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 7
Fullerene Rotaxanes
Reverse Shuttling in a Fullerene-Stoppered Rotaxane
*
Abstract
The macrocycle shuttling in a hydrogen-bonded rotaxane containing a tetraamide macrocycle, a succinamide binding station and a fullero-pyrrolidine stopper was studied in different solvents. 1H NMR and electrochemical studies revealed an unexpected switching behavior in DMSO. In this strongly hydrogen-bond disrupting solvent, the macrocycle was found to reside close to the fulleropyrrolidine stopper rather than over the alkyl chain. In similar rotaxanes in DMSO, but with another stopper in place of the fullero-pyrrolidine, the macrocycle is generally found along the alkyl chain due to the disruption of hydrogen bonds with the succinamide binding station. The 1H NMR, electrochemical and photophysical characterization of rotaxane C60R does not support the existence of strong interactions between the macrocycle and the fulleropyrrolidine stopper. Instead, the driving force for reverse switching is provided by solvophobicity of the fulleropyrrolidine in DMSO.
* This work was published in: Mateo-Alonso, A.; Fioravanti, G.; Marcaccio, M.; Paolucci, F.; Jagesar, D. C.; Brouwer, A. M.; Prato, M. Reverse Shuttling in a Fullerene-Stoppered Rotaxane. Org.
7.1
Introduction
Since the discovery of fullerenes,[1] a 3-dimensional allotropic form of carbon, these appealing structures have been a subject of extensive fundamental research. The most prominent member of the fullerene family is without doubt [60]fullerene (C60), also referred to as Buckminster fullerene. [60]Fullerene belongs to the icosahedron symmetry group and is the most abundant among fullerenes. The spherically delocalized π-electron systems of fullerenes give rise to a set of interesting electrochemical and electronic properties. [60]Fullerene is an excellent electron acceptor in the ground state and capable of reversibly accepting up to six electrons.[2]
During the past decade, [60]fullerene has also entered the active field of research on interlocked molecules. Several examples of rotaxanes with incorporated functional C60 moieties have been reported in literature. In the simplest cases, C60 functions as the stopper for the macrocyclic ring in hydrogen-bonded,[3-6] transition-metal coordinated[7] and cyclodextrin rotaxanes.[8] In other systems, C60 incorporated in the thread or the macrocycle, (also) functions as photophysically and electrochemically active component in dyads with a variety of electron donors such as ferrocene,[9-11] porphyrins[11-15] transition metal complexes[7,15-17] and triphenylamine[18]. Also, systems with [60]fullerene and multiple electron donors forming long lived charge-separated radical pair states have been reported.[11,19]
This chapter describes a hydrogen-bonded rotaxane with a fulleropyrrolidine stopper (Figure 7-1), examined in protic (dimethylsulfoxide, DMSO) and aprotic (dichoromethane, CH2Cl2 and benzonitrile, PhCN) solvents. The position of the macrocycle will be determined with NMR and cyclic voltammetry. Also, the effect of the proximity of the macrocycle on the photophysical behavior of the fulleropyrrolidine chromophore will be studied in these solvents.
The position of the macrocyclic ring in C60R is influenced by the nature of the solvent. In aprotic solvents such as tetrahydrofuran (THF), dichloromethane and chloroform (CHCl3) the macrocycle is expected to be hydrogen-bonded to the succinamide station. However, strongly hydrogen bonding solvents such as dimethylformamide (DMF) and especially DMSO disrupt hydrogen bonding between the macrocycle and the succinamide station and may force the macrocycle to move away from this station. Solvent-induced switching is a successful way to influence the position of the macrocycle and has been reported in literature for many hydrogen-bonded rotaxanes.[3,20-22]
Figure 7-1 Structures of the [60]fullerene containing thread C60T and rotaxane C60R.
7.2
Results and Discussion
7.2.1 Solvent-Induced Shuttling in
C
60R
The position of the macrocycle along the thread in C60R was monitored with 1H NMR spectroscopy using the anisotropy effect of the aromatic macrocycle over the thread. In CDCl3 and THF-d8, the aliphatic protons of the succinamide station, protons HK (the
assignments correspond to the lettering shown in Figure 7-2E) were shielded by nearly 1.4 ppm relative to the thread, showing that the macrocycle was located over the succinamide template (Figure 7-2). In DMSO-d6, the typical shielding of some protons in the aliphatic
region was expected due to the location of the macrocycle over the C10 alkyl chain. Surprisingly, the protons associated with the C10 aliphatic chain underwent negligible shielding. Instead, the fulleropyrrolidine (HD) and adjacent protons (HJ and HG) were shifted
upfield by as much as 0.8 ppm, which evidenced that the macrocycle was preferentially located in that region.
