Controlled photophysical behaviors between dibenzo-24-crown-8 bearing terpyridine moiety and fullerene-containing ammonium salt

Article abstract of DOI:10.1021/jo102311m

A novel [2]pseudorotaxane was successfully constructed by the complexation of dibenzo[24]-crown-8 (DB24C8) derivative bearing terpyridine moiety (1) with lanthanide ion (Tb3〈) and fullerene-containing ammonium salt (2), exhibiting the controlled photophys

Full text of DOI:10.1021/jo102311m

NOTE
pubs.acs.org/joc
Controlled Photophysical Behaviors between Dibenzo-24-crown-8
Bearing Terpyridine Moiety and Fullerene-Containing
Ammonium Salt
Zhi-Jun Ding, Ying-Ming Zhang, Xue Teng, and Yu Liu*
Department of Chemistry, State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, P.R. China
S Supporting Information
b
ABSTRACT: A novel [2]pseudorotaxane was successfully con-
structed by the complexation of dibenzo[24]-crown-8 (DB24C8)
derivative bearing terpyridine moiety (1) with lanthanide ion
(Tb3þ) and fullerene-containing ammonium salt (2), exhibiting
the controlled photophysical behaviors as a reversible luminescent
lanthanide switch in the presence of Kþ or 18-crown-6 (18C6).
onstruction of rotaxanes and pseudorotaxanes1 has attracted
tremendous interest because of not only their mechanically
Scheme 1. Syntheses of 1 and the Molecular Structure of 2
C
interlocked topologies2 but also their unique photophysical and
electrochemical properties that have potential applications in the
field of artificial molecular machinery, such as nanoelectronics,3
smart surface materials,4 drug-loading nanoparticles,5 and non-
viral gene-delivery vectors.6 Triggered by external stimuli,7
including pH changes, metal ions, temperature or electrochemi-
cal redox,8 these multicomponent assemblies could be modu-
lated in a precisely controlled manner. Recently, Stoddart and
co-workers9 have designed a source-connector-drain na-
noarchitectures, where [Ru(bpy)3]2þ (bpy =2,20-bipyridine)-
modified dibenzo-24-crown-8 (DB24C8) could be connected
to the viologen by benzonaphtho[36]crown-10 (BN36C10)
with a dialkylammonium ion center, displaying a bottom-up con-
struction of the photoinduced electron flow in the self-assembling
supramolecular cable. Guldi and Hirsch et al.10 have reported a
series of supramolecular porphyrin-fullerene hybrids through
hydrogen-bonding interaction, leading to the long-lived electron-
transfer products in the range of tens of nanoseconds. More
recently, we have successfully constructed a tris[2]pseudorotaxane11
and bistable [3]rotaxane,12 revealing an excellent reversible lumi-
nescent lanthanide and pH-controlled intramolecular charge-
transfer behaviors, respectively. In the present work, we would like
to report the fabrication of a well-defined [2]pseudorotaxane
comprised of DB24C8 derivative bearing a terpyridine unit (1) as
chelators for lanthanide ions and fullerene-containing ammonium
salt (2), with the aim of comprehensive investigation on the
controlled photophysical behaviors of energy transfer (ET) and
photoinduced electron transfer (PET) process in the complex of
[1 Tb]3þ and [Tb 1 2]3þ, respectively. The results obtained in
3
3 3
this work indicate that PET process could be conveniently con-
trolled by the addition of Kþ or 18-crown-6 (18C6).
