A short and general route to the substituted dipyridylcyclopentenes was explored and several new compounds belonging to this new group of diarylethenes were synthesized. The study of their photochromic and thermochromic properties shows that the rate of the thermal ring opening is strongly dependent on the polarity of the solvent.

Ben L. Feringa

Dunn A. D.

Gorodetsky B.

Gros P.

Martin Walko

Miura Y.

Walko M.

English

Walko, M

Feringa, Bernard

University of Groningen Research Database

  
 University of Groningen
Synthesis and properties of dipyridylcyclopentenes
Walko, M; Feringa, B.L.
Published in:
Molecular Crystals and Liquid Crystals
DOI:
10.1080/15421400590947379
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from
it. Please check the document version below.
Document Version
Publisher's PDF, also known as Version of record
Publication date:
2005
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Walko, M., & Feringa, B. (2005). Synthesis and properties of dipyridylcyclopentenes. Molecular Crystals
and Liquid Crystals, 431(1), 549-553. DOI: 10.1080/15421400590947379
Copyright
Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the
author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policy
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately
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Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the
number of authors shown on this cover page is limited to 10 maximum.
Download date: 10-02-2018
Synthesis and Properties of Dipyridylcyclopentenes
Martin Walko
Ben L. Feringa
Department of Organic and Molecular Inorganic Chemistry, Stratingh
Institute and Materials Science Center, University of Groningen,
Nijenborgh, Groningen, The Netherlands
A short and general route to the substituted dipyridylcyclopentenes was explored
and several new compounds belonging to this new group of diarylethenes were
synthesized. The study of their photochromic and thermochromic properties shows
that the rate of the thermal ring opening is strongly dependent on the polarity of
the solvent.
Keywords: diarylethene; photochromism; thermochromism
INTRODUCTION
Among photochromic materials a lot of attention is paid to the systems
whose behaviour is responsive to more stimuli than just light. Dithie-
nylethenes with pyridine substituents have been studied thoroughly
by several groups. They have been used to demonstrate electronic com-
munication through dithienylethene backbone [1], to construct supra-
molecular assemblies [2], and it was shown that their ring-closing
reaction can be achieved electrochemically [3].
We have synthesized two basic representatives of the new group of
diarylethenes in which the pyridine moieties are attached directly to
the cyclopentene or perfluorocyclopentene ring (Fig. 1) [4]. As it was
expected, these new switches have some unique properties arising
from the presence of nitrogens as a proton and metal binding sites
We gratefully acknowledge a research grant from the Material Science Center
(MSCþ) and the Zeon corporation for a gift of octafluorocyclopentene.
Address correspondence to Ben L. Feringa, Department of Organic and Molecular
Inorganic Chemistry, Stratingh Institute and Materials Science Center, University of
Groningen, Nijenborgh 4, 9747AG Groningen, The Netherlands. E-mail: feringa@
chem.rug.nl
Mol. Cryst. Liq. Cryst., Vol. 431, pp. 249=[549]–253=[553], 2005
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400590947379
249=[549]
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directly in the photochromic core. Not only do they show shift of the
absorption maxima upon protonation but also gated photochromism
when the protonated open form does not undergo the photochemical
ring closing, whereas the ring opening reaction stays unaffected. Com-
pound 2 can be gated also by presence of Cu(1þ) ions creating 1:1 com-
plex. To enlarge this group of photochromic switches, we were looking
for the methods that would conveniently lead to substituted dipyridyl-
