Rapid fluorescence switching by using a fast photochromic [2.2]paracyclophane-bridged imidazole dimer

Recently, we have developed a series of fast photochromic imidazole dimers with a [2.2]paracyclophane ([2.2]PC) moiety that bridge diphenylimidazole units and succeeded the acceleration of the thermal decoloration rate. The colorless [2.2]PC-bridged imidazole dimers show a photoinduced homolytic bond cleavage of the C-N bond between the imidazole rings to give a pair of colored imidazolyl radicals upon UV light irradiation, followed by the radical-radical coupling reaction to form the initial C-N bond between the imidazole rings. The decoloration reaction to give the initial imidazole dimer proceeds only thermally. The high quantum yield close to unity of the photochromic reaction and the large extinction coefficient of the radical achieve both high optical density at the photostationary state and rapid switching speed. The application to rapid fluorescence switching has been investigated to develop a new type of photochromic fluorescence switching molecule applicable to super-resolution microscopy. The widespread absorption of the colored radical lying between 500 and 900 nm enables the efficient quenching of the excited electronic state of the fluorophores by Förster resonance energy transfer (FRET) from the fluorophores to the radical moiety. We successfully developed a [2.2]PC-bridged imidazole dimer possessing a fluorescein moiety as a fluorescence unit. This photochromic dye shows fast photochromism to give a pair of imidazolyl radicals that quench the fluorescence from the fluorescent unit by the FRET mechanism. The fluorescence intensity can be switched rapidly with the fast photochromism.