Intercomponent interactions and mobility in hydrogen-bonded rotaxanes
Jagesar, D.C.Publication date 2010
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Citation for published version (APA):
Jagesar, D. C. (2010). Intercomponent interactions and mobility in hydrogen-bonded rotaxanes.
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C h a p t e r 5
Perylene Diimide Rotaxanes
Hydrogen Bonding and Macrocycle Translocation
Abstract
The hydrogen bond interactions between the macrocycles and the redox-active perylene diimide (pdi) binding station were examined in two types of [2] and [3]rotaxanes. In the rotaxanes [2]8 and [3]8, containing only a perylene diimide as possible binding station, hydrogen bonding is increasingly stronger in the neutral, radical anion and dianion oxidation state. This was signaled by changes of the reduction potentials and blue-shifts of the UV-Vis absorption maxima of the rotaxanes compared to those of the thread. The rotaxanes [2]9 and [3]9 contain two succinamide (succ) binding stations and one pdi station and behave as molecular shuttles. Macrocycle translocation from the initially occupied succ to the pdi station takes place upon electrochemical reduction of the latter. Direct detection of the resulting co-conformers, in which hydrogen bonds between the macrocycle and reduced pdi station are formed, was however only possible for the dianion. Hydrogen-bond induced blue-shifts of the UV-Vis absorption maxima were observed as well as shifts of the C=O stretching IR bands of the macrocycle and pdi dianion. Hydrogen bonding of the macrocycle with the pdi radical anion leads to significant increase of the electron affinity of the latter, which is reflected by the immediate conversion to the dianion, at almost the same potential at which the radical anion is formed.
* A part of this work was published in: Baggerman, J.; Jagesar, D. C.; Vallée, R. A. L.; Hofkens, J.; De Schryver, F. C.; Schelhase, F.; Vögtle, F.; Brouwer, A. M. Fluorescent Perylene Diimide Rotaxanes: Spectroscopic Signatures of Wheel-Chromophore Interactions. Chem. Eur. J. 2007, 13, 1291-1299.
5.1
Introduction
Perylene diimides are known for their pronounced and tuneable electrochemical and electronic properties, and outstanding thermal and photochemical stabilities.[1-3] Tuning of
the electronic and electrochemical properties can be achieved by introducing different functional groups at the bay positions of the aromatic skeleton.[4] During the past decades,
the knowledge on the tuneability of these properties has been extended, and exploited in numerous applications in which perylene diimides function as active components. Examples are: organic transistors,[5-7] photovoltaic cells,[8,9] light-harvesting arrays,[10-12] organic light emitting diodes (OLEDs)[13] and fluorescent labels and sensors.[14-17]
Electrons and photons are often used for the activation and control of the molecular motor functionality in rotaxanes. Perylene diimides are very sensitive to these external stimuli, and also provide the tools to sense their direct environment through their electrochemical and photophysical behavior. For this reason, perylene diimides would be obvious candidates to be applied in the field of molecular motors. However, until now, only one example of a perylene diimide containing rotaxane shuttle has been reported in the literature.[18] The reversible translocation of the macrocycle was triggered by light-induced cis-trans isomerization of an initially occupied fumaramide station. After translocation, and protonation, the macrocycle quenched the fluorescence of a pyrene energy donor group, which indirectly resulted in a reduced fluorescence of the perylene diimide.
In this chapter, studies will be presented of two types of [2] and [3]rotaxanes containing a functional perylene diimide (Figure 5-1). For comparison, data were obtained for the commercial dye Perylene Red (Figure 5-2). The rotaxanes [2]8 and [3]8 are examples of the Vögtle-type rotaxanes. These rotaxanes are assembled using a trapping approach.[19,20] The
negatively charged phenoxide oxygen of one of the axle precursors is aligned in the cavity of the macrocycle by hydrogen bonds with the macrocycle NH groups, before reacting with the other part of the axle.[20] In the present case, the axle is constructed by a nucleophilic substitution reaction of the 4-tritylphenoxide ion with a benzyl bromide group attached to the imide nitrogens of the chromophore. A typical feature of these rotaxanes with ether linkages is that the interaction that is applied to pre-align the axle precursors is lost once the rotaxane is formed. In the rotaxanes [2]8 and [3]8, apart from van der Waals interactions, the only possible intercomponent interactions are hydrogen bonds between the macrocycle NH groups and perylene diimide C=O groups. Thus, the macrocycle is not firmly bound to a template, as in the mechanically interlocked architectures of, e.g. Leigh[21,22] and
Stoddart.[23,24] The aim of the experiments with [2]8 and [3]8 is to investigate the effect of these hydrogen bond interactions on the electrochemical and photophysical behavior of the perylene diimide chromophore in different oxidation states.
Rotaxanes [2]9 and [3]9 contain two types of potential binding stations for the macrocycle, separated by a C12 alkyl chain: an initially occupied succinamide (succ) station
pyromellitimide molecular shuttles [2]5 and [3]5 described in Chapters 3 and 4, respectively. In rotaxanes [2]9 and [3]9 a perylene diimide station is present instead of a naphthalimide or pyromellitimide station.
Figure 5-1 Structures of one-station perylene diimide thread 8, [2]rotaxane [2]8 and [3]rotaxane [3]8. Thread 9 and rotaxanes [2]9 and [2]9 equipped with a perylene diimide (pdi) and two succinamide (succ) binding stations.