cyclopentenes.
RESULTS AND DISCUSSION
The initial attempts to directly derivatize compounds 1 and 2 by ortho-
lithiation with n-BuLi=N,N-dimethyaminoethanol [5] or ortho-
halogenation through pyridine N-oxides [6] failed. Therefore it was
necessary to prepare dipyridylethene from precursors that already
contained functional groups which were easy to substitute.
Dibromo substituted dipyridylethene 3 was prepared in 67% yield
from the known 2,6-dibromo-3,5-dimethylpyridine [7]. It can be very
conveniently converted into a variety of derivatives through halo-
gen-metal exchange and a consequent reaction with electrophiles or
through one of the many cross-coupling reactions involving arylha-
lides (Sch. 1). A disadvantage of this precursor is the presence of the
methyl group in the ortho position to bromide, which may be a source
of steric hindrance for bulkier substituents such as a phenyl group,
pushing it out of the plane of the pyridine ring and disrupting the con-
jugation.
Dibromide 3 shows only limited photochromism and dialdehyde 4 is
not photochromic at all. However, dipyridylethenes with electron rich
substituents such as thiophene 5 and thiazole 6 perform very well.
After irradiation with 313nm UV light a new peak with the absorption
maximum around 650nm appears, indicating the presence of the
closed form of the switch. The peak is extremely broad, covering
almost the whole visible spectrum up to 850nm (Fig. 2). However, both
FIGURE 1 Basic dipyridylethene photochromic switches.
250=[550] M. Walko and B. L. Feringa
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compounds 5 and 6 are thermochromic even at room temperature, a
common feature for all dipyridylethenes.
By screening the photochromic reaction in various solvents only
very weak solvatochromism (shift up to 10nm) in the open as well
SCHEME 1 Synthesis of substituted dipyridylcyclopentenes.
FIGURE 2 Absorption spectra of 5 and 6 in dichloromethane before (——)
and after (----) irradiation with 313nm light.
Synthesis and Properties of Dipyridylcyclopentenes 251=[551]
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as in the closed form was observed, without any correlation to solvent
polarity. Nevertheless, to our surprise, the thermochromic reaction
occurred to be very solvent dependent (Fig. 3). The thermal ring-open-
ing of dithienyl derivative 5 is as many as 20 times slower in acetoni-
trile than in hexane. To our knowledge, such a phenomenon has never
been reported for diarylethenes, although it may lead to a valuable
insight into the nature of the thermochromic ring-opening which is
most probably thermally forbidden conrotary electrocyclic process.
CONCLUSIONS
Many substituted dipyridylethenes are accessible in just one step from
the easily prepared precursor. Electron donating substituents are
required to achieve good photochromic properties. Some of the pre-
pared compounds show thermochromism which is dependent on the
polarity of their environment.
REFERENCES
[1] Gilat, S. L., Kawai, S. H. & Lehn, J.-M. (1995). J. Chem. Eur., 1, 275.
[2] Matsuda, K., Takayama, K., & Irie, M. (2001). Chem. Commun., 363.
[3] Gorodetsky, B., Samachetty, H. D., Donkers, R. L., Workentin, M. S., & Branda,
N. R. (2004). Angew. Chem. Int. Ed., 43, 2812.
FIGURE 3 Solvent polarity dependent rate of thermochromic reaction of 5
(——)(rate constant multiplied by 104) and 6 (----)(rate constant multiplied
by 103).
252=[552] M. Walko and B. L. Feringa
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[4] Walko, M. & Feringa, B. L. Chem. Commun., submitted.
[5] Gros, P., Fort, Y., & Caubere, P. (1997). J. Chem. Soc. Perkin. Trans. 1, 20, 3071.
[6] Miura, Y., Takaku, S., Nawata, Y., & Hamana, M. (1991). Heterocycles, 32, 1579.
[7] Dunn, A. D. & Guillermic, S. (1988). Z. Chem., 28, 59.
Synthesis and Properties of Dipyridylcyclopentenes 253=[553]
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Synthesis and properties of dipyridylcyclopentenes

