Controlling Emission Energy, Self-Quenching, and Excimer Formation in Highly Luminescent N∧C∧N-Coordinated Platinum(II) Complexes

A series of cyclometalated platinum(II) complexes have been prepared, [PtLnCl], containing N∧C∧N-coordinating,
terdentate ligands based on 1,3-dipyridylbenzene (HL1), incorporating aryl substituents at the central 5 position of
the ligand. All of the new complexes are intensely luminescent in a degassed solution at 298 K (φ = 0.46−0.65
in CH2Cl2) with lifetimes in the microsecond range (7.9−20.5 μs). The introduction of the aryl substituents leads to
a red shift in the lowest-energy, intense charge-transfer absorption band compared to [PtL1Cl] (401 nm in CH2Cl2),
in the order H < mesityl < 2-pyridyl < 4-tolyl < 4-biphenylyl < 2-thienyl < 4-(dimethylamino)phenyl (431 nm in
CH2Cl2), which correlates with the decreasing order of oxidation potentials. A similar order is also observed in the
emission maxima, ranging from 491 nm for [PtL1Cl] to 588 nm for the 4-(dimethylamino)phenyl-substituted complex.
The emission spectra of all of the complexes, except for the amino-substituted compound, are highly structured in
a dilute solution in CH2Cl2, and the emission is assigned to excited states of primarily 3LC (ligand-centered) character.
At higher concentrations, self-quenching accompanied by structureless excimer emission centered at 700 nm is
observed, but the aryl groups attenuate the self-quenching compared to the parent compound [PtL1Cl], particularly
for the most sterically hindered mesityl complex. The introduction of the strongly electron-donating 4-dimethylamino
substituent leads to a switch in the nature of the lowest-energy excited state from 3LC to one of primarily intraligand
charge-transfer (ILCT) character in CH2Cl2:  this complex displays a structureless and much broader emission
band than the other compounds and a high degree of positive solvatochromism. No excimer emission is observed
in CH2Cl2, and self-quenching is an order of magnitude lower than that for the other complexes. However, in
nonpolar solvents such as CCl4, the ILCT state is destabilized, such that the 3LC remains the lowest-energy excited
state. Reversible switching between the ILCT and 3LC states can also be achieved in a CH2Cl2 solution by protonation
of the amine, with an accompanying large change in the emission maxima of >100 nm. The X-ray structures of the
biphenylyl- and methyl-substituted complexes are reported, together with those of the 2-pyridyl- and mesityl-substituted
ligands and the key synthetic intermediate 1-bromo-3,5-di(2-pyridyl)benzene.