The synthetic route of 1 and the molecular structure of 2 were
described in Scheme 1. 2,6-Bis(20-pyridyl)-40-(p-hydroxyphe-
nyl)pyridine (4) was prepared from 4-methoxybenzaldehyde
and 1-(pyridin-2-yl)ethanone according to the reported
literature.13 Then, compound 4 reacted with 4-chloromethyldi-
benzo-24-crown-8 under basic condition to afford terpyridine
modified dibenzo-24-crown-8 (1) in 72% yield. Fullerene-con-
taining ammonium salt (2) was synthesized by Hirsch-Bingel
reaction of benzyl malonate and C60, followed by protonation
and counterion exchange.14
Received: November 23, 2010
Published: February 14, 2011
r
2011 American Chemical Society
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|

The Journal of Organic Chemistry
NOTE
Figure 1. Emission spectra of 1 (1.0 ꢀ 10-5 M) upon addition of
Figure 3. Emission spectra of [1 Tb]3þ complex (1.0 ꢀ 10-5 M) upon
3
Tb(NO3)3 6H2O (0-2.66 ꢀ 10-5 M, from a to i) in CH3CN/CHCl3
addition of 2 (0-1.33 ꢀ 10-5 M, from a to j) in CH3CN/CHCl3 (1:1,
3
v/v) solution (λex = 340 nm). Inset: fluorescence changes of [1 Tb]3þ
(1:1, v/v) solution (λex = 319 nm). Inset: fluorescence changes of 1 at
544 nm. The asterisk corresponds to 2λex signal.
3
at 544 nm upon the addition of 2.
solution of 1, the absorption peak of 1 at 285 nm was gradually
declined while the absorption peak at 345 nm was enhanced in
proportion, accompanied by an isosbestic point at 319 nm. These
phenomena described above jointly indicated the conversion of
free ligand 1 to the [1 Tb]3þ complex. Job’s plot was also
3
performed to confirm the coordination stoichiometry between 1
and Tb3þ, which showed a maximum peak at a molar fraction of
0.5, corresponding to a 1:1 1/Tb3þ stoichiometry (Figure S5,
Supporting Information). After validating the 1:1 binding stoi-
chiometry, the complex formation constant (Ks) between 1 and
Tb3þ was calculated to be (1.8 ( 0.7) ꢀ 105 M-1 by analyzing
the sequential changes in absorption intensity (ΔA) of 1 at
varying concentrations of Tb3þ using a nonlinear least-squares
curve-fitting method (Figure 2, inset), indicating the high affinity
of terpyridine and lanthanide ions.
Figure 2. Absorbance spectral changes of 1 (5.0 ꢀ 10-5 M) upon
addition of Tb(NO3)3 6H2O (0-6.0 ꢀ 10-5 M, from a to q) in
3
CH3CN/CHCl3 (1:1, v/v) solution. Inset: nonlinear least-squares
analysis of the differential intensity to calculate Ks.
As an intriguing class of molecules with unique physical and
chemical properties, fullerene and its analogues are known to
have excellent photosensitizing activities, especially under visible
and near-infrared lights.16 Many efforts have been devoted to the
preparation of various donor-acceptor systems by coordination
of fullerene derivatives bearing one pyridine or polypyridine17
unit with transition metals.18 When 1.3 equiv of compound 2
The quantitative investigation of the coordination complex of
1 with Tb3þ was examined by means of the fluorescence
spectroscopy titration. As seen in Figure 1, the fluorescence
intensity of 1 at 375 nm assigned to the terpyridine moiety was
dramatically decreased upon addition of Tb(NO3)3 6H2O to a
3
solution of 1 in CH3CN/CHCl3, accompanied with the obvious
bathochromic shift to 475 nm. In contrast, the emission of Tb3þ
at 491 nm (5D4 f 7F6), 544 nm (5D4 f 7F5), 586 nm (5D4 f
7F4), and 622 nm (5D4 f 7F3, overlapped the 2λex signal)15 was
gradually increased with a quantum yield of 0.050. According to
these observations, we can deduce that the strong fluorescence
bearing the C60 moiety was added to the solution of [1 Tb]3þ
,
3
approximately 90% of the fluorescence intensity of metal-ligand
complex was quenched at 544 nm with a quantum yield of 0.005
(Figure 3). These results were well consistent with our previous
observations.11 Considering the fullerenes are good electron
acceptors,18,19 these results indicated that there should be an
emission of [1 Tb]3þ complex may attribute to an energy
intramolecular PET process in [2]pseudorotaxane [Tb 1 2]3þ
3
3 3
transfer (ET) process, that is, the energy was transferred from
the excited state of terpyridine moiety to Tb3þ. Moreover, the
coordination stoichiometry between 1 and Tb3þ was explored by
the molar ratio method using fluorescence spectrometry. The
curve of ΔF (complex-induced changes of fluorescence in-
tensity) versus [Tb3þ]/[1] molar ratio showed an inflection
point at a molar ratio of 1, implying a 1:1 1/Tb3þ coordination
(Figure1, inset), which was further confirmed by the Job’s
experiment as described below.
