Dual-mode control of PET process in a ferrocene-functionalized [2]rotaxane with high-contrast fluorescence output

Article abstract of DOI:10.1021/ol300753d

The shuttling motion of the ferrocene-functionalized macrocycle between the dibenzylammonium and the N-methyltriazolium recognition sites in a bistable [2]rotaxane, as well as the photoinduced electron transfer process occurring between ferrocene units an

Full text of DOI:10.1021/ol300753d

ORGANIC
LETTERS
2012
Vol. 14, No. 9
2334–2337
Dual-Mode Control of PET Process in a
Ferrocene-Functionalized [2]Rotaxane
with High-Contrast Fluorescence Output
Hui Zhang, Juan Hu, and Da-Hui Qu*
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China
University of Science & Technology, Shanghai 200237, P. R. China
dahui_qu@ecust.edu.cn
Received March 23, 2012
ABSTRACT
The shuttling motion of the ferrocene-functionalized macrocycle between the dibenzylammonium and the N-methyltriazolium recognition sites in
a bistable [2]rotaxane, as well as the photoinduced electron transfer process occurring between ferrocene units and the morpholin-naphthalimide
fluorescent stopper, can be adjusted not only by acidꢀbase stimuli but also additionꢀremoval of the fluoride anion, along with remarkable, high-
contrast fluorescent intensity changes.
Bistable [2]rotaxanes, in which the competitive binding
ability of a macrocycle with two distinct, well-separated
recognition sites on the thread component can be changed
in response to an external stimuli, have attracted much
attention because of their adjustable physical and chemical
properties, applications in molecular electronics, and as
components of molecular machinery.1 The shuttling mo-
tionofthe macrocycle on the rotaxanethreadcanbedriven
by acidꢀbase stimuli,2 light,3 electrochemical potential
change,4 or ions.5 Recently, solution-phase counterion
effects6 have been harnessed to control submolecular
motion in mechanically interlocked molecules, such as
rotaxanes and catenanes. A counterion-induced switch-
ing strategy represents a very promising approach if the
counterion-induced mechanical movement can be accom-
panied with naked-eye-based property changes, such as
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2007, 46, 72. (g) Balzani, V.; Credi, A.; Venturi, M. Molecular Devices
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r
10.1021/ol300753d
Published on Web 04/19/2012
2012 American Chemical Society

