Highly Luminescent Half-Lantern Cyclometalated Platinum(II) Complex: Synthesis, Structure, Luminescence Studies, and Reactivity.

The half-lantern compound [{Pt­(bzq)­(μ-C7H4NS2-κN,S)}2]·Me2CO (1) was obtained by reaction of equimolar
amounts of potassium 2-mercaptobenzothiazolate (KC7H4NS2) and [Pt­(bzq)­(NCMe)2]­ClO4. The Pt­(II)···Pt­(II) separation in the neutral complex
[{Pt­(bzq)­(μ-C7H4NS2-κN,S)}2] is 2.910 (2) Å, this being among the shortest observed
in half-lantern divalent platinum complexes. Within the complex, the
benzo­[h]­quinoline (bzq) groups lie in close proximity
with most C···C distances being between 3.3 and 3.7
Å, which is indicative of significant π–π
interactions. The reaction of 1 with halogens X2 (X2 = Cl2, Br2, or I2) proceeds with a two-electron oxidation to give the corresponding
dihalodiplatinum­(III) complexes [{Pt­(bzq)­(μ-C7H4NS2-κN,S)­X}2] (X = Cl 2, Br 3, I 4). Their X-ray structures confirm
the retention of the half-lantern structure and the coordination mode
of the bzq and the bridging ligand μ-C7H4NS2-κN,S. The Pt–Pt distances (Pt–Pt
= 2.6420(3) Å 2, 2.6435(4) Å 3, 2.6690(3) Å 4) are shorter than that in 1 because of the Pt–Pt bond formation. Time dependent-density
functional theory (TD-DFT) studies performed on 1 show
a formal bond order of 0 between the metal atoms, with the 6pz contribution diminishing the antibonding
character of the highest occupied molecular orbital (HOMO) and being
responsible for an attractive intermetallic interaction. A shortening
of the Pt–Pt distance from 2.959 Å in the ground state
S0 to 2.760 Å in the optimized first excited state
(T1) is consistent with an increase in the Pt–Pt
bond order to 0.5. In agreement with TD-DFT calculations, the intense,
structureless, red emission of 1 in the solid state and
in solution can be mainly attributed to triplet metal–metal-to-ligand
charge transfer (3MMLCT) [dσ*­(Pt–Pt) →
π*­(bzq)] excited states. The high quantum yields of this emission
measured in toluene (44%) and solid state (62%) at room temperature
indicate that 1 is a very efficient and stable 3MMLCT emitter, even in solution. The high luminescence quantum yield
of its red emission, added to its neutral character and the thermal
stability of 1, make it a potential compound to be incorporated
as phosphorescent dopant in multilayer organic light-emitting devices
(OLEDs).