3D-Conformer of tris[60]fullerenylated cis-Tris(diphenylamino-fluorene) as photoswitchable charge-polarizer on GHz-responsive trilayered core-shell dielectric nanoparticles

Article abstract of DOI:10.3390/molecules23081873

Novel 3D-configurated stereoisomers cis-cup-tris[C₆₀>(DPAF-C₉)] and trans-chair-tris[C₆₀>(DPAF-C₉)] were designed and synthesized in good yields. The former, with three C₆₀> cages per molecule facing

Full text of DOI:10.3390/molecules23081873

molecules
Article
3D-Conformer of Tris[60]fullerenylated
cis-Tris(diphenylamino-fluorene) as Photoswitchable
Charge-Polarizer on GHz-Responsive Trilayered
Core-Shell Dielectric Nanoparticles
He Yin 1, Min Wang 1, Tzuyang Yu 2, Loon-Seng Tan 3 and Long Y. Chiang 1,
*
1
Department of Chemistry, University of Massachusetts Lowell, Lowell, MA 01854, USA;
He_Yin@student.uml.edu (H.Y.); wangmin81@gmail.com (M.W.)
Department of Civil and Environmental Engineering, University of Massachusetts Lowell, Lowell,
MA 01854, USA; Tzuyang_Yu@uml.edu
Functional Materials Division, AFRL/RXA, Air Force Research Laboratory, Wright-Patterson Air Force Base,
Dayton, OH 45433, USA; loon.tan@us.af.mil
2
3
*
Correspondence: Long_Chiang@uml.edu; Tel.: +1-978-934-3663; Fax: +1-978-934-3013
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Received: 11 July 2018; Accepted: 25 July 2018; Published: 27 July 2018
Abstract: Novel 3D-configurated stereoisomers cis-cup-tris[C60>(DPAF-C9)] and trans-chair-
tris[C60>(DPAF-C9)] were designed and synthesized in good yields. The former, with three C60>
cages per molecule facing at the same side of the geometrical molecular cup-shape, was proposed
to provide excellent binding interaction forces at the gold surface of core-shell
γ-FeOx@AuNP
nanoparticles and to direct the subsequent formation of a fullerene cage array (defined as
fullerosome). Upon photoactivation of the Au-layer and cis-cup-tris[C60>(DPAF-C9)] itself, the degree
of photoinduced intramolecular e−-transfer from DPAF to a C60> moiety was found to be largely
enhanced by the accumulated plasmonic resonance energy at the near-field surface. Distribution of
resulting negative charges along the outer (C60>)-derived fullerosome shell layer of the trilayered
NPs was correlated with the detected photoswitchable dielectric amplification phenomena using
white LED light at 1.0 GHz.
Keywords: tri[60]fullerenylated cis-cup-tris(diphenylaminofluorene); 3D-configurated cis-cup-tris(C60-
diphenylaminofluorene); trilayered core-shell gold-fullerosome nanoparticles; photoswitchable
permittivity; amplification of relative dielectric constant
1. Introduction
Electronic charge-polarization of organic substances under the externally applied field is the basic
phenomenon leading to dielectric characteristics. It can be correlated with electrons shifting within
polarizable molecules or the dielectric medium showing electronic polarizability. Certain polymers
including poly(vinylidene fluoride) (PVDF, having asymmetrical electron-withdrawing fluorine) [1,2]
polyarylene ether nitrile (PEN, having polar cyano groups) with conducting polyaniline [ ] and
azobenzene dyes (having large dipole moments of polar push-pull donor-acceptor terminal groups) [
may exhibit such behavior; however, with a low dielectric constant. Advantages of organic dielectrics
over inorganic ones, such as barium calcium titanate [ ], boron nitride on graphenes [ ], and titanium
dioxide [ ], lay on their facile chemical modification and easy processing, intrinsic excellent mechanical
performance, recyclability, and multiple components blending. Example of the latter case was
given by hybrid preparation of PEN on core-shell nanoparticles [ ]. These dielectric materials and
polymers may exhibit potential in applications of capacitor-type charge and energy storage [10 12
,
3,4
5]
6
7
8
9
–
]
Molecules 2018, 23, 1873; doi:10.3390/molecules23081873
www.mdpi.com/journal/molecules

Molecules 2018, 23, 1873
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and organic photovoltaics [13]. Recently, we found [14
–16] that the dielectric constant of a medium
can be amplified to more than 200–350% by a photoswitching technique using the construction of
multi-layered core-shell nanoparticles. They consist of two main photoresponsive components, such as
organic electron (e−)-polarizable nanostructure of C60(>DPAF-C9) (
1-C9, Figure 1) and inorganic
plasmonic gold nanocrystal hybrids in a form of core and shell layered nanomaterials. The degree
variation of dielectric amplification depends on the core-shell composition and the distribution of
polarized charges.
Figure 1. Synthetic route to the preparation of cis-cup-tris[C60(>DPAF-C9)] (cis-cup-
3-C9) from the
isolated precursor cis-cup-tris(DPAF-C9) (cis-cup-2-C9) with reaction reagents provided.
The compound
1-C9 analogous nanostructure is an electronically polarizable push-pull
donor-acceptor type conjugative compound having a C60 cage as the acceptor moiety and a
DPAF-C9 [9,9-di(3,5,5-trimethylhexyl)-2-diphenylaminofluorene] chromophore as the light harvesting
donor antenna moiety. It is a part of the general composition of C60(>light-harvesting antenna)x
exhibiting ultrafast inter- and intramolecular photoinduced electron-transfer processes going from the
electron-donating diphenylaminofluorene moiety to the electron-accepting C60 cage moiety [17
The phenomena resulted in the formation of a positively charged (DPAF)+
-C9 and a negatively
charged (C60>)−
cage moieties, respectively. These two charge states form the foundation of our
observed photoswitchable dielectric property enhancements [14 16]. Recently, we further extended
the study to design and synthesize highly restricted 3D-stereoisomers based on inter-connected three
DPAF-C9 chromophore units giving a structure of tris(DPAF-C9) or -C9 (Figure 1). The structural
–20].
·
·
–
2
modification allowed us to investigate the stereo-configuration-dependence of organic fluorophore
on the enhancement of their photophysical properties, including photoluminescence (PL) and
electroluminescence (EL) emission in solid-state thin-films [21]. The stereochemical modification
was based on the construction of 3D-geometrically branched chromophores having sterically hindered
alkyl side-chains that resulted in the physical separation of each DPAF-C9 moiety of
2-C9 from
each other. It prevents and minimizes the tendency of planar organic -conjugated fluorophore
π
molecules, such as DPAF, to undergo aromatic-aromatic stacking, overlapping, and aggregation,
via intermolecular hydrophobic-hydrophobic interactions, in solid state thin-films. The packing
action triggers concentration-dependent self-quenching effects at excited states, resulted in the

