A perylene diimide rotaxane: Synthesis, structure and electrochemically driven de-threading

Article abstract of DOI:10.1002/chem.201103090

The first example of a [2]-rotaxane in which a perylene diimide acts as a recognition site has been synthesised and characterised. The interlocked nature of the compound has been verified by both NMR studies and an X-ray structure determination. Electroch

Full text of DOI:10.1002/chem.201103090

DOI: 10.1002/chem.201103090
A Perylene Diimide Rotaxane: Synthesis, Structure and Electro
Driven De-Threading
ACHTUNGTRENUNcNG hemically
Benjamin J. Slater,[a] E. Stephen Davies,[a] Stephen P. Argent,[a] Harriott Nowell,[b]
William Lewis,[a] Alexander J. Blake,[a] and Neil R. Champness*[a]
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Chem. Eur. J. 2011, 17, 14746 – 14751

FULL PAPER
Abstract: The first example of a [2]-ro-
taxane in which a perylene diimide acts
as a recognition site has been synthes-
ised and characterised. The interlocked
nature of the compound has been veri-
fied by both NMR studies and an X-
ray structure determination. Electro-
chemical investigations confirm that
the nature of the redox processes asso-
ciated with the perylene diimide are
modified by the complexation process
and that it is possible to mono-reduce
the [2]-rotaxane to give a radical anion
based rotaxane. Further reduction of
the compound leads to de-threading of
the macrocycle from the reduced
PTCDI recognition site. Our synthetic
strategies confirm the potential of
PTCDI-based rotaxanes as viable tar-
gets for the preparation of complex in-
terlocked species.
Keywords: electrochemistry · pery-
lene diimide · redox chemistry · ro-
taxanes · X-ray structures
Introduction
The synthesis of interlocked species has long fascinated the
chemist and many elegant studies have been reported since
the original concepts of supramolecular chemistry were pro-
posed.[1] The field has developed significantly allowing the
assembly of remarkable species with highly tuned compo-
nents leading to the construction of molecular machines.[2]
In such studies components have been developed as recogni-
tion sites including a variety with redox-[3] or photo-active[4]
properties; these include the elegant work of Sanders and
co-workers who have developed the synthesis of naphthale-
nediimide (NDI) species in both catenanes and rotaxanes.[5]
It is surprising therefore that the larger analogue of NDIs,
N,N’-bisACHTUNGTRENNUNG(alkyl)-3,4,9,10-perylene tetracarboxylic acid dii-
mides (PTCDIs), have not be widely exploited for the for-
mation of interlocked species. PTCDIs display useful redox
properties, typically exhibiting two reversible reduction pro-
cesses, are highly coloured, a property that can be tuned by
chemical substituents and redox state, and are commonly
fluorescent in solution, with significant quantum yields.[6]
Indeed, the only reported examples of rotaxanes including
PTCDI units either use the PTCDI moiety as a stopper
unit[7] or exploit a sterically encumbered PTCDI group as
part of the axle such that only the imide moieties act as rec-
ognition sites.[8]
We have therefore developed a method for preparing a
PTCDI-based rotaxane using
a
ꢁslippingꢂ approach
(Scheme 1).[9] We targeted PTCDI derivatives without sub-
stitution around the periphery, or ꢁbayꢂ region, of the pery-
lene moiety to avoid complications with additional steric
bulk and/or isomerism around the PTCDI recognition site.
Scheme 1. 1) imidazole, ZnBr2, 1708C, 4 h; 2) CHCl3/MeOH (95:5),
658C, 14 days.
[a] Dr. B. J. Slater, Dr. E. S. Davies, Dr. S. P. Argent, Dr. W. Lewis,
Prof. Dr. A. J. Blake, Prof. Dr. N. R. Champness
School of Chemistry, University of Nottingham
University Park, Nottingham, NG7 2RD (UK)
E-mail: Neil.Champness@nottingham.ac.uk
Such unfunctionalised PTCDI derivatives are known to
have low solubility in many solvents and therefore synthetic
routes usually employ derivatisation to improve solubility.[6]
Imidisation of PTCDA, the corresponding dianhydride to
PTCDI, proceeds only at relatively high temperatures[10] and
is therefore an unattractive step when trying to prepare sen-
sitive species such as rotaxanes. Thus it was decided to pre-
pare the axle, introducing groups that aid solubility and act
[b] Dr. H. Nowell
Diamond Light Source, Diamond House
Harwell Science and Innovation Campus
Didcot, Oxfordshire OX11 0DE (UK)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103090.
