Triplet excitation energy transfer in porphyrin-based donor--bridge--acceptor systems with conjugated bridges of varying length: an experimental and DFT study

A series of donor--bridge--acceptor (D--B--A) systems with varying donor-acceptor distances have been studied with respect to their triplet energy transfer properties. The donor and acceptor moieties, zinc(II), and free-base porphyrin, respectively, were separated by 2-5 oligo-p-phenyleneethynylene units (OPE) giving rise to edge-to-edge separations ranging between 12.7 and 33.4 A. The study was performed in 2-MTHF at 150 K and it was established that triplet excitation energy transfer occurs with high efficiency in all of the studied D--B--A systems. The distance dependence was exponential with an attenuation factor, beta, equal to 0.45 +/- 0.015 A(-1). The experimental study was also supported by quantum mechanical DFT and TD-DFT calculations on a series of closely related model systems. A thorough analysis of the OPE-bridge conformational dynamics led to an equation that quantitatively models the distance dependence of the electronic coupling found in the experimental study.