Modulation of Unconventional Fluorescence of Novel Photochromic Perimidine Spirodimers

Fluorescence modulation by a class of photochromic perimidine spirodimers, which exhibit a characteristic fluorescence associated with their photochromic reactions, has been described. Upon irradiation using 365 nm light, these non‐fluorescent spiro molecules undergo a thermally‐reversible ring opening at their spiro junction resulting in the generation of strong fluorescence. The fluorescing species is distinctly different from both the stable ring‐closed and the ring‐opened compounds, though it appears to have been formed from and remains in equilibrium with the photochemically generated ring‐opened form. While the fluorescing species possesses a narrow absorption band with its maximum centered at 500 nm, the ring‐opened form exhibits a broad absorption across the visible region with two maxima centered at 410 and 650 nm, respectively. After initiating the photochromic reactions in these molecules using 365 nm light, purely photochemically‐controlled fluorescence modulation can be carried out using two wavelengths in the visible region, that is, 500 and 700 nm, while the equilibrium concentration of the ring‐opened form and the fluorescing species is controlled. Fluorescence modulation is attained also by controlling the ratio of the ring‐closed and ring‐opened forms by photochemical ring‐opening and thermal ring‐closing reactions. The study on the effect of substitution of these molecules suggests that by extending the conjugation of the perimidine core in the ring‐opened form the molecule is rendered non‐fluorescent and hence it can be assumed that the perimidine core forms the fluorescing entity of the molecule.