Influence of alkyl chain lengths on the properties of iridium(III)-based piezochromic luminescent dyes with triazole-pyridine type ancillary ligands

• Synthesis of Ir(III)-based dyes with different N-alkyl chains is described.
• They show naked-eye visible and reversible piezochromism.
• The chain length-dependent emissions are presented and investigated.
• The dyes exhibit high quantum yields, redox reversibility and good stability.
• The experimental results are rationalized by the theoretical calculations.

To investigate the relationship between structures and piezochromic luminescence behavior and to develop excellent materials for efficient light-emitting electrochemical cells, a series of iridium(III) complexes with different length of alkyl chains have been designed and synthesized. The results show that all complexes exhibit not only naked-eye visible and reversible piezochromic behavior, but chain length-dependent emission properties: the shorter the alkyl chain is, the more remarkable mechanochromic behavior and higher recrystallization temperature will be. In light of powder X-ray diffractometry and differential scanning calorimetric data, the interconversion between crystalline and amorphous states upon external stimuli is response for the present piezochromism. Additionally, these complexes show high quantum yields of 55–65% in neat films as well as excellent redox reversibility. Despite changed alkyl chains are introduced into these complexes, the negligible effect on emitting colors and excited-state characters of them in both solutions and neat films have been observed. Such photophysical properties are also interpreted with the help of theoretical calculations. Moreover, the theoretical results suggest that the intrinsic intramolecular π–π stacking confirmed by the crystal structures can effectively restrict the opening of the structures in metal-centered excited-state, demonstrating their stability. The structure–property relationships and the results demonstrated here will help researchers develop and design more promising iridium(III)-based piezochromic materials and attractive phosphors for optical devices.

A series of iridium(III)-based phosphorescent dyes showing naked-eye visible and reversible piezochromic luminescent behavior are successfully synthesized. Moreover, they are promising luminescent materials with high quantum yields of 55–65%, excellent redox reversibility and relative stability.Figure optionsDownload as PowerPoint slide