The electrochemical behavior of thread C60T and rotaxane C60R was investigated by cyclic voltammetry.[23] THF was used as solvent that does not interfere strongly with hydrogen bonding and DMSO as the hydrogen-bond disrupting medium. The voltammograms displayed five waves that correspond to the reduction of the fullero-pyrrolidine stopper. The CV behavior of rotaxane C60R differs from that of thread C60T: anodic shifts of the half-wave potential values (E½) for the first three cathodic processes
were observed. In THF, the E½ values of rotaxane C60R were slightly affected by the presence of the macrocycle, by comparison with those of thread C60T. This reveals weak interactions between the macrocycle and the stopper that stabilizes the electrochemically generated fullerene anions. When the electrochemical measurements were carried out in
DMSO, more distinct shifts were observed, being especially substantial in the second reduction wave (23 mV), which is in agreement with the proximity of the macrocycle to the fulleropyrrolidine stopper. The ∆E½ values suggest that π-π interactions might take place
between the negatively charged fullerene and the macrocycle. The 1H NMR and electrochemical characterization of rotaxane C60R does not support the existence of strong interactions in the neutral state between the macrocycle and the fullerene stopper, and thus, reverse switching is explained as a result of solvation and solvophobic interactions.
(E)
Figure 7-2 1H NMR spectra of (A): thread C60T in CDCl3, (B): rotaxane C60R in CDCl3, (C):
thread C60T in DMSO-d6, and (D): rotaxane C60R in DMSO-d6. The peaks highlighted with stars
correspond to residual solvent peaks. (E): Behavior of the solvent switchable rotaxane C60R in CDCl3 and DMSO-d6.
Solvation of the amide groups and the macrocycle by DMSO molecules results in de-complexation of the two components. In rotaxane C60R, the succinamide motif is connected to the fulleropyrrolidine by two methylene groups (J and G), which allows
(A) C60T CDCl3 (D) C60R DMSO-d6 (C) C60T DMSO-d6 (B) C60R CDCl3 δ / ppm
shuttling in both directions. Weak interactions between the macrocycle and the fulleropyrrolidine might be responsible for the predominance of the co-conformer in which the macrocycle is positioned over the fulleropyrrolidine. The presence of the macrocycle next to the fulleropyrrolidine apparently stabilizes the electrochemically generated anions.
1H NMR studies in DMSO-d
6 of similar rotaxanes but with a different stopper in the
place of the fulleropyrrolidine, show that the macrocycle moves away from the succinamide station and resides over the alkyl chain. This implies that the preferred location of the macrocycle in C60R is induced by the fulleropyrrolidine stopper, in other words, the translocation of the macrocycle to the fulleropyrrolidine stopper is driven by interactions between fulleropyrrolidine and macrocycle.
7.2.2 UV-Vis Absorption and Fluorescence
The preference of the macrocycle of C60R in DMSO to reside over the fulleropyrrolidine stopper implies the presence of interactions between the macrocycle and the stopper. However, such interactions could not be confirmed by the 1H NMR and electrochemical characterization. Examination of the fulleropyrrolidine photophysics, however, might be a way to reveal interactions between the macrocycle and fulleropyrrolidine stopper. For this purpose, the ground state UV-Vis absorption of the fulleropyrrolidine chromophore (neutral and radical anion) of C60R and C60T was investigated in different solvents. Also, the singlet and triplet excited states were examined by analyzing the fluorescence and triplet-triplet (T-T) absorption spectra, respectively.
