The Syntheses and Properties of Photochromic Systems based
on Dimethyldihydropyrenes
by
Subhaj
it Bandyopadhyay
B.Sc. (Hons.), University of Burdwan, India, 1995 M.Sc. Indian Institute of Technology, India, 1998
A Dissertation Submitted in Partial Fulfillment of the
Requirements for the Degree of
DOCTOR OF PHILOSOPHY
in the Department of Chemistry
O Subhajit Bandyopadhyay, 2004
University of Victoria
All
rights reserved. This dissertation may not be reproduced in whole or in part, by photocopy or other means, without the permission of the author.Supervisor: Dr. R. H. Mitchell
ABSTRACT
This thesis explores substituent effects on the photochromism of the benzo[e]dimethyldihyropyrene system 3. Thus systems with conjugated connectors, (25,
29, 33, 34), a system with conjugation and then insulating connector (31), one with an
organometallic connector (32), and a system functionalized on the benzene ring (35), were synthesized and their photochemical studies were carried out. Comparisons of the photo opening and closing rates relative to benzo[e]pyrene 3 under similar conditions were performed. All the derivatives of 3 synthesized (except for the ferrocenyl product 32) had faster visible opening rates than the parent 3; the UV closing rates for most of the derivatives were are almost the same as that of the parent 3.
Several linked photochromes having two photochromic units have been successfully synthesized from their monomeric precursors using metal mediated coupling reactions. Both photochromic units of the bis-switches, 41 and 54 open and close, although it is not
possible to address each photochrome independently. UV closing of the open-open forms of the bis-photochromes occur via the open-closed intermediate. Thus 41 and 54 act as multiple state switches. In the case of 49 only one photochromic unit of the bis- photochrome opened. However, none of the photochromic units in 38 opened.
The thermal return reactions (cyclophanediene to dihydropyrene) were studied on the benzo[e]dihydropyrene derivatives and linked bis-photochromic systems and the energy of activation of the thermal reversal and the half-lives were determined to be similar or
1-
Table of Contents
Abstract
Table of Contents List of Tables List of Figures
List of Numbered Compounds List of Abbreviations Acknowledgements Dedication Chapter 1: Introduction 1.1. Molecular switches 1.2. Molecular photoswitches
1.3. Types of photochromic reactions 1.3.1.
Z,
E-Isomerization1.3.2. Tautomerism
1.3.3. Homolytic bond cleavage 1.3.4. Heterolytic bond cleavage 1.3.5. Redox photochromism
.
. 11 iv...
Vlll ix xii xvi xvii xix1.3.6. Pericyclic reactions a. Cycloaddition Reactions b. Electrocyclic Reactions
1.3.7. Fulgides, fulgimides and fulgenates 1.3.8. Spiropyrans and spirooxazines
1.3.9. Diarylethenes and their thermal reversal 1.3.10. 1,2-Dithienylethenes
1.3.1 1. Dihydropyrenes 1.4. Research objectives
Chapter 2 : Synthesis
2.1. Background: Synthesis of dihydropyrenes 3 1
2.2. Synthesis of substituted dihydropyrenes for photochromic
property studies 3 4
2.2.1 Synthesis of 4-acetyldihydropyrene 9 3 4
2.2.2 Formylation of substituted benzo[e]dihydropyrenes 3 5
2.2.3 Synthesis of ethynyldihydropyrenes 37
2.2.4 Introduction of a propargyl alcohol group onto dihydropyrenes 39
2.2.5 Synthesis of ferrocenylbenzodihydropyrene 36 42
2.2.6 Synthesis of diphenylbenzo[e]dimethyldihydropyrene 33 43
2.2.7. Synthesis of bis-(bipheny1)benzodihydropyrene 34 4 5
2.2.8. Synthesis of the first side ring functionalized benzo[e]dihydropyrene 35 46
2.3. Syntheses of linked DHP photochromes 4 8
2.3.2. Phenylene spaced systems 2.3.3. Ferrocene spaced systems 2.3.4. Ethynyl spaced systems
Chapter 3: Photochromic studies on functionalized benzo[e]dihydropyrene systems
3.1. The parent benzo[e]dihydropyrene system 3
3.2. Comparative studies of photochromism of substituted mono-benzodihydropyrene systems with 3: General aspects. 3.3. Photochromism of formylbenzodihyropyrene 25
3.4. Photochromism of ethynylbenzodihyropyrene 29 and propargyl alcohol substituted benzodihydropyrene 31
3.5. Photochromism of diphenylbenzodihydropyrene 33 and
bis-(bipheny1)benzo-dihydropyrene 34 3.6. Photochromism of the phenol 35
3.7. Complete shut-down of photochromism
of ferrocenyl-benzodihydropyrene 36
Chapter 4: The photochromic behavior of non-annelated bis-dihydropyrene photochromic systems
4.1 Bis-photochromic systems: A general background 8 6
4.2. Photochromism of the diethynyl spaced bis-hetero switch 54 88
4.3. Photochromism of ethynyl spaced bis-benzo[e]dihydropyrene 42 94 4.4 Photochromic behavior of the phenylene spaced bis-switch 41 100
vii
Chapter 5. Thermal reversal of the CPD forms 113
Chapter 6. Non-destructive read-out of dihydropyrene systems 120
Chapter 7. Conclusion 127
Chapter 8. Experimental and X-ray structures
8.1. Instrumentation 129
8.2. Experimental conditions of X-ray crystallographic studies. 130
8.3. Photoisomerization. 131
8.4. Syntheses 132
8.5. Crystal structures of dimethyldihydropyrene 2 and benzodihydropyrene 3 160
References 169
. . . V l l l List of Tables Table 1.1. Table 1.2. Table 1.3. Table 5.1. Table 5.2. Table 5.3. Table 5.4. Table 5.5. Table 6.1.
Calculation on the thermal reversibility of 1,2-diarylethene systems
Thermal reversal of dihydropyrene systems Quantum yields of some DHP systems Data for the thermal reversal reaction of 3.
Calculation of error for the thermal reversal data
The k (46OC) and 7112 (46 O C ) values obtained directly from
the experiments with non-deareated samples. Thermal reversal rates and half-lives
The 2112 (20•‹C)values of the compounds
List of Figures
Figure 1.1. Sequential change in UV-vis absorption spectra during the
photoisomerization reaction of a photochromic compound. 2
Figure 1.2.a. Frontier molecular orbital representation of the
electrocyclization of a 1,3,5-hexatriene system.
1.2.b. Conrotatory ring closure of metacyclophanediene
to dihydropyrene.
Figure 1.3. Non-radiative decay of the dihydropyrene 2 26
Figure 1.4. Steric repulsion in the cyclophanediene form 2 8
Figure 2.1. Top: 'H NMR in acetonitrile-dj; Bottom: 'H NMR in acetonitrile-d3
+
a drop of methanol-d4.Figure 2.2. Annelated and non-annelated bis-dihydropyrene systems 4 8
Figure 2.3. Diastreomeric forms of the ethynyl spaced bis dihydropyrene 37 50
Figure 3.la. UV-vis spectra of 3(O) and 3(C);
3.lb. The NMR spectra of the internal methyl and the t-butyl
protons of 3(0) (top), and 3(C) (bottom) 66
Figure 3.2. UV-vis spectra of 25 ( 0 ) and 25 (C). 69
Figure 3.3. Visible opening of BDHP 3 (C) and BDHP-CHO 25 (C). 70
Figure 3.4. UV reversal of the open forms of BDHP 3 ( 0 )
and BDHP-CHO 25 ( 0 ) .
