Four novel diimine rhenium(i) carbonyl complexes with the formula [Re(CO)3(L)Br], where L= 2-(4-(9H-carbazol-9-yl)phenyl)-1H-imidazo[4,5-f][1,10]phenanthroline (P1), 2-(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)-1H-imidazo-[4,5-f][1,10]phenanthroline (P2), 2-(4-(6-(9H-carbazol-9-yl)-9H-3,9′-bicarbazol-9-yl)phenyl)-1H-imidazo[4,5-f][1,10]phenanthroline (D1), and 2-(4-(3′,6′-di-tert-butyl-6-(3,6-di-tert-butyl-9H-carbazol-9-yl)-9H-3,9′-bicarbazol-9-yl)phenyl)-1H-imidazo[4,5-f][1,10]phenanthroline (D2), have been successfully synthesized and fully characterized by 1H NMR, IR, and UV–Vis, etc.The luminescence quantum yields (LQYs) of the parent Re(i) complexes P1and P2are 0.13 and 0.16, respectively, which are much higher than the previously reported Re(i) dendrimers. The HOMOs and the LUMOs of P1and P2are calculated to be mainly composed of [d(Re) + π(CO + Br)] and π*(L) orbital, respectively. However, those of the Re(i) dendrimers D1(LQY = 0.066) and D2(LQY = 0.0048) are mainly localized on ligand L, indicating that the component of the metal-to-ligand charge-transfer dπ(Re) →π*(N–N) (MLCT) transitions in P1and P2should be more than those in D1and D2. As a result, the higher LQYs of P1and P2are tentatively assigned to the disturbance of the MLCT transitions during the photoluminescence process. [ABSTRACT FROM AUTHOR]