Luminescent Soft Crystals That Exhibit Color Changes in Response to Vapor

It is well-known that square-planar Pt(II) complexes exhibit characteristic luminescence when the complex units are stacked with Pt···Pt short contacts. Such assembled systems have interesting properties that the luminescence color can change sensitively in response to the surroundings such as temperature, pressure, and vapor. Therefore, the Pt(II) complexes are attractive as flexible and ordered sensing materials called soft crystals. In this presentation, the latest topics of vapochromic Pt(II) complexes are introduced.

1. Wide-range color tuning of luminescence based on the fine control of the Pt···Pt interactions.¹⁾ A series of assembled Pt(II) complexes comprising N-heterocyclic carbene and cyanide ligands was constructed using different substituent groups (Figure 1). All the complexes exhibited highly efficient photoluminescence with an emission quantum yield in the solid state at room temperature, originating from the triplet metal-metal-to ligand charge transfer (³MMLCT) state. Their emission colors cover the entire visible region from red to blue by changing the substituents. It is notable that blue ³MMLCT emission was realized firstly in this system. In addition, vapochromic response was observed based on the single-crystal-to-single-crystal transformation.

2. Fast and stable vapochromic response induced through nanocrystal formation of a luminescent platinum(II) complex on periodic mesoporous organosilica.²⁾ A hybrid vapoluminescent system exhibiting fast and repeatable response was constructed using periodic mesoporous organosilica with bipyridine moieties (Bpy-PMO) and a Pt(II) complex bearing a potentially luminescent 2-phenylpyridinato ligand. An intense red luminescence appeared when the Pt(II)-complex immobilized Bpy-PMO was exposed to methanol vapor and disappeared on exposure to pyridine vapor. The ON-OFF vapochromic behavior occurred repeatedly in a methanol/ pyridine/heating cycle. Interestingly, a rapid response was achieved in the second cycle and cycles thereafter. The unique vapoluminescence triggered by the unprecedented protodesilylation on exposure to protic solvent vapor at room temperature is attributable to Bpy-PMO being a giant ligand and an effective vapor condenser. Consequently, this hybrid system presents a new strategy for developing sensors using bulk powdery materials.

¹⁾ D. Saito et al., Angew. Chem. Int. Ed. 2020, 59, 18723–18730.

²⁾ H. Matsukawa et al., Sci. Rep. 2019, 9:15151.

Figure 1