from the excited singlet state of [1 Tb]3þ complex to C60 moiety
3
in solution. To confirm the assumption of PET mechanism in the
formation of [2]pseudorotaxane, some control experiments were
carried out. Instead of 2 with dibenzylammonium hexafluoro-
phosphate which did not contain C60 as an electron acceptor, the
luminescence of the metal-ligand complex [1 Tb]3þ was al-
3
most unchanged, which provided further evidence for the PET
process in self-assembled dyads [Tb 1 2]3þ (Figure S6, Sup-
3 3
porting Information). As shown in Figure S7, when even 100
equiv of Kþ was added in the solution of 1, the absorption
intensity was slightly changed, indicating that the interactions
between terpyridine moiety and Kþ were negligible.
1
We further performed H NMR titration to examine the
binding abilities between 1 and 2, showing that Ha and Ha’
shifted downfield 0.46 ppm, whereas Hb shifted upfield 0.19
ppm, respectively (Figure S4, Supporting Information). Detailed
In addition, cyclic voltammetry experiments were performed
in order to investigate the PET process quantitatively.18 The
peaks at -0.70, -1.22, and -1.74 V were assigned to reduction
potentials of 2, and the peak at 1.20 V was assigned to oxidation
discussion on the formation of [1 Tb]3þ complex were obtained
3
from absorption spectroscopy experiment. As discerned in
Figure 2, with the stepwise addition of Tb(NO3)3 6H2O to a
3
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The Journal of Organic Chemistry
NOTE
Figure 4. Cyclic voltammogram of [2]pseudorotaxane [Tb 1 2]3þ in
3
3
CH2Cl2 containing 0.1 M (t-C4H9)4NClO4 as the supporting electro-
lyte at 100 mVs-1 ([Tb 1 2]3þ = 5 ꢀ 10-4 M, vs Ag/AgNO3).
3
3
Figure 6. Emission spectral changes observed for [Tb 1 2]3þ
,
3
3
[Tb 1 2]3þ in the presence of Kþ, and [Tb 1 2]3þ in the presence
þ3
3
3 3
of K and 18C6, respectively. Inset: the emission changes of [Tb 1] 3þ
3
(1 ꢀ 10-5 M) in the presence of 2 (1, 1.1 ꢀ 10-5 M) and added KPF6
(2, 1.2 ꢀ 10-5 M; 4, 2.6 ꢀ 10-5 M; 6, 4.0 ꢀ 10-5 M) and 18C6 (3, 1.2 ꢀ
10-5 M; 5, 2.6 ꢀ 10-5 M) in CH3CN/CHCl3 (1:1, v:v) solution at
544 nm (λex = 340 nm).
Scheme 2. Schematic Representation of the Presumed
Coordination Mode for the [Tb 1 2]3þ Complex and
3 3
Their PET-ET Process
Figure 5. Emission spectra of the [Tb 1 2]3þ complex (1 ꢀ 10-5 M)
3
3
upon addition of Kþ (0-1.3 ꢀ 10-5 M, from a to h) in CH3CN/CHCl3
(1:1, v/v) solution (λex = 340 nm).
potential of [1 Tb]3þ, respectively (Figure 4, as well as Figures
3
S8 and 9, Supporting Information). Furthermore, the Rehm-
Weller equation (eq 1) was introduced to evaluate the Gibbs free
energy (ΔGPET) of an electron-transfer reaction, where E0,0 is
the excited singlet energies of [1 Tb]3þ (2.28 eV), Eox and Ered
3
values are the oxidation potential of [1 Tb]3þ (1.20 V) and the
3
reduction potential of 2 (-0.70 V), respectively, ε is the di-
electric constant of the solvent (εCH2Cl2 = 9.1), and d is the center
distance between C60 and terpyridine that could be estimated as
7.50 Å from the optimized molecule modulation of [Tb 1 2]3þ
3 3
(Figure S10, Supporting Information). Therefore, ΔGPET was
calculated to be -0.59 eV. Although there may be errors in
calculating of free-energy changes, due to the irreversible elec-
trochemical data in the present system,20 the ΔGPET value was
negative enough to drive the PET reaction in a thermodynami-
cally favorable way.
