Photochromic Organometallics with a Dithienylethene (DTE) Bridge, [YCCDTECCY] (Y={MCp*(dppe)}): Photoswitchable Molecular Wire (M=Fe) versus Dual Photo‐ and Electrochromism (M=Ru)

Abstract

Dinuclear acetylide-type complexes bridged by a photochromic dithienylethene unit (DTE), [Y-C[triple bond]C-DTE-C[triple bond]C-Y] 1 (Y={MCp*(dppe)}; Cp*=pentamethylcyclopentadienyl, M=Fe (1(Fe)), Ru (1(Ru))), have been prepared, and their wirelike and switching behavior, as well as their oxidation chemistry has been investigated. The DTE complexes 1 exhibit photochromic behavior in a manner similar to organic DTE derivatives; UV irradiation causes ring closure of the open isomer 1O to form the closed isomer 1C and visible-light irradiation of the resultant 1C causes reverse ring opening to regenerate 1O. But the performance is dependent on the metals. With respect to the interconversion rates and the 1C content at the photostationary state under UV irradiation, the ruthenium complex 1(Ru) is superior to the iron analogue 1(Fe). The wirelike performance is associated with the photochromic processes, and the efficient switching performance has been verified for 1(Fe) as characterized by the V(ab) values [V(ab) is the electronic coupling derived from intervalence charge-transfer (IVCT) bands: V(ab)(1(Fe)C; ON)=0.047 eV versus V(ab)(1(Fe)O; OFF)=0 eV], and are also supported by the large switching factor (SF=K(C)(C; ON)/K(C)(O; OFF)=39; K(C)=comproportionation constant). SF for 1(Ru) is determined to be 4.2. The remarkable switching behavior arises from the different pi-conjugated systems in the two isomeric forms, that is, cross-conjugated (1O) and fully conjugated pi-systems (1C). It was also found that, in contrast to the reversible redox behavior of the iron complex 1(Fe), the ruthenium complex 1(Ru)O undergoes oxidative ring closure to form the dicationic species of the closed isomer 1(Ru)C(2+) and, thus, the ruthenium system 1(Ru) shows dual photo- and electrochromism. The distinct oxidation behavior of 1(Fe) and 1(Ru) can be ascribed to the spin distribution on the diradical intermediates 1(Fe)O(2+) and 1(Ru)O(2+), as supported by DFT calculations.