A three-level luminescent response in a pyrene/ferrocene rotaxane

Article abstract of DOI:10.1021/ol303108q

A solvent switchable rotaxane equipped with a pyrene stopper and with two ferrocenyl units on the macrocycle is reported, in which three different states, two nondegenerate and one degenerate, can be obtained in different solvents at room temperature. This is accompanied by high contrast changes in fluorescence intensity of the pyrene stopper by the presence of the ferrocenyl moieties on the macrocycle, which quench the emission of pyrene more efficiently with proximity.

Full text of DOI:10.1021/ol303108q

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
A Three-Level Luminescent Response
in a Pyrene/Ferrocene Rotaxane
Aurelio Mateo-Alonso,*,†,‡ Christian Ehli,§ Dirk M. Guldi,*,§ and Maurizio Prato*,
POLYMAT, University of the Basque Country UPV/EHU, Avenida de Tolosa 72,
E-20018 Donostia-San Sebastian, Spain, Ikerbasque, Basque Foundation for Science,
E-48011 Bilbao, Spain, Center of Excellence for Nanostructured Materials
(CENMAT) and Italian Interuniversity Consortium on Materials Science and
Technology (INSTM), Unit of Trieste, Dipartimento di Scienze Farmaceutiche,
ꢀ
Universita degli Studi di Trieste, Piazzale Europa 1, 34127 Trieste, Italy, and
Department of Chemistry and Pharmacy & Interdisciplinary Center for Molecular
€
€
Materials (ICMM), Friedrich-Alexander-Universitat Erlangen-Nurnberg,
Egerlandstrasse 3, D-91058 Erlangen, Germany
amateo@polymat.eu; guldi@chemie.uni-erlangen.de; prato@units.it
Received November 12, 2012
ABSTRACT
A solvent switchable rotaxane equipped with a pyrene stopper and with two ferrocenyl units on the macrocycle is reported, in which three different
states, two nondegenerate and one degenerate, can be obtained in different solvents at room temperature. This is accompanied by high contrast
changes in fluorescence intensity of the pyrene stopper by the presence of the ferrocenyl moieties on the macrocycle, which quench the emission
of pyrene more efficiently with proximity.
Synthetic fluorescent switches and probes able to pro-
vide distinct emissive responses to different environments1
have attracted much attention because of the great poten-
tial of application in different fields which are relevant to
materials science and biology, such as molecular logic,
imaging, sensing, and characterizing microenvironments,
among others.1a,2 Bistable rotaxanes provide an excellent
framework to develop switches and probes, since the
(3) (a) Zhang, H.; Hu, J.; Qu, D. H. Org. Lett. 2012, 14, 2334–7. (b) Qu,
D. H.; Wang, Q. C.; Ren, J.; Tian, H. Org. Lett. 2004, 6, 2085–8. (c) Zhao,
Y.; Li, Y.; Lai, S. W.; Yang, J.; Liu, C.; Liu, H.; Che, C. M.; Li, Y. Org.
Biomol. Chem. 2011, 9, 7500–3. (d) Zhou, W.; Zhang, S.; Li, G.; Zhao, Y.;
Shi, Z.; Liu, H.; Li, Y. ChemPhysChem 2009, 10, 2066–72. (e) Zhou, W.;
Li, J.; He, X.; Li, C.; Lv, J.; Li, Y.;Wang, S.; Liu, H.; Zhu, D. Chem.;Eur.
J. 2008, 14, 754–63. (f) Gunbas, D. D.; Zalewski, L.; Brouwer, A. M.
Chem. Commun. 2011, 47, 4977–9. (g) Mateo-Alonso, A. Chem. Commun.
2010, 46, 9089–9099. (h) Mateo-Alonso, A.; Guldi, D. M.; Paolucci, F.;
Prato, M. Angew. Chem., Int. Ed. 2007, 46, 8120–8126. (i) Mateo-Alonso,
A.; Fioravanti, G.; Marcaccio, M.; Paolucci, F.; Rahman, G. M. A.; Ehli,
C.; Guldi, D. M.; Prato, M. Chem. Commun. 2007, 1945–1947. (j) 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,
3521–3525. (k) Li, Y.; Li, H.; Li, Y.; Liu, H.; Wang, S.; He, X.; Wang, N.;
Zhu, D. Org. Lett. 2005, 7, 4835–8. (l) Onagi, H.; Rebek, J., Jr. Chem.
Commun. 2005, 4604–6. (m) Perez, E. M.; Dryden, D. T.; Leigh, D. A.;
Teobaldi, G.; Zerbetto, F. J. Am. Chem. Soc. 2004, 126, 12210–1.
(n) Jiang, Q.; Zhang, H. Y.; Han, M.; Ding, Z. J.; Liu, Y. Org. Lett.
2010, 12, 1728–31. (o) Zhang, H.; Zhou, B.; Li, H.; Qu, D.-H.; Tian, H.
J. Org. Chem. 201210.1021/jo302107a. (p) Li, H.; Zhang, H.; Zhang, Q.;
Zhang, Q.-W.; Qu, D.-H. Org. Lett. 2012, 14, 5900–5903.
† POLYMAT, University of the Basque Country UPV/EHU.
‡ Basque Foundation for Science.
§
ꢀ
Universita degli Studi di Trieste.
€
Friedrich-Alexander-Universitat Erlangen-N€urnberg.
(1) (a) Valeur, B. Fluorescence: Principles and Applications; Wiley-VCH:
Weinheim, 2002. (b) Lakowicz, J. R. Principles of Fluorescence Spectroscopy;
Kluwer Academic/Plenum: New York, 1999.
(2) (a) Qu, D.-H.; Ji, F.-Y.; Wang, Q.-C.; Tian, H. Adv. Mater. 2006,
18, 2035–2038. (b) Qu, D. H.; Wang, Q. C.; Tian, H. Angew. Chem., Int.
Ed. 2005, 44, 5296–9. (c) Leigh, D. A.; Morales, M. A.; Perez, E. M.;
Wong, J. K.; Saiz, C. G.; Slawin, A. M.; Carmichael, A. J.; Haddleton,
D. M.; Brouwer, A. M.; Buma, W. J.; Wurpel, G. W.; Leon, S.; Zerbetto,
F. Angew. Chem., Int. Ed. 2005, 44, 3062–7. (d) Gassensmith, J. J.;
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D. B. J. Am. Chem. Soc. 2006, 128, 12229–38.
r
10.1021/ol303108q
XXXX American Chemical Society

