Third-order nonlinear optical properties of oligothiophene-based thin films investigated by electroabsorption spectroscopy: Influence of conjugated chain length and electron-withdrawing substituents

A series of thiophene-based oligomers are investigated by electroabsorption spectroscopy in terms of influence of chain length and peripheral substitution for the first time. Polycrystalline thin films of nonsubstituted oligomers, 8T, 6T and 4T, and of oligothiophenes with electron-withdrawing cyano substituents, 4T-V(CN)2 and CN-TET-CN, are measured and analyzed to determine the character of optical transitions. The results of the nonsubstituted oligomers suggest that the transitions at 500–600 nm present a neutral (Frenkel exciton) character, while those of oligothiophenes with electron-withdrawing substituents suggest that the transitions present a polar (charge-transfer exciton) one, indicating significant contribution of noncentrosymmetric substitution. Third-order nonlinear susceptibilitiy χ(3) (−ω; ω, 0, 0) is also evaluated, and it is shown that the longest unsubstituted oligomer 8T presents a high value of χ(3) ∼ 45 × 10−12 esu, and substituted one CN-TET-CN presents further higher value of χ(3) ∼ 250 × 10−12 esu. These investigations indicate multiple significance of electron-withdrawal substitution in molecular design of an oligothiophene-based nonlinear optical material.