The first excited states of platinum phthalocyanine in magnetic fields

A Zeeman study of platinum phthalocyanine in Shpol'skii matrices was performed in the temperature range of 25-77 K with magnetic fields up to ∼6·3 Tesla (T). The general phosphorescence region (950-945 nm) exhibited only one principal peak whose position is not affected by the field. An extremely weak structure that is energetically about 26 cm-1 lower than the principal peak and which originally was attributed to a different crystal site turned out to be a genuine field dependent collective response of weakly radiating spin-levels. The weakness of the radiation from these levels and the size of the subsequently confirmed zero field splitting D parameter are a unique case among all porphyrins and phthalocyanines studied to date in magnetic fields. For the first time the coefficients of the 5d (zx and yz) AO of PtPc in the egx LCAO MO were experimentally determined to be about 0·18. The Zeeman effect of the first π-π* absorption, in terms of phosphorescence excitation, was also investigated in approximately the same temperature and field range. An apparent angular momentum of Λ' ≈ 3.6ħ was deduced from the Zeeman energy in the components of the 1Eu state lifted by the crystal field. Λ', in turn, gives rise to an orbital reduction factor q ⪆ 0·9 which implies a fairly weak Jahn-Teller coupling, similar to but weaker than that found earlier in PdPc.