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Cite this: Soft Matter, 2024,
20, 6539

Azaylide-based gemini amphiphiles displaying
unique self-assembling behavior via an even–odd
effect of alkyl linker chain length†

Received 28th June 2024,
Accepted 29th July 2024

Yoshimori Akiyama, Masahiro Yamashina* and Shinji Toyota*

DOI: 10.1039/d4sm00789a
rsc.li/soft-matter-journal

Herein, we report a straightforward synthesis of azaylide-based
gemini amphiphiles using bis(diphenylphosphino)alkanes via the
Staudinger reaction. The prepared gemini amphiphiles exhibitedan even–odd effect in their self-assembly behavior depending on
the length of the alkyl linkers. Furthermore, the assembled micelles
had high host capability toward hydrophobic guests in water.

Gemini amphiphiles consist of two hydrophilic moieties and twohydrophobic moieties connected through a linker.1–5 These amphiphiles are generally highly functional compared with their monomeric amphiphiles;6–9 therefore, they have been utilized in various
applications including drug and gene delivery,10–14 antibacterial
agents,15–18 corrosion inhibitors,19,20 phase transfer catalysts,21 andmicroemulsion stabilizers.22–24 In terms of the even–odd effect,25–28
which is the oscillation in properties depending on the even or odd
number of connected repeating units (e.g., methylene group), somegemini amphiphiles comprising aliphatic moieties (e.g., n-alkane) as
building blocks display a unique self-assembling behavior.29–31
Although these examples are based on long alkyl linkers, e.g.,(CH2)n with n = 10 and 12, the even–odd effect of short alkyl chains
can be expected to dramatically change the aggregation behavior
owing to their considerable impact on the molecular structure.32,33
However, systematic studies on the self-assembly of gemini amphiphiles with short alkyl linkers are scarce.31,34 We previously reported
the facile transformation of hydrophobic triarylphosphines toward
monomeric amphiphiles via the Staudinger reaction.35,36 The
obtained azaylide-based (i.e., NQP bond) amphiphiles selfassembled into ca. 2-nm-sized spherical micelles that could
encapsulate various hydrophobic dyes in water. In addition tomonophosphines, diphosphines such as bis(diphenylphosphino)alkanes with various alkyl chains exhibit different catalytic activities depending on the alkyl chain length and are
Department of Chemistry, School of Science, Tokyo Institute of Technology,
Meguro-ku, 2-12-1 Ookayama, Tokyo 152-8551, Japan.
E-mail: yamashina@chem.titech.ac.jp, stoyota@chem.titech.ac.jp† Electronic supplementary information (ESI) available. CCDC 2342817 and
2342818. For ESI and crystallographic data in CIF or other electronic format
see DOI: https://doi.org/10.1039/d4sm00789a

This journal is © The Royal Society of Chemistry 2024

commercially available (Fig. 1a).37 In this study, we focused onazaylide-based gemini amphiphiles38 with a short alkyl linkage(NdpPn; n = 1–6, Fig. 1b) based on bis(diphenylphosphino)alkanes (dpPn) as the building blocks and investigated theunique self-assembling behavior of NdpPn in water along withthat of the corresponding monomeric amphiphile NPPh2Me(Fig. 1c and d) for comparison.
A series of azaylide-based gemini amphiphiles were successfully synthesized by simply mixing hydrophilic azide 1 andbis(diphenylphosphino)alkanes in CH3CN. To prevent residual
free hydrophilic azide 1 and intermediates (e.g., monosubstituted product), a small excess of bis(diphenylphosphino)alkanes was added to 1 two times. First, 0.45 equiv. ofbis(diphenylphosphino)propane (dpP3) was added to a CH3CN
solution of hydrophilic azide 1 (1.0 equiv.) and stirred at 60 1C
until complete consumption of dpP3 (ca. 10 min). Subsequently, a CHCl3 solution of dpP3 (0.075 equiv.) was slowly
added to the resultant solution over 10 min to aﬀord the desired

Fig. 1 Chemical structures of (a) diphosphines dpPn (n = 1–6) and (c)monophosphine PPh2Me. (b) Chemical structures of azaylide-based
gemini amphiphiles NdpPn (n = 1–6) and (d) the corresponding monomeric amphiphile NPPh2Me.

