To elucidate the reversible formation of radicals, a timedependent electron paramagnetic resonance (EPR) study was conducted under irradiation of a sample of 1c with various light sources at 300 K. While the sample (1c) exhibited no EPR signal initially, a strong signal at g = 2.005 emerged upon exposure to 370 nm light (43 W) (Figure ). This signal progressively intensified with prolonged irradiation, indicating the formation of the radical anion (1c .-). Irradiation at a wavelength where the photochromic unit isomerizes to the open form, the EPR signal was found to diminish. Upon irradiation at a wavelength, which leads to isomerization of the close form of the photochromic unit to its open form, a decrease in the EPR signal was observed. Thus, exposure at 525 nm resulted in a notable reduction in the EPR signal, indicative of the radical quenching (refer to Figure ). Additionally, in the UV-visible experiment, the absorption bands associated with the radical anion 1c .-(in the closed form of the DHP unit) exhibited a decrease upon exposure to 525 nm, implying the formation of the cyclophane (CPD) isomer with the simultaneous generation of the benzenoid band at 265 nm typical for the CPD isomer (1c .-→ 1o) (see Figure ).