The unique optical and magnetic properties of organic biradicaloids on polycyclic aromatic hydrocarbons are of fundamental interest in the development of novel organic optoelectronic materials. Open-shell π-conjugated molecules with helicity have recently attracted a great deal of attention due to the magnetic-field-dependence and spin-selectivity arising from the combination of helical chirality and electron spins. However, the molecular design for helical biradicaloids is limited due to the thermal instability and high reactivity. Herein, we achieved the fast photochromic reactions and the reversible photo-generation of biradical species using helicene-bridged imidazole dimers. [9]Helicene-bridged imidazole dimer exhibits the reversible photochromism upon UV light irradiation. The transient species produced reversibly by UV light irradiation exhibited ESR spectra with a fine structure characteristic of a triplet radical pair, indicating the reversible generation of the biradical. The half-life of the thermal recombination reaction of the biradical was estimated to be 29 ms at 298 K. Conversely, a substantial activation energy barrier was confirmed for the intramolecular recombination reaction in [7]helicene-bridged imidazole dimer, attributed to the extended pitch length of [7]helicene. The temperature dependence of the thermal back reactions revealed that the [7]helicene and [9]helicene moieties functioned as 'soft' and 'hard' molecular bridges, respectively. These findings pave the way for future advances in the development of photoswitchable helical biradicaloids.

This article is Open Access

Please wait while we load your content... Something went wrong. Try again?