Figure 3.5. The UV-vis absorption spectra of open and closed forms
of 29 (top) and 31 (bottom). 72
Figure 3.6. Opening of 3 (C) and 29 (C) with visible light (A > 490 nrn). 73
Figure 3.7. Opening of 3 (C) and 31 (C) with visible light (A > 490 nm). 73
Figure 3.8. The UV reversal of 3(O) and 29 ( 0 ) under identical condition. 74 Figure 3.9. The UV reversal of 3(O) and 31 ( 0 ) under identical condition. 75
Figure 3.10. Figure 3.1 1. Figure 3.12. Figure 3.13. Figure3.14. Figure 3.15. Figure 3.16. Figure 3.17. Figure 3.18. Figure 3.19. Figure 4.1. Figure 4.2. Figure 4.3. Figure 4.4. Figure 4.5. Figure 4.6. Figure 4.7. Figure 4.8.
Sequential UV-vis spectra of the UV closing of CPD 31 ( 0 )
to dihydropyrene 31 (C).
The UV-vis absorption spectra of open and closed forms of 33 (top), and 34 (bottom).
Opening of 3 (C), 33 (C) and 34 (C)
with visible light (h > 490 nm).
UV reversal of 3(0) and 33(0) (cyclohexane),
and 34(0) (toluene).
UV reversal of 34(O) and 3(0).
Absorption spectra of 35 (C) and 35(0).
The 'H NMR spectra of 35 ( 0 ) (top), and 35(C) (bottom).
Opening of 35(C) and 3(C) with visible light (h > 490 nrn).
UV reversal of 35(0) and 3(0).
UV-vis absorption spectrum of 32.
Intramolecular energy transfer in the excited state from open unit to the closed unit of the the bis-photochrome 57.
Opening the benzodihydropyrene photochromic unit of the bis-switch 49
Conversion of 49 (C-C) to 49 (C-0) by visible light,
monitored by NMR spectroscopy.
The internal methyl protons of the DHP unit differentiated the 49 (C-C) and 49 (GO) forms (zoomed in from Figure 4.3).
The UV reversal of 49 (C-0) to 49 (C-C).
Opening of 54 (C-C) to 54 (0-O), monitored by
UV-vis spectroscopy.
The NMR spectra of the internal methyl protons of the successive opening of
54 (C-C)
to54
(0-0).Absorption spectra of the closing of 5 ( 0 - 0 ) with
Figure 4.9. (a) Top: isosbestic point at 305 nm during the early part of UV irradiation of 54 ( 0 - 0 ) .
(b) Bottom: isosbestic point at 293 nm
during later part of UV irradiation, in cyclohexane
Figure 4.10. UV-vis spectra recorded at successive intervals during
the opening of 41 with visible light, 3L >590 nm.
Figure 4.11. A single isosbestic point was not found during the visible
light opening of the bis-photochrome 41 (C-C).
Figure 4.12. The 'H NMR spectra of the internal methyl peaks during
the opening of 41 (C-C) with >590 nrn visible light. Figure 4.13. Relative opening rates of 41 compared to parent 3.
Figure 4.14. Sequential UV-vis absorption spectra of the
UV closing of 41 (0-0).
Figure 4.15. A single isosbestic point was not observed during
the UV reversal of 41 ( 0 - 0 ) to 41 (C-C).
Figure 4.16. Sequential UV reversal of 41 ( 0 - 0 ) monitored by
NMR spectroscopy in a quartz NMR tube.
Figure 5.1. Arrhenius plots for the thermal reversal reaction of benzoDHP derivatives.
Figure 6.1. IR spectra of the closed (top) and open (bottom) forms of 3.
Figure 6.2. IR spectra of closed (top) and open (bottom) forms of 25. Figure 6.3. Oxidation waves of 3(C) and 3(0).
Figure 6.4. Oxidation waves of closed and open forms of 31. Figure 6.5. Repetitions of the switching do not decompose 60.
Figure 8.1. Schematic drawing of the irradiation set-up.
Figure 8.2. Absorption profile of the cut-off filters used for the photo-irradiation experiments.
Figure 8.3. Crystal structure of dihydropyrene 2
Figure 8.4. Crystal structure of dihydropyrene 3
xii
161
. . . X l l l
xvi
xvii List of Abbreviations Ar BDHP br. bs CI CPD 6 d dd dec. DHP or DMDHP DMF EI 0 h HRMS IR LSIMS m
Me
min arenel aryl benzo [e]dimethyldihydropyrene broad broad singlet chemical ionization metacyclophanedienechemical shift in ppm from standard doublet doublet of doublet decomposition dimethyldihydropyrene dimethylformamide electron impact quantum yield hour
high resolution mass spectrum infrared spectrum
liquid secondary ion mass spectrometry multiplet
methyl
xviii mP MS NBS NMR Ph PPm S SD t THF UV-vis melting point mass spectrum N-bromosuccinimide nuclear magnetic resonance phenyl
parts per million second or singlet standard deviation tertiary group tetrahydro furan
xix
Acknowledgement
The author wishes to express his appreciation and sincerest gratitude to his supervisor Professor Reg Mitchell for his excellent guidance, help, suggestions and constant encouragement during the time of this research and writing. The author also would like to thank Professors Cornelia Bohne, Tom Fyles and Dave Berg for their help during the course of this work. Help fiom former group members Tim Ward, Zinka Brkic and
Chapter 1. Introduction
1.1. Molecular switches
Development of nanomolecular systems, as a step towards fabrication of molecular electronic devices involving circuits of the dimension of a few nanometers, is an area of considerable interest to chemists, physicists and engineers. Several molecular components that can serve as different electronic components, such as switches, wires and rectifiers are necessary for the purpose. Compounds having specific functions, such as photochromism, conductivity, magnetism, etc are promising components for those units. Scaling down an electronic component, for example, a memory chip has obvious limitations in assembling and processing of the material. The alternate approach involves a building-up from the molecular scale and is accessible to chemists because of advancements in lithographic techniques1 or molecular processes such as self assembly.' A more important issue in these molecular machines involves addressing the machines by turning the molecular components on and off.
1.2. Molecular photoswitches
Switches are an inseparable part of any electronic system. So will they be in their nanomolecular version. Light driven molecular switches or photoswitches are promising candidates, as they could be used without donation or acceptance of electrons or any external chemicals, which might affect other parts of the molecular system or lead to formation of undesired by-products.2 Photoswitches are compounds that can reversibly undergo a photoisomerization process between two isomers having different absorption spectra ( c o l o ~ r ) . ~ , ~ This phenomenon of reversible photoisomerization is called photochromism and the compounds showing this behavior are called photochromic
compounds. For example, A and B are two photoisomers. When irradiated with light of frequency vl, A photoisomerizes to B. B, on irradiation with a different frequency vz, reverts back to A.
hv2
The irradiation wavelength depends on the absorption spectra of the isomers. An example is shown in Figure 1.1.
Figure 1.1. Sequential change in UV-vis absorption spectra during the photoisomerization reaction of a photochromic compound.
Isomer B, which has considerable absorption at higher wavelength, on irradiation with visible light transforms into isomer A. It can be seen from the absorption spectra at different intervals that with increase in irradiation time at v2 the concentration of B
decreases and the concentration of A increases until all has transformed into A. A, having stronger absorption at lower wavelength, reverts to B on irradiation with light of shorter wavelength. The wavelength at which both isomers have the same absorption (molar extinction coefficient) is called the isosbestic point. The presence of more than one isosbestic point indicates that the photoisomerization is not a single step process and does not go through any long-lived (of the order of the spectrometer timescale) intermediate species. Sometimes, the absorption spectra of A and B overlap for a large domain of wavelengths. This is undesirable because photoirradiation at any wavelength within that domain converts A to B and at the same time B to A. Thus the system reaches a photostationary state within that wavelength domain and clean conversion (1 00%) to either of the isomers does not happen under that condition.