color changes7 and fluorescence changes,8,9 which holds the
potential to function as a molecular sensing device for ions.
Here, we report the design, preparation, characterization,
and properties of a bistable [2]rotaxane 1-H, in which the
shuttling motion of the macrocycle on the rotaxane thread
can be dual-mode-driven, i.e. by not only acidꢀbase stimuli
but also additionꢀremoval of the fluoride anion, along with
remarkable, high-contrast fluorescent intensity changes, by
introducing ferrocene electron donor units into the system.
The chemical structure and preparation of rotaxane 1-H
are shown in Scheme 1. The key feature of the rotaxane-type
molecular shuttle is the introduction of two ferrocene (Fc)
moieties as electron donors into the dibenzo-24-crown-8
(DB24C8) ring, which can be chemically driven to shuttle
between the two well-separated recognition sites, namely
dibenzylammonium (DBA)8b,c,10 and N-methyltriazolium
(MTA) stations.8b,c,11 As a result, the fluorescence of the
4-morpholin-naphthalimide (MA) stopper can be switched
on and off by a tunable, distance-dependent photoinduced
electron transfer (PET) process12 that occurs between the Fc
electron donors and the excited MA fluorophore. Using a
threading-followed-by-stoppering strategy,2,4a,8,13 rotaxane
2-H was prepared in a moderate yield through the well-
known copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddi-
tion reaction14 between alkyne 5 and azide 6 in the presence
of macrocycle 7. Then the subsequent methylation of the
triazole unit followed by anion exchange with saturated
NH4PF6 solution afforded the target [2]rotaxane 1-H in a
high yield (90%). The dumbbell-shaped molecule 3-H was
also prepared for comparison in a similar strategy as shown
in Scheme 1.
Scheme 1. Preparation and Chemical Structures of Rotaxane
1-H and Dumbbell-Shaped Compound 3-H
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H.-B.; Li, Y.-L. Org. Lett. 2007, 9, 3929. (b) Hsueh, S.-Y.; Lai, C.-C.;
Chiu, S.-H. Chem.;Eur. J. 2010, 16, 2997. (c) Yasuda, T.; Tanabe, K.;
Tsuji, T.; Coti, K. K.; Aprahamian, I.; Stoddart, J. F.; Kato, T. Chem.
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X.-X.; Zhang, Q.; Qu, D.-H.; Tian, H. Org. Biomol. Chem. 2011, 9, 4051.
Rotaxanes1-H, 2-H and dumbbell compounds 3-H, 4-H
were well characterized using 1H and 13C NMR and HR-
ESI mass spectrometry. The HR-ESI mass spectrum of the
target rotaxane 1-H revealed that the most intense peak
occurred at m/z 883.8239 as a doubly charged peak, with
an isotope distribution corresponding to the consecutive
ꢀ
(9) Perez, E. M.; Dryden, D. T. F.; Leigh, D. A.; Teobaldi, G.;
Zerbetto, F. J. Am. Chem. Soc. 2004, 126, 12210.
loss of two PF6ꢀ counterions, i.e. [Mꢀ2PF6]2þ
.
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White, A. J. P.; Williams, D. J.; Wolf, R. Angew. Chem., Int. Ed. 1998,
37, 1913. (b) Zhang, C.-J.; Li, S.-J.; Zhang, J.-Q.; Zhu, K.-L.; Li, N.;
Huang, F.-H. Org. Lett. 2007, 9, 5553. (c) Wang, F.; Han,
C.-Y.; He, C.-L.; Zhou, Q.-Z.; Zhang, J.-Q.; Wang, C.; Li, N.; Huang,
F.-H. J. Am. Chem. Soc. 2008, 130, 11254. (d) Qu, D.-H.; Feringa, B. L.
Angew. Chem., Int. Ed. 2010, 49, 1107. (e) Dong, S.-Y.; Luo, Y.; Yan,
X.-Z.; Zheng, B.; Ding, X.; Yu, Y.-H.; Ma, Z.; Zhao, Q.-L.; Huang,
F.-H. Angew. Chem., Int. Ed. 2011, 50, 1905.
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(12) (a) Andersson, A.; Linke, M.; Chambron, J.-C.; Davidsson, J.;
Heitz, V.; Sauvage, J.-P.; Hammarstrom, L. J. Am. Chem. Soc. 2000,
122, 3526. (b) Andersson, A.; Linke, M.; Chambron, J.-C.; Davidsson,
J.; Heitz, V.; Sauvage, J.-P.; Hammarstrom, L. J. Am. Chem. Soc. 2002,
124, 4347. (c) Ferrer, B.; Rogez, G.; Credi, A.; Ballardini, R.; Gandolfi,
M. T.; Balzani, V.; Liu, Y.; Tseng, H.-R.; Stoddart, J. F. Proc. Natl.
Acad. Sci. U.S.A. 2006, 103, 18411. (d) Rajkumar, G. A.; Sandanayaka,
A. S. D.; Ikeshita, K.; Araki, Y.; Furusho, Y.; Takata, T.; Ito, O. J. Phys.
Chem. B 2006, 110, 6516. (e) Mateo-Alonso, A.; Ehli, C.; Rahman,
G. M. A.; Guldi, D. M.; Fioravanti, G.; Marcaccio, M.; Paolucci, F.;
Prato, M. Angew. Chem., Int. Ed. 2007, 46, 3521. (f) Mateo-Alonso, A.;
Guldi, D. M.; Paolucci, F.; Prato, M. Angew. Chem., Int. Ed. 2007, 46,
8120. (g) Sasabe, H.; Sandanayaka, A. S. D.; Kihara, N.; Furusho, Y.;
Takata, T.; Araki, Y.; Ito, O. Phys. Chem. Chem. Phys. 2009, 11, 10908.
(h) Mateo-Alonso, A. Chem. Commun. 2010, 9089.
The 1H NMR of 1-H in CDCl3 confirmed the location of
the macrocycle to be predominantly over the DBA binding
site. The peaks for the methylene protons H4, H5 on the
DBA recognition site (Figure 1c) are shifted downfield
(Δδ = 0.59 ppm) compared with those of dumbbell 3-H
(Figure 1b), meanwhile, the peaks of protons H1, H2, H3
on the dimethoxybenzene stopper shifted upfield (Δδ =
ꢀ0.13, ꢀ0.08, and ꢀ0.11 ppm, respectively) due to the
shielding effect of macrocycle 7. Moreover, the protons H14,
H15, H16, H17 on the unencircled MTA site have the same
chemical shifts as those of dumbbell 3-H. All this evidence
confirmed that the DB24C8 ring exhibits a predominant
selectivity for the encirclement of the DBA recognition site.
Addition of 2 equiv of 1,8-diazabicyclo[5.4.0]undec-7-
ene (DBU) to the CDCl3 solution of rotaxane 1-H resulted
in the migration of the DB24C8 ring to the MTA recogni-
tionsite. AsshowninFigure1d, themethylene protons H16
and H17 neighboring the naphthalimide stopper are shifted
downfield (Δδ = 0.48 and 0.13 ppm, respectively), and the
(13) Yin, J.; Chi, C.-Y.; Wu, J.-S. Org. Biomol. Chem. 2010, 8, 2594.
(14) Wang, J.-Y.; Han, J.-M.; Yan, J.; Ma, Y.-G.; Pei, J. Chem.;
Eur. J. 2009, 15, 3585.
Org. Lett., Vol. 14, No. 9, 2012
2335

