Photoswitchable Norbornadiene–Quadricyclane Interconversion Mediated by Covalently Linked C60

A Journal of

Accepted Article

Title: Switchable Norbornadiene/Quadricyclane Interconversion

Mediated by Covalently Bound C60 Patrick Lorenz[a] and

Andreas Hirsch*[a]

Authors: Andreas Hirsch and Patrick Lorenz

This manuscript has been accepted after peer review and appears as an

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using the Digital Object Identifier (DOI) given below. The VoR will be

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the VoR from the journal website shown below when it is published

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content of this Accepted Article.

To be cited as: Chem. Eur. J. 10.1002/chem.201904679

Link to VoR: http://dx.doi.org/10.1002/chem.201904679

Supported by

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Photoswitchable Norbornadiene/Quadricyclane Interconversion –

Mediated by Covalently Linked C₆₀

Patrick Lorenz^[a]and Andreas Hirsch*^[a]

Abstract: We report on the synthesis and properties of various

norbornadiene/quadricyclane (NBD/QC) fullerene hybrids. By

cyclopropanation of C₆₀with malonates carrying the NBD scaffold a

small library of NBD – fullerene monoadducts and NBD – fullerene

hexakisadducts was established. The substitution pattern of the NBD

scaffold as well as the electron affinity of the fullerene core within

these hybrid systems has a pronounced impact on the properties of

the corresponding energy rich QC derivatives. Based on this, the first

direct photoisomerization of NBD – fullerene hybrids to their QC

derivatives was achieved. Furthermore, it was possible to use the

redox-active fullerene core of a QC – fullerene monoadduct to enable

the back-reaction to form the corresponding NBD – fullerene

monoadduct. Combining these two processes enables switching

between NBD and QC simply by changing the irradiation wavelength

between 310 nm and 400 nm. Therefore, turning this usually

photo/thermal switch into a pure photoswitch. This not only simplifies

the investigation of the underlying processes of the NBD – QC

interconversion within the system but also renders such hybrids

interesting for applications as molecular switches.

weight of only 92.14 g/mol enables high storage capacities.

Moreover, the back-reaction of QC to NBD is thermally forbidden

by the WOODWARD-HOFFMANN rules for cycloadditions and

therefore, QC is remarkably stable (half-life at 140 °C > 14 h).^[3]

This makes it possible to store the accumulated energy for long

time periods. Another, often underestimated advantage of the

NBD – QC system is that both molecules are liquid, which is highly

beneficial for practical applications. Despite these advantages,

the NBD – QC valence isomerization needs further optimizations

in order to approach practical applications. The main drawbacks

of NBD are the absorption onset in the UV region of the spectra

and the low quantum yield of the direct photoisomerization,

meaning that sunlight cannot drive the isomerization directly.^[4]

Hence, a triplet photosensitizer with E_T(sensitizer) > E_T(NBD)

such as acetophenone or benzophenone is needed to enable the

isomerization of neat NBD to QC.^[5]Alternatively, it has been

shown that by functionalization of the NBD scaffold it is possible

to red-shift the absorption and simultaneously increase the

quantum yield.^[6]However, functionalization of the NBD scaffold

naturally is always accompanied with an increased molecular

weight and therefore a lower energy storage density. Next to the

photoisomerization, also the corresponding back-reaction from

QC to NBD must be considered and optimized. For practical

applications it is desirable to release the stored energy

catalytically. This can be achieved by various ways and some

research has already been done in this regard.^[7]Possible

pathways involve for example, the catalysis via coordination of

square planar transition metal complexes,^[8,9]electrochemically

induced oxidation of QC^[10]or the irradiation of charge-transfer

complexes of QC and electron acceptors.^[11]It is obvious that in

order to develop an efficient MOST energy storage and release

system based on the NBD – QC interconversion full control over

both reactions must be gained. This however is only possible if

the underlying process of this valence isomerization are well

understood. Our approach is therefore based on the design and

synthesis of model systems. Analysis of the above-mentioned

catalysts for the back-reaction of QC to NBD showed that in most

cases an oxidized form of QC is involved in the reaction

mechanism. Based on this finding the covalent linkage of the

NBD/QC moiety to electron acceptors might result in hybrid

systems with interesting ground and excited state properties. For

this purpose, we chose C₆₀fullerene as a building block for these

hybrid systems, since C₆₀is thoroughly investigated as both,

ground and excited state electron acceptor.^[12]Furthermore, the

electron affinity of the fullerene core can be altered and tuned by

changing the number of addends attached to the carbon scaffold.

While monoadducts of C₆₀can easily be reduced in the ground

and in the excited state, the electron affinity of C₆₀hexakisadducts

is much less pronounced.^[13]During the early stage of our

Introduction

Towards the search of novel renewable energy technologies,

molecular solar thermal (MOST) energy storage and release

systems offer highly attractive opportunities. In contrast to

common photovoltaics, in MOST systems the energy is not only

harvested but also stored on a molecular level. Later the stored

energy can be released upon demand.^[1]Therefore, the often-

limiting mismatch of energy production and demand does not

affect these systems. One of the most promising concepts in this

regard is the valence isomerization of norbornadiene (NBD) and

quadricyclane (QC). The NBD – QC interconversion holds great

potential for several reasons. First, the amount of energy that is

stored within the metastable framework of QC is extraordinary

high (89 kJ/mol);^[2]which in combination with the low molecular

[a]

P. Lorenz and Prof. Dr. A. Hirsch

Chair of Organic Chemistry II

Friedrich-Alexander Universität Erlangen-Nürnberg

Nikolaus-Fiebiger-Str. 10, 91058 Erlangen, Germany

E-mail: andreas.hirsch@fau.de

Supporting information for this article is given via a link at the end of

the document.

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research the work of TUKTAROV and co-workers on the “Synthesis

and Properties of Energy-Rich Methanofullerenes Containing

Norbornadiene and Quadricyclane Moieties” was published;

therefore, NBD/QC – fullerene monoadducts no longer presented

a new class of hybrid compounds.^[14]Nevertheless, we decided to

continue our investigations since some of our observations did not

coincide with the reported ones. Herein we describe the synthesis

and properties of various NBD – fullerene monoadducts as well

as unprecedented NBD – fullerene hexakisadducts. By careful

analysis of this small library of hybrid molecules it was possible to

establish synthetic strategies towards both QC – fullerene

monoadducts as well as a first QC – fullerene hexakisadduct.

Even more remarkable is the development of a switchable

NBD/QC interconversion system, based on NBD – fullerene

monoadducts. Within this system a “push-pull” substituted NBD

scaffold is covalently linked to C₆₀to give a monoadduct, direct

irradiation of the “push-pull” system with 310 nm light results in

the formation of the corresponding QC derivative in quantitative

yield. Simply changing the irradiation wavelength to 400 nm and

thereby excitation of the fullerene core leads to clean and

quantitative back-reaction. Therefore, the interconversion of such

fullerene bearing NBC-QC-couples resembles the first switchable

system based on NBD and QC, which uses a covalently linked

excited state acceptor. This not only provides mechanistic

insights into the interaction of these hybrid systems, but also

renders such systems interesting for applications such as

molecular switches.

Figure 1. NBD malonates 1-5 divided into three groups: a) alkyl-

monosubstituted NBD scaffold with varying linker length; b) alkyl-

monosubstituted NBD bismalonate; c) malonate carrying

a “push-pull”

substituted NBD scaffold.

The NBD malonates of group a) and b) carry alkyl-

monosubstituted NBD scaffolds. This substitution pattern was

chosen to keep the NBD scaffold as similar to the unfunctionalized

parent NBD molecule as possible. It has been shown that the

substitution pattern of the NBD scaffold greatly affects the

photophysical properties. Especially so called “push-pull”

substituted NBD derivatives, which can be obtained by attaching

both an electron donating group (EDG) and an electron

withdrawing group (EWG) to the opposite sides of one of the

double bonds of NBD, show interesting and often superior

photophysical properties compared to neat NBD.^[17]In course of

another project, which we recently published,^[9]we investigated

such a “push-pull” substituted NBD derivative NBD 6 (Scheme 1).

