Molecular engineering of click -phospholes towards self-assembled luminescent soft materials

Article abstract of DOI:10.1002/adfm.201302294

Inspired by the self-assembled bilayer structures of natural amphiphilic phospholipids, a new class of highly luminescent click -phospholes with exocyclic alkynyl group at the phosphorus center is reported. These molecules can be easily functionalized with a self-assembly group to generate neutral phosphole-lipids . This novel approach retains the versatile reactivity of the phosphorus center, allowing further engineering of the photophysical and self-assembly properties of the materials at a molecular level. The results of this study highlight the importance of being able to balance weak intermolecular interactions for controlling the self-assembly properties of soft materials. Only molecules with the appropriate set of intermolecular arrangement/interactions show both organogel and liquid crystal mesophases with well-ordered microstructures. Moreover, an efficient energy transfer of the luminescent materials is demonstrated and applied in the detection of organic solvent vapors. The molecular engineering of novel click -phospholes toward functional phosphole-lipid soft materials is reported. Appropriate modification of the conjugated head group results in balanced intermolecular interactions and a tailored molecular arrangement that provides access to luminescent organogels and liquid crystals. Efficient energy transfer and vapochromism using one species are also demonstrated.

Full text of DOI:10.1002/adfm.201302294

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Molecular Engineering of “Click”-Phospholes Towards
Self-Assembled Luminescent Soft Materials
Xiaoming He, Jian-Bin Lin, Wang Hay Kan, Pengcheng Dong, Simon Trudel,
and Thomas Baumgartner*
a promising new avenue for efficiently
Inspired by the self-assembled bilayer structures of natural amphiphilic phos-
pholipids, a new class of highly luminescent “click”-phospholes with exo-
cyclic alkynyl group at the phosphorus center is reported. These molecules
can be easily functionalized with a self-assembly group to generate neutral
“phosphole-lipids”. This novel approach retains the versatile reactivity of the
phosphorus center, allowing further engineering of the photophysical and
self-assembly properties of the materials at a molecular level. The results of
this study highlight the importance of being able to balance weak intermolec-
ular interactions for controlling the self-assembly properties of soft materials.
Only molecules with the appropriate set of intermolecular arrangement/inter-
actions show both organogel and liquid crystal mesophases with well-ordered
microstructures. Moreover, an efficient energy transfer of the luminescent
materials is demonstrated and applied in the detection of organic solvent
vapors.
engineering the optoelectronic properties
of conjugated materials at the molecular
level by virtue of their unique structural
(i.e., bonding) and electronic properties.
Organophosphorus materials, in par-
ticular, offer great advantages over other
main group species toward constructing
self-assembled materials, due to the versa-
tile reactivity of the P center (i.e., via mod-
ification of the lone pair with O, S, Me+,
BH3 groups, or metal complexes).[5]
The groups of Weiss and Kato pio-
neered the study of phospholipid-like,
self-assembled ionic phosphonium com-
pounds, by focusing on systems with non-
conjugated head groups.[6] Recently, we
have combined the amphiphilic features
of lipids with the valuable photophysical
features of conjugated phospholes to gen-
1. Introduction
erate several series of conjugated ‘phosphole-lipid’ and ‘phos-
phinine-lipid’ systems with π-conjugated head groups that led
to self-assembled organogels and liquid crystal mesophases
with intriguing photophysical and electrochemical behavior
(Scheme 1a, top).[7] In this context, we were also able to une-
quivocally confirm the supramolecular bilayer organization of
such lipid systems by X-ray crystallography.[7e] The strategy used
for the molecular design of these first-generation conjugated
organophosphorus-lipids, is the quaternization of the P center
with an appropriate self-assembly moiety (Scheme 1a, top),
which unfortunately blocks the P center from further function-
alization and consequently precludes any further tuning of the
optoelectronic properties via P-chemistry avenues.