A short and general route to the substituted dipyridyleyclopentenes was explored and several new compounds belonging to this new group of diarylethenes were synthesized. The study of their photochromic and thermochromic properties shows that the rate of the thermal ring opening is strongly dependent on the polarity of the solvent.</p

ARTS repository - University of Groningen

   University of GroningenSynthesis and properties of dipyridylcyclopentenesWalko, M; Feringa, BernardPublished in:Molecular Crystals and Liquid CrystalsDOI:10.1080/15421400590947379IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2005Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Walko, M., & Feringa, B. (2005). Synthesis and properties of dipyridylcyclopentenes. Molecular Crystalsand Liquid Crystals, 431(1), 549-553. https://doi.org/10.1080/15421400590947379CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 06-06-2022Synthesis and Properties of DipyridylcyclopentenesMartin WalkoBen L. FeringaDepartment of Organic and Molecular Inorganic Chemistry, StratinghInstitute and Materials Science Center, University of Groningen,Nijenborgh, Groningen, The NetherlandsA short and general route to the substituted dipyridylcyclopentenes was exploredand several new compounds belonging to this new group of diarylethenes weresynthesized. The study of their photochromic and thermochromic properties showsthat the rate of the thermal ring opening is strongly dependent on the polarity ofthe solvent.Keywords: diarylethene; photochromism; thermochromismINTRODUCTIONAmong photochromic materials a lot of attention is paid to the systemswhose behaviour is responsive to more stimuli than just light. Dithie-nylethenes with pyridine substituents have been studied thoroughlyby several groups. They have been used to demonstrate electronic com-munication through dithienylethene backbone [1], to construct supra-molecular assemblies [2], and it was shown that their ring-closingreaction can be achieved electrochemically [3].We have synthesized two basic representatives of the new group ofdiarylethenes in which the pyridine moieties are attached directly tothe cyclopentene or perfluorocyclopentene ring (Fig. 1) [4]. As it wasexpected, these new switches have some unique properties arisingfrom the presence of nitrogens as a proton and metal binding sitesWe gratefully acknowledge a research grant from the Material Science Center(MSCþ) and the Zeon corporation for a gift of octafluorocyclopentene.Address correspondence to Ben L. Feringa, Department of Organic and MolecularInorganic Chemistry, Stratingh Institute and Materials Science Center, University ofGroningen, Nijenborgh 4, 9747AG Groningen, The Netherlands. E-mail: feringa@chem.rug.nlMol. Cryst. Liq. Cryst., Vol. 431, pp. 249=[549]–253=[553], 2005Copyright # Taylor & Francis Inc.ISSN: 1542-1406 print=1563-5287 onlineDOI: 10.1080/15421400590947379249=[549]Downloaded By: [University of Groningen] At: 13:53 28 February 2011directly in the photochromic core. Not only do they show shift of theabsorption maxima upon protonation but also gated photochromismwhen the protonated open form does not undergo the photochemicalring closing, whereas the ring opening reaction stays unaffected. Com-pound 2 can be gated also by presence of Cu(1þ) ions creating 1:1 com-plex. To enlarge this group of photochromic switches, we were lookingfor the methods that would conveniently lead to substituted dipyridyl-cyclopentenes.RESULTS AND DISCUSSIONThe initial attempts to directly derivatize compounds 1 and 2 by ortho-lithiation with n-BuLi=N,N-dimethyaminoethanol [5] or ortho-halogenation through pyridine N-oxides [6] failed. Therefore it wasnecessary to prepare dipyridylethene from precursors that alreadycontained functional groups which were easy to substitute.Dibromo substituted dipyridylethene 3 was prepared in 67% yieldfrom the known 2,6-dibromo-3,5-dimethylpyridine [7]. It can be veryconveniently converted into a variety of derivatives through halo-gen-metal exchange and a consequent reaction with electrophiles orthrough one of the many cross-coupling reactions involving arylha-lides (Sch. 1). A disadvantage of this precursor is the presence of themethyl group in the ortho position to bromide, which may be a sourceof steric hindrance for bulkier substituents such as a phenyl group,pushing it out of the plane of the pyridine ring and disrupting the con-jugation.Dibromide 3 shows only limited photochromism and dialdehyde 4 isnot photochromic at all. However, dipyridylethenes with electron richsubstituents such as thiophene 5 and thiazole 6 perform very well.After irradiation with 313nm UV light a new peak with the absorptionmaximum around 650nm appears, indicating the presence of theclosed form of the switch. The peak is extremely broad, coveringalmost the whole visible spectrum up to 850nm (Fig. 2). However, bothFIGURE 1 Basic dipyridylethene photochromic switches.250=[550] M. Walko and B. L. FeringaDownloaded By: [University of Groningen] At: 13:53 28 February 2011compounds 5 and 6 are thermochromic even at room temperature, acommon feature for all dipyridylethenes.By screening the photochromic reaction in various solvents onlyvery weak solvatochromism (shift up to 10nm) in the open as wellSCHEME 1 Synthesis of substituted dipyridylcyclopentenes.FIGURE 2 Absorption spectra of 5 and 6 in dichloromethane before (——)and after (----) irradiation with 313nm light.Synthesis and Properties of Dipyridylcyclopentenes 251=[551]Downloaded By: [University of Groningen] At: 13:53 28 February 2011as in the closed form was observed, without any correlation to solventpolarity. Nevertheless, to our surprise, the thermochromic reactionoccurred to be very solvent dependent (Fig. 3). The thermal ring-open-ing of dithienyl derivative 5 is as many as 20 times slower in acetoni-trile than in hexane. To our knowledge, such a phenomenon has neverbeen reported for diarylethenes, although it may lead to a valuableinsight into the nature of the thermochromic ring-opening which ismost probably thermally forbidden conrotary electrocyclic process.CONCLUSIONSMany substituted dipyridylethenes are accessible in just one step fromthe easily prepared precursor. Electron donating substituents arerequired to achieve good photochromic properties. Some of the pre-pared compounds show thermochromism which is dependent on thepolarity of their environment.REFERENCES[1] Gilat, S. L., Kawai, S. H. & Lehn, J.-M. (1995). J. Chem. Eur., 1, 275.[2] Matsuda, K., Takayama, K., & Irie, M. (2001). Chem. Commun., 363.[3] Gorodetsky, B., Samachetty, H. D., Donkers, R. L., Workentin, M. S., & Branda,N. R. (2004). Angew. Chem. Int. Ed., 43, 2812.FIGURE 3 Solvent polarity dependent rate of thermochromic reaction of 5(——)(rate constant multiplied by 104) and 6 (----)(rate constant multipliedby 103).252=[552] M. Walko and B. L. FeringaDownloaded By: [University of Groningen] At: 13:53 28 February 2011[4] Walko, M. & Feringa, B. L. Chem. Commun., submitted.[5] Gros, P., Fort, Y., & Caubere, P. (1997). J. Chem. Soc. Perkin. Trans. 1, 20, 3071.[6] Miura, Y., Takaku, S., Nawata, Y., & Hamana, M. (1991). Heterocycles, 32, 1579.[7] Dunn, A. D. & Guillermic, S. (1988). Z. Chem., 28, 59.Synthesis and Properties of Dipyridylcyclopentenes 253=[553]Downloaded By: [University of Groningen] At: 13:53 28 February 2011

