Synthesis of novel rod-shaped and star-shaped fluorescent phosphane oxides-nonlinear optical properties and photophysical properties

Article abstract of DOI:10.1002/chem.200600464

The design of a new class of fluorophores is presented. Some push-pull chromophores (D-π:-A) containing polyphenylethynyl units and a phosphane oxide moiety were efficiently prepared from common intermediates. Straightforward syntheses gave novel one-arme

Full text of DOI:10.1002/chem.200600464

DOI: 10.1002/chem.200600464
Synthesis of Novel Rod-Shaped and Star-Shaped Fluorescent Phosphane
Oxides—Nonlinear Optical Properties and Photophysical Properties
Minh-Huong Ha-Thi,[a] Vincent Souchon,[a] Abdelwaheb Hamdi,[a] RØmi MØtivier,[a]
ValØrie Alain,[a] Keitaro Nakatani,[a] Pascal G. Lacroix,[b] Jean-Pierre GenÞt,[c]
VØronique Michelet,*[c] and Isabelle Leray*[a]
Abstract: The design of a new class of
fluorophores is presented. Some push–
pull chromophores (D–p–A) contain-
star-shaped derivatives were evaluated,
showed high fluorescence quantum
yields, and their excitation induces very
efficient charge redistribution. More-
over, thanks to their push–pull charac-
ter, the molecules exhibited significant
second-order NLO properties with
good transparency, up to 67·10ꢀ30 esu at
1907 nm, with an absorption lmax at
369 nm. The effect of the donor group
and of the number of phenylethynyl
arms have been studied in this work.
ing polyphenylethynyl units and
a
phosphane oxide moiety were efficient-
ly prepared from common intermedi-
ates. Straightforward syntheses gave
novel one-armed, rod-shaped and
three-armed, star-shaped fluorophores.
The optical properties of the resulting
Keywords: charge transfer · fluo-
rescence spectroscopy · nonlinear
optics · phosphane oxide · solvato-
chromism
Introduction
For the development of a new fluorescent molecular
sensor consisting of a recognition moiety linked to a fluores-
cent moiety, the choice of the fluorophore is of major impor-
tance. This component should convert the recognition event
into an optical signal as the result of a change in its photo-
physical characteristics caused by the perturbation of vari-
ous photoinduced processes (electron transfer, energy trans-
fer, charge transfer) by the bound species.[3] A particular ad-
vantage of fluoroionophores based on cation control of pho-
toinduced charge transfer is that the absorption and fluores-
cence spectra are shifted upon cation binding[4] so
ratiometric measurements are possible: the ratio of the fluo-
rescence intensities at two appropriate emission or excita-
tion wavelengths provides a measure of the cation concen-
tration independent of the probe concentration and insensi-
tive to incident light intensity, scattering, inner filter effects,
and photobleaching. To ensure better sensitivity, different
photophysical properties of the fluorophore—such as high
molar extinction coefficient, high fluorescence quantum
yield, and good photostability—must be considered, which
has prompted us to design new charge-transfer molecules.
Among the existing fluorophores, poly(phenylene)ethynyl-
Over the past few years significant research has been direct-
ed toward the development of organic materials for poten-
tial application in molecular photonic devices[1] and the de-
velopment of sensors.[2] Interest in these materials is primar-
ily due to the infinite numbers of possible molecular struc-
tures with the desired properties, by virtue of the tremen-
dous capabilities of organic synthesis.
[a] M.-H. Ha-Thi, V. Souchon, Dr. A. Hamdi, Dr. R. MØtivier,
Dr. V. Alain, Prof. K. Nakatani, Dr. I. Leray
CNRS UMR 8531 Laboratoire de Photophysique et Photochimie Su-
pramolØculaires et MacromolØculaires
DØpartement de Chimie, ENS-Cachan
61 avenue du PrØsident Wilson, 94235 Cachan Cedex (France)
Fax : (+33)147-402-454
E-mail: icmleray@ppsm.ens-cachan.fr
[b] Dr. P. G. Lacroix
Laboratoire de Chimie de Coordination du CNRS
205 route de Narbonne, 31077 Toulouse (France)
[c] Prof. J.-P. GenÞt, Dr. V. Michelet
Laboratoire de Synth›se SØlective Organique et Produits Naturels
E.N.S.C.P., UMR7573, 11 rue P. et M. Curie
75231 Paris Cedex 05 (France)
AHCTREeUNG nes and other arylethynyl fluorophores are very attractive
because of their high photostability, their electron-transport
abilities, and their intense fluorescence emissions.[5] A series
of push–pull chromophores (D–p–A), each containing a poly-
phenylethynyl unit as a conjugated bridge, had already been
Fax : (+33)144-071-062
E-mail: veronique-michelet@enscp.fr
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
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Chem. Eur. J. 2006, 12, 9056 – 9065

FULL PAPER
studied.[6] In these systems, photoexcitation induces substan-
tial charge separation, as evidenced by the large solvato-
chromic shifts observed in their emission spectra, and it is
possible to take advantage of the push–pull characters of
these systems for nonlinear applications.[7]
In the course of our recent ongoing program directed to-
wards the production of novel fluorophores, we have recent-
ly described the synthesis of two novel molecules 1a and 2a
(Scheme 1), each bearing a phosphane oxide as an acceptor
group and a methoxy group or groups as donor moieties.[8]
These systems exhibit high fluorescence quantum yields, and
their excitation induces very efficient charge redistribution
in the molecules. In anticipation that the donor group might
influence either the fluorescence or the nonlinear properties,
an easy route to analogues of 1a and 2a was desired. More-
over, the influence of substitution of the star-shaped phos-
phane oxides may be evaluated through the preparation of
two families bearing mono- or trisubstituted fluorescent
arms. There being no precedent for and no data relating to
such derivatives, we therefore wish to describe here our full
study involving a series of fluorescent probes, including their
synthesis, their photophysical and nonlinear optical proper-
ties, and molecular orbital calculations.
