Tuning the aggregation-induced enhanced emission behavior and self-assembly of phosphole-lipids

Article abstract of DOI:10.1039/c5tc03311j

Herein, we report on the synthesis, self-assembly, as well as the photophysical properties of a novel series of P-benzylated phosphole-lipids. In the context of this structure-property study we have systematically altered the number, position, and length

Full text of DOI:10.1039/c5tc03311j

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Tuning the aggregation-induced enhanced
emission behavior and self-assembly of
Cite this:DOI: 10.1039/c5tc03311j
phosphole-lipids†
Zisu Wang, Benjamin S. Gelfand, Pengcheng Dong, Simon Trudel and
Thomas Baumgartner*
Herein, we report on the synthesis, self-assembly, as well as the photophysical properties of a novel
series of P-benzylated phosphole-lipids. In the context of this structure–property study we have system-
atically altered the number, position, and length of the alkyl chains in the 3-,4-, and 5-position of the
benzyl group. Both the self-assembly and photophysical properties of the compounds were found to
correlate strongly with the alkyl chain length and chain arrangement of the mesogenic moiety.
Received 13th October 2015,
Accepted 12th December 2015
DOI: 10.1039/c5tc03311j
www.rsc.org/MaterialsC
Introduction
Organic materials that contain main group elements, such
as P,1a–d B,1e,f and Si,1g are experiencing a surge in interest. This
is due to the unique structural and electronic properties these
elements can impart on the organic scaffold of the material.
Among structural motifs commonly found in organo-main
group molecules, phospholes are a unique class of compounds.1a
Fig. 1 The unique electronics of phospholes. (a) General bonding patterns
of phospholes; (b) s*–p* interaction in phospholes.
Distinct from the analogous pyrroles, phospholes are partially Some of the phosphole-lipids also display gelation properties
aromatic, as the phosphorus lone pair has only little overlap in a wide range of solvents at low concentrations.2b The self-
with the p orbital of the backbone.1a The limited aromaticity of assembly features of phosphole-lipids are the results of a
the molecule actually comes from the interaction of the s* balancing act between intermolecular p–p interactions of the
orbital of the P–R single bond and the p* orbital of the phosphole backbones, the intermolecular ionic interactions
conjugated scaffold (Fig. 1). Due to these unique interactions, and the thermal disorder imparted by the flexible alkyl chains.
chemical modification of the phosphole lone pair can have a As can be seen in Fig. 2, with larger p conjugation in its
considerable effect on the optoelectronic properties of the resulting phosphole backbone, the increased intermolecular p–p inter-
compounds. By adjusting the substituent ‘E’ on the lone pair, actions in 1a led to a more crystalline soft crystal phase unlike
the emission properties of the compounds can be easily tuned the liquid crystal phase found in the smaller DTP. Therefore, we
without the need for modification of the backbone.2
became interested in further exploring this delicate balance
Over more than a decade, our group has been involved with between the driving forces and tuning the self-assembly feature
the development of functional conjugated organophosphorus of our phosphole-lipid system in more detail.
species and we have established various highly desirable optoe-
In phosphole-lipids, quaternization at the phosphorus lone
lectronic features for the materials.3 To expand the general pair affords the ionic feature of the compounds, which is a
utility of such species in a materials context, we have recently critical directing force for the self-assembly process. At the
reported on a series of self-assembled dithienophosphole- same time, due to the aforementioned s*–p* hyperconjugation,
based materials. These ‘phosphole-lipids’ exhibit liquid crystal- the benzyl moiety also heavily participates in the optical
line properties over a wide temperature range (Fig. 2).4a transitions of the molecule. As such, the rotational flexibility
of the benzyl moiety also impacts the optoelectronic behavior of
the compounds both in solution and in the solid state.4a,c We
were further able to confirm that stimuli-responsive features of
Department of Chemistry and Centre for Advanced Solar Materials, University of
Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada.
E-mail: thomas.baumgartner@ucalgary.ca
phosphole-lipids such as aggregation-induced enhanced emis-
† Electronic supplementary information (ESI) available. CCDC 1431111. For ESI
sion (AIEE) and mechanochromism also stem from this parti-
cular role of the benzyl moiety.4
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c5tc03311j
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Results and discussion
Synthesis
To gain deeper insights into the underlying parameters
required for the self-assembly and ultimately the AIEE of the
phosphole-lipid system, a variety of target compounds were
chosen for this study, in which we have selectively altered the
number and position of the benzyl alkyl chains as well as their
length (Scheme 1).