This derivative can be readily photoisomerized to its

corresponding QC isomer upon irradiation with light of a

wavelength of 310 nm. Furthermore, the metastable QC isomer

has a remarkably long half-life of about 450 days at room

temperature in tetrachloroethane. These properties render this

interconversion couple the perfect candidate for the generation of

“push-pull” substituted NBD-fullerene hybrids. The synthesis of a

malonate carrying the NBD 6 scaffold, without significantly

altering the photochemical properties of the latter, was carried out

following the reaction sequence depicted in Scheme 1. It involves

saponification of the ester moiety of NBD 6 followed by a

STEGLICH esterification with ethylene glycol to give NBD-alcohol

8. This alcohol was afterwards treated with methyl malonyl

chloride to give the desired NBD malonate 5.

Results and Discussion

Functionalization of C₆₀can be achieved by various methods,^[15]

since the BINGEL-HIRSCH reaction is very reliable and uses mild

reaction conditions^[16]we believed it to be ideal for the coupling of

the NBD moiety. Therefore, suitable malonates carrying the NBD

scaffold were needed. In total, we designed and synthesized five

different NBD malonates, which can be divided into three groups

(Figure 1). To study the effect of the linker length we synthesized

malonates 1, 2 and 3 bearing a methano-, propano- and hexano

bridge, respectively. These three malonates are easily accessible

from their corresponding NBD-alcohols and commercially

available methyl malonyl chloride (general procedure A). Apart

from the length of the linker also the number of NBD units might

have an influence, therefore, by reacting two equivalents of the

corresponding NBD alcohol with malonyl dichloride it was also

possible to obtain NBD-bismalonate 4 carrying two NBD moieties.

For detailed information see supporting information.

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Scheme 1. Synthesis of NBD malonate 5 starting from NBD 6; a): 9.0 eq. NaOH, THF/H₂O 1:1 v:v, reflux, 30 h; b): 2.0 eq. ethylene glycol, 1.0 eq. DCC, 0.1 eq.

DMAP, CH₂Cl₂, room temperature, 20 h; c): 1.1 eq. triethylamine, 1.0 eq. methyl malonyl chloride, CH₂Cl₂0 °C to room temperature, overnight.

BINGEL-HIRSCH reactions towards the desired NBD – fullerene

hybrids were conducted with malonates 1-5 (Scheme 2). The

reactions were carried out according to general procedure B, after

purification by column chromatography and precipitation with n-

pentane the desired NBD – fullerene monoadducts 9-13 were

obtained in reasonable yields varying between 37% and 53%.

Compounds 9-13 were characterized by HRMS, UV/Vis

absorption spectroscopy as well as ¹H and ¹³C NMR spectroscopy,

for detailed information see supporting information. As a

1

representative example, the H and ¹³C NMR spectra of NBD –

fullerene monoadduct 12 are depicted in Figure 2 and will be

briefly discussed.

Figure 2. NMR spectra of NBD – fullerene monoadduct 12. Top: ¹H NMR

spectra with detailed assignment of the protons of the NBD scaffold (400 MHz,

CDCl₃, rt). Bottom: ¹³C NMR spectrum of 12 (101 MHz, CDCl₃, rt).

All expected signals can be observed in the ¹H NMR spectrum of

12. The detailed assignment of the protons corresponding to the

NBD moiety is depicted in Figure 2. This assignment is based on

a comparison with the spectra of NBD 6 which we recently

published.^[9]In the ¹³C NMR spectrum eight signals can be

observed in the aliphatic region between 45-75 ppm, seven

thereof originate from the malonate and the remaining signal

corresponds to the sp³-hybridized carbon atom of C₆₀. Three of

the carbon atoms of the phenyl ring give rise to the three signals

located between 127.0 and 130.0 ppm. The remaining signal of

the phenyl ring as well as the sp²-hybridized carbon atoms of C₆₀

and three of the olefinic carbons of the NBD scaffold can be

detected between 135.0-146.0 ppm. The fourth olefinic carbon

Scheme 2. NBD – fullerene monoadducts synthesized according to general

atom is strongly shifted downfield due to the conjugation of the

procedure B a): 1.0 eq. C₆₀, 1.5 eq. malonate, 1.5 eq. CBr₄, 2.5 eq. DBU, toluene,

carbonyl group and appears together with the signals of the three

room temperature, overnight.

carbonyl groups in the region between 163.0-169.0 ppm. As

evidenced, different NBDs can easily be linked to the fullerene

core via their malonates in a standard BINGEL-HIRSCH reaction,

furthermore, the purification is straightforward without the need for

HPLC. Thus, NBD – fullerene monoadducts bearing a redox-

active fullerene are readily available.

As already stated above, the big advantage of choosing C₆₀as an

electron acceptor for our hybrid systems is that by going from a

monoadduct to a hexakisadduct it is possible to significantly

decrease the electron affinity of the fullerene core.^[13]Therefore,

NBD – fullerene hexakisadducts would serve as the perfect

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reference systems, where the electron transfer is mostly

prevented, for their corresponding monoadducts. To keep the

systems as similar as possible we decided to synthesize [5:1]

hexakisadducts, which are readily accessible via [5:0]

pentakisadducts. An efficient strategy towards these precursor

molecules was developed earlier in our group.^[18]A BINGEL-

HIRSCH reaction using the malonates 2, 4 and 5 and a [5:0] diethyl

malonate pentakisadduct gave the desired hexakisadducts in

excellent yields, (Scheme 3). To further expand the synthetic

scope of NBD – fullerene hybrids we tested how many NBD

moieties we could attach to the fullerene core. Therefore, we

performed a modified BINGEL-HIRSCH reaction using pure C₆₀and

an excess of NBD malonate 4, CBr₄and DBU to obtain a [6:0]

NBD – fullerene hexakisadduct 17 carrying now a total of twelve

NBD moieties.

Figure 3. Top: ¹H NMR spectrum of NBD – fullerene hexakisadduct 15

(400 MHz, CDCl₃, rt) with a detailed assignment of the protons of the NBD

scaffold. Bottom: ¹³C NMR spectrum of compound 15 (101 MHz, CDCl₃, rt).

1

The H NMR spectrum of 15 is dominated by the two multiplets

between 1.29-1.34 and 4.28-4.36 ppm which can be assigned to

the CH₃and the CH₂groups of the five diethyl malonates,

respectively. The integral ratio of these two multiplets and the

remaining signals of the NBD malonate 5 confirm the formation of

a [5:1] hexakisadduct. The signals corresponding to the NBD

moiety are assigned in the spectrum. In the ¹³C NMR all the

expected signals can be observed. The CH₃and the CH₂groups

of the diethyl malonates give rise to two intense signals at 14.2

and 63.0 ppm, respectively. In the aliphatic region, seven signals

can be assigned to the malonate carrying the NBD scaffold. The

sp³-hybridized carbon atoms of the fullerene appear at around

69.0 ppm. Three of the carbon atoms of the phenyl ring give rise

to signals between 127.5 to 129.0 ppm. The signal of the

remaining carbon atom of the phenyl ring and three of the olefinic

protons of the NBD scaffold are located in the region 135.0-

144.0 ppm. The sp²-hybridized carbon atoms of the fullerene core

appear in two sets of signals at 140.5-141.5 and 145.7-146.2 ppm

as is typical for such [5:1] hexakisadducts (with local T_hsymmetry).

Signals corresponding to the carbonyl carbon atoms and of the

remaining olefinic carbon atom are detected between 163.4 and

168.5 ppm.

