Synthesis and properties of photochromic difurylethenes for mechanically controlled break junctions

SYNTHESIS AND PROPERTIES OF PHOTOCHROMIC DIFURYLETHENES FOR MECHANICALLY CONTROLLED

BREAK JUNCTIONS

Dissertation

zur Erlangung des akademischen Grades

des Doktors der Naturwissenschaften (Dr. rer. nat.) an der Universität Konstanz

Matematisch-Naturwissenschaftliche Sektion Fachbereich Chemie

vorgelegt von

Dmytro Sysoiev

Tag der mündlichen Prüfung: 02. Mai 2012

Referent: Prof. Dr. Ulrich Groth Referentin: Prof. Dr. Elke Scheer

This thesis was accomplished during the period from January 2008 to February 2012 in the group of Prof. Dr. U. Groth, Chemistry Department, and in the group of Prof. Dr. E. Scheer, Physics Department, within the SFB 767 at the University of Konstanz, Germany.

First, I am grateful to my supervisor Prof. Dr. Ulrich Groth for the highly interesting interdisciplinary research project and provision of all the necessary equipment and excellent working conditions. I highly appreciate his continuous support in all matters considering various documentation formalities for international research students since my first visit in Konstanz in 2005.

Furthermore, I would like to thank my co-supervisor Prof. Dr. Elke Scheer for the opportunity for collaboration with the researchers at the Department of Physics and for a feeling that the products and results of chemical experiments have found a practical application, which I as a synthetic organic chemist find very important and pleasant.

I would like to thank cordially Dr. Thomas Huhn for support, consulting and encouragement in all scientific and common issues.

Special thanks to:

Prof. Dr. Ulrich Steiner for the interesting and helpful consulting in the UV/Vis-spectroscopy;

Dr. Thomas Huhn for the X-ray data interpretation and for constant help with manuscript editing;

Prof. Dr. Thomas Exner for quantum-chemical computations and their interpretation;

Youngsang Kim for transport measurements and interdisciplinary collaboration;

Dr. Markus Ringwald for excellent catalysts and wonderful time at "gourmet meetings";

Anke Friemel, Ulrich Haunz and Prof. Dr. Heiko Möller for measurements and discussions of the NMR spectra;

Dmitry Galetsky and Anna-Lena Steck for measurements of the mass-spectra;

Milena Quentin and Angelika Früh for all the warmth, help and support they gave me;

All members of AG Groth for pleasant and interesting working atmosphere;

Alexandra Kukharenko for fruitful discussions and editing;

Dr. Tanya Nikitina, Dr. Natasha Kopylova, Dr. Vova Semeniuchenko, Katya Gura, and Igor Fadeitsev for the wonderful moments in Konstanz, Alps, etc. etc.;

SFB 767 and Herbert-Quandt-Stiftung for financial support for my PhD research;

Table of Contents

TABLE OF CONTENTS

ABBREVIATIONS...2

1. ZUSAMMENFASSUNG...4

2. INTRODUCTION...5

2.1. Photochromic molecular switches... 5

2.1.1. Photochromism...5

2.1.2. Azobenzenes...12

2.1.3. Spiropyrans and related compounds...14

2.1.4. Diarylethenes ...15

2.2. Diarylethenes: Overview of synthetic approaches ... 25

2.2.1. BuLi/perfluorocycloalkene method ...26

2.2.2. McMurry pathway ...28

2.2.3. Other methods ...29

2.3. Conclusions... 30

3. DESIGN, DEVELOPMENT AND INVESTIGATION OF THIOPHENE-FREE DIARYLETHENES ...31

3.1. Perfuorocyclopentene-based switches (C5F) ... 31

3.1.1. Furan derivatives...31

3.1.2. Isoxazole and pyrazole derivatives ...39

3.2. Perfluorocyclobutene-based switches (C4F) ... 42

3.3. Physical aspects (charge transport investigations)... 46

4. SUMMARY ...55

5. OUTLOOK ...56

6. EXPERIMENTAL SECTION ...57

7. REFERENCES ...80

8. APPENDIX...90

Abbreviations

ABBREVIATIONS

Ac Acetyl

AcOH Acetic acid

Ac2O Acetic anhydride

Ar Aryl ArLi Aryllithium Bn Benzyl Bpin B-pinacol Bu Butyl

tBu tert-Butyl

BuLi Butyllithium c Concentration

COSY Correlation spectroscopy

CuOTf Copper triflate

δ Chemical shift

Δ Heating

DMF Dimethylformamide DMSO Dimethylsulfoxide e‾ Electron E0 Molecular level alignment

ESI Electrospray ionisation

ESR Electron spin resonance Et Ethyl et al. et alii (and others)

Et3N Triethylamine

Et2O Diethylether

ε Extinction coefficient

 Quantum yield

FRET Fluorescence resonance energy transfer

 Level broadening

Abbreviations

h hour(s)

HMBC Heteronuclear multiple-bond correlation spectroscopy HRMS High resolution mass spectroscopy

hυ Ultraviolet irradiation

IR Infrared

J Coupling constant

λabs Position of absorbtion maximum

λex Excitation wavelength

λem Emission wavelength

M Metal

MALDI Matrix assisted laser desorption/ionization Me Methyl

MeOH Methanol

MCBJ Mechanically controlled break junction

ml Milliliter

m/z Mass-to-charge ratio

nm Nanometer

NMR Nuclear magnetic resonance pH Potential Hydrogenii

ppm Parts per million

Py Pyridine

RG Reactive groups

Rf Retention factor

t Time

T Temperature

TFA Trifluoroacetic acid

THF Tetrahydrofuran

TLC Thin layer chromatography TMSBr Trimethylsilylbromide

UV Ultraviolet light

Vis Visible light

Zusamenfassung

1. ZUSAMMENFASSUNG

• Es wurden einundzwanzig neue photochrome Furan-basierte Diarylethene mit verschiedenen Seitenketten-Substituenten synthetisiert und deren Schaltverhalten untersucht (B).

