Controlling emission energy, self-quenching, and excimer formation in highly luminescent N∧C∧N-coordinated platinum(II) complexes

A series of cyclometalated platinum(11) complexes have been prepared, [(PtLCl)-Cl-n], containing N boolean AND C boolean AND 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 (phi = 0.46-0.65 in CH2Cl2) with lifetimes in the microsecond range (7.9-20.5 mu s). The introduction of the aryl substituents leads to a red shift in the lowest-energy, intense charge-transfer absorption band compared to [(PtLCl)-Cl-1] (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 [(PtLCl)-Cl-1] 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 (LC)-L-3 (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 [(PtLCl)-Cl-1], particularly for the most sterically hindered mesityl complex. The introduction of the strongly electron-donating 4-dimethylamino substituents leads to a switch in the nature of the lowest-energy excited state from (LC)-L-3 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 (LC)-L-3 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.