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2.1 Basic Concepts
Thermal irreversibility is an essential and indispensable property for the applications of molecular photoswitches to memory media and switches. Although tremendous efforts were made in the 1970–1980s to provide the thermal irreversibility to molecular photoswitches, all attempts to modify existing photoswitchable molecules failed. We had to wait until the thermally stable molecular photoswitches were serendipitously discovered. In the beginning and the middle of the 1980s, it was found that furylfulgides and diarylethenes undergo thermally irreversible photoswitching. The photogenerated colored isomers practically never revert back to the colorless isomers at room temperature. Although they undergo thermally irreversible photoswitching, the reason why the molecules show the thermal stability was not understood. It was a crucial task to reveal the reason. The basic principle behind the thermally irreversible photoswitching reactivity was elucidated using both theoretical and experimental approaches, as follows.
The 2,3‐diphenyl‐2‐butene unit, shown in Figure 1.3, underwent a thermally reversible photoswitching reaction in a deaerated solution, while the 2,3‐di(2,5‐dimethyl‐3‐thienyl)‐2‐butene unit, shown in Figure 1.4B(b) exhibited a thermally irreversible reactivity. The photogenerated closed‐ring form of 2,3‐di(2,5‐dimethyl‐3‐thienyl)‐2‐butene was found to remain stable and practically never returned to the open‐ring form at room temperature. In addition, the open‐ring isomer was stable even at elevated temperatures and no thermochromic reaction was observed. To reveal the reason why the diarylethene having phenyl rings and that having thiophene rings exhibit such a different reactivity, semiempirical modified neglect of diatomic overlap (MNDO) calculation was carried out for diarylethene derivatives 9–12 (Scheme 2.1) [1].
Scheme 2.1 Electrocyclic reactions of diarylethenes 9−12.
For electrocyclic reactions, two modes of geometrical structure changes, conrotatory and disrotatory, are distinguished, as shown in Scheme 2.2. According to the Woodward‐Hoffmann rule [2] based on π‐orbital symmetries for 1,3,5‐hexatriene (HT), which is the simplest molecular framework of the above molecules, the conrotatory cyclization reaction to cyclohexadiene (CHD) is brought about by light and the disrotatory cyclization by heat.
Scheme 2.2 Disrotatory and conrotatory cyclization reactions of hexatriene.
The cycloreversion reaction is allowed photochemically in the conrotatory mode and thermally in the disrotatory mode. From this simple symmetry consideration of the HT molecular framework, the thermal stability of the open‐ring isomer of 2,3‐di(2,5‐dimethyl‐3‐thienyl)‐2‐butene and thermal irreversibility in the cycloreversion reaction cannot be explained. A state energy calculation is necessary to discuss the thermal stability.
Figures 2.1 and 2.2 show the state correlation diagrams for the electrocyclic reactions of 9 and 11 in disrotatory and conrotatory modes, respectively. Full lines in the figures show that interconnecting states belong to the same symmetry groups. The relative ground state energy differences between the open‐ and closed‐ring isomers are shown in Table 2.1. The two heterocyclic rings were assumed to be in the parallel orientation for the disrotatory reaction and in the antiparallel orientation for the conrotatory