• Der Einfluss der Groesse des zentralen fluorierten vier- und fünfgliedrigen Ringes auf die strukturellen und photochemischen Eigenschaften von molekularen Schaltern wurde bewiesen: Gespanntere Difurylcyclobutene zeigten im Vergleich zu den cyclopenten- basierten Schaltern groessere Quantenausbeuten fuer die Ringoeffnungsreaktion.

• Einkristalle von acht neuen Diarylethenen wurden erfolgreich gezuechtet und mittels Roentgenstrukturanalyse untersucht: Eine ungewöhnliche Cycloalken- Doppelbindungstorsion in einem isoxazol-basierten Schalter wurde beobachtet.

• Vier Difurylethene mit unterschiedlichen Ankergruppen (C) wurden als molekularer Schalter in mechanisch gesteuerten Bruchkontakten (MCBJ) (A) angewandt.

Intoduction

2. INTRODUCTION

2.1. Photochromic molecular switches 2.1.1. Photochromism

Photochromism as a phenomenon is a “reversible change, induced by light irradiation, between two states of a molecule having different absorption spectra”.^1a In other words it is the ability of a compound to change reversibly, on exposure to optical irradiation, its absorption spectrum in the visible region, and also other properties.^1b

A ^h B

h 







_ or

Scheme 1

Photochromism as a reversible transformation between two forms.

Constantly rising interest towards such a phenomenon is caused by the possible access to the controlled manipulation of various physical and physicochemical processes such as membrane transport, optical switching and data storage, etc.1, 26, 101, 123, 124, 125. The possibility of switching between two principally different states of chosen system (conductive/non-conductive, coloured/colourless, paramagnetic/diamagnetic, geometry change or alternating self-organisation in colloidal medium) opens many pathways to various interdisciplinary applications (drug transport, catalysis, cell microscopy and more). For instance, a significant example of data storage application was the Bible issue in 1971, whereas 1245 pages were put on a 6 cm² area.² Photochromic molecules were known for over a hundred years: yellowish dinitroethane potassium salt crystals turn red in the daylight³; tetracene solution was orange upon storage in a dark place and bleached when its solution was exposed to the sunshine.⁴

orange colourless

h

r.t.

Intoduction

The term “photochromism” was proposed by Hirshberg⁵ in 1950 (from Greek phos (light) and chroma (colour)). According to IUPAC⁶, photochromism is a reversible transformation between two forms with different absorption spectra which occurs in one or both directions via absorption of electromagnetic irradiation. The thermodynamically more stable form transforms in its isomer by irradiation; the reverse process occurs thermally (T-type photochromism) or photochemically (P-type photochromism). Most of the organic photochromic systems exhibit positive photochromism (the initially colourless or yellowish form deepens in colour (e. g. red, blue)), whereas systems which show (for example) the relaxation via photocycloaddition (some spiropyranes, anthracene-like systems) and exist in a coloured form in the darkness exhibit negative photochromism.

Here is a brief overview of "chromisms" (e. g., most commonly observed colour change processes in organic molecules) beared from H. Duerr’s “Organische Photochromie”.⁶

Heliochromism refers to those photochromic compounds possessing a high efficiency for colouring with near-ultraviolet radiation and a low efficiency for bleaching with visible radiation, but a moderate efficiency for thermal fading at ambient temperature.⁷

Electrochromism is a reversible change of the absorption spectra induced by electrochemical reaction.⁸

Thermochromism describes a thermally induced reversible change in colour. 9,9- Bixanthylidene is colourless at the liquid nitrogen temperature, green-yellow at room temperature and blue when molten (318-320°C).

O O 9,9-Bixanthylidene

Piezochromism is the distinctive change in colour of the crystal by applying additional pressure; the induced colour returns to the initial one after storage in the darkness or when the damaged crystal is dissolved in some organic solvent, as in case of diflavylenes.^{9, 10} To the contrary, if the colour does not bleach neither in the darkness nor when dissolved in organic solvent, tribochromism occurs (non-damaged crystal exists in the metastable state).¹¹

Intoduction

O O Diphenylflavylene

O O

O

Tribochromic compound

Solvatochromism describes solvent influence on reversible changes in absorption and emission of a chemical substance.¹² Some betaine dyes (example below) exhibit halosolvatochromism¹³, that is, the chromophore remains unchanged whilst the ionic strength of the medium influenses the colour change.

