The first fulgides were synthesized and published by Stobbe [47] at the beginning of the last century. He established the te fulgide (from the Latin: fulgere = to glisten), because the first compounds were isolated as shining crystals. [25] This photochromic compounds were thermochromic, so the closed isomer (Scheme 5, C-form) was not thermally stable.
13 The photochromic reaction (Scheme 5) occurs between a colourless or pale yellow isomer (open isomer) and the coloured (from yellow to green) closed isomer. The open isomer can be found as Z-or E-isomer depending of the configuration of the double bond which connects the aromatic ring and the acid anhydride. Only the E-isomer is able to undergo the cyclization, but normally the geometrical photoisomerization between E and Z isomer takes place easily. However, it is an energy-consuming process which complicates the photochromic process. [46]
During the twentieth century, the researchers have tried to understand the photochromic process of the fulgides: the Z-E isomerization reaction and the thermal instability of the closed isomer. In 1968, the work of Becker and coworkers. [48] helped to lighten the mechanism of the photochromic process, and the research group of Heller [49,50] published a se ies of pu li atio s ith the title O e o ded Mole ules , where the chemistry of fulgides, closely related compounds and the thermal side reactions of the coloured isomers (hydrogen rearrangements and/or dehydrogenative aromatization) were dealt with. [50,51,52]
In 1981, Heller and coworkers [52,53] reported the first P-Type fulgide photochromic system, which shows neither thermal back reaction nor side reactions (Figure8, left). Furthermore, the conversion from the E-form to the C-form (Figure8, left) is close to 100 %, because the absorption of the E-form is large where the C-form does not absorb, so the back reaction by irradiation with UV light is insignificant (Figure8, right). When irradiated with white light, the C-form turns again into the E-form.
Figure 8: Fulgide molecular switch reported by Heller and coworkers [52,53] and qualitative UV-vis spectra.
In 1988, Yokoya et al. [54] investigated the effect of the R substituents (Figure8) in the E-Z isomerization. They reported it was greatly suppressed when the alkyl group R became
The azo molecular switch is one of the smallest photo-switches since it only contains two nitrogen atoms connected with a double bond, each of them carrying a non-bonding pair of electrons. [55]
The first azo-dye was synthesized by Martius in 1863 and only one year later Griess reported the coupling reaction of diazonium compounds. This important discovery opened the way to the development to the azo-dyes, the most important and versatile group of coloured organic compounds used as dyes and pigments. [11]
Krollpfeiffer et al. [56] reported in 1934 what could be the first elucidated photochemical reaction of an azobenzene derivative, isolating the fading products of an o-aminoazo compound. Following that, Hartley [57] observed in 1937, for the first time, the reversible photochemical E-Z isomerization (Figure 9) of azobenzene by irradiating a solution of the E isomer with sunlight. Furthermore he was able to isolate and identify the Z isomer.
Figure 9: Reversible switching reaction of azobenzene.
In the following years, the azo-compounds have been intensely studied due to their potential applications, among others, as a molecular switch. [58,59,59]
The organic azo-derivatives can be divided in three different types, according to the relative energetic position of the (n-π*)- and the (π-π*)-transitions: the azobenzene type, the
15 aminoazobenzene type, in which azobenzene is substituted by o- or p-amino groups, and the pseudo stilbenes (characterized by a low-lying (π-π*)-transition). [8]
Although the E-Z isomerization mechanism for each type varies, in this work only the general process, which coincide for all types, will be explained.
The E isomer is thermodynamically more stable than the Z form. However, the Z isomer is kinetically stabilized by an activation energy of isomerization. The E-Z isomerization can be initiated by UV-light stimulating a π-to-π* or a n-to-π* transition. The Z-E isomerization can be affected by means of visible light or by heat.
The geometry of the E isomer is more elongated while the Z isomer is a bent molecule and hence, more compact. This leads to different physical and chemical properties.
An example of the varied application of azo-molecular switches is the work of Shinkai et al. [60] They reversibly modified the cavity of a crown ether by switching between the E and Z configuration (Figure 10).
Figure 10: “hi kai s sup a ole ula s it hi g s ste . [60]
Different solutions of alkali metal salts of methyl orange were extracted with the azo-benzene crown ether (Figure 10). It was found that depending on the configuration of the azo group, the size of the cavity varied (being bigger in the case of the Z-isomer).The large alkali metal ions such as rubidium(I) and caesium(I) were hardly extracted by the E-isomer and good extracted by the Z-isomer. The opposite was true for the smaller cations like sodium(I). Finally, lithium(I) cation was only extracted by the E-isomer.