Over the past several decades, there have been numerous
efforts in incorporating a terminal alkynyl group into molecular
systems as building block for the construction of molecular
wires and optical materials, due to its linear geometry, struc-
tural rigidity, extended π-electron delocalization, and impor-
tantly, its diverse chemical reactivity.[8] Particularly, the Huisgen
alkynyl−azide “click” reaction has gained considerable attention
and has been widely used in the construction of organic conju-
gated materials because of its ease of synthesis, high functional
group tolerance, and excellent reaction yields.[9] It is expected
that incorporation of a terminal alkynyl group into a phosp-
hole system would generate highly valuable functional mate-
rials. However, the very few examples reported by Matano and
co-workers exclusively showcase α-alkynyl phosphole systems
(Scheme 1b).[10]
The self-assembly of luminescent materials with well-organ-
ized 1D or 2D structures has paramount importance for effi-
cient energy- and charge-transport processes in the emerging
field of organic electronics.[1,2] Among these soft materials,
supramolecular organogels and liquid crystals are of particular
interest because they can provide well-ordered bulk phases and
are easily processable at low cost.[1,2] The specific organization
into 1D- or 2D-supramolecular assemblies can be achieved
through weak intermolecular interactions such as π–π stacking,
hydrogen bonding, van der Waals forces, dipole-dipole and/or
ionic interactions.[1,2] The optoelectronic properties of these soft
materials, on the other hand, strongly rely on the π-conjugated
central cores, which have mostly been based on neutral polycy-
clic aromatic hydrocarbons to date.[1,2]
The incorporation of inorganic main-group elements (e.g.,
B,[3] Si,[4] P[5]) into π-conjugated materials has recently created
Dr. X. M. He, Dr. J.-B. Lin, W. H. Kan, P. Dong,
Prof. S. Trudel, Prof. T. Baumgartner
Department of Chemistry and Centre
for Advanced Solar Materials
University of Calgary
2500 University Drive NW, Calgary,
AB T2N 1N4, Canada
E-mail: thomas.baumgartner@ucalgary.ca
DOI: 10.1002/adfm.201302294
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Scheme 1. (a) Top: original ionic ‘phosphole-lipid’ system; Bottom: current design strategy toward neutral phosphole-lipids using a “click”-phosphole;
two conjugated heads (a and b) are used in this study. (b) Molecular structure of α-alkynyl phospholes reported by Matano et al.[10]
In this contribution, we now report a new class of longitu-
dinally alkyne-functionalized phospholes (Scheme 1a, bottom).
The installation of the alkynyl group at this position is expected
to allow for further “click” reactions with appropriately function-
alized azides, while the latitudinal direction along the building
block is open for further tuning of the photophysical proper-
ties by incorporating aromatic moieties. Along these lines, two
different π-conjugated head groups (a: dithienophosphole, b:
bis(benzothieno)phosphole) have been chosen in order to influ-
ence potential π–π stacking interactions. Functionalization with
self-assembly groups would then lead to a new class of neutral
phosphole-lipids, where the phosphorus center is available for
further engineering of the photophysical and self-assembly
properties via P-modification (E). The “click”-functionalization
of the new phospholes with azides affords an aromatic tria-
zole substituent at the P-center that should be beneficial the
overall stability of the scaffold, akin to the well-known, stable
P-phenyl congeners.[11] Due to the sterically bulky nature of the
pyramidal phosphorus center, it is not expected that this group
would play a crucial role in the self-assembly behavior of the
system. This paper now provides details on the synthesis, the
photophysical and electrochemical properties, the molecular
engineering toward self-assembly, as well as intriguing energy
transfer and vapochromism of the new species.
2. Results and Discussion
2.1. Synthesis and Functionalization of the “Click”-Phospholes
The synthesis and functionalization of the new “click”-phosp-
holes is shown in Scheme 2, and the detailed procedures are
Scheme 2. Synthesis of “click”-phospholes and further functionalization with a self-assembly moiety. Reagents and conditions: i) nBuLi, Et2O, −78 °C,
30 min, then TIPSC≡CPCl2, Et2O, rt, 2h; ii) TBAF, THF, −78 °C, 2h; iii) H2O2, THF/CH2Cl2 (1:1, v/v), rt, 2h; iv) CuI, DIPEA, THF, rt, 24h.
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described in the Supporting Information. The central starting
The 31P NMR-spectroscopic data for this family of phosp-
holes are summarized in Table 1. The trivalent alkynyl phos-
phole systems (1a,b, 2a,b) have 31P NMR resonance signals in
the range of −60 4 ppm, which are highly upfield shifted com-
pared to our previously reported trivalent dithienophosphole
systems with P-phenyl group (c.f.: −20 5 ppm).[11] Similar
upfield-shifted 31P NMR resonances were also observed with
dithienophospholes that exhibit an electron-withdrawing pen-
tafluorophenyl group at the P center (c.f.: −51.3 ppm),[11d] as well
as somewhat related ferrocenylalkynylphosphanes that were
reported by us in 2006.[13] The corresponding phosphole oxides
also show similar upfield shifts in the 31P NMR spectrum.
