ABSTRACT
The optical and electronic properties of organic molecules are determined by their energy gap, which is the energy separation between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), also called the HOMO-LUMO gap (HLG). The molecules with extended conjugation or conjugated oligomers and polymers can be described by the band structures or the linear combination of molecular orbitals of multiple repeat units. The band gap between the valence and conduction band corresponds to the energy difference between the HOMO and LUMO and is relevant for optical absorption, photoluminescence, and other photophysical behaviors [1-3]. The band gap engineering is a key in the design of low energy gap organic materials and often involves chemical modification at the molecular level. Generally speaking, when tuning the energy gap of molecules, several factors should be considered, such as conjugation length, bond length alternation, and donor-acceptor charge transfer.