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N. R. Champness et al.
as stoppers and to pursue a ꢁslippingꢂ approach to rotaxane
formation.
PTCDI interacting with the macrocyclic component 2; it is
likely that the macrocycle is shuttling over the PTCDI at
room temperature. Macrocycle protons H1, H2 and H3 all
show a direct upfield shift of approximately 1 ppm and
more complex profiles. Resonances associated with glycol
chains of 2 are also shifted compared to the values in the
free macrocycle, with the two triplets shifting upfield to 3.85
and 3.74 ppm, while the complex multiplet shifts downfield
to 4.03 ppm. NOE and ROE experiments on 3 at room tem-
perature failed to detect coupling between the axle and
macrocycle, possibly owing to the fluxional nature of the
molecule at this temperature, therefore low temperature
VT NMR spectroscopy was carried out on 3 in a CD2Cl2 so-
lution. The temperature was first lowered to 273 K and then
decreased in 10 K steps to 193 K (see the Supporting Infor-
mation). Ha/b splits at low temperature into a complex signal
that is unsymmetrical, thus indicating desymmetrisation of 3
upon cooling. A series of one-dimensional ROE experi-
ments was carried out at 193 K and through-space interac-
tions were noted. ROESY NMR studies were therefore car-
ried out at low temperature (see the Supporting Informa-
tion). Through-space interactions were detected between a
number of the resonances, notably H1 on the macrocycle
being associated with Ha/b on the axle, thus indicating that 2
resides on the PTCDI. Thus, we conclude from the NMR
experiments that 3 exists as a [2]-rotaxane in solution with
the PTCDI operating as a recognition site.
A purple single crystal of 3 was successfully grown by
layering MeOH onto a solution of the compound in toluene.
A single crystal X-ray diffraction study established that the
structure of 3 was a [2]-rotaxane in the solid state, showing
the axle 1 threaded through the macrocycle 2 and adopting
a Z conformation with one stopper above the perylene
plane and one below (Figure 2a). As expected, the macrocy-
cle naphthalene units are both involved in p–p stacking in-
teractions with opposite sides of the PTCDI core, with cent-
roid-to-plane separations of 3.43 and 3.49 ꢃ. The values are
similar to previously reported rotaxanes involving the mac-
rocycle 2,[13] although slightly longer than that reported for a
related pyromellitic diimide recognition site.[5a] The closest
points between the axle and macrocycle are via two pairs of
Results and Discussion
The target rotaxane was prepared by sequential synthesis of
the axle molecule 1 and subsequent complexation of the
macrocycle 2. Axle 1 was constructed from a PTCDI unit
and two bis(4-tert-butylphenyl)(4-ethylphenyl)(4-phenoxy)
methane stopper groups; the latter have proved to be effec-
tive in allowing slipping reactions in rotaxane formation.[11]
Axle 1 and an excess of macrocycle 2[12] were dissolved in a
small volume of CHCl3/MeOH (95:5) and heated to 658C
under N2 in a sealed pressure tube for 2 weeks (Scheme 1).
The desired product 3 was isolated by column chromatogra-
phy in a relatively low yield (8%). Attempts were made to
improve the yield and reaction time using alternative reac-
tion conditions involving a melt phase. The melting point of
the macrocycle 2, 1388C, was exploited and it was found
that heating a mixture of 1 in an excess of 2 at 1608C for
6 hours produced the desired product in yields that were
still low (8%), but more acceptable given the much reduced
overall reaction times.
