Expanding the Scope of Molecular Mixed Crystals Enabled by Three Component Solid Solutions

Crystalline solid solutions (mixed
crystals) formed from molecular
compounds are a long-known class of multicomponent solids whose composition
can be varied, at least in principle, in continuum. Should such control
occur in mixed crystals over a wide range of compositions, then it
follows that some structural and physical properties would also be
tuned in continuum. However, the potential utility of mixed crystals
as molecular materials has been hindered by their limited accessibility
across a broad range of compositions and difficulties associated with
their preparation. We address these matters herein through the use
of mechanochemistry and solution crystallization to study three-component
mixed crystals. In addition to expanding the composition range of
previously known mixed crystals, we demonstrate that anthracene (ant) and phenazine (pnz), which tend to crystallize
as separate phases, form mixed crystals with a third molecule which
is miscible in both, namely, acridine (acr). The three-component
mixed crystal is only accessible mechanochemically as solution methods
afford phase separation. If this observation is general, then it could
have broad implications for this understudied class of molecular crystals.

由分子化合物构成的结晶固溶体（混合晶体，mixed crystals）是一类历史悠久的多组分固体，其组成至少在理论上可在连续区间内实现调控。若混合晶体可在宽泛的组成范围内实现此类组成调控，则其部分结构与物理性质也可实现连续调谐。然而，混合晶体作为分子材料的潜在应用价值却受限于其在宽泛组成区间内的可制备性不足，以及制备过程中的诸多难题。本文中，我们借助机械化学与溶液结晶法开展三组分混合晶体的相关研究，以解决上述问题。除拓展了已知混合晶体的组成区间外，我们还证实：通常会分别结晶形成独立物相的蒽（anthracene, ant）与吩嗪（phenazine, pnz），可与第三种同时与二者混溶的分子——吖啶（acridine, acr）——形成混合晶体。该三组分混合晶体仅可通过机械化学手段获得，溶液法制备则会导致物相分离。若该观测结果具备普适性，则可为这一尚未得到充分研究的分子晶体类别带来广泛的研究启示。