Unraveling ultrafast dynamics of the photoswitchable bridged dithienylethenes under structural constraints.

The excited state dynamics of constrained photochromic benzodithienylethenes were addressed by considering the bridging with polyether chains (from x = 4 to 6 units) at the ortho and meta positions of the aryl group, named DTE-o and DTE-m, via time-resolved absorption spectroscopy supported with (TD)-DFT calculations. The photochromic parameters and geometrical structures of these series are discussed. A novel photocyclization pathway via a triplet state, evidenced recently (Hamdi et al., Phys. Chem. Chem. Phys., 2016, 18, 28091-28100), is largely dependent on the length and the position of the polyether chain. For the first time, by comparing the two series, we revealed, for the DTE-o series, an interconversion not only in the ground state but also between the triplet states of the anti-parallel and parallel open form conformers.