As I searched there are different formula for it which I don't know which one I should use? And what is different between them?
1) E LUMO=-(E red (onset)+4.4)
2)E LUMO=-[4.65 V-E red(onset)]
3)E LUMO=[(E red-E 1/2 (ferrocene)+4.8 ]
4)E LUMO=-(E red (onset)-4.4)
It looks like your equations all convert LUMO into eV vs vacuum reference. That isn't really a conventional thing to do for materials where you determine your electrochemical potentials using cyclic voltammetry (CV). It is more common for those working with inorganic semiconductors in a pure solid state device setup, and you would likely use the more solid state materials methods for determining the bandgap/Fermi levels, etc. CV is generally used for organic materials and referenced to things such as ferrocene or other common references.
If you are analyzing an organic/organometallic dye, and it functions by absorbing light and transferring an electron to something like TiO2, then you would performing the following experiments:
1. You would get the HOMO level as the first oxidation of the molecule using CV.
2. You would get the emission spectrum using a fluorimeter, and determine the excitation injection energy (in eV) for the molecule (singlet or triplet energy). You can find out how to find the exact value online if you have a large stokes shift.
The equation for the LUMO would then be LUMO = HOMO - Excitation energy(eV) , and the output values for HOMO and LUMO would be in Volts vs. your reference.
Note: Many people use CV to get the oxidation and reduction potentials, subtract them, and see that as the HOMO-LUMO gap. That is incorrect because the LUMO level doesn't contain an extra electron, and due to reorganizational energy differences, the LUMO value calculated this way can be way off.
But how do I get step 1 i.e., "..get the HOMO level as the first oxidation of the molecule using CV."
I guess HOMO = -[4.4+Eox(onset) (Vs Fc)] V
Eox(onset) (Vs Fc) = oxdidation onset compare Fc
"That is incorrect because the LUMO level doesn't contain an extra electron, and due to reorganizational energy differences, the LUMO value calculated this way can be way off."
Can i get detail explanation on this?..And may i get the reference. Thanks
Bradley Brennan Please could you explain more clear about your reply? I meant I have seen many papers but most of them mentioned about using ferrocene as internal reference or external reference and calculate them with formular HOMO (LUMO) = -4.8 - (Eonset - E1/2 ferrocene) or second way, tried to measure UV-VIS absorbance and emission spectra and get the Eo-o transition energy from the intersection between them, get the HOMO value from oxidation potential and LUMO=HOMO-Eo-o.
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