Structural studies, thermochromic luminescent properties and crystal-to-crystal transformation of a double-stranded...

The development of thermochromic luminescent materials with reversible chemical transformation capabilities based on earth-abundant metals is a central topic in materials chemistry. While several stimuli-responsive materials have been reported, compounds that respond to physical stimuli and can undergo single-crystal-to-single-crystal transformations (SCSC) are still rare. Herein, a new color-emitting Cu^I coordination polymer (CP), ^1D{[Cu₂(µ-I)₂(L)₂]·(CH₃CN)}, (L = synthetic ligand, N ²,N ⁶-bis(quinolin-5-yl)pyridine-2,6-dicarboxamide), CP1 was prepared, structurally described by single crystal X-ray diffraction at different temperatures and studied in detail as a multi-responsive luminescent material to temperature and mechanical force in the solid state. Upon excitation at 413 nm and low temperature of 97 K, CP1 displays intense orange emission at 618 nm which is linearly quenched up to 191 K, subsequently, its emission is weakened under ambient conditions. The cuprophilic interactions (d _Cu-Cu= 2.848 Å) formed at low temperature are particularly responsible for the intense color emission as evidenced by crystallographic studies and DFT calculations. Upon mechanical grinding of crystalline CP1, the amorphous material (CP1-G) presents a bright orange emission at 620 nm with a quantum yield of 0.51 and bi-exponential lifetime of τ ₁ = 2.17 µs and τ ₁ = 7.62 µs at ambient conditions. Based on multiple analytical tools (fluorescence, quantum yields, PXRD, IR-ATR, and lifetime) and SEM observations, this color emission is attributed to a desolvation process, involving CH₃CN molecules' release and formation of cuprophilic interactions. Mechanochromic luminescence and the crystalline phase of CP1 can be reversibly restored by exposing the amorphous material to CH₃CN vapors. Furthermore, CP1 exhibits a reversible SCSC transformation whereby the 1D-double-stranded stair polymer is converted into a 1D zigzag chain ^1D[CuI(L)], CP2, by controlled heating with drastic changes in the Cu^I coordination environment and in the crystal lattice. This work showcases the potential of Cu^I-I-bisquinoline-based materials for new applications.