Photoluminescent iridium(III) complexes with phenylvinylpyridine-base cyclometalating ligands: Synthesis, photophysical properties, and photocyclization

Photoresponsive materials have recently garnered significant research interest and have been designed using various molecules. A viable approach to develop photoresponsive materials is by integrating photoisomeric structures into the organic ligands of phosphorescent transition metal complexes. In this study, we designed two photoresponsive cyclometalating ligands, 2-phenyl-3-vinylpyridine (HL1) and trans-4-methyl-2-phenyl-3-styrylpyridine (HL2), via the replacement of a phenyl group with a pyridyl group in vinylbiphenyl. Upon photoirradiation under an argon atmosphere, both ligands underwent photocyclization and demonstrated hypochromism and bathochromic shifts in absorption. Notably, HL2 underwent additional oxidative dehydrogenation upon photoirradiation in the presence of oxygen, resulting in an elongated conjugation structure that exhibited significant fluorescence intensity increase at 363 nm. Additionally, three photoresponsive iridium(III) complexes cyclometalated with HL1 or HL2 were synthesized and characterized. The photocyclization process of the complexes was much faster than that of the free ligands because the phenylpyridine moiety became coplanar upon cyclometalation. Upon photoirradiation under an argon atmosphere, these complexes exhibited significant luminescence enhancement by about 10–20 folds because of photocyclization resulting in a rigid closed structure that limited nonradiative decay of the excited complexes. These molecules present a unique strategy to construct photocyclization complexes and provide important insights into the development of photoresponsive materials.