Light-Triggered Metal Coordination Dynamics in Photoswitchable Dithienylethene-Ferrocene System.

The C2-symmetric photochromic molecule 3, containing dithienylethene (DTE) and ferrocene units connected by an alkyne bridge, represents a unique probe where a metal (Hg2+) binds with the central DTE moiety. Both photoisomerized states of 3 (open, 3o; closed, 3c) are found to interact with Hg2+ ion by the S atoms of the DTE core; however, the binding constants (from a UV-vis study) and DFT calculations suggest that the open isomer (3o) binds with the metal ion more strongly than that of the closed isomer (3c). Notably, the course of metal binding does not perturb the inherent photoisomerization properties of the DTE core and the photoswitchability persists even in the metal-coordinated form of 3, however, with a comparatively slower rate. The quantum yields for photocyclization (Φo→c) and photocycloreversion (Φc→o) in the free form are 0.56 and 0.007, respectively, whereas the photocyclization quantum yield in the Hg2+ complexed species is 0.068, 8.2 times lower than the photocyclization quantum yield (Φo→c) of free 3o. Thus, the rate of photoisomerization can be modulated by a suitable metal coordination to the DTE core. The dynamics of photoswitchability in the metal-coordinated form of DTE has been explored by experimental means (UV-vis and electrochemical studies) as well as quantum chemical calculations.