Fluorescence photoswitching and photoreversible two-way energy transfer in a photochrome–fluorophore dyad

Abstract

A thorough photophysical study of a photochrome–fluorophore dyad (3), combining a fluorescent laser dye (DCM-type, 1, Φ₁ = 0.27) and a photochromic diarylethene (2), obtained by click chemistry, is presented. In addition to photochromism, the open form (OF) of 2 exhibits fluorescence (Φ₂2_-OF = 0.016), whereas the closed form (CF) does not. Fluorescence is switched upon alternate UV/visible irradiation of 2. The emission band of 2-OF matches the absorption band of 1 (400–550 nm), whereas the emission band of 1 overlaps the absorption band of 2-CF (550–700 nm). Therefore, a photoreversible two-way excitation energy transfer (EET), controlled by the state of the photochromic moiety, is obtained in the dyad 3. Their efficiencies are quantified as Φ^EET_OF→F = 85% and as Φ^EET_F→CF ∼ 100% from the comparison of emission and excitation spectra between 1, 2, and 3. These results are fully compatible with the shortening of fluorescence lifetimes (from τ₂2_-OF = 70 ps and 170 ps essentially to τ₃333_-OF < 10 ps) and to the values of Förster radii determined for 3 (R₀(OF → F) = 29 Å and R₀(F → CF) = 71 Å), evidencing a Förster-type resonance energy transfer mechanism (FRET). An important outcome of this two-way FRET is the possibility to quench 49% of the fluorescence in 3 at PSS upon UV irradiation, corresponding to the conversion extent of the photochromic reaction, which is different from 2 (α_CF = 91%). This is a clear example of a situation where the presence of FRET between the photochromic unit and the fluorophore affects noticeably the photochromic properties of the dyad molecule 3.