Energy transfer and structure determination of porphyrin dimers linked via a phenylenebisvinylene bridge: A time-resolved triplet electron paramagnetic resonance study

Abstract

The photoexcited triplet state in a series of homo and hetero dimers was examined by time resolved electron paramagnetic resonance (TREPR) spectroscopy. The systems studied here consist of free-base tetraxylylporphyrin ( H 2 TXP ) and Zn (II) tetraxylylporphyrin ( ZnTXP ) and held together covalently by phenylenebisvinylene bridge at different positions. Experiments were carried out on the dimers, dissolved in an isotropic matrix (toluene), and in an anisotropic matrix of a liquid crystal (LC). Analysis of the results demonstrates that the dimers exhibit different geometries, depending on the linkage. Specifically, the triplet line shape analysis indicates that the para-dimers have a planar structure, the meta-dimers reveal the existence of two structural conformers, i.e. planar and slightly bent, while the ortho-dimers are characterized by the strongly bent molecular structure. The dimers exhibit efficient intramolecular singlet energy transfer (EnT) from ZnTXP to H 2 TXP subunit. On the other hand, EnT in all hetero dimers studied here is incomplete and some contribution from ZnTXP subunit is present in all triplet spectra. Thus, the efficiency of the EnT in the present systems is determined by properties of the connecting spacer, and does not depend on specific conformation and geometry of the molecule.