1.3. Types of photochromic reactions
The photochromic systems illustrated in equation 1.1 can be classified into several groups on the basis of a photochemically induced primary step:
2, E-Isomerization
'
TautomerismHomolytic bond cleavage Heterolytic bond cleavage Electron transfer reaction Pericyclic reaction
#
When the double bond of stilbene is excited, a reversible Z to E-isomerization is o b s e r ~ e d . ~
Scheme 1.1. Photoisomerization of stilbene
The isomerization of 2-stilbene is problematic because of its ease of cyclization and
subsequent dehydrogenation to phenanthrene (Scheme 1 . 1 ) . ~ Stilbenes are not found in
modern photoswitch applications.
Azobenzene (pseudo-stilbenes) undergoes a cleaner isomerization (Scheme 1.2), although has a photostationary state with 80% Z at 3 13 nm, and 40% Z at 365 nm.7 Z- Azobenzenes undergo thermal reversal reactions. The thermal reversal is slow for azobenzene itself but is fast for amino substituted azobenzenes. 8
Interesting examples of
Z,
E-isomerization are Feringa's chiroptical switches (Scheme1.3). These switches incur helicity isomerization on irradiation.9710 The concept of
controlling directionality of rotary movement at the molecular level was achieved by
(Thermal
processes)
Scheme 1.3. Four diasteromers
I
j
(rherma,processes)
of Feringa's chiro-optical switch, and their photochemical and thermal interconversions
introduction of a stereogenic center near to the central olefinic bond of a sterically overcrowded alkene. The full rotary cycle comprises four consecutive steps: two photochemical isomerizations each followed by
a
thermal helix inversion.Retinal is an example of a Z to E-photoswitchable system found in nature (Scheme 1.4). In our eyes, 2-retinal is bound as a Schiff s base to a lysine (Lys-312) of the -40 K Dalton protein rhodopsin and absorbs at 570 nm (I,, ) with a quantum yield of -0.6, and isomerizes to the E-isomer." Although in-vivo the reverse reaction is enzyme mediated.
Scheme 1.4. E-, 2-isomerization of retinal 1.3.2. Tautomerism
Light induced prototropic tautomerism can occur in the salicylidene-aniline system (Scheme 1.5). These molecules are photochromic in the crystalline state. They possess some excellent characteristics such as picosecond scale proton transfer,'* and excellent fatigue resistance, comparable to magnetic storage technology (-1 0' cycles).13
Scheme 1.5. Phototropic tautomerism in salicylidene -aniline system
1.3.3. Homolytic bond cleavage
Homolytic bond cleavage of octaphenyl- 1 ,l '-bipyrrolyl generates a colour compound
(Scheme l.6).I4 Hexaphenyl- 1 ,l '-biimidazolyl is another example of this kind of system. These species generate coloured fiee radicals on exposure to light.I5 The photochromism is observed both in the crystalline and solution states.
Scheme 1.6. Photochromism involving homolytic bond dissociation 1.3.4. Heterolytic cleavage
Though some people consider the photochromism of spiropyrans and spirooxazines as examples of heterolytic bond cleavage,'6 strictly speaking, these heterolytic cleavages occur as a result of reverse electrocyclic
reaction^.'^
They are thus discussed in the electrocyclic reaction (Section. 1.3.6b).1.3.5. Redox photochromism
Some commercial photochromic lenses are based on redox or electron transfer photochromism. Stookey and Armistead,
''
while working for Corning in 1959, investigated the photochromic property of borosilicate glasses containing silver halide as the photochromic component. In 1962, Corning filed a US-patent on photochromic ophthalmic ~ ~ e c t a c l e s . ' ~ Silver halide particles, present in the glass, having a particle size of the order of 5 nrn and with a concentration of 500 ppm, decompose on exposure to light to metallic silver (Ago)." The halide ion, X-, which acts as the electron donor is oxidized to X' but is rapidly reduced back to halide ion, X-, by cuprous ion. In the presence of UV-light, the system reaches a photostationary state. In absence of a UV source, the Cu(1I) receives an electron back from the metallic Ag, converting it to Ag(1) and the colour bleaches. The photochromic and bleaching reactions are shown below. Photochromic reactions (in the presence of UV light):X-+hv
+
X'+e A ~ ++
e+
Ago (dark) c u + + x + cu2+ + x -Bleaching reaction (in the absence of the UV source):
A ~ O
+
cu2++
Ag++
CU+1.3.6 Pericyclic reactions
Pericyclic reactions are among the most important processes that allow the design and function of photochromic systems. Pericyclic reactions include a broad range of organic
reactions such as cycloaddition, electrocyclization, sigmatropic rearrangements and cheletropic reactions, provided they are ~oncerted.~' Most of the photochromic systems fall under the first two examples.
a. Cycloaddition reactions
Cycloaddition reactions involve two or more unsaturated bonds where the two or more unsaturated bonds combine with the formation of a cyclic adduct with a net reduction in bond multiplicity. Cycloaddition reactions are represented by the [i
+
j+
. . .] symbolism where the numbers i, j, etc. identify the numbers of electrons in the interacting units that participate in the transformation of the reactant to product. The symbol a or s (a =antarafacial, s = suprafacial) is often added as a subscript after the number to designate the stereochemistry of addition of each of the fragments. A subscript specifying the orbitals, o or 7c or n (in case of an orbital associated with a single atom only) may be
added as a subscript before the number. Thus a normal Diels-Alder reaction is a [,4,
+
,2,] cycloaddition.[2
+
21 Cycloadditions1,l'-Polymethylenebisthymines (n = 2 to 4) undergo [2
+
21 photodimerization whenirradiated at 300 nrn. The reverse reaction occurs when the product was irradiated at 254 nm (Scheme 1 . 7 ) ~ ~
Scheme 1.7. Photochromism involving a reversible [2
+
21 cycloaddition[4
+
41 Cycloadditions2-Aminopyridinium-salts can undergo [4
+
41 photodimerization. The phosphate salt of the dimeric product can be cleaved by short wavelength irradiation (Scheme 1.8a)." Acridinium salts also undergo similar reversible [4+
41 cycloaddition reactions (Scheme1.8b).'~
Scheme 1.8. Photochromism involving reversible
w
[4+4] cycloaddition.b. Electrocyclic reactions
A large class of photochromic compounds falls under the category of electrocyclic
reactions. Reports of 4nn systems are rare and most of the photoswitches are
(4n +2)
.n systems, where n=l, namely, 6n cyclizations. Spiropyrans, spiroxazines, fulgides, diarylethylenes and dihydropyrenes are examples of this type of photochromic system. These compounds have a typical 1,3,5-hexatriene molecular framework, though hetero atoms (e.g., in spirooxazines) can sometimes be a part of the framework.1.3.7. Fulgides, fulgimides and fulgenates 24
Fulgide oxidative
9
h ~ lo
*
a r o m a t i z p ~ ohv2
\ / / H.,,"
(when R = H) 0 R 0 0 FulgimideFulgides, fulgimides and fulgenates have the same general structure (Scheme 1.9).
