Large differences in the intramolecular charge-transfer fluorescence quantum yields and electroluminescence efficiencies were observed among the isomeric donor-acceptor molecules 2-(4-phenyl-2-quinolyl)-10-methylphenothiazine (2PQMPT) and 3-(4-phenyl-2-quinolyl)-10-methylphenothiazine (3PQMPT). In solution, the 2PQMPT isomer had a larger positive solvatochromism and thus a greater degree of charge transfer, whereas 3PQMPT had a larger fluorescence quantum yield (71%) compared to 2PQMPT (46%). High brightness (23750 cd/m(2)) and high efficiency (8.18 cd/A, 4.45 lm/W, 2.42% external quantum efficiency at 1015 cd/m(2)) green electroluminescence was achieved from 3PQMPT diodes. In contrast, green light-emitting diodes with lower brightness (8900 cd/m(2)) and efficiencies (4.79 cd/A, 2.36 lm/W, 1.41% external quantum efficiency at 690 cd/m(2)) were obtained from 2PQMPT. The two isomeric donor-acceptor molecules had identical HOMO (5.1 eV) and LUMO (2.4 eV) energy levels derived from electrochemistry. Density functional theory (DFT) calculations provided insights into the molecular geometry, electronic structures, and properties of the donor-acceptor isomers. These results demonstrate the pronounced influence of the donor/acceptor connection on the charge-transfer emission efficiency of donor-acceptor molecules and the performance of solid-state light-emitting devices based on them.