N

O

2,6-Diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenoxide

Gated photochromism. A very interesting type of photochromism occurs when one or both forms of the photochromic system can be reversibly transformed into a non-photochromic state (it reminds of opening/closing of a gate). Therefore, "opening" the gate is possible by means of external stimuli such as protonation, redox processes, solvation effects or temperature changes. Scheme 2 illustrates this: conrotatory photocyclisation of diarylethene is possible only from the antiparallel conformation; therefore, no cyclisation in cyclohexane occurs (the non- reactive parallel conformation is stabilized by hydrogen bonds). Indeed, upon heating up to 100 °C or adding some ethanol the photochromic reaction proceeds.¹⁴

Dual-mode photochromism occurs in complex systems and can be triggered alternatively by two different external stimuli, such as light and electric current.¹⁵

Among the whole variety of photochromic molecules one can distinguish several types (also called classes or families) of major importance for the applied sciences; they are given in the pictures below. Detailed information can be found in the selected publications given in brackets after the class name.

Intoduction

S S

CF₂ F₂ F₂C C

COOH HOOC

S S

CF2

F₂ F2C C

HOOC COOH

F₂C CF2

F₂ C

S S

O O

O O

H

H

antiparallel open f orm closed form (coloured)

parallel open form cyclohexane

ethanol or heat

Scheme 2

Gated photochromism of photochromic bis(benzthiazolyl)hexafluorocyclopentene derivative.

S S

CF₂ F₂ F₂C C

OH HO

S S

CF₂ F₂ F₂C C

OH HO

S S

CF₂ F₂ F₂C C

O O

UV Vis

+2e^- +2H⁺

-2e^- -2H⁺

Vis -2e^-

-2H⁺

Scheme 3

Potential molecular electro-optical dual-mode switch based on dithienylperfluorocyclopentene.¹⁶

Intoduction

Spiropyrans¹⁷ (C. Beyer, H.-A. Wagenknecht J. Org. Chem., 2010, 75 (8), 2752–2755)

N O NO2 N

O

NO₂ h₁

h₂or

closed form open form (merocyanine)

Spirooxazines¹⁸ (G. Berkovic, V. Krongauz and V. Weiss. Chem. Rev. 2000, 100, 1741-1753)

N O N

N N

O h₁

h₂or

closed form open form

Chromenes¹⁹ (A. Yassar, H. Jaafari, N. Rebière-Galy, M. Frigoli, C. Moustrou, A. Samat and R.

Guglielmetti 2002 Eur. Phys. J. Appl. Phys., 18, 3-8)

h₁ h₂or

closed form open form

O

O

Fulgides^{20, 21} (a)Yokoyama, Y., Chem. Rev., 2000, 100(5), 1717-1740; b)“Fulgides and Related Systems”:

H. G. Heller in Handbook of Organic Photochemistry and Photobiology, Part I (Hrsg.: W. M. Horspool, P.-S. Song), CRC, Boca Raton, FL, 1995, Kap. 13)

UV Vis

O O

O

O O

O

O

O

Intoduction

Spirodihydroindolizines²² (H. Duerr, C. Kranz, Mol. Cryst. Liq. Cryst., 1994, 246, 135-138)

N N

CN CN

CN NC N

N h₁

h₂or

closed form open f orm

Dihydroazulenes²³ (H. Goerner, C. Fischer, S. Gierisch, J. Daub, J. Phys. Chem., 1993, 97(16), 4110–17)

NC CN

CN NC

h₁ h₂or

open form closed form

Dihydropyrenes²⁴ (R. H. Mitchell et al, J. Am. Chem. Soc., 2003, 125 (10), 2974–2988)

UV Vis

open form closed form

Azocompounds²⁵ (Gorostiza P., Isacoff E. Science, 2008, 322(5900), 395-399)

N N

h₁ N N h₂or

t rans-f orm cis-form

Diarylethenes²⁶ (Diarylethenes for Memories and Switches, M. Irie, Chem. Rev., 2000, 100, 1685-1716)

N

N N N

UV Vis

open form closed form

Intoduction

Polycyclic aromatic compounds

O

O

O

O O

O h₁, O₂

h₂

h₁ h₂or

Aniles²⁷ (F. Wang, L. Qin und M. Fan, Res. Chem. Intermediat., 1993, 19 (4), 299-306)

OH N

O NH

O HN

h h





Polycyclic quinones²⁸ (N. Sokoluk et al, US5945252A, 1999)

O

O O O

O

O O

O h₁

h₂or

Perimidinespirocyclohexadienones²⁹ (Minkin, V., Komissarov, V. and Kharlanov, V., ChemInform Abstract: Perimidinespirocyclohexadienones. 2000, ChemInform, 31: no. doi: 10.1002/chin.200030282)

NH NR

O

NHR

O h₁ N

h₂or

Intoduction

Viologens³⁰ (T. Kuwabara, M. Sugiyama, M. Nanasawa, Photochem. Photobiol. 2001, 73(5), 469-472)

N

R N R R N N R

M, h

M⁺, Triarylmethanes

CN

NR₂

R₂N R₂N NR₂

CN^- h



Of course, there is a constant search on other classes of organic compounds with photochromic properties. Herewithin, four most widely used families, e. g. azobenzenes, diarylethenes, spiropyrans and furylfulgides will be described to get a comparative overview of their properties and applications.

2.1.2. Azobenzenes

Azobenzenes are the molecules containing two phenyl rings separated by an azo (-N=N-) group about which a reversible isomerisation between cis (Z) and trans (E) geometrical isomers is possible.