The dodecyl-functionalized species (2a, 3a and 4a) were
aimed at paralleling and expanding our original studies4 in
order to see if a different placement of the dodecyl chains led
to distinct self-assembly behavior and/or altered electronics.
To this end, we expected to pinpoint and systematically study
the effect of chain arrangements on the resulting solid-state
properties. The octyl-functionalized species (1b, 2b, 3b and 4b)
were targeted to study the effects of a shifted balance with
regard the potential intra- and intermolecular interactions via
van-der-Waals and p-stacking interactions.
The di(benzothieno)phosphole S1 was synthesized following
our reported procedure.6 The various alkoxybenzyl bromides
S2 were obtained in high yields by following the standard
procedure reported by Zhao et al.7 Quaternization of the
phosphorus center afforded the target compounds in modest
to good yields (Scheme 1).
To also test the counteranion as effective handle for tuning
the self-assembly properties of phosphole-lipids, a model study
was conducted with the cation of 4a and the dodecylsulfate
anion. Dodecylsulfate was chosen because its ability to promote
amphiphilic properties8 and its expected compatibility with the
dodecyl chains already present in the cationic portion of 4a.
The exchange of the bromide for dodecylsulfate was accom-
plished by repeated aqueous extraction of an equimolar solution
of the phosphole-lipid 4a and sodium dodecylsulfate (Na+DSÀ)
in dichloromethane to yield the anion-exchanged species 4a-DS
in 81% yield.
Fig. 2 Phosphole-lipid liquid crystals. S, ‘‘solid’’; Cr, crystalline state; SmX,
soft crystal; Sm, smectic phase; I, isotropic liquid.
AIEE is the phenomenon where the aggregated-state fluorescence
quantum yield of the compound exceeds that of the compound
in solution.5 Contrary to the more common aggregation-caused
quenching (ACQ) of fluorescence that is found in organic
chromophores, AIEE-enabled materials hold more appeal in
application settings such as OLED displays. As well, due to the
increased fluorescence of the compounds in the solid state, these
materials provide interesting opportunities in studying the photo-
luminescence process in the solid state.5 Tetraphenylethylene-
containing compounds and phenyl-appended siloles are two of
the most studied AIEE systems for their excellent response to
concentration variations not only in solution, but also in the
solid state (Fig. 3).5 The AIEE in both systems stems from the
free/restricted intramolecular motion of conjugated peripheral
substituents.
Photophysical properties
While the general principle of AIEE can be understood with
relative ease, the effect of self-organization on AIEE still needs
to be better defined. To this end, the well defined self-assembly
features of the phosphole-lipid system enable us to gain a
glimpse into the relationship between self-assembly and AIEE.
Herein, we report on our efforts in further exploring and
enhancing both the self-assembly and the AIEE properties of
phosphole-lipids. In the context of this study, the di(benzothieno)-
phosphole-based lipid was chosen because of the promising
packing features of the p-extended head group.4
UV-Vis absorption spectra. To establish a baseline for
the photophysics of the new compounds, UV-Vis absorption
spectra in dichloromethane solution were collected on all
Scheme 1 Synthesis of phosphole-lipids. Quaternization of S1 with various
benzyl bromides (S2) afforded the desired phosphole-lipids 1–4. The bromide
anion was exchanged with dodecylsulfate via metathesis.
Fig. 3 Examples of common AIEE chromophores. (a) Tetraphenylethylene;
(b) hexaphenylsilole.
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Table 1 Photophysical features of the phosphole-lipids
log ea
lem solution (nm)
fPL solution
lem solid-state (nm)
fPL solid-state
AIEE responsed (%)
b
c
Compound
labs (nm)
1b
2a
2b
3a
3b
4a
4b
4a-DS
350, 415
350, 416
350, 415
347, 413
348, 414
347, 417
349, 416
348, 417
4.02, 3.88
4.12, 3.99
4.11, 3.96
3.98, 3.86
4.07, 3.96
4.05, 3.90
4.07, 3.92
4.04, 3.90
n/ae
514
512
n/ae
n/ae
524
522
524
0.00
0.26
0.26
0.00
0.00
0.13
0.10
0.17
n/ae
526
529
n/ae
529
517
518
529
0.00
0.03
0.05
0.00
0.09
0.33
0.19
0.10
0
À23
À21
0
9
20
9
À7
a
b
Molar absorption coefficient (MÀ1 cmÀ1). Relative, measured against a quinine sulfate standard adjusted to the same absorbance, lex = 365 nm.