Fulgide is the common name for bismethylenesuccinic anhydrides possessing at least one aromatic ring on the methylene carbons. They undergo a reversible ring closing reaction with UV light and ring-opening reaction with visible light. The photochemical isomerization of the open form to the closed form occurs in a conrotatory fashion as predicted by the Woodward-Hoffmann rules for a 671. cyclization. When the succinic anhydride moiety is converted to an imide, it is termed as a fulgimide and when it is in the form of a diester it is termed a fulgenate. Fulgimide and fulgenates are also good photochromes. Replacing the phenyl group with a heteroaromatic ring such as thienyl, fury1 or pyrrolyl,25 or introduction of an isopropylidene moiety instead of the simple rnethylene or placing a methyl substituent on the ring forming carbon of the aromatic ring?7 all stop unwanted side reactions, such as the thermal disrotatory back reaction and oxidative aromatization. Fulgides are P-type photoswitches, i.e. they are only switched by light and show no thermal reversibility. Photochromism of fulgides has been observed in solid, in solution, in glasses23 and in
Fulgenate
Pr
Vis
~ o k o ~ a r n a ~ * ' 29 reported the first photochromic ON1 OFF fluorescence switch (Scheme
1.10). The fluorescence properties of the (I()-binapthol condensed fulgides were changed by photochromism. The closed form fluoresced in toluene, while the open form did not.
1.3.8. Spiropyrans and spirooxazines
Spiropyrans and spirooxazines are structurally very similar. Photochromic reactions of spiropyrans were discovered by Fischer and Hirshberg in 1952 (Scheme 1.11).~' By 1956 Hirshberg realized the potential for using photochromism as "photochemical erasable memory".3' Merocyanines, i.e., the open form of spiropyrans and
spirooxazines tend to revert back to their more stable closed forms thermally and thus these kind of systems fall under the category of T-type (thermally reversible type) photochromic systems.
Spiropyrm
Spirooxazine
Merocyanine
Merocyanine Scheme 1.11. Photochromism of spiropyrans and spiroxazines.
The synthesis of spiropyrans is simple and they can be obtained in high yield by refluxing the desired substituted salicaldehyde with commercially available 2-methylene- 1,3,3-trimethylindoline
or a
derivative in ethanol. Colourless spiropyrans are convertedto their intensely coloured photoisomer, the open merocyanine form, when irradiated with UV light. The merocyanine can revert back to the closed isomer thermally or on irradiation with visible light. The thermal instability of the merocyanine form is a problem and for that reason it is not always easy to record a UV-vis spectra of the pure merocyanine form." Spiropyrans were used in photochromic lenses but their photodegradation over time was a serious drawback. Recently, Frank and coworkers developed spiroxazine systems having better thermal stability.33 They incorporated a phenanthroline moiety in the spirooxazine system that could bind to a variety of divalent metals, such as, Zn(II), Fe(II), Co(II), Mn(II), Ni(I1) and Cu(II), to form a tris-switch
(Scheme 1.12). The metal stays bound to the ligand in both the open and the closed
form. The metal bound open merocyanine forms have improved thermal stability. The thermal stability of the complexed merocyanine is affected because of the decreased negative charge density in the oxazine moiety through inductive effects or an increase of change density through d-n* donation. However, these two effects vary along the divalent metal series thereby, playing a significant role in the electronic stabilization. Thus, this work by Frank has changed the popular belief of the fitilitg4 of the thermally unstable spirooxazine system.
1.3.9. Diarylethenes and their thermal reversibility
2-Stilbene, the simplest example of the diarylethene class, undergoes a photocyclization reaction to produce dihydrophenanthrene, which
in
a deaerated solution thermally reverts to 2-stilbene in the dark. It was one of the earliest examples of a diarylethene photochromic system (Scheme 1 . 1 ) ~ ~ Diarylethenes with heterocyclic aryl groups are newcomers to the photochromic but they have received much attention and importance in modern research because of several reasons discussed in section1.3.10. In 1988, Nakamura and Irie carried out theoretical calculations, which served as
the foundation for their design for diarylethene photoswitches with slowed thermal return.37 The results of the calculations are summarized in Table 1.1. AHf refers to the relative ground state energy differences between the open and the closed forms.
Table 1.1. Calculation on the thermal reversibility of 1,2-diarylethene systems
1
Diarylethene l r y l group compound Pyrrolyl di(3-pyrroly1)ethene Phenyl Thienyl di(3-thieny1)ethene 1,2-diphenylethenedisrotatory
I
conrotatory ringI
energy per ring AHf (open - closed) (kcaymol)considering
I
consideringring closing
I
closingI
(kcaymol) Loss of aromatic stabilization#
From the results summarized in Table 1.1, it was inferred that the activation barrier for
the forward and the backward reactions correlated with the ground state energy difference between the open and the closed form and thus controlled the thermal stability of the photogenerated coloured ring-closed isomers. The barrier for thermal reversal is large when the ground state energy difference between the two forms is small and vice versa. Thus when the energy difference is large, as in the case of 1,2-diphenylethene (i.e., Z-
stilbene), the energy barrier becomes small and the cycloreversion reaction readily takes place thermally. On the other hand, when the energy difference is small, the barrier of the thermal reversal reaction becomes large, as in the case of 1,2-di(3-thieny1)ethene.
There, the cycloreversion reaction virtually ceases to occur. The molecular property responsible for this difference between the ground state energy difference between the open and closed forms is the aromatic stabilization energy. Destablization due to the destruction of aromaticity of the rings during the course of cyclization increases the ground state energy of the closed ring form. This difference is highest when the aryl group is phenyl, and it is lowest in case of thienyl. Thus, in a nutshell, the diarylethene compound with the aryl groups having least aromatic stabilization energy will give the switch with the least thermal reversal, because of the most thermal stability.
@
\ I@
/\
\/'
1 1'
The dimethyldihydropyrene (DHP) system 1 and its cyclophanediene (CPD) 1' (cJ:
Section 1.3.11.) can be thought as a diarylethene system, where there is an extra ethene tether between the meta-carbons of the aryl rings. This results in a l4.n-electronic framework, which changes the electronic structure of the molecule drastically. The DHP form, because of its conjugated 14n electrons, is aromatic. The difference in energy between the DHP and the CPD forms depends upon the difference in aromatic stabilization energy, the difference in strain (-20 kcallmol), and the fact that a n bond (CPD) is converted to a o bond (DHP) (-30 kcallmol). The net effect is that the CPD is of higher energy than the DHP by about 3 kcallmol.
The CPD form, having a 6n-electronic framework for the ring closing (Figure 1.2.), reverts to the thermodynamically more stable DHP form thermally, in a conrotatory fashion. This conrotatory thermal reversal of the CDP is forbidden by the Woodward- Hofhann rules for an electrocyclic reaction, which states that orbital symmetry is conserved in concerted
reaction^.^'
Thus, only the conrotatory photochemical ring closure is allowed in a 6n system (Figure 1.2a). The thermal ring closing is allowed only if it occurs in a disrotatory fashion, which is impossible here because of thewhere the inversion of the cyclophane is not possible. structural rigidity
Electrocyclized product
Ground state Excited state
Figure 1.2.a. Frontier molecular orbital representation of the electrocyclization of a
1,3,5-hexatriene system. 1.2.b. Conrotatory ring closure of metacyclophanediene to dihydropyrene.
According to Hoffmann and Woodward there cannot be any violation of this rule.2o The apparent anomaly of the violation in the case of conrotatory thermal reversal of CPD to DHP could be solved if the process is considered as a non-concerted process.
