Spin Dynamics of a “Parachute” Shaped Fullerene−Porphyrin Dyad

Abstract

The electron spin dynamics associated with the photoinduced intramolecular electron transfer (ET) in a covalently linked malonate-strapped zinc porphyrin−fullerene (ZnP−C60) dyad with “parachute” geometry was studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy. Studies were carried out in widely different media, including solvents of different polarities, such as isotropic toluene and tetrahydrofuran, and anisotropic nematic liquid crystals, LCs (E-7 and ZLI-4389). Photoexcitation of the donor, ZnP, results in ET to the acceptor, C60, in all solvents used over a wide range of temperatures. The generated radical pairs (RPs) either decay to the ground state or produce the triplet state of the acceptor moiety, ZnP−3*C60. Temperature-dependent studies permit the determination of the RP energy levels relative to the donor and acceptor singlet and triplet states. Thus, the results enable determination of the genesis of the ET routes as a function of the solvent polarity. Due to their unique dielectric properties, the LCs behave as solvents of low polarity at low temperatures and as solvents of high polarity at elevated temperatures. Finally, the triplet EPR line shape of ZnP−3*C60 oriented in the LCs verifies the asymmetric three-dimensional structure calculated recently, while the ZnP moiety in the dyad dictates the orientation of the bulky C60 part in the LC matrixes.