Absolute, determined using an integrating sphere, lex = 365 nm. Obtained by subtracting fPL solution from fPL solid-state. Negative value indicates
c
d
e
aggregation quenching, positive value indicates AIEE. Not available due to low signal to noise ratio.
compounds (Table 1). As can be seen in Fig. 4a, the absorption highly fluorescent phosphole-lipids in solution studied to date
spectra of all the phosphole-lipid species are quite similar; two (2a fPL = 26%, 2b fPL = 26%). The 4-series of compounds, with
absorption peaks are observed (at approx. 350 nm and 415 nm, two chains in the 3,5-positions, is also quite fluorescent (4a
respectively). The extinction coefficients of the compounds fPL = 13%, 4b fPL = 9%, 4a-DS fPL = 16%). However, despite
range between 0.9 and 1.3 Â 104 MÀ1 cmÀ1 (350 nm), or 0.75 the highly analogous structural elements to the 4-series, the
and 1 Â 104 MÀ1 cmÀ1 (415 nm), respectively (Fig. 4a).
3-series of compounds, with 3,4-positioned chains, is essentially
Solution fluorescence spectra. Solution fluorescence spectra non-emissive in solution. This contrast between the 3- and
were also collected for all compounds, as it is a key component of 4-series was puzzling at first given their similar structure.
AIEE (Table 1, Fig. 4b). Compared to the previously studied 1a,4a Compound 1b mirrors the original 1a species in its low emissive
a few of the newly synthesized phosphole-lipids are significantly properties in solution.4a
more fluorescent in solution. The 2-series of compounds, with
only one chain in the 4-benzyl position, is among the most
Theoretical calculations
Mechanism of AIEE in phosphole-lipids. The AIEE features
of the phosphole-lipids relate intimately to the frontier mole-
cular orbitals (FMOs) of the molecules.4 In an earlier report,4a
the calculated FMOs of MS and M1 (Fig. 5, in which the alkyl
chains are truncated by simple methyl groups) were used
to illustrate the mechanism of AIEE in the phosphole-lipid
system. The spatial separation of the Highest Occupied Molecular
Orbital (HOMO; primarily the p orbital of the benzyl moiety) and
the Lowest Unoccupied Molecular Orbital (LUMO; primarily
the p* orbital of the dithienophosphole head group) is the
key to the AIEE of MS and related compounds. In MS, after a
Fig. 4 Solution photophysics of phosphole-lipids. (a) UV-Vis absorption
spectra of the phosphole-lipids; (b) fluorescence spectra of phosphole-
lipids (low emission species was omitted for clarity due to low signal to
Fig. 5 Mechanism of AIEE in phosphole-lipids.
noise ratio).
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photo-excitation from the HOMO into the LUMO, the relaxation
M2, on the other hand, deviates significantly from the other
process also involves the transfer of electron density spatially species in this study (Fig. 6). Both its HOMO and LUMO reside
through the methylene linker at the benzylic position (photo- mostly on the p backbone of the conjugated phosphole head
induced electron transfer, PET). The coupling between the group. In addition, while spatial separation exists between
relaxation process and the rotation of the benzylic methylene HOMOÀ1 and LUMO in M2, the weak donating character of
opens up non-radiative decay pathways and leads to quenching a single alkoxy substituent leads to a significant stabilization
of fluorescence in solution. In the solid state, however, the of HOMOÀ1, creating a considerable energy gap between
restriction of the benzylic rotation leads to enhanced fluores- HOMO and HOMOÀ1. As a result of the energy difference,
cence in the case of MS. This is the basis of AIEE in the the LUMO - HOMOÀ1 fluorescence transition is suppressed.
phosphole-lipid system.4a
Consequently, the compounds should behave much like other
In M1, however, significant overlap exists between the FMOs non-AIEE chromophores where the solution fluorescence is
of the species (Fig. 5). The PET process is therefore suppressed high. This prediction fits rather well with the observed high
in this case, which leads to the lack of AIEE observed in the fluorescence of the compounds in solution.
previously reported 1a species.
