External-stimuli responsive photophysics and liquid crystal properties of self-assembled "phosphole-lipids"

Article abstract of DOI:10.1021/ja206784f

A series of new amphiphilic phosphonium materials that combine the electronic features of phospholes with self-assembly features of lipids were synthesized. Variable concentration/temperature and 2D NMR studies suggested that the systems undergo intramole

Full text of DOI:10.1021/ja206784f

ARTICLE
pubs.acs.org/JACS
External-Stimuli Responsive Photophysics and Liquid Crystal
Properties of Self-Assembled “Phosphole-Lipids”
Yi Ren, Wang Hay Kan, Matthew A. Henderson, Paolo G. Bomben, Curtis P. Berlinguette,
Venkataraman Thangadurai, and Thomas Baumgartner*
Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada
S Supporting Information
b
ABSTRACT: A series of new amphiphilic phosphonium ma-
terials that combine the electronic features of phospholes with
self-assembly features of lipids were synthesized. Variable con-
centration/temperature and 2D NMR studies suggested
that the systems undergo intramolecular conformation changes
between a “closed” and “open” form that are triggered by
intermolecular interactions. The amphiphilic features of the
phospholium species also induce liquid crystalline and soft crystal phase behavior in the solid state, which was studied by
differential scanning calorimetry (DSC), polarized optical microscopy (POM), and variable temperature powder X-ray diffraction
(VT-PXRD). The studies revealed that both conjugated backbones and counteranions work together to organize the systems into
different morphologies (liquid crystal/soft crystal). Dithieno[3,2-b:20,30-d]phosphole-based compounds exhibit enhanced emission
in the solid state and at low temperature in solution due to aggregation-induced enhanced emission (AIEE). Photoinduced electron
transfer (PET) induced via the alkoxybenzyl group at the phosphonium center in the fused-ring systems can be effectively
suppressed through intermolecular charge transfer (ICT) processes within the main scaffold of a nonfused system, which was
confirmed by static and dynamic fluorescence spectroscopy. The dynamic features of these new materials also endow the systems
with external-stimuli responsive photophysical properties that can be triggered by temperature and/or mechanical forces.
’ INTRODUCTION
πꢀπ interactions, hydrogen bonding, electrostatic interactions,
and/or van der Waals forces, can be utilized to organize
molecules into well-ordered (1D, 2D, and 3D) micro/
nanostructures.4 Through sophisticated molecular design, ionic
interactions that are based on the amphiphilic nature of phos-
pholipid analogues have proven to be a very important and
powerful driving force for long-range, high-order, and large-area
self-assembly.5 Ionic liquid crystal systems with well-defined
structures and diverse functionalities, in particular, have attracted
significant attention recently.5b,6,7 For example, Kato et al. have
reported a series of self-assembled imidazolium salts that self-
organize into well-ordered nano/microstructures and show en-
hanced ionic conductivity, therefore, opening a general path
toward practical applications for such systems (Chart 2a).6
Mostly due to the synthetic challenges involved, mesomorphic
features of inorganic main group element-based (B, Si, P),
π-conjugated systems remain fairly limited to date, compared
to genuine and nitrogen-based organic species. Weiss and co-
workers have carried out pioneering studies on the ionic self-
assembly of ammonium and phosphonium compounds that,
albeit without involving π-conjugation (Chart 2b), indicated
some intriguing features (liquid crystallinity and gelation) that
can arise from phosphonium-based systems.7 Kato’s group
recently also reported a series of phosphoryl-based systems with
Because of the inherent electronic nature and geometric
parameters of the heteroatoms, B-, Si-, P-, and S-containing,
π-conjugated systems exhibit intriguing properties that pure
carbon-based conjugated derivatives cannot offer to such extent.1
Consequently, these “heteroatom-doped” systems have increas-
ingly become the focus for the designof highly functionalmaterials
in the area of organic/bioelectronics, such as sensors, organic
light emitting diodes (OLEDs), organic field-effect transistors
(OFETs), and organic photovoltaic cells (OPVs).2 In the case of
phosphorus-based π-conjugated systems, others2gꢀk and we3 have
demonstrated that the phosphorus center can be considered an
electronic “switch” for the conjugated moieties (Chart 1) effi-
ciently manipulating the photophysical properties (band gap,
fluorescence quantum yield, charge transfer), the redox properties,
and electron affinity of the system through a diverse chemistry
involving both, modification of E0, such as oxidation (S, O),
borylation (BH3), methylation (Me+), and metalation (Au, Pd,
Pt), as well as the electronic nature of the R groups.3 More
importantly, the phosphorus switch can be tuned reversibly
through modification of E0 highlighting the unique features of this
class of materials not seen with other heteroatoms.3a,b
Because of their tremendous potential, self-assembly properties
are a very desirable feature for heteroatom-doped π-conjugated
systems with respect to both fundamental studies and practical
applications. To install self-assembly features within organic
electronic systems, specific intermolecular interactions, such as
Received: July 20, 2011
Published: September 29, 2011
r
2011 American Chemical Society
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Chart 1. Heteroatom-Doped π-Conjugated Systems (Left) and Emission of Phosphorus-Based Conjugated Systems Based on the
Nature of E0 (Right)
Chart 2. Previously Reported Ionic Compounds and Newly
Designed Amphiphilic Phospholes
the photophysical and self-assembly properties of the system. It is
worth to mention that these phosphonium salts are the first
examples of benzyl-substituted phospholium species. The triva-
lent phospholes were transformed into the corresponding
phospholium salts via nucleophilic substitutions with 3,4,5-
tris(dodecyloxy)benzyl bromide in refluxing toluene/tetrahy-
drofuran (THF), and monitored by the observed downfield shift
in the 31P NMR spectra. The resulting phospholium bromide
species were subsequently purified through column chromato-
graphy. The nucleophilic substitutions were also attempted with
dodecyl bromide under similar conditions, but did not provide
the target phospholium salts, which is probably due to the lower
stability of the required primary dodecyl carbocation. Subse-
quently, the phospholium salts with different counteranions
(BF4ꢀ, BPh4ꢀ, and OTfꢀ) were obtained via metathesis with
the corresponding metal salts (AgBF4, NaBPh4, and AgOTf).
The easy access of counteranions with different geometric and
electronic features allowed us to control the mesomorphic proper-
ties (vide infra). The identity of all new phospholium salts was
confirmed by multinuclear (1H, 13C, and 31P) NMR spectroscopy,
high-resolution mass spectrometry, and elemental analysis.
liquid crystalline features.8 However, to the best of our know-
ledge, mesomorphic properties and static/dynamic photophysi-
cal properties of ionic phosphorus-based conjugated systems
have not been systematically studied to date.
In keeping the promising materials mentioned above in mind,
we have now targeted a series of novel amphiphilic phospholium
materials in this study that combine the electronic character of
π-conjugated phospholes with the amphiphilic character of lipids
(Chart 2c). The conjugated phosphole backbones were antici-
pated to provide intriguing photophysical properties, while the
amphiphilic lipid-like (hydrophilic head and hydrophobic tail)
features could potentially induce micro/nanosegregated bulk
structures. Most importantly, we envisioned that highly tunable
photophysical properties could also be realized in a unique way
exclusively through dynamic structural changes, without further
chemical modification of the new systems. The conformational
changes of the molecular species have been comprehensively
studied by variable concentration/temperature and 2D NMR
experiments, and the liquid crystalline properties of the materials
were determined by differential scanning calorimetry (DSC),
polarized optical microscopy (POM), and variable temperature
powder X-ray diffraction (VT-PXRD); their photophysical prop-
erties were characterized by UVꢀvis and fluorescence spectros-
copy (static and time-resolved).
