Bifunctional cross-conjugated luminescent phosphines and phosphine derivatives: Phospha-cruciforms

Article abstract of DOI:10.1039/b921895e

Cross-conjugated bifunctional species including a phosphine, phosphine oxide, phosphine sulfide, phosphonium salt, phosphorus ylide and a gold(i) phosphine complex have been prepared. The photophysical characteristics of the series of compounds have been determined experimentally and are discussed/compared with simpler analogues lacking cross-conjugated branches and rationalized on the basis of DFT calculations. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b921895e

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Bifunctional cross-conjugated luminescent phosphines and phosphine
derivatives: phospha-cruciforms†
Anshuman Mangalum and Rhett C. Smith*
Received 20th October 2009, Accepted 19th March 2010
First published as an Advance Article on the web 21st April 2010
DOI: 10.1039/b921895e
Cross-conjugated bifunctional species including a phosphine, phosphine oxide, phosphine sulfide,
phosphonium salt, phosphorus ylide and a gold(I) phosphine complex have been prepared. The
photophysical characteristics of the series of compounds have been determined experimentally and are
discussed/compared with simpler analogues lacking cross-conjugated branches and rationalized on the
basis of DFT calculations.
Introduction
Optical properties of transition metal complexes have been of
interest since the dawn of coordination chemistry, when Werner
classified complexes on the basis of their color.1,2 In more re-
cent years, the role of chromophoric ligands in the photochemistry
of transition metal complexes has been an ongoing area of
interest, and chromophore-modified complexes have found use
in diverse applications ranging from triplet emissive materials and
light-emitting diodes (LEDs)3-8 to nonlinear optics (NLO)9-15 and
photocatalysis.16-20 Because of the interest in the photophysics of
transition metal complexes21 and the widespread utility of phos-
phine ligands in coordination chemistry,22 it is surprising that there
have been relatively few studies on transition metal complexes
with visible absorbing/emitting phosphine ligands (though there
are numerous phosphole complexes).23 One reason for this may
be the added synthetic challenges of phosphorus chemistry and
the air sensitive, sometimes pyrophoric nature of intermediate
compounds used in the preparation of phosphines. Nonetheless,
significant recent progress has been made in the development of
phosphorus-containing small molecule chromophores24-27 and p-
conjugated polymers28-30 (i.e., Chart 1A), and this growing field
has recently been comprehensively reviewed.25 The development
of phosphorus-containing chromophores provides intriguing pos-
sibilities for their utility as ligands. Some examples of the relatively
limited number of fluorescent or visible-absorbing chromophore-
modified phosphine complexes that have been studied are shown in
Chart 1B.26,27,31-34 Another interesting and potentially useful aspect
Chart 1
of phosphorus-containing systems is that a variety of phosphine
derivatives can be prepared with phosphorus in +3 or +5 oxidation
that displays visible-wavelength absorption and emission.27
states, allowing access to various interchromophore geometries
LHP1 was readily prepared using air-stable, crystalline
(4-iodophenyl)P(O)Ph2 as a versatile precursor amenable to Pd-
catalyzed coupling to various chromophores under mild condi-
tions at room temperature. Following this work, we have become
interested in visible-absorbing/emitting phosphines with a variety
of absorption and emission properties for the preparation of
about the phosphorus center, as well as a range of electronic
perturbations on attached p-systems.26,35
We recently reported luminescent metallopolymers polymer-
ized through a chromophoric diphosphine (LHP1, Chart 1A)
Department of Chemistry and Center for Optical Materials Science and
Engineering Technologies (COMSET), Clemson University, Clemson, SC,
29634. E-mail: rhett@clemson.edu; Fax: 1 864 656 6613; Tel: 1 864 656
6112
† Electronic supplementary information (ESI) available: NMR spectra.
See DOI: 10.1039/b921895e
additional organophosphorus species, photophysically interesting
phosphine-metal complexes and emissive coordination polymers.
