Photochromism and photofluorochromism of arylvinylene phenanthridines

Abstract

Novel phenanthridine-based light-induced, multicolour photochromic π-conjugated compounds (PC₁–PC₃) have been developed. PC₁–PC₃ were synthesised via Knoevenagel condensation on phenanthridine using various aromatic aldehydes [triphenylaminyl (TPA) and pyrenyl (PY)] in the presence of benzoic acid and benzoic anhydride in 63–70% yields. The compounds absorb strongly in the visible region (λ_max = 360–408 nm for PC₁, λ_max = 360 nm for PC₂, λ_max = 385 nm for PC₃). Interestingly, the solutions of these compounds undergo photo-transformation (under 254 nm UV light) with a colour change from yellow to purple, orange, and brown, respectively, for PC₁–PC₃, accompanied by a redshift of 120–140 nm in the absorption maxima. This transformation is rapid (within 60 seconds) and reverts to its original state by heating the solution at 35 °C for 30 minutes. In addition, PC₁–PC₃ also exhibit variations in fluorescence, with PC₁ and PC₂ red-shifting by 150 nm (yellow to red emission) and PC₃ undergoing complete quenching. We attribute this transformation to a photoexcited partial proton transfer, leading to a metastable hydrogen bonding state, as supported by ¹H NMR and DFT calculations.