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. The real picture can be more complicated, since the energy of an unoccupied state is changed upon occupation. In principle charge transport refers to free carriers hence the electrical bandgap should be of importance, but it depends on which kind of process/device you are interested in.

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. The real picture can be more complicated, since the energy of an unoccupied state is changed upon occupation. In principle charge transport refers to free carriers hence the electrical bandgap should be of importance, but it depends on which kind of process/device you are interested in.

This was not stated in the question, but be sure the optical bandgap was measured from the absorption edge of a film of the material rather than in solution as this often leads to a redshift in pi-stacked materials thus affecting your optical bandgap calculation. In addition to Ilias's comments, you may also want to consider ultraviolet photoelectron spectroscopy to help assess the energy levels in the solid state. The term "bandgap" as measured from solution is really a misnomer in the first place, but everyone seems to use it anyway. There's no way for energy bands to develop between isolated molecules in solution. The parameter measured by electrochemistry (CV or DPV) or solution UV-Vis spectroscopy is really just the electrochemical or optical gap rather than bandgap.

Thank you very much Matthew and Ilias!!!

You must consider the effect of solvation as electrochemical bandgap in all most all cases is invariably larger. There are approximate methods to calculate the solvent contribution and if charge transfer is in presence of electrolyte ions and solvent, I guess electrochemical bandgap is more important.

You find more information below:

1- Fundamentals of Semiconductor Electrochemistry and Photoelectrochemistry. -Krishnan Rajeshwar The University of Texas at Arlington, Arlington.

(http://www.wiley-vch.de/bard/eoe/pdf/v06_1.pdf)

2- Electrochemistry of CdS Nanoparticles: A Correlation between Optical and Electrochemical Band Gaps. - Santosh K. Haram, Bernadette M. Quinn, and Allen J. Bard Department of Chemistry and Biochemistry The UniVersity of Texas at Austin, Texas

(http://bard.cm.utexas.edu/resources/Bard-Reprint/694.pdf)

3-Electrochemical and optical studies of the band gaps of alternating polyfluorene copolymers. - Shimelis Admassie, Olle Inganäs, Wendimagegn Mammo,

Erik Perzon, Mats R. Andersson. Biomolecular and Organic Electronics, IFM, Center of Organic Electronics, Link¨opings Universitet,Sweden.

(http://www.sciencedirect.com/science/article/pii/S0379677906000567)

4-About of optical absorption you will find something in: Physics of Amorphous Semiconductors by Kazuo Morigaki. World Scientific Chapter 8

5-Book entitled: Electrochemistry at Metal and Semiconductor Electrodes by Norio Sato. Elsevier- Chapter 2.

Regards.

May I add one more question? Is there any possibility that the value of optical gap is larger than electrochemical gap?

What would be a reason for the optical gap be larger than the electrochemical gap (for the case of isolated molecules in solution)?