Competition between charge transfer via superexchange and thermally activated energy transfer in porphyrinheterodimerquinone systems

Electron Transfer in Porphyrin Complexes in Different Solvents

“…Each of the electronic states has its own vibrational substructure. As a rule for the porphyrin-containing systems the time of vibrational relaxation is two orders of magnitude faster than the characteristic ET time [16]. Because of this we assume that only the ground vibrational states play a role and we do not include the vibrational substructure.…”

“…Donor and acceptor can exchange their charges only through B. In the present investigation the supermolecule consists of free-base tetraphenylporphyrin (H 2 P) as donor, zinc substituted tetraphenylporphyrin (ZnP) as bridge, and p-benzoquinone as acceptor [16]. In each of those molecular blocks we consider only two molecular orbitals (m = 0, 1), the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) [53].…”

“…When incoherent effects such as dephasing dominate the transfer is mainly sequential [12,23], i. e., the levels are occupied mainly in sequential order [5,12,13,16]. The dependence on the differences between the energy levels is exponential [9,10].…”

“…Changing a building block of the complex [9,16,17] or changing the environment [16,18] can modify which mechanism is mainly at work. Actually, there is still a discussion in literature whether sequential transfer and superexchange are limiting cases of one process [19] or whether they are two processes which can coexist [7].…”

“…In experiments this is done by substituting parts of the complexes [9,10,16,17,20] or by changing the polarity of the solvent [16]. Also the geometry and size of the bridge block can be varied, and in this way the length of the subsystem through which the electron has to be transfered [9,11,[20][21][22][23] can be changed.…”