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.