When transforming the alkynyl moiety (2a-O: −10.9 ppm, 2b-
O: −12.0 ppm) into triazole (3a-O: +6.24 ppm, 3b-O: +5.1 ppm),
the 31P NMR signals underwent a significant downfield shift by
ca. +18 ppm, which was used as diagnostic tool for monitoring
this reaction.
materials, the corresponding dibromoaryl precursors[11] and
[12]
TIPSC≡CPCl2
(TIPS = triisopropylsilyl) were prepared
according to reported procedures. With the precursors in hand,
two “click”-phospholes with different π-conjugated head groups,
1a and 1b, were synthesized in 45–60% yields according to our
established procedures for P-phenyl phospholes[11] that involve
the lithiation of the dibromo-precursors, followed by the addi-
tion of TIPSC≡CPCl2 at low temperature. Deprotection of the
TIPS group in compounds 1a and 1b was performed with
tetrabutylammonium fluoride (TBAF) to afford 2a and 2b in
80–90% yield. The trivalent species (2a, 2b) were then quanti-
tatively oxidized with excess H2O2 to yield the respective phosp-
hole oxides 2a-O and 2b-O.
In order to install self-assembly properties within the mate-
rials, the 3,4,5-tris(dodecyl-1-oxy)benzyl-group was successfully
attached via Huisgen alkynyl-azide click reaction involving the
corresponding azide, as well as the phosphole oxides 2a-O and
2b-O. The reaction proceeds under mild conditions by using
CuI as catalyst and diisopropylethylamine (DIPEA) as base
to generate the corresponding 3a-O and 3b-O in high yields
(70–80%). It should be noted in this context that the click reac-
tions using the trivalent species (2a and 2b) were unsuccessful,
probably due to the poisoning of CuI catalyst by coordination
to the P center. All new compounds were fully characterized by
1H, 13C, and 31P NMR spectroscopy, as well as high-resolution
mass spectrometry. Moreover, the solid-state structure of 2b-O
was confirmed by X-ray crystallography; its discussion follows
in a subsequent section.
2.2. Modification of the P center
In order to further explore the availability of phosphorus chem-
istry avenues, the trivalent phosphole species (3a and 3b) are
necessary intermediates. The deoxygenation of the oxides 3a-O
and 3b-O could be achieved in relatively high yield (90–92%)
by reaction with trichlorosilane in refluxing toluene.[10b] With
the trivalent species in hand, we could then further modify the
phosphorus center to evaluate the effect on the self-assembly
properties. Three typical modifications (gold-complex formation,
Table 1. 31P NMR, photophysical, electrochemical and calculation data for the new phosphole compounds.
31P NMR
Media
a)
Compd
λabs
λem
[nm]
φPL
[nm] (ε [dm3 mol−1 cm−1])
1a
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Solid
289 (8260), 328 (8700)
282 (7800), 329 (9200)
372 (5730)
429
423
459
410
449
452
461
452
490
433
435
438
505
434
482
498
512
523
535
0.24
0.35
0.69
0.75
−56.0
−59.6
−10.9
−52.3
2a
2a-O
3a
283 (9538), 331 (9500)
3a-O
+6.2
CH2Cl2
Gel
364 (5980)
0.55
Solid
3a-O-Me
3a-Au
1b
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Solid
388 (5630)
0.33
0.71
0.48
0.57
0.82
0.35
−0.4
352 (5920)
−18.5
−59.0
−63.1
−12.0
−52.2
339 (14820), 365 (14990)
338 (11610), 367 (13960)
336 (9210), 412 (7960)
340 (18920), 368 (22120)
2b
2b-O
3b
3b-O
+5.1
CH2Cl2
Gel
333 (9160), 403 (9710)
0.58
0.35
Solid
3b-O-Me
CH2Cl2
346 (14370), 431 (9200)
−2.1
a)Fluorescence quantum yield, relative to quinine sulfate (0.1 M H2SO4 solution); excitation at 365 nm.
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Scheme 3. A family of new phosphole compounds through P-modification. Reagents and conditions: i) HSiCl3, toluene, 110 °C, 2h; ii) H2O2, THF, rt,
2h; iii) MeOTf, CH2Cl2, rt, 12h; iv) Au(tht)Cl, CH2Cl2, rt, 2h.
methylation, oxidation) were carried out to elaborate the poten-
tial reactivity of the trivalent phosphorus center (Scheme 3).
