Molecules with low highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) gaps are of particular importance due to their ability to easily donate (from HOMO) or accept (from LUMO) an electron. A low HOMO/LUMO gap can be endowed in organics either by extending π-conjugation in the molecules or by construction of covalent D-A compounds (where D is a π-electron donor and A is a π -electron acceptor) in which the HOMO and LUMO orbitals can be tuned relatively independently. In the former strategy, narrow HOMO-LUMO gaps are achieved in oligo(acenes) and in fullerenes. The former strategy is effective with tetrathiafulvalenes (TTFs), including, TTF-acenes, conjugated TTF-acceptor compounds (TTF-σ-A), and TTF-σ -fullerene systems. The resultant materials find applications in organic field effect transistors.
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