Scheme 2. Retrosynthesis for phosphane oxide fluorescent probes.
(Scheme 3). Phosphane oxide 1a was prepared by the previ-
ously described procedures,[8] whilst the easy route to phos-
phane oxide 1b was based on the synthesis of iodide 6, start-
ing from commercially available products 7 and 8. The So-
nogashira coupling was optimized in the presence of
9 mol% of [Pd
oxide in 70% isolated yield. The known 4-iodo-N,N-di
AHCTREUNG
ylaniline (9)[10] could also be treated with triarylphosphane
ACHTREUNG
ACHTREUNG
1d to provide the desired product 1c efficiently in 74%
yield. We therefore had a route to three star-shaped triaryl-
phosphane oxides in good overall yields and in a straightfor-
ward synthesis.
Aiming to examine the influence of mono- and trisubstit-
AHCTREuUNG ted fluorescent arms, we also envisaged the preparation of
Results and Discussion
the monosubstituted arylphosphane oxides 2a and 2b
(Scheme 4). Phosphorylation with the Grignard reagent de-
rived from the commercially available 1-(4-bromophenyl)-2-
(trimethylsilyl)acetylene (4) in the presence of chlorodiphe-
nylphosphane was followed by phosphorus oxidation and
desilylation reaction, this high-yielding, three-step process
giving the desired adduct 2d in 47% overall yield. We,[8]
and others,[11] have previously reported alternative strategies
starting from chlorodiphenylphosphane and diphenylphos-
phane, respectively, but these both afforded the diphenylar-
Synthesis: We turned our attention to the preparation of the
dimethylamino derivatives 1b and 2b and the triaryl deriva-
tive 1c (Scheme 1), as the NMe2 group is known to be a
stronger donor than the methoxy group. An anthracenyl
adduct 3 was also targeted, this being particularly attractive
from the point of view of its ability to absorb in the visible
region.[9] We envisaged an easy route to such derivatives
starting from the triarylphosphane oxides 1d[8] or 2d, which
could be prepared from commercially available 4-bromo-
phenylacetylene (4) (Scheme 2).
AHCTREyUNG lethynylphosphane in lower isolated yields. The Sonoga-
shira couplings of the phosphane oxide 2d were carried out
either with iodo derivatives 5 and 6 or with the known bro-
monaphthyl 10,[9] the corresponding substituted phosphane
The triarylphosphane oxides 1a–c were synthesized from
1d[8]
through
Pd-catalyzed
Sonogashira
couplings
Scheme 1. Phosphane oxide fluorescent probes.
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I. Leray, V. Michelet et al.
are given in Figure 1a and b,
respectively. Time-resolved flu-
orescence measurements were
performed by the single-
photon counting method with
picosecond laser excitation;
the fluorescence decays for the
different phosphane oxides are
shown in Figure 2. Satisfactory
fits can be obtained by consid-
ering
a
single exponential
(cR2 <1.25). The radiative and
nonradiative rate constants are
related to the corresponding
emission quantum yield and
lifetime by kr = F/t and knr
(1ꢀF)/t (Table 1).
=
The fluorophores containing
phenylethynyl bridges (1 and
2) each show good transparen-
cy in the visible region and an
intense absorption band in the
near UV/blue visible range,
which can be attributed to an
intramolecular charge transfer
(ICT) upon S0!S1 excita-
tion.[11] The tolane skeleton
presents the advantage of
avoiding the chemically and
photochemically readily in-
duced cis/trans isomerizations
that can occur in the corre-
sponding stilbenes, whilst the
acetylenic triple bonds also
induce hypsochromic shifts rel-
ative to the corresponding
Scheme 3. Synthesis of triarylphosphane oxides.
chromophores
containing
double bonds, producing an
enhancement of the transpar-
Scheme 4.
ency range, a highly desirable
oxides 2a, 2b, and 3 being isolated in moderate to good
yields (50–83%).
property when NLO properties are considered. The absorp-
tion maxima depend both on the natures of the peripheral
Having prepared the one-
armed, rod-shaped and three-
armed, star-shaped fluorescent
probes, we next turned our at-
tention to the photophysical
properties of these novel deriv-
atives.
Table 1. Photophysical properties of the phosphane oxide derivatives in chloroform. Maxima of one-photon
absorption labs [nm] and of steady-state emission lem [nm], molar absorption coefficient e [104 mꢀ1 cmꢀ1], fluo-
rescence quantum yield FF, fluorescence lifetime t [ns], radiative kr [108 sꢀ1] and nonradiative rate knr [108 sꢀ1
]
constants, together with computed optical data (absorption maxima lmax [nm], and oscillator strength f).
[b]
Product labs [nm] e [104 mꢀ1 cmꢀ1
]
f
lem [nm] FF
t [ns] kr [108 sꢀ1
]
knr [108 sꢀ1 lmax[d][nm] f[c,d]
]
1a
1b
1c
2a
2b
2c
338
369
367
335
369
481
17.7
12.5
6.5
5.9
4.1
4.06 392
2.29 478
1.49 428
1.42 388
0.86 471
0.61 573
0.77 0.74
10.41
3.68
3.09
9.74
4.34
2.78
3.11
2.89
2.23
3.08
2.24
0.78
315
317
294
312
319
418
6.1
6.1
4.1
2.4
2.4
0.56 1.52
0.58 1.88
0.76 0.78
0.66 1.52
0.78 2.81
Photophysical properties: The
photophysical properties of the
related phosphane oxide deriva-
tives are given in Table 1, whilst
the absorption and emission
spectra of monosubstituted and
3.1
1.43
[a] The detailed HOMO–LUMO transitions and the various compositions of CI expansion are reported in the
Supporting Information. [b] Fluorescence quantum yield with a 10% random error. [c] Computed optical data.
star-shaped phosphane oxides [d] Overestimated value.