A relatively new class of photochemically interesting materials
is comprised of materials featuring multiple p-systems that overlap
at a central junction. Chromophores with such cross-conjugated
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branches,36-40 sometimes called cruciforms,41-45 are a class of
molecules that can exhibit the unusual feature of having HOMOs
and LUMOs that are geometrically separate. Some examples perti-
nent to the current study are shown in Chart 2. Certain conjugated
polymers have also shown geometrically separate HOMO and
LUMO levels.46,47 By appending stimuli-responsive functionalities
to cross-conjugated constructs, the energy of one frontier orbital
may be perturbed while that of the other remains fairly constant.
Researchers have taken advantage of this property to rationally
design sensors with anticipated responses characterized by a larger
signal transduction relative to that observed from traditional p-
conjugated chromophores.44,48-51
Table 1 Select properties of 8-13 and model M1
lp–p*
log e
l
emit¢/nm
U
31P NMR (ppm)
8
325.0
330.0
330.0
330.0
325.0
320.0
322.4
5.19
5.27
5.12
5.33
4.99
—
418.5
419.4
449.4
423.3
422.5
406.0
394.4
0.415
0.049
0.217
0.294
0.405
—
0.773
29.3
-4.3
22.6
33.7
43.7
20.5
none
9
10
11
12
13
M1
a
5.50
a Quantum yield too low to accurately measure.
Results and discussion
Synthesis
The first phospha-cruciform, phosphine oxide 8 (46%), was
prepared from 727 and commercial precursors via the procedure
outlined in Scheme 1, culminating in Sonogashira-Miyaura type
coupling of 6 and 7 to create the cross-conjugated cruciform
architecture and install the phosphorus center. With 8 in hand,
transformation to other phosphorus-bearing species proceeded
smoothly by classic methods (Scheme 2). Reduction of 8 to
phosphine 9 was affected by the action of HSiCl3 in toluene in
52% yield. Compound 9 was readily converted to phosphonium
salt 10 (62%) by reaction with methyl iodide, to the gold complex
11 (89%) by reaction with (tht)AuCl (tht = tetrahydrothiophene),
and to phosphine sulfide 12 (69%) via reaction with S8. All of these
compounds were found to have good air stability in the solid state,
though samples of phosphine 9 in solution were observed to slowly
transform to oxide at rates comparable to PPh3 (as gauged by
31P NMR spectroscopy). We attempted to convert phosphonium
salt 10 to the phosphorus ylide by deprotonation with KOtBu
in THF. Although we were unable to isolate 13 in pure form
due to its instability compared to the other phospha-cruciforms,
the conversion of 12 to 13 was reasonably clean by 31P NMR
spectroscopy, with the resonance at 20.5 ppm attributed to the
bis(ylide). The 31P NMR resonances are summarized in Table 1.
Chart 2
The particularly impressive success of cruciform-type molecules
for metal-ion detection, coupled with our ongoing interest in
unique chromophoric ligands for coordination control and mate-
rials assembly52-54 have lead us to explore cross-conjugated diphos-
phines. Herein, we describe the preparation of phosphorus(III) and
(V)-bearing cruciforms and a gold complex (Schemes 1 and 2),
with accompanying photophysical characterization and density
functional theory calculations.
Density-functional theory calculations
Because many of the stimuli-responsive properties of a cruciform
derive from the geometric distribution of its HOMO and LUMO,
materials 8, 9 and 10 were examined by Density Functional Theory
(DFT) calculations at the B3LYP/6-31G(d) level.51 The calculated
Scheme 1 Synthesis of phosphine oxide 8 (structure provided in Scheme 2).
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Scheme 2 Synthesis of compounds 9-13 and structure of phosphorus-free model M1.
Fig. 1 HOMO, LUMO and band gap (Eg) parameters from DFT calculations (B3LYP-6-31G* level) for compounds 8 (left), 9 (center) and 10 (right).
HOMOs and LUMOs for 8, 9, and 10 are provided in Fig. 1.
The calculated HOMO–LUMO gaps (Eg) are in good agreement
with the optical bandgap estimated by the red edge absorption
onset in absorption spectra for 8 and 9, though for phosphonium
salt 10 the calculated bandgap (2.49 eV) was somewhat lower
than the experimental optical bandgap (2.81 eV). This is probably
due to the fact that calculations do not account for counterion
effects or solvent effects, both of which are expected to be much
more influential in the ionic 10 versus neutral compounds 8 and 9.