The conversion to the gold complex 3a-AuCl was readily
achieved by reaction of 3a with Au(tht)Cl (tht = tetrahydrothio-
phene) in CH2Cl2 at room temperature. Moreover, previous work
by others and our group has revealed that ionic interactions of cati-
onic phosphonium salts can provide the necessary driving force for
the formation of soft materials, such as liquid crystals and organo-
gels.[6,7] To this end, trivalent species 3a and 3b were reacted with
methyl triflate (MeOTf) in order to quaternize the P center. How-
ever, N-methylation of the triazole unit was observed exclusively
(Scheme 3), even in the presence of a large excess of MeOTf (10
eq.), which indicates a much higher Lewis basicity of nitrogen
over that of phosphorus. Similar results were also observed for
trivalent azadibenzophosphole systems.[14] The methylation also
occurs at the same nitrogen position with the phosphole oxide
species 3a-O and 3b-O. The reactivity of 3a and 3a-O towards
MeOTf was investigated in greater detail by titration experiments
and monitored via 31P NMR spectroscopy (Figure S1 and S2).
After methylation, the 31P NMR signals for both the trivalent and
the phosphole oxide species underwent an upfield shift by ca.
7–14 ppm (Table 1), which could be attributed to electronic
effects.[11d] In addition, the reactivity of the
trivalent 3a is much higher than that of 3a-O,
in line with the reduced Lewis basicity of the
N-atom in 3a-O due to the increased electron-
accepting features of the P=O moiety vs. a
trivalent P-center. On the other hand, the oxi-
dation of the trivalent species 3a-Me could be
achieved easily by reaction with H2O2. Overall,
as shown in Scheme 3, a family of new phosp-
hole derivatives can be prepared, allowing us to
further investigate the potential tuning of their
photophysical, electrochemical, as well as self-
assembly properties.
2.3. Solid-State Structure
The molecular arrangement and packing
in the solid state play an important role
for the generation of self-assembled mate-
rials. Single crystals of compound 2b-O
that were suitable for X-ray crystallography
were obtained from the slow evaporation of
a concentrated CH2Cl2 solution. The molec-
ular structure and solid-state packing of 2b-O
are shown in Figure 1. The crystal data and
structure refinement are summarized in
Table S1 in the Supporting Information. The
Figure 1. (a) Molecular structure of 2b-O in the solid state (50% probability level), Selected
bond lengths [Å] and angles [°]: P1-O1 1.483(6), P1-C2 1.785(6), P1-C9 1.749(9), C1-C2 1.370(8),
C1-C1a 1.441(9), S1-C1 1.718(6), S1-C4 1.749(6), O1-P1-C2 118.8(3), O1-P1-C9 113.4(4);
(b) Molecular packing of 2b-O; (c) Space-filling representation of the edge-to-face herringbone-
type arrangement with weak O···S interactions between neighboring molecules in the structure
of 2b-O (H atoms omitted for clarity).
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single molecule is highly symmetrical and all bond lengths
and angles are comparable to those observed for previously
reported P-phenyl heteropentacene dithienophospholes.[11c]
In terms of molecular packing, intermolecular π−π interac-
tions between neighboring conjugated backbones (at about
3.5 Å) are observed. In addition, the molecules are organized
in a highly ordered one-dimensional edge-to-face herringbone-
type arrangement, similar to that observed for many other
pentacene derivatives.[15] Remarkably, similar to the P-phenyl
congener,[11c] a peculiar O···S interaction is observed for 2b-O
between the P=O group with the two thiophene S-atoms of a
neighboring molecule. However, the distance of (3.84 Å) in the
current case is larger than in the previously reported P-phenyl
dithienophosphole heteropentacene (c.f. 2.80 Å), probably due
to electronic effects from the different exocyclic substituents at
the phosphorus center.
Notably, such kind of herringbone organization is not
observed for either the regular dithienophospholes, or other
related pentacene phosphole derivatives with different P-mod-
ification (P=S and P-Me+). The heteropentacene phosphole
oxide species thus show a solid-state packing that is unique for
the whole family, and it also strongly impacts the self-assembly
properties of 3b-O (vide infra).
P-phenyl substituted dithienophosphole oxide (λabs = 361 nm,
λem = 446 nm)[11a] and heteropentacene dithienophosphole
oxide (λabs = 385 nm, λem = 483 nm),[11c] both the absorption
and emission maxima of corresponding “click”-phospholes
2a-O (λabs = 372 nm, λem = 459 nm) and 2b-O (λabs = 403 nm,
λ
em = 505 nm) show a modest red shift. After transformation of
the alkynyl moieties to the triazole species, slightly blue-shifted
absorption and emission were observed for 3a-O (λabs = 364 nm,
λem = 452 nm) and 3b-O (λabs = 403 nm, λem = 498 nm). Gold
complex 3a-AuCl (λabs = 352 nm, λem = 433 nm) shows blue-
shifted optical features compared with that of 3a-O and no tran-
sitions resulting from aurophilic interactions are observed.
