Smaller or vanishing value of semiconductor energy band-gap (Eg ) is a good factor that determines the conductivity of semiconductor polymers and it has high correlation with the intrinsic electronic, optical and magnetic properties of materials. Squaraine dyes because of their unique optical properties have been shown to be an ideal candidate as one of the building blocks organic dyes with physical property of low HOMO (Highest occupied molecular orbital)-LUMO (lowest unoccupied molecular orbital) separation. This research dealt with the correlation of the change in theoretical Eg values computed for different models using BLYP, SVWN, B3LYP and ROHF functional and the excited properties with the available experimental Eg. The study showed that increasing the strength of an electron releasing group or use of stronger electronic donor that is bonded to squaric unit led to increasing HOMO width and consequently reduced Eg and Increased UV absorption maximal.
Key words: HOMO (highest occupied molecular orbital), LUMO (lowest unoccupied molecular orbital), band gap, UV absorption, DFT (density functional theory) and HF (Hybrid functional) methods, excited state property.
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