Absorption spectra of C60R and C60T were recorded in three different solvents: dichloromethane (CH2Cl2), benzonitrile (PhCN) and dimethylsulfoxide (DMSO). The scaled spectra in CH2Cl2, PhCN and DMSO are depicted in Figure 7-3. The spectra were scaled to the absorption maximum between 430 and 440 nm. The absorption spectra of both C60T and C60R show the characteristics of functionalized [60]fullerenes: strong absorption in the UV region with molar absorption coefficients (
ε
) in the order of 105 L mol-1 cm-1. The visible region, on the other hand, exhibits relatively weak symmetry-forbidden transitions (ε
~ 102 L mol-1 cm-1). In the non-hydrogen bond disrupting solvents CH2Cl2 and PhCN, no spectral differences between thread and rotaxane were found. A very small solvatochromic red-shift for both C60T and C60R was observed in PhCN compared to CH2Cl2.The fluorescence spectra, recorded with excitation at 340 nm, also show the characteristic features of functionalized [60]fullerenes. Two weak bands are observed. The relatively weak fluorescence is a combined result of very efficient and rapid intersystem crossing to the excited triplet state and the symmetry-forbidden nature of the lowest S1 → S0 transition. The maxima are red-shifted in PhCN (718 and 797 nm) compared to CH2Cl2 (713 and 790 nm). The spectra of C60Tand C60Rare essentially identical in both solvents.
(A) (D)
(B) (E)
(C)
Figure 7-3 Scaled absorption (A, B and C) and fluorescence (D and E) spectra of C60T (····)
and C60R (—) in CH2Cl2, PhCN and DMSO. The fluorescence spectra were recorded with
The lack of interactions between the fulleropyrrolidine chromophore and the macrocycle is consistent with the results obtained with 1H NMR studies in CDCl
3 and THF-d8.
[23] The macrocycle was found to reside on the succinamide station as evidenced by the shielding of the aliphatic protons of this station.
In DMSO on the other hand, a different picture is observed. In the absorption spectrum of the rotaxane, a broad absorption band between 420 – 800 nm is observed. This band is not present in the spectrum of the thread, implying that the fulleropyrrolidine chromophore is influenced by the presence of the macrocycle. However, this result could not be reproduced. In duplicate experiments, performed with new sample solutions, the broad band also appeared in the spectrum of C60T. This means that the broad band is not specific for the rotaxane and does not arise due to interactions with the macrocycle. Instead, it must be caused by a different phenomenon.
The broad band in the spectra of C60Tand C60Rin DMSO is probably due to absorption of the fulleropyrrolidine radical anion which is formed in a photochemical reaction with DSMO. It is reported in literature that under oxygen-free conditions, DMSO acts as a sacrificial electron donor and can be oxidized by photo-excited [60]fullerene.[24] The formed DMSO radical cation is labile and decomposes, which under oxygen-free conditions results in an accumulation of [60]fullerene radical anions (C60●
–
, Figure 7-4D). The sample solutions of C60Tand C60Rwere prepared with dry solvent. For this purpose the solvent was distilled under vacuum from CaH2, directly into the sample cuvette. A side effect of this procedure is that the prepared solutions are degassed during distillation under vacuum and consequently, oxygen-free solutions are obtained.
The radical anion of C60Tand C60Rcould indeed be photogenerated in freshly prepared solutions in degassed and dry DMSO by irradiation at 435 nm during 20 minutes. After irradiation, the characteristic absorption band of the fulleropyrrolidine radical anion was observed in the spectra of both the rotaxane and the thread (Figure 7-4). The position of the maximum (992 nm) and the band shape are identical for C60T●
–
and C60R● –
, implying that the proximity of the macrocycle does not affect the electronic energy levels of the fulleropyrrolidine radical anion. Upon aeration of the solutions by simply exposing the solution to air, the spectra of the neutral molecules are restored. The broad band between 600 – 900 nm in the spectrum of the rotaxane probably originates from decomposition products of the radical anion.
(D)
Figure 7-4 Absorption spectra of C60T and C60R in degassed DMSO, (A): after irradiation at
435 nm and (B): after aeration of the irradiated solution and (C): difference spectra of C60T and C60R (radical anion vs. neutral). (D): Electron transfer from DMSO to the photo-excited fulleropyrrolidine chromophore, leading to the formation of C60●
–
and the labile radical cation of DMSO.