Since the AIEE mechanism relies on the delicate energy
Static DFT calculations. To gain insights into the frontier orbital balance between various orbitals, the conformation of the alkoxy
energies and the optical transitions of the di(benzothieno)- substituents on the benzyl moiety was also investigated. The
phosphole-lipids in this study and to provide a rationalization for optimized structure (M4) placed the alkoxy group in plane with
the peculiar emission properties of the compounds in solution, the benzene ring, which is quite reasonable as this conformation
theoretical calculations at the B3LYP/6-31G+(d) level of theory (PCM permits the conjugation between the p orbitals of the oxygen
solvation model in CH2Cl2) were conducted on model molecules and the benzene ring. This conformation is also observed in
M2, M3 and M4 using the Gaussian09 suite of programs (Fig. 6).9 the single crystal X-ray data of compound 4a (vide infra). DFT
Contrary to the observed photophysics, M3 and M4 are calculations were also conducted based on the X-ray crystallo-
electronically quite similar to each other according to the graphic data of 4a (Fig. 7, M4-XRD with the dodecyl chains
calculations. The LUMO (primarily the p* orbital of the con- truncated to methyl for efficiency). With the endo-conformation
jugated phosphole head group), HOMO (primarily the p orbital of the benzyl substituent, there exists significant orbital com-
of the benzyl moiety) and HOMOÀ1 (primarily the p orbital of munication between the benzyl substituent and the phosphole
the phosphole head group) all have the same distribution in backbone. As a result, both the HOMO and the HOMOÀ1 were
both species. A minor stabilization of the HOMO exists in M4, slightly stabilized compared to the optimized structure. This
which can be attributed to the weaker p-donating nature of the enlargement of the HOMO–LUMO gap was reflected in the
meta-alkoxy substituent. Similar to MS, M3 and M4 are both slightly blue-shifted fluorescence of 4a in the solid state as
PET-enabled making them strong candidates of AIEE features. compared to the solution (Dl = 7 nm). To our surprise, a
Combining this result with the solution fluorescence observa- profound effect was observed, when the alkoxy group was rotated
tions, we can tentatively draw the conclusion that the significantly 901 with respect to the benzene plane, suppressing the lone pair
enhanced emissions of the 4 compounds do not originate from p-conjugation (Fig. 7, M4-alkoxy rotated). The lack of p donation
electronics but rather from different self-assembly in solution.
Fig. 7 DFT calculations of M4 as optimized (left), based on X-ray data
Fig. 6 Frontier orbitals of model phosphole-lipids.
(middle) and M4 with the alkoxy groups rotated (right).
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results in a significant stabilization (À0.58 eV) of the benzyl (B415 nm) and the high-energy (B350 nm) absorption bands
p orbital, which leads to the promotion of the phosphole are dominated by the p–p* transition of the phosphole head
p orbital to become the HOMO. In this configuration, the AIEE group. Benzyl involvements were rather limited in the excitation
should be suppressed. However, considering the possibility processes. Since the di(benzothieno)phosphole head groups are
of somewhat free rotation of the alkoxy groups in solution the same in all compounds, it is no surprise that the absorption
(at least to some extent), would explain the observed AIEE, but spectra are similar in all cases.
also accounts in part the residual fluorescence of compounds
4a,b in solution. This finding also indirectly supports the distinct
emission features of the 3-series in solution, as the ortho-
relationship of the two alkoxy chains seems to restrict their free
rotation, in contrast to the 3,5-arrangement in 4a,b.
Time-dependent DFT calculations. While static calculations
can provide a basis for discussion of the photophysical features
of the compounds, time-dependent (TD) DFT calculations
are needed to gain the full picture regarding the dynamic
opto-electronic transition processes. The calculated oscillator
strengths and the nature of the key transitions are summarized
in Table 2.
As can be seen in Fig. 6, the HOMO–LUMO energy gaps
appear to be quite varied. However, the UV-Vis spectra of the
compounds were rather similar to each other (Fig. 4a). This
discrepancy can be easily resolved by considering the TD-DFT
calculation results. According to TD-DFT, both the low energy
Solvatochromism
To substantiate the PET process in the new phosphole-lipids,
solvatochromism studies were carried out for two representa-
tive species 2a and 4a. These compounds were chosen for their
contrasting FMO arrangements (Fig. 6, M2 and M4).
Since the intramolecular PET process relies on the polar
charge-separated excited state, polar solvents are more capable
of stabilizing the excited state. Therefore it follows that, with a
polar solvent, a bathochromic shift of the absorption peaks
should be observed compared to a nonpolar solvent if the PET
is at play in the relaxation process.
As can be seen in Fig. 8, 4a indeed demonstrates significant
solvatochromism where the growth of the low-energy charge-
transfer bands is accompanied by the increase of solvent polarity.
On the contrary, 2a had limited response to solvent polarity.
As such, the solvatochromism of 4a in conjunction with the DFT
results provides evidence of the presence of PET in phosphole-
lipids with suitable FMO arrangements.
