Fast Negative Photochromism of 1,1'-Binaphthyl-Bridged Phenoxyl-Imidazolyl Radical Complex

Negative photochromism, in which a thermally stable colored form isomerizes to the transient colorless form by light irradiation and the back reaction occurs thermally, is advantageous in its applications for photoswitching materials because visible light can cause the photochromic color change of the materials. Moreover, the photochromic color change can be induced even on the inside of the materials due to the absence of the reabsorption of the visible excitation light by the photogenerated colorless species. While several negative photochromic compounds have been reported, the time scales of the back reaction are still slower than minutes, and no available fast responsive negative photochromic compounds have been reported. Here, we developed a negative photochromic 1,1'-binaphthyl-bridged phenoxyl-imidazolyl radical complex (BN-PIC) which enables fast photoswitching by visible light. The stable colored BN-PIC shows instantaneous decoloration by visible light irradiation, and the photogenerated colorless form thermally reverts to the initial colored form with a half-life of 1.9 s at room temperature. BN-PIC can also cause the drastic change in the chiroptical properties by the photochromic reaction, and the rate of the thermal back reaction is affected by the chirality of the solvent. Since the negative photochromic reaction can occur on the inside of the materials, the fast negative photochromism is expected to have an impact in the fields of photoresponsive materials of solid states and molecular aggregates.