes [for example, Ph2P(O)(C ꢀ CPh),[17]
Mes2P(C ꢀ CPh),[9f] and P(CCPh)3].[18] In addition, the aryl
rings bearing alkyne moieties are fairly close to coplanar
(angle between planes: 9.18). Similarly, in 3·O the angle
between the central arylene ring and one of the P-Ph rings is
23.68. The slight shortening of P-C and slight lengthening of
C ꢀ C bonds combined with the tendency for towards co-
planarity of the aryl groups in 2·O and 3·O may be reflective
of conjugative effects through the phosphorus moieties.
Although crystal packing effects could also contribute to the
relative orientations of the aryl rings, intermolecular pi-
stacking does not appear to be significant since there are no
close contacts between the aryl or arylene moieties of
adjacent molecules. The closest intermolecular contacts are
between the oxygen and the P-phenyl hydrogens in 2·O
(O1···H3 = 2.39 Š) and between the oxygen and the C16-CH2
hydrogen of the hexyl substituent in 3·O (O1···H = 2.49 Š).
Further insight into the electronic properties of PPYPs
may be gained by examining their photophysical properties.
The UV/Vis absorption spectra of polymers 1b and 1b·O
were recorded along with models 2, 2·O and 3·O. Each species
shows four characteristic bands in the UV region as does 1,4-
diethynyl-2,5-di-n-hexylbenzene. The absorption spectra for
the polymers are shown in Figure 4 (top). Importantly, the
two highest energy bands are significantly bathochromically
shifted for polymer 1b (lmax = 289 and 305 nm; e ꢁ 2 ”
104 mÀ1 cmÀ1) relative to model 2 (lmax = 257 and 267 nm; e
ꢁ 4 ” 104 mÀ1 cmÀ1) and 1,4-diethynyl-2,5-di-n-hexylbenzene
(lmax = 265 and 274 nm; e
ꢁ
3 ” 104 mÀ1 cmÀ1). The two
lower energy bands of 1b (lmax = 317, 330 nm; e ꢁ 2 ”
104 mÀ1 cmÀ1) are significantly more intense but less red-
shifted than in 2 (lmax = 306, 328 nm; e ꢁ 4 ” 103 mÀ1 cmÀ1) and
1,4-diethynyl-2,5-di-n-hexylbenzene (lmax = 297, 308 nm; e
ꢁ 6 ” 103 mÀ1 cmÀ1). A significant red shift in the absorbance
maxima is also observed for the oxidized PPYP (1b·O) when
compared to models (2·O and 3·O). Consistently, a low energy
tail that extends into the visible region is observed for the
PPYPs. In addition, each displays an additional weak
absorption (1b: 384 nm, 1b·O: 382 nm). A similar feature
has also been observed for poly(p-phenylene phosphane)s.[4g]
We have also investigated the emission properties for
these new PPYPs. Perhaps most striking is that a THF
solution of 1b·O shows a strong blue emission under a black
(UV) light whilst no such emission is observed for 1b.[19] For
1b·O, irradiation at 318 nm gives a broad emission spectrum
(lem = 339 nm) that extends well into the visible region
(Figure 4, bottom). Interestingly, model 2·O is not fluorescent
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for phosphanes bearing alkyne substituents;[9l] this low barrier
may be a consequence of some conjugation.
In summary, we have disclosed the synthesis and charac-
terization of the first examples of poly(p-phenylenediyne
phosphane)s and their respective molecular model com-
pounds. PPYPs are an exciting new class of phosphorus-
containing macromolecule bearing functional phosphane and
alkyne groups. Although the study of the fascinating photo-
physical properties is at an early stage, preliminary evidence
for s-p-conjugation within the polymer backbone has been
obtained. Future work will further explore the novel elec-
tronic properties of PPYPs and their potential application as
sensor materials. Additionally, we will attempt to explore the
possibility of post-polymerization modification of PPYPs at
either the alkyne and/or the phosphane functional groups.
Keywords: alkynes · conjugated polymers · inorganic polymers ·
phosphanes · phosphorus
How to cite: Angew. Chem. Int. Ed. 2015, 54, 11438–11442
Angew. Chem. 2015, 127, 11600–11604
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Figure 4. UV/Vis absorption spectra (top); emission and excitation
spectra (bottom) of 1b and 1b·O in THF solution (10À5 m). The
excitation spectrum labelled Ex339 was recorded for the emission
maxima at 339 nm, respectively. The emission spectra were recorded
with excitation wavelength 318 nm. The abbreviation cps stands for
counts per second.
whereas 3·O is. Importantly, the emission spectrum of 1b·O
does not change with excitation wavelength (range: 260–
330 nm). Moreover, the excitation spectrum of 1b·O in THF
at the emission maximum (339 nm) shows similar features to
the absorbance spectrum. The excited state lifetime (t =
0.34 ns) is consistent with a fluorescence mechanism of
emission.[20] The quantum yield (f) for the emission was
0.12 relative to that of anthracene (f = 0.27).[21] Comparisons
are difficult to find since most P-containing polymers are
either not fluorescent or the quantum yield has not been
measured. Of note, the quantum yields for several phosphole
polymers with biphenyl spacers (f = 0.08–0.12 in solution)[2i]
are similar to that obtained herein for PPYP 1b·O.
Taken together, the molecular structures of the model
compounds and the photophysical properties of the PPYPs
1b and 1b·O are consistent with p-conjugation within the p-
phenylene-di-yne moieties in combination with hyperconju-
gation with phosphorus. Such effects are known with poly-
silanes[22] and polymers containing organic p-systems and
saturated heavier main-group element centers (e.g. silicon
and phosphorus) in the main chain.[1e, 23] In addition, a low
barrier to inversion at P has been observed spectroscopically
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[12] Although PhP(C ꢀ CPh)Cl has not been reported previously, we
have gained evidence for it by adding a THF solution of PhC ꢀ
CLi dropwise to a solution of PhPCl2 (4 equivs) in THF at
À788C. Specifically, the 31P NMR spectrum of the reaction
mixture showed singlet resonances at 53.0 ppm and À60.1 ppm
in a 5:6 ratio and assigned to PhP(C ꢀ CPh)Cl and previously
known PhP(C ꢀ CPh)2, respectively (See the Supporting Infor-
mation).
[13] Attempts to analyze 1b (and 1b·O) using GPC equipped with
multi-angle light scattering (MALS) were unsuccessful since the
absorption and/or emission properties of the PPYP seemed to
interfere with light-scattering analysis.
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Received: May 17, 2015
Revised: January 0, 0000
Published online: July 14, 2015
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