Table 2 Calculated key photo-transitions
Calculated absorption bands
Nature of the transition
M1
HOMO - LUMO (95%)a
HOMOÀ1 - LUMO (4%)
(436 nm, fb = 0.2821)
p
p
phos–p*phos
benzyl–p*phos
HOMOÀ3 - LUMO (88%)a
HOMOÀ4 - LUMO (10%)
(358 nm, fb = 0.1636)
p
phos–p*phos
HOMOÀ4 - LUMO (88%)a
HOMOÀ3 - LUMO (10%)
(352 nm, fb = 0.1226)
p
phos–p*phos
M2
M3
HOMO - LUMO (99%)a
(441 nm, fb = 0.2657)
p
phos–p*phos
HOMOÀ2 - LUMO (97%)
p
p
p
phos–p*phos
benzyl–p*phos
phos–p*phos
(360 nm, f = 0.1252)
HOMOÀ3 - LUMO (97%)
(358 nm, f = 0.182)
HOMOÀ1 - LUMO (99%)
(439 nm, f = 0.2889)
p
phos–p*phos
HOMOÀ2 - LUMO (98%)
p
p
p
phos–p*phos
benzyl–p*phos
phos–p*phos
(364 nm, f = 0.077)
HOMOÀ3 - LUMO (97%)
(357 nm, f = 0.1814)
HOMOÀ4 - LUMO
p
p
phos–p*phos
benzyl–p*phos
(347 nm, f = 0.04)
M4
HOMOÀ1 - LUMO (99%)
(435 nm, f = 0.2818)
p
phos–p*phos
HOMOÀ2 - LUMO (94%)
HOMOÀ3 - LUMO (3%)
(363 nm, f = 0.1355)
HOMOÀ2 - LUMO (2%)
HOMOÀ3 - LUMO (93%)
HOMOÀ4 - LUMO (2%)
(357 nm, f = 0.1849)
p
p
phos–p*phos
benzyl–p*phos
p
p
phos–p*phos
benzyl–p*phos
Fig. 8 Solvatochromism of 2a (top) and 4a (bottom). All experiments
were conducted at 10À5 M to mitigate effect of aggregation. (a) Absorption
of 2a in various solvents; (b) absorption of 4a in various solvents, note the
disappearance of the high-energy band at 300 nm and the appearance of
the low energy bands at 350 nm and 420 nm, respectively.
a
Contribution of the particular transition to the absorption band.
f = oscillator strength of the transition as calculated by TD-DFT.
b
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Self-organization and AIEE
As discussed above, the AIEE of phosphole-lipids relies heavily
on the intramolecular rotation of the benzyl moiety. Therefore,
self-organization of the molecules both in solution and in the
solid state plays an important role in tuning the AIEE properties
of the compounds. To better understand and further explore
and leverage this important relationship, the self-organization
of the phosphole-lipids was studied with an array of methods.
Concentration-dependent NMR studies and solution fluores-
cence. The molecular self-organization of the compounds in solution
1
was studied by concentration-dependent 31P and H NMR spectro-
scopy (see ESI† for spectra).
As can be seen in Table 3, the 31P NMR signal shows distinct
shifts as a function of both concentration and species investigated. Fig. 9 (a) Proton assignments on the benzo moiety; (b) the proposed
dimerization process of phosphole-lipids. Note the position of proton Ha.
As concentration differences lead to conformational changes at
the benzylic methylene,4 the shift of the 31P NMR signals was
correspondingly assigned to conformational variations.
potential for response to intermolecular p–p interactions (Fig. 9). At
With a shift of only 77 ppb, 4a (3,5-C12 chains) experiences the low concentrations, the molecules appear more monomeric with
least changes. This small response to a relatively large concen- little intermolecular interactions. With increased concentration,
tration range (10À3 – 10À1 M) indicates a more rigid conforma- increased intermolecular p-interactions between two phosphole
tional environment around the phosphorus center, where the head groups occur (Fig. 9b). Since in almost all phosphole systems
rotation of the benzylic methylene group is restricted. This rigidity p stacking almost always exists in a slip-stack fashion, proton Ha
of 4a in solution was also confirmed by the fluorescence quantum
yield of the compound in solution. While theory predicted 4a to ment with increased p–p interaction.10
should experience the most change in its chemical environ-
1
have limited fluorescence intensity in solution, in actuality the
compound exhibited significant fluorescence in solution (fPL
The results of the concentration-dependent H NMR spectro-
scopy studies are summarized below. As can be seen in Table 4, 4a
=
13%). Compound 4b, the octyl analogue of 4a displays compar- displayed the least amount of changes to concentration variation.