The emission properties of the 3 series were further inves-
tigated in detail by comparing the CH2Cl2 solution data with
those in the solid state, in order to get some deeper insight into
the molecular organization of the species in the different states.
As shown in Figure 2, the emission of all compounds shows
a large red shift in the solid state. According to our previous
studies, this observation can be ascribed to the presence of π−π
interaction in the solid state.[7d]
The trivalent species 3a (Δλem = 39 nm) and 3b (Δλem
=
38 nm) show larger red-shifted emissions in the solid state than
the corresponding phosphole oxides 3a-O (Δλem = 0 nm) and
3b-O (Δλem = 25 nm), indicating an efficient intermolecular
π−π interaction for the trivalent species, whereas the data for
3a-O (with small conjugated core) suggest the absence of such
interactions. Overall, these data provide valuable information
for the molecular packing in the solid state, which will be fur-
ther discussed in the next section.
2.4. Photophysical and Electrochemical Properties
The photophysical data of the highly fluorescent new phosp-
hole compounds are summarized in Table 1. Compared to the
Figure 2. Emission spectra of 3a (a), 3a-O (b), 3b (c) and 3b-O (d) in different states (CH2Cl2, solid, and gel).
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years.[1] Most of the research to date has focused on the com-
bination of various chromophores with self-assembly units,[1]
however, much less efforts have been invested in tuning the
intermolecular weak interactions to control the self-assembly
behavior by simple molecular modifications. The family of new
compounds reported herein thus provides a great opportunity
towards addressing this feature via molecular engineering of
the conjugated head groups, modification of the P-center, as
well as the introduction of an ionic charge at the N-atom. Inter-
estingly, only the two neutral phosphole oxides 3a-O and 3b-O
form organogels in hydrocarbon solvents (i.e., hexane and hep-
tane). Gel formation requires the heating of the heterogeneous
solvent/gelator mixture to access a homogeneous clear solution
phase that upon cooling to room temperature in air forms an
organogel. The critical gel concentration (CGC) for 3b-O (ca.
4.0 mg mL−1) with large π-conjugated core was determined to
be much smaller than that of 3a-O (ca. 8.5 mg mL−1) with small
core, conceivably resulting from its potential for stronger π−π
interaction from the larger aromatic core. In stark contrast to
the opaque organogel of 3a-O, 3b-O forms a fully transparent
gel, which suggests a highly organized structure in the gel state
of the latter. In addition, the gel formation process of 3b-O (ca.
0.5 hour) is much faster than 3a-O (ca. 2 hours), indicating a
slower molecular reorganization and weaker intermolecular
interactions for 3a-O in the gel state. Both organogels of 3a-O
and 3b-O exhibited thermally reversible sol-to-gel phase tran-
sitions and the thermal stability of the two species (as solids)
was evaluated by thermogravimetric analysis (TGA) (Figure S5).
The decomposition temperatures (defined as the temperatures
of 5% mass loss) were determined to lie over 300 °C, indicating
high thermal stability.
When exposed UV irradiation (365 nm), the organogels of 3a-O
and 3b-O exhibit strong blue and green luminescence, respectively
(Figure 4a and b); the emission spectra are shown in Figure 2. For
3b-O, the emission maximum of the gel (λem = 512 nm) lies in
between that of the solution (λem = 498 nm) and the solid state
(λem = 523 nm), indicating somewhat weakened intermolecular
interactions in the gel, conceivably due to the presence of the sol-
vent. By contrast, the emission maximum of 3a-O in the gel state
exhibits a red shift (λem = 461 nm) compared to the solid state.
To further elucidate the self-assembly process and mor-
phology of the materials, scanning electron microscopy (SEM)
was performed on the dried gels of 3a-O and 3b-O, which were
prepared by placing a slice of the freshly prepared gel onto
a carbon-tape coated standard aluminum stub. In fact, the
micromorphology of the gels is strongly dependent on the π-
conjugated cores. Interestingly, as shown in Figures 4c and d,
3b-O forms a very thin and smooth film on the surface with
some rod-like wrinkles. A closer look reveals that both the
smooth film and wrinkles are actually composed of a series
of highly regular 1D self-assembled fibers with an average
diameter of about 70 nm. The presence of 1D nanofibers in
the organogel of 3b-O from hexane was further confirmed
by confocal fluorescence microscopy (Figure S6). Notably,
this morphology was not observed for 3a-O, whose xerogel
rather forms a 3D network of microscopically entangled 1D
fibers (Figure S7). This observation clearly illustrates the cru-
cial effects of the different π-conjugated scaffolds on the self-
assembly behavior of these two materials.
