Singlet Energy Transfer in Porphyrin-Based Donor-Bridge-Acceptor Systems: Interaction between Bridge Length and Bridge Energy

Artikel i vetenskaplig tidskrift, 2006

Singlet excitation energy transfer is governed by two donor-acceptor interactions, the Coulombic and exchange interactions giving rise to the Foerster and Dexter mechanisms, resp., for singlet energy transfer. In transfer between colliding mols. or between a donor (D) and acceptor (A) connected in donor-bridge-acceptor (D-B-A) system by an inert spacer (B), the distinction between these two mechanisms is quite clear. However, in D-B-A systems connected by a p-conjugated bridge, the exchange interaction between the donor and acceptor is mediated by the virtual low-lying excited states (unoccupied orbitals) of that bridge and, as a consequence, becomes much more long-range in character. Thus, the clear distinction to the Coulombic mechanism is lost. This so-called superexchange mechanism for singlet energy transfer has been shown to make a significant contribution to the energy transfer rates in several D-B-A systems, and its D-A distance as well as D-B energy gap dependencies have been studied. We here demonstrate that in a series of oligo-p-phenyleneethynylene (OPE) bridged porphyrin-based D-B-A systems with varying D-A distances the Foerster and through-bond (superexchange) mechanisms both make considerable contributions to the obsd. singlet energy transfer rates. The donor is either a zinc porphyrin or a zinc porphyrin with a pyridine ligand, and the acceptor is a free base porphyrin. By comparison to a homologous series where only the D-B energy gaps varies, a sepn. between the two energy transfer mechanisms was possible and, moreover, an interplay between distance and energy gap dependencies was noted. The distance dependence was shown to be approx. exponential with an attenuation factor b = 0.20 .ANG.-1. If the effect of the varying D-B energy gaps in the OPE series was taken into account, a slightly higher b-value was obtained. Ground-state absorption, steady-state, and time-resolved emission spectroscopy were used. The exptl. study is accompanied by time-dependent d. functional theory (TD-DFT) calcns. of the electronic coupling, and the exptl. and theor. results are in excellent qual. agreement (same distance dependence).