Origin of Photochemical Modification of the Resistivity of Ag(DMe-DCNQI)₂ Studied by X-ray Absorption Fine Structure

Abstract

The Ag(DMe-DCNQI) 2 (DMe-DCNQI ) 2,5-dimethyl-N,N′-dicyanoquinonediimine, or DM) charge-transfer salt is a promising material for production by photolithography as it displays a unique photoinduced change in conduction. Photoproducts (β1 and γ) of Ag(DM) 2 were investigated using the X-ray absorption fine structure (XAFS) technique in order to understand the origin of their conduction properties. In contrast to the metallic conduction exhibited by a pristine sample (R), β1 is a semiconductor, whereas γ is an insulator, even though the original Ag(DM) 2 composition is maintained in both the β1 and the γ forms. A redox mechanism has been postulated to explain the photoinduced change in conduction. However, measurement of the Ag L 3edge XANES (X-ray absorption near-edge structure) did not provide any evidence of electron transfer from the DM radical anion to the Ag cation in the β1 and γ products. Ag K-edge extended X-ray absorption fine structure (EXAFS) data demonstrated that β1 is a mixture of the original pristine R phase and a newly found R′ phase, which has a shorter bond distance between the Ag cations and the N atoms of DM molecules. The conduction electrons remain in the DM column of R and R′, but the domain boundaries between R and R′ present an activation barrier for the conduction electrons to cross these boundaries, which provides a rationale for the semiconductive behavior of β1. The Ag K-edge EXAFS results showed that the γ photoproduct has a different local structure from R, with a shorter distance and smaller coordination number of the Ag-N bond. The DM radical anions in the γ compound form covalent bonds between themselves, resulting in a loss of the columnar structure. The complex local structure around Ag cations explains the insulating behavior of the γ compound.

Development of a Control Method for Conduction and Magnetism in Molecular Crystals

“…Fourier-transformed Ag K-edge extended X-ray absorption fine structure (EXAFS) of (a) α, (b) β (β1), and (c) γAg(DM) 2 , measured at 15 K in order to reduce thermal fluctuations. 101 The results show interatomic distances between Ag and N, L(AgN). The first (at ³1.8 ¡), second (at ³2.7 ¡), and third (at ³4.1 ¡) peaks respectively correspond to the N, C, and N atoms of the (Ag)N¸CN= group in DM.…”

“…The α¤ has the same lattice constants as those of α within experimental error but contains slightly shorter AgN bonds (0.228 nm in α vs. 0.219 « 0.005 nm in α¤ based on EXAFS). 101 The second and third shell FT peaks in EXAFS indicate that the interatomic distance between Ag and C in AgN¸C remains unchanged and that the distance between Ag and farther N in Ag(N¸C)N changes in the transformation of α to α¤. The bond angle = À AgN¸C decreased from 164°(α) to 160°(α¤).…”

“…Based on EXAFS and XANES data, the mole fraction can be estimated for the sample β in Figure 13; α:α¤ = (0.83 « 0.18): (0.17 « 0.005). 101 After 40-h irradiation, the powder XRD patterns revealed that another new structure (ᤤ) appeared in β. 100 In order to distinguish these two β-phases, we should call α irradiated for <40-h β1, and α irradiated for >40-h β2.…”

“…It has been concluded that atmosphere itself does not matter in irradiation effects on α; the important factor is whether sufficient energy is provided or not by light and/or heat. They were characterized by elemental analysis (Table 2), high resolution solid-state 13 C NMR (powder) (Figure 8), 100 IR absorption (KBr) spectra, 104,105 UVvis diffuse reflectance spectra (KBr), 104,105 MALDI-TOF MS (Figure 9), 100 N 1s and Ag 3d XPS (Figure 10), 103 Ag MNN Auger spectra (Figure 11), Ag L 3 X-ray absorption near edge structure (XANES) (Figure 12), 99,101 Ag extended X-ray absorption fine structure (EXAFS) (Figure 13), 101 electrical resistivity (Figure 14), magnetic susceptibility (Figure 15), 80,81,105 and X-ray diffraction (XRD) powder patterns ( Figure 16). The Ag + ions are generally known to respond to UV, but their actual photochemistry sensitively depends on coexistent chemical species.…”

“…As described below, γ can be Figure 12. Ag L 3 -edge X-ray absorption near edge structure (XANES) spectra for α, β (β1), γ, δ, ε and Ag metal (Ag foil) in total electron mode (solid curves) 99, 101 and Ag LMM Auger electron yield mode (dotted curves). Total electron yield mode monitors bulk Ag states, while Auger electron yield mode probes surface (23 nm) chemical states.…”