The goal of the study on rotaxanes [2]9 and [3]9 is to investigate the suitability of the perylene diimide chromophore to induce and detect macrocycle switching between the succ and pdi stations. Electrons will be used as fuel to drive the macrocycle translocation. The electrochemical and electronic properties of the electrochemically reduced perylene diimide station will be used to detect the macrocycle translocation. Also, in order to get more insight into the changes in hydrogen bond interactions between the macrocycle and stations upon switching, the changes in the vibrational frequencies of the involved amide and imide groups of the macrocycle, the succinamide station and the perylene diimide station will be analyzed.
5.2
Results and Discussion
The spectroelectrochemical studies on the naphthalimide (Chapter 3) and pyromellitimide (Chapter 4) systems were performed in tetrahydrofuran (THF). In order to facilitate comparison, for the present investigation of the perylene diimide systems, THF would also be the preferred solvent. Unfortunately, this was not possible due to poor solubility of the perylene diimide compounds in THF. Therefore, the (spectro)-electrochemical behavior of these perylene diimide based systems was examined in dichloromethane (CH2Cl2).
5.2.1 Perylene Diimide Rotaxanes
[2]8
and[3]8
Electrochemical Characterization
The electrochemical behavior of thread 8, [2]rotaxane [2]8 and [3]rotaxane [3]8 was examined by means of cyclic voltammetry. The voltammograms in CH2Cl2 solutions
containing supporting electrolyte NBu4PF6 and internal standard ferrocene (Fc) are depicted
in Figure 5-2. The reported potentials are relative to the half-wave potential (E½) of the Fc/Fc+ redox couple.[a]
The voltammogram of thread 8 exhibits an electrochemically reversible first reduction to the radical anion. The reduction peak potential is -1.20 V and the back-oxidation occurs at -1.11 V. Experiments with Perylene Red resulted in similar values: Ered = -1.21 V and Eox = -1.12 V. These potentials are in agreement with those reported in the literature for similar tetraphenoxy substituted perylene diimides.[4,25]
For [2]8, the reduction potential is shifted +0.10 V to -1.10 V compared to the thread 8. The higher electron affinity of [2]8 is explained by hydrogen bonding of the perylene diimide C=O groups to the macrocycle NH groups. Consequently, this effect is expected
[a] Conversion of the reported values vs. Fc/Fc+ to the saturated calomel electrode (SCE) scale is possible by
Perylene Red
Figure 5-2 Cyclic voltammograms of Perylene Red, thread 8, [2]rotaxane [2]8 and [3]rotaxane
[3]8. The voltammograms were recorded with a sweep rate of 100 mV s-1 in CH2Cl2 solutions
containing supporting electrolyte NBu4PF6 and internal standard ferrocene. The E½ of Fc/Fc
+
is 0.475 V vs. SCE.[26]
Table 5-1 Peak potentials (V) of reduction and back-oxidation of Perylene Red, thread 8, [2]rotaxane [2]8 and [3]rotaxane [3]8 and half-wave potentials of the reversible reductions in CH2Cl2. All values are versus E½ of Fc/Fc
+
. For conversion to the SCE scale, add 0.475 V.[26]
Compound 1st reduction 2nd reduction
Ered Eox E½ Ered Eox E½
Perylene Red -1.21 -1.12 -1.17 -1.40 -1.32 -1.36 Thread 8 -1.20 -1.11 -1.15 -1.34 -1.26 -1.30 [2]Rotaxane [2]8 -1.10 -0.86 n.r. [a] -1.34 -1.14 n.r. [a]
[3]Rotaxane [3]8 -0.88 -0.67 n.r. [a] -1.18 -0.94 n.r. [a] [a] Electrochemically not reversible.
to be stronger in [3]8 because of the possibility to form two sets of hydrogen bonds between the perylene imides and the two macrocycles. Indeed, the reduction peak potential is shifted even more (+0.32 V) to -0.88 V. The reduction to the radical anion of both [2]8 and [3]8 is an electrochemically irreversible process. The separations between reduction and corresponding oxidation waves are 0.24 V and 0.21 V respectively, which are much larger than the theoretically expected value of 0.060 V for electrochemically reversible processes.[27] The irreversible behavior is obviously a result of stabilization by hydrogen
bonds between macrocycle and perylene diimide, which are stronger if the latter is in the radical anion state. The stabilization of the radical anion is larger for [3]8 than for [2]8.
The dianion of thread 8 is formed at a potential of -1.34 V, back-oxidation to the radical anion takes place at -1.26 V. The E½ value (-1.30 V) is slightly more positive than that for Perylene Red reduction (E½ = -1.36 V). For both [2]8 and [3]8 the second reduction is electrochemically irreversible. The reduction potentials are -1.34 V for [2]8 and -1.18 V for [3]8. Back oxidation occurs at -1.14 V and -0.94 V, respectively. The difference between reduction and oxidation waves is larger for [3]8 (0.24 V) than for [2]8 (0.20 V). This is consistent with the conclusions drawn for the radical anion: the dianion is more stabilized in [3]8 than in [2]8 due to the formation of an additional set of hydrogen bonds.
