Exciton coupling effects in the absorption and photoluminescence of sexithiophene derivatives

Absorption and photoluminescence properties in thiophene oligomers can be controlled through chemical engineering of the molecules. In the case of sexithiophene, the end or side disubstitution with hexyl groups modulates the absorption, emission, and photoluminescence characteristics. These effects are discussed in terms of structural data on the molecular organization in thin films obtained from X-ray diffraction spectra. The low quantum yields of photoluminescence observed in thin films are associated with the close-packed arrangement of the molecules in the solid state. Such an arrangement leads to the splitting of the excited level into two exciton bands, according to Davydov's rule, and the lowest energy transition becomes forbidden. On the other hand, when hexyl groups are substituted as pendent substituents, the spreading out of the oligomer molecules in the solid state leads to higher photoluminescence yields.