With this small library of NBD – fullerene hybrids at hand the next

step was the synthesis of their corresponding QC derivatives. In

this regard, two different pathways are conceivable. First a

BINGEL-HIRSCH reaction with the corresponding QC malonates

and second the photoisomerization of the NBD – fullerene hybrid

systems. Regarding the first pathway suitable malonates

containing the QC scaffold were required. The simplest way

towards the desired QC malonates is to photoisomerize the

corresponding NBD malonates. Direct photoisomerization of the

mono-alkyl functionalized NBD derivatives is not possible due to

unfavorable absorption properties and low quantum yields.

Therefore, an additional triplet photosensitizer is needed.

According to the literature, acetophenone is ideal for this

purpose.^[5]Irradiation of a mixture of malonate 2 and 5 mol%

acetophenone in diethyl ether resulted in the formation of the

Scheme 3. Top: Synthesis of the three [5:1] hexakisadducts 14, 15 and 16.

Conditions a): 1.0 eq. [5:0] pentakisadduct, 1.5 eq. malonate, 2.0 eq. CBr₄,

3.0 eq. DBU in toluene. Bottom: Synthesis of the [6:0] hexakisadduct 17 carrying

twelve NBD moieties. Conditions b): 1.0 eq. C₆₀, 10 eq. 4, 100 eq. CBr₄, 10 eq.

DBU in toluene.

Structural characterization was done by mass spectrometry and

¹H NMR as well as ¹³C NMR spectroscopy (Figure 3). As an

example, the spectra of 15 will be briefly discussed. In the MALDI-

TOF mass spectrum the molecular ion peak can be detected at

the expected value of m/z = 1864.

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desired QC malonate. To our surprise the successive BINGEL-

HIRSCH reaction with this malonate did not give the expected

product. Instead NBD – fullerene hybrid 10 was obtained in small

amounts as well as an inseparable mixture of side products

(Scheme 4). ¹H NMR analysis suggests that side reactions

occurred at the QC scaffold.

competing absorption of the fullerene core must be considered as

well, showing once more how efficient the photoisomerization of

this NBD – QC couple is.

Scheme 4. Photoisomerization of NBD malonate 2 to QC malonate 18 (5 mol%

acetophenone, diethyl ether, mercury vapor lamp); followed by the attempted

synthesis of QC – fullerene monoadduct 19 a): 1.0 eq. C₆₀, 1.0 eq. 18, 1.2 eq.

CBr₄, 2.5 eq. DBU, toluene, room temperature, 4 h.

For malonates 1, 3 and 4 it was not possible to obtain the pure

corresponding QC malonates. This is most probably due to the

low stability of these alkyl-monofunctionalized QC derivatives.

Therefore, we pursued the second pathway via the

photoisomerization of the already described NBD – fullerene

hybrids. Again, acetophenone was used as

a

triplet

photosensitizer. TUKTAROV et al. reported on the formation of a

poorly soluble precipitate during the irradiation of toluene

solutions of NBD – fullerene hybrids and acetophenone.^[14]

Although, we were able to circumvent this problem by working

under the strict exclusion of oxygen, we were not able to obtain

the corresponding QC derivatives of compound 9, 10, 11, 13, 14

or 16. Most likely the triplet sensitizer is more efficiently quenched

by the fullerene core than by the NBD scaffold and therefore, no

isomerization can occur. Since our attempts towards the QC –

fullerene hybrids carrying alkyl-monofunctionalized NBD scaffolds

failed, we investigated the two “push-pull” substituted NBD

fullerene hybrids 12 and 15. As described previously^[9]the “push-

pull” NBD system which is implemented in the hybrid molecules

12 and 15 undergoes clean and quantitative photoisomerization

to its QC species upon direct irradiation with light of 310 nm. To

test, whether this also works when the system is linked to the

fullerene core, a quartz glass cuvette was charged with 10 mg of

compound 12 dissolved in 2.5 mL of properly degassed CDCl₃.

The solution was irradiated using a UV LED emitting light of a

wavelength of 310 nm, the reaction was followed by ¹H NMR

analysis (Figure 4).

The ¹H NMR spectra of 20, depicted in Figure 4, unambiguously

show that the NBD scaffold undergoes clean and complete

isomerization to its metastable QC form 20 upon irradiation. This

is most obvious, when the vanishing signals of the olefinic protons

of NBD located around 7.00 ppm and the new arising signals

between 1.50-2.70 ppm which correspond to the QC scaffold, are

taken into account. By comparing the integral ratio of the NBD and

the QC signals it is possible to estimate the ratio of NBD/QC

during the isomerization. Already after one hour, the reaction

mixture comprises 60% QC – fullerene hybrid 20. After two hours

80%, three hours 88%, five hours 95% and after ten hours the

conversion from 12 to 20 is completed. The fact that complete

isomerization to 20 can be observed is very remarkable since the

Figure 4. Top: ¹H NMR spectra of the isomerization of NBD – fullerene hybrid

12 to its QC derivative 20. Bottom: ¹H and ¹³C NMR spectra of 20 measured in

CDCl₃at room temperature.

Additionally, to the ¹H NMR spectra also a ¹³C NMR spectrum of

20 was measured to corroborate the structure of 20. As expected,

twelve signals can be detected in the aliphatic region between

20.0 and 75.0 ppm. The carbon atoms of the QC scaffold give rise

to seven signals between 20.0 and 40.0 ppm. The remaining four

signals corresponding to the malonate are located between 50.0

and 65.0 ppm. The sp³-hybridized carbon atoms of C₆₀are

observed within the typical range at 71.5 ppm. As already

observed for compound 12, also in the ¹³C NMR of 20 three

signals in the range of 125.0-130.0 ppm can be assigned to the

carbon atoms of the phenyl ring. The fourth carbon atom of the

phenyl ring and the signals of the sp²-hybridized carbon atoms of

the fullerene core are located between 136.0 and 146.0 ppm. As

expected, three signals are observed low-field corresponding to

the three carbonyl carbon atoms of the malonate. Based on the

spectral data, it can be concluded that the photoisomerization of

12 to 20 is the first successful transformation of an NBD –

fullerene hybrid into its corresponding metastable QC derivative.

This direct and quantitative photoisomerization of NBD – fullerene

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hybrids significantly simplifies the synthesis of the highly

interesting QC – fullerene hybrids since tedious purification by

HPLC is not necessary.

Simultaneously, the six characteristic signals corresponding to

the QC scaffold of 21 arise in the region between 1.50 and

2.70 ppm. The integral ratio of NBD/QC during irradiation

estimates to: one hour 45/55%, two hours 13/87%, three hours

10/90%, five hours 7/93% and after ten hours full conversion to

QC – fullerene hybrid 21 was observed. Further evidence for the

formation of 21 comes from the ¹³C NMR spectrum. When

comparing the ¹³C NMR spectra of 15 and 21 this becomes most

obvious, all seven signals of the NBD scaffold can no longer be

observed in the spectra of 21, instead seven new signals located

in the aliphatic region between 21.0 and 38.0 ppm prove the

successful isomerization to QC. The remaining signals of the

malonates and C₆₀are only slightly affected by the isomerization

and can be detected in the typical regions. Even though the NMR

spectroscopic data of 20 and 21 provides solid structural evidence,

final proof for the successful photoisomerization was achieved by

catalytically induced back-conversion of the QC – fullerene

hybrids 20 and 21 to 12 and 15, respectively. For this purpose,

we used the magnetic catalyst [Fe₃O₄-CoSalphen] which was

recently developed in our group.^[9]The great advantage of this

catalyst is that it can easily be removed from the reaction mixture

by the action of an external magnet, rendering tedious purification

unnecessary. Addition of the catalyst [Fe₃O₄-CoSalphen] to

solutions of 20 and 21 in CDCl₃resulted in quantitative back-

conversion into 12 and 15, respectively (Scheme 5); as apparent

from ¹H NMR analysis. For more information see supporting

information.

