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, , Φ(1) = 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 (Φ(-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 τ(-OF) = 70 ps and 170 ps essentially to τ(-OF) < 10 ps) and to the values of Förster radii determined for 3 (R(0)(OF → F) = 29 Å and R(0)(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.

Multichromophoric sugar for fluorescence photoswitching

“…The latter one has been neglected in the following discussion, since it is very close to the time-resolution of our instrument, and associated with a very low fraction of intensity (0.02). The two other time-constants are attributed to the E -isomer (τ 1 ) and the Z -isomer (τ 2 ) of the fluorophores, which is consistent with our previous studies [4]. The fluorescence decays of 2 -OF and 2 after irradiation at 335 nm are plotted in Fig.…”

“…3), associated with a fluorescence quantum yield Φ F = 0.12. These spectroscopic characteristics are rather comparable to the absorption and fluorescence features of the fluorophore 5 [4]. The photophysical properties of the photochromic model compound 9 have been described in a previous report [4].…”

“…Bifunctional molecules built on two distinct (photochrome and fluorophore) moieties have also been reported by other groups [5–9]. The fluorophore vs photochrome ratio in reported bifunctional systems is 1:1 [4–7] or 2:1 [8–9]. Other strategies to assemble interacting fluorophores and photochromes (supramolecular systems, nanoparticles, polymer materials) are also reported [10–13].…”

“…These spectroscopic characteristics are rather comparable to the absorption and fluorescence features of the fluorophore 5 [4]. The photophysical properties of the photochromic model compound 9 have been described in a previous report [4]. Briefly, the diarylethene 9 is originally in its colorless open form ( 9 -OF), with an absorption spectrum in acetonitrile located in the UV range (λ max = 317 nm, Fig.…”

“…2) combining a DCM fluorophore (4-dicyanomethylene-2- tert -butyl-6-( p -dialkylaminostyryl)-4 H -pyran) [3] with a photochromic diarylethene (DAE) which showed a photoreversible two-way FRET controlled by the state of the photochromic moiety, and 49% quenching of the fluorescence upon UV irradiation [4]. Bifunctional molecules built on two distinct (photochrome and fluorophore) moieties have also been reported by other groups [5–9].…”