TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preparation (claimed): 2 Methods for preparation of the compound (I) are given.(1) Process (A): nitration of 1,4-disubstituted-2,3-difluorobenzene compound of formula (II) to give 1,4-disubstituted-5,6-dinitro-2,3-difluorobenzene compound of formula (III); reduction of two nitro groups in compound (III) to two amino groups with reducing metals such as iron under acidic conditions to form corresponding diamino compound of formula (IV) which can react with other compound to form fused ring structure (I); and(2) Process (B): nitration of 2,3-difluoro-1,4-dibromobenzene to give 1,4-dibromo-2,3-difluoro-5,6-dinitro-benzene followed by converting the bromine atoms to X1 and X2 groups by a coupling reaction e.g. a Stille reaction to form 1,4-disubstituted-5,6-dinitro-2,3-difluorobenzene compound of formula (III); and reduction of two nitro groups in compound (III) to two amino groups with reducing metals such as iron under acidic conditions to form corresponding diamino compound of formula (IV) which can react with other compound to form fused ring structure (I).Preferred Components: The electron-donating co-monomer is at least one phenyl, thiene, fluorene, carbazole, benzodithiophene, pyrrole, indenofluorene, indolocarbazole, dibenzosilole, dithienosilole, benzo(1,2-b;3,4-b)dithiophene, benzo(2,1-b:3,4-b')dithiophene, cyclopenta(2,1-b:3,4-b')dithiophene, thieno(3,2-b)thiophene, thieno(3,4-b)thiophene or dithieno(3,2-b:2',3'-d)pyrrole (all optionally substituted by at least one Cl, Br, I, 6-18C aryl or 3-14C heteroaryl having at least one N, O or S in the ring) (preferably fluorene, carbazole or benzodithiophene). The electron-accepting monomer other than the monomer (I) is a benzothiadiazole, thienopyrazine, quinoxaline, dihydropyrrolo(3,4-)pyrrole-1,4-dione or thieno(3,4-b)thiophene (all optionally substituted by at least one Cl, Br, I, 6-18C aryl or 3-14C heteroaryl having at least one N, O or S in the ring).    TECHNOLOGY FOCUS - POLYMERS - Preferred Oligomer: The oligomer or polymer further comprises at least one electron-accepting monomer other than the monomer (I). The oligomer or polymer has a ratio of electron-accepting monomer to electron-donating co-monomer of 1:99-99:1 mol.% or 40:60-60:40 mol.%. The oligomer or polymer comprises 2-20000 (preferably 10-10000) monomeric units. Preferred Definitions:    X1 and X2=Br, brominated thienyl or octyl substituted brominated thienyl;    R1 and R2=H, 1-20C alkyl, 2-20C alkenyl, 2-20C alkynyl, 6-20C aryl, 7-24C alkaryl, 1-20C alkoxy, 2-20C alkenoxy, 2-20C alkynoxy, 6-20C aryloxy, 1-20C alkylamino, 2-40C dialkylamino, 1-20C alkamido, 2-20C carboxy or 1-20C carbonyl;or    R1 and/or R2 taken together with Y1=6-20C carbocyclic or 3-24C heterocyclic group;and    Y1=S or Se.    X1 and X2 are the same.    SPECIFIC COMPOUNDS - 5 Compounds i.e. 5,6-difluoro-4,7-di(5-bromo-2-thienyl)-2,1,3-benzothiadiazole (I'), 4,7-dibromo-5,6-difluoro-benzo(1,2,5)thiadiazole, 4,7-bis-(5-bromo-4-octyl-thiophen-2-yl)-5,6-difluoro-benzo(1,2,5)thiadiazole, 4,7-bis-(5-bromo-thiophen-2-yl)-5,6-difluoro-benzo(1,2,5)selenadiazole, 4,7-bis-(5-bromo-4-octyl-thiophen-2-yl)-5,6-difluoro-benzo(1,2,5)selenadiazole, are disclosed as the compound (I).    EXAMPLE - In a flask, concentrated sulfuric acid (50 ml) was added and cooled to 0-5 degrees C in an ice water bath. Fuming nitric acid (50 ml) and 2,3-difluoro-1,4-dibromo-benzene (10 g) were slowly added. Then, the flask was heated to 65 degrees C for 14 hours. After work up, 2,3-difluoro-1,4-dibromo-5,6-dinitro-benzene (3.5 g, yield 26%) was obtained. 2,3-Difluoro-1,4-dibromo-5,6-dinitro-benzene (3.62 g), 2-tributyl stannyl-thiophene (8.21 g) and dichlorobis(triphenylphosphine) palladium (0.28 g) were added into a flask. After degassing and purging with argon (Ar) three times, dry tetrahydrofuran (THF) (100 ml) was added. Then the mixture heated to reflux under Ar for 54 hours. THF was removed. After work up 2,3-difluoro-1,4-di(2-thienyl)-5,6-dinitro-benzene as yellow powder (3 g, yield 82%) was obtained. 2,3-Difluoro-1,4-di(2-thienyl)-5,6-dinitro-benzene (3 g), iron powder (5.5 g) and acetic acid (100 ml) were stirred at 45 degrees C for 4 hours. Then the mixture was poured into cold 5% NaOH solution (250 ml) and extracted. After work up, 2,3-difluoro-1,4-di(2-thienyl)-5,6-diamino-benzene as yellow powder (2.1 g, yield 84%) was obtained. 2,3-difluoro-1,4-di(2-thienyl)-5,6-diamino-benzene (1.2 g) was added into a small flask and purged with Ar three times. Then, dry pyridine (24 ml), N-thionylaniline (1.08 g) and chlorotrimethylsilane (0.76 g) were added. The mixture was stirred at 80 degrees C for 16 hours before poured into ice water. After work up, 5,6-difluoro-4,7-di(2-thienyl)-2,1,3-benzothiadiazole as a yellow solid (1.3 g, yield 98%) was obtained. 5,6-Difluoro-4,7-di(2-thienyl)-2,1,3-benzothiadiazole (1.07 g), N-bromosuccinimide (1.132 g) and ortho-dichlorobenzene (20 ml) were stirred at 55 degrees C for 3 hours. Then ortho-dichlorobenzene was removed by vacuum distillation and the remaining solid was washed. After work up, yellow crystals of 5,6-difluoro-4,7-di(5-bromo-2-thienyl)-2,1,3-benzothiadiazole (I') (1.32 g, yield 84%) was obtained.