N N

N N h₁

h₂or

t rans-f orm cis-form

A) Switching mechanism

The isomerisation of azobenzene proceeds smoothly, it is completely reversible and free from side reactions, thus one of the cleanest photoreactions known.³¹ The thermodynamically by approximately 50 kJ/mol more stable³² trans form is planar, whereas the cis-isomer of azobenzene assumes a geometry with the phenyl rings twisted at right angles to the C-N=N-C plane.³³ The mechanism of azobenzene isomerisation was unclear^34(a-c) and was supposed to be a superposition of two competing mechanisms depending on the spectral class of the chromophore

Intoduction

and the local environment, a rotation about the N-N bond with disruption of the double bond, or inversion with a semi-linear transition state. Recently P. Tegeder et al^34(d) investigated the azobenzene switching mechanism at Au (111) in detail. An alternative explanation of the isomerisation process involving the bending of both CNN bond angles simultaneouly was given recently by Diau.³⁵ The barrier to photoisomerisation is approximately 200 kJ/mol.

As the Z-isomer is thermodynamically unfavourable and in most cases by UV-irradiation only a photostationary state with mixtures of the E- and Z-isomer is obtained³⁶, Dreiding and co- workers synthesized cyclotrisazobenzene in an overall yield of 2.6%³⁷ expecting, therefore, an extension of the lifetime of the unstable Z-isomer by incorporation of the switching unit in a strained macrocyclic structure.

N N N N

N N

cyclotrisazobenzene

Unfortunately, neither irradiation at different wavelengths nor the flash photolysis triggered the switching of all-Z-"cyclotrimeric" azobenzene.³⁶

B) Applications

The most fascinating feature of the azobenzene moiety is the controllable shape change which can be linked to macro-scale changes in properties of various materials; for example, disperse dyes in textile industry.³⁸ As it can be seen on Scheme 4, irradiation induces a significant geometry change in the molecule of azobenzene.

N N

N N R

R

R

R h₁

h₂

9,0 A° _{5,5 A}°

Scheme 4

Intoduction

Even mimicking of a well-known "bulk" object as scissors is possible on the molecular level using the azobenzene-linked ferrocene derivative (Scheme 5).

N N Fe

N N

R Fe

h₁ h₂

Scheme 5

Light-driven molecular scissors.⁴⁰

Other applications, such as control of physical properties of polymers and surfaces, photoregulation of biochemical activity of peptides (photocontrol over α-helix and β-sheet structures) ⁴¹, enzyme activity modulation⁴² are reviewed elsewhere.^{43, 44}

2.1.3. Spiropyrans and related compounds

Spiropyrans (discovered as photochromic compounds in 1952⁴⁵ and closely related to them spirooxazines (X=N))

N O X

NO₂ N

X

NO₂ h₁

h₂or

closed form open form (merocyanine) O

X=CH, N

undergo reversible photochemical cleavage of the CO bond, allowing the interconversion between the open (merocyanine) form with absorption bands within the visible range and the

Intoduction

closed (spyro) form. A variety of applications of this phenomenon includes plastic photochromic lenses technology⁴⁶, polyamide fabrics dyeing (Ref. [47] and references therein) and fluorescence switching as shown in Scheme 6.

N O NO₂ N

O

NO2

h₁ h₂ O

HN

HN O

O HN

HN O

fluorophore fluorophore

no FRET FRET

emission

excitation excitation no emission

Scheme 6

Photoswitching of fluorophores by spiropyrans: fluorescence resonance energy transfer (FRET)

"quenches" the fluorescence upon switching the spiropyrane to its merocyanine form.⁴⁸

2.1.4. Diarylethenes

Diarylethenes belong to thermally irreversible photochromic compounds and posess a very important feature for their application in various technologies: fatigue resistance. It means that the switching cycle with consequent UV/Vis irradiation can be repeated several times without notable side-reactions or formation of by-products:

X X X X

R R R R

UV VIS

open closed

Selected properties of diarylethenes will be discussed further, whereas a general overview of these photochromic compounds is given in references [26, 49, 50, 51].

A) Switching mechanism

The photochemical cyclization of diarylethenes can be described as electrocyclic cyclization of the 1,3,5-hexatriene moiety into cyclohexadi-1,3-ene involving 6 -electrons. In

Intoduction

accordance with Woodward-Hoffmann rules⁵², it can proceed only in conrotatory manner (Scheme 7).

conrotatory

photochemically allowed

disrotatory

C_2v C_2h

h₁ h₂

Scheme 7

Photoisomerisation 1,3,5-hexatriene/cyclohexadi-1,3-ene

h₁ h₂

C C

O O R

C C

O O

R R R

C C

O O

R R

C C

O O

R R

"antiparallel" "parallel"

C_s closed form

Scheme 8

Possible conformers of open form of difurylethene (C2- and Cs-symmetrical). Only "antiparallel"

isomer can be "switched by irradiation: reacting centres (marked with "C") are close enough to interact.

It is only possible when the reacting carbon atoms of neighbouring aryl rings of diarylethene occur in a favourable position towards each other, e. g. close enough to interact;

therefore, the flexible open form of diarylethene can undergo a ring closure from the

Intoduction

"antiparallel" conformation (Scheme 8). Other possible conformers existing at equilibrium cannot react to the closed form of diarylethene.

