Intense fluorescence of 1-aryl-2,3,4,5-tetraphenylphosphole oxides in the crystalline state

Article abstract of DOI:10.1039/c0nj00155d

A series of 2,3,4,5-tetraphenylphosphole oxides with various aryl groups on the phosphorus atoms were synthesized and their fluorescence properties in the crystalline state were investigated. Some of these compounds showed an intense fluorescence with the

Full text of DOI:10.1039/c0nj00155d

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New Journal of Chemistry
An international journal of the chemical sciences
www.rsc.org/njc
Volume 34 | Number 8 | August 2010 | Pages 1493–1784
Themed Issue: Main Group chemistry
ISSN 1144-0546
LETTER
Shigehiro Yamaguchi et al.
Intense fluorescence of
1-aryl-2,3,4,5-tetraphenylphosphole
oxides in the crystalline state

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LETTER
www.rsc.org/njc | New Journal of Chemistry
Intense fluorescence of 1-aryl-2,3,4,5-tetraphenylphosphole oxides
in the crystalline statewzy
Aiko Fukazawa, Yasunori Ichihashi and Shigehiro Yamaguchi*
Received (in Montpellier, France) 28th February 2010, Accepted 17th May 2010
DOI: 10.1039/c0nj00155d
A series of 2,3,4,5-tetraphenylphosphole oxides with various aryl
groups on the phosphorus atoms were synthesized and their
fluorescence properties in the crystalline state were investigated.
Some of these compounds showed an intense fluorescence with
the quantum yields of 0.75–0.91. These intense emissions are
attributable not only to the restricted rotation of the peripheral
phenyl rings, but also to the long intermolecular distances
between the adjacent molecules in the crystal packing.
state have not been fully clarified, despite their high potentials
as light emitting materials.
Among the various phosphole derivatives, we now focused
our attention on the oxidized derivatives, i.e., the phosphole
oxides, because of their narrower HOMO–LUMO gap and
their higher chemical stability compared to the s3, l3-phospholes.
In addition, we chose the peripherally all-phenyl-substituted
phosphole skeleton, taking into account the similarity to a
highly emissive silicon analogue. Recently, Tang and co-workers
extensively studied the intense solid-state fluorescence of the
2,3,4,5-tetraphenylsiloles, in which the restricted motion of the
peripheral phenyl groups plays a crucial role in attaining an
enhanced emission upon aggregation.4,6 We now report
the crystalline-state fluorescence properties of a series of
1-aryl-2,3,4,5-tetraphenylphosphole oxides, which include the
all-phenylated 1, bulky aryl (mesityl and 2,4,6-triisopropyl-
phenyl)-substituted 2 and 3, and electronically different aryl
(p-MeOC6H4 and p-CF3C6H4)-substituted 4 and 5 (Fig. 1). We
explored the origin of their intense solid-state emission based
on a study of their structure–property relationships.
p-Conjugated molecules are undoubtedly key components in
the field of organic electronics. Their solid-state fluorescence
properties are of particular importance for optoelectronic
applications, such as organic LEDs and organic lasers. While
there are many p-conjugated molecules that exhibit an intense
fluorescence in a dilute solution, their fluorescence is usually
substantially quenched in the solid state.1 So far, several
approaches to gain an intense solid-state emission have been
reported, such as dendritic substituent protection,2 cross
dipole stacking,3 aggregation-induced emission,4–6 enhanced
intramolecular charge transfer transition,7 and J-aggregate
formation.8
A series of 1-aryl-2,3,4,5-tetraphenylphosphole oxides 2–5
were prepared by the modified procedure of the previously
reported phosphole synthesis, as shown in Scheme 1.13 Thus,
an excess amount of PCl3 was added to a frozen solution of the
in situ-generated 1,4-dilithio-1,2,3,4-tetraphenylbutadiene at
Meanwhile, the incorporation of the main group elements
into a p-conjugated framework is a powerful strategy to
develop new p-electron materials with intriguing properties.9
Phosphole, a heavier analogue of pyrrole, has especially
attracted attention due to its unique electronic structure, i.e.,
its weak aromaticity and low-lying LUMO.10 One fascinating
feature of the phospholes is the chemical functionalization
diversity on the phosphorus atom, which significantly affects
the optical and electrochemical properties. A variety of phosphole-
containing p-conjugated compounds have been synthesized
and their interesting properties have been revealed. For
example, Reau and co-workers reported the photo- and
´
Fig. 1 Structures of peripherally phenylated phosphole oxides and
electroluminescences of the 2,5-diarylphospholes, including
the phosphole sulfide derivatives and gold complexes.11
Recently, an enhanced fluorescence of the dendritic phosphole
derivatives was also demonstrated.12 However, the fundamental
photophysical properties of the phosphole derivatives, in
particular, the detailed fluorescence properties in the solid
related compounds.