Sheepwash speculates on the possibility of existence of an intermediate CPD-singlet biradical species, but its existence was not detected.38
The thermal reversal of the CPD to the DHP form in the [a]-annelated series is fast where the relative ground state energy differences between the closed and the open forms (calculated) are large.39
[el-fused DHP
+
vis-
P uv or/\
Ar group in the [el-fused systems
2 Absent 2'
Scheme 1 .l3. [el-fused dihydropyrene systems5
The thermal reversal data of the CPD
+
DHP colouration reaction for the [el-annelated systems with different fused aromatic spacers (Scheme 1.13) is given in Table 1.2.~'AHqclosed - open) refers to the relative ground state energy differences between the closed and the open forms calculated using the semiempirical AM1 method.40
Table 1.2. Thermal reversal of dihydropyrene systems
However, when compared with Table 1.1, it becomes obvious that the same general
correlation between the relative ground state stabilization energy and thermal stability does not hold any more for these systems. Unlike Irie7s systems (cf. Table 1.1) or the
[a]-annelated series,39 in the [el-annelated series, the AHf values are small. It is not obvious why the benzo-[el-annelated system 3 having a higher energy than the naphtho- system 4 and the anthro-system 5 has a slower thermal return than both of them.
The diarene annelated derivatives of DHP behave very differently from either the [a]-
annelated or the [el-annelated systems. In this case the CPD form, because of the higher resonance stability of four benzene rings (AHf (closed - ,,,,I = -1 8 kcal/mol) is thermally more stable and so, the DHP form 6 is expected to revert to the CPD form, 6' at room
Annelated arenes Absent Benzene Naphthalene Anthracene Compound 2 * Half-life at 46 OC (hours) 1.88 -AHf (closed - open)
(kcaymol) 2.27 3 4 5 Eact (kcaY mol) 23.0 1.47 1.12 0.96 24.5 22.1 19.1 5.75 1.15 0.33
temperature. Indeed, when 6' is irradiated with UV-light, the DHP photoisomer 6 formed in the reaction reverts rapidly to the CPD form (Scheme 1.14).~'
_.____)
t---
vis, A
Scheme 1.14. Photochromism of dibenzoDHP 6 1.3.10. 1,2-Dithienylethenes
The most commonly used heteroaromatic diarylethenes are the dithienylperfluoro- cyclopentenes (scheme1 .15).41
~olourless Coloured
Scheme 1.15. Dithienylethene photochromic system
They belong to the thermally irreversible P-type photochromic compounds. The most striking feature of these compounds is their fatigue resistance. The colouration -
decolouration cycle, depending on the heterocycle, could be repeated more than lo4
times. Both thermal irreversibility and fatigue resistance properties, are indispensable for applications to photon mode RAM-type memories and switches. Both the colouration and decolouration processes take place very rapidly in less than a few
picoseconds in solution as well as in the solid matrices, such as in polymer films and crystals. The absorption spectrum is dependent on the types of aryl groups, the substitution positions of the aryl groups with respect to the ethene moiety, and the upper cycloalkene (ethene unit) structures. The open-ring isomers so far synthesized have absorption maxima ranging from 230 to 460 nm and the closed-ring isomers from 425 to 830 nm. The ability of these systems in memory devices has been d e m ~ n s t r a t e d . ~ ~ Lehn proposed the use of fluorescence or infrared absorption as readout signals to avoid destructive r e a d o ~ t . ~ ~ , ~ ~ A diarylethene having dithieno(thi0phene) units 7 (Scheme 1.16) shows fluorescence in the open-ring form 7a but not in the closed form 7b.
Irradiation of the main band of the open-ring isomer 7a at 400-500 nm gave no reaction, while the cyclization reaction proceeded by irradiation at wavelengths less than 400 nrn.
Coloured 7b
Scheme. 1.16. (a) Lehn and (b) Irie's fluorescence switch.
The closed-ring isomer 7b had an intense broad absorption band at 704 nm. The cycloreversion reaction occurred on irradiation with >600 nm light. The open-ring isomer was found to have a strong emission at 589 nm when excited in the 400-500 nm region. In contrast, the closed-ring isomer displayed only a weak fluorescence (less than 3% of the intensity of 7a). Because of the large fluorescence intensity change between the open- and closed-ring isomers, it was possible to use the change as the readout signal. When fluorescence is detected without influencing the ratio of the two isomers, the
readout method becomes nondestructive. The absorption band at 459 nrn of the open- ring isomer was photochemically inactive. However, the analogous band of the closed- ring isomer at I,, = 454 nm had low activity; a slow conversion of less than 10% per hour was observed. When the irradiation was performed with a narrow range of the excitation wavelength, the cycloreversion reaction was very slow (<2% per 30 min irradiation). Although, the system does not have perfect performance of nondestructive readout, a very large number of fluorescence readings can, in principle, be performed before significant change takes place. In 2002, Irie and coworkers synthesized a diarylethene molecule (Scheme. 1.14) whose fluorescence can be detected at even the
single-molecular
Outside the photoactive region, where the difference between the open- and closed- ring isomers can be detected by electromagnetic radiation of energy that cannot cause any molecular change, the recorded information can be read many times without destruction. Branda used optical rotation measurements for this purpose.45 Although the closed form of dithienylethenes are chiral, the closing reaction of the open form yields racemates (S,S) and (R,R). Branda and coworkers used a double stranded stereochemically pure Cu(1) complex formed by chiral ligands attached to a 1,2-dithienylthiophene. The optical rotatory dispersion spectra of the photochromic reaction of a stereoisomer showed several spectral regions where the difference between the open and closed form (complexed) is significant and provide potential for non-destructive read-outs.
Irie, in 2003 has demonstrated non-destructive readout of the photochromic reactions of diarylethene derivatives using infrared light.46
Current erasable optical data storage systems are based on magneto-optic and phase- changes, utilizing heat-mode effects. In these systems, a focused beam of light is converted into heat energy on the recording medium and increases the medium temperature above the Curie-point or melting temperature. Physical property changes accomplished by the heating are used as the memories. Photon-mode recording has various advantages over heat-mode recording in terms of resolution, speed of writing, and multiplex recording capability. The fast photonic switching of the thermally stable dithienylethenes along with the efficient non-destructive methods of reading their states make them potential candidates as materials for memory devices.
1.3.1 1. Dihydropyrenes
Dihydropyrenes, though rarely considered in the category of diarylethenes, can be thought as a special kind of 1,2-diarylethene, where a second ethene group is tethered to
the meta-positions of the two aryl-groups (Scheme 1.17.).
Cyclophanediene (CPD) Dimethyldihydropyrene (DHP)
Scheme 1.17. Dihyropyrenes are tethered diarylethenes
This kind of system is one of the very few negative or inverse photochromic systems, where the closed dihydropyrene (DHP) form, is the coloured stable state, and the open, colourless, metacyclophanediene (CPD) form is the thermally less stable state.
Irradiation of DHP with visible light converts it to the corresponding CPD; irradiation with ultraviolet light closes it back to the green DHP. The CPD
+
DHP reversal reaction also occurs thermally and it is therefore, a T-type photochromic system.The trans- 1 Ob, 1 Oc-dimethyl- 1 Ob, 1 Oc-dihydropyrene, 1, has a DHP
3
CPD opening quantum yield(a)
of about 0.006 at 380 (previously reported as 0.015 to 0.02 at 466 nm48349). This is considerably lower than for the dithienylethenes.37 The 2,7-di alkyl substituted DHP, 2 has even a lower quantum yield than the parent system, 1(a
= 0.0015)~'. The alkyl groups of 2 increases the non-radiative decay from the firstsinglet excited state of the closed form, thereby opening a pathway of draining the energy of the absorbed photon by vibronic relaxation.
[DHP (S1) ]
.
