Poly(isothianaphthene) is the first quinoid-enhancing conjugated polymer developed by Wudl et. al. thirty years ago with an intriguing small HOMO-LUMO gap (HLG) of 1.0-1.2 eV that can be attributed to the stabilized LUMO orbital by fusing pseudoaromatic unit at 3,4-positions of thiophene.1 Thieno[3,4-b]thiophene (TT) is a unique building block for effective construction of low-bandgap (LBG) polymers. PTB7 based on TT moiety is one of the most successful p-type low-bandgap polymer materials for organic photovoltaics.2 We report herein our recent work on the development of new TT-based functional materials. (1) By overcoming two key issues: the control of regioregularity and the stability related to the unsymmetrical and quinoid TT moiety, a series of regioregular thieno[3,4-b]thiophene (rr-OTTs) with well-defined planar structures is synthesized successfully via direct C-H arylation.3 rr-OTTs show light absorptions ranging from UV-Vis to near-infrared(near-IR) region with extremely long effective conjugation length of twenty repeating units and amphoteric redox properties. Combining Raman spectroscopic and theoretical investigations, we find that the quinoidization effect in rr-OTTs exhibits distinct structural features comparing with the conventional oligo(thiophene)s (OTs). The BLA decreasing and quinoidization tendency in rr-OTTs are consistent with each other in narrowing energy bandgaps. (2) By combining TT and 5-alkyl-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (TPD) units with relatively large -surfaces, we developed two-dimensional (2D)-expanded quinoid terthiophenes, 2DQTT-i and 2DQTT-o for n-type organic thin-film transistors (OTFTs).4 2DQTT-o shows favorable 2D brick-layer arrangement in single-crystal state. The minor change of thiophene orientation in 2DQTT-i and 2DQTT-o significantly affected film morphology that resulted in different OTFT performances: 0.44 cm2 V–1 s–1 (2DQTT-i) compared with 3.0 cm2 V–1 s–1 (2DQTT-o). 2DQTT-o is the first example among quinoid oligothiophenes (QOTs) that exhibits an electron mobility surpassing 1.0 cm2 V–1 s–1, revealing the potential of 2DQOT-framework for constructing n-type organic semiconductors that can rival state-of-the-art rylene diimides. (3) Based on TBTT pi-building blocks, we successfully developed TPD-type LBG-polymers combining two classical approaches to polymer design: D-A and quinoid strategies (TBTT: TPD flanked with two unsymmetrical TT moieties regioselectively). Inverted PSC devices based on LBG TPD-polymers show promising Jsc of up to 18.2 mA/cm2 with PCE values of up to 7.50% without requiring additional device treatment or additives, which is among the best reported for LBG-polymers to date. (4) Despite of the strong near-IR absorptions, QOTs are almost nonemissive. The nonradiative process of QOTs may originate from the S1→T2 intersystem crossing because of the small negative energy difference between S1 and T2 states, E(T2-S1). We developed quinoid fluorescent motifs, QBTT-Ars, exhibiting unusually high near-IR fluorescence. QBTT-Ars can be viewed as a combination of cruciform-fused aromatic (E)-1,2-bis(5-arylthiophen-2-yl)ethane (BTE-Ars) and quinoid bithiophene (QBT) subunits. E(T2-S1) is properly modulated, which restrains the ISC nonradiative process. We demonstrate the possibility of developing highly emissive near-IR dyes based on quinoid molecules. References: (1) Wudl, F.; Kobayashi, M.; Heeger, A. J. J. Org. Chem. 1984, 49, 3382. (2) Liang, Y.; Yu, L. Acc. Chem. Res. 2010, 43, 1227. (3) Zhu, X., Casado, J. et al. manuscript in preparation. (4) Zhang, C.; Zang, Y.; Gann, E.; McNeill, C. R.; Zhu, X.;* Di, C.-a.;* Zhu, D. J. Am. Chem. Soc. 2014, 136, 16176.