Tricolour luminescence in an Au(i) complex controlled by polymorphism and mechanical stress

Abstract

The aggregation-induced emission (AIE) behaviour of gold(I) complexes has been extensively reported. In particular, Au complexes that exhibit colour tunability towards changes in aggregated structures are becoming increasingly attractive in the field of materials science. Herein, we report CP, a rod-shaped Au complex with a cyclohexylphenyl moiety and an isocyanide ligand. The photophysical behaviour of CP in various states is discussed based on their primary and aggregated structures. CP emitted room-temperature phosphorescence (RTP) with high-contrast polymorph-dependent colours of blue (CP-B), green (CP-G), and yellow (CP-Y) depending on the recrystallisation conditions. CP-Y, which has the shortest Au–Au distance, exhibits an efficient direct S₀–T_n transition, leading to an enhanced RTP quantum yield (Φ_RTP) of 38% under S₀–T_n excitation. Interestingly, X-ray diffraction analysis revealed that CP-B and CP-G exhibit identical crystal structures despite their distinct emission colours. Photophysical and differential scanning calorimetry analyses revealed that the presence of small crystal domains, such as those in the ground samples of CP, is responsible for the overall green emission observed in bulk CP-G, indicating that the crystal structure and size determine the RTP colour. Moreover, CP also exhibits a liquid-crystalline (LC) nature and mechanochromic luminescence. Our findings demonstrate that the aggregated structure and AIE properties of Au complexes can be effectively controlled through crystalline-to-LC or crystalline-to-crystalline phase transitions of the complexes.