Electron Donating Group Effect on C─C Bond Scission Ability of Indanedione Dimers in Their Solution and Solid State

Indanedione dimers bearing electron-donating thiophene groups show low-temperature C─C bond scission in solution via the captodative effect. This feature was significantly changed in the solid state, showing much higher bond scission temperature in the dimethylthiophene derivative, while the one with methylbenzothiophene unit showed comparative bond scission temperature and even melted via glass formation.

Abstract

Light, heat and mechanical stress-responsive molecules are of great interest in the field of modern materials chemistry. In this work, indanedione dimers 1–3 introduced with an electron-donating thiophene group were designed with the expectation of easy cleavage of the central C─C single bond by the captodative effect. Our synthetic approach led to the successful preparation of dimethylthiophenyl and methylbenzothiophenyl derivatives 1 and 2, while benzothiophenyl compound 3 was not formed; instead, the bridged product 3' was obtained. The sequential UV–vis absorption and ESR measurements of 1 and 2 in the solution state confirmed the effectiveness of the presence of an electron-donating group, which stabilized the radical form to realize a relatively low bond cleavage temperature. Meanwhile, it was also clarified that the bond scission event was strongly affected by the packing structure; much harder in the tightly packed crystalline 1 than its solution state, while almost the same as the solution state in the glassy solid of 2, in which the intermolecular interaction should be much weaker and each molecule could move as easily as in the solution state. Further investigation of 3' revealed the loss of bond scission ability by bridging.

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