Tetracyanoplatinate Chain Complexes-Pt CN

Pt(CN) chains exhibit intense, broad emission bands at ambient pressure. The emission has been attributed to a LUMO-HOMO charge transfer transition. The LUMO is comprised primarily of 6p (Pt) and tt (CN) orbitals and the HOMO consists primarily of 5d (Pt) and 6s (Pt) orbitals. The intrachain Pt-Pt bond distances are sufficiently short to permit overlap of Pt orbitals. As a result, the LUMO orbitals combine to form a conduction band, the HOMO orbitals combine to form a valence band, and the emission can be viewed as an excitonic recombination. Distinct emission bands have been reported for delocalized (free) and localized (self-trapped) excitons [215, 216, 219]. The free exciton emission is polarized parallel to the Pt(CN) chains ( 11 c) and the self-trapped exciton emission is polarized perpendicular to the chains ( J. c). Ambient pressure emission studies have shown that the energy of emission decreases with decreasing intrachain Pt-Pt distance [215,216,220]. 

Yersin and colleagues have investigated the effect of pressure on the polarized emission properties of a number of Pt (CN) chain systems over the past several years [215,216,219-223]. The objective of their studies is to use pressure to examine the effect of intrachain Pt-Pt distance on the energy and intensity of exciton emission. In contrast to the effect of variations in the chemical identity of charge balancing cations, high pressure provides a method to continuously decrease the intrachain Pt-Pt distance. 

Stock and Yersin [221] reported polarized emission spectra up to 23 kbar for single crystals of Ba[Pt(CN)4] 4H2O. At ambient pressure, the intrachain Pt-Pt separation is 3.32 A and the peaks of the free exciton and self-trapped emission 

At ambient pressure and room temperature, the dicyanoaurates exhibit broad emission bands in the UV-visible region of the spectrum. At low temperature, the emission bands possess shoulders and it has been proposed that each distinct emission band is due to a different Au+ structural environment [230, 235]. Yersin and Riedl [219] have proposed that localized, spatially isolated [Au(CN)2]n clusters are present in the Au(CN)2 layers. They argued that Au+-Au+ distances in the clusters would differ from the distances observed in the homogeneous, unclustered portions of the layer. As a result, multiple emission bands are expected. According to the model of Yersin and Riedl, emission bands from the clusters correspond to excited states that are delocaHzed over the region of the cluster and to self-trapped states of these clusters. These self-trapped excitons are similar to those that are observed in [Pt(CN)4] chains. 

Strasser et al. [236] reported a similar red shift ( -160 cm Vkbar) for T1 [Au(CN)2] up to 20 kbar. This system differs from those studied by Yersin and Riedl [219] because TU has 6p orbitals available that are capable of covalently interacting with the 5d orbitals of Au [232,237]. The similar pressure shifts reported for Au(CN)2 emission in compounds with TU, Cs, K, and Na indicates that the effect of pressure is limited primarily to compression within the Au(CN)2 layers and that interlayer compression effects are negligible up to 20 kbar. 

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