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Fluorescent Phosphane Oxides
FULL PAPER
NACHTRE(UNG CH3)2). Furthermore, it was shown that red shifting in the
absorption band is observed with increasing conjugation
length (1c compared with 1b) due to an extension of the p-
conjugation. In the case of the anthracenyl-based fluoro-
phore, the absorption is shifted by about 100 nm, an effect
interpreted in terms of better p-orbital delocalization due to
the presence of the anthracenyl bridge. All of these mole-
cules are strongly fluorescent in chloroform, with fluores-
cence quantum yields ranging from 0.55 to 0.78. The value
of the quantum yield and the radiative rate constant are
similar to those reported for the strongly emitting fluoro-
phores based on rod-shaped oligo(p-phenyleneethyny-
lenes).[12] As also observed in the absorption, bathochromic
shifting of the emission spectra in chloroform is observed
with increasing donating group power (lem for 1a = 388
and lem for 1b = 471 nm). This effect is more pronounced
for the emission than for the absorption, suggesting an en-
hancement of the dipole moment in the excited state. The
shape, molar absorption coefficient (normalized to the same
number of fluorophores), fluorescence quantum yield, and
fluorescence lifetime are similar for compounds 1a and 2a,
indicating weakinteraction between the fluorophores; the
same tendency has been observed for compounds 1b and
2b. Thus, from these experimental data, it can be considered
that the emitting states in the star-shaped fluorophores 1a
and 1b are located on single branches of the fluorophores.
Similar effects have previously been observed in the case of
multibranching dipolar chromophores.[13] The emission effi-
ciencies (FF, kr) correlate with the electron-donating abili-
ties of the donor groups and the p-extensions, the kr values
being found to be about three times higher for 1a than for
1b and for 2a than for 2b.
Figure 1. a) Absorption of phosphane oxide derivatives in chloroform
(left ordinate: rod-shaped phosphane oxides 2a, 2b, and 3; right ordi-
nate: star-shaped phosphane oxides 1a and 1b). b) Corrected normalized
emission spectra of phosphane oxide derivative in chloroform.
These experimental features can be interpreted further by
use of semiempirical procedures at the intermediate neglect
of differential overlap (INDO) level; the experimentally ob-
tained data are compared to the semiempirical (INDO)
spectra in Table 1. Both experimentally measured and calcu-
lated spectra are dominated by intense and low-lying bands,
with a tendency towards slight blue shifting (about 50 nm)
on passing from experimentally measured to calculated
values. Calculation indicates that the origins of these bands
seem to be slightly different in the one-armed (2a, 2b) and
three-armed molecules (1a, 1b). While single, HOMO–
LUMO-based (1!2) electron transitions contribute to the
intense bands in the one-armed chromophores, sets of two
(1!2 and 1!3) transitions mixing excitations between six
orbitals are involved in the description of the bands in
three-armed systems (see Supporting Information for details
on electron transition bands). This may be explained by the
fact that C3 symmetry axes were postulated for the three-
armed molecules. The six orbitals involved in the charge-
transfer transitions of the three-armed derivatives are there-
fore reminiscent of the HOMOs and LUMOs of the one-
armed parent molecules, and finally the overall physical
properties are equivalent in any case. Thus, apart from these
differences between experimentally measured and calculat-
ed data, there is satisfactory correlation between the theory-
Figure 2. Fluorescence decays of phosphane oxide derivatives in chloro-
form.
substituents and on the lengths of the conjugated arms. As
would be expected, bathochromic shifts are observed with
increasing electron density of the donating groups (OCH3 <
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9059

I. Leray, V. Michelet et al.
derived values and the data re-
lating to UV/Vis absorption
maxima and oscillator strength
of the transition (Table 1). The
observation that the presence
of a better electron donor (di-
methylamino) lowers the ener-
gies of the transitions strongly
supports a push–pull character
in these systems, and hence the
potential for sizeable quadratic
molecular hyperpolarizabilities
(b) in these molecules, as dis-
cussed in the next section.
HOMO and LUMO orbitals
for 2a and 2b are shown in
Figure 3a and b. In each case a
charge transfer from the R-
ꢁ
ꢁ
C6H4-C C- and the -C C-
C6H4P(O)(C6H5)2 fragments is
ACHTREUNG
evident, with a tendency to-
wards stronger effects in the
case of the Me2N-containing
molecule, consistently with the
more strongly electron-donat-
ing character of the amine.
Solvatochromism effects of
the new fluorophores (1b, 1c,
2b, and 3) were studied and
compared with the previously
observed phosphane oxide
data[8] (Table 2). In contrast
with the small bathochromic
shifts observed in the absorp-
tion spectra, an important red
shifting of the emission spectra
was observed (Figure 4 for
2b). The Stokes shift for 2b in
CH3CN, for example, is
10521 cmꢀ1, indicating that the
dipole moment of the phos-
phane oxide is much larger in
the excited state than in the
ground state.
According to the Lippert–
Mataga equation,[14] the Stokes
shift can be related to the dif-
ference in dipole moment be-
tween the ground and the ex-
cited states:
2
hca3
2
naꢀnf
¼
ðmeꢀmgÞ Df
ð1Þ
þconstant
Figure 3. HOMOs (bottom) and LUMOs (middle) for 2a (a) and 2b (b) with the associated charge transfer
(top). The white (black) contributions in the charge-transfer drawing correspond to increases (decreases) in
the electron density upon excitation.
where na and nf are the fre-
quencies of the absorption and
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Fluorescent Phosphane Oxides
FULL PAPER
[a]
Table 2. Solvatochromism of 1a, 1b, 1c, 2a, 2b, and 3. labs [nm], lem [nm], and FF as the function of the ori-
entation polarizability Df.
er the slope and the higher the
enhancement of the dipole
moment in the excited state.
Interestingly the slope is the
same for compounds contain-
ing the same donor (compari-
son between 1a and 2a and be-
tween 1b and 2b). This can
again be explained by the
emitting excited states having
the same nature in the case of
the star-shaped molecule and
in the case of the rod-shaped
molecule. It should also be
noted that the slopes for 1b
and 1c are the same. In the
case of 3 the slope is smaller,
which can be explained in
terms of a minor delocalization
of the charge in the excited
state due to the presence of
the anthracenyl moiety.