Figure 2. Partial 1H NMR spectra (400 MHz, 298 K, CDCl3) of
(a) [2]rotaxane 1-H, (b) the solution obtained after adding
2 equiv of TBAF to the solution of (a), and (c) the solution
obtained after adding 4 equiv of Ca(PF6)2 to the solution of (b).
through the precipitation of CaF2 provided a spectrum
identical to the original one of rotaxane 1-H (Figure 2c),
indicating the macrocycle moved back to the DBA station.
To study the selectivity of rotaxane 1-H for the fluoride
anion, the 1H NMR spectra in the same pattern as that of
rotaxane 1-H were obtained upon addition of TBACl,
TBABr, and TBAI, indicating no shuttling movement of
the macrocycle 7 and good selectivity for the fluoride anion
Figure 1. Partial 1H NMR spectra (400 MHz, 298 K, CDCl3) of(a)
macrocycle 7, (b) dumbbell 3-H, (c) [2]rotaxane 1-H, (d) deproto-
nation with addition of 2 equiv of DBU to sample c and (e)
reprotonation with addition of 4 equiv of TFA to sample d. The
assignments correspond to the structures as shown in Scheme 1.
1
(Figure S2). Thus, by H NMR spectroscopic measure-
peaks for the N-methyltriazolium protons are shifted due
to association with the DB24C8 ring, for H14, H15 with Δδ
of ꢀ0.19, ꢀ0.07 ppm, respectively. All these changes
indicated that the macrocycle moved to the MTA recogni-
tion site. Reprotonation of the ꢀNHꢀ center with the
addition of 4 equiv of CF3CO2H resulted in the return of
the DB24C8 ring to the DBA station as evidenced by the
regeneration of the original 1H NMR spectrum (Figure 1e).
Similar experiments performed in CD3COCD3 (Figure S1)
also proved the reversible shuttling motion of the macro-
cycle between the two stations.
ments, reversible acidꢀbase-driven and fluoride-anion-
driven shuttling motions of the macrocycle along the
rotaxane thread have been demonstrated.
Next, we focused on the photophysical properties of 1-H
and 3-H in response to chemical stimuli. Upon addition of
2 equiv of DBU or 2 equiv of TBAF to the solution of
rotaxane 1-H or dumbbell 3-H, almost no absorption
spectral changes were observed (Figure S3). The fluores-
cence intensity of dumbbell 3-H upon addition of DBU or
TBAF exhibited a small magnitude of change due to the
deprotonation of the DBA moiety or the counteranion
exchange from PF6ꢀ to Fꢀ, respectively.
Very remarkable fluorescence changes were observed in
rotaxane 1-H in response to chemical stimuli. Upon addi-
tion of 2 equiv of DBU to the dichloromethane solution of
rotaxane 1-H, the emission intensity at 517 nm decreased
90% compared with the original spectrum (Figure 3a);
furthermore, time-resolved fluorescence became a biexpo-
nential decay with a lifetime of 2.23 ns (64%) and 0.35 ns
(36%) from the original monoexponential decay with a
lifetime of 1.97 ns. The shorter lifetime can be attributed to
electron transfer between the Fc units and MA fluoro-
phore, while the longer lifetime is similar to the intrinsic
lifetime of the MA fluorophore. Similar results were obtained
when 2 equiv of TBAF were added to the dichloromethane
solution of rotaxane 1-H. The fluorescent intensity decreased
As previously studied by Chiu et al.,15 the interactions
between the DBA station and DB24C8 are affected by the
nature of the counteranions. After adding 2 equiv of
tetrabutylammonium fluoride (TBAF) to a CDCl3 solu-
1
tion of 1-H, significant changes were observed in the H
NMR changes(Figure2b), whichissimilar tothespectrum
obtained by addition of 2 equiv of DBU (Figure 1d),
indicating the migration of the DB24C8 ring to the MTA
recognition site, due to the stronger hydrogen bond be-
tween the fluoride anion and the hydrogen atom on the
DBA station. Subsequent addition of 4 equiv of Ca(PF6)2
to the TBAF-added solution to remove the fluoride anion
(15) Chuang, C.-J.; Li, W.-S.; Lai, C.-C.; Liu, Y.-H.; Peng, S.-M.;
Chao, I.; Chiu, S.-H. Org. Lett. 2009, 11, 385.
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Org. Lett., Vol. 14, No. 9, 2012

