"調控含2,3-diethylhexylthieno[3,4-b]pyrazine之低能隙共軛高分子的能階結構"

Abstract

本論文以thienopyrazine衍生物為受體，利用suzuki coupling 及stille coupling設計並合成了6種不同結構的予體-受體(D-A)共聚高分子。而在予體的部分，利用fluorene衍生物及thiophene衍生物兩個系統及不同的化學結構以研究高分子主鏈的共平面性、橋接原子、強拉電子基對於高分子光學吸收、電子能帶結構及光化學穩定性的影響。 所有高分子皆具有雙峰光學吸收光譜且能隙低於2eV，予體為thiophene系統的高分子相較於fluorene系統來說具有較紅的光學吸收、較小的能隙及較好的光學穩定性。其原因是thiophene為基礎的高分子具有較好的共平面性，所以能夠在對應到intra-charge transfer之長波長光譜有較強的吸收且具有較好的穩定性。利用cyclopentadithiophene的稠環結構能夠更進一步的減低能隙。所有高分子的最低未填滿分子軌域(LUMO)皆由TP受體決定，因此變化不大；而最高已填滿分子軌域(HOMO)則隨著共平面性的加強而有顯著的提升。在thiophene環上加上強拉電子的氰基能夠在維持低光學能隙的同時，使高分子HOMO有明顯的下降。

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.