Solvent
cyclohexane
ꢀ0.001
dioxane
0.021
CHCl3
0.149
CH2Cl2
0.219
DMSO
0.265
EtOH
0.290
CH3CN
0.306
Product
Df
1a[b]
labs [nm]
lem [nm]
FF
335
366
0.78
364
336
379
0.89
369
464
0.56
359
418
0.43
333
374
0.97
367
459
0.73
480
567
0.60
338
392
0.77
370
478
0.57
367
428
0.58
335
388
0.76
369
471
0.66
481
570
0.78
338
406
0.73
373
518
0.63
364
463
0.44
334
402
0.78
370
511
0.69
489
603
0.51
338
443
336
418
334
428
0.71
0.86
0.79
[c]
[c]
[c]
1b
1c
2a[b]
2b
3
labs [nm]
lem [nm]
FF
[c]
[c]
[c]
[c]
[c]
[c]
409
[b]
labs [nm]
lem [nm]
FF
364
374
0.46
333
364
0.77
363
399
371
529
0.06
336
434
0.75
377
615
0.04
501
690
0.03
366
502
0.07
333
412
0.81
367
579
0.10
479
633
0.48
362
516
0.04
331
422
0.94
367
600
0.07
485
665
0.08
labs [nm]
lem [nm]
FF
labs [nm]
lem [nm]
FF
0.73
[b]
labs [nm]
lem [nm]
FF
[b]
[b]
[a] Fluorescence quantum yield with a 10% random error. [b] Data previously reported in ref. [8] [c] Not solu-
ble.
*
*
Figure 5. Lippert–Mataga correlations of fluorophores 1a ( ), 2a ( ), 1b
Figure 4. Corrected emission spectra of 2b in different solvents. a) Cyclo-
hexane. b) Dioxane. c) CHCl3. d) CH2Cl2. e) EtOH. f) CH3CN. g) DMSO.
~
&
(”), 2b (&), 2c ( ), and 3 ( ).
To determine the enhancements of the dipolar moments
in the excited states, one key parameter was the determina-
tion of the Onsager cavities. In the case of an elongated
molecule, overestimation of the radius cavity can often
occur through the use of the distance between donor and ac-
ceptor, which gives rise to uncertainty in the determination
of the enhancement of the dipole moment in the molecule,
so the use of an ellipsoidal cavity model is more appropri-
ate.[15] The Dm values for 2a and 2b were also evaluated by
INDO[16] and were found to be 10 D and 17 D, respectively:
lower than, but in a satisfactory agreement with, the experi-
mentally measured values (Table 3).
fluorescence maxima, respectively, h is Planckꢁs constant, c
is the velocity of light, a is the radius of the cavity in which
the solutes resides, and Df is the orientation polarizability,
defined as:
eꢀ1
n2ꢀ1
ð2Þ
Df ¼
ꢀ
2e þ 1 2n2 þ 1
where e is the static dielectric constant of the solvent and n
is the optical refractive index of the solvent.
As shown in Figure 5, plots of the orientation polarizabili-
ties (Df) against the Stokes shifts in various solvents are
linear for the different compounds. The slopes depend on
the natures of the donors: the stronger the donor, the steep-
The solvent effects on fluorescence efficiency were then
studied for compounds 2a and 2b. Table 4 displays the ob-
tained fluorescence quantum yields, fluorescence lifetimes,
and the corresponding radiative and nonradiative rate con-
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9061

I. Leray, V. Michelet et al.
Table 3. Experimentally measured dipole moments and hyperpolarizabil-
the number of arms resulted in a slight increase in b, which
will be discussed further.
ities (b in 10ꢀ30 esu) at 1907 nm for 1a, 1b, 1c, 2a, 2b, and 3.
Product
m [D]
Dm [D]
b [10ꢀ30 esu]
b0 [10ꢀ30 esu]
1a
1b
1c
2a
2b
3
5.1
7.1
6
4.2
4.8
15
20
15
15
22
12
40
67
46
24
51
32
54
NLO response in chromophores 2a and 2b: According to
the simplified, but widely used “two-level” description of
the NLO response, the hyperpolarizabilities of “push–pull”
one-dimensional molecules (e.g., 2a and 2b) have their
origin in intense low-lying transitions of energy E and oscil-
lator strength f, involving a charge-transfer character be-
tween a ground (g) and an excited (e) state (dipole moment
change Dm = meꢀmg), according to the following relation-
ship:[19,20]
20
43
stants. Whilst the fluorescence quantum yield of 2a did not
change very much with increasing solvent polarity, a signifi-
cant decrease in the fluorescence quantum yield with in-
creasing solvent polarity was observed in the case of 2b, and
this result was accompanied by an enhancement of the non-
radiative rate constant. Such an effect can be interpreted in
terms of specific interaction between the amino nitrogen
group of 2b and the solvent in the excited state, as previous-
ly observed with similar compounds.[17]
3e2ꢀhfDm
2mE3
E4
ð3Þ
b ¼
Â
2
2
ðE2ꢀð2ꢀhwÞ ÞðE2ꢀðꢀhwÞ Þ
in which ꢀhw is the energy of the incident laser beam. In the
cases of compounds 2a and 2b the lowest energy transitions
(1!2) are very intense (large f
Table 4. Photophysical properties of the phosphane oxides 2a and 2b in different solvents. Fluorescence quan-
tum yields FF, fluorescence lifetimes t [ns], radiative kr and nonradiative knr rate constants [108 sꢀ1].
