1D Polymeric Platinum Cyanoximate: A Strategy toward Luminescence in the Near-Infrared Region beyond 1000 nm

We
report the synthesis and properties of the first representative of
a new class of PtL2 complexes with ambidentate mixed-donor
cyanoxime ligands [L = 2-cyano-2-oximino-N,N′-diethylaminoacetamide, DECO (1)].
Three differently colored polymorphs of “Pt­(DECO)2” (3–5) were isolated, with
the first two being crystallographically characterized. The dark-green
complex [Pt­(DECO)2]n (5) spontaneously forms in aqueous solution via aggregation
of yellow monomeric complex 3 into the red dimer [Pt­(DECO)2]2 (4), followed by further oligomerization
into coordination polymer 5. A spectroscopic and light-scattering
study revealed a “poker-chips”-type 1D polymeric structure
of 5 in which units are held by noncovalent metallophilic
interactions, forming a Pt---Pt wire. The polymer 5 shows
a broad absorption at 400–900 nm and emission at unusually
long wavelengths in the range of 1000–1100 nm in the solid
state. The near-infrared (NIR) emission of polymer 5 is
due to the formation of a small amount of nonstoichiometric mixed-valence
PtII/PtIV species during synthesis. A featureless
electron paramagnetic resonance spectrum of solid sample 5 recorded at +23 and −193 °C evidences the absence of
PtIII states, and the compound represents a “solid
solution” containing mixed-valence PtII/PtIV centers. Exposure of KBr pellets with 5% 5 to Br2 vapors leads to an immediate ∼30% increase in the
intensity of photoluminescence at 1024 nm, which confirms the role
and importance of mixed-valence species for the NIR emission. Thus,
the emission is further enhanced upon additional oxidation of PtII centers, which improves delocalization of electrons along
the Pt---Pt vector. Other polymorph of the “Pt­(DECO)2“ complexmonomerdid not demonstrate luminescent
properties in solutions and the solid state. An excitation scan of 5 embedded in KBr tablets revealed an emission only weakly
dependent on the wavelength of excitation. The NIR emission of quasi-1D
complex 5 was studied in the range of −193 to
+67 °C. Data showed a blue shift of λmax and
a simultaneous increase in the emission line intensity with a temperature
rise, which is explained by analogy with similar behavior of known
quasi-1D K2[Pt­(CN)4]-based solids, quantum dots,
and quantum wells with delocalized carriers. The presented finding
opens a route to a new class of platinum cyanoxime based NIR emissive
complexes that could be used in the design of novel NIR emitters and
imaging agents.