Two novel blue emitters, 10-ethyl-3-(1,4,5-triphenyl-1H-imidazol-2-yl)-phenothiazine (PTHBI) and 10-ethyl-3-(1-phenyl-1H-phenanthro[9,10-d]imidazole-2-yl)-phenothiazine (PTHPI), employing a donor (10-ethyl-phenothiazine) and an acceptor (1,4,5-triphenyl-1H-imidazole and phenanthro[9,10-d]imidazole, respectively), were designed and synthesized. By breaking the C‒C bond of the phenanthro[9,10-d]imidazole acceptor, we obtain the acceptor, 1,4,5-triphenyl-1H-imidazole, with a more twisted conformation. This subtle molecular structure change leads to large differences in the optoelectronic properties of PTHBI and PTHPI. The fluorescent organic light-emitting diodes (OLEDs) were fabricated and used as the emitting layer, and all of the OLEDs showed strong blue emission. Thus, the PTHBI-based non-doped device exhibited an emission peak at 460 nm, which is bluer than the emission peak (476 nm) for the PTHPI-based device. However, the PTHPI-based OLED device exhibited a maximum current efficiency of 7.20 cd A⁻¹, maximum power efficiency of 4.00 lm W⁻¹, and external quantum efficiency of 3.41%, which are better than the corresponding values for the PTHBI-based device (luminous efficiency = 3.44 cd A⁻¹ and external quantum efficiency = 2.38%). This is caused by the twisting degree of the acceptor units. This is the first observation of blue emission using a phenothiazine-based derivative in non-doped devices. This study demonstrates a strategy toward blue emitters by tuning the acceptor pattern.