CPDFigure 1.3. Non-radiative decay of the dihydropyrene 2
The thermal reversal rate of the DHP 2 is 3.1 x 10" mid' at 40 Substituent effects on the thermal reaction show weak trends.50 Electron withdrawing substituents on the DHP slowed down the thermal reversal rates of the CPD, though no Harnmett substituent correlation was found. Thus the CPD forms of the compounds 8, 9, 10 (Scheme 1.18)
Scheme 1.18. Thermal reversal of some DHP compounds
It is interesting that the thermal ring closing rates of the CPD form increases with increase in bulkiness of the alkyl substituent at the 10b,10c-position of the DHP ring, though the opening rate remains unchanged.48 The possible reason for that is the steric repulsion between the alkyl groups and the opposite benzene ring increases with increase in their steric bulk and thus, they prefer to revert to the DHP form. There the steric repulsion is less, as the alkyl groups protrude out from the opposite faces of the ring
(Figure 1.4.).
Thus the thermal reversal rate of CPD form of the compounds 2 , 1 1 , 1 2 and 13 (Figure 1.14) at 40 O C are 3.1, 4.4, 4.7 and 12.0 (x 10" ) mid' respectively. At the same time,
the alkyl groups undergo a [1,5] sigmatropic migration more readily with increase in their steric bulk, thereby destroying the photochromic properties of the systems.50
(The pi-bonds are not shown to avoid clumsiness)
Figure 1.4. Steric repulsion in the cyclophanediene form.
Amelation of the dihydropyrenes results in systems with different quantum yields and thermal reversal rates (cJ: section 1.3.9.). The [a]-benzannelated DHP, 14, is not a good
photochrome (< 4% opening on vis-irradiation) whereas the [el-benzannelated dihydropyrene (BDHP) 15, is an excellent photochromic system with fast photobleaching, and slow thermal reversal (0.00078 mid1 at 22 OC) (Scheme 1.19).~~,~'
Initially it was not clear why the [a]-amelated systems were not good photoswitches. Later, laser flash studies suggested that they did open, but the thermal reversal of their open forms, as expected from high values of calculated AHf (,I,,, - o,,,), were extremely
fast and thus obtaining the open forms of those systems in significant amounts at room temperature was difficult.52
vis
__
A
vis h-
uv
om
Scheme 1.19. Photochromism of [a]- and [el-fused DHP systems
Quantum yield (@) measurements provide a quantitative means of measuring the efficiency of a photochromic system. Equation 1.1 defines the quantum yield of a
photochromic system.
Moles ofphotoisomerizedproduct formed @ =
Moles ofphotons absorbed
Quantum yields of some of the DHP systems were studied by ~ h e e ~ w a s h . ' ~ Table 1.3
includes some important systems relevant to this thesis.
Table 1.3. Quantum yields of some DHP systems. Compound @ D H P ~ C P D at 480 nm
1.4. Research objectives
The DHP - CPD photochromic systems are less explored because they are synthetically more challenging than some of the other photochromic systems. The parent di-t- butyldimethyldihydropyrene 2 has a low photoisomerization quantum yield and also the thermal reversal of the corresponding CPD form is fast. However, the benzo-[el- annelated system, 3, has a higher photoisomerization quantum yield, a slower thermal
reversal of the open form and is thus a more efficient photoswitch.
The main objective of this research was to develop efficient photochromic systems based on dihydropyrenes. We also wanted to fictionalize the systems through suitable non- annelated linkers so that the photochromic systems could be attached to other systems, such as, (i) annulenes, to study aromaticity, (ii) fluorophores, to see if fluorescence could be switched, (iii) electron acceptor, to see if charge transfer could be switched photonically, and (iv) other photochromic units, to see if multiple state photoswitchable systems could be realized.
The potential of DHP systems as promising photoswitches was realized soon after their discovery in the late 1960's. But, besides NMR spectroscopy, no fast methods were employed to "read" the states (oped closed) of these switches in a non-destructive fashion. In this research, we also investigate the possibility of reading the systems in a non-destructive fashion in different modes.
Chapter 2.
Synthesis
2.1. Introduction: The synthesis of dihydropyrenes.
Although Mitchell and ~oekelheide" developed a greatly improved synthesis of the dimethyldihydropyrene parent molecule 1 utilising thiacyclophane intermediates, the synthesis was still long, 10 steps in 28% overall yield, and more importantly required the lengthy synthesis of 2,6-bis(bromomethy1) toluene at the start of it. ~ a s h i r o ~ ~ made a real improvement to the sequence by starting with 4-t-butyl toluene which he converted in one step to 2,6-bis(chloromethy1)-4-t-butyl toluene, and thus overall shortened the sequence by 3 steps. In this case of course, the product was the 2,7-di-t-butyl derivative 2.
Our group has fbrther improved this sequence (Scheme 2.1)~' (see Supporting Information of the ref.) using a bromomethylation (instead of chloromethylation) with trioxane, freshly prepared anhydrous zinc bromide and 30% hydrobromic acid, glacial acetic acid mixture (90%). The dibromide 16 was converted to the dithiol 17 using thiourea and sodium hydroxide in
-
90 - 95 % yield. The dithiol 17 and the dibromide 16 were condensed under high dilution conditions and gave the corresponding dithiacyclophane 18 in 70 - 90% yield. A Wittig rearrangement of 18, followed bymethylation with methyl iodide yielded the corresponding ring contracted cyclophane 19. Methylation of 19 with Borsch reagents6 converted it to the corresponding dimethylated salt 20, which on Hoffmann elimination afforded the cyclophanediene 2' that
spontaneously underwent 67c electrocyclization to give the dihydropyrene 2. Overall from 14 g of the thiacyclophane 18, 8 g of DHP 2 was obtained, in
-
60-
65 % yield over the last 3 steps.4
(60-65%
from 18)
18 19
Borsch reagent, C HoC ~
Scheme 2.1. Synthesis of 2,7-bis-t-butyl-trans-1 Ob, 1 Oc-dimethyl-1 Ob, 1 0c-dihydropyrene
The parent 2 was converted to the benzo derivative 3 as shown in Scheme 2.2.
Bromination of 2 with NBSI DMF in dichloromethane gave bromide 21 in 92% yield. 57 Use of only dichloromethane led to a longer reaction time. The bromide 21 was then
converted to the furan adduct 22 via the corresponding aryne using NaNH2/ t-BuOK in
THF and furan. Deoxygenation of the adduct 22 was achieved with diironnonacarbonyl
in benzene at reflux temperature and gave the benzoDHP (BDHP) 3. The latter could be
converted either to the monobromide 23 or the dibromide 24. With one equivalent of
NBSI DMF in dichloromethane at -78 OC, the monobromide 23 was obtained in 90 - 95% yield. The solvents must be dry to achieve this. Wet DMF tends to give a pink compound that is soluble in water, difficult to extract and has not been fully characterized
yet, though IR spectroscopy indicates the possibility of the presence of a carbonyl group. NBS, DMFl CH2CI2 ____) - 7 8 O C (92%) Fe2(CO)d benzene
n
NBS (2 eqv.), DMFl CH 2C12 (50%) NBS (I eqv.), DMFI CH 2CI2 C -78 OC (90 - 95%) 24Scheme 2.2 : Synthesis of mono- and dibromo-benzo-[el-DHP
Scheme 2.2. Synthesis of benzo[e]dihydropyrene (3) and its mono- and dibromination.