The HOMO and LUMO become significantly more geometrically
distinct as the electron-withdrawing nature of the phosphorus
functionality are increased from phosphine 9 and phosphine
oxide 8 to cationic phosphonium 10. The lack of significant
HOMO–LUMO geometric separation in 9 is rather expected
because the electronegativity of P and C are quite similar, such
that the phosphine-bearing and t-butyl bearing arms will have
similar electronic and energetic profiles. Furthermore, the orbital
size/energy mismatch between P and C leads to only limited n-p
interaction.28-30 This is in contrast to nitrogen containing systems,
such as polyaniline or amino-derivatised cruciforms, in which
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the lone pair can engage in delocalization with an appended p-
system.55,56 Because of the cationic nature of the substituents in
10, however, the significant inductive withdrawing effect leads
to predictably notable perturbation of orbital energies. The
phosphorus-centered lone pair in 9 shows the greatest contribution
to the HOMO-1 orbital (not shown in Fig. 1), and plays a limited
role in the photophysical properties as discussed below.
relative to the ground state. Molar absorptivities of 8-12 are on the
order of 105–106, consistent with the anticipated p–p* transitions.
Absorption spectra for 8-12 each feature a lp–p* band at around
330 nm in addition to a shoulder to the red at around 350–400 nm.
The shoulder is attributed to a charge transfer band between the
p orbital on one of the cross-conjugated segments and the p*
of the other p-conjugated segment, a feature observed in many
tetraethynylbenzene derivatives.57 The absorption maxima of the
higher energy band for phosphine-containing 8-12 are slightly red-
shifted compared to that of M1 because of the small extension of
conjugation through the phosphorus centres30 and the electron-
withdrawing nature of many of the phosphorus derivatives.
Although the ylide 13 was not isolable in the solid state, the
reaction solutions were examined by absorption spectroscopy
and the data are summarized in Table 1. The ylide-containing
solution did not exhibit appreciable fluorescence, but does exhibit
a violet color. Although the ylide can be drawn in a resonance
structure wherein there is a P–C double bond, the orbital makeup
of phosphorus ylides suggests that this will not lead to any
appreciable extension of p-p type conjugation.58-61 The change
in the absorption features are likely promulgated through n-p
interactions.
The fluorescence emission spectra of the series show a single
band regardless of whether excitation is provided into the p-p*
band at ca. 330 nm or into the charge transfer band. All of
the phosphorus-containing species 8-12 have emission maxima
that are bathochromically shifted versus M1 despite very similar
absorption maxima. The larger Stokes shift in 8-12 versus M1
reflects the polarizability and electron withdrawing nature of
the phosphorus centers, as most dramatically illustrated by the
considerably larger shift for cationic 10 (lem = 449 nm), with its
large Stokes shift of 119 nm.
Photophysical properties
Normalised absorption (Fig. 2A) and emission spectra (Fig. 2B)
for the series of compounds 8-12 are shown in Fig. 2 along
with those for the benchmark compound 1,2,4,5-tetrakis(2-(4-
tert-butylphenyl)ethynyl)benzene (M1) for comparison. M1 was
selected for comparison because it features the same p-system
as phosphine 9, but lacks the phosphorus substituents, and the
substitution of phosphino substituents with tert-butyl groups
meant that the electronegativity of atom directly attached to
the p-system remained relatively constant. As noted previously,
the spectral data are in line with the properties anticipated
from DFT calculations, though caution must be exercised in
comparing ground state calculations with photophysical data that
are impacted by expected changes in excited state geometries
The quantum yields are highest (40% and 41%, respectively)
for the phosphine oxide (8) and phosphine sulfide (12). The
trend in photoluminescent quantum yields (U) for other phos-
phorus species mirror those reported for tris(stilbenyl)phosphine26
and LHP124 (Chart 1A) derivatives. Just as was observed for
tris(stilbenyl) phosphorus species, the phosphine exhibits signif-
icantly lower U than the phosphonium, phosphine oxide or d10
metal complex. This is attributable to the ability of the polarizable
phosphorus lone pair to facilitate nonradiative decay processes.