UV/Vis Absorption
The effect of hydrogen bonding on the electronic properties of the neutral perylene diimide was revealed by analyzing the characteristics of the electronic absorption and fluorescence spectra.[28] Both the absorption and fluorescence maxima of the perylene
diimide chromophore in rotaxanes [2]8 and [3]8 show substantial hydrogen-bond induced red-shifts compared to the thread. The red-shifts are more pronounced in non-polar aprotic solvents (e.g. 17 nm in CH2Cl2, Figure 5-3A, and Table 5-2) and decrease in strongly
hydrogen bonding solvents (e.g. DMSO) due to disruption of hydrogen bonds between the macrocycle and the perylene diimide. Furthermore, an increasing enhancement of the non-radiative decay of the excited singlet state was observed for the [2]rotaxane and the [3]rotaxane, evidenced by decreased fluorescence quantum yields and reduced excited state lifetimes.
In order to explore the effect of hydrogen bonds on the radical anion and dianion species, UV-Vis spectroelectrochemical experiments were performed with the thread and the [2]rotaxane. The [3]rotaxane [3]8 was not subjected to this analysis because the available amount was too small to obtain good results. The spectra of the neutral and reduced species are depicted in Figure 5-3; the absorption maxima are collected in Table 5-2. The UV-Vis spectra of the radical anions of thread 8 and [2]rotaxane [2]8 are almost identical. The maximum at 790 nm shows a slight red-shift to 791 nm for [2]8. The other two maxima are blue-shifted and appear at 974 nm (shift = 2 nm) and 1075 nm (shift = 7 nm). A blue-shift is observed for those transitions in which the hydrogen bonding is stronger in the ground
(A) (B)
Figure 5-3 Changes in the UV-Vis absorption spectra of thread 8 and [2]rotaxane [2]8 in CH2Cl2 upon electrochemical reduction with a scan rate of 2 mVs
-1
(supporting electrolyte NBu4PF6). (A): One-electron reduction and (B): reduction to the dianion.
Table 5-2 Absorption maxima λmax (nm) of the neutral, radical anion and dianion of thread 8
and rotaxane [2]8 in CH2Cl2.
Compound Neutral Radical anion Dianion
λ
maxλ
maxλ
maxλ
maxλ
maxThread 8 580 790 976 1082 682
[2]Rotaxane [2]8 597 791 974 1075 670
state than in the excited state, which is the case when excitation removes electron density from the singly or doubly occupied highest energy orbitals. The difference between ground state and excited state stabilization of the dianion by hydrogen bonds is larger than that of the radical anion: for the dianion [2]82– a larger blue-shift of 12 nm is observed.
5.2.2 Perylene Diimide Shuttles
[2]9
and[3]9
The positions of the macrocycles in rotaxanes [2]9 and [3]9 were determined with 1H
NMR spectroscopy.[29] In CDCl
3, the macrocycle was found to be hydrogen-bonded to the
succinamide station. This was concluded from the shielding of the aliphatic protons of one succinamide station in [2]9 and both stations in [3]9. In addition, the chemical shifts of the perylene diimide protons were identical for 9, [2]9 and [3]9. In this respect, these rotaxanes behave similarly to the naphthalimide rotaxane 1 (Chapter 3) and the pyromellitimide rotaxanes [2]5 and [3]5 (Chapter 4). Just as in those rotaxanes, the macrocycles in [2]9 and [3]9 are expected to switch position from the succinamide station to the perylene diimide station upon one and two-electron reduction. The changes of the interactions between the macrocycle and the two stations associated with macrocycle translocation upon electro-chemical reduction were studied with UV-Vis and IR spectroelectroelectro-chemical experiments.
UV-Vis Spectrolelectrochemistry
The absorption spectra of 9, [2]9 and [3]9 were recorded at different stages of electrochemical reduction at potentials between 0 to ca. -0.65 V vs. the Ag/Ag+ pseudo-reference electrode (Figure 5-4). The potentials were not converted to the Fc/Fc+ scale
because the E½ value of Fc/Fc
+
under these conditions was not measured. Thread 9 undergoes clean reduction to the radical anion 9●–; maximum conversion is reached at a potential of -0.66 V vs. Ag/Ag+. Quantitative conversion to the dianion is reached at a potential of -0.80 V for 9 and -0.85 V for [2]9 and [3]9.
The visible absorption spectra of the neutral molecules 9, [2]9 and [3]9 are identical and show the characteristic features of the perylene diimide chromophore (Figure 5-4A, the absorption maxima are summarized in Table 5-3). This agrees with the absence of (hydrogen bond) interactions between the macrocycle and perylene diimide station in the neutral molecules, as expected because of the strong predominance of the succ-co-conformer (Scheme 5-1). The UV-regions of the absorption spectra of the rotaxanes [2]9 and [3]9 exhibit additional absorptions from the macrocycles.