Encouraged by this result, the photoisomerization of the NBD –

fullerene hexakisadduct 15 to the first QC – fullerene

hexakisadduct 21 was attempted. According to the established

method, a quartz glass cuvette was charged with 10 mg of

compound 15 dissolved in 2.5 mL of properly degassed CDCl₃.

The solution was irradiated using a UV LED emitting light of a

wavelength of 310 nm, the reaction was followed by ¹H NMR

analysis (Figure 5).

Scheme 5. Catalytic back-reaction of the QC – fullerene hybrids 20 and 21 to

the corresponding NBD – fullerene hybrids 12 and 15, respectively. The

magnetic nanoparticle catalyst [Fe₃O₄-CoSalphen], enabled quantitative

conversion in both cases, followed by the removal of the catalyst by an external

magnet.

As stated above, photoisomerization is only one of the two

possible pathways to obtain QC – fullerene hybrids. Alternatively,

it is possible to run a BINGEL-HIRSCH reaction with the

corresponding QC malonates. Photoisomerization of NBD

malonate 5 with the 310 nm UV-LED proceeded smoothly in

CDCl₃and gave the corresponding QC malonate 24 in

quantitative yield without further purification (Scheme 6). The

synthesis of QC – fullerene monoadducts from QC malonates and

C₆₀has been reported previously.^[14]Therefore, we were confident,

that the BINGEL-HIRSCH reaction with QC malonate 22 and C₆₀

would give the desired QC – fullerene hybrid 20.

Figure 5. Top: ¹H NMR spectra of the isomerization of NBD – fullerene hybrid

15 to its QC derivative 21. Bottom: ¹H and ¹³C NMR spectra of 21 measured in

CDCl₃at room temperature.

Also, in this case, a clean and complete photoisomerization of the

NBD – fullerene hexakisadduct 15 to its QC derivative 21 can be

observed as is evident from the ¹H NMR spectra depicted in

Figure 5. Apart from the two large multiplets around 1.30 and

4.30 ppm which correspond to the diethyl malonates, the same

changes as for compound 12 can be observed during irradiation.

The signal assigned to the olefinic protons of the NBD scaffold

decreases, until after ten hours it can no longer be observed.

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Scheme 8. Synthesis of QC – fullerene hexakisadduct 21 from QC malonate 22

and the [5:0] diethyl malonate pentakisadduct; a) 1.0 eq. [5:0] diethyl malonate

pentakisadduct, 1.5 eq. 22, 2.0 eq. CBr₄, 3.0 eq. DBU, toluene, room

temperature, 2 d.

Scheme 6. Photoisomerization of NBD malonate 5 to QC malonate 22,

quantitative isomerization was achieved by direct irradiation of a CDCl₃solution

of 5 with a 310 nm UV-LED. The successive BINGEL-HIRSCH reaction with QC

malonate 22 and C₆₀gave next to the desired product 20 also a small amount

of NBD – fullerene monoadduct 12 and inseparable side products; a): 1.0 eq.

C₆₀, 1.5 eq. 22, 1.5 eq. CBr₄, 2.5 eq. DBU, toluene, room temperature, 3 h.

As a prerequisite, the back-reaction from QC to NBD within the

system has to be quantitative and without any side reactions. To

prevent the competing irradiation and consequently the

photoisomerization of the incorporated NBD “push-pull” system

(λ_onset= 350 nm) it is crucial to use a wavelength well above

350 nm to excite the fullerene core. Furthermore, to avoid side

reactions caused by oxygen the used solvent was degassed

thoroughly by the freeze-pump-thaw method. A cuvette was

charged with 3 mg of QC – fullerene monoadduct 20 dissolved in

2.5 mL of CDCl₃and irradiated with 400 nm light. The reaction

However, in direct contrast to the already published work, we

observed next to the desired product, also partial formation of

NBD – fullerene monoadduct 12 and inseparable side products

(Scheme 6). ¹H NMR analysis (Figure S13) suggests that the side

reactions predominantly occur at the QC scaffold. This, in

combination with the fact that QC – fullerene hybrid 20 is known

to be stable under inert conditions in the dark, makes it possible

to explain the formation of side products. Although, the BINGEL-

HIRSCH reaction is carried out under the exclusion of light and

under inert atmosphere, it is inevitable that oxygen and diffuse

light are present during the work-up and purification. Therefore,

we propose that the redox active C₆₀when irradiated by diffuse

light is able to oxidize QC. The thereby generated QC radical

cation then undergoes side reactions (Scheme 7).

1

was followed by H NMR (Figure S5 to Figure S8), as apparent

from the NMR, excitation of the fullerene core at 400 nm results

in clean and quantitative back conversion of QC – fullerene

monoadduct 20 to NBD – fullerene monoadduct 12. Thus, it is

possible to switch between the NBD and the QC state in this

hybrid system simply by variation of the excitation wavelength

(Scheme 9).

Scheme 7. Proposed explanation for the formation of side products. Excitation

of the fullerene core by ambient light is followed by the oxidation of QC, the

generated QC^•+is highly reactive and can undergo side reactions.

To further test this hypothesis, we also ran a BINGEL-HIRSCH

reaction with malonate 22 and the [5:0] pentakisadduct carrying

five diethyl malonates in order to obtain the QC – fullerene

hexakisadduct 21. Since the electron affinity of the formed

hexakisadduct is much lower compared to the monoadduct 20 we

expected less or no side reactions. And indeed, we were able to

obtain QC – fullerene hexakisadduct 21 after purification by

column chromatography (Scheme 8). The fact that C₆₀might

function as an excited state electron acceptor in these QC –

fullerene hybrid systems is highly interesting, since this, in

combination with the photoisomerization of the corresponding

NBD – fullerene hybrids, would enable the design of switchable

systems.

Scheme 9. The switchable NBD/QC – fullerene hybrid system. Irradiation at a

wavelength of 310 nm results in the excitation of the “push-pull” system and

consequently in the valence isomerization to form 20. The reverse reaction is

observed, if the fullerene core is excited at a wavelength of 400 nm. According

to ¹H NMR analysis both reactions are quantitative and no side products are

formed.

The interconversion of NBD – fullerene monoadduct 12 and QC –

fullerene monoadduct 20 resembles, to the best of our knowledge,

the first switchable NBD/QC system which uses a covalently

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linked excited state electron acceptor to initiate the back-

conversion of QC to NBD. The next subject of our experiment was

to repeat the reaction with the QC – fullerene hexakisadduct 21.

Due to the weaker electron affinity of the fullerene core no or a

much less efficient back-conversion of QC to NBD was expected.

Under the same experimental parameters, the ratio of QC/NBD

after five hours of irradiation was only about 85/15 %,

corroborating our previously described assumptions.

Yield 1.06 g 4.77 mmol, 74%.

¹H NMR (400 MHz, CDCl₃)  = 6.80-6.74 (m, 2H), 6.57-6.56 (m,

1H), 4.83-4.76 (m, 2H), 3.75 (s, 3H), 3.58-3.58 (m, 1H), 3.44-3.44

(m, 1H), 3.40 (s, 2H), 2.06-1.99 (m, 2H) ppm.

¹³C NMR (101 MHz, CDCl₃)  = 167.1, 166.5, 152.2, 143.5,

142.7, 140.6, 74.0, 64.2, 52.6, 51.6, 50.5, 41.5 ppm.

HRMS (ESI, CH₂Cl₂/acetonitrile/MeOH) [M+Na]⁺

m/z = 245.0784 (calcd.), 245.0783 (found).

NBD malonate 2

Conclusions

Compound 2 was prepared according to the general procedure A,

after purification by column chromatography (hexanes/ethyl

acetate 60/40 v:v, SiO₂) the product was obtained as a colorless

oil.

