Linker effects on optoelectronic properties of alternate donor-acceptor conjugated polymers

In this study, three conjugated polymers (DA1, DA2 and DA3) with different linkers between an electron-rich unit (3,4-didodecylthiophene as electron donor, D) and an electron-deficient unit (benzothiadiazole as electron acceptor, A) were synthesized via Suzuki, Heck and Sonogashira polycondensation, respectively. The D and A groups are connected in three different ways: without any linker (DA1), by a vinylene linker (DA2), and by an ethynylene linker (DA3). Among these three polymers, DA2 exhibited the lowest bandgap (E _g) at 1.65 eV and the highest planarity evidenced by the smallest dihedral angle (1°) between the donor and acceptor units. The femtosecond up-conversion fluorescence measurement showed that DA1 and DA3 exhibited a rise process for the energetically downhill excitonic energy transfer (EET) in a timescale of sub-ps to several ps, while DA2 exhibited a decay process in such downhill EET. In addition, DA2 showed the shortest time constant of ∼4.6 ps for torsional relaxation with the highest amplitude at ∼43.1% among all the three polymers. This suggested that the vinylene linker has the fastest torsional relaxation from a flexible ground-state structure to a more rigid planar geometry, which may reduce conformational defects and improve exciton migration. This work studied the linker effects between the D and A units on the alternate D-A polymers and may shed light on the design of new D-A polymers with improved properties.