Remarkably, all compounds (1aꢀd, 2a,b, 3aꢀc, and 4a) show
1
concentration-dependent changes in their H NMR spectra in
CDCl3 from 10ꢀ4 to 10ꢀ2 M (see Supporting Information),
which were tentatively assigned to the flexible nature of the
benzyl group allowing for conformational changes about the
phosphorus center. Figure 1a shows the variable concentration
1H NMR spectra of 2a in CDCl3 as representative example.
Upon increasing the concentration of 2a from 10ꢀ4 to 10ꢀ2 M at
298 K, the proton chemical shifts of Hb and He experienced slight
upfield shifts, while Hd close the trisalkoxyphenyl ring, experi-
enced a significant downfield shift. In a control experiment using
reference compound 2a0 without flexible benzyl group under
otherwise similar conditions (10ꢀ4 to 10ꢀ2 M at 298 K in CDCl3;
Figure S13), the chemical shift of Hd (ca. 8.05 ppm) only
experienced a very small change upon increasing the concentra-
tion, which strongly suggests that the chemical shift change of
Hd in 2a is due to intramolecular conformational changes. In fact,
the 31P NMR signal of 2a also experiences a downfield shift
upon increasing the concentration from 10ꢀ4 to 10ꢀ2 M in
CDCl3 (Figure 1b), indicating changes of the environment of the
phosphorus center.
’ RESULTS AND DISCUSSION
Synthesis and Conformation Studies. The π-conjugated
phosphole-containing scaffolds L1, L2, and L3, used for this
study, have been synthesized following our reported protocols
(Scheme 1).3a,d,f L4 was obtained through the reduction of the
corresponding oxide species, which has also been reported by our
group.3b The variable conjugated backbones ranging from small
dithieno- to larger bis(benzo[b]thieno)-entities were expected to
offer an opportunity for revealing the conjugation effect on both
To better understand the fluxional processes in solution, and
to also study the fundamental structural elements of the new
systems in the solid state, the model compound M1 (Scheme 1)
without alkoxy-substituents was synthesized. Its solid-state orga-
nization revealed important insights into the conformational
differences that very likely apply to all of the new phospholium
salts, confirming the fluxional nature of the system. Notably, we
were able to obtain two different crystal types, blocks and
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Scheme 1. Synthesis of Amphiphilic Phospholes
Figure 1. 1H (a) and 31P (b) NMR study of 2a at varying concentrations (CDCl3, 298 K).
needles, through slow evaporation of different solvents (CH2Cl2
or CHCl3) at room temperature, respectively (see Figure 2). In
the block-shaped crystals, obtained from CH2Cl2, the benzyl ring
is close to the main dithieno-scaffold (M1c). By contrast, the
benzyl ring is removed from the dithieno-scaffold in the needle-
shaped crystals obtained from CHCl3 (M1o). The existence of
both forms of M1, closed and open, in the solid state clearly
supports the dynamic structural features of these phospholium
systems.
For both conformations, hydrogen bonding between the
bromide counteranion and the benzylic H-atoms is observed
(2.81, 2.94 Å in M1o and 2.84, 3.11 Å in M1c). InM1o, twoadjacent
dithienophosphole moieties exhibit intermolecular πꢀπ inter-
actions (3.49 Å); consequently, the distance between the methylene
carbon atom (C20) and the central five-membered phosphole
plane (C3ꢀC4ꢀC5ꢀC6) is larger in the open conformation of
M1o (1.618 vs 1.336 Å in M1c; see Supporting Information).
An ionic interaction between the cationic phospholium center
and the anionic bromide was also observed in both structural
isomers (M1c, P+ Brꢀ = 4.34 and 4.45 Å; M1o, P+ Brꢀ =
3 3 3
3 3 3
4.34 and 4.37 Å), which is comparable to similar phosphonium
bromide salts (P+ Brꢀ: 4.40 Å).7a Futhermore, the similar
3 3 3
concentration-dependent 1H NMR changes between M1 and the
1 series further support the hypothesis of the conformation
changes in all phospholium species upon increasing the concen-
tration (vide infra).
On the basis of 2D-NMR experiments of compounds 1ꢀ4
(see Supporting Information) and conformational studies of
model compound M1, a plausible mechanism for the concentra-
tion-dependent conformation changes of this flexible system was
proposed (see Figure 3). At low concentration, the alkoxy-
substituted benzyl ring is in close proximity to the main scaffold
of the π-conjugated moieties (closed form) showing weak intra-
molecular electrostatic interactions between the electron-poor
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Figure 2. (a) Molecular structure of M1c in the solid state (50% probability, H-atoms and solvent molecules omitted for clarity); (b) crystal appearance
of M1c; (c) molecular packing of M1c (benzylic H-atoms are shown in green); (d) molecular structure of M1o in the solid state (50% probability,
H-atoms omitted for clarity); (e) crystal appearance of M1o; (f) molecular packing of M1o (benzylic H-atoms are shown in green).
Figure 3. Plausible conformation changes in solution: arrow indicates the trialkoxyl benzyl group leaving away from conjugated phosphole cores (insets:
side view of the conformation changes).
conjugated backbone and the electron-rich alkoxy-substituted
phenyl ring.9a,b At higher concentration, intermolecular interac-
tions (ionic and/or H/πꢀπ) take effect pushing the benzyl
group of an adjacent molecule away from the π-conjugated main
scaffold (open form), which eliminates the shielding of Hd by the
alkoxyphenyl ring.
This process is supported by NOESY NMR experiments
carried out at both low (10ꢀ4 M) and high concentration
(10ꢀ2 M; see Supporting Information). Interestingly, although
having a similar environment to Hd in the 2 series, Ha of the
symmetric 3 series did not experience a similar shielding from the
alkoxyphenyl ring. Ha of 3a only changed from 8.31 to 8.37
ppm upon increasing the concentration from 10ꢀ4 to 10ꢀ2 M at
298 K in CDCl3 (see Supporting Information). These features
can be explained by the 3 series adopting a form in which the
alkoxy-substituted phenyl ring is not as close as in the 1 series at
low concentration, which is probably due to electronic and/or
steric effects of the conjugated backbone.9c,d Therefore, increas-
ing the concentration did not cause a significant conformation
change as observed for the other series.
Similar conformation changes can also be oberserved at dif-
ferent temperatures (see Supporting Information). The NMR
studies of the 1 series further confirm that ionic interactions
between the π-conjugated cations and the respective counter-
anions (Brꢀ, BF4ꢀ, BPh4ꢀ, and OTfꢀ) are influenced by the
electronic and geometric nature of the latter (see Supporting
Information). Consequently, the systematic studies on the first
examples of conjugated benzyl phospholium derivatives clearly
support that the discovered dynamic (intra/intermolecular)
features can be channeled through the conjugated phos-
phonium cores as well as the counterions, providing sys-
tems with stimuli-responsive and self-assembly properties
(vide infra).
Self-Organization in the Solid State. As anticipated, the
amphiphilic nature of the phospholium salts indeed offers
desirable self-assembly features for the new materials. DSC
experiments revealed several thermal phase transitions for each
compound (see Figure 4 and Table S1), which were further
identified via POM and VT-PXRD experiments.