When two equivalents of NBS were used at room temperature (- 20 O C ) , the dibromide
24 was obtained in about 50% yield, as well as a compound which was colourless but
fluoresced under UV light, and is possibly a pyrene derivative. This suggested that the internal methyl groups had migrated. This undesired side reaction destroyed the
dihydropyrene framework and thus gave a low yield for the dibromination reaction. The decomposition could not be avoided even when other solvent systems such as, pure dichloromethane or dichloromethane-chloroform mixture were used. This side reaction proceeded at all temperatures between -78 OC to
+
25 OC. Use of wet DMF also caused the discolouration of the BDHP system, presumably because of the formation of the 4 3 - quinone and subsequent ring opening or migration of the internal methyl groups.2.2 Synthesis of substituted dihydropyrenes for photochromic property studies
Substituents have a marked effect on the photochromic properties of the parent DHP7s
1 and 2. For example, 2-nitro-DHP has a better quantum yield of photoisomerization than
the parent 1 but also has a 40 times faster thermal return rate. 47'48 Data for the benzo-[el-
series is almost unknown. So we decided to prepare a variety of targets with different substituents, for example formyl, ethynyl, diphenyl, bis-biphenyl, ferrocenyl, etc.
We also synthesized some derivatives of the parent 2 for photophysics studies.
2.2.1 Synthesis of the 4-acetyldihydropyrene 9
Sheepwash and ~ o h n e ~ ' wanted to investigate the photochromic properties of 4- acetyl-DHP (9). Tashiro reported the synthesis of 9 in 1982~', and again in 1 9 9 1 ~ ~ with some modification. Some of the data reported in the two papers were inconsistent. For example, the former paper reported the colour of 9 as blue but the latter reported it as brown. We synthesized 9 using a Friedel-Crafts reaction of 2 with acetic anhydride and
boron trifluoride etherate, in a slightly modified fashion than reported by Tashiro
Scheme 2.3. Acetylation of dimethyldihydropyrene 2.
Compound 9 was obtained as brown crystals in 49 % yield, mp 184 - 185 OC,
consistent with the reported melting point. Two singlets for the internal methyl groups were found in the proton NMR at 6 -3.93 and -3.94, indicating the asymmetric structure of 9. This was contrary to Tashiro's results of a single peak at 6 -3.89 (at 100 MHZ)", though the two singlets for the t-butyl protons at 6 1.69 and 1.68 were consistent with the reported values. In the
13c
NMR spectrum the internal methyl groups were at 6 14.95and 14.48, also differentiated by the acetyl group at position 4- of the DHP ring.a The characteristic carbonyl carbon appeared at 6 202.14.
2.2.2 Formylation of substituted benzo[e]dihydropyrenes
The formyl derivative 25 is the logical precursor to an ethynyl conjugated dihydropyrene system, using Wittig, aldol or McMuny reactions. However, the introduction of the formyl group turned out to be quite challenging for the reasons described below.
Boekelheide formylated the parent DHP 1 using dichloromethoxymethane and
titanium (IV) chloride as Lewis acid.60 Wang attempted to formylate BDHP 3 using the
same methodology but surprisingly obtained mono- and dichloro- substituted products instead of f ~ r m ~ l a t i o n . ~ ' Later, other Lewis acids such as, BF3.0Et2, anhydrous ZnC12, and AlC13 were also tried with dichloromethoxymethane by this author without much success. Benzo-DHP 3 did not even survive formylation using N,N-dimethylformamide
and POC13, the Vilsmeier-Haack reaction. In most cases the internal methyl group of 3
migrated to the periphery and resulted in destruction of the dihydropyrene framework. So the methodology was changed from a direct electrophilic substitution that resulted in a positive charge on the dihydropyrene system, to an indirect substitution involving a carbanionic intermediate, obtained from the bromide precursor 23, described as follows. We thought that a lithium halogen exchange reaction with n-BuLi and subsequent attack by DMF might give the formylbenzodihydropyrene 25. Lithium halogen exchange of 23 at -78 "C and quenching of the lithiated intermediate with dry DMF did indeed afford, in 98% yield, the desired formyl-BDHP 25 as dark purple crystalsb (Scheme 2.4), mp 178
-
179 "C.
Scrupulous attention to anhydrous conditions was necessary. Lower yields were
obtained when the DMF was not properly dried, the glassware was not flame dried or an old batch of n-BuLi was used.
1. n-BuLi / THF 478 O C )
2. DMF
w
(70
-
98 %)Scheme 2.4. Synthesis of formylbenzo[e]dihydropyrene
The structure was proven by NMR spectroscopy. The formyl proton appeared at 6 10.79 and the formyl carbon at 6 189.05. The internal methyl protons were at 6 -1.34 and -1.44, the carbons were at 6 18.78 and 18.12. The t-butyls peaks were at 6 1.54 and
1.47, and 6 30.70, 30.68 in the 'H and
I3c
NMR spectra respectively. The IR spectrum confirmed the presence of C=O group at 1678 om-'. The mass spectrum confirmed themolecular formula of 25 as C31H340 by EI MS (M+) at m/z 422 and a satisfactory HRMS was also obtained.
2.2.3 Synthesis of ethynyldihydropyrenes
Introduction of an acetylene moiety to dihydropyrene systems required protected acetylenes that could react with a halogenated dihydropyrene, and could subsequently be deprotected.
Sheepwash used Sonogashira coupling reactions with the iodide 26 or the bromide 21
via the alcohol 27, and its subsequent deprotection with NaH in 95% yield to synthesize
the ethynyldihydropyrene 28 (Scheme 2.5).47 Use of other bases such as sodium
OH
NaH, toluene
EtNH, DMF reflux, 4 h
PdC12(PPh3)2, CUI (95%)
Scheme 2.5. Synthesis of ethynyldihydropyrene by Sheepwash
But when the BDHP bromide 23 was subjected to similar conditions, the reaction
yielded only 9% of the desired product 29. Most of the starting bromide was recovered by chromatography. Several reaction conditions (different mixed solvents, different arnine bases, different palladium catalyst and different temperatures) did not improve the yield significantly. However, reaction of 23 with excess of TIPS-acetylene in a
minimum volume of 3:2 DMF: diisopropylamine (see Experimental section) at 75 O C for
36 h gave the desired TIPS-substituted intermediate (Scheme 2.6).
qBr
I
/ I . . . 3 :2 DMF: Pr2NH@
\ ,,... / (1.0 M solution T B , ~ F / in THF) + &H \ ,,,~. // reflux
75 O C , 36 h / 6 h A
I
\
(79% over two steps)(not isolated)
29 Scheme2.6. Synthesis of ethynylbenzo[e]dihydropyrene 29 via a Sonogashira
coupling and removal of the protecting group.
Even after several washes with water, column chromatography and heating under reduced pressure, the product was difficult to separate from the unreacted TIPS-acetylene
or
itscoupled dimer. So after column chromatography (to make sure no Cu/ Pd catalysts were present) the product was used directly in the next step where the crude triisopropylsilyl compound was dissolved in diethylether and heated under reflux with tetrabutylammonium fluoride (1.0 M solution in THF) under argon for 6 h. After purification by chromatography, a red crystalline product was obtained in 79 % yield (over two steps), mp 158-1 59 OC.
The proton NMR spectrum showed peaks at 6 -1.43 and -1.45 for the internal methyl protons indicating the asymmetry. The acetylenic proton was found at 6 3.50. The two acetylenic carbons found at 6 83.9 K = C ) and 81.6 (=s-H) in the 13c NMR spectrum
were assigned on the basis of DEPT and the HETCORR NMR experiments. Peaks at 33 10 cm-' in the infrared spectrum showed the characteristic acetylenic C-H stretch and at 2090 cm-', the characteristic terminal acetylene CrC stretch. FAB MS showed a peak at
m/z
318.2 corresponding to the molecular ion of C32H34. A satisfactory elemental analysis confirmed the composition of 29.2.2.4 Introduction of a propargyl alcohol group onto the dihydropyrenes.