Molecules 2018, 23, 1873
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reduction of photophysical properties at the condensed phase. In the case of tris(DPAF-C9),
attachment of large bulky groups at the vicinity of planar molecular benzene core region can
restrict and reduce the degree of freedom in intramolecular DPAF-C9 rotation via steric hindrance,
and thus the tendency of intermolecular aggregation. During the synthesis of tris(DPAF-C9),
two stereoisomeric forms were isolated, namely, trans- and cis-tris(DPAF-C9) defined as trans-chair-
2-C9
and cis-cup- -C9 [22], respectively, as shown in Figure 1. We selected the latter cup-form to undergo
2
further attachment of a C60> cage as the end group of each DPAF-C9 arm that resulted in a
nanostructure of tris[60]fullerenylated cis-tris(diphenylaminofluorene) as cis-cup-tris[C60(>DPAF-C9)]
or cis-cup-3-C9 for the photoswitchable dielectric property amplification study. This design of
new stereo-isomeric structures may be beneficial for uses as positive charge carriers in enhancing
photoinduced dielectric characteristics [23] and as the precursor building blocks in the synthesis
of several C60- and C70-based ultrafast photoresponsive nanomaterials in correlation to our recent
studies [17–20]. Therefore, we applied cis-cup-3-C9 as a photoswitchable dielectric charge-polarizer in
the fabrication of microwave-responsive plasmonic trilayered core-shell nanoparticles.
2. Results and Discussion
An electromagnetic (EM) wave consists of both components of perpendicular electric-fields and
magnetic-fields. Most organic photoresponsive materials are non-magnetic. Therefore, our main
design consideration of organic microwave-responsive materials in tuning a material’s refractive
index was based on the electric susceptibility, in terms of the degree of dielectric polarization in
response to photoactivation instead of an applied electric field. In general, for disordered amorphous
charge-transfer complex solids, the charge transfer events will not lead to significant alternation of the
dielectric property of solids. Our approaches were to achieve the ferroelectric-like characteristics by
firstly demonstrating ultrafast photoinduced intramolecular electron-transfer rate going from DPAF
donor moiety to the C60> acceptor moiety of C60(>DPAF-C9) (1-C9) within <130 femtoseconds [17–20],
resembling nearly spontaneous photoinduced charge-polarization action. Secondly, we then fabricated
the C60> cage assembly into an aligned partial bilayered fullerosome membrane structure [24]. This type
of fullerosome consists of a C60-C60 cage aligned array into a nano-layer to host the polarized negative
charges at the inner membrane area and all DPAF donor-antennae moieties facing outside surface
of the fullerosome to host the polarized positive charges. The self-assemble action was induced by
strong hydrophobic-hydrophobic (C60>)-(C60>) interaction forces among fullerene cages. The resulting
layered, polarized charges distribution affords a close resemblance to the characteristics of dielectrics,
also as nano-capacitor-like assemblies, upon LED irradiation [14–16].
In the material’s design, we replaced
1-C9 by cis-cup-tris[C60(>DPAF-C9)] (cis-cup-3-C9,
Figure 1) as the outer shell donor-acceptor nanoconjugates on the construction of core-shell
nanoparticles. It was derived from a C3-symmetrical 1,3,5-triaminobenzene ring as the central core
for connecting three fused 2-diphenylaminofluorene moieties, forming a new class of 3D-stereomeric
tris(fluorenylphenylamino)-benzene as tris(DPAF-C9) 2-C9. These 3D-stereomers faces high torsional
stress at the central benzene core region that forces all fluorene chromophore groups located at
the outer-edge area to be oriented in a non-coplanar 3D-configuration [22]. Two stereoisomeric
forms are possible, namely, cis-cup-2-C9 and trans-chair-2-C9 (Figure 1). They can be separated
by thin-layer chromatographic technique with the former as the major product. Based on the
density functional theory (DFT) calculation with the geometries optimized at the B97-D3/SVP
level of theory [21], the cis-cup-form having three bis(30,50,50-trimethylhexyl)fluorene moieties on
1,3,5-tris(phenylamino)benzene was found to be essentially C3 symmetric with a higher formation
tendency than the corresponding trans-chair-form. It is owing to the influence by strong dispersion
interactions within the alkyl chains that are enhanceable in the presence of more polar media. Upon a
180◦ -rotation of one of three amino groups, the structure changes to produce a conformational isomer
in a trans-chair-form which is less cup-like than the cis-cup-form. In principle, subsequent attachment
of three C60> cages on cis-cup-2-C9 should lead to cis-cup-3-C9 having all C60> moieties facing outward

Molecules 2018, 23, 1873
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from the central benzene core at the same side with each other. This will facilitate the cup-attachment
on the gold layer of core-shell nanoparticles induced by the strong C60-Au binding force [25 26] and,
,
thus, enhance the distribution of polarized negative charges at gold surface bound monolayered
fullerosome region, as the positive contribution to the photoswitchable relative dielectric properties.
2.1. Synthesis of Electron-Polarizable Tris[C60(>DPAF-C9)] Stereoisomer cis-cup-3-C9
Synthetic procedure for the preparation of tris(DPAF-C9) with the detailed spectroscopic
characterization was reported previously by us [21
cis-cup- -C9 was clearly isolated from the trans-chair-stereoisomer and confirmed by the use of high
performance liquid chromatography (HPLC, m, 35 300 mm, mobile phase:
Porasil™ 125 Å, 10
hexane-EtOAc (9.0:1.0, v/v), flow rate: 1.0 mL/min, detector: UV at 350 nm) in an analytical
,22]. Geometrically conformational isomer
2
µ
µ
×
λ
scale. For the scale-up separation, column chromatography (CC) using the same solvent system
did not provide satisfactory results. However, the application of thin-layer chromatography (TLC)
technique using the solvent eluent in slightly lower polarity (hexane-ethylacetate/9.5:0.5, v/v) than
those of HPLC or the CC formed an overlap band of the stereoisomers cis-cup-2-C9 and trans-chair-2-C9.
By cutting the corresponding fluorescent TLC band of tris(DPAF-C9) into two portions and performing
several repeated TLC procedures, we were able to significantly increase the degree of product purity
sufficiently enough to allow our confirmation of their entity by 1H NMR spectrum [21
,
22]. It indicated
a higher Rf value for cis-cup-2-C9 than that of trans-chair-2-C9. With a C3 symmetry in structure,
the former displayed a differentiable singlet peak for three central benzene protons (cis-Hb) at
(Figure 2(Aa)), while the latter with a less symmetry resulted in two proton peaks (trans-Hb) at
(1H) and 6.55 (2H).
δ
δ
6.55
6.48
Figure 2.
(b) cis-cup-tris(BrDPAF-C9), (c) cis-cup-tris[C60(>DPAF-C9)] (cis-cup-
as reference for comparison.
13C NMR spectra of (a) cis-cup-tris(BrDPAF-C9),
-C9), showing in (b) the main group
140–147 that indicated the attachment of C60> cages on -C9,
(A
)
1H NMR spectra (CDCl3) of (a) cis-cup-tris(DPAF-C9) (cis-cup-
-C9), and (d) C60(>DPAF-C9)
2-C9),
3
a
(B)
(b) cis-cup-tris[C60(>DPAF-C9)], and (c) C60(>DPAF-C9) (
1
of fullerenyl sp2 carbon peaks at
as compared with those of (Bc).
δ
2
Prior to the attachment of three C60> cages on cis-cup-
Friedel–Crafts acylation at C7 position of diphenylaminofluorene moiety with
in the presence of aluminum chloride (6.0 eq.) in 1,2-dichloroethane at 0 ◦ C to ambient temperature
overnight to afford the corresponding -bromoacetylfluorene derivative, as the intermediate step
of reactions. It resulted in yellow semi-solids in 53% yield of cis-N1,N3,N5-tris(7-
-bromoacetyl-9,9-
2
-C9, it was functionalized by the
α
-bromoacetyl bromide
α
α
di(30,50,50-trimethyl-10-hexyl)fluoren-2-yl)-1”,3”,5”-tris(phenylamino)benzene as cis-cup-tris(BrDPAF-
C9). It was purified by either column or TLC chromatography (silica gel, hexane–toluene, 3:2, Rf = 0.3
on TLC). The functional structure of cis-cup-tris(BrDPAF-C9) was verified by both infrared (FT-IR)
1
and H NMR spectra with the former showing a strong carbonyl (-C=O) stretching absorption