Generally, oblong and mosaic textures were observed (Figure 5,
top; and Supporting Information) for most compounds after
cooling from the isotropic state, which are indicative of smectic
phases (liquid crystal and soft crystal).10a As a general feature
for the ionic liquid crystals, dark homeotropic areas were also
observed due to the vertical alignments of the compounds
between two glass slides.5b
As for the 1 series with the smaller conjugated dithieno-
moiety, smectic liquid crystal properties could be observed
for 1a and 1b within a very broad temperature range (1a,
76ꢀ205 °C; 1b, 41ꢀ168 °C; Figure 4), which was further
confirmed by VT-PXRD experiments. For example, 1a with a
bromide anion exhibited reflections corresponding to d-spacingsat
26.1, 13.2, 8.89, and 6.39 Å, which are consistent with the 1:1/2:
1/3:1/4 peak ratios characteristic for a lamellar phase (Figure 5).10b
In addition, a broad peak at the wide angle range (17° ∼ 25°) for
1a and 1b (Figure 5 and Supporting Information) indicates the
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Figure 4. DSC traces of compounds, 2nd heating, 5 °C/min; S, “solid” (precise nature could not be determined due to temperature limitations with the
PXRD instrument); Cr, crystalline state; SmX, soft crystal; Sm, smectic phase; I, isotropic liquid; UP, unidentified phase; G, glass transition.
Figure 5. (Top) Polarized optical micrographs of 1a at 195 °C and 1b at 168 °C; (top right) glass like solid of 3a after fast cooling to ca. ꢀ20 °C;
(bottom left) DSC traces of 1a with the heating and cooling rate at 5 °C/min (negative heat flow values indicate an endothermic process); (bottom
right) powder X-ray diffraction of 1a at 180 °C.
liquid-like state of the long alkyl chains.10b As very important
tunable parameter for ionic liquid crystals, the counteranions were
also found to impact the molecular organization in the solid state.
First, the clearing temperature decreased with the inꢀcreasing size of
counteranions in the order Brꢀ (205 °C) > BF4 (168 °C) >
these phospholium compounds can very likely be attributed to a
Smectic B phase.10
In addition to the counteranions, the different conjugated
backbones are another important aspect with potential to tune
the systems’ mesomorphic properties. 2a with the somewhat
extended asymmetric conjugated backbone also exhibits a smec-
tic phase but with a lower clearing point than 1a (173 vs 205 °C
for 1a). As for 3a,b and 4a, only soft crystal phases and different
crystalline phases can be observed at higher temperatures.
Generally, the rigid anisotropic molecular structure can promote
efficient molecular alignment in the liquid crystal phase.10b The
smaller conjugated backbone, such as the dithieno-moiety, can
be considered a small spherical shape, which does not diminish
the rod-like structural feature of the system combining the
BPh4 (108 °C), OTfꢀ (98 °C). Second, 1c and 1d with larger
ꢀ
counteranions (OTfꢀ and BPh4ꢀ) only exhibited smectic soft
crystal phases instead of liquid crystal phases at higher temperature,
which was supported by the presence of several weak peaks at wide
angle in VT-PXRD experiments (see Supporting Information).
These results indicate that the bulkier counteranions exhibit a steric
and a more delocalized electronic effect, which suppresses the
effective intermolecular ionic interactions.5b,11 On the basis of the
POM and PXRD experiments, the observed liquid crystal phase of
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Figure 6. Proposed organization of the phospholium compounds: (a) molecular organization of 1c in the soft crystal phase; (b) molecular organization
of 1a in the both soft crystal and liquid crystal phase; (c) molecular organization of 3a in the soft crystal phase (d, the thickness of layer; L, the length of
molecule estimated by theoretical calculation: MM2).
alkoxybenzyl group. As the size of the conjugated backbone
increases, the anisotropic features of the molecular structure are
diminished; the decreased aspect ratio usually prevents dense
molecular packing. This could explain why 2a exhibits a lower
clearing point than 1a. As the size of conjugated backbone
further increases, such as in the case of 3a and 4a, the rod-like
structural feature is transformed into a T-shape, which desta-
bilizes the mesophase at higher temperatures. Moreover, in 4a
with larger conjugated backbone, the intermolecular ionic
interaction could be further weakened through the delocalization
of the positive charge within the bulky conjugated backbone.5b,11
It is worth to mention that 3a,b and 4a only exhibited very small
or broad exothermic transitions during the cooling process and a
broad endothermic glass transition in the heating process. These
observations indicate that a metastable amorphous state could be
induced due to the slow kinetics of crystallization during the
cooling process (Figure 5, top right); VT-PXRD indicated that
a certain degree of order is still preserved within the amorphous
state (see Supporting Information).
According to VT-PXRD experiments (see Supporting Infor-
mation), most compounds (1aꢀd, 2a, and 3a) show a lamellar
organization in their liquid crystalline state and in the soft crystal
phase; the proposed molecular organizations of these com-
pounds are shown in Figure 6. For the 1 series with smaller
conjugated moieties, 1a, 1b and 1d show a lamellar structure
where the thickness of the layers is close to the length of
individual molecule (Table S2) in both liquid crystalline and
This could be explained by stronger intermolecular interactions
(πꢀπ and/or CH-π) between the positive conjugated head and
negative tetraphenyl borate that not only reduce the free space
within the ionic layer, but also repel the long alkyl chains from
adjacent molecules resulting in less interdigitation of the alkyl
chains and ultimately lead to the increased layer thickness
(Figure 6a). This stronger cationꢀanion interaction is also
supported by the NMR experiments of 1c (vide supra). 3a with
a more extended conjugated backbone exhibits a layer in the soft
crystal phase with a thickness of about 1.4 times of the molecule
length. This similar organization is also likely due to intermole-
cular interactions (πꢀπ and/or CHꢀπ) of the conjugated
moiety within intralayers (Figure 6c). It is worth to mention
that the highly ordered lamellar structures are also preserved at
room temperature for some compounds, that is, 1aꢀd and 2a.
Such well-defined organization in the solid state could potentially
enhance the charge-, ion-, and energy-transfer, which are cur-
rently investigated in our laboratory.
Photophysical Properties and External Stimuli Responses.
Photophysical Properties. The varying conjugated backbones
further allowed us to systematically study the substituent effect
on the photophysical properties of the systems (Table 1). As
observed for related phospholes before,3a,d,f,g the absorption and
emission features are red-shifted by increasing the conjugation
length of systems (1 < 2 < 3 < 4). Remarkably, the new
compounds with the trisalkoxyphenyl ring generally exhibit a
blue-shifted emission compared to the methyl derivatives, which
is probably due to the higher electron density on the phospho-
nium center imparted by the donating nature of the benzyl group.
Notably, a high fluorescence quantum yield is usually observed
for corresponding phosphole oxide (E0 = O) and phospholium
(E0 = Me) derivatives (cf.: ϕPL = 0.3ꢀ0.5) in both solution and
the solid state, due to the polar/bulky phosphorus center.3
However, the fused-ring phospholium species (1, 2, and 3 series)
the soft crystal phase. In addition, the layer thickness decreases
with increasing size of the counteranion in the order Brꢀ > BF4
>
ꢀ
OTfꢀ, which is probably due to the weakened intermolecular
ionic/hydrogen-bonding interactions.5b,12
Previous studies of ionic liquid crystal systems have suggested
that the intermolecular ionicinteractionbetweencationand anion,
as well as halogen-hydrogen bonding within the ionic layer, can
stabilize the ordered micro/nanostructures in the mesomorphic
phase.5b,7 NMR spectroscopy and model studies of the new
phospholium salts also led us to conclude that the ionic interac-
tions and halogen-hydrogen bonding are the main driving forces
for stabilizing the lamellar mesomorphic phase. Therefore, alter-
nating distribution of cation and anion with the alkoxybenzyl rings
between adjacent molecules pointing to the opposite direction of
ionic layer is proposed for 1a, 1b, and 1d (Figure 6b). Besides, the
full interdigitation of alkyl long chains further stabilizes the ionic
double layer. In the case of 1c with the tetraphenylborate anion, an
increased layer thickness was observed in the soft crystal phase.
were found to only exhibit very low quantum yields (ϕPL
=
0.003ꢀ0.005) in CH2Cl2. Remarkably, compounds 1aꢀd ex-
hibit much more intense emission features (ϕPL = 0.09ꢀ0.19) in
the solid state (Figure 7) with, for example, ∼40 times enhanced
quantum yield for 1c when compared to its value in solution.
