Light-Emitting Tridentate Cyclometalated Platinum(II) Complexes Containing σ-Alkynyl Auxiliaries: Tuning of Photo- and Electrophosphorescence

The synthesis and structural, photophysical, electrochemical, and electroluminescent properties
of a class of platinum(II) complexes bearing σ-alkynyl ancillary ligands, namely [(C∧N∧N)Pt(C⋮C)nR]
[H(C∧N∧N) = 6-aryl-2,2‘-bipyridine; n = 1−4; R = aryl, alkyl, or trimethylsilyl], have been studied. Substituents
with different steric and electronic properties were introduced into the tridentate cyclometalating and
arylacetylide ligands, and the π-conjugation length of the oligoynyl moiety was homologously extended
from ethynyl to octatetraynyl. The X-ray crystal structures of several derivatives confirm the Pt−(C⋮C)
ligation and reveal various intermolecular interactions, such as π−π, Pt···Pt, and C−H···F−C. The complexes
display good thermal stability and intense phosphorescence in fluid and glassy solutions with high quantum
yields and microsecond lifetimes. Their emission energies are sensitive to solvent polarity, the electronic
affinities of the substituents on both the cyclometalating and arylacetylide groups, and the length of the
oligoynyl ligand. By choosing appropriate cyclometalating and σ-alkynyl ligands, the emission color of this
class of platinum(II) complexes can be tuned from green-yellow to saturated red. In addition to 3MLCT
[Pt(5d) → π*(C∧N∧N)] and 3IL(C∧N∧N), intriguing 3IL(alkynyl) excited states localized on −(C⋮C)4− and
−(C⋮Cpyrenyl-1) moieties that afford narrow-bandwidth emissions have been observed. Selected Pt(II)
complexes were doped into the emissive region of multilayer, vapor-deposited organic light-emitting diodes.
The tunable electrophosphorescence energy resembles that recorded in fluid solutions for these emitters,
and the devices exhibit high luminance and efficiencies (up to 4.2 cd A-1).