Solvent- and Vapor-Induced Isomerization between the Luminescent Solids [CuI(4-pic)]₄ and [CuI(4-pic)]∞ (pic = methylpyridine). The Structural Basis for the Observed Luminescence Vapochromism

Exposure of the polymeric solid [CuI(4-pic)] ∞ (pic = picoline = methyl pyridine) to liquid or vapor toluene leads to disappearance of its room-temperature blue emission (λ max 437 nm) and the appearance of a yellow emission (λ max 580 nm) characteristic of the [CuI(4-pic)] 4 tetramer. The process is reversed when the latter is exposed to liquid or vapor n -pentane. Analogous transformations between the tetrameric and polymeric forms do not occur when the 3-picoline analogues [CuI(3-pic)] x are similarly treated. Single-crystal X-ray diffraction studies on the compounds [CuIL] ∞ and [CuIL] 4 (L = 3-, 4-pic) indicate that the 4-pic tetranuclear isomer incorporates toluene into its solid phase to give a material with the composition [CuI(4-pic)] 4 ·2C 6 H 5 CH 3, but the other three phases are solvent-free. The chains in the two polymeric phases exhibit double-zigzag configurations, also commonly observed in zeolitic tetrahedral structures. In both polymeric phases, the chains propagate along the monoclinic b axis. The 3-pic tetrameric phase can be described as a close-packed structure of [CuIL] 4 units, whereas tetramers in the 4-pic phase form infinite columns along the unique tetragonal c axis segregated by four columns of toluene pairs. These structural differences explain the different behaviors during the phase transformation between tetrameric and polymeric polymorphs of the 3-pic and 4-pic compounds.