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Fig. 2 (a) Synthesis of azaylide-based gemini amphiphile NdpP3 via the
Staudinger reaction. (b) 1H NMR (500 MHz, DMSO-d6, 298 K) and (c) 31P
NMR (202 MHz, DMSO-d6, 298 K) spectra of dpP3 (top) and NdpP3(bottom). (d) ESI-TOF MS (CH3CN/CH3OH) spectrum of NdpP3. (e) X-ray
crystal structure of NdpP3I. Thermal ellipsoids are set at 50% probability. (f)Time-course of the decomposition of NdpP3 and nNdpP3 in D2O.

gemini amphiphile NdpP3 in excellent yield (96%), as confirmed by
NMR and MS analyses (Fig. 2a). After the formation of the azaylide(NQP bond), a 31P NMR signal attributable to NdpP3 appeared at
7.3 ppm, which shifted downfield by +25.1 ppm relative to that offree dpP3, and the 1H NMR signals of the dpP3 moiety showed
downfield shift (Dd = +0.69 ppm, Fig. 2b and c). The mass spectrumof dicationic NdpP3 showed a prominent signal at m/z = 496.18 for[M 2Cl]2+ (Fig. 2d). The other gemini amphiphiles were synthesized in a similar manner in 87–98% yields (Fig. S1, ESI†).
Interestingly, the synthesis of NdpP1 required a longer reaction
time (3 h), higher concentration (113 mM), and higher temperature(80 1C) due to the increased steric repulsion between the diphenylphosphine moieties. The crowded aromatic rings and phosphine moieties exerted synergistic shielding effects, resulting in aremarkable downfield shift of the 1H signal of the methylene linker
and an upfield shift of the 31P signal compared with those of other
NdpPn.
A single-crystal X-ray diﬀraction (SXRD) analysis of NdpP3I,
in which the counter ions in NdpP3 were replaced with two
iodide ions, unambiguously revealed the chemical structure ofgemini amphiphile NdpP3 (Fig. 2e and Fig. S2, ESI†), which
consisted of two hydrophilic parts connected to the dpP3
moiety in the same direction, i.e., in the syn-conformation.
One phenyl group of dpP3 and a phenyl azaylide moiety formeda trans-stilbene-like structure.35 NdpP3 displayed an absorption
band (lmax = 318 nm) and a weak blue emission (lem = 450 nm)

6540 | Soft Matter, 2024, 20, 6539–6543

Soft Matterin CH3CN (Fig. S4, ESI†), similar to those of a previously
reported azaylide-based amphiphile with PPh3.35 The otherazaylide compounds NdpP1 and NdpP5 showed excellent watersolubility. Non-chlorinated nNdpP3 was extremely fragile in
D2O, exhibiting a lifetime (t1/2) of 14 min (Fig. 2f and Fig. S5and S6, ESI†), whereas a triphenylphosphine-based azaylide35
showed a relatively long t1/2 of 9.8 h. This result indicates thatelectron-rich yet less sterically hindered alkyl phosphines accelerate the hydrolysis of the azaylide moiety. Nevertheless, chlorinated NdpP3 exhibited high stability against hydrolysis evenin water (Fig. 2f and Fig. S7, ESI†). Thus, our strategy can beapplied to bis(diphenylphosphino)alkanes with reactive
phosphine sites.
Next, the self-assembling behavior of the gemini amphiphiles was investigated in water. First, the amphiphiles with an
odd number of methylene linkers, i.e., NdpP1, NdpP3, andNdpP5, were examined. When NdpP3 was dissolved in D2O(2.0 mM), (NdpP3)m micelle aggregates were quantitatively andspontaneously formed (Fig. 3a (top)). The 1H and 31P NMR
signals of NdpP3 in D2O were substantially broadened owing to
the dynamic behavior (Fig. S8 and S9, ESI†). A diﬀusion-ordered
spectroscopy (DOSY) analysis revealed a single band at a small
diﬀusion coeﬃcient (D) of 2.28  10 10 m2 s 1 (Fig. S20, ESI†).
According to the Stokes–Einstein equation, this D value corresponds to aggregates with a diameter of ca. 1.7 nm. A dynamic
light scattering (DLS) analysis revealed the formation ofFig. 3 (a) Schematic of diﬀerences in assembling behavior into micelles
and layered structures in water. (b) DLS charts of a 2.0 mM aqueous
suspension of NdpP3 and NdpP2 (298 K). AFM images of (c) micelles of
NdpP3 and (d) the layered structure of NdpP2. (e) X-ray packing structureof NdpP2. (f) PXRD patterns of a white dispersion of NdpP2 and simulation
based on the results of SXRD of NdpP2.