University of Groningen

   University of GroningenSynthesis and properties of dipyridylcyclopentenesWalko, M; Feringa, BernardPublished in:Molecular Crystals and Liquid CrystalsDOI:10.1080/15421400590947379IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2005Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Walko, M., & Feringa, B. (2005). Synthesis and properties of dipyridylcyclopentenes. Molecular Crystalsand Liquid Crystals, 431(1), 549-553. https://doi.org/10.1080/15421400590947379CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 12-11-2019Synthesis and Properties of DipyridylcyclopentenesMartin WalkoBen L. FeringaDepartment of Organic and Molecular Inorganic Chemistry, StratinghInstitute and Materials Science Center, University of Groningen,Nijenborgh, Groningen, The NetherlandsA short and general route to the substituted dipyridylcyclopentenes was exploredand several new compounds belonging to this new group of diarylethenes weresynthesized. The study of their photochromic and thermochromic properties showsthat the rate of the thermal ring opening is strongly dependent on the polarity ofthe solvent.Keywords: diarylethene; photochromism; thermochromismINTRODUCTIONAmong photochromic materials a lot of attention is paid to the systemswhose behaviour is responsive to more stimuli than just light. Dithie-nylethenes with pyridine substituents have been studied thoroughlyby several groups. They have been used to demonstrate electronic com-munication through dithienylethene backbone [1], to construct supra-molecular assemblies [2], and it was shown that their ring-closingreaction can be achieved electrochemically [3].We have synthesized two basic representatives of the new group ofdiarylethenes in which the pyridine moieties are attached directly tothe cyclopentene or perfluorocyclopentene ring (Fig. 1) [4]. As it wasexpected, these new switches have some unique properties arisingfrom the presence of nitrogens as a proton and metal binding sitesWe gratefully acknowledge a research grant from the Material Science Center(MSCþ) and the Zeon corporation for a gift of octafluorocyclopentene.Address correspondence to Ben L. Feringa, Department of Organic and MolecularInorganic Chemistry, Stratingh Institute and Materials Science Center, University ofGroningen, Nijenborgh 4, 9747AG Groningen, The Netherlands. E-mail: feringa@chem.rug.nlMol. Cryst. Liq. Cryst., Vol. 431, pp. 249=[549]–253=[553], 2005Copyright # Taylor & Francis Inc.ISSN: 1542-1406 print=1563-5287 onlineDOI: 10.1080/15421400590947379249=[549]Downloaded By: [University of Groningen] At: 13:53 28 February 2011directly in the photochromic core. Not only do they show shift of theabsorption maxima upon protonation but also gated photochromismwhen the protonated open form does not undergo the photochemicalring closing, whereas the ring opening reaction stays unaffected. Com-pound 2 can be gated also by presence of Cu(1þ) ions creating 1:1 com-plex. To enlarge this group of photochromic switches, we were lookingfor the methods that would conveniently lead to substituted dipyridyl-cyclopentenes.RESULTS AND DISCUSSIONThe initial attempts to directly derivatize compounds 1 and 2 by ortho-lithiation with n-BuLi=N,N-dimethyaminoethanol [5] or ortho-halogenation through pyridine N-oxides [6] failed. Therefore it wasnecessary to prepare dipyridylethene from precursors that alreadycontained functional groups which were easy to substitute.Dibromo substituted dipyridylethene 3 was prepared in 67% yieldfrom the known 2,6-dibromo-3,5-dimethylpyridine [7]. It can be veryconveniently converted into a variety of derivatives through halo-gen-metal exchange and a consequent reaction with electrophiles orthrough one of the many cross-coupling reactions involving arylha-lides (Sch. 1). A disadvantage of this precursor is the presence of themethyl group in the ortho position to bromide, which may be a sourceof steric hindrance for bulkier substituents such as a phenyl group,pushing it out of the plane of the pyridine ring and disrupting the con-jugation.Dibromide 3 shows only limited photochromism and dialdehyde 4 isnot photochromic at all. However, dipyridylethenes with electron richsubstituents such as thiophene 5 and thiazole 6 perform very well.After irradiation with 313nm UV light a new peak with the absorptionmaximum around 650nm appears, indicating the presence of theclosed form of the switch. The peak is extremely broad, coveringalmost the whole visible spectrum up to 850nm (Fig. 2). However, bothFIGURE 1 Basic dipyridylethene photochromic switches.250=[550] M. Walko and B. L. FeringaDownloaded By: [University of Groningen] At: 13:53 28 February 2011compounds 5 and 6 are thermochromic even at room temperature, acommon feature for all dipyridylethenes.By screening the photochromic reaction in various solvents onlyvery weak solvatochromism (shift up to 10nm) in the open as wellSCHEME 1 Synthesis of substituted dipyridylcyclopentenes.FIGURE 2 Absorption spectra of 5 and 6 in dichloromethane before (——)and after (----) irradiation with 313nm light.Synthesis and Properties of Dipyridylcyclopentenes 251=[551]Downloaded By: [University of Groningen] At: 13:53 28 February 2011as in the closed form was observed, without any correlation to solventpolarity. Nevertheless, to our surprise, the thermochromic reactionoccurred to be very solvent dependent (Fig. 3). The thermal ring-open-ing of dithienyl derivative 5 is as many as 20 times slower in acetoni-trile than in hexane. To our knowledge, such a phenomenon has neverbeen reported for diarylethenes, although it may lead to a valuableinsight into the nature of the thermochromic ring-opening which ismost probably thermally forbidden conrotary electrocyclic process.CONCLUSIONSMany substituted dipyridylethenes are accessible in just one step fromthe easily prepared precursor. Electron donating substituents arerequired to achieve good photochromic properties. Some of the pre-pared compounds show thermochromism which is dependent on thepolarity of their environment.REFERENCES[1] Gilat, S. L., Kawai, S. H. & Lehn, J.-M. (1995). J. Chem. Eur., 1, 275.[2] Matsuda, K., Takayama, K., & Irie, M. (2001). Chem. Commun., 363.[3] Gorodetsky, B., Samachetty, H. D., Donkers, R. L., Workentin, M. S., & Branda,N. R. (2004). Angew. Chem. Int. Ed., 43, 2812.FIGURE 3 Solvent polarity dependent rate of thermochromic reaction of 5(——)(rate constant multiplied by 104) and 6 (----)(rate constant multipliedby 103).252=[552] M. Walko and B. L. FeringaDownloaded By: [University of Groningen] At: 13:53 28 February 2011[4] Walko, M. & Feringa, B. L. Chem. Commun., submitted.[5] Gros, P., Fort, Y., & Caubere, P. (1997). J. Chem. Soc. Perkin. Trans. 1, 20, 3071.[6] Miura, Y., Takaku, S., Nawata, Y., & Hamana, M. (1991). Heterocycles, 32, 1579.[7] Dunn, A. D. & Guillermic, S. (1988). Z. Chem., 28, 59.Synthesis and Properties of Dipyridylcyclopentenes 253=[553]Downloaded By: [University of Groningen] At: 13:53 28 February 2011

