Shortwave infrared luminescent Pt-nanowires: a mechanistic study of emission in solution and in the solid state

Abstract

Several complexes of “PtL₂” composition containing two cyanoxime anions – 2-oximino-2-cyano-N-piperidineacetamide (PiPCO⁻) and 2-oximino-2-cyano-N-morpholylacetamide (MCO⁻) – have been obtained and characterized both in solution and in the solid state. Complexes exist as two distinct polymorphs: monomeric yellow complexes and dark-green [PtL₂]_n 1D polymers, while for the MCO⁻ anion a red, solvent containing dimeric [Pt(MCO)₂·DMSO]₂ complex has also been isolated. The interconversion of polymorphs was investigated. The monomeric PtL₂ units are arranged into anisotropic extended solid [PtL₂]_n polymers with the help of Pt⋯Pt metallophilic interactions. Crystal structures of monomeric PtL₂ (L = PiPCO⁻, MCO⁻) and red dimeric [Pt(MCO)₂·DMSO]₂ complexes were determined and revealed the cis-arrangement of cyanoxime anions. The Pt–Pt distance in the “head-to-tail” red dimer was found to be 3.133 Å. The structure of the polymeric [Pt(PiPCO)₂]_n compound was elucidated using the EXAFS method and evidenced the formation of Pt-wires with ∼3.15 Å intermetallic separation. The EPR spectra of both 1D polymers at variable temperatures indicate the absence of Pt(III) species. Both pure dark-green [PtL₂]_n polymers showed a considerable room temperature electrical conductivity of 20–30 S cm⁻¹, which evidences the formation of a mixed valence Pt(II)/Pt(IV) system. We discovered that these 1D polymeric [PtL₂]_n complexes show an intense NIR fluorescence beyond 1000 nm, while yellow monomeric PtL₂ complexes are not emissive at all. The room temperature excitation spectra of 1D polymeric [PtL₂]_n complexes demonstrated their strong emission beyond 1000 nm regardless of the used excitation wavelength between 350 and 800 nm, which is typical of systems with delocalized charge carriers. For the first time the formation of mixed valence “metal wires” held together by metallophilic interactions is directly linked both with an intense fluorescence in the NIR region of the spectrum and with the electrical conductivity. The effect of the concentration of [PtL₂]_n complexes dispersed in the dielectric salt matrix on the photoluminescence wavelength and intensity was investigated. Both polymers show a quantum yield that is remarkably high for this region of the spectrum, reaching ∼2%. Variable temperature emission of polymeric [PtL₂]_n in the −190–+60 °C range was studied as well.