This phenomenon can be related to an aggregation-induced
enhanced emission (AIEE), in which intramolecular rotation
(IR) of a flexible structural element is restricted in the solid state,
leading to increased photoluminescence efficiency.13 The disap-
pearance of the component with shorter excited-state lifetime
(ca. 0.1 ns) in the solid state further supports the restricted
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Table 1. Photophysical Properties of the Phospholium Salts
compound
λabs [nm]a
ε[L molꢀ1 cmꢀ1 b
]
λem[nm] (CH2Cl2)
λem[nm] (solid)
ϕPL (CH2Cl2)c
ϕPL (solid)c
τ [ns] (CH2Cl2)d
3
3
M1
372
6384
463
465
0.060
0.60
0.75 (15.59%)
6.81 (84.41%)
0.11 (66.76%)
2.72 (8.78%)
1a
369
12610
454
467
0.0048e
0.11
10.78 (24.46%)
0.096 (70.17%)
9.95 (29.83%)
0.085 (72.01%)
9.52 (27.99%)
0.099 (72.89%)
1.70 (4.21%)
1b
1c
1d
369
370
369
7250
4910
9998
457
455
456
463
458
459
0.0031e
0.0050e
0.0039e
0.10
0.19
0.09
11.51 (22.90%)
0.025 (7.57%)
2.26 (5.35%)
2a
337
7710
421f
n.d.
n.d.
n.d.
11.21 (87.08%)
0.019 (19.49%)
10.12 (80.51%)
0.064 (21.80%)
9.97 (78.20%)
0.062 (53.97%)
8.82 (46.03%)
4.90 (35.44%)
7.56 (64.56%)
2b
3a
3b
4a
339
415
417
438
9040
13470
14650
20260
423f
501
503
536
n.d.
516
518
538
n.d.
n.d.
0.03
0.04
0.58
0.05
0.06
0.30
a The lowest absorption measured in CH2Cl2. b ε: Molar absorption coefficient. c Fluorescence quantum yield was determined by a calibrated integrating
e
sphere system. d Fluorescence lifetime. Fluorescence quantum yield cannot be determined by a calibrated integrating sphere system (<0.01), and
f
determined by using quinine sulfate (0.1 M H2SO4 solution). Emission spectra measured at ca. 10ꢀ4 M CH2Cl2 due to the low intensity, others
measured at ca. 10ꢀ5 M in CH2Cl2. n.d. : not detectable.
intramolecular rotations for 1aꢀd (see Table S3 in the Support-
ing Information).13 In addition, model compound M (ϕPL = 0.06
in CH2Cl2; ϕPL = 0.60 in the solid) with the simple benzyl group,
exhibits a stronger emission in both solution and the solid state
than the 1 series compounds with the trisalkoxybenzyl group
(ϕPL = 0.003ꢀ0.005 in CH2Cl2; ϕPL = 0.10ꢀ0.20 in the solid).
Such observation suggests that the photoinduced electron trans-
fer (PET) from the alkoxybenzene (donor) to the phospholium
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chromophore’s main scaffold (acceptor) is likely another non-
radiative decay channel.14 However, it seems that such PET
channel is suppressed for the 1 series in the solid state due to the
conformational changes, as indicated by the NMR studies (vide
supra). Quite different from the 1 series, the quantum yields for
the compounds of the 2 series are too low to be detected in either
CH2Cl2 or the solid state. This observation suggests that intra or/
intermolecular PET is still active in the 2 series in the solid state,
despite the IR process being naturally restricted. Compounds 3a
and 3b exhibit relatively higher quantum yields in solution (3a,
ϕPL = 0.03; 3b, ϕPL = 0.04) than their congeners of the 1 and 2
series, probably due to the different electronic (redox potential,
band gap of their conjugated acceptor backbone) or geometric
parameters (distance between the donor and acceptor moiety;
structural flexibility), likely suppressing PET and/or IR pro-
cesses.14 On the other hand, both 3a and 3b show enhanced
emission in the solid state. By contrast, compound 4a with the
nonfused, extended π-conjugated moiety shows a significantly
higher quantum yield in CH2Cl2 (ϕPL = 0.58) and the solid state
(ϕPL = 0.30) compared to the other species in this study.
Importantly, however, the structural flexibility of the benzyl unit
is still preserved in 4a according to the NMR studies. Moreover,
the higher quantum yield in solution and absence of the shorter
excited-state lifetime (τ1 = 4.90 ns, τ2 = 7.56 ns) component
suggests that the PET is not active in 4a.
To gain some deeper understanding for the observed photo-
physics, we have performed time-dependent (TD) DFT-calcula-
tions at the B3LYP/6-31G(d) level of theory using a PCM-
solvation model to counterbalance the omission of the anions in
the calculations.15 To furthermore reduce the computation time,
the dodecyloxy moieties were truncated with methoxy groups
(Figure 8). We found that all model compounds (1#, 2#, 3# and
4#) show the typical σ*ꢀπ* orbital coupling of dithienophos-
pholes in the LUMO orbital.3 For the fused-ring compounds 1#
and 3#, HOMO-1 and HOMO energy levels are very close
(ca. 0.1 eV), and fully degenerate for 2# according to the TD-
DFT calculations. It is worth to mention that, both HOMO and
HOMO-1 of 2# are mainly localized in electron-rich trimethoxy-
benzyl ring, while only the HOMO of 1# shows a similar
distribution; by contrast the HOMO-1 represents the π-con-
jugated scaffold. However, both HOMO and HOMO-1 of 3#
show a strong contribution from the conjugated backbone as well
as the benzyl group, which is believed to be one major reason for
the different observed quantum yields (3 series > 1 series > 2
series) in solution. Importantly, the HOMO representing the
conjugated backbone is significantly destabilized in compound 4#
due to the extension of the π-system by the terminal phenyl rings,
as commonly observed for such materials.
Furthermore, based on the TD-DFT calculations, a weak
charge-transfer character within the conjugated backbone of 4#
can also be found from the outer phenyl rings as weak donors
(HOMO) to the centeral phospholium as an acceptor (LUMO).
The stabilized HOMO-1, representing the electron-rich tri-
methoxybenzyl ring in 4#, suggests that PET is indeed suppressed
Figure 7. Quantum yields of the phospholium species M1 and 1ꢀ4;
(inset top right) plausible nonradiative decay channels (IR = Intramo-
lecular Rotation; PET = Photoinduced Electron Transfer); (inset
center) fluorescence pictures of selected compounds in CH2Cl2 and
the solid state; n.d., not detectable.
Figure 8. Frontier orbitals of model compounds 1#, 2#, 3#, and 4# (B3LYP/6-31G(d), PCM (solvent = dichloromethane)).15 PET = Photoinduced
Electron Transfer; ICT = Intramolecular Charge Transfer.
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in this system. The theoretical results further support that this
weak intramolecular charge transfer competes with the excited
state electron transfer in 4a, leading to the higher luminescence
quantum yield for 4a when compared to the other phospholium
species (Figure 8; Table S4, Supporting Information).