Crossref

Synthesis and Properties of Dipyridylcyclopentenes

   University of GroningenSynthesis and properties of dipyridylcyclopentenesWalko, M; Feringa, BernardPublished in:Molecular Crystals and Liquid CrystalsDOI:10.1080/15421400590947379IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2005Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Walko, M., & Feringa, B. (2005). Synthesis and properties of dipyridylcyclopentenes. Molecular Crystalsand Liquid Crystals, 431(1), 549-553. https://doi.org/10.1080/15421400590947379CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 24-04-2023Synthesis and Properties of DipyridylcyclopentenesMartin WalkoBen L. FeringaDepartment of Organic and Molecular Inorganic Chemistry, StratinghInstitute and Materials Science Center, University of Groningen,Nijenborgh, Groningen, The NetherlandsA short and general route to the substituted dipyridylcyclopentenes was exploredand several new compounds belonging to this new group of diarylethenes weresynthesized. The study of their photochromic and thermochromic properties showsthat the rate of the thermal ring opening is strongly dependent on the polarity ofthe solvent.Keywords: diarylethene; photochromism; thermochromismINTRODUCTIONAmong photochromic materials a lot of attention is paid to the systemswhose behaviour is responsive to more stimuli than just light. Dithie-nylethenes with pyridine substituents have been studied thoroughlyby several groups. They have been used to demonstrate electronic com-munication through dithienylethene backbone [1], to construct supra-molecular assemblies [2], and it was shown that their ring-closingreaction can be achieved electrochemically [3].We have synthesized two basic representatives of the new group ofdiarylethenes in which the pyridine moieties are attached directly tothe cyclopentene or perfluorocyclopentene ring (Fig. 1) [4]. As it wasexpected, these new switches have some unique properties arisingfrom the presence of nitrogens as a proton and metal binding sitesWe gratefully acknowledge a research grant from the Material Science Center(MSCþ) and the Zeon corporation for a gift of octafluorocyclopentene.Address correspondence to Ben L. Feringa, Department of Organic and MolecularInorganic Chemistry, Stratingh Institute and Materials Science Center, University ofGroningen, Nijenborgh 4, 9747AG Groningen, The Netherlands. E-mail: feringa@chem.rug.nlMol. Cryst. Liq. Cryst., Vol. 431, pp. 249=[549]–253=[553], 2005Copyright # Taylor & Francis Inc.ISSN: 1542-1406 print=1563-5287 onlineDOI: 10.1080/15421400590947379249=[549]Downloaded By: [University of Groningen] At: 13:53 28 February 2011directly in the photochromic core. Not only do they show shift of theabsorption maxima upon protonation but also gated photochromismwhen the protonated open form does not undergo the photochemicalring closing, whereas the ring opening reaction stays unaffected. Com-pound 2 can be gated also by presence of Cu(1þ) ions creating 1:1 com-plex. To enlarge this group of photochromic switches, we were lookingfor the methods that would conveniently lead to substituted dipyridyl-cyclopentenes.RESULTS AND DISCUSSIONThe initial attempts to directly derivatize compounds 1 and 2 by ortho-lithiation with n-BuLi=N,N-dimethyaminoethanol [5] or ortho-halogenation through pyridine N-oxides [6] failed. Therefore it wasnecessary to prepare dipyridylethene from precursors that alreadycontained functional groups which were easy to substitute.Dibromo substituted dipyridylethene 3 was prepared in 67% yieldfrom the known 2,6-dibromo-3,5-dimethylpyridine [7]. It can be veryconveniently converted into a variety of derivatives through halo-gen-metal exchange and a consequent reaction with electrophiles orthrough one of the many cross-coupling reactions involving arylha-lides (Sch. 1). A disadvantage of this precursor is the presence of themethyl group in the ortho position to bromide, which may be a sourceof steric hindrance for bulkier substituents such as a phenyl group,pushing it out of the plane of the pyridine ring and disrupting the con-jugation.Dibromide 3 shows only limited photochromism and dialdehyde 4 isnot photochromic at all. However, dipyridylethenes with electron richsubstituents such as thiophene 5 and thiazole 6 perform very well.After irradiation with 313nm UV light a new peak with the absorptionmaximum around 650nm appears, indicating the presence of theclosed form of the switch. The peak is extremely broad, coveringalmost the whole visible spectrum up to 850nm (Fig. 2). However, bothFIGURE 1 Basic dipyridylethene photochromic switches.250=[550] M. Walko and B. L. FeringaDownloaded By: [University of Groningen] At: 13:53 28 February 2011compounds 5 and 6 are thermochromic even at room temperature, acommon feature for all dipyridylethenes.By screening the photochromic reaction in various solvents onlyvery weak solvatochromism (shift up to 10nm) in the open as wellSCHEME 1 Synthesis of substituted dipyridylcyclopentenes.FIGURE 2 Absorption spectra of 5 and 6 in dichloromethane before (——)and after (----) irradiation with 313nm light.Synthesis and Properties of Dipyridylcyclopentenes 251=[551]Downloaded By: [University of Groningen] At: 13:53 28 February 2011as in the closed form was observed, without any correlation to solventpolarity. Nevertheless, to our surprise, the thermochromic reactionoccurred to be very solvent dependent (Fig. 3). The thermal ring-open-ing of dithienyl derivative 5 is as many as 20 times slower in acetoni-trile than in hexane. To our knowledge, such a phenomenon has neverbeen reported for diarylethenes, although it may lead to a valuableinsight into the nature of the thermochromic ring-opening which ismost probably thermally forbidden conrotary electrocyclic process.CONCLUSIONSMany substituted dipyridylethenes are accessible in just one step fromthe easily prepared precursor. Electron donating substituents arerequired to achieve good photochromic properties. Some of the pre-pared compounds show thermochromism which is dependent on thepolarity of their environment.REFERENCES[1] Gilat, S. L., Kawai, S. H. & Lehn, J.-M. (1995). J. Chem. Eur., 1, 275.[2] Matsuda, K., Takayama, K., & Irie, M. (2001). Chem. Commun., 363.[3] Gorodetsky, B., Samachetty, H. D., Donkers, R. L., Workentin, M. S., & Branda,N. R. (2004). Angew. Chem. Int. Ed., 43, 2812.FIGURE 3 Solvent polarity dependent rate of thermochromic reaction of 5(——)(rate constant multiplied by 104) and 6 (----)(rate constant multipliedby 103).252=[552] M. Walko and B. L. FeringaDownloaded By: [University of Groningen] At: 13:53 28 February 2011[4] Walko, M. & Feringa, B. L. Chem. Commun., submitted.[5] Gros, P., Fort, Y., & Caubere, P. (1997). J. Chem. Soc. Perkin. Trans. 1, 20, 3071.[6] Miura, Y., Takaku, S., Nawata, Y., & Hamana, M. (1991). Heterocycles, 32, 1579.[7] Dunn, A. D. & Guillermic, S. (1988). Z. Chem., 28, 59.Synthesis and Properties of Dipyridylcyclopentenes 253=[553]Downloaded By: [University of Groningen] At: 13:53 28 February 2011

