A series of stable heteroleptic Ir complexes bearing a pair of 2-phenylpyridine as main ligands and 2-(diphenylphosphino)phenol as an auxiliary ligand were synthesized to achieve efficient luminescence upon photochemical isomerization. Photoisomerization of an N,N-trans isoform, where a pair of nitrogen atoms of 2-phenylpyridine ligands were located at the trans position, using a high-pressure Hg lamp