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Soft Matteraggregates with a diameter of ca. 2.1 nm in water with a narrowsize distribution (Fig. 3b and Fig. S21, ESI†). Atomic force
microscopy (AFM) measurements confirmed the existence of
spherical aggregates (NdpP3)m with a height of ca. 2–3 nm(Fig. 3c and Fig. S28, ESI†). These results indicate that the sizeof the (NdpP3)m micelle aggregates corresponds to four molecules of NdpP3, as estimated by molecular mechanics (MM)calculations (Fig. S22, ESI†). The critical micelle concentration(CMC) of NdpP3 was measured using concentration-dependent1
H NMR and DOSY analyses. The D values suddenly decreased
when the concentration reached 1.0 mM, which was accompanied by a broadening of the 1H NMR signals (Fig. S23 and S26,
ESI†). Thus, the CMC of NdpP3 was determined to be 1.0 mM,
which is considerably lower than that of the monomericamphiphile NPPh2Me (10 mM, Fig. S25, ESI†). The otheramphiphiles NdpP1 and NdpP5 also formed micelle aggregatesin water with relatively small CMC values (1.0 and 0.20 mM,
respectively, Fig. S10–S13, S20–S22, S24, and S26, ESI†). Owing
to the increased hydrophobicity, the CMC values gradually
decreased with increasing methylene linker length.
Diﬀerent results were obtained for the amphiphiles with aneven number of methylene linkers, i.e., NdpP2 and NdpP4.
When NdpP2 (4.0 mmol) was added to H2O (2.0 mL), a whitesuspension was obtained instead of a clear aqueous solution(Fig. 3a (bottom)). A DLS analysis of the suspension revealed
the existence of two kinds of aggregates, one type with adiameter of ca. 2.6 nm and a narrow distribution and anothertype with a diameter of ca. 1.4 mm and a broadened distribution(Fig. 3b). 1H NMR analyses showed sharp signals with a large D
value of 5.25  10 10 m2 s 1 at 298 K in water (Fig. S14, S15 andS20, ESI†). The fact that NdpP2 has low water-solubility (ca.
0.41 mM) suggests that NdpP2 is in a monodisperse state or
forming premicelles in water. Apart from these NdpP2, mostof the NdpP2 molecules formed a white suspension consistingof ca. 1–2 mm aggregates in water. AFM and transmission
electron microscopy (TEM) observations showed large aggregates (ca. 1–2 mm) and spherical small aggregates (ca. 3–10 nm),which are consistent with the DLS results (Fig. S28 and S29,
ESI†). In particular, the AFM image revealed that the large
aggregates exhibit a multilayered structure (Fig. 3d), implying
the formation of microcrystals.39,40
To gain more insight into the self-assembled structures,
X-ray diﬀraction analyses were conducted. Slow evaporationof a CH3CN/CHCl3 solution of NdpP2 resulted in plate-shaped
single crystals, whose SXRD analysis revealed that the twohydrophilic moieties possessed an anti-conformation, diﬀeringfrom that of NdpP3 (Fig. 2e). In the crystalline packing,elongated NdpP2 molecules were aligned to form a layered
structure; specifically, the amphiphiles formed 2D-sheets
through multiple CH–p interactions along the a- and c-axes,
and these 2D-sheets stacked in the b-axis (Fig. 3e). The powder
X-ray diffraction (PXRD) pattern of white dispersions of NdpP2
in water was consistent with the simulation pattern of NdpP2
based on the SXRD analysis (Fig. 3f). A similar PXRD pattern
was found for NdpP4 (Fig. S27, ESI†). Thus, unlike the micelleformation of NdpP1, NdpP3, and NdpP5, both NdpP2 andThis journal is © The Royal Society of Chemistry 2024