In the solid state, both counteranions and conjugated moieties
were found to have a significant impact on the overall emission
properties of the materials (Figure 9). The increasing size of the
counteranions (Brꢀ < BF4ꢀ < BPh4ꢀ, OTfꢀ) clearly reduces the
shifts between the emission wavelengths in solution and the solid
state (1a, Δλem = 13 nm; 1b, Δλem = 6 nm; 1c, Δλem = 3 nm; 1d,
Δλem = 3 nm), due to reduced intermolecular interactions of the
π-conjugated moieties. The higher quantum yield of 1c in the
solid state also supports the less efficient intermolecular electro-
nic communication due to the much bulkier counteranion
(BPh4ꢀ). On the other hand, both 3a and 3b show a more
exhibit a hyperchromic emission at 213 K (for 1aꢀd about 30
times) compared to the intensity at 298 K, likely due to restricted
intramolecular rotation at low temperature (Figure 10).13 It needs
to be mentioned that the overall less enhanced emission of 4a
(compared to the other congeners) indicates that intramolecular
rotation is not a effective quenching channel, as opposed to the 1
series compounds, probably due to the steric interaction between
the bulky trisalkoxybenzyl ring and the twisted terminal phenyl rings
in 4a. Upon reducing the temperature from 298 to 213 K,
compounds 2a and 3a furthermore show a ∼20 nm, or ∼10 nm
red shift in CH2Cl2, respectively (Figure 11). Remarkably, the
thermal emission responses of the 1 (ca. 7 nm), the 2 (ca. 19 nm),
and the 3 series (ca. 10 nm) exhibit a comparable red shift to the
chemical modification of the phosphorus center in the P1 (14 nm),
P2 (17 nm), and P3 (22 nm) systems reported before
(Figure 11).3a,d,f
pronounced red shift in the solid-state emission (3a, Δλem
=
The red-shifted emission at such low concentration (10ꢀ5 M)
is very likely due to the intramolecular conformation changes
rather than intermolecular associations, which are further sup-
ported by VT NMR experiments of the 1, 2, and 3 series (see
Supporting Information). Recalling the typical coupling in silole
and phosphole species between the σ*-orbital of exocyclic
heteroatom (Si/P)-carbon bonds and the π*-orbital of the
conjugated backbones, this red-shifted emission can be attributed
to the change of the σ*ꢀπ* orbital overlap during the conforma-
tional changes.3,16
Flexible structures are known to induce polymorphism in
the solid state. Moreover, the transition between the stable and
the metastable state triggered through external (thermal and
mechanical) stimuli can also induce altered photophysical
properties that have drawn a lot attention recently.17 Remark-
ably, compound 4a was also found to show an intriguing
mechanical photoluminescence response in the solid state.
Upon grinding the solid powder of 4a deposited on a glass
slide, its fluorescence changed from green (λem = 538 nm, τ =
3.89) to yellow (λem = 560 nm, τ = 2.70 (7.51%), τ2 = 7.67
(92.49%); Figure 12, left). Importantly, the original green
fluorescence features could be recovered after thermal anneal-
ing (ca. 80 °C) and this process (ca. Δλem = 20 nm) could be
repeated several times without compromising the response
(Figure 12, right). To emphasize the unique behavior of 4a, the
fused systems 1a,b, 2a,b, and 3a,b with smaller conjugated
cores and counteranions did not exhibit any significant me-
chanically responsive emission under similar conditions, which
supports a possible eximer formation for 4a upon mechanical
grinding potentially accessible via a planarized extended con-
jugated backbone.
15 nm; 3b, Δλem = 15 nm), which could be correlated with more
efficient intermolecular πꢀπ interactions of the larger, more
exposed π-conjugated main scaffold. To our surprise, however,
the solid-state emission wavelength of 4a (λem = 538 nm) with
extended phenyl groups is almost the same as in solution (λem
=
536 nm). Compared to 3a,b with a comparable effective con-
jugation length, the different emission features of 4a could be
explained by less efficient planarization of the conjugated back-
bone and intermolecular association in the solid state, which are
due to twisted terminal phenyl rings3b and the bulky pyr-
amidal phospholium center.
External Stimulus Response. The flexible nature of the sys-
tems was also reflected in the observed temperature-dependent
emission properties (see Supporting Information). All compounds
Figure 9. Solution vs solid-state emission (red circle, solid state; blue
circle, CH2Cl2 solution).
Figure 10. (Left) Emission intensity ratio between 213 and 298 K; (right) emission spectrum of 1a in 10ꢀ5 M CH2Cl2 solution (red, at 213 K; blue, at
298 K).
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Figure 11. (Left) Normalized emission of 3a in CH2Cl2 10ꢀ5 M solution (red, at 298 K; blue, at 213 K); (middle) emission wavelengths in 10ꢀ5 M
CH2Cl2 solution (red, at 213 K; blue, at 298 K); (right) emission changes through the chemical modification on the phosphorus in P1, P2, and P3
systems.
Figure 12. (Left) Excitation (ex, dots) and emission (em, solid) spectra of 1b; (right) responding cycles; AT = after thermal annealing; AG = after
grinding.
To elucidate the mechanism for the mechanochromism of 4a,
variable concentration H NMR spectroscopy in CDCl3 was
τ2 = 6.81 ns) is also red-shifted and broadened compared to the
emission of isomer M1o in the crystal state, which can be
considered as sum of the two isomers’ emissions in the crystals
(Figure 14, left). Notably, mechanical force also caused the
emission changes for the two M1 isomer crystals (see Supporting
Information). These results indicate that (i) strong intermole-
cular πꢀπ interactions could induce an excimer emission even
with the bulky phosphonium center, (ii) emission in solution
reflects the average emission of the different isomers that is
consistent with the multiexponential decay of 1, 2, 3, and 4 series,
and (iii) different conformations of the flexible phospholium
species can lead to a different emission wavelengths, which can be
triggered through mechanical forces.
To reveal the nature of the intermolecular interactions in the solid
state, PXRD experiments of 4a were carried out before and after
mechanical grinding. As shown in Figure 14 (right), the mechanical
force induces a completely different PXRD pattern. Moreover, the
significantly reduced diffraction intensity indicates that 4a becomes
more amorphous upon mechanical grinding. Similar observations
could also be detected in other mechanically responsive systems.17
Finally, 4a was also doped into a polystyrene (PS) matrix at different
concentrations (0.05ꢀ20 wt %) to solidify intra/intermolecular
interactions in the solid state as cause for the red-shifted photo-
physics. For the higher doping concentration (20 wt.%), the
polymer matrix shows similar absorption and emission to 4a in
solution (see Supporting Information). However, the 0.05%
doped polymer matrix shows a noticeably blue-shifted emission
(λem = 529 nm, τ1 = 3.37 ns, τ2 = 6.62 ns) compared to the
1
performed that confirmed a similar behavior as observed for the
other compounds indicating the alkoxybenzyl ring moving away
from π-conjugated main scaffold upon intermolecular association-
induced conformation change (Figure 13a). 2D-NOESY experi-
ments (see Supporting Information) also revealed an interaction
between Ha and Hi atlower concentration (8.71 ꢁ 10ꢀ4 M), and a
different HeꢀHi interaction at higher concentration (1.12 ꢁ
10ꢀ2 M). Consistent with the concentration-dependent NMR
experiments, both absorption and emission spectra of 4a showed a
noticeable red-shifted shoulder peak upon increasing the concen-
tration (Figure 13b,c). Such red-shifted shoulder was not observed
in 1a, 2a, and 3a indicating that the intermolecular interactions
may play a role in this concentration-dependent emission.