7.2.3 Transient Absorption
Triplet State
The T-T absorption spectra of C60T and C60R, recorded in PhCN and DMSO exhibit maxima at 370 and 700 nm (Figure 7-5A and B). In both solvents, the spectra are similar to those reported in literature.[25] In PhCN, no difference is observed between rotaxane and thread. This means, that the fulleropyrrolidine in the triplet excited state does not interact with the macrocycle. In DMSO, a slight band broadening is observed in the spectrum of C60R. However, great care must be taken with the interpretation of this band broadening, because of the photoreduction process in DMSO. The observed spectral differences could originate from the radical anion or decomposition products.
The lifetimes of triplet states (τT) of C60T and C60R in PhCN were determined (Figure 7-5C and D). A longer triplet lifetime was found for the rotaxane. This might be due to shielding of the fullerene chromophore from the solvent by the macrocycle, leading to a reduced non-radiative decay rate. This shielding may also be related with the slight band broadening of the T-T absorption of the rotaxane compared to the thread.
(A) (B)
(C) (D)
Figure 7-5 Scaled T-T absorption spectra of C60T and C60R in (A): DMSO and (B): PhCN recorded after laser-excitation at 435 nm (4 mJ pulse-1). Decay of the triplet state absorption of (C): C60T and (D): C60R in PhCN, recorded at 700 nm after photoexcitation at 355 nm.
After a typical transient absorption experiment significant decomposition of both C60T and C60Rwas observed, also evidenced by the colour change of the solutions from purple to grey. Because of the observed poor photostability of the compounds, the conclusions drawn from the transient absorption experiments must be treated with suspicion.
PhotoInduced Electron Transfer
The radical anion of C60Tcould also be photogenerated in PhCN. Upon laser-excitation (λexc = 435 nm) of a PhCN solution containing C60T and 5.3 mM phenothiazine (PTZ, see Figure 7-6) as electron donor, the characteristic absorption from PTZ●+
with maxima at 439 and 520 nm was observed (Figure 7-6).[26,27] The absorption between 700 – 800 nm is not from PTZ●+ and can be attributed to C60T●
–
photochemically generated radical anion. Unfortunately, the absorption band at 990 nm of the formed radical anion C60T●
–
could not be detected, because wavelengths above 800 nm are not accessible in our transient absorption setup.
Figure 7-6 Transient absorption spectrum of a PhCN solution containing C60T and PTZ,
recorded 60 ns after laser-excitation (λexc = 435 nm, 1.8 mJ pulse -1
).
7.2.4 Driving Force for Shuttling
The NMR studies clearly revealed that in DSMO the macrocycle of C60R resides close to the fulleropyrrolidine stopper. The proximity of the macrocycle to the fulleropyrrolidine stopper was confirmed by comparing the electrochemical behavior of the stopper in C60T and C60R. The effect on the first reduction of the fulleropyrrolidine was weak, but substantial for the second reduction. From the photophysical experiments, no clear indications of interactions between macrocycle and stopper were found in the form of changes of the maxima and shapes of the absorption and emission bands.
Combining these results, the conclusion can be drawn that in the neutral rotaxane, the interaction between the fulleropyrrolidine stopper and the macrocycle is very weak. Therefore, it is unlikely that these interactions between the macrocycle and the fulleropyrrolidine contribute substantially to the driving force needed for the observed reverse shuttling behaviour in C60R. So, in order to find an adequate explanation for this behaviour, external factors such as solvent polarity, must be considered.
The co-conformer of rotaxane C60R in DMSO, with the macrocycle close to the fulleropyrrolidine stopper (Figure 7-2E), show structural resemblance with host-guest complexes between C60 and host molecules. In the literature, many examples of host-guest complexes of fullerenes with resorcinarenes[28] and calixarenes[29,30] have been reported. Studies of the solvent dependence of association constants between C60 and the host
molecules suggest that the formation of these host-guest complexes is dominated by the desolvation energy rather than the host-guest interactions themselves. This conclusion is drawn from the fact that the association constants decrease with increasing solubility of C60.
[29,31] A similar effect might be expected for C
60R in DMSO. In the highly polar DMSO, which is not able to efficiently solvate the hydrophobic fulleropyrrolidine stopper, such solvophobic interactions are very likely to exist. This means that the system will strive to minimize its energy by reducing the unfavorable solvent-fulleropyrrolidine contacts. This is achieved by translocation of the macrocycle to the fulleropyrrolidine stopper. In this way the fulleropyrrolidine is partly shielded from the solvent by “solvation” by the macrocycle.
