FORBIDDEN ELECTRONIC-TRANSITIONS BETWEEN THE SINGLET GROUND-STATE ANDTHE TRIPLET EXCITED-STATE OF PT(II) COMPLEXES

Direct access to the triplet emitting state from the ground state is o bserved for Pt(II) complexes containing heterocyclic (C boolean AND C' , C boolean AND N, N boolean AND N') and bis(diphenylphosphino)alkane (P boolean AND P') ligands. Extinction coefficients for such transitio ns are in the range 4-25 M-1 cm(-1). Emission quantum yields resulting from singlet-to-triplet excitation are as high as 61-77 times the emi ssion quantum yields resulting from singlet-to-singlet excitation at 2 96 K. The intersystem crossing quantum yield from the singlet excited state to triplet emitting state is lower than 2% at 296 K but is great ly enhanced at 77 K. The forbidden electronic transition observed for Pt(II) complexes is attributed to result from spin-orbit coupling due to the presence of Pt(II) in the skeleton structure. The importance of excitation spectra on the computation of emission quantum yields is d iscussed.