Further insights on a plausible mechanism for this mechanical
response were obtained from the solid-state photophysical proper-
ties of two isomers of the model compound M1 (Figure 14, left).
The crystals of M1c (cocrystallized with CH2Cl2) show intense
blue emission (λem = 456 nm, τ = 7.94 ns). A notable red shift
(Δλem = 9 nm) was observed for its structural isomer M1o
(cocrystallized with CHCl3; λem = 465 nm, τ = 9.90 ns) that
can be attributed to the intermolecular interactions in M1o (vide
supra). TD-DFT calculations performed on the dimers of each
isomer (M1o and M1c), using the X-ray crystallographic data as
input, also confirmed the intermolecular orbital coupling in the
dimer of M1o (Figure 14, middle). It is worth to mention that
the emission of M1 in CH2Cl2 (λem = 461 nm, τ1 = 0.75 ns,
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Figure 13. (a) Concentration-dependent 1H NMR spectra of 4a (CDCl3); (b) concentration-dependent absorption spectra of 4a, 10ꢀ3 M (blue line)
∼ 10ꢀ9 M (red line) in CHCl3; (c) concentration-dependent emission spectra of 4a 10ꢀ3 M (blue line) ∼ 10ꢀ9 M (red line) in CHCl3.
Figure 14. (Left) Emission spectra of model M1 (blue dash, emission in 10ꢀ6 M CH2Cl2 solution; orange, emission M1o (crystal); red, emission M1c
(crystal). (Middle) Differences in intermolecular orbital coupling of M1o (top) and M1c (bottom). (Right) PXRD of 4a; red, before grinding; blue, after
grinding.
20% doped matrix (λabs = 536 nm, τ1 = 2.23 ns, τ2 = 4.38 ns; see
Supporting Information). At the low concentration level (0.05 wt
%), the blue-shifted emission of 4a can be rationalized as monomer
emission with trapped conformation in the polymer matrix as
opposed to the higher degree freedom of the monomer in solution,
which is consistent with model studies.
On the basis of these studies, the mechanism of the mechani-
cally responsive emission of 4a as shown in Figure 15 is proposed.
Upon mechanical grinding, adjacent molecules come close to each
other (with adjacent conjugated cores coming together and the
trisalkoxybenzyl group moving away) through cooperative inter-
molecular interactions (ionic and/or H/πꢀπ), which potentially
induces excimer emission; in turn, the terminal phenyl rings are
consequently forced into a more planar conformation with
restricted motion, resulting in the red-shifted emission. During
thermal annealing, the motion of the flexible trisalkoxybenzyl ring
likely provides more free space for twisting of the terminal phenyl
rings resulting in an interruption of the conjugation. This motion
can also interrupt the excimer formation in the solid state via
pushing adjacent conjugated phosphonium cores away from each
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Figure 15. Proposed mechanism for the mechanical and thermal response of 4a.
other.3g According to NMR spectroscopy and the polymer
matrix studies, however, a change in the σꢀπ interaction can
also not be fully ruled out at the current stage.
our laboratory, as these well-organized structures could potentially
enhance energy-, charge-, andion-transferprocesses that arehighly
desirable for practical applications.
’ ASSOCIATED CONTENT
’ CONCLUSIONS AND REMARKS
S
Supporting Information. Full experimental details for
b
In conclusion, we have synthesized a series of novel “phos-
phole-lipids” combining both conjugated phosphole backbones
and amphiphilic lipid structures through very effective phos-
phorus chemistry approaches. The amphiphilic character of the
new species allowed accessing self-assembly features, as well as
intriguing photophysical properties. Systematic NMR studies
(1H, 31P, VT, VC, 2D) suggested that these newly synthesized
flexible systems undergo conformational changes from “closed”
to “open” forms via increasing concentration or temperature,
respectively. The isomeric structures of the model compound
M1 further support the proposed conformational changes. Our
results reveal that the intermolecular ionic interaction is a very
effective driving force for stabilizing lamellar liquid crystal and
soft crystal phases in these ionic phosphorus-based π-conjugated
systems. The well-preserved and highly ordered organization of
these systems at room temperature make them very promising
candidates for the application of organic electronics. The newly
discovered dynamic structural features induce an unusual en-
hanced emission for the 1 series compounds in the solid state (up
to 40 times for 1c) due to aggregation-induced enhanced emission
(AIEE). We have shown that, through careful molecular design, the
fluorescence of the system (such as 4a) can be significantly
enhanced via an intramolecular charge transfer within the conju-
gated backbone in both solution (ϕPL = 58%) and the solid state
(ϕPL = 30%). DFT calculations also supported the photophysical
processes occurring in these systems. Importantly, these flexible
structures endow the system with very interesting thermally
responsive emission in solution that is comparable to the chemical
modification of phosphorus center. In the solid state, 4a, with
extended conjugated backbone, also displays a mechanically
responsive emission that can be switched back via thermal
treatment.
the synthesis of all new compounds. Additional 1H, 31P, VC/VT
and 2D NMR data, TD-DFT data, photophysical data, DSC
traces, POM images, VT-PXRD data; complete ref 15. CIF files
of all structures determined by X-ray crystallography and inter-
molecular packing interactions of all crystallographically char-
acterized compounds. This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
Thomas.baumgartner@ucalgary.ca
’ ACKNOWLEDGMENT
Financial support by NSERC of Canada and the Canada
Foundation for Innovation (CFI) is gratefully acknowledged.
We also thank Alberta Ingenuity now part of Alberta Innovates -
Technology Futures for a graduate scholarship (Y.R.) and a New
Faculty Award (T.B.). Thanks to Prof. T. Sutherland for helpful
discussions with regard to the liquid crystal properties.
’ REFERENCES
(1) Boron:(a) Mercier, L. G.; Piers, W. E.; Parvez, M. Angew. Chem.,
Int. Ed. 2009, 48, 6108–6111. (b) Caruso, A., Jr.; Tovar, J. D. Org. Lett.
2011, 13, 3106–3109. (c) Wakamiya, A.; Mori, K.; Yamaguchi, S. Angew.
Chem., Int. Ed. 2007, 46, 4273–4276. (d) Yamaguchi, S.; Akiyama, S.;
Tamao, K. J. Am. Chem. Soc. 2001, 123, 11372–11375. (e) Elbing, M.;
Bazan, G. C. Angew. Chem., Int. Ed. 2008, 47, 834–838. Silicon:
(f) Yamaguchi, S.; Tamao, K. J. Chem. Soc., Dalton Trans. 1998,
3693–3702. (g) Zhan, X.; Barlow, S.; Marder, S. R. Chem. Commun
2009, 1948–1955. (h) Zhan, X.; Risko, C.; Amy, F.; Chan, C.; Zhao, W.;
Barlow, S.; Kahn, A.; Brꢀedas, J.-L.; Marder, S. R. J. Am. Chem. Soc. 2005,
127, 9021–9029. (i) Yamaguchi, S.; Xu, C.; Tamao, K. J. Am. Chem. Soc.
2003, 125, 13662–13663. (j) Xu, C.; Wakamiya, A.; Yamaguchi, S. J. Am.
Chem. Soc. 2005, 127, 1638–1639. Phosphorus:(k) Fukazawa, A.;
Yamaguchi, S. Chem.—Asian J. 2009, 15, 1386–1400. (l) Baumgartner,
T; Rꢀeau, R. Chem. Rev. 2006, 106, 4681–4727. correction: Chem.
Rev. 2007, 108, 303. (m) Dillon, K. B.; Mathey, F.; Nixon, J. F.
Phosphorus: The Carbon Copy; Wiley: New York, 1998. (n) Mathey, F.
