A series of bithiophene and terthiophene derivatives and their respective polymers have been prepared. The absorption and fluorescence spectra, quantum yields, and lifetimes of these derivatives in solution are studied and compared. The bandwidths of the absorption and fluorescence profiles of the different derivatives are used to discuss the geometry changes occurring from one molecule to the other in the ground and excited states. Increasing the chain length and/or substitution of bithiophene and terthiophene at both ends of the molecule with a carbonyl chloride group increases the planarity of the molecules in both their ground and excited states by improving the electronic delocalization throughout the molecular frame. Comparison between the absorption and fluorescence spectra has proved to be a good tool to discover conformational changes occurring during the relaxation of the excited state. All the molecules investigated show geometrical changes from a twisted ground state to a more planar conformation in the excited state. A radiationless torsional mechanism and intersystem crossing processes are involved in the excited state relaxation of,the various molecules. Incorporation of substituted bithiophene and terthiophene units in aromatic polyesters have been made and the optical and photophysical properties of these polymers are studied. It is shown that the bithiophene and terthiophene units are perfectly isolated in the polymeric chain. The photophysical properties of the polyesters make these materials good candidates for the fabrication of efficient blue-light-emitting diodes.