Highly Luminescent Platinum Complexes for Light Emitting Devices and Bio-Imaging

Abstract

Intensely luminescent cyclometallated platinum(II) complexes have been prepared
containing N^C^N-coordinating ligands, based on 1,3-dipyridylbenzene. Exceptionally
high quantum yields ( 0.85, degassed DCM, 298 K) are rationalised in terms of the
particularly short Pt-C bond, which raises the energy of the normally deactivating d-d*
state. Terdentate ligand functionalisation has been employed to obtain complexes with
tuneable emission. Fluorination at the 3,5-aryl positions and/or placing electronreleasing
substituents at the 4-pyridyl position gave complexes with blue-shifted
emission, whilst retaining a high quantum yield. Substitution of the chloride ancillary
ligand for cyanide led to further blue-shifts.
In general, these complexes undergo intermolecular interactions in solutions of higher
concentration to form excimers. The excimers are emissive in their own right, and
display broad, structureless emission bands at lower energy than the isolated molecules
(monomers). The influence of substituents in the terdentate ligand and the identity of
the coligand on the energy and relative intensity of excimer emission has been
elucidated.
Some complexes have been used as phosphorescent dopants in the fabrication of high
efficiency organic light emitting devices (OLED)s. The colour of a device depends on
the monomer-to-excimer emission ratio, which is controlled by varying the dopant
concentration. At intermediate concentrations, white OLEDs, with properties
approaching pure white light, have been obtained.
Preliminary investigations into the utility of the complexes as bio-imaging agents in live
cells are reported. The complexes which contain small substituents on the terdentate
ligand enter cells rapidly, retaining their phosphorescence. These Pt(II) complexes
appear to localise simultaneously in both the nucleus and mitochondria of cells, where
their emission characteristics differ: the application of appropriate filter sets allows
separate visualisation of the two organelles. A selection of iridium(III) complexes were
also investigated as bio-imaging agents. These complexes localise in cytoplasm-based
organelles, and do not appear to enter the nucleus. Both classes of organometallic
complexes are shown to be suitable candidates for time resolved emission imaging
microscopy (TREM).