Vapochromic Luminescent π-Conjugated Systems with Reversible Coordination-Number Control of Hypervalent Tin(IV)-Fused Azobenzene Complexes.

We report dynamic and reversible changes of coordination numbers between five and six in solution and solid states based on hypervalent tin(IV)-fused azobenzene (TAz) complexes. It was found that the TAz complexes showed deep-red emission owing to the hypervalent bond composed of an electron-donating three-center four-electron (3c-4e) bond and an electron-accepting nitrogen-tin (N-Sn) coordination. Furthermore, hypsochromic shifts in optical spectra were observed in Lewis basic solvents because of alteration of the coordination number from five to six. In particular, vapochromic luminescence was induced by attachment of dimethyl sulfoxide (DMSO) vapor to the coordination point at the tin atom accompanied with a crystal-crystal phase transition. Additionally, the color-change mechanism and degree of binding constants were well explained by theoretical calculation. To the best of our knowledge, this is the first example of vapochromic luminescence by using stable and variable coordination numbers of hypervalent bonds.