Formation of [2]-rotaxane 3 was confirmed by NMR spec-
troscopy through key changes observed in the 1H NMR
spectrum (Figure 1). The typical PTCDI double doublet of
Ha/b is replaced by a broad singlet at 8.5 ppm due to the
À
C H···O contacts (H···O=2.67, 2.79 ꢃ; Figure 2b) between
perylene hydrogen atoms and a glycol oxygen atom.
The redox behaviour of 1 and 3 was analysed by cyclic
voltammetry (CV), bulk electrolysis and UV/Vis spectroe-
lectrochemistry and that of 2 by CV only. As anticipated,[6]
1
undergoes two chemically reversible one-electron reduction
processes, at E1/2 =À0.95 and À1.10 V, versus Fc+/Fc, based
on the PTCDI moiety (see Figure 3a and the Supporting In-
formation for further discussion). In contrast, the [2]-rotax-
ane 3 displays a single asymmetric couple (E1/2 ꢀÀ1.19 V vs.
Fc+/Fc) the shape of which is essentially independent of
scan rate (Figure S10 in the Supporting Information) and
occurs at a potential more negative than the first reduction
of the parent PTCDI. The shift in potential may be rational-
ised by an increase in electron density on the perylene core
due to proximity of electron-rich naphthyl groups of the
Figure 1. The aromatic region of the 1H NMR spectra of i) axle 1; ii) [2]-
rotaxane 3 and ii) macrocycle 2 at 258C.
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Perylene Diimide Rotaxanes
FULL PAPER
Figure 2. Views of a) the X-ray structure of 3 confirming the [2]-rotaxane
À
structure; b) C H···O interactions between the perylene core and macro-
cycle glycol chain. Primed atoms are related to their unprimed equiva-
lents by the inversion symmetry operator (Àx, Ày, Àz) and the stopper
groups are omitted for clarity.
macrocycle, but the presence of a single-reduction process
was unexpected and demonstrates a clear change in electro-
chemical properties of the PTCDI moiety as a result of com-
plexation. Typically for PTCDI molecules, two one-electron
reductions are present[14] and the difference in potentials be-
tween the first and second reductions results from Coulom-
bic repulsion between the added electrons acting through
the p framework of the aromatic core. Therefore, the obser-
vation of a single process for 3 may result from either two
one-electron reductions with no significant interactions or
the presence of only a single-electron reduction process. For
1 the presence of two one-electron processes, that rapidly
developed equilibrium, was confirmed by a bulk electrolysis
experiment. However, the analogous experiment for 3 gave
rapid electrolysis, initially to an approximately 1 electron re-
duced form, followed by a slow reduction characterised by
an almost linear current profile (Figure S11 in the Support-
ing Information). Cyclic voltammograms recorded for 3
prior to electrolysis and then after a reduction–oxidation
cycle indicated the presence of an additional couple at a po-
tential consistent with the first reduction of 1 (Figure 3b),
thus indicating that a de-threading process had occurred.
The formation of 1 was confirmed by MALDI-MS and
NMR analyses (Figure S12 in the Supporting Information)
of the redox cycled solution following removal of electrolyte
present in the electrochemical experiment. Thus, upon re-
duction, 3 begins a de-threading process. It is postulated
Figure 3. a) Cyclic voltammograms for 1, 2 and 3. All cyclic voltammo-
grams were recorded at ambient temperature in CH2Cl2 containing
[Bu4N]
sis (solid line) and after re-oxidation (dotted line) in CH2Cl2 containing
[Bu4N][BF4] (0.4m) at 273 K.
ACHTUNGTREN[NUGN BF4] (0.4m); b) Cyclic voltammograms of 3 before bulk electroly-
AHCTUNGTRENNUNG
that the reduced recognition site of 3 is an unfavourable
binding site and repels the electron-rich macrocycle 2.
To determine the nature of fully reduced 3, both 1, acting
as the reference compound, and 3 were monitored by in situ
UV/Vis spectroelectrochemistry at an optically transparent
electrode. The results for 1 were consistent with previous
studies of PTCDI-based species and demonstrated the
highly characteristic profiles associated with mono- and di-
anions (Figure S13–S15 and Table S2 in the Supporting In-
formation).[15] For 3, at 273 K exhaustive reduction showed
the initial development of bands associated with a mono-
anionic PTCDI which depleted slowly as electrolysis pro-
gressed and were replaced by new bands characteristic of di-
reduction of the PTCDI core, thus confirming the formation
of a 32À species (Figure S16 in the Supporting Information).
