TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preparation (Claimed): The preparation of (I), comprises (i) preparing N,N'-disubstituted-6,6'-dibromoisoindigo compounds of formula (III) from 6,6'-dibromoisoindigo (II), and (ii) carrying out Suzuki coupling reaction or Stille coupling reaction of (III) to obtain final product. Preferred Method: The method comprises: either (i) reacting (II) with halohydrocarbon of formula (RBr) in presence of potassium carbonate in dimethylformamide, and (ii) carrying out Suzuki coupling reaction to obtain 6,6'-Bis(hexylthiophen-2-yl)-N,N'-di(2-ethylhexyl)-isoindigo, 6,6'-bis(bithiophen-2-yl)-N,N'-di(2-ethylhexyl)-isoindigo or 6,6'-bis(terthiophen-2-yl)-N,N'-di(2-ethylhexyl)-isoindigo, or carrying out Stille coupling reaction to obtain 6,6'-Bis(thiophen-2-yl)-N,N'-di(2-ethylhexyl)-isoindigo or 6,6'-bis(dihydrothieno(3,4-b)(1,4)dioxin-2-yl)-N,N'-di(2-ethylhexyl)-isoindigo; or (i) reacting (II) with acyl chloride compounds of formula (RCOCl) in presence of sodium hydride in dimethylformamide, and (ii) carrying out Suzuki coupling reaction or Stille coupling reaction to obtain (E)-1,1'-bis(2-ethylhexanoyl)-6,6'-bis(thiophen-2-yl)-(3,3')biindolinyliden-2,2'-dione, (E)-6,6'-bis((2,2')bithiophen-5-yl)-1,1'-bis(2-ethylhexanoyl)-(3,3')biindolinyliden-2,2'-dione or (E)-6,6'-bis((2,2':5',2'')-terthiophen-5-yl)-1,1'-bis((2-ethylhexyl)hexanoyl)-(3,3')biindolinyliden-2,2'-dione. SPECIFIC COMPOUNDS - 11 Compounds (I) are specifically claimed e.g. 6,6'-bis(4-hexylthiophen-2-yl)-N,N'-di(2-ethylhexyl)-isoindigo (Ia), 6,6'-bis(bithiophen-2-yl)-N,N'-di(2-ethylhexyl)-isoindigo, 6,6'-bis(terthiophen-2-yl)-N,N'-di(2-ethylhexyl)-isoindigo, 6,6'-bis(thiophen-2-yl)-N,N'-di(2-ethylhexyl)-isoindigo and 6,6'-bis(dihydrothieno(3,4-b)(1,4)dioxin-2-yl)-N,N'-di(2-ethylhexyl)-isoindigo.    EXAMPLE - 450 mg 1-Bromo-2-ethylhexane, 840 mg 6,6'-dibromoisoindigo and 1.7 g potassium carbonate were reacted in 40 ml dimethylformamide at 100 degrees C for 15 hours to obtain 6,6'-dibromo-N,N'-di(2-ethylhexyl)-isoindigo. Then 223 mg 6,6'-dibromo-N,N'-di(2-ethylhexyl)-isoindigo and 250 mg 4-hexylthiophene-2-boronic acid pinacol ester were mixed in 8 ml toluene, mixture of 7 mg tris(dibenzylideneacetone)dipalladium(0) and 5 mg tri(o-tolyl)phosphine, and tetraethylammonium hydroxide were added, reacted at 85 degrees C under argon gas atmosphere for 20 hours, and obtained product was purified by silica gel column chromatography to obtain 6,6'-bis(4-hexylthiophen-2-yl)-N,N'-di(2-ethylhexyl)-isoindigo with yield of 75%.