The absorption spectrum of 9●– exhibits one strong (787 nm) and two weak transitions (977 and 1080 nm). The absorption spectrum of the radical anion of the [2]rotaxane, [2]9●–
, displays maxima at 786, 971 and 1069 nm. These spectral features are also observed for similar perylene diimide derivatives reported in the literature.[30,31] The lowest-energy transitions of [2]9●– are blue-shifted with respect to those of 9●–. This is an indication for macrocycle translocation: the reduced pdi station is hydrogen-bonded to the macrocycle. Blue-shifts were also observed for the radical anions of the naphthalimide (1, Chapter 2) and pyromellitimide rotaxanes ([2]5 and [3]5, Chapter 4). The blue-shift is explained by the fact that stabilization of the radical anion, provided by hydrogen bonding to the macrocycle, is larger for its ground state than for its excited state. The strong absorption band at 786 nm is broadened and, although slightly, also blue-shifted with respect to that of 9●–. This broadening can occur if a mixture of both co-conformers exists i.e. if co-conformer succ-[2]9●– is not fully converted to pdi-[2]9●– (Scheme 5-2A). The result would be that the observed absorption band is composed of a blue-shifted band of pdi-[2]9●–
and an unperturbed contribution of succ-[2]9●–. A small equilibrium constant k1/k-1(Scheme 5-2B) implies that the difference in hydrogen bonding affinity of the succ and pdi radical anion is small. A similar band broadening was also observed for a naphthalene diimide rotaxane.[29]
In that case an equilibrium constant k1/k-1 = 5 was estimated from CV studies.
[32]
The absorption maximum of the dianion 92– is observed at 683 nm. The dianions of the rotaxanes, [2]92– and [3]92–, exhibit increasingly blue-shifted maxima at 671 nm (shift 12 nm) and 662 nm (shift 21 nm), respectively. Again, the blue-shift is attributed to hydrogen bonding between macrocycle and perylene diimide dianion. The larger blue-shift for [3]92– is clear evidence that both macrocycles switch position to form hydrogen bonds with the perylene diimide dianion (Scheme 5-1).
(A) (B)
(C) (D)
Figure 5-4 Changes in the absorption spectra of thread 9 and rotaxanes [2]9 and [3]9 in CH2Cl2 during electrochemical reduction. (A): one-electron reduction, the indicated potentials
are versus the Ag/Ag+ pseudo-reference electrode. (B): second reduction. Scaled absorption spectra of (C): radical anions and (D): dianions of 9, [2]9 and [3]9. The strong absorption below 710 nm in the radical anion absorption spectra (C) is from the dianion, which is formed immediately after the radical anion.
Interestingly, the blue-shifts of the three absorption maxima in [2]9●– relative to 9●– (1, 6 and 11 nm) are larger than for [2]8●– relative to 8●– (-1, 2 and 7 nm, Table 5-2). This suggests that hydrogen bonding is stronger in [2]9●– than in [2]8●–. An explanation is that the macrocycle of [2]9●– is smaller than that of [2]8●– and can offer a more favorable relative
orientation (distance and angle) of the diimide C=O and macrocycle NH groups, i.e., the perylene diimide station fits better into the macrocycle of [2]9●–. The spectrum obtained for radical anion [3]9●– is identical to that of 9●–; this unexpected result will be discussed below.
Table 5-3 Absorption maxima λmax (nm) of the neutral, radical anion and dianion of thread 9
and rotaxanes [2]9 and [3]9 in CH2Cl2.
Compound Neutral Radical anion Dianion
λ
maxλ
maxλ
maxλ
maxλ
maxThread 9 578 787 977 1080 683
[2]Rotaxane [2]9 578 786 971 1069 671 [3]Rotaxane [3]9 578 787 976 - 662
Scheme 5-1 Proposed structures of three different co-conformers of [3]rotaxane[3]9resulting from translocation of one or both macrocycles.
Remarkably, while for 9 the formations of 9●‒ and 92‒are well separated, for [2]9 and [3]9
these two reductions overlap. The dianion already arises before quantitative conversion to the radical anion is reached. This effect is stronger for [3]9 than for [2]9, as can be seen in
Figure 5-4. This phenomenon can be explained by translocation of the macrocycle from the succinamide to the reduced perylene diimide station. In [2]9, this leads to the formation of co-conformer pdi-[2]9●– from succ-[2]9●– (Scheme 5-2A). So, the species which is reduced to form the dianion is pdi-[2]9●– rather than succ-[2]9●–.
The perylene diimide radical anion in pdi-[2]9●– is hydrogen-bonded to the macrocycle, which increases the electron affinity. Hence, the reduction potential of co-conformer pdi-[2]9●– (E2pdi) is less negative than that of succ-[2]9●– (E2succ). More importantly, the conclusion can be drawn thatE2pdiis similar to the reduction potential of succ-[2]9 (E1succ). This conclusion can be justified by comparison with the electrochemical behavior of 8 and [2]8: hydrogen bonding causes the reduction potential of [2]8 to shift +10 mV compared to that of 8. In pdi-[2]9●– hydrogen bonding is stronger, which will cause an even larger shift of the reduction potential of pdi-[2]9●–. The proposed scheme with the electrochemical reductions of each co-conformer followed by macrocycle translocations (with forward rate constants k0, k1 and k2) is depicted in Scheme 5-2B.
(A) (B) 0 0 1 1 2 2 k k k k k k − − − > > pdi 2 E > succ 2 E pdi 2 E ≈ succ 1 E
Scheme 5-2 (A): Macrocycle translocation in [2]9●‒ leading to the formation of co-conformer
pdi-[2]9●‒ from succ-[2]9●–. (B): Formation of the radical anions and dianions of the possible co-conformers of the corresponding oxidation states (vertical) and macrocycle translocation equilibrium in each state (horizontal). The solid arrows indicate the preferred interconversion pathways.
