Towards a rationalization of electronic properties of (thiophene+fluorene) semiconductors : modulating the HOMO-LUMO gap by modyifing the fluorene core

Within our current exhaustive research program on multithiophene-based semiconductors, a frequently employed building block is the popular fluorene one. Aiming to find new rationalization and molecular design criteria, we have synthesized and electrochemically characterized many systematic series, like the present one, including two parallel series of three monomers corresponding to three different modifications in the fluorene core, with symmetric terminal thiophene or bithiophene groups, respectively (see Figure). Our voltammetric investigation showed a remarkable modulating effect of the X substituent on the LUMO level, which is centered on the fluorene core, while the effect of the terminal groups remains only slightly perceivable; On the contrary, a significant modulating effect of the terminal group can be perceived on the HOMO level while the effect of the X substituent remains this time nearly negligible. The effective conjugation in the terminal groups is especially determining on the oligomerization ability, which is very poor for Series 1 and good for Series 2. Interesting electrochromic effects have also been observed for some terms in both series, and studied in detail by spectroelectrochemical experiments.