Re-oxidation of 32À at this temperature failed to generate
the exact spectral profile of 3 (Figure S17 in the Supporting
Information) and new bands, in positions consistent with 1
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N. R. Champness et al.
were noted, thereby confirming the results of the bulk elec-
trolysis/CV experiment and indicating the limited de-thread-
ing of 3 upon reduction. Repeating the experiment at 230 K
(Figure S18 in the Supporting Information) allowed the sta-
bilisation of reduced 3 to at least the 3À state (slight over-re-
duction to 32À could also be reversed but full reduction to
32À was not pursued). Reduction of 3 resulted in the major
absorption band shifting to lower energy but a broad feature
centred at 530 nm was retained. The latter feature was as-
signed to a charge transfer band resulting from macrocyclic
complexation, thus implying that the [2]-rotaxane is intact
upon a one-electron reduction under these conditions.
may not be surprising given that reduced 3 shows some in-
stability with respect to de-threading under these conditions
(see above), and it was noted that the spectrum appeared to
lose some resolution with time. The spectral shape was best
reproduced from parameters obtained by converging values
assigned to the two largest hyperfine couplings used in the
simulation of the EPR spectrum of 1À (Ha and Hb) whilst
maintaining similar couplings for two equivalent nitrogen
atoms and the four hydrogen atoms assigned as the a-
carbon of the imide groups (Figure 4). This result may imply
a more even distribution of the electron density about the
PTCDI core and confirms the influence of the macrocycle in
the reduced form of 3.
Complementary to the low temperature NMR studies for
3, evidence for the influence of the macrocycle on the axle
in 3À was noted using EPR spectroscopy. The electrochemi-
cal one-electron reduction of 1 and 3 produced paramagnet-
ic species consistent with the generation of radical anions.
Hyperfine splitting was observed for 1À which could be si-
mulated as the coupling of the unpaired electron to the
nuclei of two nitrogen atoms and three sets of four hydrogen
atoms (Figure 4). These may be accounted for by the two
sets of four equivalent hydrogen atoms (Ha and Hb) on the
perylene core (a4H =0.56ꢄ10À4 cmÀ1 and 1.68ꢄ10À4 cmÀ1, re-
spectively) and the four hydrogen atoms at the a-carbon of
the imide groups (a4H =0.13ꢄ10À4 cmÀ1). The giso of 2.0033
was consistent with literature values for a PTCDI anion.[14]
The ambient temperature EPR spectrum of 3À was similar
to that of 1À in respect to position (giso of 2.0034 for 3À) and
spectral width (ca. 13 G), thus suggesting that a radical
anion based on the PTCDI is present, but differed in spec-
tral shape. Attempts to reproduce all features in the experi-
mental spectrum of 3À by simulation were unsuccessful. This
Conclusion
In conclusion, we have successfully synthesised and charac-
terised a perylene diimide based [2]-rotaxane that represents
the first example of such a species. The PTCDI moiety suc-
cessfully acts as a recognition site for complexation of a
naphthyl-based macrocyclic species. Importantly, the proper-
ties of the PTCDI group are modified by complexation, in-
cluding the nature of the redox processes. One-electron re-
duction of 3 leads to the formation of 3À, a radical anion
based [2]-rotaxane, representing a switchable state of the
PTCDI based [2]-rotaxane. The second reduction of 3 ini-
tiates a de-threading process in the [2]-rotaxane, presumably
as a result of electrostatic repulsion of the electron-rich
macrocycle from the reduced PTCDI recognition site. This
observation, in addition to the synthetic strategies employed
in this study, confirms the potential of PTCDI-based rotax-
anes as viable targets for the preparation of more complex
interlocked species including molecular machines.