Theoretical investigations on the switching mechanism⁵³ include evidence of adiabatic channels in photoisomerization of cis-diarylethenes⁵⁴, femtosecond reaction dynamics of the ring closure^{55, 56}, reversibility of photoswitching in diarylethenes attached to the gold surface⁵⁷, discussion on energy levels tuning⁵⁸, and multiphoton gating of cycloreversion reactions.⁵⁹

B) Absorption spectra

In the UV/Vis spectrum, the open form of a diarylethene usually exhibits a narrow band at approx. 250-330 nm; the closed form is characterized by additional absorption band (broad) at longer wavelengths region (450 nm - 850 nm) depending on substituents on the aryl groups.

3 0 0 4 0 0 5 0 0

0 ,0 5 ,0 x1 0³ 1 ,0 x1 0⁴ 1 ,5 x1 0⁴ 2 ,0 x1 0⁴ 2 ,5 x1 0⁴

, n m

 / cm2 mol-1

o p e n fo rm c lo s e d fo rm

Figure 1

Schematic representation of UV/Vis spectra of open and closed forms of a diarylethene.

Upon irradiation with UV light diarylethenes undergo a photocyclization; the corresponding dynamic development in the UV/Vis spectrum is shown below (Fig. 2).

Theoretical studies on absorption spectra of diarylethenes, dealing with their nonlinear optical properties, excitation energies, band gaps and so forth are presented by P. Patel and A.

Masunov⁶⁰; NMR, Raman, IR and ESR spectroscopic data of diarylethenes with theoretical

Intoduction

-0,1 0,1 0,3 0,5 0,7 0,9 1,1 1,3

190 290 390 490 590 690 790

Wavelength, nm

Absorption

0 s 60 s 120 s 180 s 240 s 300 s 360 s 420 s 540 s 660 s 840 s 1140 s 1440 s 1740 s 2040 s 2400 s 2760 s 3120 s 3480 s

Figure 2

Spectrokinetic series for irradiation of diarylperfluorocyclopentene in MeOH at 313 nm. Time intervals of irradiation are given in the figure. Concentration c₀ = 2.12*10^-5 M, photon irradiance I₀ =

2.23*10^-8 Einstein*cm^-2*s^-1.

C) Stability, temperature dependence and fatigue resistance

In general, thermal stability of the closed form of diarylethene depends strongly on the aryl part of the molecular switch and, in particular, on its substitution pattern, e. g. aromatic stabilization energy of the aryl fragment allows conrotatory cycloreversion thus making the closed form of diarylethene thermally unstable.⁶² Diarylethenes which consist of furan, thiophene, selenophene or thiazole have thermally stable closed isomers and do not "open" even at 80°C, whereas pyrrole, phenyl or indole derivatives having high aromatic stabilization energies are thermally unstable in the closed form.^{63, 64} Dulic, Feringa et al. showed⁶⁵ a significant temperature dependence of the ring opening and observed suppression of photochemistry below 130°C in dithienylethene derivatives. Kitagawa, Sasaki and Kobatake⁶⁶ systematized the influence of different structural properties of diarylethene fragments on the stability against the thermal cycloreversion reaction; aromatic stabilization energy of the aryl

Intoduction

groups, electron-withdrawing substituents on the aryl group and steric hindrance of the substituents at the reactive carbon atoms (marked with "C") were taken into account:

O O N

H N

NC H CN

 

stable 32 min at 293K 1.5 min at 293K aromatic stabilization energy of the aryl group

S S



stable 573 min at 333K 247 min at 333K electron/withdrawing substituents

CF₂ F₂ F₂CC

S S

OHC CHO

CF2

F₂ F2CC

S S

CF₂ F₂ F₂CC

S S

CF₂ F₂ F₂C C

N N NC CN NC CN

3.3 min at 333K





S R S R

stable 23 days at 373K

R=CH₃

40 h at 373K R=CH₂CH₃

0.33 h at 373K R=CH(CH₃)₂ steric hindrance of the subtituents

CF₂ F₂ F₂C C



S R S R

CF₂ F₂ F₂C C

S R S R

CF₂ F₂ F₂CC



Scheme 9

Thermal stability (halflife time) of diarylethene closed-ring isomers (adapted from ref. [66]).

Stability and fatigue of photochemical "open-close" reactions of diarylethenes is influenced by side processes, especially photodegradation and formation of by-products (Scheme 10).

Intoduction

S

S S S

S S

UV Vis

open closed

byproduct 1 CF2

F2

F2C C CF₂

F2

F₂C C

CF2

F₂ F2C C

O O O O O O O O

O O O O

S S F2

F₂C C

O O

O O

byproduct 2 F UV

UV -HF

Scheme 10

Photochromic reaction of thien-1,1-dioxide-3-yl-based diarylethene with formation of two by- products (adapted from Ref. [68]).

S

S S S

S S

S S

S S

S S

UV Vis

R R R R

R R

R R R R

R R

open closed

diradical

byproduct

R=H, CH₃

Scheme 11

Byproduct formation in diarylethenes (proposed mechanism; adapted from Ref. [71]).