Walko, Martin,

Feringa, Ben L.,

University of Groningen Digital Archive

Synthesis and Properties of Dipyridylcyclopentenes
Martin Walko
Ben L. Feringa
Department of Organic and Molecular Inorganic Chemistry, Stratingh
Institute and Materials Science Center, University of Groningen,
Nijenborgh, Groningen, The Netherlands
A short and general route to the substituted dipyridylcyclopentenes was explored
and several new compounds belonging to this new group of diarylethenes were
synthesized. The study of their photochromic and thermochromic properties shows
that the rate of the thermal ring opening is strongly dependent on the polarity of
the solvent.
Keywords: diarylethene; photochromism; thermochromism
INTRODUCTION
Among photochromic materials a lot of attention is paid to the systems
whose behaviour is responsive to more stimuli than just light. Dithie-
nylethenes with pyridine substituents have been studied thoroughly
by several groups. They have been used to demonstrate electronic com-
munication through dithienylethene backbone [1], to construct supra-
molecular assemblies [2], and it was shown that their ring-closing
reaction can be achieved electrochemically [3].
We have synthesized two basic representatives of the new group of
diarylethenes in which the pyridine moieties are attached directly to
the cyclopentene or perfluorocyclopentene ring (Fig. 1) [4]. As it was
expected, these new switches have some unique properties arising
from the presence of nitrogens as a proton and metal binding sites
We gratefully acknowledge a research grant from the Material Science Center
(MSCþ) and the Zeon corporation for a gift of octafluorocyclopentene.
Address correspondence to Ben L. Feringa, Department of Organic and Molecular
Inorganic Chemistry, Stratingh Institute and Materials Science Center, University of
Groningen, Nijenborgh 4, 9747AG Groningen, The Netherlands. E-mail: feringa@
chem.rug.nl
Mol. Cryst. Liq. Cryst., Vol. 431, pp. 249=[549]–253=[553], 2005
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400590947379
249=[549]
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1directly in the photochromic core. Not only do they show shift of the
absorption maxima upon protonation but also gated photochromism
when the protonated open form does not undergo the photochemical
ring closing, whereas the ring opening reaction stays unaffected. Com-
pound 2 can be gated also by presence of Cu(1þ) ions creating 1:1 com-
plex. To enlarge this group of photochromic switches, we were looking
for the methods that would conveniently lead to substituted dipyridyl-
cyclopentenes.
RESULTS AND DISCUSSION
The initial attempts to directly derivatize compounds 1 and 2 by ortho-
lithiation with n-BuLi=N,N-dimethyaminoethanol [5] or ortho-
halogenation through pyridine N-oxides [6] failed. Therefore it was
necessary to prepare dipyridylethene from precursors that already
contained functional groups which were easy to substitute.
Dibromo substituted dipyridylethene 3 was prepared in 67% yield
from the known 2,6-dibromo-3,5-dimethylpyridine [7]. It can be very
conveniently converted into a variety of derivatives through halo-
gen-metal exchange and a consequent reaction with electrophiles or
through one of the many cross-coupling reactions involving arylha-
lides (Sch. 1). A disadvantage of this precursor is the presence of the
methyl group in the ortho position to bromide, which may be a source
of steric hindrance for bulkier substituents such as a phenyl group,
pushing it out of the plane of the pyridine ring and disrupting the con-
jugation.
Dibromide 3 shows only limited photochromism and dialdehyde 4 is
not photochromic at all. However, dipyridylethenes with electron rich
substituents such as thiophene 5 and thiazole 6 perform very well.
After irradiation with 313nm UV light a new peak with the absorption
maximum around 650nm appears, indicating the presence of the
closed form of the switch. The peak is extremely broad, covering
almost the whole visible spectrum up to 850nm (Fig. 2). However, both
FIGURE 1 Basic dipyridylethene photochromic switches.
250=[550] M. Walko and B. L. Feringa
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1compounds 5 and 6 are thermochromic even at room temperature, a
common feature for all dipyridylethenes.
By screening the photochromic reaction in various solvents only
very weak solvatochromism (shift up to 10nm) in the open as well
SCHEME 1 Synthesis of substituted dipyridylcyclopentenes.
FIGURE 2 Absorption spectra of 5 and 6 in dichloromethane before (——)
and after (----) irradiation with 313nm light.
Synthesis and Properties of Dipyridylcyclopentenes 251=[551]
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1as in the closed form was observed, without any correlation to solvent
polarity. Nevertheless, to our surprise, the thermochromic reaction
occurred to be very solvent dependent (Fig. 3). The thermal ring-open-
ing of dithienyl derivative 5 is as many as 20 times slower in acetoni-
trile than in hexane. To our knowledge, such a phenomenon has never
been reported for diarylethenes, although it may lead to a valuable
insight into the nature of the thermochromic ring-opening which is
most probably thermally forbidden conrotary electrocyclic process.
CONCLUSIONS
Many substituted dipyridylethenes are accessible in just one step from
the easily prepared precursor. Electron donating substituents are
required to achieve good photochromic properties. Some of the pre-
pared compounds show thermochromism which is dependent on the
polarity of their environment.
REFERENCES
[1] Gilat, S. L., Kawai, S. H. & Lehn, J.-M. (1995). J. Chem. Eur., 1, 275.
[2] Matsuda, K., Takayama, K., & Irie, M. (2001). Chem. Commun., 363.
[3] Gorodetsky, B., Samachetty, H. D., Donkers, R. L., Workentin, M. S., & Branda,
N. R. (2004). Angew. Chem. Int. Ed., 43, 2812.
FIGURE 3 Solvent polarity dependent rate of thermochromic reaction of 5
(——)(rate constant multiplied by 10
4) and 6 (----)(rate constant multiplied
by 10
3).
252=[552] M. Walko and B. L. Feringa
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1[4] Walko, M. & Feringa, B. L. Chem. Commun., submitted.
[5] Gros, P., Fort, Y., & Caubere, P. (1997). J. Chem. Soc. Perkin. Trans. 1, 20, 3071.
[6] Miura, Y., Takaku, S., Nawata, Y., & Hamana, M. (1991). Heterocycles, 32, 1579.
[7] Dunn, A. D. & Guillermic, S. (1988). Z. Chem., 28, 59.
Synthesis and Properties of Dipyridylcyclopentenes 253=[553]
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28, 59. Synthesis and Properties of Dipyridylcyclopentenes 253=[553] D o w n l o a d e d B y : [ U n i v e r s i t y o f G r o n i n g e n ] A t : : F e b r u a r y

FIGURE 3 Solvent polarity dependent rate of thermochromic reaction of 5 (——)(rate constant multiplied by 10 4) and 6 (----)(rate constant multiplied by

A short and general route to the substituted dipyridyleyclopentenes was explored and several new compounds belonging to this new group of diarylethenes were synthesized. The study of their photochromic and thermochromic properties shows that the rate of the thermal ring opening is strongly dependent on the polarity of the solvent

