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 c ase of sexithiophene, the end or side disubstitution with hexyl groups modulates the absorption, emission, and photoluminescence characteris tics. These effects are discussed in terms of structural data on the m olecular organization in thin films obtained from X-ray diffraction sp ectra. The low quantum yields of photoluminescence observed in thin fi lms 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, wh en hexyl groups are substituted as pendent substituents, the spreading out of the oligomer molecules in the solid state leads to higher phot oluminescence yields.