A series of seven new platinum(II) complexes PtL
n
Cl have been prepared, where L
n
is an NCN-coordinating ligand comprising
a benzene
ring 1,3-disubstituted with two different azaheterocycles. In PtL1–5Cl, one heterocycle is a simple pyridine ring, while
the other is an isoquinoline, a quinoline, a pyrimidine (L1, L2, L3), or a p-CF3- or p-OMe-substituted pyridine (L4 and
L5). PtL6Cl incorporates both a p-CF3 and a p-OMe-substituted pyridine.
The synthesis of the requisite proligands HL
n
is achieved using Pd-catalyzed cross-coupling methodology.
The molecular structures of six of the Pt(II) complexes have been
determined by X-ray diffraction. All the complexes are brightly luminescent
in deoxygenated solution at room temperature. The absorption and emission
properties are compared with those of the corresponding symmetrical
complexes featuring two identical heterocycles, PtL
nsymCl, and of the parent Pt(dpyb)Cl containing two unsubstituted
pyridines [dpybH = 1,3-di(2-pyridyl)benzene]. While the absorption
spectra of the nonsymmetrical complexes show features of both PtL
nsymCl and Pt(dpyb)Cl, the emission generally
resembles that of whichever of the corresponding symmetrical complexes
has the lower-energy emission. PtL1Cl differs in thatat
room temperature but not at 77 Kit displays emission bands
that can be attributed to excited states involving both the pyridine
and the isoquinoline rings, despite the latter being unequivocally
lower in energy. This unusual behavior is attributed to thermally
activated repopulation of the former excited state from the latter,
facilitated by the very long-lived nature of the isoquinoline-based
excited state. At elevated concentrations, all the complexes show
an additional red-shifted emission band attributable to excimers.
For PtL1Cl, the excimer strikingly dominates the emission
spectra at all but the lowest concentrations (<10–5 M). Trends in the energies of the excimers and their propensity
to form are compared with those of the symmetrical analogues.