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

Farley, S.J.; Rochester, D.L.; Thompson, A.L.; Howard, J.A.K.; Williams, J.A.G.

Authors

S.J. Farley

D.L. Rochester

A.L. Thompson

J.A.K. Howard



Abstract

A series of cyclometalated platinum(II) complexes have been prepared, [PtLnCl], containing NCN-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 ( = 0.46-0.65 in CH2Cl2) with lifetimes in the microsecond range (7.9-20.5 s). 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.

Citation

Farley, S., Rochester, D., Thompson, A., Howard, J., & Williams, J. (2005). Controlling emission energy, self-quenching, and excimer formation in highly luminescent N C N-coordinated platinum(II) complexes. Inorganic Chemistry, 44(26), 9690-9703. https://doi.org/10.1021/ic051049e

Journal Article Type Article
Publication Date 2005-11
Deposit Date Apr 25, 2007
Journal Inorganic Chemistry
Print ISSN 0020-1669
Electronic ISSN 1520-510X
Publisher American Chemical Society
Peer Reviewed Peer Reviewed
Volume 44
Issue 26
Pages 9690-9703
DOI https://doi.org/10.1021/ic051049e