Dear colleagues,
What is the accepted method to extract the HOMO-LUMO band gap from electronic absorption spectrum with no fluorescence? I know people taking the wavelength at 10 % of the maximum absorption or the onset of the absorption by tracing a line of the linear (more or less) part of the band. Both give more or less the same value. But can this be applied to broad charge transfer bands (this is the reason of having no fluorescence)? From the molecular modelling it seems that the experimental value is underestimated by about 0.08 eV so that the experimental value should be taken at about 50% of the maximum absorption. But these are just models. So what is your opinion?
Thanks
Dumitru,
Your question is discussed in a recent paper that I co-wrote - see particularly pages 14 and 15 of the manuscript in the following link:
Article Molecular electronic spectroscopy: From often neglected fund...
Dumitru,
Your question is discussed in a recent paper that I co-wrote - see particularly pages 14 and 15 of the manuscript in the following link:
Article Molecular electronic spectroscopy: From often neglected fund...
Thank you Andy.
So the answer is for broad featureless bands it is quite difficult to get these values. But still having to decide will use the method iv-2. For me it gives the same value as 0.2 of maximum absorption.
I owe you a citation.
In my humble opinion, the simple, albeit quite disappointing answer would be:
" No, you can't get the HOMO-LUMO gap from the absorption spectrum, as the one-electron approximation that underlies the orbital picture is simply too crude",
Also, the onset of the band gives the energy difference between the molecule(s) in two different geometries (the relaxed geometries for the excited electronic state and for the ground state, respectively).
The second moment of the absorption band (which can also be called the centre of 'weight' of the band, 'weight' stands here for intensity) gives the vertical excitation energy (for the initial=ground state geometry), but it is also not equal to the HOMO-LUMO gap, but the the energy difference of two electronic states. The difference would be associated to the correlation effects, that can be very different for different systems.
The phenomenological, approximate estimations may be attempted, but they qould be strongly dependent on the quantum chemistry method that is used for the orbital energy calculations, as the orbital energies in the absolute sense has no meaning.
Would not be better to speak of the optical or electrical energy gap, as a difference between the energies of molecular states instead of orbitals. Or to estimate the CT state energies as the IP-EA+polarizarion energy of the surrounding matrix or crystal?
Yours sincerely
Marcin
This link may help,
http://www.instanano.com/characterization/theoretical/uv-vis-spectroscopy-band-gap-calculation/
Dear Dheeraj, this simple calculation is not correct. The energy difference between the HOMO and LUMO does not necessarily correspond to the absorption peak.
The electrochemical bandgap can be determined during processes of reduction (addition of an electron to the Lowest Unoccupied Molecular Orbital, LUMO) or oxidation (addition of a hole=removal of an electron from the Highest Occupied Molecular Orbital, HOMO) in a molecular species. In a photoexcitation, where one probes the optical bandgap, a strongly bound electron-hole pair is created by a transfer of an electron from the HOMO to the LUMO (or from the ground state to an excited state if you like). This Coulombic interaction is what makes the optical bandgap in general smaller than the electrical one. so they are not the same.
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