values) and can be assumed to
be dominant in the description
of the NLO properties. Fur-
thermore, the data gathered in
the Supporting Information in-
dicate that each transition is
based on the corresponding
single HOMO!LUMO exci-
tation, contributing at levels of
89 and 83% of the effect in 2a
and 2b, respectively. There-
2a
2b
Solvent
FF
t [ns]
kr [108 sꢀ1
]
knr [108 sꢀ1
]
FF
t [ns]
kr [108 sꢀ1
]
knr [108 sꢀ1
]
cyclohexane
dioxane
CHCl3
CH2Cl2
DMSO
EtOH
0.77
0.92
0.76
0.78
0.75
0.81
0.89
0.63
0.73
0.78
0.87
1.17
1.03
1.13
12.22
12.60
9.74
8.97
6.41
7.86
7.88
3.65
1.10
3.08
2.53
2.14
1.84
0.97
0.76
0.73
0.66
0.69
0.06
0.18
0.11
0.87
1.43
1.52
2.12
0.42
0.76
0.61
8.74
5.10
4.34
3.25
1.43
2.37
1.80
2.76
1.89
2.24
1.46
22.38
10.79
14.59
CH3CN
Nonlinearoptical por petries : Measurements of the molecu-
lar hyperpolarizabilities of the phosphane oxides were
made, in order to investigate and thus illustrate potential
optical applications.
fore, it can readily be assumed that the description of the
charge transfer associated with the HOMO!LUMO excita-
tion provides the rationale for understanding of the micro-
scopic origin of the NLO response in these “push–pull”
chromophores.
Since EFISH gives the scalar product (mb) of the dipole
moment by the hyperpolarizability, it was then possible to
obtain the projection of b on the dipole moment direction
(noted b for sake of simplicity) from independent measure-
ment of m. In both the one-armed and the three-armed
series we had observed an increase in b through the replace-
ment of methoxy groups by dimethylamino moieties
(Table 3). As would be expected from the better electron-
donating capacity of the latter group, the same tendency
was observed for the m value. Although compound 1c has
weaker b and m values than its analogue 1b, arising from a
shorter conjugated chain between the push and pull moiet-
ies, it competes well with other push–pull diphenylacety-
lenes reported in the literature (e.g., 46”10ꢀ30 cm5 esuꢀ1 for
4,4’-dimethylaminonitrodiphenylacetylene[7b]), but is signifi-
cantly smaller than that obtained for 1,3,5-triazine substitut-
ed with a dimethylaminophenylethynyl group.[18] Such an
effect can be explained by the fact that the 1,3,5-triazine
group is a better withdrawing group than the nitro or the
phosphane oxide group. To the best of our knowledge, only
powder tests of molecules possessing three phenyl rings and
two acetylene functions have been reported.[7a] Multiplying
Experimentally measured and computed values also sup-
port the idea that the charge transfer (and hence the hyper-
polarizability) is enhanced when the strength of the donat-
ing substituent is stronger. Within the approximation of the
two-level description [Eq. (3)], the current set of Dm, f, and
E parameters gives computational b values of 43 and 84”
10ꢀ30 cm5 esuꢀ1 for 2a and 2b, respectively. These values are
somewhat larger than the experimentally ascertained 24 and
51”10ꢀ30 cm5 esuꢀ1 values, but it is well known that the two-
level description frequently gives an overestimation of mo-
lecular hyperpolarizabilities.[21]
NLO response in three-armed 1a and 1b and comparison
with the one-armed analogues: Because of the presence of
the phosphane oxide moieties the three arms are not copla-
nar, resulting in a dipolar (not octupolar) geometry. If a
one-dimensional model along the C3 symmetry axis is con-
sidered, the projection of the dipole moment change (Dm)
associated with the two degenerate (1!2 and 1!3) transi-
tions should be viewed as the resulting contribution to the
NLO response. This approach gave hyperpolarizabilities
9062
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Chem. Eur. J. 2006, 12, 9056 – 9065

Fluorescent Phosphane Oxides
FULL PAPER
equal to 15 and 22”10ꢀ30 cm5 esuꢀ1 for 1a and 1b, respec-
tively. As would be expected, these values are significantly
lower than the experimentally measured (42 and 67”
10ꢀ30 cm5 esuꢀ1) data because they arise from an oversimpli-
fied picture in which b = bzzz and do not take into account
the contribution of the bzxx and bzyy tensor components,
which will probably be important in the current geometry.
Unlike in the situation encountered in the simple “push–
pull” 2a and 2b chromophores, this analysis therefore sug-
gests that a model based on simple charge-transfer processes
may not be fully reliable in these systems.
ting state in each star-shaped derivative is located on a
single branch. Because of their strong push–pull characters,
the chromophores exhibit sizeable NLO responses in solu-
tion, with a tendency towards larger hyperpolarizabilities in
the three-armed series.
Experimental Section
General procedure: Reagents were commercially available from Acros,
Aldrich, or Avocado and were used without further purification unless
otherwise stated. Cyclohexane, dioxane, chloroform, dichloromethane, di-
methylsulfoxide, and acetonitrile (Aldrich, spectrometric grade or SDS,
spectrometric grade) were employed as solvents for absorption and fluo-
rescence measurements. Column chromatography was performed with
E. Merck0.040–0.063 mm Art. 11567 silica gel. 1H NMR, 13C NMR, and
31P NMR were recorded on Bruker AV 300 or AV 400 instruments. All
signals were expressed as ppm downfield from Me4Si for 1H and
13C NMR and from H3PO4 for 31P NMR used as an internal standard (d).
Coupling constants (J) are reported in Hz and refer to apparent peak
multiplicities. Melting points were measured in open capillary tubes.
Mass spectrometry analyses were performed at the Ecole Nationale Su-
pØrieure de Chimie de Paris by using a Hewlett–Packard HP 5989 A in-
strument. Direct introduction experiments were performed by chemical
ionization with ammonia. Elemental analyses were performed at the In-
stitut de Chimie des Substances Naturelles and high-resolution mass
spectra were done at the University of Paris XI (Orsay).