The low scan rate (2 mV s-1) during the electrochemical reduction is much smaller than
the macrocycle translocation rate (typically of the order of 105 – 106 s-1 for aromatic imide
rotaxanes).[29,33] So, once the radical anion
succ-[2]9●– is formed, shuttling (formation of pdi-[2]9●–) takes place before the bare radical anion (succ-[2]9●–) can be converted to the dianion (succ-[2]92–). And also, because of the small difference between E2pdiandE1succ, a substantial
part of pdi-[2]9●– is already converted to the dianion pdi-[2]92–. As a result, no accumulation of radical anions is obtained. At any given potential, an equilibrium mixture will be obtained of succ-[2]9●–, pdi-[2]9●– and pdi-[2]92–. Thus, the observed UV-Vis absorption of [2]9●– will be that of a mixture of co-conformers succ-[2]9●‒ and pdi-[2]9●–. The positions of the absorption maxima are therefore determined by the population ratio of both co-conformers. Along the same lines, the almost identical absorption maxima of 9●– and [3]9●– can be explained: the main contribution is from the co-conformer succ-succ-[3]9●–, because co-conformers pdi-succ-[3]9●– and pdi-pdi-[3]9●– are immediately converted to the dianions. For the same reason, detection of co-conformer pdi-[2]9●– with UV-Vis spectroelectro-chemistry is also impossible. The stronger effect for the [3]rotaxane is explained by the presence of two macrocycles.
An elegant way to directly detect the shuttling in the radical anion state would be through selective generation of the radical anion with photoinduced electron transfer from a suitable electron donor and subsequent time-resolved monitoring of the absorption maximum of the radical anion. This approach was not attempted, because the spectral detection range of our experimental transient absorption setup (350 – 800 nm) was not adequate to capture the absorption of the radical anion species.
Infrared Spectroelectrochemistry
The Amide I regions of the IR spectra of the neutral thread 9 and rotaxanes [2]9 and [3]9 exhibit two strong bands from the perylene diimide station, with maxima at 1695 and 1658 cm-1 (Figure 5-5). The high frequency band at 1695 cm-1 is assigned to the symmetric C=O
stretching [
ν
s(CO)pdi]; the low frequency band at 1658 cm-1
originates from antisymmetric C=O stretching [
ν
as(CO)pdi] of the diimide. The band at 1587 cm-1 and the shoulder (ca. 1591cm-1) are associated with vibrations of the perylene ring [
ν
(Ar)pdi]. The intensity increase of this band in [2]9 and [3]9 is due to overlap with the underlying broad Amide II band (centred around 1540 cm-1) of the macrocycle. These spectral features are in agreement with those reported in the literature for similar systems.[34-37] In thread9, the amide groups of the succinamide station give rise to one strong
ν
(CO)succ band at 1671 cm-1
, which is visible as a shoulder on the
ν
as(CO)pdi peak. This characteristic band is also found in the related imide systems discussed in the previous chapters. In the naphthalimide thread 2 in CH2Cl2, thisband appears at 1672 cm-1 and in the pyromellitimide thread
5 in THF it is at 1677 cm-1.
Compared to the thread, several spectral changes are observed for the rotaxanes. The C=O stretching band of the macrocycle amide groups [
ν
(CO)mac] fully overlaps with theν
as(CO)pdi at 1658 cm-1
. This is in agreement with the result obtained for the naphthalimide rotaxane 1 in CH2Cl2: in this case the
ν
(CO)mac band appears at 1659 cm-1 (Chapter 3).
The rotaxanes [2]9 and [3]9 contain one or two macrocycles which are hydrogen-bonded to the same number of succinamide stations. This will give rise to a new
ν
(CO)succ band belonging to hydrogen-bonded C=O of the succinamide station. This band is found at ca.1640 cm-1 for both
[2]9 and [3]9. The wavenumber is the same as for the naphthalimide rotaxane 1 in CH2Cl2. Rotaxane [2]9 contains one macrocycle which is hydrogen-bonded to
one succinamide station. So, compared to the thread, the intensity of the unperturbed
ν
(CO)succ of the rotaxane [2]9 (at 1671 cm-1) is expected to decrease by roughly a factor of 2.
Due to the intensity decrease, this band is only observed as a weak shoulder on the relatively more intense
ν
as(CO)pdi andν
(CO)mac bands. In rotaxane [3]9, both succinamide stations are hydrogen-bonded to the macrocycle, which results in further intensity decrease of the freeν
(CO)succ band.Figure 5-5 Partial IR spectra of thread 9 (—) and rotaxanes [2]9 (····) and [3]9 (- - -) in CH2Cl2. The spectra were scaled to the intensity of the νs(CO)pdi band at 1695 cm
-1
in order to facilitate comparison. The irregularity at 1605 cm-1 and increased baseline below 1700 cm-1 are caused by solvent peaks which could not be subtracted adequately. These artefacts are however eliminated in the difference spectra (Figure 5-6 and Figure 5-7).