Molecules 2018, 23, 1873
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band centered at 1675 cm−1 indicating this functional group being linked on a phenyl moiety since
a clear band absorption shift from 1725 cm−1, normally detectable for an alkyl carbonyl group,
was evident. In the case of 1H NMR spectrum, tris(Br-DPAF-C9) displayed characteristic new peak
signals of two methylene protons (Hα) next to the carbonyl group of the
at 4.46 (Figure 2(Ab)). Subsequence attachment of a C60> cage to each of three DPAF moieties
was made via a cylopropylacetyl bridge by the treatment of three -bromoacetylfluorene moieties
α-bromoacetyl moiety
δ
α
of cis-cup-tris(BrDPAF-C9) with C60 in toluene in the presence of DBU at ambient temperature for
8.0 h. An excessive amount of C60 (4.5 eq.) was applied to avoid the formation of C60-bisadducts
or trisadducts during the cyclopropanation reaction. The product of cis-cup-tris[C60(>DPAF-C9)]
stereoisomer as cis-cup-3-C9 was confirmed by the evidence of changing solubility characteristics
matching those of C60 and a slight shift of cyclopropyl keto group absorption in IR spectrum to υmax
1627 cm−1, which was assigned to the carbonyl stretching band. It was also accompanied by two typical
fullerenyl cage signals at υmax 575 (w) and 529 (s) cm−1. These two bands are characteristic absorptions
used to provide evidence of (C60>)-related monoadduct with relative intensity ratio differentiable from
those of starting C60 itself. They will disappear from the spectrum once becoming a (C60>)-bisadduct.
Therefore, this IR technique was applied often in the [60]fullerenyl product structure verification.
In addition, disappearance of
α
-proton peaks of the
α-bromoacetyl group at δ 4.46 (Figure 2(Ab))
with the appearance of new peaks centered at
δ
5.67 (Figure 2(Ac)) can be indicative of successful
formation of the cyclopropanyl keto-bridge between a C60> cage and the fluorene moiety, using
previously reported C60(>DPAF-C9) characterization (Figure 2(Ad)) as the reference [17]. The latter
peak
down-fielded shift from the normal chemical shift value of a alkyl acetyl proton at ~
the influence of strong [60] fullerenyl current in the close vicinity. Accordingly, the detection of
δ
5.67 was assigned to the chemical shift of
α
-proton (Hα) of
3
-C9 indicated in Figure 1. Its large
2.1–2.6 is due to
5.67
δ
δ
peak can also be served as the verification of short covalent-bonding between C60> and DPAF moieties.
The half-height width of Hα peak in Figure 2(Ac) is slightly wider than that of Figure 2(Ab) that
revealed less symmetrical environment among three Hα protons of cis-cup-3-C9. Similarly, by using
13C NMR spectrum of C60(>DPAF-C9) (Figure 2(Bc)) as the reference for comparison, we were able to
detect calculated 29 fullerenyl sp2 carbon peaks corresponding to the C60> moiety (C2 symmetry) of
cis-cup-tris[C60(>DPAF-C9)] as a matching multi-peak band in the range of δ 140–147 (Figure 2(Bb)).
The reduced spectrum resolution is due to the increasing molecular complexity along with the reduced
solubility of cis-cup-3-C9.
2.2. Synthesis of cis-cup-Tris[C60(>DPAF-C9)]-Encapsulated Core-Shell Nanoparticles
Synthetic procedures for the preparation of core-shell
γ
-FeOx@AuNP nanoparticles (4-NPs) in
an average particle diameter of ~20 nm were reported by us recently [14
,
15]. A slightly modified
method was applied in this study to afford similar results based on the verification by transmission
electron microscopy (TEM) micrographs and microanalyses of energy dispersive x-ray spectra (EDS).
Subsequent encapsulation of
of trilayered core-shell NPs (Figure 3) was performed by mixing these two components in a predefined
ratio in solvent under ultrasonication. The initial binding force of cis-cup- -C9 to -FeOx@AuNP
was governed by strong binding interaction forces of C60> cages to the gold surface [26 27] that
should result in the partial replacement of 1-octanethiol capping molecules on the gold surface via
alkylthiol ligand-C60> exchange induced by sonochemical energy to invert cis-cup- -C9 molecules
γ-FeOx@AuNPs by cis-cup-tris[C60(>DPAF-C9)] leading to the formation
3
γ
,
3
under ultrasonication. The exchange should form a monolayer of cis-cup-tris[C60(>DPAF-C9)] with
the orientation of all three C60> cages per nanomolecule in a cup-capping manner facing on the
surface of Au layer with interlinked three DPAF-C9 moieties facing outward. Attachment of additional
cis-cup-3-C9 nanomolecules will follow the strong hydrophobic-hydrophobic interactions among
(C60>)-(C60>) cages to form a partial trilayered fullerosome membrane resembling that of reported
fullerenyl nanovesicle [24]. Since all encapsulated magnetic nanoparticles were physically removed
from the container solution by the assistance of an external permanent magnet and washed repeatedly

Molecules 2018, 23, 1873
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by ethanol and diethyl ether, we were able to ensure the products being free from residual non-binding
cis-cup- -C9 molecules at the NP surface. We assigned the resulting trilayered core-shell nanostructures
3
of (γ-FeOx@AuNP)@{cis-cup-tris[C60(>DPAF-C9)]}n as 5-NPs.
TEM micrographs of Figure 3 showed the morphology and topography of NPs that indicated a
roughly homogeneous narrow size distribution of the parent
~20 nm, on average. Subsequent deposition of solid gold nanocrystals on the surface of
structure of core-shell -FeOx@AuNP ( -NP) was controlled to a shell layer thickness of 6.0–8.0 nm by
γ
-FeOx NPs (Figure 3a) at a diameter of
γ
-FeOx NPs to a
γ
4
the calculated quantity of HAuCl4 applied. This thickness was measured by the comparison of average
particle size between Figure 3a,b, where the latter displayed much higher contrast of the Au layer
shell. With the encapsulation of 4-NPs by cis-cup-3-C9-derived fullerosome (array of C60> cages) as
the outer shell layer, the morphology of soft organic materials in lighter contrast covering all hard
nanoparticles can be observed that allowed us to measure an average thickness of roughly 10 nm
(Figure 3c), matching well with the weight ratio of
nanoparticle fabrication.
4-NPs and cis-cup-3-C9 applied in the core-shell
Figure 3.
and ( ) (
TEM micrograph images of
-FeOx@AuNP)@{cis-cup-tris[C60(>DPAF-C9)]}n (5
(
a
)
γ
-FeOx NPs,
(b) γ-FeOx@AuNP (4-NPs),
c
γ
-NPs), showing evolution of particle
morphology changes and the soft organic material encapsulation on dark nanoparticles on the
latter micrograph.
2.3. Physical Properties of cis-cup-Tris[C60(>DPAF-C9)] and Its Encapsulated Core-Shell Nanoparticles
Photophysical properties of ci−s-cup-tris[C60(>DPAF-C9)] are dominated by two components,
namely, C60> cages as the electron (e )-acceptors and light-harvesting DPAF antenna units as electron
(e−)-donors. Both components are photoresponsive at a different wavelength range. For example,
optical absorption of C60> cages occurs mainly the band centered at 335 nm (1.76
cm−1) [17], whereas the band centered at 402 nm (4.19 104 L mol−1 cm−1) is attributed to the
absorption of DPAF moieties, as shown in the UV–vis spectra of Figure 4c. Characteristics of the
latter band were compared with that of cis-cup- -C9 (Figure 4a), cis-cup-tris(BrDPAF-C9) (Figure 4b),
and C60(>DPAF-C9) ( -C9, Figure 4d) showing a clear bathochromic shift of the 353-nm band of the
former to 395 nm (1.10
105 L mol−1 cm−1), which is matching roughly with the 404-nm band
of -C9 and the 402-nm band of cis-cup- -C9 for the peak assignment. This assignment was also
consistent with the observation of 2.9-folds higher in the absorption extinction coefficient ( ) value for
cis-cup-tris(BrDPAF-C9), having three DPAF moieties per molecule, as comparing with that of -C9
×
105 L mol−1
×
2
1
×
1
3
ε
1
with one DPAF moiety per molecule. Upon the attachment of three C60> cages, optical absorptions of
fullerene moieties of cis-cup-3-C9 became dominating in the spectrum with a much higher ε value for
the 335-nm band (Figure 4c). It was accompanied by a weak characteristic (forbidden) steady state
absorption band of the C60> moiety appearing at 692 nm (the insert of Figure 4c) that provided further
confirmation of a fullerene conjugate structure.

