Zero field splitting of the triplet state of porphyrins. II

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Abstract
The zero field splitting (zfs) for porphyrin is calculated for possible transitions b2u(π), a1u(π), a2u(π) → eg(π*). Calculations are done with pi−electron orbital coefficients determined from extended Hückel calculations on the deprotonated dianion, the free base, and the acid dication. The spin dipole−dipole couplings are evaluated exactly using a Gaussian−lobe expansion. The calculations are compared to the reported zfs of zinc and free base porphin and to the zfs reported herein of Zn etioporphyrin in various lattice sites in an octane matrix and of Zn tetraphenylporphin. The calculations fit experimental D and E values within 10% under the following assumptions: (i) For Zn etioporphyrin and Zn porphin the triplet state is 3[a1u(π) → eg(π*)], but for Zn tetraphenylporphin it is 3[a2u(π) → eg(π*)]; (ii) the larger value for D observed in free base porphin is due to configuration interaction between these two transitions; (iii) variable axis directions set up by the lattice crystal field cause the variation in E value observed for Zn porphin and Zn etioporphyrin in the various lattice sites.