NARCIS 

   University of GroningenSynthesis and properties of dipyridylcyclopentenesWalko, M; Feringa, BernardPublished in:Molecular Crystals and Liquid CrystalsDOI:10.1080/15421400590947379IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2005Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Walko, M., & Feringa, B. (2005). Synthesis and properties of dipyridylcyclopentenes. Molecular Crystalsand Liquid Crystals, 431(1), 549-553. https://doi.org/10.1080/15421400590947379CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 11-09-2023Synthesis and Properties of DipyridylcyclopentenesMartin WalkoBen L. FeringaDepartment of Organic and Molecular Inorganic Chemistry, StratinghInstitute and Materials Science Center, University of Groningen,Nijenborgh, Groningen, The NetherlandsA short and general route to the substituted dipyridylcyclopentenes was exploredand several new compounds belonging to this new group of diarylethenes weresynthesized. The study of their photochromic and thermochromic properties showsthat the rate of the thermal ring opening is strongly dependent on the polarity ofthe solvent.Keywords: diarylethene; photochromism; thermochromismINTRODUCTIONAmong photochromic materials a lot of attention is paid to the systemswhose behaviour is responsive to more stimuli than just light. Dithie-nylethenes with pyridine substituents have been studied thoroughlyby several groups. They have been used to demonstrate electronic com-munication through dithienylethene backbone [1], to construct supra-molecular assemblies [2], and it was shown that their ring-closingreaction can be achieved electrochemically [3].We have synthesized two basic representatives of the new group ofdiarylethenes in which the pyridine moieties are attached directly tothe cyclopentene or perfluorocyclopentene ring (Fig. 1) [4]. As it wasexpected, these new switches have some unique properties arisingfrom the presence of nitrogens as a proton and metal binding sitesWe gratefully acknowledge a research grant from the Material Science Center(MSCþ) and the Zeon corporation for a gift of octafluorocyclopentene.Address correspondence to Ben L. Feringa, Department of Organic and MolecularInorganic Chemistry, Stratingh Institute and Materials Science Center, University ofGroningen, Nijenborgh 4, 9747AG Groningen, The Netherlands. E-mail: feringa@chem.rug.nlMol. Cryst. Liq. Cryst., Vol. 431, pp. 249=[549]–253=[553], 2005Copyright # Taylor & Francis Inc.ISSN: 1542-1406 print=1563-5287 onlineDOI: 10.1080/15421400590947379249=[549]Downloaded By: [University of Groningen] At: 13:53 28 February 2011directly in the photochromic core. Not only do they show shift of theabsorption maxima upon protonation but also gated photochromismwhen the protonated open form does not undergo the photochemicalring closing, whereas the ring opening reaction stays unaffected. Com-pound 2 can be gated also by presence of Cu(1þ) ions creating 1:1 com-plex. To enlarge this group of photochromic switches, we were lookingfor the methods that would conveniently lead to substituted dipyridyl-cyclopentenes.RESULTS AND DISCUSSIONThe initial attempts to directly derivatize compounds 1 and 2 by ortho-lithiation with n-BuLi=N,N-dimethyaminoethanol [5] or ortho-halogenation through pyridine N-oxides [6] failed. Therefore it wasnecessary to prepare dipyridylethene from precursors that alreadycontained functional groups which were easy to substitute.Dibromo substituted dipyridylethene 3 was prepared in 67% yieldfrom the known 2,6-dibromo-3,5-dimethylpyridine [7]. It can be veryconveniently converted into a variety of derivatives through halo-gen-metal exchange and a consequent reaction with electrophiles orthrough one of the many cross-coupling reactions involving arylha-lides (Sch. 1). A disadvantage of this precursor is the presence of themethyl group in the ortho position to bromide, which may be a sourceof steric hindrance for bulkier substituents such as a phenyl group,pushing it out of the plane of the pyridine ring and disrupting the con-jugation.Dibromide 3 shows only limited photochromism and dialdehyde 4 isnot photochromic at all. However, dipyridylethenes with electron richsubstituents such as thiophene 5 and thiazole 6 perform very well.After irradiation with 313nm UV light a new peak with the absorptionmaximum around 650nm appears, indicating the presence of theclosed form of the switch. The peak is extremely broad, coveringalmost the whole visible spectrum up to 850nm (Fig. 2). However, bothFIGURE 1 Basic dipyridylethene photochromic switches.250=[550] M. Walko and B. L. FeringaDownloaded By: [University of Groningen] At: 13:53 28 February 2011compounds 5 and 6 are thermochromic even at room temperature, acommon feature for all dipyridylethenes.By screening the photochromic reaction in various solvents onlyvery weak solvatochromism (shift up to 10nm) in the open as wellSCHEME 1 Synthesis of substituted dipyridylcyclopentenes.FIGURE 2 Absorption spectra of 5 and 6 in dichloromethane before (——)and after (----) irradiation with 313nm light.Synthesis and Properties of Dipyridylcyclopentenes 251=[551]Downloaded By: [University of Groningen] At: 13:53 28 February 2011as in the closed form was observed, without any correlation to solventpolarity. Nevertheless, to our surprise, the thermochromic reactionoccurred to be very solvent dependent (Fig. 3). The thermal ring-open-ing of dithienyl derivative 5 is as many as 20 times slower in acetoni-trile than in hexane. To our knowledge, such a phenomenon has neverbeen reported for diarylethenes, although it may lead to a valuableinsight into the nature of the thermochromic ring-opening which ismost probably thermally forbidden conrotary electrocyclic process.CONCLUSIONSMany substituted dipyridylethenes are accessible in just one step fromthe easily prepared precursor. Electron donating substituents arerequired to achieve good photochromic properties. Some of the pre-pared compounds show thermochromism which is dependent on thepolarity of their environment.REFERENCES[1] Gilat, S. L., Kawai, S. H. & Lehn, J.-M. (1995). J. Chem. Eur., 1, 275.[2] Matsuda, K., Takayama, K., & Irie, M. (2001). Chem. Commun., 363.[3] Gorodetsky, B., Samachetty, H. D., Donkers, R. L., Workentin, M. S., & Branda,N. R. (2004). Angew. Chem. Int. Ed., 43, 2812.FIGURE 3 Solvent polarity dependent rate of thermochromic reaction of 5(——)(rate constant multiplied by 104) and 6 (----)(rate constant multipliedby 103).252=[552] M. Walko and B. L. FeringaDownloaded By: [University of Groningen] At: 13:53 28 February 2011[4] Walko, M. & Feringa, B. L. Chem. Commun., submitted.[5] Gros, P., Fort, Y., & Caubere, P. (1997). J. Chem. Soc. Perkin. Trans. 1, 20, 3071.[6] Miura, Y., Takaku, S., Nawata, Y., & Hamana, M. (1991). Heterocycles, 32, 1579.[7] Dunn, A. D. & Guillermic, S. (1988). Z. Chem., 28, 59.Synthesis and Properties of Dipyridylcyclopentenes 253=[553]Downloaded By: [University of Groningen] At: 13:53 28 February 2011