controlled motion of the macrocycle from one recognition
site on the thread (“station”) to another can be used to
develop a distinct environmental emissive response2,3 that
can be detected with a wide variety of accessible spectro-
scopic techniques, both locally and remotely, and in some
cases even with the naked eye.2c,3a,3m Herein we report a
solvent switchable molecular shuttle (rotaxane 1) with a
pyrene stopper and with two ferrocenyl units on the
macrocycle, in which three different states, two non-
degenerate and one degenerate, can be obtained in different
solventsat room temperature. This is accompanied by high
contrast changes in fluorescence intensity of the pyrene
stopper by the presence of the ferrocenyl moieties on the
macrocycle, which quench the emission of pyrene more
efficiently with proximity.
Scheme 2. Solvent Switchable Behavior of Rotaxane 1 in
Different Solvents at Room Temperature
Rotaxane 1 consists ofa macrocycle, functionalizedwith
two ferrocenyl units, interlocked to a thread, equipped
Scheme 1. Synthesis of Rotaxane 1
the macrocycle. Conversely, in DMSO-d6, the macrocycle
decomplexes from the 1,4-diamide station and switched to
sit on top of the alkyl chain (translational isomer 1B); as a
consequence, the protons N of the alkyl chain are shielded
for as much as 1.0 ppm, while protons E and A and are
nearly deshielded (Figure 1, Table 1). The situation is
different in DMF-d7 where the macrocycle shuttles back
and forth between the two stations that become energeti-
cally degenerate, as reflected by the simultaneous and less
efficient shielding of the protons of the alkyl chain (M and
N) and of the 1,4-diamide station (E and A) (Figure 1,
Table 1).
Absorption measurements carried out on thread 5 and
rotaxane 1 are dominated by the absorption of 1-substituted
pyrene with matching maxima for both compounds in the
samesolvent. Inthecaseofrotaxane1, anadditionalbroad
absorption band (400À550 nm) was observed that corre-
sponds to the ferrocenyl moieties (Figure S9).
On the other hand, steady-state fluorescence measure-
ments reveal an interesting trend (Figure 2, Table 2). The
pyrene fluorescence (325 nm excitation), which seems to be
unaffected in thread 5 (i.e., 100%), is heavily quenched in
rotaxane 1 (i.e., 13% in CHCl3; 9% in THF). Time-
resolved fluorescence measurements (295 nm excitation)
further confirm this, since for the major emitting compo-
nents lifetimes were detected on the order of hundreds of
nanoseconds for thread 5 (i.e., 194 ns) and of tenths of
nanoseconds for rotaxane 1 (i.e., 54 ns) in Ar-saturated
THF. The situation changes completely in polar solvents,
and the emission is quenched more strongly with increas-
ing hydrogen bond basicity (i.e., 8% in DMF; 4% in
DMSO). In time-resolved fluorescence measurements
with (i) a pyrene unit, which acts as both a fluorophore and
a stopper; (ii) an alkyl station; (iii) a 1,4-diamide station;
and (iv) a diphenyl stopper.
Thread 5 was synthesized from 1-pyrenebutanol (2)
as shown in Scheme 1. First, compound 2 was esterified
with 11-Boc-aminoundecanoic acid in the presence of 1-
ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride
(EDC) and 4-(dimethylamino)pyridine (DMAP). The
terminal amine of 3 was deprotected in the presence of
trifluoroacetic acid (TFA) and subsequently acylated with
44 upon activation with EDC and DMAP to yield thread 5.
Rotaxane 1 was synthesized employing a modification3j,5 of
Leigh’s protocol6 by adding 5-ferroceneacetoxyisophthaloyl
chloride 65 and p-xylylenediamine to thread 5 under high
dilution conditions.
Rotaxane 1 presents two different translational isomers
in different solvents as observed by NMR spectroscopy
(see Scheme 2 and Figure 1). In relative nonpolar solvents
(i.e., CDCl3, THF-d8), the macrocycle is located primarily
over the 1,4-diamide station (translational isomer 1A)
confirmed by the shielding of protons E and A with those
of thread 5 due to the anisotropy of the aromatic rings on
(4) Hannam, J. S.; Lacy, S. M.; Leigh, D. A.; Saiz, C. G.; Slawin,
A. M.; Stitchell, S. G. Angew. Chem., Int. Ed. 2004, 43, 3260–4.
(5) Mateo-Alonso, A.; Prato, M. Eur. J. Org. Chem. 2010, 1324–
1332.
(6) Leigh, D. A.; Murphy, A.; Smart, J. P.; Slawin, A. M. Z. Angew.
Chem., Int. Ed. 1997, 36, 728–732.
(7) In addition, a 125 ns long-lived component is seen. This is
attributed to slow conformational changes due to the increased viscosity
of the solvent and the flexibility of the butyl linker.
B
Org. Lett., Vol. XX, No. XX, XXXX