Figure 3. Cyclic voltammograms of compounds 3a-O, 3a-O-Me, 3b-O
and 3b-O-Me in CH2Cl2 with 0.1M tetrabutylammonium hexafluorophos-
phate as a supporting electrolyte. Scan rate: 100 mVs−1. Ferrocene was
added as internal standard and referenced to 0 V.
N-methylation of 3a-O (Δλabs = 24 nm, Δλem = 37 nm) and
3b-O (Δλabs = 28, Δλem = 37 nm) leads to further red-shifted
UV-vis absorption and emission values (Table 1 and Figure S4).
In particular, the fluorescence of 3a-O and 3b-O in CH2Cl2 solu-
tion visibly changes from blue to green (for 3a-O-Me), and from
green to yellow (for 3b-O-Me), respectively. This feature can be
attributed to the generation of a stronger electron-accepting
moiety upon methylation; a similar red-shifted fluorescence
emission was observed for the dithienophosphole system upon
introducing a electron-withdrawing pentafluorophenyl group
at phosphorus (Δλem = 56 nm).[11d] To further demonstrate the
increased electron-accepting properties, the electrochemical
properties of 3a-O and 3b-O, as well as two methylated com-
pounds (3a-O-Me and 3b-O-Me) were investigated by cyclic vol-
tammetry (vs. ferrocene/ferrocenium; Figure 3). Indeed, the
methylated compounds 3a-O-Me (Ered = −1.85, −2.15 V) and
3b-O-Me (Ered = −1.58, −1.84 V) show lower reduction poten-
tials than their corresponding non-methylated congeners 3a-O
(Ered = −2.17 V) and 3b-O (Ered = −1.92 V), indicating a consid-
erable increase in the electron-accepting ability upon methyla-
tion. In contrast to the non-methylated species that only have
one reduction peak, the methylated species exhibit two reduc-
tion processes, probably due to their cationic nature that can
help counterbalance the negative charge formed during the
process.[7e] However, among these four compounds, only 3b-O
shows a fully reversible reduction process.
2.5. Self Assembly (Organogel)
Organogels are interesting soft materials that can entrap large
volumes of solvents in self-assembled networks through var-
ious balanced weak intermolecular interactions. Moreover,
organogelators based on functional chromophores and dyes
have also attracted significant interest as novel functional
materials for a variety of optoelectronic applications in recent
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Figure 4. (a) Top: Opaque organogel of 3a-O in hexane before (left) and after (right) UV irradiation; Bottom: Proposed molecular organization for
3a-O in the self-assembled gel phase. (b) Top: Transparent organogel of 3b-O before (left) and after (right) UV irradiation; Bottom: Proposed molecular
organization for 3b-O in the self-assembled gel phase. (c) and (d) Variable-pressure SEM images of a xerogel of 3b-O obtained from hexane (landing
energy = 20 kV, ambient pressure = 30 Pa). (e) Polarized optical micrograph of the xerogel of 3b-O at room temperature. (f) Polarized optical micro-
graph of neat 3b-O at 110 °C.
The proposed molecular organizations for the organogels of
3a-O and 3b-O are shown in Figures 4a and b. Based on the
solid-state packing of 3b-O determined by X-ray crystallography,
it is inherently plausible to propose that the well-organized
film results from multiple synergetic driving forces, such as
O···S interactions, π−π stacking and intermolecular hydro-
phobic interactions between the long alkoxy chains; the O···S
interactions in the gel state of 3b-O is also supported by IR spec-
troscopy (Figure S9). However, 3a-O with small π-conjugated
core only has the latter two driving forces, highlighting the ben-
efits of the weak O···S interactions in the formation of self-
assembled soft materials that has never been noticed before.
As mentioned above, gel formation was not observed for the
trivalent species in this family (3a and 3b), further indirectly
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validating the importance of the O···S interaction. In addi-
tion, the trivalent species likely do have stronger π−π interac-
tions than their P-oxo counterparts. All of the results high-
light the crucial importance of correctly balancing the delicate
intermolecular interactions for the formation of soft materials.
Interestingly, when the concentrated hexane solution of 3b was
exposed to air (i.e., O2), a partial gel with strong green emission
was formed, as the result of the oxidation of 3b to 3b-O. This
“off–on” gel formation consequently may offer a promising
approach to detect oxygen.
We were also interested in investigating the effect of ionic
interactions on the self-assembly behavior of the new materials
via 3a-O-Me and 3b-O-Me. However, these two compounds
were found to also not form organogels, suggesting that inter-
molecular ionic interactions are in fact a counterproductive fea-
ture for creating self-assembled organogels with these species.
gel sample moreover showed liquid (or soft) crystal properties
at room temperature, and the POM texture indicated a highly
organized 1D-fiber assembly, consistent with the observation
from SEM and confocal fluorescence microscopy (Figure 4e),
which could generally provide for an easy processing of liquid
crystal materials from self-assembled gels.