University of GroningenSynthesis and properties of dipyridylcyclopentenesWalko, M; Feringa, BernardPublished in:Molecular Crystals and Liquid CrystalsDOI:10.1080/15421400590947379IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2005Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Walko, M., & Feringa, B. (2005). Synthesis and properties of dipyridylcyclopentenes. Molecular Crystalsand Liquid Crystals, 431(1), 549-553. https://doi.org/10.1080/15421400590947379CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 23-12-2025Synthesis and Properties of DipyridylcyclopentenesMartin WalkoBen L. FeringaDepartment of Organic and Molecular Inorganic Chemistry, StratinghInstitute and Materials Science Center, University of Groningen,Nijenborgh, Groningen, The NetherlandsA short and general route to the substituted dipyridylcyclopentenes was exploredand several new compounds belonging to this new group of diarylethenes weresynthesized. The study of their photochromic and thermochromic properties showsthat the rate of the thermal ring opening is strongly dependent on the polarity ofthe solvent.Keywords: diarylethene; photochromism; thermochromismINTRODUCTIONAmong photochromic materials a lot of attention is paid to the systemswhose behaviour is responsive to more stimuli than just light. Dithie-nylethenes with pyridine substituents have been studied thoroughlyby several groups. They have been used to demonstrate electronic com-munication through dithienylethene backbone [1], to construct supra-molecular assemblies [2], and it was shown that their ring-closingreaction can be achieved electrochemically [3].We have synthesized two basic representatives of the new group ofdiarylethenes in which the pyridine moieties are attached directly tothe cyclopentene or perfluorocyclopentene ring (Fig. 1) [4]. As it wasexpected, these new switches have some unique properties arisingfrom the presence of nitrogens as a proton and metal binding sitesWe gratefully acknowledge a research grant from the Material Science Center(MSCþ) and the Zeon corporation for a gift of octafluorocyclopentene.Address correspondence to Ben L. Feringa, Department of Organic and MolecularInorganic Chemistry, Stratingh Institute and Materials Science Center, University ofGroningen, Nijenborgh 4, 9747AG Groningen, The Netherlands. E-mail: feringa@chem.rug.nlMol. Cryst. Liq. Cryst., Vol. 431, pp. 249=[549]–253=[553], 2005Copyright # Taylor & Francis Inc.ISSN: 1542-1406 print=1563-5287 onlineDOI: 10.1080/15421400590947379249=[549]Downloaded By: [University of Groningen] At: 13:53 28 February 2011directly in the photochromic core. Not only do they show shift of theabsorption maxima upon protonation but also gated photochromismwhen the protonated open form does not undergo the photochemicalring closing, whereas the ring opening reaction stays unaffected. Com-pound 2 can be gated also by presence of Cu(1þ) ions creating 1:1 com-plex. To enlarge this group of photochromic switches, we were lookingfor the methods that would conveniently lead to substituted dipyridyl-cyclopentenes.RESULTS AND DISCUSSIONThe initial attempts to directly derivatize compounds 1 and 2 by ortho-lithiation with n-BuLi=N,N-dimethyaminoethanol [5] or ortho-halogenation through pyridine N-oxides [6] failed. Therefore it wasnecessary to prepare dipyridylethene from precursors that alreadycontained functional groups which were easy to substitute.Dibromo substituted dipyridylethene 3 was prepared in 67% yieldfrom the known 2,6-dibromo-3,5-dimethylpyridine [7]. It can be veryconveniently converted into a variety of derivatives through halo-gen-metal exchange and a consequent reaction with electrophiles orthrough one of the many cross-coupling reactions involving arylha-lides (Sch. 1). A disadvantage of this precursor is the presence of themethyl group in the ortho position to bromide, which may be a sourceof steric hindrance for bulkier substituents such as a phenyl group,pushing it out of the plane of the pyridine ring and disrupting the con-jugation.Dibromide 3 shows only limited photochromism and dialdehyde 4 isnot photochromic at all. However, dipyridylethenes with electron richsubstituents such as thiophene 5 and thiazole 6 perform very well.After irradiation with 313nm UV light a new peak with the absorptionmaximum around 650nm appears, indicating the presence of theclosed form of the switch. The peak is extremely broad, coveringalmost the whole visible spectrum up to 850nm (Fig. 2). However, bothFIGURE 1 Basic dipyridylethene photochromic switches.250=[550] M. Walko and B. L. FeringaDownloaded By: [University of Groningen] At: 13:53 28 February 2011compounds 5 and 6 are thermochromic even at room temperature, acommon feature for all dipyridylethenes.By screening the photochromic reaction in various solvents onlyvery weak solvatochromism (shift up to 10nm) in the open as wellSCHEME 1 Synthesis of substituted dipyridylcyclopentenes.FIGURE 2 Absorption spectra of 5 and 6 in dichloromethane before (——)and after (----) irradiation with 313nm light.Synthesis and Properties of Dipyridylcyclopentenes 251=[551]Downloaded By: [University of Groningen] At: 13:53 28 February 2011as in the closed form was observed, without any correlation to solventpolarity. Nevertheless, to our surprise, the thermochromic reactionoccurred to be very solvent dependent (Fig. 3). The thermal ring-open-ing of dithienyl derivative 5 is as many as 20 times slower in acetoni-trile than in hexane. To our knowledge, such a phenomenon has neverbeen reported for diarylethenes, although it may lead to a valuableinsight into the nature of the thermochromic ring-opening which ismost probably thermally forbidden conrotary electrocyclic process.CONCLUSIONSMany substituted dipyridylethenes are accessible in just one step fromthe easily prepared precursor. Electron donating substituents arerequired to achieve good photochromic properties. Some of the pre-pared compounds show thermochromism which is dependent on thepolarity of their environment.REFERENCES[1] Gilat, S. L., Kawai, S. H. & Lehn, J.-M. (1995). J. Chem. Eur., 1, 275.[2] Matsuda, K., Takayama, K., & Irie, M. (2001). Chem. Commun., 363.[3] Gorodetsky, B., Samachetty, H. D., Donkers, R. L., Workentin, M. S., & Branda,N. R. (2004). Angew. Chem. Int. Ed., 43, 2812.FIGURE 3 Solvent polarity dependent rate of thermochromic reaction of 5(——)(rate constant multiplied by 104) and 6 (----)(rate constant multipliedby 103).252=[552] M. Walko and B. L. FeringaDownloaded By: [University of Groningen] At: 13:53 28 February 2011[4] Walko, M. & Feringa, B. L. Chem. Commun., submitted.[5] Gros, P., Fort, Y., & Caubere, P. (1997). J. Chem. Soc. Perkin. Trans. 1, 20, 3071.[6] Miura, Y., Takaku, S., Nawata, Y., & Hamana, M. (1991). Heterocycles, 32, 1579.[7] Dunn, A. D. & Guillermic, S. (1988). Z. Chem., 28, 59.Synthesis and Properties of Dipyridylcyclopentenes 253=[553]Downloaded By: [University of Groningen] At: 13:53 28 February 2011

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Synthesis and Properties of Dipyridylcyclopentenes

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