quenching when adding the pristine 18C6 to the solution. The
reversibility can be recycled in several times (Figure 6). Thus, a
reversibly connected donor-acceptor conjugate of metal-ligand
complex and fullerene derivative based on a [2]pseudorotaxane
between DB24C8 and secondary dibenzylammonium ions was
successfully constructed and the electronic communications be-
tween donor and acceptor sites could be efficiently adjusted via an
ion-controlled binding and release strategy.
14:4
ΔGPET ¼ Eox - Ered - ΔE0,0
-
ð1Þ
εd
Taking advantages of the different binding abilities, it is well-
documented that the assembling/disassembling processes of
[2]pseudorotaxane based on DB24C8 and secondary dialkylam-
monium salts can be reversibly governed by adding Kþ and 18C6
in series, which is proved to be a simple and accessible strategy for
constructing supramolecular switch systems.7,8,11 When KPF6
was added to the solution of [2]pseudorotaxane, the lanthanide
The possible mechanism for the switch may be explained as
follows. Before complexation with fullerene-containing ammo-
nium salt 2, the terpyridine moiety as a sensitizer or an antenna
can absorb the excitation light with a larger absorption coefficient
and then transfer its energy to the lanthanide ion through
intersystem crossing, by which Tb3þ enters its emission states
(Scheme 2a). When the highly emissive lanthanide complex
emission of [1 Tb]3þ complex was observed, suggesting that the
3
dialkylammonium ion was expelled from the cavity of DB24C8
by Kþ. Subsequently, the intramolecular PET process from
[1 Tb]3þ to fullerene was suppressed and the ET process was
3
restored (Figure 5). In addition, [2]pseudorotaxane could be
regenerated along with the reproduction of the luminescence
[Tb 1]3þ was coordinated with acceptor 2, the energy transfer
3
(ET) process was synchronously suppressed to a certain extent,
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NOTE
leading to an intramolecular PET process from [Tb 1]3þ to 2 as
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3 3
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3
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’ EXPERIMENTAL SECTION
Preparation of 40-[4-(4-Hydroxymethyldibenzo-24-crown-
8)phenyl]-2,20:60,200-terpyridine (1). Compound 4 (65 mg, 0.2
mmol) was dissolved in dry DMF (20 mL), and to this solution were
added 4-chloromethyldibenzo-24-crown-8 (100 mg, 0.2 mmol) and
K2CO3 (55.2 mg, 0.4 mmol). The resulting mixture was kept under reflux
in an inert atmosphere for 24 h. The reaction mixture was evaporated to
dryness, the residue was partitioned between CH2Cl2 (50 mL) and H2O
(50 mL), and the aqueous layer was extracted with CH2Cl2 (3 ꢀ 50 mL).
The combined organic extracts were dried with Na2SO4, filtered under
vacuum, and evaporated to dryness. The crude product was purified by flash
column chromatography (SiO2) eluting with EtOAc to afford 1 as a white
solid (113 mg, 72%): 1H NMR (400 MHz, CDCl3, 298 K) δ 3.84 (m, 8H),
3.90-3.95 (m, 8H), 4.12-4.20 (m, 8H), 5.03 (s, 2H), 6.83-6.92 (m, 5H),
6.99 (d, 2H), 7.09 (d, 2H), 7.35 (t, 2H), 7.84-7.92 (m. 4H), 8.64-8.75
(m, 6H); 13C NMR (CDCl3) δ 69.5, 69.9, 71.3, 113.7, 114.0, 114.2, 115.3,
118.3, 120.8, 121.4, 123.8, 128.5, 129.8, 131.0, 136.7, 149.1, 149.7, 155.8,
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: yuliu@nankai.edu.cn.
’ ACKNOWLEDGMENT
This work was supported by the 973 Program (2011CB932500)
and NNSFC (Nos. 20932004 and 20972077), which are gratefully
acknowledged.
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