Experimental Section
Details of and information relating to further synthetic procedures are
given in the Supporting Information. Macrocycle 2 was prepared by a lit-
erature method.[12] All reactions were carried out under an atmosphere
of dinitrogen or argon as noted below. Column chromatography was per-
formed on silica gel (Merck silica gel 60, 0.2–0.5 mm, 50–130 mesh).
1H NMR spectra were measured using a Bruker DPX 400 MHz spec-
trometer unless otherwise stated. Microanalyses were performed by Ste-
phen Boyer, London Metropolitan University, London, UK. MS spectra
(MALDI-TOF-MS) were determined on Voyager-DE-STR mass spec-
trometer.
Synthesis of 3: Compound 1 (41.7 mg, 0.030 mmol, 1 equiv), 2 (95 mg,
0.149 mmol, 5 equiv) and LiBr (10.3 mg, 0.149 mmol, 5 equiv) were dis-
solved in CHCl3/MeOH (95:5, 1.5 mL) and heated to 708C for 14 days in
a sealed pressure tube. Solvent was removed in vacuo and the dark red
residue purified by column chromatography (SiO2: gradient elution,
CHCl3 to 0.1% MeOH in CHCl3) to yield the title compound. Rf =0.2,
(0.1% MeOH in CHCl3) as a purple solid (4.9 mg, 0.0024 mmol, 8%);
1H NMR (400 MHz, CD2Cl2): d=8.47 (s, 8H) 7.31 (d, J=8.28 Hz, 2H)
7.17–7.26 (m, 10H) 7.13 (d, J=8.66 Hz, 10H) 7.03–7.09 (m, 6H) 6.99 (d,
J=9.03 Hz, 4H) 6.72 (d, J=8.41 Hz, 4H) 6.14 (td, J=7.90, 5.10 Hz, 4H)
5.64 (dd, J=7.53, 4.27 Hz, 4H) 4.71 (t, J=6.30, 4H) 4.49 (t, J=5.60, 4H)
4.03 (brdd, J=15.12, 4.58 Hz, 16H) 3.88 (brd., 8H) 3.74 (brd, J=
Figure 4. a) Experimental EPR spectrum of 1À (bold black line) and sim-
ulation (grey line) using giso =2.0033; a4H =1.68, 0.56 and 0.13ꢄ10À4 cmÀ1
and a2N =0.53ꢄ10À4 cmÀ1 and a Lorentzian lineshape with a linewidth of
0.12 G. b) Experimental EPR spectrum of 3À (bold black line) and simu-
lation (grey line) using giso =2.0034; a4H =1.46, 0.79 and 0.16ꢄ10À4 cmÀ1
and a2N =0.55ꢄ10À4 cmÀ1 and a Gaussian lineshape with a linewidth of
0.18 G.
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Perylene Diimide Rotaxanes
FULL PAPER
3.89 Hz, 8H) 2.61 (q, J=7.53 Hz, 4H) 1.29 (s, 32H) 1.22 ppm (t, J=
7.59 Hz, 6H); 13C NMR (126 MHz, CDCl3): d=164.12, 156.97, 156.58,
152.70, 148.75, 148.24, 144.51, 144.32, 144.07, 141.87, 141.34, 141.14,
139.97, 139.03, 134.65, 134.60, 132.31, 131.01, 130.85, 130.65, 129.28,
128.92, 128.63, 128.02, 127.27, 126.61, 125.16, 125.03, 124.71, 124.03,
123.80, 123.10, 121.69, 114.62, 114.43, 114.05, 113.52, 113.11, 105.67,
103.04, 71.51, 71.39, 71.20, 71.02, 70.79, 70.67, 70.55, 70.07, 69.84, 67.93,
67.61, 64.84, 63.11, 55.63, 53.42, 43.57, 39.27, 34.67, 34.33, 34.23, 31.93,
31.59, 31.37, 31.32, 30.32, 29.70, 29.36, 29.06, 28.18, 25.28, 22.66, 22.11,
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:
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Acknowledgements
We thank the EPSRC and the University of Nottingham for support and
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Received: October 3, 2011
Published online: December 1, 2011
Chem. Eur. J. 2011, 17, 14746 – 14751
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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14751