The exceptionally low U of 9 allows the observation of a very
weak (U < 0.01) red-shifted emission feature in Fig. 2B that may
be attributable to some very weak triplet emission or emission
from aggregates in solution. The later would not be unexpected
due to the limited solubility of the compounds, and it should be
stressed that this emission band accounts for < 1% of absorbed
photons.
Conclusions
In summary, we have successfully synthesized and characterized
six novel phosphorus-containing cross-conjugated cruciform sys-
tems and examined their photophysical properties. The presence
of phosphorus centers on the tetraethynylbenzene core lead to a
decrease in the bandgap and increased Stokes shifts versus the
hydrocarbon model compound. DFT calculations and optical
bandgap estimates indicate that band gap energies (Eg) for these
materials are within the range of other materials that have
Fig. 2 Absorption (A) and photoluminescence (B) spectra for com-
pounds 8-12 and M1 in dichloromethane. The trace for 9 is cut off before
reaching baseline resolution because it overlaps the expected overtone peak
at double the excitation wavelength at 660 nm (lex = 330 nm).
5148 | Dalton Trans., 2010, 39, 5145–5151
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found utility in organic optoelectronic applications. Extensions
of these molecules to the preparation of various luminescent
charge-transfer polymers, polyelectrolytes and metallopolymers
are currently being carried out in our lab.
Synthesis of 1,4-bis(2-(4-tert-butylphenyl)ethynyl)-2,5-bis(2-(4-
(diphenylphosphino) phenyl)ethynyl)benzene (9). Under an atmo-
sphere of dry nitrogen, 1,4-bis(2-(4-tert-butylphenyl)ethynyl)-2,5-
bis(2-(4-(diphenylphosphoryl)phenyl)ethynyl)benzene 8 (0.1 g,
0.1 mmol) was dissolved in 1 : 1 mixture of THF and toluene
(30 mL) followed by addition of triethylamine (1 mL). Trichlorosi-
lane (1.34 mL, 3.00 mmol) was added and the mixture was refluxed
overnight. After cooling to room temperature, degassed saturated
aqueous sodium bicarbonate (10 mL) was slowly added. A yellow
precipitate was filtered off and the organic layer was further
extracted with saturated aqueous sodium bicarbonate (3 ¥ 50 mL).
The organic layer was collected, dried over sodium sulfate and all
volatiles were removed under reduced pressure. The residue was
dissolved in minimal dichloromethane, followed by slow addition
of pentane (20 mL) to yield the product as a yellow crystalline solid
upon cooling, filtering and drying in vacuo (50 mg, 52%). 1H NMR
(300 MHz, CDCl3): d = 1.34 (s, 18H, 6 ¥ CH3), 7.39 - 7.27 (m, 28H,
Aromatic), 7.56–7.49 (m, 8H, Aromatic), 7.76 (s, 2H, Aromatic).
13C NMR (75.4 MHz, CDCl3): d = 31.1, 34.8, 86.8, 88.7, 95.1,
95.7, 119.8, 123.2, 125.1, 125.4, 125.4, 128.5, 128.6, 128.9, 131.4,
131.5, 131.6, 133.3, 133.5, 133.7, 134.0, 134.9, 136.5, 136.6, 138.5,
138.6, 152.3. 31P NMR (121.4 MHz, CDCl3): d = –4.3. HRMS
(M+H)+: calc. for C70H57P2, 959.3936; found, 959.3957. UV-vis:
lmax 330 nm (e = 190,000 M-1 cm-1).