changes are due to the exchange of counteranions, not the
movement of the macrocycle. This is in accordance with the
1
result obtaind from the H NMR study that C1ꢀ, Brꢀ Iꢀ
could not result in the shuttling motion of the macrocycle. In
addition, the chemical oxidation of the Fc units was per-
formed using Fe(ClO4)3 as an oxidant (Figure S8). Upon the
addition of Fe(ClO4)3 to the DBU-added solution of 1-H, the
emission intensity enhanced dramatically. This phenomenon
could be ascribed to the oxidation of the Fc units; after
oxidation, the electron-donating abilities of the Fc units are
reduced, and the PET reaction would be arrested, leading to
fluorescence enhancement.
In conclusion, a novel “bright” [2]rotaxane has been
prepared and well-characterized. The shuttling motion of
the functionalized DB24C8 macrocycle between the DBA
station and the MTA station can be driven by not only
acidꢀbase stimuli but also additionꢀremoval of the fluo-
ride anion. By introducing two Fc moieties into the
macrocycle, the efficiency of the PET process occurring
between Fc units and the MA fluorescent stopper could be
altered due to the well-defined large-amplitude positional
change in response to two kinds of chemical stimuli,
generating remarkable, high-contrast fluorescent intensity
changes observed by the naked eye. Most importantly, the
system can distinguish fluoride anion from other halide
anions, which holds the potential to function as a novel
kind of fluorescent molecular sensing device, translating
molecular recognition into mechanical movement of the
subunits, and remarkable fluorescence outputs.
Figure 3. Fluorescence spectral changes in CH2Cl2 (a) from
[2]rotaxane 1-H to the mixture obtained after adding 2 equiv of
DBU to the solution of 1-H, (b) from dumbbell 3-H to the mixture
obtained after adding 2 equiv of DBU to the solution of 3-H, (c)
from [2]rotaxane 1-H to the mixture obtained after adding 2 equiv
of TBAF to the solution of 1-H, and (d) from dumbbell 3-H to the
mixture obtained after adding 2 equiv of TBAF to the solution of
3-H. Excitation wavelength of all fluorescence spectra was 398 nm.
88% (Figure 3c), and the fluorescent decay in this case
obeyed biexponential function with lifetimes of 2.44 ns
(61%) and 0.52 ns (39%). After the addition of 4 equiv of
Ca(PF6)2, rotaxane 1-H was regenerated, and the fluores-
cence spectra recoverd. The fluorescence changes can even be
observed by the naked eye, as shown in the Supporting
Information. Both of these cases indicate that the distance-
dependent PET process from the Fc electron donors to the
excited state of the MA fluorophore became more efficient
after the functional macrocycle moved from the DBA station
to the MTA station by deprotonation with DBU or addition
of the fluoride anion, after which the spatial distance between
the Fc units and the MA fluorophore became much closer
compared with the original state. Most importantly, the
shuttling motion of the macrocycle driven by acidꢀbase
stimuli could be repeated many times without obvious
degradation, as evidenced by the reversible fluorescent
change cycles (Figure S6). To further explore the influence
of counteranions, we conducted experiments of addition of
other halide anions to the solution of rotaxane 1-H and
dumbbell 3-H, respectively. The same change tendencies
were observed for 1-H and 3-H upon addition of 5 equiv of
TBACl, TBABr, and TBAI, indicating that the fluorescence
Acknowledgment. We thank the NSFC/China (20902024
and 21190033), National Basic Research 973 Program
(2011CB808400). Dr. D.-H. Qu thanks the Foundation for
the Author of National Excellent Doctoral Dissertation of
China (200957), the Shanghai Pujiang Talent Program
(10PJ1402700), the Fok Ying Tong Education Foundation
(121069), SRFDP (20090074120017), the Shanghai Chen-
guang project (09CG26), and the Fundamental Research
Funds for the Central Universities (WJ1014001) for financial
support.
Supporting Information Available. Experimental pro-
cedures and characterization data for new compounds,
the absorption and fluorescence spectra of 1-H. This
material is available free of charge via the Internet at
http://pubs.acs.org.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 9, 2012
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