2. INTRODUCTION
2.1. Photochromic molecular switches
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
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accordance with Woodward-Hoffmann rules52, it can proceed only in conrotatory manner (Scheme 7).
conrotatory
photochemically allowed
disrotatory
C2v C2h
h1 h2
Scheme 7
Photoisomerisation 1,3,5-hexatriene/cyclohexadi-1,3-ene
h1 h2
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"
Cs 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
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"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 closure55, 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
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190 290 390 490 590 690 790
Wavelength, nm
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
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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 steric hindrance of the subtituents
CF2
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).
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Photochromic reaction of thien-1,1-dioxide-3-yl-based diarylethene with formation of two by-products (adapted from Ref. [68]).
S
Byproduct formation in diarylethenes (proposed mechanism; adapted from Ref. [71]).
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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
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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.
Diarylethene ring closure: stereochemical considerations.
S S
Diastereoselective ring-closure reaction of chiral diarylethene mediated by Cu+.74
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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
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
Stabilization of photochemically inactive form of diarylethene by complexation.
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Areephong, Feringa et al.78 showed that the conductive polymers can be doped with diarylethenes thus making electropolymerizability switchable.
S
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
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]).
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Fluorescence switching also can be achieved by applying fluorophore-modified diarylethenes.80-88An 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.