shorter-lived component for rotaxane 1 in THF is reduced
to 8.7 ns in DMSO.7
To ensure that any effect observed is undoubtedly owed
to shuttling and not related to the solvent, 1-pyrenebutyl-
ferroceneacetate 78 was synthesized and used as a reference
to explore the impact of shuttling on the emissive proper-
ties (Scheme S1). The effects of the solvent can be deduced
by examination of the fluorescence intensities of reference
7 since the relative position of the ferrocene and pyrene
moieties is maintained, irrespective of the solvent used
because of the covalent linkage bridging both units. The
fluorescence intensities of 7 decrease with increasing hy-
drogen bond basicity except for DMF (Table 2). However
a larger emission intensity is observed for 1 than for 7 in
both CHCl3 and THF. This is consistent with the larger
relative separation in CHCl3 and THF between pyrene and
ferrocene, where translational isomer 1A is mainly pres-
ent, in contrast to the relative shorter separation in 7.
Figure 2. Comparison of the fluorescence intensities of rotaxane
1 in different solvents with matching absorbance at the excita-
tion wavelength (325 nm).
Conversely, in DMSO the emission intensity of 1 (4%) is
remarkably lower than in CHCl3 and closer to the values
measured for 7 (i.e., 1%) (Table 2). Such differences in
emission intensity cannot be rationalized solely on the basis
of the solvent without taking into account the effects derived
from shuttling. The reduction in the positional integrity
of the macrocycle at the 1,4-diamide station, which occurs
as the hydrogen bonds are weakened by an incremental
increase in hydrogen-bond basicity, causes an increase in
the shuttling rate toward the formation of 1B. This is in
strong agreement with the NMR experiments. In fact, the
low emission intensities observed in DMSO, where
Figure 1. NMR of thread 5, and rotaxane 1 in THF-d8 (top),
DMF-d7 (center), and DMSO-d6 (bottom). The assignments
correspond to the lettering shown in Schemes 1 and 2.
Table 1. Shielding (Δδ) of the Indicated Protons of Rotaxane
1 versus Thread 5
solvent
ΔδE (ppm)
ΔδA (ppm)
ΔδN (ppm)
CDCl3
À1.3
À1.2
À0.6
À0.1
À0.9
À0.9
À0.3
À0.1
À0.1
À0.2
À0.6
À1.0
(8) Structure of reference
Information).
7
(additional details in Supporting
THF-d8
DMF-d7
DMSO-d6
carried out in Ar-saturated DMSO, thread 5 gives rise to a
fairly long-lived component (i.e., 118 ns) while the 54 ns
Org. Lett., Vol. XX, No. XX, XXXX
C

