We report a new family of photochromic (1-(4-R-phenyl) − 4,4′-bipyridinium; R = F, Cl, Br, I), R-pbpy-Cl (R = F, Cl, Br, I) compounds, which undergo rapid and reversible light-driven transformation in both the solid state (powders and crystals) and embedded in polymer matrices. Upon UV irradiation at 365 nm, the bromine-substituted derivative (Br-pbpy-Cl) undergoes a pronounced colour change from yellow to green, attributed to the to the formation of the corresponding radical cation, as evidenced by UV–Vis and EPR spectroscopies. In polymer films, the photochromic response is strongly concentration-dependent: at low chromophore loadings, isolated radicals initially dominate and progressively convert into a second species associated with dimers, whereas higher concentrations favour radical–radical interactions that directly lead to this second species. In the presence of oxygen, both species, radicals and dimers are stabilized over extended timescales (from several hours to days), allowing detailed spectroscopic characterization. This oxygen-mediated, concentration-dependent interplay between radical recombination and dimer stabilization offers a tuneable handle over the kinetics and optical response of these photochromic materials. Altogether, these findings highlight Br-pbpy-Cl derivatives as a versatile platform for designing reversible, and fast-responsive solid-state and polymeric photochromic materials.