The uranyl complex U$^{VI}$O$_2$Cl(L$^{Mes}$) of the redox-active, acyclic dipyrrin–diimine anion L$^{Mes}$– [HL$^{Mes}$ = 1,9-di-tert-butyl-imine-5-(mesityl)dipyrrin] is reported, and its redox property is explored and compared with that of the previously reported U$^{VI}$O$_2$Cl(LF) [HLF = 1,9-di-tert-butyl-imine-5-(pentafluorophenyl)dipyrrin] to understand the influence of the meso substituent. Cyclic voltammetry, electron paramagnetic resonance spectroscopy, and density functional theory studies show that the alteration from an electron-withdrawing meso substituent to an electron-donating meso substituent on the dipyrrin ligand significantly modifies the stability of the products formed after reduction. For UVIO2Cl(LMes), the formation of a diamond-shaped, oxo-bridged uranyl(V) dimer, [U$^V$O$_2$(L$^{Mes}$)]$_2$ is seen, whereas in contrast, for UVIO2Cl(LF), only ligand reduction occurs. Computational modeling of these reactions shows that while ligand reduction followed by chloride dissociation occurs in both cases, ligand-to-metal electron transfer is favorable for U$^{VI}$O$_2$Cl(L$^{Mes}$) only, which subsequently facilitates uranyl(V) dimerization.