Weakly fluorescent (Z)-4-arylidene-5-(4H)-oxazolones (1), ΦPL<
0.1%, containing a variety of conjugated aromatic fragments and/or
charged arylidene moieties, have been orthopalladated by reaction
with Pd(OAc)2. The resulting dinuclear complexes (2) have the oxazolone ligands bonded as a C^N-chelate, restricting
intramolecular motions involving the oxazolone. From2, a variety of mononuclear derivatives, such as [Pd(C^N-oxazolone)(O2CCF3)(py)] (3), [Pd(C^N-oxazolone)(py)2](ClO4) (4), [Pd(C^N-oxazolone)(Cl)(py)]
(5), and [Pd(C^N-oxazolone)(X)(NHC)] (6,7), have been prepared and fully characterized. Most of complexes3–6are strongly fluorescent in solution
in the range of wavelengths from green to yellow, with values of ΦPLup to 28% (4h), which are among the highest
values of quantum yield ever reported for organometallic Pd complexes
with bidentate ligands. This means that the introduction of the Pd
in the oxazolone scaffold produces in some cases an amplification
of the fluorescence of several orders of magnitude from the free ligand1to complexes3–6. Systematic
variations of the substituents of the oxazolones and the ancillary
ligands show that the wavelength of emission is tuned by the nature
of the oxazolone, while the quantum yield is deeply influenced by
the change of ligands. TD-DFT studies of complexes3–6show a direct correlation between the participation of the
Pd orbitals in the HOMO and the loss of emission through non-radiative
pathways. This model allows the understanding of the amplification
of the fluorescence and the future rational design of new organopalladium
systems with improved properties.