The liquid crystalline nature and phase transitions of 3b-O
were further supported by VT-PXRD experiments (Figure 5b).
The sample was first melted at its isotropic liquid phase state
(150 °C), where no signals were observed, suggesting an amor-
phous phase as would be expected for melts. Upon cooling to
110 °C, a single sharp peak appeared at 2θ = 2.2, corresponding
to a d-spacing value of 40.1 Å, which was assigned to a smectic
phase indexed to the (100) plane, according to our previous
studies.[7] However, the thickness of the layer (d-spacing) of
3b-O is much larger than our previously reported phosphole-
lipid or phosphinine-lipid system (d ≈ 26–27 Å),[7] suggesting
weaker van der Waals interactions in current system. On the
other hand, a similarly clear phase transition was not observed
for 3a-O in the VT-PXRD experiments (Figure S10).
2.6. Self Assembly (Liquid Crystals)
As mentioned in the introduction, another intriguing form of
self-assembled functional materials are liquid crystals.[2] After
screening this family of new phospholes for this property,
only compound 3b-O showed liquid crystalline behavior. The
liquid crystal properties of 3b-O were investigated by differen-
tial scanning calorimetry (DSC), polarized optical microscopy
(POM) and variable-temperature powder X-ray diffraction (VT-
PXRD). The DSC experiments revealed several thermal phase
transitions (Figure 5a). Upon heating 3b-O from −50 °C, one
endothermic (at 103 °C) and one exothermic (at 110 °C) tran-
sition were observed, which were tentatively assigned to a
solid-to-liquid crystal, and liquid crystal-to-liquid crystal tran-
sition, respectively. As the temperature increased to 143 °C,
compound 3b-O started to exhibit phase transitions to an iso-
tropic liquid, where a dark homeotropic area was observed. The
return cooling showed two exothermic signals at 121 °C (small)
and 74 °C (intense), which were assigned to the liquid-to-liquid
crystal, and liquid crystal-to-soft crystal transitions. Multiple
DSC scans revealed reversibility, supporting high stability of
the compound. The formation of a liquid crystal phase of 3b-O
was also confirmed by POM (Figure 4f). Surprisingly, the dried
2.7. Energy Transfer and Vapochromism
Luminescent organogels provide a high degree of self-organ-
ization for luminophores, which has been recognized as cru-
cial for efficient energy-transfer species that mimic natural
light-harvesting systems.[1a,16] Given the broad spectral overlap
between the fluorescence of 3b-O (as donor) and the absorp-
tion of Rhodamine B (as acceptor) (Figure S11), it was expected
that efficient fluorescence resonance energy transfer (FRET)
would occur in a mixture of the two species.[1a,7b,16] Because
of the low solubility of Rhodamine B in the hydrocarbon sol-
vents used for the gelation, the energy-transfer process was
studied in the solid state instead of the gel state.[17] Figure 6a
shows the emission spectral changes of a drop-cast film of 3b-O
(from CH2Cl2 solution) blended with 1 mol% of Rhodamine B.
Remarkably, the energy-transfer process was found to heavily
rely on the presence of solvent residues. Upon prolonged
evaporation times, the emission from Rhodamine B gradually
increased, while the emission of 3b-O concomitantly decreased.
Figure 5. (a) DSC traces of 3b-O with the heating and cooling rate at 5 °C min−1 (negative heat flow values indicate an endothermic process); (b) powder
X-ray diffraction of 3b-O at variable temperatures.
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Figure 6. a) Emission spectra of 3b-O:Rhodamine B (100:1) mixture in CH2Cl2 solution (solid line) and the solid state (dotted, dashed lines). b) Picture
showing the experimental setting (left) and fluorescent vapochromism (right).
otherwise noted, used without further purification. Solvents were dried
using an MBraun solvent purification system prior to use. Dibromo-
Complete energy transfer was found to occur after ca. 12 h.
Such sensitivity towards trace of solvent residue is quite useful
and could be utilized in the detection of volatile organic com-
pound (VOC) vapors.[18] Indeed, as shown in Figure 6b, four
spots composed of 3b-O:Rhodamine B (100:1) film were pre-
pared by drop-casting and dried overnight for efficient energy
transfer to occur.
[12]
precursors[11] and TIPSC≡CPCl2 were prepared according to reported
procedures. The detailed synthetic procedures and characterization
for the new compounds are described in the Supporting Information.
