PANI shows absorption peak at 320 nm and at 626 nm in the spectrum. The absorption band at 626 nm is assignable to the transition of electron from the HOMO of the benzenoid part to the LUMO of the quinoid structure of PANI.

Polymers 2011, 3(2), 875-885

A Possibility for Construction of an Iodine Cleaning System Based on Doping for π-Conjugated Polymers

Polymer 2008, 49, 3070–3075.

Dear Chen 

Which peak should be consider for band gap? 

An described by Shen-Ming, these transitions derive from localised Molecular like orbitals, and the terminology of solid state semiconductor band theory should not be applied.

Dear Anil,

It seems this paper will be useful to reply your question:

 https://www.researchgate.net/publication/235446875_Spectroscopy_and_defect_states_in_polyaniline

Article Spectroscopy and defect states in polyaniline

Thanks Dear, Hugh J Byrne and Alexander Pud, Your valuable suggestions will be beneficial for me. Once again thanks a lot.

Anil Kumar 

We always take into consideration 320 nm peak corresponding to pi-pi* transition for the band gap evaluation.

Yes, as S.K Dhawan suggested  it is the pi -pi* transition band.

For the calculation of band gap energy you may apply 'Tauc's equation.

Dear Dhawan Sir and Yahya Ismail,

Thank you very much for your kind reply. Please tell me, Is their any specific reason to consider this transition only ?

Anil Kumar

Delocalization of pi-electrons in conducting polymers creates charge carriers like polarons/bipolarons or solitions which give rise to electronic conductivity. Enclosing herewith a case of PPP and polyacetylene which may clarify your doubts to some extent.