Redox-modulated stepwise photochromism in a ruthenium complex with dual dithienylethene-acetylides

Achieving stepwise photochromism in a combined molecule to access all of the possible ring-open/closed isomers is a challenge due to facile energy transfer from ring-open dithienylethene (DTE) to an adjacent ring-closed moiety that prohibits further photocyclization. The preparation, characterization, and photochromic properties of a bis(σ-acetylide) bonded ruthenium(II) complex 2oo and its oxidized form 2oo(+) with two identical DTE-acetylides (L1o) are described. Stepwise and dual photochromic reactions are successfully achieved in both 2oo and 2oo(+), in which the ring-closing absorption band of 2oo(+) shows an obvious blue-shift relative to 2oo. It is demonstrated that stepwise photochromic reactions 2oo→2co→2cc are more facile than 2oo(+)→2co(+)→2cc(+). The lower electronic density at the reactive carbon atoms upon oxidation of Ru(II) to Ru(III) causes photocyclization to have more difficulty proceeding in oxidized species 2oo(+)/2co(+). Upon dual ring-closure, 2cc/2cc(+) exhibits significant electronic interaction between two identical ring-closed DTE units across trans-Ru(dppe)(2) spacer. The interconversion processes among six states are unambiguously demonstrated by NMR, UV-vis-NIR, and IR spectroscopic, and electrochemical and computational studies.