All reactions and manipulations were carried out under a dry nitrogen
atmosphere employing standard Schlenk techniques.
Instruments and Methods: 31P{1H} NMR, 1H NMR, and 13C{1H}
NMR were recorded on Bruker DRX400 and Avance (-II,-III) 400 MHz
spectrometers. Chemical shifts were referenced to external 85% H3PO4
(31P) or residual non-deuterated solvent peaks (1H, 13C). Elemental
analyses were performed in the Department of Chemistry at the University
of Calgary. Mass spectra were run on a Finnigan SSQ 7000 spectrometer
or a Bruker Daltonics AutoFlex III system. All photophysical experiments
were carried out on a Jasco FP-6600 spectrofluorometer and UV/vis−NIR
Cary 5000 spectrophotometer. Cyclic voltammetry analyses were performed
on an Autolab PGSTAT302 instrument, with a polished glassy carbon
electrode as the working electrode, a Pt-wire as counter electrode, and
an Ag/AgCl/KCl 3M reference electrode, using ferrocene/ferrocenium as
internal standard. If not otherwise noted, cyclic voltammetry experiments
were performed in dichloromethane solution with tetrabutylammonium
hexafluorophosphate (0.1 M) as supporting electrolyte. Crystal data and
details of the data collection are provided in Table S1. Diffraction data were
collected on a Nonius Kappa CCD diffractometer, using Mo Kα radiation
(λ) 0.71073 Å (graphite monochromator). The structures were solved by
direct methods (SHELXTL) and refined on F2 by full-matrix least-squares
techniques. CCDC-949092 contains the supplementary crystallographic
data of 2b-O for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif. Scanning electron microscopy was performed
using a Zeiss Σigma VP microscope equipped with a field-emission gun.
Xerogels of 3a-O and 3b-O in hexane were prepared by placing a slice of
the freshly prepared gel onto a carbon-tape coated standard aluminum
stub, and imaging the samples as rapidly as possible following loading
into the instrument chamber. For all samples secondary electron images
were acquired with a landing energy of 20 kV in variable pressure mode
(P = 29 to 30 Pa of air) to mitigate the effects of charging. Fluorescence
confocal microscopy was performed on Leica inverted confocal TCS SP2
with water immersion lense. Powder XRD was measured by using Bruker
Advance D8 powder X-ray diffractometer (CuKα, 40 kV, 40 mA). Thermal
analyses were performed using TA-Q200 DSC instrument.
When exposed to various solvent vapors (CH2Cl2, CHCl3,
diethyl ether, acetone and methanol), the lower spots change
their emission color very quickly from orange to green, while
the top spots without exposure to vapor keep their original
orange emission color. Despite the lack of selectivity, this pro-
cess could find potential utility in general air quality testing
indicating the presence of VOCs.
3. Conclusion
In conclusion, the synthesis of a family of novel “click”-
phospholes and their molecular engineering toward highly
luminescent phosphole-lipid soft materials is reported. The
photophysical, electrochemical, and particularly self-assembly
properties can be tuned by tailoring the π-conjugated phos-
phole heads, as well as simple modifications of the P- and
N-centers. Among this family, compound 3b-O shows the best
self-assembly behavior leading to highly ordered structures in
the presence, but also absence of solvents. The self-assembled
microstructures were investigated with SEM, VT-PXRD, con-
focal fluorescence microscopy and POM. These results suggest
that 3b-O has the most balanced intermolecular interactions
(π−π stacking, O···S interaction, hydrophobic–hydrophobic
interactions) that lead to a suitable molecular arrangement for
the formation of both organogels and liquid crystals. A practical
application of the intriguing self-assembly and luminescent
properties of 3b-O involves the energy transfer studies between
3b-O and Rhodamine B in a thin film. The energy transfer is
found to be strongly dependent on the presence of solvent resi-
dues, and can successfully be applied in organic solvent vapor
detection.
Preparation of Materials for Vapor Detection:
A CH2Cl2 solution
containing 3b-O (1 mM) and Rhodamine B (1.0 × 10−5 M) was prepared,
followed by deposition of the solution to 4 spots on a glass slide by drop
casting. Then the glass slide was air-dried overnight before the experiment.
Supporting Information
4. Experimental Section
Supporting Information is available from the Wiley Online Library or
from the author.
Materials and Synthesis: All chemical reagents were purchased from
commercial sources (Sigma-Aldrich, Alfa Aesar, Strem) and were, unless
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Acknowledgements
Financial support by the NSERC of Canada, as well as the Canada
Foundation for Innovation (CFI) is gratefully acknowledged. We thank
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