Porphyrins. XIX. Tripdoublet and Quartet Luminescence in Cu and VO Complexes

Luminescence spectra, excitation spectra, and lifetimes taken on Cu and VO porphyrins in polymethylmethacrylate on a Cryo‐Tip between 80 and 10°K confirm the prediction that the unpaired metal $d$ electron couples with the porphyrin triplet to give “tripdoublet” and quartet states. Four Cu porphyrins show loss of the main short‐wavelength luminescence peak and a lifetime increase from ∼ 100 to ∼ 1000 μsec over the temperature range 80–10°K that is consistent with a change from tripdoublet to quartet luminescence. CuTPP and VOEtio show no spectral change, but similar lifetime increases for CuTPP around 80°K and for VOEtio around 15°K suggest that luminescence from the former is quartet at all temperatures studied and from the latter tripdoublet. VOTPP gives anomalously short lifetimes between 8 and 16 μsec. Nonexponential decay is observed in all cases but VOTPP, and is attributed to slow spin–lattice relaxation among the quartet sublevels.