Molecules 2018, 23, 1873
7 of 16
Interestingly, distinctly 3.3-folds lower in the
ε
value of the 402 nm peak in Figure 4c than that of
1
-C9, making it as a shoulder band to the main C60> absorption, was interpreted by a phenomenon
likely caused by the specific molecular 3D-conformation of cis-cup-tris[C60(>DPAF-C9)]. Owing to
good solubility and compatibility of fullerenes to toluene solvent used, the orientation of three C60>
cages was likely to face outward into the solvent phase while DPAF moieties were hidden inside
the partially aggregated cluster nanoparticle. The hypothesis was also verified by observation of the
distinguishable solubility decrease of cis-cup-3-C9 in CHCl3; however, increase in toluene and CS2
matches well with the solubility characteristics of C60.
Figure 4. UV-vis spectra of (a) cis-cup-tris(DPAF-C9) (cis-cup-2-C9), (b) cis-cup-tris(BrDPAF-C9),
(c) cis-cup-tris[C60(>DPAF-C9)] (cis-cup-
3
-C9), (d) C60(>DPAF-C9) (
1
-C9), (e) ( -NP)@(cis-cup- -C9)n
4
3
(
5
-NPs), and (f) white light emission spectrum, where (a) and (b) were taken in EtOAc and (c), (d),
−5
and (e) were taken in toluene. The concentration of all samples is 1.0 × 10 M.
Our study of photoswitchable dielectric amplification phenomena is based on photoexcited
plasmonic resonance energy transfer to induce the intramolecular charge polarizati−on of
cis-cup-tris[C60(>DPAF-C9)] forming the corresponding dielectric ion-radical components (C60>)
·
and
DPAF+ -C9. In this event, C60> served as an electron (e−)-acceptor, whereas light-harvesting DPAF
·
antenna worked as an electron (e−)-donor. Accordingly, we first investigated the characteristics of
redox potentials of both C60> and DPAF moieties in the 3D-configurated nanostructure. Measurements
of the cyclic voltammetry (CV) were carried out on the sample of cis-cup-3-C9 in a mixture of
−
toluene and (n-butyl)4N+-PF6 electrolyte, using Pt as both working and counter electrodes and
Ag/AgCl as the reference electrode. To provide a clear data interpretation, CV characteristics of C60
(>DPAF-C9) were applied as the reference under variation of cyclic oxidation and reduction voltages
vs. Ag/Ag+ from
−
2.0 to 1.5 V. It displayed one reversible oxidation (1Eox of 0.95 V)-reduction
(1Ered of 1.07 V) cycle wave at positive voltages and at least two reversible reduction (1Ered of
2
2
−0.43 V and Ered of −0.88 V)-oxidation (1Eox of
−
0.62 V and Eox of
−
1.10 V) cycle waves at
negative voltages (Figure 5(Ac)). These potential values were compared by those of a set of separated
molecular components using a C60> monoadduct and cis-cup-tris(DPAF-C9) as models for confirmation,
1
2
3
showing reversible Ered/1Eox of
−
0.40/
1.62 V vs. Ag/Ag+ for the C60> monoadduct (Figure 5(Aa)) and one reversible cyclic wave
of 1Eox/1Ered as 0.93/1.03 V for cis-cup-tris(DPAF-C9) (Figure 5(Ab)). Once the compound
-C9 was
molecularly tripled to cis-cup- -C9, the cyclic redox wave corresponding to DPAF moieties at positive
voltages fully disappeared, as shown in Figure 5(Ad). However, two clearly reversible 1Ered/1Eox
−
0.71 V, Ered/2Eox of
−
0.82/
−
1.14 V, and Ered/3Eox of
−
1.33/
−
1
3
(
−
0.23/
0.97/
−
0.55 V) and 2Ered/2Eox
(
−
0.75/
−
0.99 V) with one less obvious cyclic wave of 3Ered/3Eox
-C9
(
−
−
1.15 V) vs. Ag/Ag+ corresponding to the redox characteristics of C60> moieties of cis-cup-
3

Molecules 2018, 23, 1873
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were observed. This revealed the occurrence of a partial one-electron oxidation process of e−-donor
DPAF-C9 moiety in the structure of cis-cup- -C9 in the presence of three C60> cages. This inhibited
3
the further electron-oxidation event in a clear manner. Since C60> is capable of undergoing at least
three reversible reduction/oxidation waves (Figure 5(Aa)), partial e−-transfer from DPAF may not
give significant influence on its CV diagram, except the first and second reduction potentials being
modified from −0.43 to −0.23 and −0.88 to −0.75 V vs. Ag/Ag+, respectively.
Most importantly, we were able to apply the white LED light irradiation (30 min) concurrent
with the CV measurement to obtain the insight of redox characteristics of cis-cup-3-C9 under similar
photoexcitation conditions used in the dielectric property measurements of the same compound
in a form of core-shell nanoparticles, as described below. As a result, Figure 5(Ae) displayed a
similar cyclic wave profiles as those of Figure 5(Ad) showing two clearly reversible waves with
1Ered/1Eox
(
−
0.32/
−
0.55 V) and 2Ered/2Eox
(
−
0.70/
0.99 V) vs. Ag/Ag+ that was attributed to the
−
redox characteristics of C60> moieties. No clear oxidation waves of DPAF moieties were detected
that was indicative of an effective intramolecular e−-transfer from DPAF-C9 to C60> upon light
irradiation to produce three ion-radical pairs per molecule as cis-cup-tris[C60
polarized charge-form, DPAF moieties were not able to undergo further electron-oxidation process,
consistent with the observed data. Furthermore, these CV results may be correlated with or indicative
−
·
(>DPAF-C9)+
]. At this
·
of similar charge-polarization phenomena to occur in the solid state of (cis-cup-
core-shell 5-NPs.
3-C9)-encapsulated
Figure 5. (A) Cyclic voltammograms (CV) of (a) C60>, (b) cis-cup-tris(DPAF-C9), (c) C60 (>DPAF-C9)
(1-C9), (d) cis-cup-tris[C60(>DPAF-C9)] (with no light), and (e) cis-cup-tris[C60(>DPAF-C9)] (after white
LED lig−ht3 irradiation for 30 min) at different voltages vs Ag/Ag+ in a solution concentration of
−
1.0 × 10
M
in CH3CN-CH2Cl2, containing (n-butyl)4N+-PF6 electrolyte (0.2 M), using Pt as working
and counter electrodes and Ag/AgCl as the reference electrode at a scan rate of 10 mV/s. (
time-dependent relative dielectric constant (εr ) amplification of trilayered core-shell nanoparticles of (a)
B) Irradiation
0
(γ-FeOx@AuNP)@[C60(>DPAF-C9)]n (6-NPs) and (b) (γ-FeOx@AuNP)@{cis-cup-tris[C60(>DPAF-C9)]}n
(5-NPs) at the frequency of 1.0 GHz. White LED light irradiation period was 60 min.
The reflectivity of a subject is controlled by the refractive index of the subject surface medium
that is a function of the product of permittivity (εr) in a complex form and permeability (µ). Both are
relevant material parameters in response to incident electromagnetic waves. The former complex
0
0
permittivity can be presented by the equation εr* = εr −
i
εr”, where εr is the real part as the relative
0
dielectric constant and εr” is the imaginary part as the loss factor. The former parameter εr is dependent
on charge-polarization of the material at the wavelength of measurements. In the experimental data
collection, the complex relative electric permittivity (and) values were measured in terms of complex
reflection coefficient of electromagnetic waves in proportion to a complex scattering parameter, defined