The ratios between the m and b values of the three-armed
star-shaped molecules and their one-armed analogues range
between 1.2 and 1.7. Theoretically, the relationship between
the two types of molecules should be:[22]
m3 ¼ 3m1cos q
b3 ¼ 3b1cos q
ð4Þ
ð5Þ
where the indices 1 and 3 refer to the one- and three-armed
molecules, respectively, and q to the angle between each
arm and the C3 axis. The terms m3 and b3 are along the C3
axis, whereas m1 and b1 are along the moleculeꢁs main axis,
and are the components involved in EFISH data.
Diphenyl-[4-trimethylsilylethynyl)phenyl]phosphane oxide: Hydrogen
peroxide (30% solution, 1.7 mL) was slowly added to a solution of di-
phenyl-[4-trimethylsilylethynyl)phenyl]phosphane (2 g, 5.58 mmol) in a
mixture of dichloromethane (50 mL) and methanol (50 mL). The result-
ing mixture was stirred at room temperature for 2 h and was then
quenched with aqueous Na2SO3 solution and extracted with dichlorome-
thane. The combined organic phases were dried (MgSO4), filtered, and
concentrated under reduced pressure to give a white solid (2 g, 96%).
M.p. 1398C; 1H NMR (300 MHz, CHCl3): d = 7.65–7.46 (m, 14H; Har),
0.25 ppm (s, 9H; CH3); 13C NMR (75 MHz, CHCl3): 132.7 (C-P), 132.4
(2”C-P), 132.2 (2”CHar), 132.2 (4”CHar), 132.0 (2”CHar), 131.9 (2”
CHar), 128.7 (4”CHar), 127 (C), 104.0, 97.6 (2”C, C=C), 0.0 ppm (3”
CH3); 31P NMR (121 MHz, CDCl3): d = 30.0 ppm; MS (CI, NH3): m/z:
375 [M+H]+; ES HRMS: m/z: calcd for C46H46O2P2Na: 771.26; found:
771.24 [2M+Na]+.
In the calculated structures of the three-armed chromo-
phores, each dipolar arm makes an angle of q = 668 with
the C3 axis, so the above ratio would be 1.22. The experi-
mentally obtained results are therefore consistent with the
theory within the experimental errors of the EFISH meas-
urements.
Conclusion
We have synthesized a series of new phenylethynyl phos-
phane oxides bearing different donor groups. To study vari-
ous degrees of donor and acceptor strength, the influence of
the substitution and the nature of the p-conjugation element
were investigated through the preparation of six analogues.
The geometries of these fluorophores were optimized, re-
sulting in one-armed, rod-shaped and three-armed, star-
shaped derivatives. The absorption and emission spectra re-
vealed that the electronic properties of these fluorophores
were strongly affected by the nature of the donor, the
length, and the nature of the p-conjugation. The internal
charge-transfer characters of the transitions were investigat-
ed by solvatochromism measurements and it was found that
highly efficient charge redistribution occurred upon excita-
tion both for the one-armed, rod-shaped fluorophores and
for their three-armed, star-shaped counterparts. Estimations
of the enhancements of the dipole moments in the excited
states were performed, showing that enhancement is higher
in cases of compounds bearing the more strongly electron-
donating dimethylamino group. The photophysical proper-
ties of the star-shaped fluorophores and rod-shaped fluoro-
phores were found to be the same, indicating that the emit-
Diphenyl-(4-ethynylphenyl)phosphane oxide (2d): Potassium carbonate
(256 mg, 1.8 mmol) was added to a solution of diphenyl-[4-(trimethylsilyl-
AHCTREeUNG thynyl)phenyl]phosphane oxide (2 g, 5.34 mmol) in a mixture of di-
chloromethane (40 mL) and methanol (60 mL). The resulting mixture
was stirred at room temperature for 2 h, quenched with water, and then
extracted with dichloromethane. The combined organic layers were dried
(MgSO4), filtered, and concentrated under reduced pressure to give a
white solid (1.4 g, 85%). M.p. 1448C; 1H NMR (300 MHz, CHCl3): d =
7.69–7.53 (m, 10H; Har), 7.50–7.43 (m, 4H; Har), 3.20 ppm (s, 1H; HC
=
CH); 13C NMR (75 MHz, CHCl3): d = 133.1 (C-P), 132.1 (2”C-P), 132.1
(4”CHar), 132.0 (2”CHar), 131.9 (2”CHar), 128.6 (4”CHar), 126.9 (2”
CHar), 125.9 (C), 82.6, 79.9 ppm (2”C, C=C); 31P NMR (121 MHz,
CDCl3): d = 28.7 ppm; MS (CI, NH3): C20H15OP: m/z: 303 [M+H]+; ES
HRMS: m/z: calcd for C20H15OPNa: 325.0753; found: 325.0765 [M+Na]+
.
[4-(10-Bromoanthracen-9-ylethynyl)-phenyl]-dimethyl-amine (10): CuI
(26.3 mg, 0.14 mmol) and Pd
a solution of 9,10-dibromoanthracene (2.3 g, 6.89 mmol) and 4-ethynyl-
N,N-dimethylbenzylamine (1 g, 6.89 mmol) in mixture of toluene
ACHTRE(UNG PPh3)4 (159 mg, 0.14 mmol) were added to
a
(70 mL) and triethylamine (20 mL). The resulting mixture was stirred for
24 h at 508C, and was then allowed to cool to room temperature and con-
centrated under vacuum. The residue was purified by silica gel chroma-
tography (dichloromethane/cyclohexane 90:10) to give an orange solid
(1.3 g, 50%). M.p. 2368C; 1H NMR (300 MHz, CHCl3): d = 8.73–8.70
Chem. Eur. J. 2006, 12, 9056 – 9065
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www.chemeurj.org
9063

I. Leray, V. Michelet et al.
(m, 2H; H1ar), 8.57–8.54 (m, 2H; H2ar), 7.66–7.57 (m, 6H; H2ar, H3ar, H5ar),
6.76 (d, 9.2 Hz, 2H;
6ar), 3.05 ppm (s, 6H; CH3); 13C NMR
(75 MHz, CHCl3): d = 150.6 (C), 133.0 (2”CHar), 132.8 (2”C), 130.5
(2”C), 128.2 (2”CHar), 127.7 (2”CHar), 127.5 (2”CHar), 126.5 (2”
CHar), 122.9 (C), 119.7 (C), 112.1 (2”CHar), 110.2 (C), 103.8 (C), 84.4
(C), 40.4 ppm (CH3); MS (CI, NH3): C24H18BrN: m/z: 402 [M+H]+.
