Mediated electronic coupling: Singlet energy transfer in porphyrin dimers enhanced by the bridging chromophore

We have studied singlet electronic energy transfer (EET) in two donor-bridg e-acceptor series (D-B -A), in which the donor (zinc porphyrin or its pyrid ine complex) and the acceptor (free base porphyrin) were covalently connect ed by a geometrically well-defined bridging chromophore. We have investigat ed how the medium between a donor and an acceptor influences EET by separat ing the influence of the electronic structure of the bridging chromophore f rom other effects known to influence the energy transfer. The electronic st ructure of the bridging chromophore was varied by changing the central unit (bicyclo[2.2.2]octane, benzene, naphthalene, or anthracene) in the bridgin g chromophore. In all systems the excited state energy separation donor-bri dge and bridge-acceptor is large enough to prevent stepwise singlet energy transfer. In addition, the systems were designed to minimize conjugation to preserve the identity of the separate chromophores (donor, bridge, accepto r). Compared with the rate constant expected from the Forster theory, the b ridging chromophore with bicyclo[2.2.2]octane as the central unit did not s ignificantly enhance the energy transfer rate constant. However, the bridgi ng chromophores with benzene and naphthalene as the central unit showed a m oderate increase, whereas the bridging chromophore with anthracene as the c entral unit showed the largest increase in energy transfer rate constant. T his increase is ascribed to a mediating effect of the bridging chromophore and it is proposed to be strongly correlated to the energy splitting betwee n the singlet excited states of donor and bridging chromophores.