A small library of NBD – fullerene hybrid molecules, including

monoadducts as well as unprecedented hexakisadducts was

synthesized and investigated with regard to their corresponding

QC derivatives. The tailor-made NBD scaffold incorporated into

the two hybrid systems 12 and 15 made it possible to, for the first

time ever, directly photoisomerize these two NBD – fullerene

hybrids quantitatively to the QC – fullerene hybrids 20 and 21,

Yield 354 mg, 1.41 mmol, 85%.

¹H NMR (300 MHz, CDCl₃)  = 6.78-6.73 (m, 2H), 6.17-6.16 (m,

1H), 4.13-4.08 (m, 2H), 3.75 (s, 3H), 3.51-3.50 (m, 1H), 3.38 (s,

2H), 3.27-3.27 (m, 1H), 2.35-2.17 (m, 2H), 2.00-1.93 (m, 2H),

1.82-1.73 (m, 2H) ppm.

respectively.

A

comparison

of

these

energy

rich

methanofullerenes revealed a strong interaction of the QC moiety

and the fullerene core within the monoadduct 20; whereas the

interaction in the hexakisadduct 21 is much weaker. Based on this

observation, we were able to show that the fullerene core of

monoadduct 20 can be efficiently used to facilitate the back-

reaction from 20 to 12. This enabled the development of the

“switchable” interconversion couple 12/20 (Scheme 9) in this

system it is possible to switch between the NBD and QC state

simply by changing the excitation wavelength from 310 to 400 nm.

This not only simplifies the investigation of the QC – NBD back-

reaction, but also renders such hybrid systems interesting with

regard to molecular switches. The covalent linkage of the

NBD/QC moiety to fullerenes has been shown to produce model

systems with fascinating properties. Further investigations

regarding hybrid systems featuring NBD/QC and other electron

acceptors such as rylene-dyes are currently underway in our

laboratories.

¹³C NMR (101 MHz, CDCl₃)  = 167.1, 166.6, 157.3, 144.0,

142.3, 134.5, 73.6, 65.2, 53.6, 52.5, 50.2, 41.5, 27.7, 26.2 ppm.

HRMS (ESI, CH₂Cl₂) [M+Na]⁺m/z = 273.1097 (calcd.),

273.1094 (found).

NBD malonate 3

Compound 3 was prepared according to the general procedure A,

after purification by column chromatography (hexanes/ethyl

acetate 90/10 v:v, SiO₂) the product was obtained as a colorless

oil.

Yield 2.08 g 7.11 mmol, 86%.

¹H NMR (400 MHz, CDCl₃)  = 6.76-6.73 (m, 2H), 6.11-6.10 (m,

1H), 4.15-4.11 (m, 2H), 3.75 (s, 3H), 3.48-3.48 (m, 1H), 3.38 (s,

2H), 3.26-3.26 (m, 1H), 2.22-2.13 (m, 2H), 1.97-1.92 (m, 2H),

1.67-1.58 (m, 2H), 1.45-1.25 (m, 6H) ppm.

¹³C NMR (101 MHz, CDCl₃)  = 167.2, 166.7, 158.9, 144.0,

142.5, 133.5, 73.6, 65.8, 53.6, 52.6, 50.2, 41.6, 31.5, 29.0, 28.5,

27.2, 25.7 ppm.

HRMS (APPI, acetonitrile/toluene) [M+H]⁺m/z = 293.1747

(calcd.), 293.1746 (found).

Experimental Section

General procedure A (NBD malonates)

Under inert atmosphere the corresponding NBD alcohol (1.0 eq.)

was dissolved in CH₂Cl₂and the solution was cooled to 0 °C.

Successively, triethylamine (1.1 eq.) and methyl malonyl chloride

(1.0 eq.) were added. The reaction mixture was stirred overnight

(0 °C to rt). Afterwards the crude product was rotated onto silica

and purified by column chromatography.

NBD malonate 4

Under inert atmosphere, NBD alcohol 21 (2.0 eq., 3.33 mmol,

500 mg) was dissolved in 50 mL CH₂Cl₂and cooled to 0 °C. Then,

TEA (2.2 eq., 2.66 mmol, 508 µL) was added. Afterwards,

malonyl dichloride (1.0 eq., 1.66 mmol, 162 µL) dissolved in

4.0 mL CH₂Cl₂was added dropwise via a syringe over 1 h. Once

the addition was complete, the reaction mixture was first stirred at

0 °C for one hour and then three hours at room temperature. After

this time, water (100 mL) was added and the phases were

NBD malonate 1

Compound 1 was prepared according to the general procedure A,

after purification by column chromatography (hexanes/ethyl

acetate 85/15 v:v, SiO₂) the product was obtained as a colorless

oil.

separated,

the

organic

phase

was

washed

with

water (4 × 100 mL), dried over MgSO₄and evaporated. The crude

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product was purified by column chromatography (hexanes/ethyl

acetate, 90:10 v:v, SiO₂), to yield the product as a colorless oil.

HRMS (MALDI-TOF, dctb) [M]⁺m/z = 940.0730 (calcd.),

940.0706 (found).

Yield 246 mg, 0.667 mmol, 40%.

NBD – fullerene monoadduct 10

¹H NMR (300 MHz, CDCl₃)  = 6.77-6.73 (m, 4H), 6.16-6.16 (m,

2H), 4.12-4.08 (m, 4H), 3.50-3.50 (m, 2H), 3.36 (s, 2H), 3.27-

3.27 (m, 2H), 2.33-2.19 (m, 4H), 1.99-1.93 (m, 4H), 1.82-1.72 (m,

4H) ppm.

¹³C NMR (75 MHz, CDCl₃)  = 166.7, 157.4, 144.1, 142.4, 134.5,

73.7, 65.3, 53.7, 50.3, 41.8, 27.8, 26.3 ppm.

HRMS (APPI, CH₂Cl₂) [M+H]⁺m/z = 369.2060 (calcd.),

369.2059 (found).

Compound 10 was prepared according to general procedure B,

after purification by column chromatography (toluene, SiO₂) and

precipitation from CS₂with n-pentane the product was obtained

as a brown solid.

Yield 99 mg, 0.102 mmol, 37%.

¹H NMR (400 MHz, CDCl₃)  = 6.79-6.76 (m, 2H), 6.23-6.22 (m,

1H), 4.48-4.44 (m, 2H), 4.09 (s, 3H), 3.53-3.53 (m, 1H), 3.31-3.31

(m, 1H), 2.46-2.32 (m, 2H), 2.03-1.93 (m, 4H) ppm.

¹³C NMR (101 MHz, CDCl₃)  = 164.1, 163.5, 156.9, 145.3,

145.2, 145.2, 145.2, 145.1, 145.1, 145.1, 144.9, 144.7, 144.7,

144.6, 144.6, 144.6, 144.6, 144.0, 143.9, 143.9, 143.1, 143.1,

143.0, 143.0, 142.9, 142.2, 142.1, 141.9, 141.9, 140.9, 140.9,

139.1, 138.9, 134.7, 73.6, 71.5, 66.9, 54.0, 53.5, 52.1, 50.2, 27.7,

26.2 ppm.

NBD malonate 5

Compound 5 was prepared according to the general procedure A,

after purification by column chromatography (hexanes/ethyl

acetate 90/10 to 70/30 v:v, SiO₂) the product was obtained as a

colorless oil.

HRMS (MALDI-TOF, dctb) [M]⁺m/z = 968.1043 (calcd.),

968.1043 (found).

Yield 122 mg 0.475 mmol, 76%.

¹H NMR (400 MHz, CDCl₃)  = 7.52-7.49 (m, 2H), 7.38-7.29 (m,

3H), 7.00-6.91 (m, 2H), 4.32-4.27 (m, 4H), 4.07-4.06 (m, 1H),

3.86-3.85 (m, 1H), 3.73 (s, 3H), 3.36 (s, 2H), 2.27-2.07 (m, 2H)

ppm.

NBD – fullerene monoadduct 11

Compound 11 was prepared according to general procedure B,

after purification by column chromatography (toluene, SiO₂) and

precipitation from CS₂with n-pentane the product was obtained

as a brown solid.

