Tuning the energy levels of low band gap conjugated polymers containing 2,3-diethylhexylthieno[3,4-b]pyrazine
Date Issued
2012
Date
2012
Author(s)
Shih, Chun-Han
Abstract
In this thesis, six donor-acceptor low band gap conjugated copolymers containing thienopyrazine derivative as acceptor unit were designed and synthesized via Suzuki coupling and Stille coupling. Various donors, based on fluorene and thiophene derivatives, with different chemical structures were employed to investigate the effects of coplanarity, bridging atom and strong electron withdrawing group on the optical absorptions, the electronic band structures and the stability of the copolymers.
All the copolymers synthesized showed bimodal absorption spectra with small band gap (< 2 eV). Comparing to the counterparts containing fluorene derivatives, the copolymers with thiophene based donors exhibited relative reddish optical absorptions, smaller band gaps and better photochemical stability. Considering the thiophene based copolymers, the better coplanarity led to stronger optical absorption at long wavelengths corresponding to enhanced intra-charge transfer as well as better stability. The introduction of cyclopentadithiophene with fused ring structures would further lower the band gap. The lowest unoccupied molecular orbital (LUMO) was similar for all the copolymers, which should be determined by the thienopyrazine unit. The highest occupied molecular orbital (HOMO) would be significantly elevated by improving the coplanarity. The introduction of strong electron withdrawing cyano group on thiophene could significantly lower the HOMO while retaining low optical band gap.
Subjects
thienopyrazine
low band gap polymer
band structure engineering
photochemical stability
Type
thesis
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