able fluorescence (fPL = 10%). The rigidity of the molecule along This is in line with the 31P NMR spectroscopy studies and
with the free rotation of the alkoxy substituent (vide supra) confirms the rigidity of the compounds in solution. Even at rather
combined, give the 4-series of compounds increased solution high concentration (10À1 M), 4a still exhibited significant mono-
fluorescence as compared to other phosphole-lipids.
meric characteristics. The concentration response of 3a clearly
By contrast, the 31P NMR signal of 3a (3,4-C12 chains) experiences contrasts with 4a. This prominent response of 3a also confirms
a shift of 360 ppb, indicating a considerably more flexible conforma- the 31P NMR results and indicates a highly flexible system, where
tional environment. In this case, this flexibility was mirrored by the rotation of the benzyl group can easily accommodate the
the non-emissive nature of the compound in solution; the octyl dimerization of the compounds. Compound 2a also appears to be
analogue 3b was also non-emissive in solution.
more flexible than 4a, albeit to a lesser degree than 3a.
While 2a (4-C12 chain) was comparable to 3a in terms of 31
P
The results of concentration-dependent 31P and 1H NMR
NMR shift (232 ppb), the solution fluorescence of the com- studies demonstrate empirically that the attachment pattern of
pound was quite intense (fPL = 26%) and that of 2b (4-C8 chain)
the alkyl chains has considerable impact on the solution self-
was equally strong. According to the theoretical prediction, assembly process. The 3,5-bisalkyl arrangement (4a) displays
the PET process was suppressed in this compound. As such, significant rigidity and resistance to intermolecular interactions in
the high fluorescent quantum yields of the 2-series confirm the solution, while the 3,4-bisalkyl and 4-alkyl arrangement appear to
result of the DFT-calculated results.
be rather flexible in their scaffold. Despite the fact this difference
While 31P NMR spectroscopy revealed the intramolecular in rigidity being reflected by the solution emission properties of
interactions of the compounds in solution, 1H NMR spectro- these compounds, the newly synthesized phosphole-lipids were
scopy was able to provide key information on the intermolecular also studied in the solid state to better understand the mechanism
p–p interactions of the compounds at various concentrations.
for these self-assembly processes.
The benzo-proton Ha was selected for examination due to its
Solid-state self-organization and fluorescence. The self-
organization of the compounds in the solid state was studied
Table 3 Concentration-dependent 31P NMR shifts of the phosphole-
lipids
Table 4 Concentration-dependent 1H NMR shifts of Ha in the phosphole-lipids
d1H (10À1 M)/ppm
d1H (10À3 M)/ppm
Dd1H/ppb
Compound
d31P (10À1 M)/ppm
d31P (10À3 M)/ppm
Dd31P/ppb
Compound
2a
3a
4a
19.6769
20.4754
20.5358
19.9097
20.8383
20.6126
233
363
77
2a
3a
4a
8.1214
8.1066
8.2226
8.1411
8.1819
8.2080
20
75
15
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Table 5 Self-assembly features and solid-state fluorescence quantum
yields of phosphole-lipids
a
Compound
Self-assembly features
Solid-state fPL (%)
1b
2a
Viscous liquid/liquid crystal
Amorphous
0
5
2b
3a
Soft crystal/liquid crystal
Amorphous
3
0
3b
4a
4b
4a-DS
Amorphous
Crystalline
Crystalline
Soft crystal/liquid crystal
9
33
19
11
a
Absolute, determined using an integrating sphere.
by differential scanning calorimetry (DSC), polarized optical
microscopy (POM) and powder X-ray diffractometry (PXRD).
The results are summarized in Table 5 (see ESI† for details on
thermograms, POM pictures, and powder diffractograms).
As can be seen in Table 5, the self-assembly features of the newly
synthesized phosphole-lipids range from the highly crystalline
4-series of compounds to the highly isotropic 1b species. As a
Fig. 10 Scanning electron microscopy of 3-series of phosphole-lipids.
(a) Phosphole-lipids 3a and 3b; (b) crystallites on the surface of 3b film;
(c) amorphous surface of 3a film; (d) amorphous surface of 3b film with
crystallites on the surface.
result, the fluorescence of the compounds in the solid state also the diminutive size of the crystallites, PXRD of the film did not
varies significantly according to the differences in morphology. support any increased crystallinity for 3b.