Experimental
Reagents and general methods
Reagents were obtained form Acros, Aldrich Chemical Co.,
TCI America, or Alfa Aesar and used as received from the
suppliers without further purification. Solvents were purified
by passage through alumina column under a N2 atmosphere
employing an MBraun solvent purification system. All the air
sensitive manipulations were performed in an MBraun dry
box or using standard Schlenk techniques under N2 atmo-
sphere. 1,4-dibromo-2,5-diiodobenzene (2),62 1,4-dibromo-2,5-
bis(2-(4-tert-butylphenyl)ethynyl)benzene (4),63 and 1,4-bis(2-(4-
tert-butylphenyl)ethynyl)-2,5-diethynylbenzene (6)64 were synthe-
sized by modification of reported methods. Compounds were
1
characterized by H, 13C, and 31P NMR spectra using a Bruker
Avance 300 spectrometer operating at 300 MHz for 1H, 75.4 MHz
for 13C, and 121.5 MHz for 31P nuclei. All the spectra were collected
at 25 ◦C and referenced to TMS or residual solvent peak for
1H and 13C and to 85% phosphoric acid for 31P. Absorption
spectra were recorded using Varian Cary 50 spectrophotometer
and photoluminescence (PL) data were recorded using Varian
Eclipse spectrofluorimeter in quartz cuvettes with a pathlength
of 1 cm in DCM. Quantum yield (U) for all compounds were
calculated relative to quinine bisulfate in 0.1 M H2SO4 (aq) (U =
0.546).27
Synthesis of phosphonium salt (10). Under a nitrogen atmo-
sphere, 9 (25.0 mg, 0.026 mmol) was dissolved in toluene (20 mL)
followed by addition of methyl iodide (16.0 mg, 0.100 mmol).
The reaction mixture was refluxed for 16 h. All the volatiles were
removed under reduced pressure and the residue was triturated
with pentane (10 mL) to yield the yellow solid (20.0 mg, 62.5%).
1H NMR (300 MHz, CDCl3): d = 1.33 (s, 18H, 6 ¥ CH3), 3.35
(d, 6H, JHP = 12.0 Hz), 7.43 (d, 4H, J = 9.0 Hz, Aromatic), 7.52
Synthesis of 1,4-bis(2-(4-tert-butylphenyl)ethynyl)-2,5-bis(2-
(d, 4H, J = 9.0 Hz, Aromatic), 7.67–7.84 (m, 30H, Aromatic). 13
C
(4-(diphenylphosphoryl) phenyl)ethynyl)benzene (8). In
a
NMR (75.4 MHz, CDCl3): d = 31.1, 34.9, 86.2, 92.7, 93.2, 96.6,
118.2, 119.4, 124.7, 125.7, 125.8, 128.5, 128.7, 130.2, 130.5, 130.7,
131.4, 132.0, 132.1, 133.1, 133.3, 133.3, 133.4, 133.5, 135.3, 135.6,
152.6. 31P NMR (121.4 MHz, CDCl3): d = 22.6. MS (M-2I-)2+:
calc. for C72H62P2, 988.4; found, 988.4. UV-vis: lmax 330 nm (e =
130,000 M-1 cm-1).
round bottom flas, 1,4-bis(2-(4-tert-butylphenyl)ethynyl)-2,5-
diethynyl benzene 6 (0.34 g, 0.78 mmol) and (4-iodophenyl)-
diphenylphosphineoxide) 7 (0.85 g, 2.1 mmol) were degassed
and taken into the dry box. Compound 7 was dissolved in THF
(100 mL) followed by the addition of tetrakis(triphenylphosphine)
palladium(0) (250 mg, 0.24 mmol) and copper iodide (45 mg,
0.24 mmol). In a vial, 1,4-bis(2-(4-tert-butylphenyl)ethynyl)-
2,5-diethynylbenzene 6 was dissolved in diisopropyl amine
(20 mL) and added into the reaction mixture drop wise. The
reaction mixture was stirred for 48 h at room temperature.
Dichloromethane (100 mL) was added into the reaction mixture,
which was then extracted with saturated sodium bicarbonate
solution (4 ¥ 100 mL). The combined organics were collected,
dried over sodium sulfate and all volatiles were removed under
reduced pressure. The residue was washed recursively with
acetonitrile to afford a yellow solid (0.35 g, 46%). 1H NMR
(300 MHz, CDCl3): d = 1.34 (s, 18H, 6 ¥ CH3), 7.40 (d, 4H,
J = 6.8 Hz, Aromatic), 7.54–7.48 (m, 12H, Aromatic), 7.60–7.57
(m, 4H, Aromatic), 7.74–7.66 (m, 16H, Aromatic), 7.78 (s, 2H,
Aromatic). 13C NMR (75.4 MHz, CDCl3): d = 31.1, 34.9, 86.6,
90.0, 94.3, 96.1, 119.6, 125.0, 125.5, 125.6, 126.7, 126.7, 128.5,
128.7, 131.4, 131.5, 131.6, 132.0, 132.1, 132.1, 132.8, 133.5, 135.0,
152.3. 31P NMR (121.4 MHz, CDCl3): d = 29.3. HRMS (M+H)+:
calc. for C70H57O2P2: 991.3834; found, 991.3837. UV-vis: lmax
325 nm (e = 160,000 M-1 cm-1).