¹³C NMR (101 MHz, CDCl₃)  = 168.0, 166.8, 166.4, 165.0,

143.9, 140.9, 138.7, 135.7, 128.8, 127.9, 127.8, 70.7, 63.3, 61.6,

58.9, 53.1, 52.7, 41.2 ppm.

HRMS (APPI, acetonitrile) [M]⁺m/z = 356.1254 (calcd.),

356.1255 (found).

Yield 149 mg, 0.148 mmol, 53%.

¹H NMR (400 MHz, CDCl₃)  = 6.74-6.71 (m, 2H), 6.11-6.10 (m,

1H), 4.49-4.46 (m, 2H), 4.08 (s, 3H), 3.47-3.47 (m, 1H), 3.25-3.25

(m, 1H), 2.25-2.14 (m, 2H), 1.99-1.94 (m, 2H), 1.87-1.80 (m, 2H),

1.52-1.34 (m, 6H) ppm.

General procedure B (NBD – fullerene monoadducts)

Under inert atmosphere and the exclusion of light, C₆₀(1.0 eq.)

was dissolved in toluene (1 mL per mg C₆₀) under sonication. The

solution was degassed with nitrogen for 10 min. Afterwards 1.5

eq. of the corresponding malonate, CBr₄(1.5 eq.) and DBU

(2.5 eq.) were added. The reaction mixture was stirred overnight,

concentrated and purified by column chromatography. The

products were obtained after precipitation from CS₂with n-

pentane.

¹³C NMR (101 MHz, CDCl₃)  = 163.7, 163.2, 158.3, 145.3,

145.3, 145.2, 145.2, 145.2, 145.2, 145.1, 145.1, 144.9, 144.7,

144.7, 144.6, 143.9, 143.9, 143.8, 143.1, 143.0, 143.0, 143.0,

143.0, 142.2, 142.2, 142.2, 141.9, 141.9, 141.0, 139.1, 139.0,

133.6, 73.6, 71.5, 67.3, 53.7, 53.7, 52.1, 50.3, 31.6, 29.1, 28.7,

27.3, 26.0 ppm.

HRMS (MALDI-TOF, dctb) [M]⁺m/z = 1010.1513 (calcd.),

1010.1529 (found).

NBD – fullerene monoadduct 9

NBD – fullerene monoadduct 12

Compound 9 was prepared according to general procedure B,

after purification by column chromatography (toluene, SiO₂) and

precipitation from CS₂with n-pentane the product was obtained

as a brown solid.

Compound 12 was prepared according to general procedure B,

after purification by column chromatography (toluene/ethyl

acetate 95/5 v:v, SiO₂) and precipitation from CS₂with n-pentane

the product was obtained as a dark brown solid.

Yield 102 mg, 0.108 mmol, 39%.

¹H NMR (400 MHz, CDCl₃)  = 6.86-6.75 (m, 3H), 5.19- 5.08 (m,

2H), 4.09 (s, 3H), 3.65-3.63 (m, 1H), 3.58-3.56 (m, 1H), 2.16-2.03

(m, 2H) ppm.

¹³C NMR (101 MHz, CDCl₃)  = 164.2, 163.6, 151.5, 145.4,

145.4, 145.4, 145.3, 145.3, 145.3, 145.3, 145.1, 144.8, 144.8,

144.8, 144.0, 143.5, 143.2, 143.2, 143.2, 142.8, 142.5, 142.4,

142.1, 142.0, 142.0, 141.1, 139.2, 139.2, 74.3, 71.7, 65.8, 54.2,

52.3, 51.9, 50.7 ppm.

Yield 65 mg, 6.05*10^-5mol, 44%.

¹H NMR (400 MHz, CDCl₃)  = 7.59-7.56 (m, 2H), 7.42-7.31 (m,

3H), 7.01-6.90 (m, 2H) 4.71-4.69 (m, 2H), 4.49-4.46 (m, 2H) 4.09-

4.08 (m, 1H) 4.04 (s, 3H), 3.87-3.87 (m, 1H), 2.25-2.05 (m,

2H) ppm.

¹³C NMR (101 MHz, CDCl₃)  = 168.6, 164.9, 164.0, 163.6,

145.4, 145.4, 145.3, 145.2, 145.2, 145.2, 145.2, 145.1, 144.8,

144.8, 144.8, 144.8, 144.1, 144.0, 143.9, 143.2, 143.2, 143.2,

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143.2, 143.2, 143.1, 143.1, 142.4, 142.1, 141.1, 141.1, 140.9,

139.3, 139.1, 138.4, 135.6, 129.1, 128.1, 127.9, 71.5, 70.7, 65.0,

61.7, 58.9, 54.2, 53.2, 51.9 ppm.

HRMS (MALDI-TOF, dctb) [M]⁺m/z = 1074.1103 (calcd.),

1074.1098 (found).

NBD – fullerene hexakisadduct 15

Compound 15 was prepared according to general procedure C,

after purification by column chromatography (toluene/ethyl

acetate 90/10 to 80/20 v:v, SiO₂) and removal of the solvent in

vacuo the product was obtained as a yellow solid.

NBD – fullerene monoadduct 13

Yield 26 mg, 1.42*10^-5mol, 71%.

Compound 13 was prepared according to general procedure B,

after purification by column chromatography (toluene/ethyl

acetate 95/5 v:v, SiO₂) and precipitation from CS₂with n-pentane

the product was obtained as a brown solid.

¹H NMR (400 MHz, CDCl₃)  = 7.53-7.51 (m, 2H), 7.38-7.30 (m,

3H), 7.01-6.90 (m, 2H), 4.45-4.43 (m, 2H), 4.36-4.29 (m, 22H),

4.07-4.06 (m, 1H) 3.86-3.85 (m, 1H) 3.76 (s, 3H), 2.26-2.06 (m,

2H), 1.34-1.29 (m, 30H) ppm.

¹³C NMR (101 MHz, CDCl₃)  = 168.4, 164.9, 164.2, 163.9,

163.9, 163.9, 163.9, 163.7, 146.1, 146.0, 146.0, 146.0, 146.0,

145.9, 145.9, 145.9, 145.9, 145.8, 145.8, 144.0, 141.3, 141.3,

141.3, 141.3, 141.2, 141.2, 141.1, 140.9, 140.9, 140.8, 138.6,

135.6, 129.0, 127.9, 127.9, 70.7, 69.2, 69.2, 69.2, 69.2, 69.2,

69.0, 64.5, 63.0, 61.5, 58.8, 53.7, 53.1, 45.5, 45.5, 45.0,

14.2 ppm.

Yield 50 mg, 4.60*10^-5mol, 22%.

¹H NMR (400 MHz, CDCl₃)  = 6.79-6.76 (m, 4H), 6.22-6.22 (m,

2H), 4.47-4.44 (m, 4H) 3.53-3.53 (m, 2H), 3.30-3.30 (m, 2H), 2.46-

2.31 (m, 4H), 2.03-1.93 (m, 8H) ppm.

¹³C NMR (101 MHz, CDCl₃)  = 163.7, 157.1, 145.5, 145.4,

145.3, 145.3, 145.0, 144.8, 144.8, 144.8, 144.8, 144.1, 144.0,

143.2, 143.2, 143.1, 142.4, 142.3, 142.1, 141.1, 139.1, 134.9,

73.8, 71.8, 67.0, 53.7, 52.5, 50.3, 27.9, 26.4 ppm.

HRMS (APPI, CH₂Cl₂/toluene) [M]⁺m/z = 1086.1826 (calcd.),

1086.1828 (found).

HRMS (MALDI-TOF, dctb) [M]⁺m/z = 1864.3999 (calcd.),

1864.3993 (found).