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NdpP4 exhibited high crystallinity with packing structures
similar to that shown in Fig. 3e in water. These differences
stem most likely from the dipole moments of NdpP3 (38.66 D)
and NdpP2 (4.84 D), with the latter resulting in a strong
packing force.
Contrary to our expectations, NdpP6 with an even number ofmethylene linkers was highly soluble in water, resulting in theformation of micelles with a diameter of ca. 2–3 nm instead of alayered structure (Fig. S18–S21, ESI†). Increasing the length of
the alkyl linker chain should endow the gemini amphiphiles
with conformational flexibility, leading to the formation of
micelle aggregates in water. This observation indicates thatthe remarkable even–odd effect of the alkyl linker on theassembly behavior is unambiguous for the amphiphiles withshort linkers up to n = 5.
Finally, the host capability of the gemini amphiphiles wasinvestigated. NdpP3 (4.0 mmol) and perylene (Per; 1.0 equiv.)
were ground in a mortar for 3 min, followed by the addition ofwater (2.0 mL), and the mixture was stirred vigorously for 1 h.
After removal of excess guest powders via filtration, a yellow
solution of (Per)x(NdpP3)m was obtained (Fig. 4a). The UV-vis
spectrum clearly displayed absorption bands around 401 nm
ascribable to encapsulated hydrophobic Per (Fig. 4b). Encapsulated Per exhibited weak excimer emission (Fig. S30, ESI†). The
host–guest complex (Per)x(NdpP3)m was ca. 2.7 nm in diameter, as confirmed by DLS measurement (Fig. 4c). The
1
H NMR spectrum of a DMSO-d6 solution of the solid revealed
that the amount of encapsulated Per was 0.87 mM in 2.0 mM of
NdpP3 aqueous solution (Fig. S36, ESI†). Considering theseresults, MM calculation provided (Per)2(NdpP3)5 as an average
host–guest complex model (Fig. 4a (right) and Fig. S39, ESI†).
Similarly, NdpP3 successfully encapsulated other hydrophobic

Fig. 4 (a) Schematic of the encapsulation of hydrophobic guests (pyrromethene 546 (BP), perylene (Per), and fullerene (C60)) by NdpP3 in waterand the optimized structure of (Per)2(NdpP3)5. (b) UV-vis spectra and (c)DLS charts of (Per)x(NdpP3)m, (BP)x(NdpP3)m, and (C60)x(NdpP3)m (H2O,
298 K, 2.0 mM based on NdpP3). (d) Comparisons of the uptake quantity of
Per by NPPh2Me and NdpPn (n = 1, 3, 5, 6). The numbers above the bars
indicate relative uptake ratio to NPPh2Me (note: NPPh2Me was used 4.0
mM to unify the concentration of diphenylphosphino moiety).

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guests such as pyrromethene 546 (BP) and fullerene (C60) inwater (Fig. 4a and Fig. S30–S39, ESI†). It should be noted thatthe guest uptake ability of gemini amphiphile NdpP3 toward
Per was much higher than that of the monomeric amphiphile
NPPh2Me (0.072 mM, Fig. 4d). Interestingly, the host capability
toward Per of the other gemini amphiphiles, i.e., NdpP1,
NdpP5, and NdpP6, was similar regardless of the length of
the alkyl linker (Fig. 4d and Fig. S30, S33, S36, S39, ESI†). Thistendency was observed in the case of BP (Fig. S37 and S38, ESI†).
Considering that aromatic moieties can effectively encapsulate
guest molecules in water,9,35,36,41 the bound hydrophobic cores
may play a key role in the effective molecular encapsulation.
In conclusion, we have successfully synthesized a series ofazaylide-based gemini amphiphiles from bis(diphenylphosphino)alkanes via the Staudinger reaction. The gemini amphiphiles exhibited an even–odd effect of the alkyl chain length on their assembly
behavior. Our findings provide useful guidance for designing new
types of multilinked amphiphiles (e.g. gemini and bola41 amphiphiles) in the near future.
This study was supported by a JSPS Grant-in-Aid for Scientific
Research (20H02721), Grants-in-Aid for Early-Career Scientists(22K14662), and the Cooperative Research Program of ‘‘NJRC
Mater. & Dev. (MEXT).’’ The authors thank Tokyo Tech. OpenFacility Center. The X-ray analysis was performed under theapproval of the Photon Factory Program Advisory Committee(2021G046, PF-BL5a). We appreciate Prof. Masaki Kawano (Tokyo
Tech.) for X-ray analysis and Prof. Kei Goto (Tokyo Tech.) for the
FTIR measurements. We also appreciate valuable discussions with
Dr Eiji Tsurumaki (Tokyo Tech.).

Data availability
The data underlying this study are available in the publishedarticle and its ESI.†

Conflicts of interest
There are no conflicts to declare.

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