Theoretical methods: The all-valence INDO (intermediate neglect of dif-
ferential overlap) method[23] was employed for the calculation of the elec-
tronic spectra of the six molecules. The monoexcited configuration inter-
action (MECI) approximation was employed to describe the excited
states. The 100 lowest-energy one-electron transitions between the 10
highest occupied molecular orbitals and the 10 lowest unoccupied ones
were chosen to undergo CI mixing. Calculations were performed with the
aid of the commercially available MSI software package ZINDO.[24] In
the absence of molecular structures, metrical parameters used for the
INDO calculations were obtained from gas-phase geometry optimization
performed at the PM3 level (MOPAC) in the Gaussian98 package.[25] The
starting fragments used to build the molecules were taken from previous-
ly reported related crystal structures.[26] Molecules 1a, 1b, and 1c were
found to possess roughly C3 symmetry axes, within ranges of uncertainty
of 0.20, 0.35, and 0.15 Š, respectively, so strict C3 symmetries were im-
posed for these three molecules in the final optimization processes. The
calculated structures have been included as Supporting Information.
J
=
H
Diphenyl-{4-[10-(4-dimethylaminophenylethynyl)anthracen-9-ylethynyl]-
phenyl}phosphane oxide (3): CuI (1.91 mg, 0.01 mmol) and PdACHTREU(NG PPh3)4
(12 mg, 0.01 mmol) were added to a solution of 10 (145 mg, 0.33 mmol)
and the phosphane oxide 2d (100 mg, 0.33 mmol) in a mixture of toluene
(15 mL) and triethylamine (4 mL). The resulting mixture was stirred at
room temperature for 24 h at 508C, and was then allowed to cool to
room temperature and concentrated under vacuum. The residue was pu-
rified by silica gel chromatography (dichloromethane/acetone 95:5) to
give an orange solid (70 mg, 30%). M.p. > 3008C; 1H NMR (300 MHz,
CHCl3): d = 8.73–8.61 (m, 4H; H1ar), 7.84–7.50 (m, 6H; Har), 6.76 (d, J
= 8.8 Hz, 2H; Har), 3.06 ppm (s, 6H; CH3); 13C NMR (75 MHz, CHCl3):
d = 150.7 (C-N), 133.1 (2”CHar), 132.6 (C), 132.5 (2”CHar), 132.4 (2”
C), 132.3 (4”CHar), 132.2 (2”C), 131.9 (2”C), 131.6 (2”CHar), 128.8
(4”CHar), 127.8 (2”CHar), 127.5 (C), 127.2 (2”CHar), 127.0 (2”CHar),
126.6 (2”CHar), 120.8 (C), 116.3 (C), 112.1 (2”CHar), 110.1 (C), 105.1,
Spectroscopic measurements: UV/Vis absorption spectra were recorded
on a Varian Cary5E spectrophotometer and corrected emission spectra
were obtained on a Jobin–Yvon Spex Fluorolog 1681 spectrofluorimeter.
The fluorescence quantum yields were determined by using quinine sul-
fate dihydrate in sulfuric acid (0.5n; FF = 0.546[27]) or coumarin C153 in
ethanol (FF = 0.38[28]) as standards. For the emission measurements, the
absorbances at the excitation wavelengths were below 0.1 and so the con-
102.0, 89.7, 85.0 (4”C, C C), 40.4 ppm (CH3); 31P NMR (121 MHz,
ꢁ
CDCl3): d = 28.9 ppm; MS (CI, NH3): C44H32NOP: m/z: 622 [M+H]+.
Diphenyl-(4-{4-[(4-dimethylaminophenyl)ethynyl]phenylethynyl}-phe-
nyl)phosphane oxide (2b): Compound 2b was obtained by the same ex-
perimental procedure as used to prepare 3, starting from a solution of [4-
(4-iodophenylethynyl)phenyl]-dimethylamine (6, 527 mg, 1.51 mmol) and
the phosphane oxide 2d (100 mg, 0.33 mmol) in a mixture of toluene
(18 mL) and triethylamine (4 mL) with CuI (14 mg, 0.07 mmol) and [Pd-
centrations were below 10ꢀ5 molLꢀ1
.
The oscillator strength of a transition is given by the following equation:
½29ꢂ
f
¼ 4:315 Â 10ꢀ9eDE
ACHTREUNG(PPh3)4] (39 mg, 0.03 mmol), to afford a yellow solid (316 mg, 67%). M.p.
269–2718C; 1H NMR (300 MHz, CHCl3): d = 7.70–7.47 (m, 18H; Har),
7.40 (d, J = 9 Hz, 2H; Har), 6.66 (d, J = 9 Hz, 2H; Har), 2.99 ppm (s,
6H; CH3); 13C NMR (100 MHz, CHCl3): d = 150.4 (C-N), 132.9 (2”Car),
132.4 (C-P), 132.3 (2”C-P), 132.2 (4”CHar), 132.2 (2”CHar), 132.1 (2”
CHar), 131.7 (2”CHar), 131.5 (2”CHar), 131.3 (2”CHar), 128.7 (4”CHar),
Fluorescence intensity decays were obtained by the single-photon timing
method with picosecond laser excitation by use of a Spectra-Physics set-
up composed of a titanium sapphire Tsunami laser pumped by an argon
ion laser, a pulse detector, and doubling (LBO) and tripling (BBO) crys-
tals. Light pulses were selected by optoaccoustic crystals at a repetition
rate of 4 MHz. Fluorescence photons were detected through a long-pass
filter (375 nm) by means of a Hamamatsu MCP R3809U photomultiplier,
connected to a constant-fraction discriminator. The time-to-amplitude
converter was purchased from Tennelec. Data were analyzed by a nonlin-
ear least-squares method with the aid of Globals software (Globals Un-
limited, University of Illinois at Urbana-Champaign, Laboratory of Fluo-
rescence Dynamics).