Molecules 2018, 23, 1873
9 of 16
as S11. The scattering parameter S11 was measured by a high-performance coaxial probe (Agilent
85070E) [27]. It was then converted to complex relative electric permittivity (εr*) values using the
0
Nicolson-Ross algorithm [28]. From this measurement and calculation, the value of εr was derived.
Surface plasmon resonance (SPR) energy phenomena has been demonstrated and applied in
many technological areas including the use of it as an alternative means to increase either light
absorption or scattering in a thin film to enhance solar cells efficiency [29,30]. In principle, SPR energy
occurs as the result of collective oscillation of surface electrons in gold nanoparticles that is induced
by the interaction with the incident light. It leads to polarization with the formation of polaritons
leading to the influence of absorption cross-section enhancement and light emitting [31]. In our study,
we applied core-shell nanoparticles to investigated the possibility of tuning the SPR energy band to
match with absorptions of dielectric materials for the induced intramolecular electron polarization
that led to the modulation of dielectric constant and related physical property of the material layer
on core-shell nanoparticles of
5
-NPs. The approach coincides with our recent results of ultrafast
photoinduced intramolecular e−-transfer phenomena in femtoseconds [19
,
20]. Therefore, we found
that a near-field effect within a few nanometers was crucial to enhance SPR energy accumulation
during the energy-transfer event between plasmonic AuNPs and C60-(antenna)x core-shell layers
by direct contacts. Accordingly, we assembled a multiple layered structure of NPs having an inner
magnetic γ-FeOx@AuNP core encapsulated by an outer cis-cup-tris[C60(>DPAF-C9)] shell, as described
above. The NP fabrication was coupled by the control of the layer thickness of cis-cup-
3-C9 in terms of
the quantity weight ratio to that of γ-FeOx@AuNP (4-NPs).
We first evaluated the degree difference of dielectric property amplification between
-NPs) and cis-cup- -C9 (giving -NPs) using the same plasmonic core-shell -FeOx@AuNP core. As a
result, Figure 5B showed irradiation time-dependent relative dielectric constant (εr0) curves of
-NPs
1-C9 (giving
6
3
5
γ
5
and 6-NPs taken at the microwave frequency of 1.0 GHz during and after a 60-min white LED light
illumination (corresponding to a total light fluence of 12.8 J/cm2). Apparently, microwave EM wave
showed a higher sensitivity to the NP surface materials for us to note the detected initial dielectric
0
constant value at the time zero min being comparable in value to that of organic
1
-C9 (εr 0min = 1.55)
0
and cis-cup-
NPs having a much higher εr
3
-C9 (εr
= 2.48) even though the inner shell layer of AuNPs and the core
γ
-FeOx
0min
0
0
(=9.0) value. A slightly higher εr
for the latter is interesting
0min
0min
since higher dielectric constant organics may be advantageous in energy devices. In all experiments,
◦
white light illumination led to a slight rise in temperature to 35–45 C in the first 15 min. Slight increase
of chamber temperatures away from 25 ◦C resulted in a small increase of the εr value in a similar
degree among these two samples. The εr value was then remaining relatively steady during the
0
0
rest of irradiation period up to 60 min. Upon turning-off the light at the end of 60-min irradiation,
0
we observed a large sharp raise of the εr value for both samples while the chamber temperature
dropped quickly back to 25 ◦C. Sharp increase of the permittivity at the light-off stage was found to
0
reach a peak maximum (εr max) value of 5.75 for
5
-NPs (Figure 5(Bb)) and 3.75 for
6
-NPs (Figure 5(Ba)).
0
0
They can be accounted by a ratio of εr max
respectively. Similar calculations based on the ratio of εr max
were found to be 1.76- and 1.74-fold increase in the εr value for
/
εr 0min in a 2.35- and 2.42-fold increase for
5
-NPs and -NPS,
6
0
0
0
/
εr
(the εr value at time 60-min)
60min
0
0
5-NPs (εr 60min = 3.26, Figure 5(Bb))
0
and
6
-NPs (εr
= 2.16, Figure 5(Ba)), respectively. Apparently, fabricated core-shell structural
60min
configurations were capable of inducing permittivity amplification at a RF-frequency of 1.0 GHz.
We also investigated the weight ratio (cis-cup- -C9/ -FeOx@AuNP)-dependent dielectric property
of trilayered ( -FeOx@AuNP)@{cis-cup-tris[C60(>DPAF-C9)]}n -NPs) for the comparison at the
3
γ
γ
(5
frequency of 1.0 GHz. Similar sharp increas0e of the permittivity at the light-off stage was found
for all samples to reach a peak maximum (εr max) value of 5.46 (Figure 6(Aa)), 6.60 (Figure 6(Ab)),
7.03 (Figure 6(Ac)), and 8.39 (Figure 6(Ad)) for the weight ratio of01:2, 1:1, 2:1, and 3:1, respectively.
0
0
0min
They ca0n be accounted by a0 corresponding ratio of εr max
/
εr
in a 2.80- (εr
= 1.95),
0min
0
2.75- (εr
= 2.40), 2.42- (εr
= 2.90), and 1.90-fold (εr
= 4.42) increase, respectively.
0min
0min
0min
Interestingly, a higher quantity of cis-cup-3-C9 applied led to a progressively increase of the initial

