Trans-2 Addition Pattern to Power Charge Transfer in Dendronized Metalloporphyrin C60 Conjugates

Abstract

Coordinating different transition metals--manganese(III), iron(III), nickel(II), and copper(II)--by a dendronized porphyrin afforded a new family of redox-active metalloporphyrins to which C(60) was attached as a ground-state electron acceptor. Such a strategy introduced an additional center of redoxactivity, that is, a change of the oxidation state of the metal. Cyclic voltammetry and absorption/fluorescence measurements provided support for mutual interactions between the redox-active constituents in the ground state. In particular, slightly anodic shifted reduction potentials/cathodic shifted oxidation potentials and the occurrence of new charge transfer features in the 700-900 nm range prompt to sizable electronic coupling in the range of 300 cm(-1). Photophysical means--steady-state/time-resolved fluorescence and transient absorption measurements--shed light on the excited-state interactions. To this end, we have added pulse radiolytic investigations to characterize the radical cation (i.e., metalloporphyrins) and radical anion (i.e., fullerene) characteristics. Pi-pi stacking of the excited state electron donor and the electron acceptor is key to overcome the intrinsically fast deactivation of the excited states in these metalloporphyrins and to power an exothermic charge transfer. The lifetimes of the rapidly and efficiently generated radical ion pair states, which range from 15 to >3000 ps, revealed several important trends. First, they were found to depend on the solvent polarity. Second, the nature of the transition metal plays a similarly decisive role. It is important that the product of charge recombination, namely tripmultiplet excited states versus ground state, had a great impact. Finally, a correlation between the charge transfer rate (i.e., charge separation and charge recombination) and the free energy change for the underlying reaction reveals a parabolic dependence with parameters of the reorganization energy (0.84 eV) and electronic coupling (70 cm(-1)) closely resembling that seen for the zinc(II) and free base analogues.