Limits of Molecular Dithienylethene Switches Caused by Ferrocenyl Substitution Alejandra Escribano, Torben Steenbock, Carmen Herrmann, and Jürgen Heck* [a] 1. Introduction Molecular switches are molecules that are capable of intercon- verting reversibly between two (or more) (meta)stable states upon application of an external stimulus. If the switching reac- tion is induced by light, these molecules are known as photo- chromic switches. The different chemical and physical proper- ties of these (meta)stable states can be applied in photonic devices. [1, 2] Dithienylethene (DTE) molecular switches are characterized by their thermal stability and resistance to fatigue. [1] These two properties make them superior to other groups of molecular switches and perfect candidates for the development of new responsive materials that could be applied in fields such as op- tical data storage [1, 3] and electronic communication through molecular wires. [4] Diarylethene molecular switches are formed by two aromatic subunits, normally thiophene rings, which are linked through an alkene bridge or a cycloalkene ring (Scheme 1). If the open state of the dithienylethene switch is irradiated with UV light of the appropriate wavelength, photo- cyclization occurs. A new bond between the methylated carbon atoms of the thiophene units is formed, and the entire p-bonding arrangement is fixed in one plane (Scheme 1, right). In the resulting closed state the p conjugation extends over the entire molecule and links both functional R groups elec- tronically. The back reaction is activated by visible (Vis) light. In the open state (Scheme 1, left), the p conjugation is restricted to the aromatic thiophene units. More than 2000 papers focused on this group of molecular switches [5] have been published, and this is indicative of the importance of these compounds in current scientific research. Hundreds of derivatives from those photochromic molecules have been studied so far: symmetrically [6, 7] and nonsymmetri- cally [8] substituted ones, with different aromatic rings instead of thiophene, [9] with different cycloalkene sizes connecting both terminal aromatic rings, [10, 11] and with different organic [7] and organometallic substituents. [12] Their reversible photo- switching behavior has been studied not only in solution but also in the crystalline state, [8, 13] as liquid crystals, [14] and even as gels. [1, 15] The concept of combining organometallic complexes with diarylethene molecular switches is relatively new, [16] but a long list of such coordination compounds and their properties have already been studied. [16–24] The photochromic behavior of this type of molecular switch depends on the substituents attached to the thiophene units (5’-positions, Scheme 1) and on the nature of the cycloalkene unit that connects the two aromatic rings. [10, 19] This permits systematic modification of the switch- ing properties. By introducing a metal center into the substitu- ents attached to the switching core (normally at the 5’-posi- tion), it is possible not only to vary the switching reaction but also to combine it with the photophysical, [25] electrochemi- cal, [21–23, 25] optical, [24] and magnetic [16, 20, 22] properties of the co- ordination compounds. [16] In addition, combining transition- metal units with molecular switches can change their excited- state reactivities and spectroscopic properties. [22, 24] Different examples of metal coordination modulating elec- tron-transfer, [24] fluorescence, [26] nonlinear optical, [16] and emis- sion [17, 24] responses have been described. One of our groups The efficiency of photochromic switches can be modified by attaching organic or organometallic groups to the photochro- mic 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-ferro- cene 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 sug- gested 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-sub- stituted switches have one energetically more isolated state that is responsible for the switching. The increase in the avail- able excited states in the ferrocene-substituted switches was attributed to mixing between orbitals from the ferrocene units and the p system of the bridge. [a] Dr. A. Escribano, T. Steenbock, Prof. Dr. C. Herrmann, Prof. Dr. J. Heck Institut für Anorganische und Angewandte Chemie, Universität Hamburg Martin-Luther-King-Platz 6, 20146, Hamburg (Germany) E-mail : heck@chemie.uni-hamburg.de Supporting Information and the ORCID identification number(s) for the author(s) of this article can be found under http://dx.doi.org/10.1002/ cphc.201600085. Invited contribution to a Special Issue on Molecular Machines ChemPhysChem 2016, 17, 1881 – 1894 # 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1881 Articles DOI: 10.1002/cphc.201600085