Molecules 2018, 23, 1873
10 of 16
0
dielectric εr 0min value up to 4.42 higher than that of many organic materials and polymers. This higher
0
0
0
0
εr
value resulted in a lower degree of amplification εr max εr 0min. However, a high dielectric
εr max value of 8.39 (Figure 6(Ad)) achieved in organic-inorganic hybrid nanomaterials is significant
for potential applications. Finally, similar calculations based on the ratio of εr max
to be 1.82- (εr
Figure 6(Ac)), and 1.42-fold (εr
constant ratios of either εr max
/
0min
0
0
/
εr
were found
= 4.21,
60min
60min
0
0
0
= 3.0, Figure 6(Aa)), 1.80- (εr
= 3.67, Figure 6(Ab)), 1.67- (εr
60min
60min
0
= 5.91, Figure 6(Ad)) increase, respectively. Relative dielectric
60min
0
0
0
0
0
as the εr value shown at each peak maximum
60min
/
εr
or εr max
/
εr
0min
vs that at either 0 min or 60 min, respectively, in Figure 6B indicated a progressive increase of the
degree of dielectric amplification upon the quantity increase of cis-cup-
3
-C9 up to a weight ratio of
cis-cup-3-C9/γ-FeOx@AuNP as 2.0/1.0.
Weight Ratio (cis-cup-3-C9 : 4-NPs)
(A)
8
(d) 3:1
(c) 2:1
(b) 1:1
(a) 1:2
(B)
3.0
2.5
2.0
1.5
7
6
5
4
3
2
light-off
light-off
light-on
light-on
εr’max / εr’0min
εr’max / εr’60min
(d)
(c)
(b)
(a)
0.5
1.0
1.5
2.0
Weight Ratio (cis-cup-3-C9 : 4-NPs)
0
20
40
60
80
Time (min)
Figure 6.
core-shell nanoparticles of (
weight ratio of cis-cup- -C9 vs
LED light irradiation period of 60 min. (
(
A
) Irradiation time-dependent relative dielectric constant (εr0) amplification of trilayered
-FeOx@AuNP)@{cis-cup-tris[C60(>DPAF-C9)]}n ( -NPs) with a different
-FeOx@AuNP ( -NPs) at the frequency of 1.0 GHz with a white
) Relative dielectric constant (εr0) ratios of the values at
γ
5
3
γ
4
B
peak maximum vs that at either 0 min or 60 min, showing a progressive increase of the degree of
dielectric amplification.
3. Experimental Section
3.1. Chemicals and Reagents
Reagents of aluminum chloride (AlCl3), tris(dibenzylideneacetone)dipalladium(0) [Pd2(dba)3(0)],
0
0
α
-bromoacetyl bromide, rac-2,2 -bis(diphenylphosphino)-1,1 -binaphthyl (BINAP), sodium t-butoxide,
and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) were purchased from Aldrich Chemicals (St. Louis, MO,
USA) and used without further purification. A C60 sample with a purity of 99.0% was purchased
from Term USA, Inc. (Fort Bragg, CA, USA) The anhydrous grade solvent of THF was refluxed over
sodium and benzophenone overnight and distilled under reduced pressure (10−1 mmHg). Sodium
sulfate was used as the drying agent. The precursors 1,3,5-tris(N-phenylamino)benzene (TPAB)
and 2-bromo-9,9-bis(30,50,50-trimethyl-10-hexyl)fluorene (BrF-C9) were synthesized according to our
previous procedures [22].
3.2. Instruments for Spectroscopic Measurements
Infrared spectra were recorded as KBr pellets on a Thermo Nicolet AVATAR 370 FTIR spectrometer
(Thermo Scientific Nicolet, Waltham, MA, USA). UV-vis spectra were recorded on a PerkinElmer
Lambda 750 UV spectrometer (PerkinElmer, Shelton, CT, USA). 1H NMR and 13C NMR spectra were
recorded on Bruker & Spectrospin Avance 500 (Bruker, Billerica, MA, USA). The high performance

Molecules 2018, 23, 1873
11 of 16
liquid chromatography (HPLC) was performed using the
µ µm,
PorasilTM 125 Å Column (10
35 × 300 mm, Waters, Milford, MA, USA) with a mixture of hexane-EtOAc as the mobile phase
and the flow rate of 1.0 mL/min. The eluted compounds were detected by their UV absorption at
350 nm.
λ
Cyclic voltammetry (CV) was record on EG&G Princeton Applied Research 263A
Potentiostat/Galvanostat (Ametek Inc., Berwyn, PA, USA) using Pt metal as the working electrode,
Ag/AgCl as the reference electrode, and Pt wire as the counter electrode at a scan rate of 10 mV/s.
The solution for CV measurements was prepared in a concentration of 1.0
×
10−3 M in CH3CN–CH2Cl2,
containing the electrolyte Bu4N+-PF6 (0.2 M). Transmission electron microscopy (TEM) measurements
were carried out on Philips EM400T Transmission Electron Microscope (FEI Co., Hillsboro, OR,
USA). In the sample preparation, a carbon-Copper film grid in a 200-mesh size was used as the
−
supporting plate for directly coating of a nanoparticle sample in a solution of 1.0
grid. All solvents were removed and dried by the freeze-dry technique to prevent the solvent-removal
induced particle aggregation.
×
10−6 M on the
3.3. Synthesis of N1,N3,N5-Tris(9,9-di(30,50,50-trimethyl-10-hexyl)fluoren-2-yl)-1”,3”,5”-
tris(phenylamino)benzene, tris(DPAF-C9) as 2-C9
Synthetic procedure of tris(DPAF-C9) was followed by and slightly modified from the recently
reported methods [22]. A typical procedure was given as follows. A mixture of BrF-C9 (10.0 g,
20.3 mmol, excess), TPAB (1.16 g, 3.30 mmol), Pd2(dba)3(0) (0.023 g, 0.25 mol%), BINAP (0.046 g,
0.75 mol%), and sodium t-butoxide (1.94 g, 20.3 mmol) taken in anhydrous toluene (75 mL) was
heated to refluxing temperature under nitrogen for a period of 72 h. After being cooled to room
temperature, the reaction mixture was washed with water for three times and dried over sodium
sulfate. A crude brown colored paste was obtained after evaporating the solvent. It was subjected to
column chromatography purification using silica gel as the stationary phase and hexane-ethylacetate
(9:1) as the eluent. The product of tris(DPAF-C9) was collected at Rf = 0.8 as light yellow solids in 83%
yield (4.35 g).
Since the HPLC indicated a mixture of tris(DPAF-C9) stereoisomers, further separation and
isolation were carried out by thin-layer chromatography (TLC), using silica gel as the stationary phase
and hexane-ethylacetate (9.5:0.5, v/v) as the eluent. A mid-broad band at Rf 0.55 0.7 was identified
−
as the tris(DPAF-C9) products that can be cut evenly into two parts at Rf 0.55–0.63 (as the bottom
1
cut) and Rf 0.63–0.7 (as the top cut). Based on their corresponding H NMR spectra, the top cut
was confirmed to be the pure cis-cup-tris(DPAF-C9) stereoisomer and the bottom cut was consisting
of a mixture of trans-chair-tris(DPAF-C9) and cis-cup-tris(DPAF-C9) stereoisomers in the ratio of
roughly 2:3. This fraction was subjected to further TLC separation and elution two more times
using hexane-ethylacetate (9.7:0.3, v/v) as the eluent for the first run and the similar mixture (9.8:0.2,
v/v) for the second run to obtain cis-cup-tris(DPAF-C9) in improved purity and the pure form of
trans-chair-tris(DPAF-C9) fraction.
Spectroscopic data of cis-cup-tris(DPAF-C9): FT-IR (KBr) υmax 3064 (w, aromatic C-H stretching),
3038 (w), 3011 (w), 2956 (vs, aliphatic C-H stretching), 2908 (m), 2865 (s), 1596 (m), 1584 (s, C=C),
1495 (s, anti-symmetric deformations of CH3 groups and scissor vibrations of CH2 groups), 1468 (m),
1450 (s), 1363 (m, symmetric deformations of CH3 groups), 1296 (m, asymmetric stretching vibrations
of C-N-C), 1246 (m, asymmetric stretching vibrations of C-N-C), 1215 (w), 1176 (w), 1157 (w), 1030 (w),
933 (w), 826 (w), 754 (m), 736 (s, C-H out-of-plan deformation), 710 (m, C-H out-of-plan −d1eformation),
−5
694 (m), and 508 (w) cm−1; UV-vis (EtOAc, 1.0
×
10 M) λmax
(
δ
ε
) 323 (5.62
×
104 L mol cm−1) and
353 nm (5.65
×
104 L mol−1 cm−1); 1H NMR (500 MHz, CDCl3)
(m, 9H), 7.09–7.01 (m, 18H), 6.82 (t, 3H), 6.55 (s, 3H), 1.92–1.67 (m, 12H), 1.10 (m, 6H), and 0.97–0.39
(m, 96H); 13C NMR (500 MHz, CDCl3)
151.7, 150.0, 149.2, 147.7, 146.5, 141.2, 136.8, 129.2, 126.6, 126.6,
124.0, 122.9, 121.7, 120.0, 119.3, 115.6, 54.5, 51.0, 50.6, 50.6, 38.3, 38.1, 37.4, 37.6, 33.1, 32.7, 31.1, 30.2,
7.58 (s, 3H, br), 7.52 (d, 3H), 7.32–7.21
δ