Department of Chemistry, Graduate School of Science,
Nagoya University, Furo, Chikusa, Nagoya, 464-8602, Japan.
E-mail: yamaguchi.shigehiro@b.mbox.nagoya-u.ac.jp;
Fax: +81 52 789 5947; Tel: +81 52 789 2291
w Dedicated to the late Professor Pascal Le Floch.
Scheme 1 Reagents and conditions: (i) Li, Et2O, rt; (ii) PCl3, ꢀ196 1C
then rt; (iii) ArLi (MesLi for 2, TipLi for 3, 4-MeOC6H4Li for 4, and
4-CF3C6H4Li for 5), THF, 0 1C then rt; (iv) 30% H2O2 aq. for 2, 3,
and 4, and mCPBA for 5, 0 1C then rt. Mes = 2,4,6-trimethylphenyl;
Tip = 2,4,6-triisopropylphenyl.
z This article is part of a themed issue on Main Group chemistry.
y Electronic supplementary information (ESI) available: Experimental
details, spectral data, and crystallographic data. CCDC reference
numbers 767155–767158 (2–5). For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c0nj00155d
ꢁc
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depending on the P-aryl groups. As the aryl group became
bulkier from 1 to 3, the fluorescence band shifted to a shorter
wavelength with a decrease in the Stokes shift from 6600 cmꢀ1
for 1 to 6000 cmꢀ1 for 3. This may be simply due to the less
effective p conjugation in the 2,5-diarylphosphole moiety in
the excited state, caused by the restricted conformational
change from the Franck–Condon state due to the bulkier aryl
group on the phosphorus atom. As for the electronic effect, as
the aryl group became more electron-withdrawing, the lem
shifted to a longer wavelength. This may be attributed to the
enhanced charge transfer nature in the excited state by the
change in the electron-accepting character of the phosphole
oxide moiety.
Fig. 2 ORTEP drawing of compound 2 (50% probability for thermal
Although compounds 1–5 showed only a faint fluorescence
in solution, all of them exhibited an intense yellowish green or
yellow fluorescence in the crystalline state. We evaluated their
fluorescence quantum yields (FF) in the crystalline state
using a calibrated integrating sphere system equipped with a
multichannel spectrometer (Hamamatsu C9920-02). Since a
slight size dependence of the FF was observed,15 we adopted
the highest value as the inherent value for the FF of the
compound. Significant enhancements in the FF from solution
to the crystalline state were observed. For instance, the
all-phenylated 1 has the FF value of 0.49 in the crystalline
state, which is ca. 880 times higher than that in the THF
solution (FF = 0.00056). This enhancement in FF may be
mainly attributed to the suppression of rotation of the phenyl
rings at the 3,4-positions in the solid state (vide infra), as has
already been well discussed for the peripherally phenylated
silole derivatives.4,6 It is worth noting that the all-phenylated 1
has a higher FF in the crystalline state than the 3,4-dialkylated
analogue 6 (FF = 0.33). This comparison demonstrates the
benefit of the peripherally phenylated structure for attaining
the intense solid-state fluorescence.11,12
ellipsoids). Hydrogen atoms are omitted for clarity.
ꢀ196 1C, then the mixture was gradually warmed to room
temperature. The produced P-chlorophosphole intermediate
was subsequently reacted with the corresponding aryllithiums
and H2O2 or mCPBA to produce the 1-aryl-substituted phosphole
oxides 2, 3, 4 and 5 in 30, 3, 44 and 19% yields, respectively.
The low yield for 3 is likely due to the steric hindrance of the
Tip group. The pentaphenylphosphole oxide 1 was synthesized
according to the literature.14 All compounds were unequivocally
characterized by NMR spectroscopies and finally by X-ray
crystallography (see ESIy). As a representative example, an
ORTEP drawing of 2 is shown in Fig. 2, which reveals the
propeller-like arrangement of the peripheral phenyl rings. In
all the compounds, no significant intermolecular p-stacking
interaction was observed, except for 4, in which a slipped
p-stacking between the phenyl rings of the molecules adjacent
to each other was observed.
The photophysical data for the phosphole oxides 1–5 both
in solution (THF) and in the crystalline state are summarized
in Table 1. All the compounds have sufficient photo-stability
and no decomposition was observed during the measurements.