Phosphorus-Carbon Heterocyclic Chemistry: The Rise of a New Domain;
Pergamon Press: Amsterdam, 2001. Sulfur:(o) Curtis, M. D.; Cao, J.;
Kampf, J. W. J. Am. Chem. Soc. 2004, 126, 4318–1055. (p) Kim, E.-G.;
Coropceanu, V.; Gruhn, N. E.; Sꢀanchez-Carrera, R. S.; Snoeberger, R.;
These initial studies have successfully demonstrated that
“smart phospholes” can be realized through sophisticated mo-
lecular engineering of the dynamic features on the phosphorus
center, providing unique photophysical and self-assembly proper-
ties for phosphorus-based conjugated materials that can be
modified through external stimuli (temperature/concentration,
or host and guest interaction). However, in order to further
amplify the external stimuli response (absorption and emission),
the σꢀπ orbital coupling likely needs to be maximized, and
further engineering toward efficient intermolecular interactions
are also required, which is currently under detailed investigation in
17025
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Journal of the American Chemical Society
ARTICLE
Matzger, A. J.; Brꢀedas, J.-L. J. Am. Chem. Soc. 2007, 129, 13072–13081.
(q) Suzuki, Y.; Okamoto, T.; Wakamiya, A.; Yamaguchi, S. Org. Lett. 2008,
10, 3393–3396. (r) Amir, E.; Rozen, S. Angew. Chem., Int. Ed. 2005,
44, 7374–7378. (s) Barbarella, G.; Favaretto, L.; Zambianchi, M.; Pudova,
O.; Arbizzani, C.; Bongini, A.; Mastragostino, M. Adv. Mater. 1998,
10, 551–554. (t) Tedesco, E.; Sala, F. D.; Favaretto, L.; Barbarella, G.;
Albesa-Jovꢀe, D.; Pisignano, D.; Gigli, G.; Cingolani, R.; Harris, K. D. M.
J. Am. Chem. Soc. 2003, 125, 12277–122783.
(g) Yazaki, S.; Funahashi, M.; Kagimoto, J.; Ohno, H.; Kato, T. J. Am.
Chem. Soc. 2010, 132, 7702–7708.
(7) (a) Ma, K.; Lee, K.-M.; Minkova, L.; Weiss, R. G. J. Org. Chem.
2009, 74, 2088–2098. (b) Abdallah, D. J.; Robertson, A.; Hsu, H.-F.;
Weiss, R. G. J. Am. Chem. Soc. 2000, 122, 3053–3062. (c) Chen, H.;
Kwait, D. C.; G€onen, Z. S.; Weslowski, B. T.; Abdallah, D. J.; Weiss, R. G.
Chem. Mater. 2002, 14, 4063–4072. (d) Ma, K.; Somashekhar, B. S.;
Gowda, G. A. N.; Khetrapal, C. L.; Weiss, R. G. Langmuir 2008,
24, 2746–2758. (e) Ma, K.; Shahkhatuni, A. A.; Somashekhar, B. S.;
Gowda, G. A. N.; Tong, Y. Y.; Khetrapal, C. L.; Weiss, R. G. Langmuir
2008, 24, 9843–9854. (f) Gowda, G. A. N.; Chen, H.; Khetrapal, C. L.;
Weiss, R. G. Chem. Mater. 2004, 16, 2101–2106.
(8) (a) Hatano, T.; Kato, T. Chem. Commun. 2006, 1277–1279.
(b) Kimura, M.; Hatano, T.; Yasuda, T.; Morita, J.; Akama, Y.; Minoura,
K.; Shimomura, K.; Kato, T. Chem. Lett. 2009, 38, 800–801.
(9) (a) Rashkin, M. J.; Hughes, R. M.; Calloway, N. T.; Waters, M. L.
J. Am. Chem. Soc. 2004, 126, 13320–13325. (b) Rashkin, M. J.; Waters,
M. L. J. Am. Chem. Soc. 2002, 124, 1860–1861.(c) Compared to 2a0
(8.05 ppm), the significant downfield shift of Hd in 2a (7.40 ppm) supports
the shielding effect from the trisalkoxyphenyl ring at low concentrations.
(d) In fact, the 3 series shows a downfield shift compared with the methyl
derivative 3a0 (Ha: 8.00 ppm; Supporting Information). The NOESY
spectrum of 3a did not show any coupling between Hh and Ha/Hd at low
concentration (10ꢀ4 M, CDCl3; see Supporting Information)
(10) (a) Dierking, I. Texture of Liquid Crystals; Wiley-VCH: Weinheim,
2003. (b) Demus, D.; Goodby, J.; Gray, G. W.; Spiess, H.-W.; Vill, V.
Handbook of Liquid Crystals; Wiley-VCH: Weinheim, 1998.
(11) (a) Cardinaels, T.; Lava, K.; Goossens, K.; Elisseva, S. V.;
Binnemans, K. Langmuir 2011, 27, 2036–2043. (b) Goossens, K.;
Wellens, S.; Hecke, K. V.; Meervelt, L. V.; Cardinaels, T.; Binnemans,
K. Chem.—Eur. J. 2011, 17, 4291–4306.
(12) (a) Kouwer, P. H. J.; Swager, T. M. J. Am. Chem. Soc. 2007,
129, 14042–1452. (b) Wiggins, K. M.; Kerr, R. L.; Chen, Z.; Bielawski,
C. W. J. Mater. Chem. 2010, 20, 5709–5714.
(2) Boron:(a) J€akle, F. Chem. Rev. 2010, 110, 3985–4022. (b) Wade,
C. R.; Broomsgrove, A. E. J.; Aldridge, S.; Gabbaï, F. P. Chem. Rev. 2010,
110, 3958–3984. (c) Hudson, Z. M.; Wang, S. Acc. Chem. Res. 2009,
42, 1548–1596. Silicon:(d) Chu, T.-Y.; Lu, J.; Beauprꢀe, S.; Zhang, Y.;
Pouliot, J.-R.; Wakim, S.; Zhou, J.; Leclerc, M.; Li, Z.; Ding, J.; Tao, Y.
J. Am. Chem. Soc. 2011, 125, 4250–4253. (e) Hou, J.; Chen, H.-Y.;
Zhang, S.; Li, G.; Yang, Y. J. Am. Chem. Soc. 2008, 130, 16144–16145.
(f) Sohn, H.; Sailor, J. M.; Magde, D.; Trogler, W. C. J. Am. Chem. Soc.
2003, 125, 3821–3830. Phosphorus:(g) Matano, Y.; Saito, A.; Fukush-
ima, T.; Tokudome, Y.; Suzuki, F.; Sakamaki, D.; Kaji, H.; Ito, A.;
Tanaka, K.; Imahori, H. Angew. Chem., Int. Ed. 2011, 50, 8016–8020.
(h) Romero-Nieto, C.; Durben, S.; Kormos, I. M.; Baumgartner, T. Adv.
Funct. Mater. 2009, 19, 3625–3631. (i) Fadhel, O.; Gras, M.; Lemaitre,
N.; Deborde, V.; Hissler, M.; Geffroy, B.; Rꢀeau, R. Adv. Mater. 2009,
21, 1261–1265. (j) Tsuji, H.; Sato, K.; Sato, Y.; Nakamura, E. J. Mater.
Chem. 2009, 19, 3364–3366. (k) Su, H.-C.; Fadhel, O.; Yang, C.-J.; Cho,
T.-Y.; Fave, C.; Hissler, M.; Wu, C.-C.; Rꢀeau, R. J. Am. Chem. Soc. 2006,
128, 983–995. (l) Fave, C.; Cho, T.-Y.; Hissler, M.; Chen, C.-W.; Luh,
T.-Y.; Wu, C.-C.; Rꢀeau, R. J. Am. Chem. Soc. 2003, 125, 9254–9255.
