Electron-Transfer Kinetics within Supramolecular Assemblies of Donor Tetrapyrrolytic Dyes and an Acceptor Palladium...

9,18,27,36-Tetrakis[meso-(4-carboxyphenyl)]tetrabenzoporphyrinatozinc(II) (TCPBP, as a sodium salt) was prepared in order to compare its photoinduced electron-transfer behavior toward unsaturated cluster Pd3(dppm)3(CO)(2+) ([Pd3(2+)]; dppm = Ph2PCH2PPh2 as a PF6(-) salt) with that of 5,10,15,20-tetrakis[meso-(4-carboxyphenyl)]porphyrinatozinc(II) (TCPP) in nonluminescent assemblies of the type dye···[Pd3(2+)]x (x = 0-4; dye = TCPP and TCPBP) using femtosecond transient absorption spectroscopy. Binding constants extracted from UV-vis titration methods are the same as those extracted from fluorescence quenching measurements (static model), and both indicate that the TCPBP···[Pd3(2+)]x assemblies (K14 = 36000 M(-1)) are slightly more stable than those for TCPP···[Pd3(2+)]x (K14 = 27000 M(-1)). Density functional theory computations (B3LYP) corroborate this finding because the average ionic Pd···O distance is shorter in the TCPBP···[Pd3(2+)] assembly compared to that for TCPP···[Pd3(2+)]. Despite the difference in the binding constants and excited-state driving forces for the photoinduced electron transfer in dye*···[Pd3(2+)] → dye(•+)···[Pd3(•+)], the time scale for this process is ultrafast in both cases (<85 fs). The time scales for the back electron transfers (dye(•+)···[Pd3(•+)] → dye···[Pd3(2+)]) occurring in the various observed species (dye···[Pd3(2+)]x; x = 0-4) are the same for both series of assemblies. It is concluded that the structural modification on going from porphyrin to tetrabenzoporphyrin does not greatly affect the kinetic behavior in these processes.