In solution, the series of compounds 1–5 showed almost
identical absorption spectra, indicating that the P-aryl groups
do not affect the transition energy for the absorption.
In contrast, in the fluorescence spectra, their fluorescence
maximum wavelengths (lem) varied in the range of 509–528 nm
The noteworthy issue is the substituent effects of the P-aryl
groups on the lem and FF in the crystalline state. Not only
the bulkier aryl-substituted derivatives 2 and 3, but also
the electron-donating p-MeOC6H4-substituted 4 exhibited
significantly intense fluorescences with the FF values of 0.91,
0.75, and 0.91, respectively, whereas the all-phenylated 1 and
the electron-withdrawing p-CF3C6H4-substituted 5 showed a
moderately intense fluorescence. This fact clearly demonstrates
that the fluorescence efficiency in the solid state is not simply
dependent on the degree of the steric congestion in the
molecule or the electronic effects of the substituents. Notably,
while the moderately emissive compounds 1 and 5 showed a
considerably red-shifted fluorescence in the crystalline state
compared to those in solution, the highly emissive compounds,
particularly 2 and 4, showed a blue-shifted lem. This trend
suggests that the intermolecular interaction in the excited state
is another important factor to determine the fluorescence
efficiency in the solid state.
Table 1 Photophysical data for 1-aryl-2,3,4,5-tetraphenylphosphole
oxides and their related compounds
Solutiona
abs/nm log e
Crystal
c
l
lem/nm
lem/nm FF
ts/nsd
Compound
1
2
3
4
5
6
7
387
389
391
388
3.92
3.85
3.90
3.91
3.89
520b
517
509
517
528
538
509
520
508
535
538
471
0.49 5.0
0.91 10.6
0.75 10.0
0.91 10.7
0.25 3.1
0.33 2.6
0.76 0.74/5.7
389
370e
363f,g
3.93e 505e
—
371f,g
To gain a deeper insight into the excited state dynamics in
the crystalline state, we conducted the fluorescence lifetime (ts)
measurement using several crystals of each sample. The highly
emissive compounds 2, 3, and 4 have longer fluorescence
lifetimes than the less emissive compounds 1 and 5. Fig. 3
shows a comparison of the radiative (kr) and non-radiative
(knr) decay rate constants, calculated from FF and ts, for a
a
b
In THF. The relative fluorescence quantum yield determined with
c
quinine sulfate as a standard was 0.00056. Absolute quantum yield
determined by calibrated integrating sphere system equipped
with a multichannel spectrometer. All the decay profiles were fitted
reasonably well using single exponential function except for
a
d
a
e
f
g
7. Ref. 12b. In cyclohexane. Ref. 6.
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Fig. 4 A plot of the fluorescence quantum yield FF as a function of
the intermolecular Pꢃ ꢃ ꢃP distance d in the crystalline state.
difference in the packing mode might be an important factor.
To examine the effect of the packing structure, we focused on
the intermolecular Pꢃ ꢃ ꢃP distances (d) between the closest
adjacent phosphole rings in the crystal structures as an index.
Fig. 4 shows a plot of the FF values as a function of the Pꢃ ꢃ ꢃP
distances. Notably, a rough correlation between the Pꢃ ꢃ ꢃP
distance and FF was observed, that is, the FF becomes higher
as d increases. These results suggest that the longer d, i.e., less
spatial proximity, may be an important factor for a more
efficient suppression of the intermolecular excited state energy
migration, resulting in the suppression of the non-radiative
decay process and thus an enhancement in FF.
In summary, the 2,3,4,5-tetraphenylphosphole oxides with
various aryl groups on the phosphorus atoms exhibited intense
fluorescences in the crystalline state. While the enhanced
fluorescence compared to those in solution is mainly due to
the restricted rotation of the peripheral phenyl groups,
whether they can show a significantly intense solid-state
emission or not is highly dependent on the packing mode in
the crystal structure. Some of the phosphole compounds
indeed showed very high fluorescence quantum yields of
0.75–0.91. Notably, these values are comparable or even
higher when compared to that of a silole analogue, 1,2,3,4,5-
pentaphenyl-1-methylsilole 7 (FF = 0.76).6,16 These results
well demonstrate the high potentials of the phosphole oxide
derivatives as a new class of the light emitting materials for
organic electronics.
