A series of novel fluorescent isomers were designed and synthesized by using 4-azafluoren-9-one (IP) as electron acceptor and carbazole as electron donor. The substitution position of carbazole on IP was adjusted to form IP-6-PhCz, IP-7-PhCz, IP-8-PhCz and IP-9-PhCz. The photoluminescent, electroluminescent and electrochemical properties of the four fluorescent dyes were characterized by UV absorption spectroscopy, fluorescence emission spectroscopy, electrochemical analysis. Experiment result shows that the substitution position of Cz on IP leads to different molecular conformation and determines the luminescent behavior of these isomers. The isomers IP-6-PhCz and IP-9-PhCz with larger torsion angles due to strong steric hindrance exhibited thermally activated delayed fluorescence (TADF) feature, while IP-7-PhCz and IP-8-PhCz emit only normal fluorescence. IP-7-PhCz showed higher photoluminescent quantum yields of 29% and realized the best performance with a maximum external quantum efficiency of 2.8% in organic light-emitting diode.