NBD – fullerene hexakisadduct 16

General procedure C (NBD – fullerene [5:1] hexakisadducts)

Compound 16 was prepared according to general procedure C,

after purification by column chromatography (toluene/ethyl

acetate 90/10 v:v, SiO₂) and removal of the solvent in vacuo the

product was obtained as a yellow solid.

Under inert atmosphere and the exclusion of light, [5:0] diethyl

malonate pentakisadduct (1.0 eq.) was dissolved in toluene

(1.5 mL per mg pentakisadduct) under sonication. The solution

was degassed with nitrogen for 10 min. Afterwards 1.5 eq. of the

corresponding malonate, CBr₄(2.0 eq.) and DBU (3.0 eq.) were

added. The reaction mixture was stirred for approx. two days.

Whereas, after one day another 2.0 eq. CBr₄and 3.0 eq. DBU

were added. Once TLC analysis indicated that all starting material

was consumed, the solvent was evaporated, and the crude

product was purified by column chromatography. The products

were obtained after removal of the solvent.

Yield 63 mg, 3.36*10^-5mol, 91%.

¹H NMR (400 MHz, CDCl₃)  = 6.75-6.71 (m, 4H), 6.15-6.14 (m,

2H), 4.35-4.30 (m, 20H), 4.22-4.19 (m, 4H), 3.49-3.49 (m, 2H),

3.25-3.25 (m, 2H), 2.32-2.17 (m, 4H), 1.98-1.92 (m, 4H), 1.84-

1.75 (m, 4H), 1.37-1.28 (m, 30H) ppm.

¹³C NMR (101 MHz, CDCl₃)  = 164.0, 163.9, 163.9, 163.9,

157.0, 145.9, 145.9, 145.9, 144.0, 142.3, 141.3, 141.2, 134.7,

73.7, 69.2, 69.2, 69.2, 66.5, 63.0, 53.6, 50.3, 45.6, 45.5, 45.5,

27.7, 26.1, 14.2 ppm.

HRMS (MALDI-TOF, dctb) [M]⁺m/z = 1876.4727 (calcd.),

1876.4721 (found).

NBD – fullerene hexakisadduct 14

Compound 14 was prepared according to general procedure C,

after purification by column chromatography (toluene/ethyl

acetate 80/20 v:v, SiO₂; hexanes/ethyl acetate 60/40 v:v to

DC/ethyl acetate 60/40 v:v, SiO₂) and removal of the solvent in

vacuo the product was obtained as a yellow solid.

NBD – fullerene hexakisadduct 17

Under inert atmosphere and the exclusion of light, C₆₀(1.0 eq.,

6.94*10^-5mol, 50 mg) was dissolved in 15 mL of ortho-

dichlorobenzene under sonication. The solution was degassed

with nitrogen for 10 min. Afterwards the NBD malonate 4 (10 eq.,

6.94*10^-4mol, 256mg), CBr₄(100 eq., 6.94 mmol, 2.30 g) and

DBU (10 eq., 6.94*10^-4mol, 104 μL) were added. The reaction

mixture was stirred overnight and another 20 μL of DBU were

added. After another day, the reaction mixture was concentrated

and purified by column chromatography (hexanes/ethyl acetate,

90/10 v:v, SiO₂). The product was obtained after removal of the

solvent in vacuo.

Yield 26 mg, 1.48*10^-5mol, 64%.

¹H NMR (400 MHz, CDCl₃)  = 6.76-6.72 (m, 2H), 6.16-6.16 (m,

1H), 4.36-4.30 (m, 20H) 4.23-4.20 (m, 2H), 3.86 (s, 3H), 3.49-3.49

(m, 1H), 3.26-3.26 (m, 1H), 2.33-2.18 (m, 2H), 1.99-1.92 (m, 2H),

1.85-1.77 (m, 2H), 1.35-1.30 (m, 30H) ppm.

¹³C NMR (75 MHz, CDCl₃)  = 164.5, 163.9, 163.9, 163.9, 157.1,

146.0, 146.0, 146.0, 145.9, 145.9, 145.9, 145.9, 145.9, 144.0,

142.4, 141.3, 141.3, 141.3, 141.3, 141.2, 141.2, 134.8, 73.7, 69.3,

69.3, 69.3, 69.2, 69.2, 66.5, 63.0, 53.7, 53.6, 50.3, 45.6, 45.6,

45.5, 27.7, 26.2, 14.2 ppm.

HRMS (APPI, CH₂Cl₂) [M]⁺m/z = 1758.3939 (calcd.), 1758.3915

(found).

Yield 21 mg, 7.23*10^-6mol, 10%.

¹H NMR (400 MHz, CDCl₃)  = 6.76-6.72 (m, 24H), 6.16-6.15 (m,

12H), 4.21-4.18 (m, 24H), 3.49-3.49 (m, 12H), 3.25-3.25 (m, 12H),

2.32-2.18 (m, 24H), 1.98-1.93 (m, 24H), 1.85-1.77 (m, 24H) ppm.

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¹³C NMR (101 MHz, CDCl₃)  = 163.9, 157.1, 145.9, 144.0,

142.4, 141.3, 134.7, 73.7, 69.3, 66.6, 53.6, 50.3, 45.6, 27.8, 26.1

ppm.

HRMS (APPI, CH₂Cl₂/toluene) [M]⁺m/z = 2917.0981 (calcd.),

2917.0954 (found).

139.9, 135.9, 127.8, 126.9, 125.5, 68.3, 68.3, 68.2, 68.1, 63.5,

62.0, 62.0, 60.2, 52.8, 44.5, 44.5, 44.1, 36.8, 32.0, 31.3, 31.0,

30.5, 29.1, 20.0, 13.2 ppm.

Acknowledgements

QC – fullerene monoadduct 20

We gratefully thank the German Research Council (DFG) for

funding through the SFB 953 “Synthetic Carbon Allotropes” and

the project 391585168 “Photochemisch und magnetochemisch

ausgelöste Speicherung/Freisetzung von Sonnenergie in

gespannten organischen Verbindungen”.

QC

–

fullerene monoadduct 20 was prepared by

fullerene

photoisomerization of the corresponding NBD

–

monoadduct 12. For this purpose, 10 mg of compound 12 were

dissolved in thoroughly degassed CDCl₃(2.5 mL). The solution

was transferred into a quartz cuvette which was sealed under

argon atmosphere. Afterwards, the cuvette was irradiated with a

20 mW UV-LED emitting light of a wavelength of 310 nm. The

Keywords: Norbornadiene • fullerenes • energy conversion •

1

reaction was followed by H NMR spectroscopy, which indicated

full conversion after ten hours.

molecular switches, photoswitch

[1]

K. Moth-Poulsen, D. Ćoso, K. Börjesson, N. Vinokurov, S. K. Meier, A.

Majumdar, K. P. C. Vollhardt, R. A. Segalman, Energy Environ. Sci. 2012,

5(9), 8534–8537.

¹H NMR (400 MHz, CDCl₃)  = 7.31-7.27 (m, 4H), 7.21-7.19 (m,

1H), 4.55-4.49 (m, 2H), 4.37-4.30 (m, 2H), 4.08 (s, 3H), 2.63-2.62

(m, 1H), 2.52-2.51 (m, 1H), 2.39-2.36 (m, 1H), 2.26-2.25 (m, 1H),

2.16-2.14 (m, 1H), 1.73-1.72 (m, 1H) ppm.

¹³C NMR (101 MHz, CDCl₃)  = 171.6, 164.1, 163.6, 145.5,

145.5, 145.4, 145.4, 145.3, 145.3, 145.2, 145.2, 145.2, 145.1,

144.9, 144.8, 144.8, 144.8, 144.8, 144.1, 144.1, 143.3, 143.3,

143.2, 143.2, 143.2, 143.2, 143.2, 143.2, 143.2, 142.4, 142.1,

142.1, 142.1, 141.2, 141.2, 139.3, 139.3, 139.1, 139.1, 136.9,

128.8, 127.9, 126.5, 71.5, 65.0, 61.3, 54.3, 51.9, 37.9, 33.1, 32.4,

32.0, 31.5, 30.3, 21.2 ppm.

