Synthesis, photophysics, and electrochemistry of thiophene-pyridine and thiophene-pyrimidine dyad comonomers

A series of new pi-conjugated donor (D) and acceptor (A) dyad comonomers were prepared using Suzuki coupling protocols. The D A comonomers consisting of thiophene/bithiophene as donors and pyridine/pyrimidine as acceptors were prepared to investigate their photophysical and electrochemical properties. The dyads were spectroscopically confirmed to be highly conjugated. This was further supported by the X-ray crystal structure of the bithophene-pyridine dyad that showed all the heterocycles to be coplanar. It was further found that the fluorescence yields (Phi(fl)) of the dyads were highly dependent on the number of thiophenes. The bithiophene derivatives exhibited Phi(fl) values >= 0.3 while the thiophene derivatives did not fluoresce. The suppressed fluorescence observed for the thiophene derivatives was due to their higher triplet energy resulting in efficient intersystem crossing (ISC) to the triplet state with Phi(ISC) >= 0.8. This was confirmed both by time-resolved and steady-state measurements. The singlet excited state of both thiophene and bithiophene dyads was deactivated solely by either fluorescence and (or) ISC. Owing to their donor and acceptor character, the dyads could be oxidized and reduced both electrochemically and photochemically to afford the radical cation and anion, respectively.