Surprisingly, the
ν
(CO)mac band is almost equally intense for [2]9 and [3]9, while on the basis of an extra macrocycle present in [3]9 one would expect an intensity approximately twice as high as for [2]9. The experimental result can be explained by the underlyingν
(CO)succ bands of a free and hydrogen-bonded succinamide unit in [2]9. For the succinamide model compound 4 (see Scheme 3-2), two overlappingν
(CO)succ bands at 1676 and 1669 cm-1 were found (Figure 3-4). The presence of two distinct bands implies that theC=O transition moments are not antiparallel to each other but almost perpendicular. This was confirmed with B3LYP calculations, which predicted a dihedral angle of 122°. In the case of the rotaxanes the bifurcated sets of hydrogen bonds with the macrocycle NH groups require an antiparallel configuration of the C=O groups,[38] and indeed, for rotaxane
1 only one
ν
(CO)succ band was observed (see Figure 3-6B). In [2]9, one succinamide station is unoccupied, which results in two correspondingν
(CO)succ bands. In [3]9, both succinamidestations are hydrogen-bonded. The required antiparallel configuration of the C=O’s thus results in only one
ν
(CO )succ band, the shoulder at 1640 cm-1 in Figure 5-5.
Upon electrochemical reduction of thread 9 to form 9●–, five new bands of the perylene diimide station grow in (Figure 5-6). The
ν
s(CO)pdi andν
as(CO)pdi bands shift to 1635 and 1568 cm-1 respectively (shifts: -60 and -90 cm-1). The two overlappingν
(Ar)pdi bands also shift to lower frequency and split up in two distinct bands at 1522 cm-1 and 1489 cm-1. The
red-shifts are 55 and 103 cm-1 respectively. The origin of the band at 1607 cm-1 is unclear.
This band is also present in the spectra of N,N’-dioctyl perylene diimide shown by Kaake et al., but not mentioned in the text of that paper.[35] Possibly, this band originates from a
vibrational mode that is formally symmetry-forbidden in the neutral molecule, but gains intensity in the radical anion due to conformational changes of the aromatic core. Also, as in the case of the pyromellitimide systems (Chapter 4), the appearance of overtones due to Fermi resonances cannot be excluded.
Figure 5-6 (A): Partial IR spectra of neutral, radical anion and dianion of perylene diimide thread 9 in CH2Cl2. Difference spectra (B): radical anion vs. neutral and (C): dianion vs. neutral.
The dianion 92– exhibits a more complex pattern of bands. Eight bands are found in the difference spectrum (Figure 5-6C) at 1620, 1601, 1583, 1561, 1550, 1518, 1508 and 1482 cm-1. The new IR bands of the perylene diimide could not be unambiguously assigned. As
was proposed above, changes in the perylene core upon electrochemical reduction might lead to an increase of the number of IR modes with detectable intensity, or overtones/Fermi resonances might come into play.
It was concluded from the UV-Vis spectroelectrochemical study that macrocycle translocation takes place in the radical anions of rotaxanes [2]9 and [3]9, but the resulting co-conformer is difficult to detect because of the reduction to the dianion at almost the same potential. This effect was also observed during the IR spectroelectrochemical experiments. Therefore, the focus of the IR spectroelectrochemical study of the rotaxanes will be on the dianion.
The Amide I regions of the IR spectra of the dianions of 9, [2]9 and [3]9, and the corresponding difference spectra (dianion vs. neutral) are collected in Figure 5-7. As expected, negative
ν
(CO)pdi peaks at 1695 and 1658 cm-1 are observed in the difference
spectra. Also, a negative peak is observed at 1636 cm-1 for the rotaxanes. For the neutral
rotaxanes, this band was assigned to stretching of the C=O groups of the succinamide station which is hydrogen-bonded to the macrocycle NH groups. The presence of this negative band is a clear indication of macrocycle translocation: hydrogen bonds between succinamide station and macrocycle are broken in the dianion. Furthermore, the intensity of this negative band is ca. 1.7 times larger for the [3]rotaxane than for the [2]rotaxane. This means that, as was also concluded from the UV-Vis spectra, in the [3]rotaxane both macrocycles undergo translocation from the succinamide to the dianion of the perylene diimide station.
(A) (B)
Figure 5-7 (A): Partial IR spectra of dianions of perylene diimide thread 9 (—) and rotaxanes
[2]9 (····) and [3]9 (- - -) in CH2Cl2. (B): Difference spectra (dianion vs. neutral). All spectra were
The precise position of the
ν
(CO)mac band could not be determined in the spectra of the neutral rotaxanes due to overlap with theν
as(CO)pdi band. In the spectra of the dianions [2]92– and [3]92– however, this band is separated from the perylene diimide bands and appears at 1654 cm-1 (Figure 5-7A). In the neutral form of the naphthalimide rotaxane1 in CH2Cl2, the
ν
(CO)mac band was found at 1659 cm-1. This red-shift of 5 cm-1 for
[2]92– and [3]92– suggest stronger hydrogen bonding in the dianions [2]92– and [3]92– than in the neutral rotaxanes. The explanation for this observation is macrocycle translocation: in the resulting co-conformers, the macrocycle forms hydrogen bonds with the C=O groups of the reduced pdi station (Scheme 5-1).