Molecules 2018, 23, 1873
12 of 16
29.6, 29.4, 27.2, 22.9, and 22.4; MALDI–TOF MS calcd for C117H153N3, m/z 1600.21; found, m/z 1601.26
(MH+), 1602.30, 1603.29, 1260.95, 1092.81, 752.23 and 508.79 (PhN+H + fluorene-C9).
Spectroscopic data of trans-chair-tris(DPAF-C9): FT-IR (KBr) υmax 3062 (w, aromatic C-H
stretching), 3037 (w), 3012 (w), 2955 (vs, aliphatic C-H stretching), 2925 (vs., aliphatic C-H stretching),
2855 (s), 1630 (vs), 1588 (s, C=C), 1495 (s, anti-symmetric deformations of CH3 groups and
scissor vibrations of CH2 groups), 1450 (s), 1362 (m, symmetric deformations of CH3 groups),
1294 (m, asymmetric stretching vibrations of C-N-C), 1250 (m, asymmetric stretching vibrations
of C-N-C), 1218 (w), 1158 (w), 1035 (w), 803 (w), 763 (m), 736 (m, C-H out-of-plan deformation),
709 (w, C-H out-of-plan deformation), 691 (m), and 510 (w) cm−1; UV-vis (EtOAc, 1.0
×
10−5 M) λmax
7.58 (s, 3H,
(
ε
) 322 (5.50 δ
×
104) and 353 nm (5.68 104 L mol−1 cm−1); 1H NMR (500 MHz, CDCl3)
×
br), 7.51 (d, 3H), 7.34–7.20 (m, 9H), 7.09–7.01 (m, 18H), 6.84 (m, 3H), 6.55 (m, 2H), 6.48 (m, 1H), 2.02–1.67
(m, 12H), 1.11 (m, 6H), and 0.97–0.39 (m, 96H); 13C NMR [similar to those of cis-cup-tris(DPAF-C9)];
MALDI
TOF MS calcd for C117H153N3, m/z 1600.21; found, m/z 1601.37 (MH+), 1602.36, 1603.35,
−
1604.40, 1260.00, 1093.86, and 508.73 (PhN+H + fluorene-C9).
3.4. Synthesis of N1,N3,N5-Tris(7-a-bromoacetyl-9,9-di(30,50,50-trimethyl-10-hexyl)fluoren-2-yl)-1”,3”,5”-
tris(phenylamino)benzene, cis-cup-Tris(BrDPAF-C9)
◦
To a suspension of AlCl3 (1.0 g, 7.5 mmol) in 1,2-dichloroethane (40 mL) at 0 C was added
cis-cup-tris(DPAF-C9) (0.7 g, 0.44 mmol) with vigorously stirring. The compound of α-bromoacetyl
bromide (1.0 g, 5.0 mmol) was then slowly added over 10 min while maintaining the temperature
◦
between 0–10 C. The mixture was warmed to ambient temperature and stirred overnight. The reaction
was quenched by slow addition of water (50 mL) while maintaining the temperature below 45 ◦C.
The organic layer was washed sequentially with dil. HCl (1.0 N, 50 mL) and water (50 mL
×
2),
and dried over sodium sulfate and then concentrated in vacuo to give the crude product as viscous
yellow semi-solids. It was purified by column chromatography (silica gel) using hexane–EtOAc
(9:1, v/v) as eluent to afford tris(Br-DPAF-C9) in 53% yield (0.46 g). Spectroscopic data: FT-IR (KBr)
υmax 3066 (w), 3035 (w), 2954 (s), 2907 (m), 2864 (m), 1733 (w), 1675 (s), 1594 (s), 1584 (s), 1492 (m),
1464 (m), 1429 (m), 1393 (w), 1364 (w), 1343 (w), 1288 (s), 1262 (m), 1205 (w), 1180 (m), 1107 (w),
1022 (w), 819 (m), 748 (m), 713 (m), and 696 (m) cm−1; UV-vis (EtOAc, 1.0
×
10−5 M) λmax
(ε) 304
1
(8.88
×
104 L mol−1 cm−1) and 395 nm (1.10
×
105 L mol−1 cm−1); H NMR (500 MHz, CDCl3)
δ
7.94 (m, 3H), 7.88 (m, 3H), 7.63 (m, 3H), 7.56 (m, 3H), 7.22 6.92 (m, 21H), 6.56 (m, 3H), 4.46 (m, 6H),
−
2.11−1.67 (m, 12H), 1.10 (m, 6H), 0.99–0.26 (m, 96H).
3.5. Synthesis of N1,N3,N5-Tris(7-(1,2-dihydro-1,2-methanofullerene[60]-61-carbonyl)-9,9-di(30,50,50-
trimethyl-10-hexyl)fluoren-2-yl)-1”,3”,5”-tris(phenylamino)benzene, cis-cup-Tris[C60>(DPAF-C9)] as
cis-cup-3-C9
A homogeneous solution of C60 (0.70 g, 0.97 mmol)) in anhydrous toluene (700 mL) was prepared
by ultrasonication for a period of 1.0 h that was then stirred overnight under nitrogen. To this
was added by cis-cup-tris(BrDPAF-C9) (0.40 g, 0.20 mmol) and 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU, 0.21 g, 1.38 mmol) sequentially and stirred at room temperature for a period of 8.0 h. At the
end of stirring, the reaction mixture was concentrated to a 10%-volume. Methanol (100 mL) was
added to effect precipitation of the crude product, which was isolated by centrifugation. Further
purification by column chromatography (silica gel) using a solvent mixture of hexane–toluene (3:2) as
the eluent afforded cis-cup-tris[C60>(DPAF-C9)] as brown solids in 68% yield (0.52 g). Spectroscopic
data: FT-IR (KBr) υmax 2951 (m), 2923 (m), 2861 (w), 1681 (m), 1627 (s), 1594 (s), 1585 (s), 1490 (m),
1462 (m), 1429 (w), 1361 (w), 1317 (m), 1292 (m), 1247 (m), 1213 (m), 1184 (m), 1093 (w), 1035 (w),
908 (m), 819 (m), 732 (s), 698 (m), 575 (w), and 529 (s) cm−1; UV-vis (toluene, 1.0
335 nm (1.76
105 L mol−1 cm−1) and 402 nm (shoulder band, 4.19 104 L mol−1 cm−1); 1H NMR
(500 MHz, CDCl3) 8.46 (m, 3H), 8.30 (m, 3H), 7.81 (m, 3H), 7.63 (m, 3H), 7.31 6.70 (m, 21H), 6.60 (m,
3H), 5.67 (m, 3H), 1.92
1.67 (m, 12H), 1.14 (m, 6H), 0.70 (m, 96H); 13C NMR (500 MHz, CDCl3)
×
10−5 M) λmax
(ε)
×
×
δ
−
−
δ