Sulfur:(m) Xiao, K.; Liu, Y.; Qi, T.; Zhang, W.; Wang, F.; Gao, J.; Qiu,
W.; Ma, Y.; Cui, G.; Chen, S.; Zhan, X.; Yu, G.; Qin, J.; Hu, W.; Zhu, D.
J. Am. Chem. Soc. 2005, 127, 13281–13286.
(3) (a) Ren, Y.; Baumgartner, T. J. Am. Chem. Soc. 2011, 133, 1328–
1340. (b) Dienes, Y.; Durben, S.; Kꢀarpꢀati, T.; Neumann, T.; Englert, U.;
Nyulꢀaszi, L.; Baumgartner, T. Chem.—Eur. J. 2007, 13, 7487–7500.
(c) Ren, Y.; Baumgartner, T. Chem.—Asian J. 2010, 5, 1918–1929.
(d) Baumgartner, T.; Neumann, T.; Wirges, B. Angew. Chem., Int. Ed.
2004, 43, 6197–6201. (e) Durben, S.; Dienes, Y.; Baumgartner, T. Org. Lett.
2006, 8, 5893–5893. (f) Dienes, Y.; Eggenstein, M.; Kꢀarpꢀati, T.; Sutherland,
T. C.; Nyulꢀaszi, L.; Baumgartner, T. Chem.—Eur. J. 2008, 14, 9878–9889.
(g) Ren, Y.; Dienes, Y.; Hettel, S.; Parvez, M.; Hoge, B.; Baumgartner, T.
Organometallics 2009, 28, 734–740. (h) Romero-Nieto, C.; Kamada, K.;
Cramb, D. T.; Merino, S.; Rodríguez-Lꢀopez, J.; Baumgartner, T. Eur. J.
Org. Chem. 2010, 27, 5225–5231. (i) Durben, S.; Linder, T.; Baum-
gartner, T. New J. Chem. 2010, 34, 1585–1592. (j) Romero-Nieto, C.;
Merino, S.; Rodríguez-Lꢀopez, J.; Baumgartner, T. Chem.—Eur. J. 2009,
15, 4135–4145.
(4) (a) Zhang, L.; Che, Y.; Moore, J. S. Acc. Chem. Res. 2008, 41,
1596–1608. (b) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A.
P. H. J. Chem. Rev. 2005, 105, 1491–1546. (c) Diegelmann, S. R.; Gorham,
J. M.; Tovar, J. D. J. Am. Chem. Soc. 2008, 130, 13840–13841. (d) Guerzo,
A. D.; Olive, A. G. L.; Reichwagen, J.; Hopf, H.; Desvergne, J.-P. J. Am. Chem.
Soc. 2005, 127, 17984–17985. (e) Ajayaghosh, A.; Praveen, V. K. Acc. Chem.
Res. 2007, 40, 644–656.
(5) (a) Faul, C. F. J.; Antonietti, M. Adv. Mater. 2003, 15, 673–683.
(b) Binnemans, K. Chem. Rev. 2005, 105, 4148–4204. (c) Ariga, K.; Hill,
J. P.; Ji, Q. Phys. Chem. Chem. Phys. 2007, 9, 2319–2340. (d) Haketa, Y.;
Sasaki., S.; Ohta, N.; Masunaga, H.; Ogawa, H.; Mizuno, N.; Araoka, F.;
Takezoe, H.; Maeda, H. Angew. Chem., Int. Ed. 2010, 49, 10079–10083.
(e) Wu, D.; Zhi, L.; Bodwell, G. J.; Cui, G.; Tsao, N.; M€ullen, K. Angew.
Chem., Int. Ed. 2007, 46, 5417–5420. (f) Wu, D.; Liu, R.; Pisula, W.;
Feng, X.; M€ullen, K. Angew. Chem., Int. Ed. 2011, 50, 2791–2794.
(6) (a) Kato, T. Angew. Chem., Int. Ed. 2010, 49, 7847–7848.
(b) Ichikawa, T.; Yoshio, M.; Hamasaki, A.; Mukai, T.; Ohno, H.; Kato,
T. J. Am. Chem. Soc. 2007, 129, 10662–10663. (c) Yoshio, M.; Mukai, T.;
Ohno, H.; Kato, T. J. Am. Chem. Soc. 2004, 126, 994–995. (d) Shimura,
H.; Yoshio, M.; Hoshino, K.; Mukai, T.; Ohno, H.; Kato, T. J. Am. Chem.
Soc. 2008, 130, 1759–1765. (e) Yazaki, S.; Funahashi, M.; Kagimoto, J.;
Ohno, H.; Kato, T. J. Am. Chem. Soc. 2010, 132, 7702–7708. (f) Tanabe,
K.; Yasuda, T.; Yoshio, M.; Kato, T. Org. Lett. 2007, 9, 4271–4274.
(13) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Chem. Commun 2009,
4332–4353.
(14) (a) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd
ed.; Springer: New York, 2006. (b) Valeur, B. Molecular Fluorescence,
Principles and Applications, Wiley-VCH, Weinheim, 2002.
(15) Frisch, M. J.; et al. Gaussian 03, Revision E.01; Gaussian, Inc.:
Wallingford, CT, 2007 (see Supporting Information for full reference).
(16) Toassesstheconformationaleffects on the σ*ꢀπ* orbital coupling
in the 1, 2, and3series, Mulliken charges at phosphorus have been calculated
for both the optimized structures (“closed” and “open”) and the crystal
structures of the model compounds (different isomers). The calculations
confirm the conformational effects on the Mulliken charge distribution at
the phosphorus centers, and consequently the HOMO and LUMO energy
levels. These results will be published elsewhere.
(17) (a) Sagara, Y.; Kato, T. Nat. Mater. 2009, 4, 605–610.
(b) Sagara, Y.; Kato, T. Angew. Chem., Int. Ed. 2008, 47, 5175–5178.
(i) Sagara, Y.; Yamane, S.; Mutai, T.; Araki, K.; Kato, T. Adv. Funct.
Mater. 2009, 19, 1869–1875. (j) Zhang, G.; Lu, J.; Sabat, M.; Fraser, C. L.
J. Am. Chem. Soc. 2010, 132, 2160–2162. (k) Nguyen, N. D.; Zhang, G.;
Lu, J.; Sherman, A. E.; Fraser, C. L. J. Mater. Chem. 2011, 21, 8409–8415.
(c) Yoon, S.-J.; Chung, J. W.; Gierschner, J.; Kim, K. S.; Choi, M.-G.;
Kim, D.; Park, S. Y. J. Am. Chem. Soc. 2010, 132, 13675–13683.
(d) Kunzelman, J.; Kinami, M.; Crenshaw, B. R.; Protasiewicz, J. D.;
Weder, C. Adv. Mater. 2008, 20, 119–122. (e) Crenshaw, B. R.; Weder,
C. Chem. Mater. 2003, 15, 4717–4724. (f) L€owe, C.; Weder, C. Adv.
Mater. 2002, 14, 1625–1629. (g) Sagara, Y.; Mutai, T.; Yoshikawa, I.;
Araki, K. J. Am. Chem. Soc. 2007, 129, 1520–1521. (h) Sase, M.;
Yamaguchi, S.; Sagara, Y.; Yoshikawa, I.; Mutai, T.; Araki, K. J. Mater.
Chem. 2011, 21, 8347–8354.
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