Intoduction

Mechanism of formation of by-product 1 was theoretically investigated by P. Patel et al⁶⁹; experimental data was reported by Irie and co-workers⁷⁰, and it is the closed from of diarylethene, from which the by-product is formed (the distinctly higher fatigue resistance of methyl-substituted diarylethene (R=CH3) can apparently be explained by steric repulsion between methyl group and its neighbouring CF2-fragment) (Scheme 11).

When designing a specified diarylethene molecule for applied purposes, possible increasing of fatigue resistance by introducing a bulky substituent (in the position marked with R on Scheme 11) should be taken into account!

D) Quantum yields

Usage of molecular memory devices, data storage materials, or industrially important dyes based on photochromic molecules requires large quantum yields of photochemical conversion between open and closed forms. It is supposed that the diarylethene molecule in solution exists predominantly in two conformations, "parallel" (reactive) and "antiparallel" (non- reactive), both in constant equilibrium with each other. Therefore, the "efficiency" of photochemical reaction should depend on the ratio between these conformers. For most frequently used diarylethens with general structure

X

R X R

X=S, O, NR

"parallel" and "antiparallel" conformers exist in a ratio 1/1 due to the free rotation at the C-C bond linking the aryl group with cycloalkene, thus limiting the highest value of quantum yield by 0.5 (it means that every absorbed quantum efficiently switched the reactive conformer to the closed form). If one of the conformations is preferred (hindered rotation caused by bulky substituents at the reactive carbon atoms or by enlarging the central ring size, intramolecular interaction of substituents via hydrogen bonds, etc.), the quantum yield can be modulated, also decreased (if diarylethene is forced to stay in the non-reactive conformation, for example, when put into a narrow cyclodextrine cavity.⁷²

Also, electron-withdrawing substituents at -positions of the aryl groups affect the cycloreversion quantum yield and make the photochemical reaction of diarylethene thermally reversible.⁷³

Intoduction

E) Reversible switching of properties of molecules and materials

Diarylethenes can be switched between two geometrically and electronically different states by irradiation with light. This feature can be transferred to other objects on molecular and macroscopic levels.

S S S S

S S

*

*

S * * S UV

VIs

UV VIs

R,R-isomer

S,S-isomer P-helix isomer

M-helix isomer

Scheme 12

Diarylethene ring closure: stereochemical considerations.

S S

CuOTf O

N N

O

Bn Bn

S S

O

N N

O

Bn Bn

S S O

N N O Bn Bn

Cu^{+ +}Cu

S S

O

N N

O

Bn Bn

S S O

N N O Bn Bn

Cu^{+ +}Cu

S S

O

N N

O

Bn Bn

NH₄OH

313 nm

Scheme 13

Diastereoselective ring-closure reaction of chiral diarylethene mediated by Cu⁺.⁷⁴

Intoduction

The ring closure reaction of diarylethene molecule produces two chiral centers on the reactive carbon atoms.

In general case, two stereoisomers in 1/1 ratio are formed. However, N. Branda and co- workers managed⁷⁴ to obtain a single diastereoisomer of the closed form of diarylethene by attaching the chiral dihydroisoxazoline ligand to the aryl groups (Scheme 13) and forced diastereoselective switching controlled by metal complexation at the ligands.

Achiral liquid crystals obtain tunable transmission and reflection when doped with dithienylcyclopentene switch with axial chirality.⁷⁵

S S

O O

C₈H₁₇O O O OC₈H₁₇

6 6

Scheme 14

Diarylethene with high helical twisting power in self-organizing liquid crystals (adapted from Ref. [75]).

Diarylethenes can be used in ion sensing techniques.⁷⁶ Substituted with crown ethers⁷⁷, they exhibit strong coordinating effect in photochemically inactive "antiparallel" conformation.

S S

CF₂ F₂ F₂CC

UV Vis O

O O

O O O O

O O O

S S

CF₂ F₂ F₂CC O

O O

O O O O

O O O

S S

CF₂ F₂ F₂C C

O O O O O

O O

O O O

Rb⁺ Rb⁺

S S

CF₂ F₂ F₂C C

O O O O O

O O

O O O

Scheme 15

Stabilization of photochemically inactive form of diarylethene by complexation.

Intoduction

Areephong, Feringa et al.⁷⁸ showed that the conductive polymers can be doped with diarylethenes thus making electropolymerizability switchable.

S S

CF₂ F₂ F₂CC

UV Vis

S S

S S

S S

CF₂ F₂ F₂CC

S S

S S

S S

CF₂ F₂ F₂CC

S S

S S S S

CF₂ F₂ F₂CC

S S

S S

S S

S S

S S CF₂

F2

C F₂C

n open state

open state²⁺

closed state

closed state²⁺



slow

-2e^- +2e^- -2e^-

+2e^-

-2H⁺

Scheme 16

Electropolymerizability switching (adapted from Ref. [78]).

An interesting way of Lewis acidity modulation was introduced by N. Branda, V.

Lemieux and co-workers: a phenylboronic ester was incorporated in a diarylethene switch bridging unit, and its acidity was successfully switched between "neutral" and acidic states.

S S

B O O

R

R S S

B O O

R R

UV Vis Lewis "neutral"

4n+2-electrons

Lewis acidic

Scheme 17

More pronounced aromatic character of the dioxaborole ring in the open form of diarylethene then of that in the closed form causes decrease of boron atom's Lewis acidity (adapted from Ref. [79]).