An alcohol is a flexible linking unit, chemically different from an alkyne or an aldehyde, and so, to give variety to the synthetic linking reaction we decided to use a propargyl alcohol substituted benzo[e]dihydropyrene 31, we synthesized the less expensive analog of the parent dihydropyrene 30. A Sonogashira coupling of 4-iodo- DHP 26 with propargyl alcohol was carried out in diisopropylamine at 70 OC for 36 h. The microcrystalline green product 30 was obtained in 94% yield (Scheme 2.7), mp 116- 117 OC.
Pd(PPh3>2C 12, CuI,
N H C P ~ ~
70 OC, 36 h. (94%)
Scheme 2.7. Sonogashira coupling of 2-butynol with iododimethyldihydropyrene 26
In the proton NMR spectrum, the internal methyl groups appeared as a singlet at S -3.93, while the t-butyl groups appeared as two separate peaks at S 1.69 and 1.66. This is akin to the dimethyl analog 27 47 (DHP-CeC-CMe20H), where the two methyl
protons appeared as a singlet at 6 -3.93 but the two t-butyl protons appeared as two peaks at 6 1.64 and 1.55. However, the two methyl carbons are different in the
13c
spectrum since two peaks at 6 15.03 and 14.71 were found. The methylene protons appeared at6 4.81. Signals for the acetylene carbons were found at 6 92.28 and 86.60. The infrared
spectrum showed the characteristic broad alcohol peak at 3402 cm-' and the acetylenic C-C stretch at 2206 cm-'. The peak corresponding to the molecular ion (M') was found by EI mass spectrometry at d z 398. High-resolution mass spectrometry gave a molecular weight of 398.2612, consistent with the calculated value of 398.2654 for C29H340.
Unfortunately, unlike the parent DHP-system, the iodide was not available and direct introduction of the propargyl alcohol group via a Sonagashira coupling of bromide 23 to give the desired dihydropyrene containing coupled species, 31, proceeded extremely
Pd(O)/Pd(II) CuI, i- Pr2NH b
't'
1. n-BuLi at O•‹C 2. HCHO (gas)+
(49%)Scheme 2.8. Incorporation of a 2-butynol group onto benzoDHP (a) via a direct Sonogashira coupling; (b) from ethynyl derivative 29 and formaldehyde.
An indirect route was thus taken where 23 was first converted to the ethynyl derivative 29 using our previously developed procedure.62 This was then deprotonated with n-BuLi at 0 OC, and reacted with dry gaseous formaldehyde (generated by pyrolysis of paraformaldehyde at 200 OC in a stream of argon), and gave the red alcohol 31, mp 83-84 OC, in 49% yield (Scheme 2.8).
In the 'H NMR spectrum, the internal methyl groups of the alcohol appeared at 6
-1.44 and -1.46. The methylene protons appeared at 6 4.67. In the NMR spectrum the two acetylene carbons appeared at 9 1.76 (CHI-GEC)), 85.60 (CH~CECJ. In the IR spectrum the weak acetylenic C r C stretch appeared at 2164 cm-' and the characteristic
broad 0-H stretch at 3439 cm". The molecular ion (M') was found by FAB mass spectrometry at m/z 448.2. High-resolution mass spectrometry gave a molecular weight at 448.2766 for C33H360 (the calculated value was also 448.2766).
2.2.5. Synthesis of ferrocenylbenzodihydropyrene 32
A metal coupled DHP has the possibility of an oxidation state change for the metal, and we thought it would be interesting to see if this affects the photoswitching. We chose to synthesize ferrocenyl-BDHP 32. This was synthesized using a Suzuki coupling reaction with the bromide 23 and 1,l'-ferrocenediboronic acid in 1: 1 ratio using tetrakis(triphenylphosphine)palladium(0) as the catalyst, sodium carbonate as base, and DME as s01vent.~ Refluxing the product with propionic acid for 24 h hydrolyzed the residual boronic acid moiety. The reaction mixture was neutralized with K2CO3 solution, extracted with ether, and on purification of the product by column chromatography over deactivated silica gel using 5% ethylacetatel hexanes as eluant, red crystals of 32 were obtained in 28% yield (Scheme 2.9) mp > 330 OC.
G + 3 ( 0 ~ ) 2 &B(OH)* \/ 2 Pd(PPh3)4, 1(M) Na2C03, DME Br 2. EtCOOH _____t (28%)
Instead of 1,l '-ferrocenediboronic acid a more logical synthon for the Suzuki coupling to synthesize 32 was 1 -ferroceneboronic acid. Attempted synthesis of 1-ferroceneboronic acid, by deprotonation of ferrocene with n-BuLi and TMEDA, and subsequent reaction of the lithiated intermediate with trimethylborate always gave a mixture of the unsubstituted, mono- and di-substituted product.
Scheme 2.9. Synthesis of ferrocenyldihydropyrene via a Suzuki coupling and subsequent hydrolysis of the residual boronic acid moiety
The structure of 32 was characterized by NMR spectroscopy. The internal methyl
protons for the compound 32 appeared at 6 -0.94 and -0.97 indicating the asymmetry. The substituted Cp-ring showed multiplets and the unsubstituted Cp-ring showed a sharp singlet. In the 13c NMR spectum five peaks were obtained for the substituted Cp-ring
and a single peak for the non-substituted Cp. It is interesting to note that the ortho- and ortho'- protons of the substituted Cp ring showed a NOE with H-3 of the dihydropyrene, whereas the unsubstituted Cp protons showed NOE with H-5. This indicates that in solution, at room temperature, the compound prefers a geometry where the unsubstituted Cp ring is in close proximity to H-5. A peak at m/z 578 (M+) in the mass spectrum (EI) supported the molecular composition of C ~ O H ~ ~ F ~ , and as well a satisfactory elemental analysis was obtained for 32.
2.2.6 Synthesis of the diphenylbenzo[e]dimethyldihydropyrene 33
Phenyl groups were introduced at the 4 and 5 positions of BDHP 3 by using a one-pot
double Suzuki reaction of the BDHPdibromide 24 with two equivalents of commercially
available phenyl boronic acid using sodium carbonate as base, tetrakis(triphenylphosphine)palladiurn(O) as catalyst and DME as solvent (Scheme 2.10).
Scheme 2.10. Synthesis of diphenylbenzo[e]dimethyldihydropyrene 33 via a one-pot
double Suzuki coupling reaction.
@B(OH)L
The reaction was complete within 2 hours. Purification of the product by column chromatography over deactivated silica gel using benzene/ hexanes (15: 85) as eluant gave red crystals of 33 in 70 % yield.
The 'H NMR spectrum of 33 indicated the symmetric structure; single peaks for both the internal methyl and the t-butyl groups at 6 -1.28 and 1.33 respectively were observed
\/
as well as an AA'XX' pattern for the fused benzene ring. It was difficult to assign each Pd(PPh3),, DME
aq. K2CO3
-
reflux, 2 hBr
(70%)
of the protons of the phenyl substituents as the signals were broadened because of the restricted rotation of the phenyl groups. Assignments of the 13c peaks in the NMR spectrum are done mostly on the basis of assignment of the 'H NMR peaks first; for this compound, 13c peak assignment was difficult because of the broadness of some of the proton peaks. EI mass spectrometry supported the molecular composition with m/z at
546 (M+) and a satisfactory elemental analysis was also obtained.
