A series of cyclometalated platinum(II) complexes have been prepared, [PtL(n)Cl], containing N--C--N-coordinating, terdentate ligands based on 1,3-dipyridylbenzene (HL(1)), 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 CH(2)Cl(2)) with lifetimes in the microsecond range (7.9-20.5 micros). The introduction of the aryl substituents leads to a red shift in the lowest-energy, intense charge-transfer absorption band compared to [PtL(1)Cl] (401 nm in CH(2)Cl(2)), in the order H < mesityl < 2-pyridyl < 4-tolyl < 4-biphenylyl < 2-thienyl < 4-(dimethylamino)phenyl (431 nm in CH(2)Cl(2)), which correlates with the decreasing order of oxidation potentials. A similar order is also observed in the emission maxima, ranging from 491 nm for [PtL(1)Cl] 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 CH(2)Cl(2), and the emission is assigned to excited states of primarily (3)LC (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 [PtL(1)Cl], 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 (3)LC to one of primarily intraligand charge-transfer (ILCT) character in CH(2)Cl(2): 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 CH(2)Cl(2), and self-quenching is an order of magnitude lower than that for the other complexes. However, in nonpolar solvents such as CCl(4), the ILCT state is destabilized, such that the (3)LC remains the lowest-energy excited state. Reversible switching between the ILCT and (3)LC states can also be achieved in a CH(2)Cl(2) 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.

Platinum(II) complexes containing the strong π‐acceptor N,N‐chelating ligand phenylazopyridine (Ph‐azpy) [Pt(p‐R‐Ph‐azpy)X2], R = H, NMe2 or OH, X = Cl or N3, have been synthesized and characterized to explore the effects of monodentate ligands and phenyl substituents on their absorption spectra and photoactivation. Time‐dependent density functional theory calculations showed that the complexes have a low‐lying unoccupied orbital with strong σ‐antibonding character toward the majority of the coordination bonds. The UV–visible absorption bands were assigned as mainly ligand‐centered or metal‐to‐ligand charge‐transfer transitions, with strong contributions from the chlorido and azido groups. In complexes with substituted Ph‐azpy ligands, σ‐donation from NMe2 and OH/O– groups results in a redshift of the main absorption bands compared with unsubstituted Ph‐azpy complexes. The diazido complexes are photoactive in solution upon irradiation with either UVA or visible light for R = H or NMe2, or UVA only when R = OH/O–. Intriguingly, the phenolate group of the latter complex undergoes very slow protonation in solution. Biological screening was limited by poor solubility; however, initial tests showed that the phenolato diazido complex is rapidly taken up into the nuclei of HaCaT keratinocytes, which are stained intensely blue, and its cytotoxicity is increased upon irradiation with UVA light.