translational isomer 1B exists preferentially, confirm a
much closer pyreneÀferrocene separation, which is in
between that of 1A and 7 as expected. Although at first
glance the emission of 1 in DMF is closer to that of THF,
the measurements carried out on reference 7 indicate that
emission is particularly higher in relative terms in DMF,
which justifies the relative high fluorescence intensity of
1 and is also consistent with a degenerate state and, thus,
with a relative separation between pyrene and ferrocene
and a emission intensity that falls in between 1A and 1B.
This is also supported by time-resolved emission experi-
ments, where shorter-lived and similar lifetimes were
observed for reference 7 in Ar-saturated THF (i.e., 10.7
and 2.1 ns in 34% and 66%, respectively) and DMSO
(i.e., 3.3 and 0.5 ns in 26% and 74%, respectively), in
contrast to the lifetimes measured for rotaxane 1 in THF
(i.e., 54 ns) and DMSO (i.e., 8.7 ns).
high contrast changes in the fluorescence intensity of the
pyrene stopper are undoubtly owed to the relative position
of the macrocycle with the two ferrocenyl units along the
thread, as demonstrated by a combination of NMR,
steady-state, and time-resolved emission assays that verify
the strong pyrene/ferrocene interactions and their tunability
through a systematic change of the environment (i.e., solvent
polarity and/or hydrogen bond basicity). The quenching
mechanism is very complex and cannot be simplified solely
in terms of electron transfer or energy transfer, since it
involves the motion of the macrocycle, different solvents,
and heavy atoms. Further time-resolved absorption spectro-
scopy studies (femto- and nanosecond), which are currently
underway, will shine light on such a complex mechanism and
will be reported in due time. In any case, we believe that this
emission trend is essential to develop future ultrasensitive
and multiresponsive luminescent molecular systems to
probe different nano-, micro-, and macroenvironments.
Acknowledgment. This work was carried out with sup-
port of the University of Trieste (Giovani Ricercatori Pro-
gram), the Italian Ministry of Education MIUR (Cofin Prot.
2010N3T9M4 and FIRB prot. RBAP11C58Y), INSTM, the
Verband der Chemischen Industrie (SK 185/13), POLY-
MAT, and Ikerbasque. Moreover, Solar Technologies Go
Hybrid is greatly appreciated.
Table 2. Integrated Fluorescence Intensities of Rotaxane 1
Referenced to Thread 5
solvent
thread 5
rotaxane 1
reference 7
CHCl3 2.56 Â 107 100% 3.39 Â 106 13% 8.05 Â 105 3%
THF
6.82 Â 107 100% 6.09 Â 106 9%
4.68 Â 107 100% 3.93 Â 106 8%
1.38 Â 106 2%
3.20 Â 106 7%
1.24 Â 106 1%
DMF
DMSO 1.06 Â 108 100% 4.29 Â 106 4%
Supporting Information Available. Full synthetic pro-
cedures, including structural and photophysical charac-
terization. This material is available free of charge via the
Internet at http://pubs.acs.org.
In conclusion, we have reported the synthesis of a
solvent switchable rotaxane equipped with a pyrene stop-
per and with two ferrocenyl units on the macrocycle that
exists in three states in different solvents at room tempera-
ture: two nondegenerate and one degenerate. The observed
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX