Synthesis and structure property relationship of several dicyanomethylene malononitrile derivatives

Based on the intramolecular charge-transfer (ICT) molecule 2-[bis-(4'-diethylamino-biphenyl-4-yl)-methylene]- malononitrile (D7), two novel malononitrile derivatives (C1 and C2) with 2,7- and 3,6-substituted fluorene as conjugated backbones, respectively, were synthesized, which had the same electro-donor and acceptor groups as D7. The effects of the conjugated backbone and substitution site on optical properties of these compounds were investigated. 3,6-substituted fluorene is a better conjugated unit for ICT compared to 2,7 substitution site, whereas it is still weaker than diphenylmethyl unit in D7. However, due to the rigidity of fluorene, C2 possesses much higher fluorescence quantum yield. The results are supported by TD-DFT calculation. This study develops theoretical basis for designing effective optical or electronic materials.