Limits of Molecular Dithienylethene Switches Caused by Ferrocenyl Substitution.

The efficiency of photochromic switches can be modified by attaching organic or organometallic groups to the photochromic core. We studied ferrocene-substituted dithienylethene switches differing by the size of the cycloalkene ring bridging the two thiophene groups. The results were compared with their chlorine-substituted counterparts and an ethynyl-ferrocene substituted switch published earlier by Guirado and co-workers. From the measured UV/Vis spectra, both ferrocene-substituted compounds were found to be considerably less likely to switch than the corresponding chlorine-substituted ones. Kohn-Sham density functional theory calculations suggested that this is due to a multitude of energetically close-lying excited states in the former, which may offer multiple pathways for excitation and relaxation, out of which only one leads to ring opening or closing. By contrast, the chlorine-substituted switches have one energetically more isolated state that is responsible for the switching. The increase in the available excited states in the ferrocene-substituted switches was attributed to mixing between orbitals from the ferrocene units and the π system of the bridge.