Comparing the perylene diimide with the naphthalimide system, one would expect similar electron densities on the oxygen atoms of the perylene diimide dianion and the naphthalimide radical anion and consequently similar hydrogen bond acceptor strength. Also, both imides have the same spatial arrangements of the hydrogen bond accepting C=O groups, so differences due to conformational effects can be excluded. The red-shift of the
ν
(CO)mac band in [2]92–
and [3]92– (5 cm-1: 1659 cm-1 → 1654 cm-1) upon shuttling is however
smaller than the 7 cm-1 (1667 cm-1 → 1660 cm-1) observed for the radical anion of the
naphthalimide rotaxane 1 in THF. This seems to conflict with the expected similarity in hydrogen bond accepting strengths of the naphthalimide radical anion and perylene diimide dianion. The different shifts for the naphthalimide rotaxane in THF and the perylene rotaxanes in CH2Cl2 can, however, be due to solvent effects. In CH2Cl2, the
ν
(CO)mac band of the macrocycle, which is hydrogen-bonded to the succinamide station, appears at 1659 cm-1, while in THF this band is observed at 1667 cm-1. This shift is related to the moreelectrophilic nature of CH2Cl2 compared to THF. This was demonstrated in Chapter 3 by
the dependence of the wavenumber of this band on the solvent acceptor number (AN, for CH2Cl2 AN = 20.4 and for THF AN = 8). Our hypothesis is that the solvent effect on the
ν
(CO)mac wavenumber is weaker if the macrocycle is hydrogen-bonded to the reduced diimide station. Thus, a weaker dependence of theν
(CO)mac wavenumber on AN is expected and the red-shift of theν
(CO)mac frequency in CH2Cl2 compared to THF will be smaller.Comparison of the IR spectroelectrochemistry results of 1 in THF and PrCN (AN = 13) supports this hypothesis (see Chapter 3). The
ν
(CO)mac band (1658 cm-1) of co-conformer
ni-1●‒ (Scheme 3-1) in THF and PrCN appeared at the same wavenumber (1658 cm-1), while in
succ-1 a pronounced solvent effect was observed (THF: 1667 cm-1 and PrCN 1663 cm-1). A
systematic study of the solvent effect on the
ν
(CO)mac band of the macrocycle, which is hydrogen-bonded to a reduced imide station, would be required to test this hypothesis further.The IR bands of the perylene diimide station of 92– can be clearly recognized in the spectra of [2]92– and [3]92– (Figure 5-8). The most prominent observation in the spectra of [2]92– and [3]92– is the red-shift of the bands of thread 92– at 1599 and 1585 cm-1 to 1593 and
red-shift of the bands at 1561 and 1545 cm-1 to 1556 and 1537 cm-1, respectively. In the
[2]rotaxane, these red-shifted bands are weaker and much more difficult to resolve. Obviously, these shifted bands are from co-conformers in which the reduced perylene diimide is stabilized by hydrogen bonds with the macrocycle.
The intensities of the red-shifted bands are larger for [3]92– than for [2]92–, indicating a larger extent of hydrogen bonding in [3]92–, due to translocation of both macrocycles, leading to formation of co-conformer pdi-pdi-[3]92– (Scheme 5-1). This is consistent with the conclusion drawn from the relative intensities of the
ν
(CO)mac andν
(CO)succ bands of [2]92–
and [3]92–.
Figure 5-8 IR bands of the two-electron reduced perylene diimide station of thread 9 (—) and rotaxanes [2]9 (····) and [3]9 (- - -) in CH2Cl2 (zoom-in of the spectra in Figure 5-7A).
For both [2]92– and [3]92–, also the unperturbed bands of the perylene diimide dianion are present in the spectra as shoulders at 1599, 1585, 1561 and 1545 cm-1. This means that
not all C=O groups of the perylene diimide dianion are all the time involved in hydrogen bonding. Steric hindrance between the macrocycle and bulky p-tert-butylphenoxy substituents at the bay positions might be a cause for this. Thus, the hydrogen bonds between macrocycle and perylene diimide dianion are not exactly as sketched in Scheme 5-1. The presence of both unperturbed and hydrogen bond associated C=O stretching bands was also observed for the naphthalimide rotaxane 1.
It was already mentioned in Chapter 4 that binding of one macrocycle to a reduced diimide station might result in distortion of the characteristic band pattern of this station. The reason for this is that the diimide station contains two imide groups and the macrocycle only binds two carbonyl groups at the same time, which can lead to changes of the electron densities on different carbonyls and thus to a lowering of the symmetry. This effect is,
however, not observed in the spectra of [2]92–. In the case of the pyromellitimide rotaxane [2]5 this was tentatively explained by the possibility that both imide groups are involved in hydrogen bonding with the macrocycle (chair binding mode, Figure 4-6). In the present case, however, movement of the macrocycle over the aromatic core is blocked by the bulky p-tert-butylphenoxy substituents at the bay positions of the perylene core. We can therefore conclude that apparently binding to the macrocycle does not modify the electron density of the perylene imide dianion to such an extent that it can be detected as a lowering of the symmetry. In other words, the delocalization of the excess electrons over the large aromatic core is not affected much by hydrogen bonding with the macrocycle. An alternative explanation is that in pdi-[2]92– hydrogen bonding of the free imide groups with the succinamide station occurs in a folded conformation, similar to the one detected for the naphthalimide thread in THF (Scheme 3-3). The presence of such folded conformations is however not supported by the observation that in [2]92– only free succinamides are present, as evidenced by the unperturbed
ν
(CO)succ bands.5.3
Conclusion
The hydrogen bond interactions in two types of perylene diimide rotaxanes were studied. Rotaxanes [2]8 and [3]8 contain only one binding station. The macrocycles in these rotaxanes are hydrogen-bonded to the perylene diimide station in the neutral, radical anion and dianion states. The hydrogen bonding strengths increase in this order of oxidation states. This was concluded from the electrochemical behavior and UV-Vis spectra of the respective species. Compared to the thread, substantial hydrogen-bond induced shifts of the reduction potentials and absorption maxima were observed for the rotaxanes. These shifts are larger for the [3]rotaxane than for the [2]rotaxane, because of an additional set of hydrogen bonds with the second macrocycle. Furthermore, the rotaxanes exhibited irreversible electrochemical behavior. This must mean that substantial structural reorganizations occur upon reduction, even though the macrocycle is already interacting with the imide groups in the neutral molecules.