ꢁ
127.1 (C), 124.8 (C), 121.4 (C), 111.9 (C), 109.6, 93.3, 89.9, 87.3 (4”C, C
C), 40.3 ppm (2”CH3); 31P NMR (121 MHz, CDCl3): d = 30.0 pm; MS
(CI, NH3): C36H28NOP: m/z: 522 [M+H]+; elemental analysis calcd (%)
for C36H28NOP: C 82.90, H 5.41, N 2.69; found: C 82.88, H 5.45, N 2.65.
Tris-(4-{4-[(4-dimethylaminophenyl)ethynyl]phenylethynyl}phenyl)phos-
phane oxide (1b): Compound 1b was prepared by the same experimental
procedure as used to prepare 3, starting from a solution of [4-(4-iodophe-
nylethynyl)phenyl]dimethylamine (6, 600 mg, 1.75 mmol) and tris-(4-
ethynylphenyl)phosphane oxide (1d, 121 mg, 0.35 mmol) in a mixture of
toluene (20 mL) and triethylamine (4 mL) with CuI (16 mg, 0.07 mmol)
EFISHG measurements: Measurements were performed at 1907 nm.
This wavelength was generated by focusing the 1064 nm fundamental
beam of a nanosecond Nd:YAG pulsed laser in a Raman-shifting hydro-
gen cell (40 bar, 1 m long) and used as the fundamental beam for SHG
measurements. The SHG intensity was detected by use of a photomulti-
plier (Hamamatsu) and the signal was read on an oscillator (Tektronic
TDS 620). The centrosymmetry of the solution was broken by application
of a pulsed electric field of 5 kV for 5 ms (Lasermetrics). A solution of
MNA (2-methyl-4-nitroaniline, mb = 71”10ꢀ48 esu at 1907 nm)[30] served
as reference. These measurements were performed for each molecule
with increasing concentrations in chloroform. Detailed set-up and data
analysis method have been described elsewhere.[31]
and PdACHTREUNG(PPh3)4 (44 mg, 0.035 mmol), to afford a yellow solid (261 mg,
74%). M.p. 246–2488C; 1H NMR (300 MHz, CHCl3): d = 7.67–7.61 (m,
12H; Har), 7.50–7.47 (m, 12H; Har), 7.41 (d, J = 7.2 Hz, 6H; Har), 6.66
(d, J = 7.2 Hz, 6H; Har), 3.00 ppm (s, 18H; CH3); 13C NMR (100 MHz,
CHCl3): d = 150.4 (3”C-N), 132.9 (6”CHar), 132.1 (6”CHar), 131.7 (6”
ꢀ
CHar), 131.7 (6C), 131.7 (3”C P), 131.3 (6”CHar), 127.5 (3C), 124.9
ꢁ
(3C), 121.4 (3C), 111.9 (6C), 109.7 (3C), 93.3, 92.4, 89.8, 87.3 (4”C, C
C), 40.3 ppm (6”CH3); 31P NMR (121 MHz, CDCl3): d = 28.2 ppm; ele-
mental analysis calcd (%) for C72H54N3OP: C 85.77, H 5.40, N 4.17;
found: C 85.69, H 5.66, N 4.16.
Ground-state dipole moments: Dielectric constants and refractive in-
dexes of solutions of increasing concentration were measured on an
HP 4192 A impedance analyzer and an Abbe refractometer (Carl Zeiss),
respectively. From these data, Guggenheimꢁs method was used to deter-
mine the ground state dipole moments.[32]
Tris-[4-(4-dimethylaminophenylethynyl)phenyl]phosphane oxide (1c):
Compound 1c was obtained by the same experimental procedure as used
to prepare 3, starting from a solution of (4-iodophenyl)-dimethylamine
(9, 500 mg, 2 mmol) and tris-(4-ethynylphenyl)phosphane oxide (1d,
177 mg, 0.5 mmol) in a mixture of toluene (26 mL) and triethylamine
(7 mL) with CuI (19 mg, 0.1 mmol) and [Pd
ACHTREUNG
to afford yellow solid (110 mg, 30%). M.p.
a
>
(300 MHz, CHCl3): d = 7.60–7.55 (m, 12H; Har), 7.40 (d, J = 9 Hz, 6H;
Har), 6.65 (d, J = 9 Hz, 6H; Har), 2.99 ppm (s, 18H; CH3); 13C NMR
(75 MHz, CHCl3): d = 150.5 (3”C-N), 133.1 (6”CHar), 132.0 (6”CHar),
130.8 (3”C-P), 128.4 (3C), 111.9 (6”CHar), 109.3 (3C), 94.1, 86.9 (2”C,
Acknowledgements
This workwas supported by the French Research Ministry program “ACI
Jeune Chercheur 2004–2006”. We are grateful to J.-P. Lefevre and J.-J.
Vachon for their assistance in tuning the single-photon timing instru-
ments. The authors acknowledge B. Valeur for fruitful discussions. M.-H.
C C), 40.3 ppm (6”CH3); 31P NMR (121 MHz, CDCl3): d = 28.7; MS
ꢁ
(CI, NH3): C48H42N3OP: m/z: 709 [M+H]+.
9064
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Chem. Eur. J. 2006, 12, 9056 – 9065

Fluorescent Phosphane Oxides
FULL PAPER
Ha-Thi is grateful to the Minist›re de l’Education et de la Recherche for
a grant (2004–2007).
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Received: April 3, 2006
Revised: June 13, 2006
Published online: September 6, 2006
Chem. Eur. J. 2006, 12, 9056 – 9065
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9065