Intoduction

Fluorescence switching also can be achieved by applying fluorophore-modified diarylethenes.^80-88

An interesting example of potential application of diarylethenes gave V. Lemieux and N.

Branda⁸⁹: a specially constructed dithienylcyclohexadiene with reactivity-gated photochromism can be used as a sensor for toxic dienophiles (Lewisite (2-chlorovinyldichloroarsine), 1,3- dichloropropene etc.)

S

S S S

dienophile

S S

UV Vis

Scheme 18

Diarylethene-based "trap" for toxic compounds bearing a dienophilic character.

2.2. Diarylethenes: Overview of synthetic approaches

The diarylethene moiety as a valuable photochromic fragment can be generally synthesized via carbon-carbon bond formation between three already available rings A, B and C properly substituted with reactive groups or atoms (RG), such as boronic acid derivatives, alkylzinc and alkyltin substituents, or halogen atoms (Scheme 19).

S O

RG RG

RG RG A

B

C S O

Scheme 19

Schematic representation of (A+B+C)-diarylethene construction.

An alternative synthetic method concerns attaching of fragment B via various ring- closure reactions to the previously prepared A-C conjugate (Scheme 20).

Intoduction

Of course, organic chemistry and, especially, its organometallic area offer more procedures and pathways to construct a selected photochromic molecule. However, two of them are most frequently used in the common laboratory practice.

S O

A C S O

N N O

O O

B

KOH

Scheme 20

Example of (AC+B)-diarylethene construction

2.2.1. BuLi/perfluorocycloalkene method

The affinity of a fluorine atom attached to the sp²-hybridized carbon atom towards nucleophilic substitution⁹⁰ was successfully used already in 1993 by J.-M. Lehn for obtaining various substituted photochromic diarylethenes⁹¹ (Scheme 21).

S S R

Br Li

R

S S

CF₂ F₂ F₂C C

R R

F₂C F2C CF2

F F

-LiF BuLi

Scheme 21

(A+B+C)-formation of substituted photochromic dithienylethene

Of course, the "tricomponent" reaction with lithiated aryls and perfluorocyclopentene can be proceeded stepwise in order to obtain nonsymmetrical species; however, the problem of parallel formation of mono- and disubstituted cycloalkenes requires additional separation procedures. In order to simplify the obtaining of nonsymmetrical (e. g., A≠C) diarylethenes, a microflow system was introduced by Ushiogi and co-workers.⁹²

Intoduction

As almost every synthetic method (probably except OH^- + H⁺ = H2O reaction), also BuLi/perfluorocyclopentene approach has its disadvantages due to several reasons: the expensive and highly volatile starting material perfluorocyclopentene itself, and several possible side- reactions which decrease the overall yield of the diarylethene. Indeed, not only the substitution of olefine-bound fluorine atoms is possible, but also of those in the aliphatic part (CF2) of the central cycloalkene ring of the switching molecule (Scheme 22).

F₂C CF₂ F₂ C

F F

F₂C CF₂ F2

C

F R

F₂C F₂ C

R F

F₂C CF₂ F2

C

R R

R ArLi

phosphonium methylide

and/or

Scheme 22

Reaction of perfluorocyclopentene with nucleophiles (from Ref. [93]).

Moreover, even in already formed diarylperfluorocyclopentene there is a possibility of further undesirable substitution of fluorine atoms of the CF2 groups (which will be discussed in the "Results"-section).

To deal with aforementioned unconveniences, Hiroto and co-workers proposed a synthetic protocol for diarylperfluorocyclopentenes through Suzuki-Miyaura coupling using a 1,2-dichloro- instead of 1,2-difluorocyclopentene (Scheme 23) in 2011.

F₂C CF₂ F₂ C

F F

F₂C CF₂ F₂ C

Ar Ar

F₂C CF₂ F2

C

Cl Cl ArLi

-78^C

Ar-Bpin or Ar-B(OH)₂ (storable)

less volatile

Suzuki-Miyaura coupling catalytic

non-cryogenic Scheme 23

Intoduction

Advantages are higher reaction temperature (compared to -78°C of the conventional method), less expensive and easy-to-handle dichloro building block, and, definitely, more stable boronic acid derivatives as educts (compared to generated in situ lithiated intermediates in the conventional method).

2.2.2. McMurry pathway

As an alternative to the perfluorocyclopentene-based switches (with good photochemical properties but hardly modifiable at the cycloalkene unit) two technically simplier and potentially more diverse procedures were thoroughly investigated by the groups of Feringa and Fan.^{95, 96, 97} New fluorine-free diarylethenes were obtained and later a compatitive study with analogous perfluorocyclopentenes was held.⁹⁸ Of course, "conventional" perfluorinated derivatives were also available using McMurry reaction with TiCl4/Zn mixture for a key step of C=C-bond formation in cyclopentene ring closure.⁹⁹

Cl S

Li

MeOOC(CF₂)₃COOMe S Cl

S Cl O O

CF₂ C F₂ F₂ C

S S

CF₂ F₂ F₂C C

Cl Cl

low-valent titanium

Scheme 24

A new synthetic route to symmetrical photochromic diarylperfluorocyclopentenes (McMurry).

