Synthesis and Properties of Dinuclear Complexes with a Photochromic Bridge: An Intervalence Electron Transfer Switching “On” and “Off”

Abstract

Two cyclometallated Ru(bpy)2(pp) units (bpy = 2,2′-bipyridine; pp = 2-phenylpyridine) were grafted via ethynyl spacers onto the photochromic cores of the norbornadiene or dithienylethene families, to give the bimetallic ruthenium(II) complexes 1 and 2a, respectively. Two aspects of these compounds were studied: (i) photoisomerization (norbornadiene to quadricyclane for 1, open form to closed form of the dithienylethene core for 2a), and (ii) intervalence transitions in the mixed-valence ruthenium(II−III) state, with determination of the metal−metal electronic coupling (Vab) by Hush’s equation. Moderate metal−metal electronic coupling (0.068 eV) was found for 1, but 1 cannot be isomerized into the quadricyclane form. On the other hand, 2a can be reversibly photoisomerized to the closed form 2b. An iodine(III) reagent, the triflate salt of bis(pyridyl)phenyliodonium, was used for the oxidation of 2a and 2b. During these oxidations, an intervalence band due to intramolecular electron transfer between ruthenium(II) and ruthenium(III) was detected for 2b, but not for 2a. This switching “on” or “off” of the intervalence transition is rationalized with extended Huckel calculations. Finally, the molecular orbital calculation explains the observed tendency of the closed form 2b to reopen upon oxidation.