Electrochemistry of photochromic [2.2]paracyclophane-bridged imidazole dimers: rational understanding of the electronic structures.

[2.2]Paracyclophane-bridged imidazole dimers, which show unique fast photochromism, have various practical applications in industry. To put them to practical use, it is necessary to prepare various types of the imidazole dimers which have different color, reaction rate, sensitivity, etc. One of the simple methods for modulating the optical properties is to add substituents and sensitizers. However, it is difficult to estimate the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the imidazole dimers by optical spectroscopy because the LUMO of the imidazole dimers are optically inactive. In the present study, we applied electrochemistry and density functional theory to reveal the effect of substituents on the electronic states of the imidazole dimers. We revealed that the HOMO and LUMO of the imidazole dimers are localized over only one of the imidazole rings of the imidazole dimer. By comparing the measured LUMO energies of the imidazole dimers and calculated LUMO energies of several visible sensitizers, we found which visible sensitizers work in the imidazole dimer systems. These fundamental insights provide useful information for understanding the electronic structures of the imidazole dimers and give a strategy for designing novel fast photochromic molecules whose photochromism is triggered by visible light.