Cl S

Cl(O)C(CH₂)₃C(O)Cl

S Cl

S Cl O O

S

Cl S Cl

TiCl₃(THF)_3,Zn

Scheme 25

Friedel-Crafts/McMurry procedures applied to diarylcyclopentene synthesis.

Intoduction

2.2.3. Other methods

M. Krayushkin and co-workers developed a pathway to a variety of diarylethenes with different central "ethene" units; Friedel-Crafts-reaction was often used to connect the aryl fragment with modifiable substituent, and a large family of alternatively "bridged" diarylethenes was prepared and investigated^{100, 101} (Scheme 26).

Ar

Ar

=

RN

O

O O

O

O O O

O

O R

O O

O

S N

Ph R

N O

N Ar N

HN NH

O O

S

O O

Cl Cl

AlCl₃/Py

S S

O O

S S

O O O

S S RN

O O

H2O2 RNH₂

S S S

S S

RC(O)NHNH₂ Ac₂O

SnCl₄

S O

S

O O HO O

S S

O O N

N N

R

SeO₂ SnCl₄ CuSO₄

Py

Scheme 26

Friedel-Crafts-reaction as a synthetic route to diarylethenes.

Intoduction

2.3. Conclusions

Diarylethenes are very interesting objects for applied sciences because of their prominent ability to be switched between two geometrically and electronically different states by irradiation with UV/Vis light. Data encoding, readout and storage, photosensitive polymeric films, ophthalmic lenses are possible applications. Understanding of switching mechanism and prediction of correlation of photophysical and photochemical properties of diarylethenes with modulations in their structure is therefore very important.

Furan-based diarylethenes were chosen for investigation of structure-to-property correlation in this thesis. The objectives were as follows:

 development of synthetic approach towards substituted difurylcycloalkenes;

 investigation of the switching process between open and closed forms and correlation with spectroscopic data;

 predicted modification of photochemical properties of diarylethenes;

 synthesis of molecular switches for further application in mechanically controlled break- junctions.

Design, Development and Investigation of Thiophene-Free Diarylethenes

3. DESIGN, DEVELOPMENT AND INVESTIGATION OF THIOPHENE- FREE DIARYLETHENES

Dithienylethenes are the most well-known class of diarylethenes; the majority of scientific publications is dedicated to thiophene-based molecular switches because of their synthetic availability and excellent photochemical properties (thermally irreversible photoswitching, stability, fatigue resistance). Nevertheless, there are specific areas of application of photochromic compounds where sulphur-containing fragments (with sp²-hybridized sulphur, as heteroatom in thiophene or methylthio group SCH3) are doubtful due to the possible non-specific interaction with gold electrodes in MCBJ experiments.

Furan with its oxygen atom does not show any affinity to gold atoms (compared to thiophene with sulphur atom) similarly to isoxazole- and pyrazole-based heterocyclic aryl fragments. The research efforts of this thesis were thus focused on the design of a sulphur-free, stable and switchable diarylethene "core unit" equipped with chemically active substituents, thus allowing further modification of the molecular switch according to the practical requirements.

3.1. Perfuorocyclopentene-based switches (C5F)

While thiophene is rather similar compared to benzene in its chemical properties (higher degree of aromatic stabilization, electrophilic substitution as common reaction example), furan being an aromatic compound still exhibiting a diene-like character and, moreover, being labile against mineral acids (also addition and ring-opening reactions are favoured). That is why general approaches and synthetic procedures suitable for constructing of thiophene-, pyrrole-, and thiazole-based diarylethenes often appear invalid for their furan-based analogues.

3.1.1. Furan derivatives

Abovementioned specific features of furan derivatives provide restrictions on the synthetic pathways to difurylethenes. Another important detail is the necessity of modifiable substituents in the switching unit for further construction of desired photochromic objects. The "ideal" starting fragment must look like

Design, Development and Investigation of Thiophene-Free Diarylethenes

O (1)CH₃

H₃C(2)

R

(3)

with its methyl group (1) (small enough not to enhance cycloreversion) to avoid the dehydrogenization followed by aromatization to the inactive furobenzofuran derivative; methyl group (2) for steric hindrance caused by its repulsion with cycloalkene fragments (to avoid the by-product formation (for details see 1.1.4.C); substituent R for further modification (if absent, the reactive -hydrogen atom on its place will be attacked, for example, by BuLi required for later steps, or by electrophile in case of Friedel-Crafts approach), and a vacant position (3) either for bromination with subsequent metallation and coupling with perfluorocyclopentene, or for acylation/alkylation in case of the "McMurry"

synthetic route.

O H O H

O H O H

R R R R

UV

VIS -H2

O O

R R

Scheme 27

Irreversible oxidative transformation of diarylethene lacking substituents at the reactive carbon atoms.

One promising starting compound was 2,4-dimethylfuran, which should be further transformed according to Scheme 28 using the different activity of two bromine atoms towards substitution.

O Br O

Br

O O

R R

CF2

F₂ F2C C Br₂

Scheme 28

Unfortunately, all attempts to brominate the 2,4-dimethylfuran ended up with spontaneous polymerization of the reactant, most likely caused by protonation of the furan ring and ring-opening to dicarbonyl compound. A significant role in rejecting this compound played also its poor availability (the practically useful procedure in ref. [102] unfortunately did not allow upscaling from milligramm to gramm quantities).