Tuning the optical properties of thiophene oligomers toward infrared emission: A theoretical study

The structural and electronic effects of various substituents on thiophene- based oligomers have been studied using a density-functional-based tight-bi nding scheme. Optical absorption spectra have been calculated applying the semiempirical Hartree-Fock-based ZINDO/S method. Electron withdrawing or do nating substituents have been placed on the two terminal thiophene units in alpha, beta, and gamma positions with respect to the sulfur atom of the th iophene. The presence of substituents has in general only minor effects on the geometry of the parent molecules, but strong influence on the optical p roperties. Substitution in alpha and gamma positions always leads to a reds hift. The effect of beta substitution is much weaker and electron withdrawi ng groups even cause a modest blueshift. The calculated shifts can be expla ined in terms of modifications of energy and shape of the frontier orbitals . (C) 2001 American Institute of Physics.