Inorganic Electrochemistry |
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UV-Vis
Gives us information about the electronic energy levels of a compound
The best way to obtain information about the electronic energy levels of a compound is Uv/Vis spectroscopy. It is especially useful to us if we can record the UV/Vis spectra not only of a compound we make but also of the products of its various redox reactions. This can be achieved using chemical reducing agents but these are messy and unpredictable. We have modified the Uv/Vis apparatus so that redox products can be generated in-situ at low temperatures (Fig. 1).
Fig. 1 - Experimental set-up for in situ UV/Vis/NIR spectroscopy
e.g. Reduction of 4-NO2-bpy (bpy=2,2´-bipyridine)
4-NO2-bpy
4-NO2-bpy gives a single UV/Vis peak as shown in Fig. 2 (blue line). When a potential of -1.5 V is applied, the other curves shown in Fig. 2 grow in over 1 h and stop changing at the red curve. First of all, we can see that there are two peaks in the spectrum of the neutral compound. These correspond to the two p-p* electronic transitions within the two different pyridine moieties in 4-NO2-bpy. It is known that the p-p* transition of py (py = pyridine) occurs at 40,000 cm-1. Hence the higher energy band in the UV/Vis spectrum of 4-NO2-bpy is assigned to a p-p* transition based on py and the lower energy band to a p-p* transition based on 4-NO2-py.
![UV/Vis/NIR absorption spectra of 4-NO2-bpy (blue) and [4-NO2-bpy]1- (red), in 0.1M [TBA][BF4]/DMF showing partially reduced curves, T = 233 K. Potential held at -1.5 V vs. Ag/AgCl.](./../techniques/images/uv-vis2.gif "UV/Vis/NIR absorption spectra of 4-NO2-bpy (blue) and [4-NO2-bpy]1- (red), in 0.1M [TBA][BF4]/DMF showing partially reduced curves, T = 233 K. Potential held at -1.5 V vs. Ag/AgCl.")
Fig. 2 - UV/Vis/NIR absorption spectra of 4-NO2-bpy (blue) and [4-NO2-bpy]1- (red), in 0.1M [TBA][BF4]/DMF showing partially reduced curves, T = 233 K. Potential held at -1.5 V vs. Ag/AgCl.
The two peaks in the Uv/Vis spectrum of [4-NO2-bpy]1- are similar to those in the spectrum of [4-NO2-py]1-. Thus they are assigned as p-p* based on the 4-NO2-py moiety. They also compare well to peaks in the Uv/Vis spectrum of [4,4'-(NO2)2-bpy]1-/2- . Thus the reductions of 4,4'-(NO2)2-bpy can be assigned as based on each 4-NO2-py moiety as opposed to being spread over both rings.
Electrochemistry - EPR - UV-Vis
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