Porphyrins XIV. Theory for the luminescent state in VO, Co, Cu complexes

Abstract

The normal triplet state of closed shell metal porphyrins becomes a “tripdoublet” and a quartet in systems with one unpaired d electron. Exchange integrals between metal d and porphyrin π-electrons give intensity to the tripdoublet and set the tripdoublet-quartet energy gap. These integrals are evaluated theoretically giving tripdoublet radiative lifetimes between 26 and 1450 μsec and tripdoublet-quartet energy gaps in the range 156 to 639 cm−1 for the metals and porphyrin skeletons considered. Spinorbit coupling similarly gives intensity to the quartet and causes a zero field splitting dependent on a parameter Z which has value 0.7, 3.5, 20.4, 21.6 cm−1 for Zn (triplet), Cu, Co and VO complexes. Theory predicts that the intensity ratio of 0−1 to 0−0 bands will be very low for the tripdoublet but stronger for the quartet. Existing experimental evidence shows that luminescence comes from both the tripdoublet and the quartet states, their relative importance varying with metal, porphyrin skeleton, and temperature.