[2]

[3]

X. An, Y. Xie, Thermochim. Acta 1993, 220, 17–25.

a) G. S. Hammond, N. J. Turro, A. Fischer, J. Am. Chem. Soc. 1961,

83(83), 4674–4675; b) H. Hogeveen, H. C. Volger, J. Am. Chem. Soc.

1967, 89(10), 2486–2487;

[4]

[5]

C.‐L. Sun, C. Wang, R. Boulatov, ChemPhotoChem 2019, 3, 268–283.

H. Taoda, K. Hayakawa, K. Kawase, J. Chem. Eng. Jpn. 1987, 20(4),

335–338.

[6]

a) A. Dreos, Z. Wang, J. Udmark, A. Ström, P. Erhart, K. Börjesson, M.

B. Nielsen, K. Moth-Poulsen, Adv. Energy Mater. 2018, 8(18), 1703401;

b) T. Toda, E. Hasegawa, T. Mukai, H. Tsuruta, T. Hagiwara, T. Yoshida,

Chem. Lett. 1982, 11(10), 1551–1554; c) V. A. Chernoivanov, A. D.

Dubonosov, V. A. Bren, V. I. Minkin, A. N. Suslov, G. S. Borodkin, Mol.

Cryst. Liq. Cryst. Sci. Technol., Sect. A. Cryst. Liq. Cryst. Sci. Technol.,

Sect. A 1997, 297(1), 239–245; d) M. Jevric, A. U. Petersen, M. Mansø,

S. Kumar Singh, Z. Wang, A. Dreos, C. Sumby, M. B. Nielsen, K.

Börjesson, P. Erhart, K. Moth-Poulsen, Chem. - Eur. J 2018, 24(49),

12767–12772;

QC – fullerene hexakisadduct 21

QC – fullerene hexakisadduct 19 is accessible via a BINGEL-

HIRSCH reaction using the corresponding QC malonate 22 or by

photoisomerization of the NBD – fullerene hexakisadduct 15.

[7]

[8]

V. A. Bren', A. D. Dubonosov, V. I. Minkin, V. A. Chernoivanov, Russ.

Chem. Rev. 1991, 60(5), 451–469.

The BINGEL-HIRSCH reaction was carried out according to general

procedure C, using the QC malonate 22. After purification by

column chromatography (toluene/ethyl acetate 90/10 v:v, SiO₂)

and removal of the solvent in vacuo the product was obtained as

a) Z. Wang, A. Roffey, R. Losantos, A. Lennartson, M. Jevric, A. U.

Petersen, M. Quant, A. Dreos, X. Wen, D. Sampedro, K. Börjesson, K.

Moth-Poulsen, Energy Environ. Sci. 2019, 12, 187–193; b) J. Manassen,

J. Catal. 1970, 18(1), 38–45; c) S. Miki, T. Ohno, H. Iwasaki, Z. Yoshida,

Tetrahedron Lett. 1985, 26(29), 3487–3490;

a

yellow solid. For detailed information see supporting

information.

[9]

T. Luchs, P. Lorenz, A. Hirsch, ChemPhotoChem 2019, 309, 548.

[10] a) O. Brummel, F. Waidhas, U. Bauer, Y. Wu, S. Bochmann, H.-P.

Steinrück, C. Papp, J. Bachmann, J. Libuda, J. Phys. Chem. Lett. 2017,

8(13), 2819–2825; b) K. Yasufuku, K. Takahashi, C. Kutal, Tetrahedron

Lett. 1984, 25(43), 4893–4896; c) F. Waidhas, M. Jevric, L. Fromm, M.

Bertram, A. Görling, K. Moth-Poulsen, O. Brummel, J. Libuda, Nano

Energy 2019, 63, 103872;

For the photoisomerization, 10 mg of NBD

– fullerene

hexakisadduct 15 were dissolved in 2.5 mL of properly degassed

CDCl₃. The solution was transferred into a quartz cuvette which

was sealed under argon atmosphere. Afterwards, the cuvette was

irradiated with a 20 mW UV-LED emitting light of a wavelength of

310 nm. The reaction was followed by ¹H NMR spectroscopy,

which indicated full conversion after ten hours.

[11] G. Jones, W. G. Becker, Chem. Phys. Lett. 1982, 85(3), 271–274.

[12] a) S. Leach, M. Vervloet, A. Desprès, E. Bréheret, J. P. Hare, T. John

Dennis, H. W. Kroto, R. Taylor, D. R.M. Walton, Chem. Phys. 1992,

160(3), 451–466; b) D. M. Guldi, M. Prato, Acc. Chem. Res. 2000, 33(10),

695–703;

¹H NMR (400 MHz, CDCl₃)  = 7.27-7.24 (m, 4H), 7.21-7.15 (m,

1H), 4.37-4.26 (m, 22H), 4.20-4.18 (m, 2H), 3.83 (s, 3H), 2.62-

2.60 (m, 1H), 2.49-2.47 (m, 1H), 2.40-2.37 (m, 1H), 2.26-2.25 (m,

1H), 2.17-2.13 (m, 1H), 1.72-1.70 (m, 1H), 1.35-1.30 (m, 30H)

ppm.

¹³C NMR (101 MHz, CDCl₃)  = 170.6, 163.3, 163.0, 163.0,

162.9, 162.9, 162.7, 145.1, 145.0, 145.0, 144.9, 144.9, 144.9,

144.9, 144.8, 140.4, 140.3, 140.3, 140.3, 140.3, 140.3, 140.2,

[13] C. Boudon, J.-P. Gisselbrecht, M. Gross, L. Isaacs, H. L. Anderson, R.

Faust, F. Diederich, Helv. Chim. Acta 1995, 78(5), 1334–1344.

[14] A. R. Tuktarov, A. R. Akhmetov, A. A. Khuzin, U. M. Dzhemilev, J. Org.

Chem. 2018, 83(7), 4160–4166.

[15] A. Hirsch, M. Brettreich in Fullerenes -Chemistry and Reactions; Wiley-

VCH, Weinheim, 2004.

10.1002/chem.201904679

Chemistry - A European Journal

FULL PAPER

[16] a) C. Bingel, Chem. Ber. 1993, 126(8), 1957–1959; b) X. Camps, A.

Hirsch, J. Chem. Soc, Perkin Trans. 1 1997(11), 1595–1596;

[17] A. D. Dubonosov, V. A. Bren, V. A. Chernoivanov, Russ. Chem. Rev.

2002, 71(11), 917–927.

[18] a) F. Hörmann, W. Donaubauer, F. Hampel, A. Hirsch, Chem. - Eur. J.

2012, 18(11), 3329–3337; b) M. E. Pérez-Ojeda, I. Wabra, C. Böttcher,

A. Hirsch, Chem. - Eur. J. 2018, 24(53), 14088–14100;

10.1002/chem.201904679

Chemistry - A European Journal

FULL PAPER

Herein we report on the synthesis of

several norbornadiene (NBD) –

fullerene adducts. Within these new

hybrid systems, the photochemical

isomerization of NBD is combined with

the electron affinity of C₆₀. This

enables the selective light induced

isomerization of the NBD derivative to

its corresponding quadricyclane (QC)

counterpart and the back-conversion

of the latter mediated by the

Patrick Lorenz, Andreas Hirsch*

Page No. – Page No.

Switchable

Norbornadiene/Quadricyclane

Interconversion – Mediated by

Covalently Linked C₆₀

photochemically excited fullerene

core.

Article Doi

Photoswitchable Norbornadiene–Quadricyclane Interconversion Mediated by Covalently Linked C60

DOI: 10.1002/chem.201904679

Source and publish data:

Authors:

Article abstract of DOI:10.1002/chem.201904679

Full text of DOI:10.1002/chem.201904679

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