Molecules 2018, 23, 1873
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148.67, 148.56, 148.46, 147.89, 147.70, 147.18, 145.96, 145.82, 145.72, 145.68, 145.58, 145.47, 145.24, 145.13,
145.05, 144.73, 144.60, 144.48, 144.35, 144.28, 144.18, 144.14, 144.03, 143.94, 143.87, 143.72, 143.50, 143.41,
143.20, 142.92, 142.65, 142.50, 141.64, 141.35, 139.92, 137.07, 134.75, 134.03, 133.86, 129.61, 129.47, 124.34,
124.13, 123.53, 119.78, 116.61, 84.81, 72.88, 55.60, 51.55, 51.32, 51.02, 50.89, 45.07, 38.33, 38.02, 37.85,
37.67, 33.33, 31.31, 30.37, 29.93, 27.66, and 22.99.
3.6. Preparation of cis-cup-Tris[C60>(DPAF-C9)]-Encapsulated
γ-FeOx@AuNP Yielding Trilayered Core-Shell
Nanoparticles, (γ-FeOx@AuNP)@{cis-cup-tris[C60(>DPAF-C9)]}n (5-NPs)
Nanoparticles of -FeOx and the corresponding plasmonic metallic gold-coated
nanoparticles ( -NPs) were synthesized according to a slightly modified procedure reported
previously [14 15]. A large scale preparation was carried out for the subsequent fabrication procedure
using the same batch of materials to give consistency of -FeOx@AuNP characteristics in terms of
the particle size and the Au-layer thickness. Spectroscopy data of -NPs: FT-IR (KBr) νmax 3027 (w),
2956 (w), 2923 (m), 2856 (m), 1631 (s), 1492 (m), 1461 (s), 1378 (m), 1328 (w), 1263 (w), 1166 (w), 1128 (w),
1076 (w), 1037 (w), 804 (m), 728 (m), and 588 (s) cm−1
Encapsulation of cis-cup-tris[C60>(DPAF-C9)] on nanoparticles of
by the method as follows. A mixture of -FeOx@AuNPs (100 mg) and cis-cup-
γ
γ-FeOx@AuNP
4
,
γ
4
.
γ
-FeOx@AuNP was carried out
-C9 in a predefined
γ
3
weight ratio amount were dissolved in toluene (30 mL) with stirring for 30 min and then ultrasonicated
for an additional >30 min until a clear solution being obtained showing a homogenized nanoparticles
dispersion. The solution was concentrated via rotary evaporation to less than 3.0 mL in volume to
increase the molecular contact of cis-cup-3-C9 to 4-NPs. It was then diluted by toluene to a volume of
30 mL with stirring and subsequent sonication for 10 min to dissolve cis-cup-3-C9 fully. All encapsulated
magnetic nanoparticles were physically removed from the container solution by the assistance of an
external permanent magnet. The separated nanoparticles were washed repeatedly by ethanol and ether,
followed by drying in vacuo to afford dark brown solids of 5-NPs. Spectroscopy data: FT-IR (KBr)
νmax 3089 (w), 3064 (w), 3031 (w), 2952 (m, aromatic C-H stretching), 2927 (s, aliphatic C-H stretching),
2860 (m), 1679 (m, -C=O), 1636 (m), 1587 (s, -C=C-), 1492 (m), 1463 (m), 1430 (m), 1363 (w), 1291 (m,
asymmetric stretching vibrations of C-N-C), 1247 (w), 1214 (m), 1187 (m), 1126 (w), 1091 (w), 1035 (m),
904 (m), 815 (w), 730 (m, C-H out-of-plan deformation), 695 (m), 576 (m), and 526 (s, C60>) cm−1
.
0
3.7. Measurements of Dielectric Properties and Permittivity as Relative Dielectric Constant (εr )
Microwave-responsive dielectric property measurements were carried out by the use of an
Agilent Network Analyzer (Agilent Technologies, Inc., Santa Clara, CA, USA) equipped with an
open-ended Agilent 85070E dielectric probe kit in a detection range of 200 MHz to 50 GHz. Prior to
each measurement, the system base-line calibration was conducted by using open-ended, close-ended,
and attenuated calibrators to ensure the minimization of signal instability that was induced by the cable
connection and system drift errors. In the data collection, a complex scattering parameter, defined as
S11, was measured and converted to relative complex electric dielectric constant values using Agilent
85071E Materials Measurement Software. This complex form of dielectric constant (
a real (εr ) and an imaginary (εr”) part. The former term represents the value of dielectric constant and
the latter is defined as the loss factor.
ε
*) consists of both
0
All permittivity measurements were performed in a custom-built chamber that was conducted
under a circumferentially uniform illumination environment. The uniformness was achieved by the
installation of a reflective half-circular aluminum plate at the back-wall side surrounding the testing
tube which was located at the center of the chamber. The light source used in this measurement
included a collimated white LED light with an output power of 2.0 W (Prizmatix, Southfield, MI, USA).
The light beam was allowed to pass through a small window opening at the front side of the chamber
for the sample irradiation. Some reflected light beams were designed to refocus from the back-side
aluminum mirror plate back to the sample-containing tube. We installed four small fans with two at the
top and one at each side-wall of the chamber to control and prevent the temperature increase inside the

Molecules 2018, 23, 1873
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chamber during the experiment. The illumination period was fixed at 60 min. Poly(dimethylsiloxane)
(PDMS, 1.0 g) semi-solid was applied as a polymer matrix host which is capable of forming a paste-like
material sample in mixing with a predefined quantity of nanoparticles. In a typical preparation,
a mixture of PDMS and core-shell nanoparticle materials (100 mg) were blended by dissolving both
components in ethyl acetate (20 mL) in a testing tube under sonication. It was followed by solvent
evaporation to yield paste-like semi-solid materials filling in the tube.
4. Conclusions
We designed and synthesized novel 3D-configurated stereoisomers cis-cup-tris[C60>(DPAF-C9)]
and trans-chair-tris[C60>(DPAF-C9)] in good yields. Efficient chromatographic separation was carried
out to obtain both stereoisomers in a pure form with their corresponding nanostructure verified by
spectroscopic techniques. The former with three C60> cages per molecule facing at the same side of
the geometrical molecular cup-shape was proposed to provide excellent binding interaction forces
at the gold surface of core-shell γ-FeOx@AuNP nanoparticles to direct the subsequent formation
of a fullerene cage array (defined as fullerosome). Upon photoactivation of the Au-layer and
cis-cup-tris[C60>(DPAF-C9)] itself, the level of photoinduced intramolecular e−-transfer from DPAF
to C60> moieties was found to be largely enhanced by the accumulated plasmonic resonance energy
at the near-field surface. Distribution of resulting negative charges along the outer (C60>)-derived
fullerosome shell layer of the trilayered NPs was correlated with the detected photoswitchable dielectric
amplification changes using white LED light at 1.0 GHz. This class of new materials exhibits potential
in microwave applications as its photoswitching responsive wavelength range is being extended to
cover WiFi-bands over 2.4/3.6/5.0 GHz.
Author Contributions: All authors contribute a significant effort on this work. H.Y. and M.W. carried out the
main synthetic works, spectroscopic characterization, data analysis, and permittivity measurements; T.Y. setup
the permittivity and reflectivity measurements and carried out dielectric properties conversion; T.Y., L.-S.T. and
L.Y.C. participated in the discussion and experimental studies and contributed to a part of manuscript writing;
All authors read and approved the final manuscript.
Funding: The authors at UML thank the financial support of Air Force Office of Scientific Research (AFOSR)
under the grant number FA9550-14-1-0153.
Conflicts of Interest: The authors declare no conflicts of interest.
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Sample Availability: Samples of all synthetic compounds including cis-cup-tris(BrDPAF-C9) and
cis-cup-tris[C60>(DPAF-C9)] are available from the authors..
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2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
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(CC BY) license (http://creativecommons.org/licenses/by/4.0/).