Energy transfer and fine structure axes determination in a hybrid porphyrin dimer oriented in a liquid crystal. Time resolved triplet EPR spectroscopy

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Abstract
A study of laser photoexcitation-EPR diode detection of the transient triplet state of the hybrid covalenty linked porphyrin dimer [ZnP–H2P] in a uniaxial liquid crystal and a glass matrix at 100 and 10 K, is reported. This time resolved technique enables monitoring the transient behavior of the magnetization until thermal relaxation sets in. Selective singlet excitation of the dimer components reveals a singlet energy transfer process [ZnP*−H2P]–––ket→ZnP–H2P*, whose rate at 10 K is calculated to be [1.0±0.2]×109 s−1. Triplet line shape analysis of the subunit monomer triplet and the dimer yields: (a) the location of the fine structure tensor axes in the molecular frame (X, Y along the pyrrole nitrogens and Z perpendicular to the porphyrin plane); consequently, the signs of the parameters D and E are found to be positive for the monomeric free base and the dimer; (b) the ratio of the sublevel population rates; and (c) the spin-lattice relaxation rate.