Orientation and Polarized Optical Emission Properties of Platinum(II) Complexes in Smectic Liquid Crystals

We have prepared a series of platinum complexes with a rodlike ligand and smectic (Sm) liquid crystals (LCs) to demonstrate polarized phosphorescence. The ligands for the Pt complexes 12F2PPyO8 [2-(4-dodecyl-5,6-difluorophenyl)-5-octyloxypyridinato-N,C2] and 12F2PPyO12 [2-(4-dodecyl-5,6-difluorophenyl)-5-dodecyloxypyridinato-N,C2] with long alkyl and alkoxy chains exhibited the Smectic A (SmA) phase, however no liquid crystallinity was evident in Pt complexes with LC ligands. The host LCs 1-(4-butyl-2,3-difluoro-phenyl)-4-(trans-4-pentylcyclohexyl)benzene (4F2PPCH5) and 2-(2,3-difluoro-4-hexylphenyl)-6-octyloxynaphthalene (6F2PNaO8) with long flexible chains showed wide SmA phase temperature ranges. Pt complex/Sm LC mixtures were oriented on rubbed polyimide substrates or by applying a magnetic field. The orientation coefficients of the magneto-oriented samples were recorded to be over 0.6 in the Sm LC temperature range. The oriented samples exhibited polarized optical emissions assigned to the monomer and excimer states, which were perpendicular to the orientation direction. This indicates that the transition dipole moment of the Pt complex is oriented from the ligand to the Pt atom. Pt complexes with rodlike ligands with long alkyl and alkoxy chains and host Sm LC with such chains are much more likely to obtain high order parameters and generate highly polarized optical emissions. In the mixtures, the polarization ratios of the monomer and excimer in the SmA phase were estimated to be approximately 2.52.9 and 4.65.4, respectively.