Implementing additional optical (luminescent) properties into the well-known class of single-molecule magnets (SMMs) is considered as a promising route toward obtaining the next generation of optomagnetic materials for quantum information storage and computing. Herein, we report a joint optical and magneto-structural study for the two novel series of lanthanide(iii) complexes of general formula Bu4N[LnIII(HL)4(dmso)]·nH2O where H2L = N-(4-Xphenyl)oxamic acid with X = Cl and n = 2 [Ln = Eu (1_Cl), Gd (2_Cl), Dy (3_Cl), and Tb (4_Cl)] and X = F and n = 3 [Ln = Eu (1_F), Gd (2_F), Dy (3_F), and Tb (4_F)]. All these compounds are mononuclear species with each lanthanide(iii) cation in a low-symmetry nine-coordinate environment (LnO9) which is constituted by four didentate monoprotonated oxamate groups and one dmso molecule. Magnetic measurements show the occurrence of field-induced SMM behavior for the Gd3+ (2_Cl and 2_F), Dy3+ (3_Cl and 3_F), and Tb3+ complexes (4_Cl and 4_F). Solid-state photophysical measurements for the Eu3+ (1_Cl and 1_F) and Tb3+ complexes (4_Cl and 4_F) reveal that both monoprotonated chloro- and fluoro-substituted phenyl(oxamate) ligands are able to sensitize the lanthanide(iii)-based luminescence in the visible region, through an energy transfer process ("antenna effect"), as supported by theoretical calculations for Eu3+ compounds. In particular, 1_Cl and 1_F present a quantum efficiency of approximately 50%, being potentially suitable as efficient light conversion molecular devices (LCMDs).