Rotaxanes [2]9and [3]9 contain two binding stations: an initially occupied succinamide station and a perylene diimide station. It is demonstrated that these rotaxanes behave as molecular shuttles. Translocation of the macrocycle, induced by one or two-electron reduction of the perylene diimide station, was studied by analyzing the UV-Vis and IR spectra of the resulting species. Shuttling was found to take place in the radical anion state of [2]9 and [3]9 but could not be detected directly, because of the immediate reduction of the resulting co-conformer to the dianion, at almost the same reduction potential. On the other hand, shuttling in the dianion could be detected in a direct manner. Compared to 9, blue-shifts of the UV-Vis absorption maxima of the dianions [2]92– and [3]92– were observed. This blue-shift, which is larger for [3]92– than for [2]92– is attributed to hydrogen
bonding with the macrocycle. Macrocycle translocation could also be detected by IR spectroelectrochemical experiments. Bond weakening of the perylene diimide dianion C=O groups induced by hydrogen bonding with the macrocycle results in red-shifts of IR frequencies of the perylene diimide station. These shifts are much smaller than for the naphthalimide rotaxane 1, indicating weaker hydrogen bond interactions in [2]92– and [3]92–. In the [3]rotaxane [3]9, both macrocycles move to the two-electron reduced perylene diimide station.
A possible way to directly detect the shuttling in the radical anion state would be through selective generation of the radical anion with photoinduced electron transfer from a suitable electron donor and subsequent time-resolved monitoring of the absorption maximum of the radical anion. As an alternative, time-resolved IR detection would be attractive.
5.4
Experimental Details
Materials
Solutions for cyclic voltammetry and spectroelectrochemical experiments were prepared in Schlenk flasks by dissolving weighed amounts of compound under dry N2 in freshly
distilled CH2Cl2 in order to avoid the presence of water in the samples. Dry
tetrabutylammonium hexafluorophosphate (Bu4NPF6), crystallized twice from methanol,
was used as supporting electrolyte. CH2Cl2 was pre-dried with CaCl2 and distilled under N2
from CaH2. The solvent was added with a syringe, the volume of the obtained solution was
known only approximately. The sample solutions were then transferred to the CV or OTTLE cell with a syringe.
Cyclic Voltammetry
CV measurements were performed using an EG&G PAR Model 283 potentiostat and a gas-tight sample compartment three electrode cell under a nitrogen atmosphere. A platinum disk (apparent surface area of 0.42 mm2) was employed as the working electrode. Before
each experiment the disk electrode was polished with a diamond paste (0.25 µm grain) Silver and platinum wires were used as pseudo-reference and auxiliary electrodes respectively. The redox potentials were measured against the ferrocene/ferrocenium (Fc/Fc+) redox couple used as an internal standard.[39,40] The problem of incompatibility of
water-based reference electrodes, such as the saturated calomel electrode (SCE) and normal hydrogen electrode (NHE), with organic solvents is generally avoided by using a silver wire as internal pseudo-reference electrode and the Fc/Fc+ redox couple as internal standard.
The Fc/Fc+ redox couple is recommended by the IUPAC as solvent independent reference
to report potentials in non-aqueous solvents. We obtained an E½ value of the Fe/Fe
+
couple in CH2Cl2 of 0.39 V vs. the Ag/Ag
+ pseudo-reference system. The literature E
value of the Fe/Fe+ couple in CH
2Cl2 with supporting electrolyte NBu4PF6 is 0.475 V vs.
SCE.[26] So, for conversion to the SCE scale, 0.475 V must be added to our reported data
versus Fc/Fc+.
Spectroelectrochemistry
The spectroelectrochemical experiments were carried out in an OTTLE-cell.[41] The
UV-Vis spectra were recorded on a single beam HP 8453 diode array spectrophotometer. A blank with pure solvent was recorded before each experiment. The IR spectra were recorded on a Bruker Vertex 70 FTIR spectrometer. For further details of sample preparation and the spectroelectrochemical experiments, see experimental section 3.5.
Acknowledgements
The compounds were obtained from Dr. J. Baggerman (University of Amsterdam) who described the synthesis, purification and characterization of both types of rotaxanes in his PhD thesis.[29] C. Mahabiersing and F. Hartl are thanked for their assistance with the CV
and spectroelectrochemical experiments.
5.5
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