7.3
Conclusion
The macrocycle shuttling in a hydrogen-bonded rotaxane with tetraamide macrocycle and a fulleropyrrolidine stopper was studied in different solvents. 1H NMR and electrochemical studies revealed an unexpected phenomenon. In the strongly hydrogen-bond disrupting DMSO, the macrocycle was found to reside close to the fulleropyrrolidine stopper rather than over the alkyl chain. The latter is observed for similar rotaxanes, but without the fulleropyrrolidine stopper. When these rotaxanes are dissolved in DMSO, the macrocycle is generally found along the alkyl chain due to the hydrogen-bond disrupting ability of DMSO.
Unfortunately, the photophysics of the fulleropyrrolidine chromophore could not be used to sense the proximity of the macrocycle: no clear effects on the properties of the ground and excited singlet and triplet state of the fulleropyrrolidine stopper were observed. The examination of the excited triplet state was hindered by the photochemical formation and accumulation of C60 radical anions in DMSO under the oxygen-free conditions required to avoid quenching of the excited triplet state.
The 1H NMR, electrochemical and photophysical characterization of rotaxane C
60R does not support the existence of strong interactions in the neutral state between the macrocycle and the fulleropyrrolidine stopper, and thus, reverse switching is explained as a result of solvophobic interactions. The system minimizes its energy by reducing the unfavorable DMSO-fulleropyrrolidine contacts. This is achieved by translocation of the macrocycle to the fulleropyrrolidine stopper. In thus formed co-conformer, the hydrophobic fullero-pyrrolidine is partly shielded from the polar solvent.
7.4
Experimental Details
Sample Preparation
Sample solutions (ca. 2 × 10-5 M) were prepared by vacuum distillation of spectroscopy grade solvents from CaH2 directly into the sample cuvette. T-shaped cells, consisting of a bulb in which the solutions can be degassed and a 1 cm quartz cell for spectroscopic measurements, were used. The samples for the transient absorption and irradiation experiments were degassed by applying three freeze-pump-thaw cycles. All measurements were carried out at room temperature (293 K).
UV-Vis Absorption and Fluorescence
Ground state electronic absorption spectra were recorded in quartz cuvettes (optical path length 1 cm) on a double beam Varian Cary 3E Spectrophotometer or on a single beam HP 8453 diode array spectrophotometer. Steady-state fluorescence spectra were recorded on a Spex Fluorolog 3 system. This spectrometer was equipped with double monochromators in the excitation and emission channels. The excitation light source was a 450 W Xe lamp and the detector was a Peltier cooled R636-10 Hamamatsu photomultiplier tube. The emission was detected in a right angle geometry. Optical filters were used to block the second-order scattered excitation light.
Irradiation Experiment
Irradiations experiments were carried out with degassed solutions of C60T and C60R in DMSO. The light source for the irradiations was the excitation beam of the Spex Fluorolog 3 system described above. The sample solutions were irradiated at 435 nm during 20 minutes.
Transient Absorption
Nanosecond-to-microsecond transient absorption spectra were recorded on the same setup as described in experimental section 2.5. The sample was excited using a Nd:YAG laser (Coherent Infinity XPO/SHG, FWHM 2 ns) operated with a repetition rate of 5 Hz. The triplet lifetimes were determined from single wavelength transient absorption measurements. In the used setup, the probe light was provided by a continuous light source (Xe lamp) in a 90º angle geometry. The probe light was led into a monochromator and detected by a photomultiplier connected to a digital oscilloscope. The triplet lifetime was obtained after fitting of the T-T absorption decay at 700 nm to a mono-exponential function.
Acknowledgements
The work described in this chapter is the result of collaboration with Dr. A. Mateo-Alonso in the group of Prof. M. Prato (Dipartimento di Scienze Farmaceutiche, Università degli Studi di Trieste) who synthesized rotaxane C60Rand thread C60T and performed the
1 H NMR-studies, and Dr. G. Fioravanti, Dr. M. Marcaccio and Prof. F. Paolucci (Dipartimento di Chimica “G. Ciamician”, Universitá di Bologna) who performed the electrochemical characterizations. Their contributions are gratefully acknowledged.
7.5
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