Kurihara et al. [61] studied azoferrocene, where two ferrocenes are linked by an azo bridge (Figure 11, left). The E-Z isomerization is induced by UV light through π-π* transition of the azo group, and the back reaction occurs by irradiating the sample with visible light, through a metal-to-ligand charge transfer (MLCT) transition. This reaction leads to a change of the intrinsic properties of both isomers. In cyclic voltammetry measurements two reversible one-electron redox waves were found for the E form (Figure 11, right, (a)), indicating a stable
16
mixed valence cationic complex and therefore a strong coupling between the ferrocene units. After irradiation with UV light, an additional two-electron redox wave was found for the Z compound, indicating that in this isomer no electronic communication exist between the iron centers (Figure 11, right, (b)). In the photostationary state three different redox waves were found because both isomers were present.
Figure 11: Azoferrocene switching reaction (left); cyclic voltammograms of E-azoferrocene (a) and Z-azoferrocene (b) obtained by irradiation with UV light of the E-isomer. (Solvent = benzonitrile;
electrolyte = [Bu4N][ClO4]) (right). [61]
In 2012 Stein et al. [10] reported the modification of Profopol, the most widely used intravenous anaesthetic. [62] They designed an azobenzene derivative of the famous drug (Figure 12) and observed that the substitution of the azobenzene unit with electron-donating substituents greatly decreased the thermal stability of the Z isomer, so it reverted to the E form in the dark. They took advantage of this fact and used it to control the anaesthetic effect within light irradiation. Given that both isomers react differently with certain receptors, it played an important role in the anaesthetic process.
Figure 12: Reversible switching cycle of Profopol. [10]
17 2.1.1.4. 1,2-Diarylethylenes
Stilbene is the simplest representative of this class of compounds. The first step of the reaction is the E-Z isomerization of stilbene due UV light irradiation. It occurs from both the singlet and the triplet state of the molecules. By irradiating the Z isomer with UV light, a cyclization via a conrotatory process and from the first excited singlet state [63] takes place and the 4a,4b-dihydrophenanthrene (DHP) is formed. [64] This compound can thermally or photochemically evolve to stilbene (exclusively in the Z-isomer). A following reaction to phenanthrene by dehydrogenation with oxygen is also possible (Figure 13). [24,65]
Figure 13: Photocyclization and dehydrogenation of stilbene. [65]
When the positions 2 and 6 of the phenyl rings and the positions 1 and 2 of the connecting double bond were substituted with methyl groups (Figure 14), the hydrogen-elimination was avoided and the DHP underwent only the back photochromic reaction, even in the presence of oxygen. However, the lifetime of the closed isomer was three minutes in the dark at 30 °C, so the system was not suitable as molecular switch. [66]
Figure 14: Photoreaction of a methyl-substituted stilbene. [66]
This photocyclization-dehydrogenation process can be used in the synthesis of condensed aromatic ring systems. [24] It was in one of this synthesis when Kellogg et al. [67] observed the lifetime of the dihydro-type intermediates were longer when, instead of the phenyl rings of the stilbene, thiophene rings were used.
In an attempt to improve the stability of the dihydro-form and based on the work of Kellogg, [67] Mohri [66] replaced the phenyl rings by other aromatic rings, i.e. thiophene and furane, and the lifetime of the dihydro-form was much longer than in the case of stilbene
18
derivatives. In addition, in the presence of oxygen, said dihydro-intermediates did not undergo to condensed rings.
In order to shift the absorption maxima of the coloured compounds to longer wavelengths, the cyano and maleic anhydride derivatives were synthesized (Figure 15). The maleic anhydride derivative also avoided the E-Z isomerization of the double bond, which competes with the cyclization reaction.
Figure 15: Cyano (top) and maleic anhydride (bottom) derivatives.
The open isomers reacted under UV light irradiation to the meta-stable closed isomers. The photogenerated isomers did not react back in the dark during three months, nor at 80 °C a thermochromic reaction was observed. Otherwise, when they were irradiated with visible light, the open isomers were obtained again.
The difference with respect to the stilbene is the aromaticity of the rings. The lower the aromatic stabilization energy, the more stable is the closed isomer, because the ring-closure process requires the loss of the aromatic character of the before mentioned rings. More than 100 colouring and bleaching cycles were carried out showing an increased resistance to fatigue.
These are the first examples of thermally irreversible diarylethene photochromic molecular switches. [24] They laid the foundation for this important group of molecular switches, whose characteristics, thermal stability and high resistance to fatigue, make it superior to the others.
19 Electrocyclic reactions. Woodward-Hoffmann rules
Electrocyclic reactions are those where a new single bond between the termini of a linear system containing K π electrons is formed, and the converse process (Scheme 6). [68]