Fig. 4. (A) Representation of the reversible photochromism of c-H3. (B) Absorption spectral profile of 30 μM c-H3 (in CH₃CN) upon irradiation with blue light (blue LED, 2.25 mW cm⁻²). (C) Absorption spectral profile of 30 μM c-H3′ + c-H3′′ (in CH₃CN) upon irradiation with red light (red LED, 1.55 mW cm⁻²). (D) Reversibility of the photochromism of c-H3 (10 μM in CH₃CN). The absorbance value at λ_max = 654 nm was monitored for switching study upon consecutive irradiation with blue light for 3 min (green dots) and red light for 45 min (red dots). (E) Schematic diagram of photochromic film fabrication of c-H3 in the PMMA gel matrix (2.3 wt%) on a PET substrate. (F) Reversibility of the photochromism of c-H3 in the PMMA gel matrix coated on a PET substrate. The absorbance value at λ_max = 654 nm was monitored for switching study upon consecutive irradiation with blue light (blue LED, 5.10 mW cm⁻²) for 3 min (green dots) and heating at 100 °C for 2 min (red dots). (G) Optimized structures along with the helical pitch and inter-planar angle between the two wings of the helical rim values of the ring-opened and ring-closed isomers of c-H3 (calculated by DFT at the B3LYP/6-31g(d,p) level of theory).