In general, crystalline compounds display higher fluorescence
intensity due to the close packing in the crystalline phase. crystallinity of the 4-series was confirmed by the relatively sharp
When crystallinity decreases, so does solid-state fluorescence. melting and crystallization transitions on DSC, large crystalline
Contrary to the amorphous nature of the 2- and 3-series, the
Compound 1b, when newly prepared, appeared to be a highly domains on POM and the prominent sharp peaks in PXRD
viscous liquid at room temperature, however, which solidified graphs (see ESI†). This significantly increased crystallinity of
after several months. While the eventual solid displayed certain the 4-series is the likely reason for its considerable solid-state
degrees of long-range order, as evident by PXRD (see ESI†), it is its fluorescence (Table 5). In fact, 4a and 4b proved to be two of the
liquidity that dominates the self-assembly of the compound on a most highly emissive phosphole-lipid species observed to date
more accessible time scale. This highly disordered feature was (4a fPL = 33%, 4b fPL = 19%). Furthermore, we were able to obtain
also found in another short-chain-decorated phosphole species in single crystals of 4a by slow evaporation of solvent mixture (dichloro-
an earlier study.11 With truncated chain length and enlarged methane and hexane) at room temperature (Fig. 11).
conjugated head group, both the shape anisotropy of the mole-
Due to the thermal disorder generally imparted by the highly
cule and charge density decrease. Since these forces are integral in flexible dodecyl chains, reports of single crystal X-ray crystallographic
the ordering of the compounds in aggregation, decreased shape data of compounds possessing multiple dodecyl substituents
anisotropy and charge density are likely the cause for the high are few and far between. Therefore, the single crystal X-ray data
liquidity of 1b. Like its previously studied cousin 1a, the species of 4a afforded us a unique opportunity in understanding the self-
was essentially non-emissive in the aggregated state (fPL = 0.17%). assembly feature of the phosphole-lipid system.‡
While 2a was amorphous in nature, 2b displayed some
One of the most significant features of 4a in the crystalline
soft crystal/liquid crystal features as evident by it PXRD results. state is the anti-parallel arrangement of the two dodecyl chains.
The low solid-state fluorescence of the compounds is, however, Contrary to our expectation that the chains would lie parallel to
more likely due to ACQ rather than the AIEE effect. This conclusion each other and form a linear molecule with polar and non-polar
is based on the particular FMOs arrangements where the PET regions on each end, the two dodecyl chains in crystalline 4a
effect was suppressed (Fig. 6, M2).
adopt a ‘split’ stance with the chains pointing away from each
While also being amorphous solids, akin to 2a, films of the other (Fig. 11a). In a recent study, we also reported on the single
3-series can be obtained by drop casting and were studied by crystal X-ray data of a ‘phosphinine lipid’ KP (Fig. 11, KP).12
scanning electron microscopy (SEM, Fig. 10); films formed
by both 3a and 3b appear to be of similar thickness (B20 mm,
‡ Crystal data and structure refinement for 4a (C53H70BrO2PS2): Mr = 914.09,
see ESI†). In both cases, amorphous defects were found evenly
%
temperature = 173(2) K, triclinic, space group P1, a = 9.5341(3), b = 14.4282(4), c =
distributed over the entire surface. However, whereas the film 18.4781(4) Å, a = 85.0625(17), b = 77.5714(16), g = 80.3446(13)1, V = 2443.82(12) Å3,
Z = 2, rcalc = 1.242 g cmÀ3, m = 0.998 mmÀ1, F(000) = 972.0, crystal size = 0.070 Â
of 3a appeared to be relatively homogeneous, rectangular
crystallites can be found embedded through out the surface
of the film of 3b. The increased crystallinity of 3b was reflected
by increased solid-state fluorescence quantum yield of 3b
(fPL = 9%) as compared to 3a (fPL = 0%). However, due to difference peak and hole 0.79 and À0.40 e ÅÀ3
0.050 Â 0.020 mm3, l = 0.71073 Å, 2Y range for data collection = 4.8921 to 55.1241,
index ranges = À12 r h r 12, À18 r k r 18, À23 r l r 24, 20 775 measured
reflections, 11 196 [Rint = 0.0484] independent reflections, GoF on F2 = 1.117, R1
=
0.0682, wR2 = 0.1164 [I Z 2s(I)], R1 = 0.1021, wR2 = 0.1309 [for all data], largest
.