Fig. 3 Comparison of radiative (kr) and non-radiative (knr) decay
rate constants among the various phosphole oxides in the crystalline
state. Rate constants were calculated with FF and ts according to the
formula of kr = FF/ts and knr = (1 ꢀ FF)/ts.
series of the phosphole oxides. Whereas the kr values
are comparable to one another for all the compounds
(7.4–9.8 ꢂ 107 sꢀ1), the knr values of the highly emissive 2, 3,
and 4 were significantly lower compared to those of 1 and 5
(8.2 ꢂ 106 sꢀ1, 2.6 ꢂ 107 sꢀ1, 8.1 ꢂ 106 sꢀ1 for 2, 3, and 4, and
1.0 ꢂ 108 sꢀ1, 2.4 ꢂ 108 sꢀ1 for 1 and 5, respectively). Thus, the
high FF values for compounds 2, 3, and 4 are attributable to
the suppression of the non-radiative decay processes.
To elucidate the origin of the difference in FF in the
crystalline state among compounds 1–5, we evaluated the
fluorescent properties in a polymer matrix, the data of which
are summarized in Table 2. All the phosphole oxides 1–5
dispersed in a poly(methyl methacrylate) (PMMA) matrix
exhibited intense emissions. This fact demonstrates that not
the intermolecular interaction, but the restricted rotation of
the peripheral phenyl rings in the solid state may be a primary
factor for their high FF values. However, it should be also
noted that, in contrast to the observed significant substituent
effects on both lem and FF in the crystalline state, almost
identical fluorescent properties were observed for all the
compounds in the polymer matrix. Namely, the FF values of
1–5 were comparable (FF = 0.41–0.56). Moreover, the lem
values (507–511 nm) were comparable to one another and all
blue-shifted when compared to those in solution. These facts
revealed that the difference in FF in the crystalline state among
compounds 1–5 should be attributed not to such an intra-
molecular effect as the restricted rotation of the peripheral
phenyl groups, but to another effect. We assumed that the
Experimental
1-Mesityl-2,3,4,5-tetraphenyl-1H-phosphole-1-oxide (2): typical
procedure for the preparation of 1-aryl-2,3,4,5-tetraphenylphosphole
oxides
To a suspension of lithium (granular, 139 mg, 20 mmol) in dry
Et2O (20 mL) was added diphenylacetylene (3.74 g, 21 mmol)
at room temperature. After stirring for 23 h, the reaction
mixture was cooled to ꢀ196 1C with liquid N2. After complete
freeze of this mixture, PCl3 (3.5 mL, 40 mmol) was added. The
reaction mixture was allowed to warm to ambient temperature
and stirred for 24 h. After removal of Et2O and the remaining
PCl3 under reduced pressure, dry THF (10 mL) was added and
the mixture was cooled to 0 1C. Mesityllithium prepared in situ
from bromomesitylene (1.99 g, 10 mmol) and t-BuLi (1.57 M
solution in pentane, 12.8 mL, 20 mmol) in dry THF (10 mL) at
ꢀ78 1C was added dropwise at 0 1C and the mixture was
Table 2 Photophysical data for compounds 1–5 in a polymer matrixa
b
Compound
lem/nm
FF
1
2
3
4
5
508
507
507
507
511
0.41
0.48
0.56
0.42
0.42
a
b
1 wt% in PMMA. Absolute quantum yield determined by a
calibrated integrating sphere system.
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allowed to warm to ambient temperature. After stirring for
24 h, H2O2 aq. (30%, 4.5 mL) was added at 0 1C and the
resulting mixture was allowed to warm to ambient temperature
again. After stirring for 18 h, the reaction was quenched by the
addition of Na2SO3 aq. (10%, 50 mL) at 0 1C. After removal
of THF under reduced pressure, Na2SO3 aq. (10%, 150 mL)
was added and the organic layer separated. The aqueous layer
was extracted with CH2Cl2. The combined organic layer was
washed with water and brine, and dried over Na2SO4. After
concentration of the organic layer, the residual crude product
was purified by column chromatography (PSQ100B, 20/1
CH2Cl2/AcOEt, Rf = 0.27) to afford 1.57 g of 2 (3.0 mmol,
30% yield) as yellowish green solids: mp 192–193 1C. 1H NMR
(CDCl3, 400 MHz) d 2.26 (s, 3H), 2.70–2.82 (br, 6H),
6.88–6.90 (m, 6H), 7.04–7.17 (m, 16H). 31P{1H} NMR
(CDCl3, 162 MHz) d 50.60. HRMS calcd for C37H31OP:
522.2113. Found: 522.2115.
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1540 | New J. Chem., 2010, 34, 1537–1540