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 degrees) 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 similar to 4.6 ps for torsional relaxation with the highest amplitude at similar to 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.
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