Photo-magnetic properties and dynamics of multi-spin coupled systems

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Spin multiplicity of the molecular system largely influences its dynamics involving electronic states. In coupled spin systems, the total spin-multiplicity of the system is not simply defined because the two spins are not always coupled enough strongly or weakly to give a single spin-multiplicity. Especially in moderately coupled triplet-doublet system, the degree of mixing of the basis-wavefunctions depends on the angle between the magnetic field and molecular axis due to orientation dependence of dipolar interaction. In a p-bipheyl-linked copper porphyrin-free base porphyrin dimer, the lowest electronic excited state of the dimer is composed of the excited triplet state of the free base and the ground state doublet of the copper porphyrin. Laser irradiation at 532 nm leads to the excitation of the dimer followed by the relaxation to the lowest electronic excited state. Time-resolved EPR spectra in toluene at 40K of this dimer suggest that the observed excited state is a moderately coupled triplet-doublet system. Nutation frequency measurements of partially oriented samples at different magnetic filed positions indicate that spin-multiplicity of the system largely depends on the resonance magnetic field and also the orientation of the molecule relative to the magnetic field.