Photo- and electro-chromic organometallics with dithienylethene (DTE) linker, L2CpM-DTE-MCpL2: Dually stimuli-responsive molecular switch

Abstract

Photochromic dithienylethene (DTE) derivatives, M-DTE-M (M: M(η⁵-C₅R₅)L₂; M = Fe, Ru; R = H, Me; L = CO, phosphine), with direct σ-bonded, redox-active organometallic attachments are prepared and their response to photo- and electro-chemical stimuli as well as wire-like and switching performance has been investigated. These properties turn out to be dependent on the metal and the auxiliary ligands. The DTE complexes with the MCp(CO)₂ and RuCp(CO)(PPh₃) fragments undergo reversible photochemical ring-closing and -opening of the DTE moiety upon UV and visible-light irradiation, respectively, whereas the other FeCp(CO)(PPh₃) and Fe(η⁵-C₅R₅)(dppe) derivatives are virtually inert with respect to the photochemical ring closing process. Electrochemical analysis of the DTE complexes reveals that 2e-oxidation of the open isomer O also brings about the ring closure of the DTE moiety to afford the Fischer-carbene-type, dicationic closed derivatives C²⁺ with the π-conjugated system different from that in the neutral ones C obtained photochemically. Subsequent reduction of C²⁺ furnishes the neutral closed species C. Thus the ring closure is mediated not only by the conventional photochemical process but also by the sequential oxidation–reduction process, i.e. the organometallic DTE complexes are found to be dually photo- and electro-chromic. It is notable that the oxidative procedures are viable for the photochemically inert derivatives. Wire-like and switching performance has been evaluated on the basis of the comproportionation constant K_C for the 1e-oxidized mixed valence monocationic species obtained by the electrochemical analysis and the switching factor SF (K_C(C)/K_C(O)), respectively. The K_C(C) (7.5 × 10⁴) and SF values (5.4 × 10³) for phosphine-substituted derivatives are significantly large, as a result of the distinct π-conjugated systems of the DTE moieties involved in the O- (with cross-conjugation) and C-forms (fully conjugated). Compared to the previously reported acetylide-type complexes bridged by a DTE linker, (dppe)Cp*M-CC-DTE-CC-MCp*(dppe), both parameters have been significantly improved by factors of ∼150. Time-dependent DFT analysis for the photochemical processes has revealed that the ring-closing process occurs not only via the ligand centered singlet excited state but also via the ligand centered triplet state resulting from energy transfer processes between the ligand- and metal-centered excited states and that this proposed mechanism can account for the photochemical reactivity of ruthenium complexes superior to that of the corresponding iron derivatives.