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Instead of the p–p dimers commonly observed in other dithieno-
phosphole systems, 4a appears to remain monomeric, even in the
crystalline state. This remarkable ‘rigidity’ of the molecule was
also mirrored in the concentration-dependent 31P NMR studies
discussed previously. Although the corresponding 4b also
appears crystalline, crystals of sufficient quality for single-
crystal X-ray crystallography could not be obtained.
Effect of the counter anion. To mediate the crystallinity
and introduce amphiphilic features to the self-assembly of 4a,
the bromide anion was substituted for the more flexible
dodecylsulfate. Whereas 4a is highly crystalline, the increased
thermal disorder introduced by the dodecylsulfate anion
results in a decreased crystallinity of 4a-DS in the solid state.
The differential thermogram of 4a-DS, shows one extra thermal
transition close to the crystal melting transition, suggesting
additional phase transitions apart from the melting/crystallization
transition (see ESI†). Moreover, the addition of the dodecylsulfate
anion resulted in significantly smaller domains observed via POM
when heated (Fig. 13). However, due to the diminutive nature of
the domains, visual assignment of phases was unfortunately
unsuccessful. PXRD results also confirmed the decreased
crystallinity of compound (broad high angle ‘bumps’ instead
of sharp peaks, see ESI†). The decreased crystallinity of 4a-DS
also resulted in the decreased solid-state fluorescence of the
Fig. 11 Single crystal XRD data of phosphole-lipid (4a) and phosphinine-
lipid (KP). (a) and (b) solid-state structures of 4a as monomers; (c) the
intermolecular packing of 4a (dodecyl chains are truncated for clarity);
(d) solid-state packing of KP.9
By comparing 4a and KP with two different chain attachment
patterns (3,5- and 3,4,5-), we can better understand the peculiar
geometry of the 3,5-chain arrangement in 4a. While the dodecyl
chains in KP were aligned parallel to each other in the solid state
(Fig. 11d), the dodecyl chains in 4a are arranged antiparallel to
each other (Fig. 11a). This difference in chain arrangement can
be attributed to the lack of van-der-Waals interchain interaction
in 4a. The presence of the 4-dodecyl substituents in KP very likely
acts as a ‘glue’ to pull the other two chains together resulting in a
parallel arrangement. This particular arrangement of the dodecyl
chain in crystalline 4a can also shed light on the self-assembly
processes of the compound in solution (Fig. 11b, Tables 2 and 3).
Without an alkyl substituent in the 4-position, the 3- and 5-dodecyl
chains are more or less free floating in solution. From the
crystallographic data, it can be deduced that the dodecyl chains
would be found primarily on the periphery of the molecule,
creating a barrier preventing the approach of another molecule
(Fig. 12). It is reasonable to believe this is the basis for the
apparent reluctance of 4a to aggregate in solution.
Another important feature of 4a is the absence of p–p interac-
tions in 4a (Fig. 11c), which is commonly found in related phosphole
compounds. In most of the other non-lipid phospholes,6,10 includ-
ing closely related ionic model compound M1,4c p–p interactions
in the solid state are predominantly of the intermolecular variety
where the phosphole p-scaffolds arrange in a face-to-face fashion.
In 4a however, intermolecular p-stacking interactions between the
phosphole scaffolds are almost completely suppressed (Fig. 11c).
Fig. 13 Polarized optical microscopy of 4a (a) and 4a-DS (b). Note the
Fig. 12 Proposed mechanism for intermolecular repulsion of 4a in solution.
differences in domain size between the two micrograph.
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compound. Due to the high solution fluorescence quantum (Z. W.). Thanks to Profs. T. Sutherland and C. C. Ling for helpful
yield of 4a-DS (fPL = 16%), the addition of the dodecylsulfate discussion and the use of the polarized optical microscope.
anion was enough to partially switch off the solid-state fluores-
cence and therefore the AIEE of this compound. This is likely due
to the increased thermal disorder in the solid state introduced
Notes and references
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relaxation pathways in the solid state. The result of the dodecyl-
sulfate species 4a-DS provided us with another unique avenue in
tuning the self-organization and opto-electronic features of the
phosphole-lipid system.
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In this study, we successfully demonstrated the tuning of AIEE
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Acknowledgements
Financial support by NSERC of Canada and the Canada Foundation
for Innovation (CFI) is gratefully acknowledged. We also thank 12 X. He, J.-B. Lin, W. H. Kan and T. Baumgartner, Angew.
Alberta Innovates – Technology Futures for a graduate scholarship
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J. Mater. Chem. C