Synthesis of phosphine gold complex (11). Under a nitro-
gen atmosphere, 9 (20.0 mg, 0.021 mmol) was dissolved in
dichloromethane (15 mL) followed by addition of (tht)AuCl
(14.0 mg, 0.044 mmol, tht = tetrahydrothiophene). The reaction
mixture was stirred for 16 h. All the volatile was removed under
reduced pressure and the residue was triturated with pentane
(5 mL) and dried in vacuo to yield 11 as a yellow solid (26 mg,
89%). 1H NMR (300 MHz, CDCl3): d = 1.35 (s, 18H, 6 ¥ CH3),
7.42 (d, 4H, J = 6.0 Hz, Aromatic), 7.49–7.67 (m, 32H, Aromatic),
7.80 (s, 2H, Aromatic). 13C NMR (75.4 MHz, CDCl3): d = 31.1,
34.9, 86.5, 90.6, 93.9, 96.2, 119.6, 124.9, 125.6, 125.6, 126.8, 126.9,
127.9, 128.6, 128.7, 129.3, 129.4, 131.4, 132.1, 132.1, 132.2, 132.2,
133.9, 134.1, 134.1, 134.2, 135.2, 152.5. 31P NMR (121.4 MHz,
CDCl3): d = 33.7. MALDI: calc. for C70H56Au2Cl2P2 (M+H)+,
1423.3; found, 1423.1. UV-vis: lmax 330 nm (e = 210,000 M-1 cm-1).
Synthesis of 1,4-bis(2-(4-tert-butylphenyl)ethynyl)2,5-bis(2-(4-
diphenylphosphorothioyl)phenyl)ethynyl)benzene (12). Under ni-
trogen atmosphere, compound 9 (15 mg, 0.016 mmol) was
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dissolved in dry THF (5 mL) followed by addition of S8 (1.6 mg,
0.048 mmol). The reaction mixture was stirred for 16 h, after
which all the volatiles were removed under reduced pressure. The
residue was further purified by dissolution in CHCl3 and filtering
through a 0.4 mm PTFE syringe filter to remove excess sulfur.
CHCl3 was removed under pressure, and the solid was rinsed
with pentane and dried in vacuo to yield the product as a yellow
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18H, 6 ¥ CH3), 7.41 - 7.38 (m, 4H, Aromatic), 7.57–7.46 (m,
16H, Aromatic), 7.66–7.62 (m, 4H, Aromatic), 7.80–7.71 (m, 13H,
Aromatic). 13C NMR (75.4 MHz, CDCl3): d = 31.2, 35.0, 86.6,
90.2, 94.4, 96.2, 119.7, 125.0, 125.6, 126.4, 128.6, 128.8, 131.5,
131.6, 131.7, 131.8, 132.0, 132.2, 132.4, 132.7, 133.1, 133.8, 135.1,
152.4. 31P NMR (121.4 MHz, CDCl3): d = 43.7. HRMS (M+H)+:
calc. for C70H57Au2Cl2P2, 1023.3377; found, 1023.3417. UV-vis:
lmax 330 nm (e = 100,000 M-1 cm-1).
Synthesis of phosphorus ylide (13). Under a nitrogen atmo-
sphere, compound 10 (19.0 mg, 0.015 mmol) was dissolved in
THF-d8 (1 mL) followed by addition of potassium tert-butoxide
until compound 10 completely dissolved. Upon addition of the
first portion of the base, an immediate color change from pale
yellow to a light violet-brown color was observed. Consumption
of 10 was completed within 30 min as determined by NMR
spectroscopy. 31P NMR (121.4 MHz, CDCl3): d = 20.5 (attributed
to 13); smaller resonances were present at d = 25.0, 23.9, 13.6 and
11.2. The spectrum is shown in the ESI.
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Acknowledgements
This work was funded by a National Science Foundation
CAREER Award (CHE-0847132), and Clemson University.
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