New low band gap compounds comprised of naphthalene diimide and imine units

New arylene bisimide derivatives, both low molecular compounds and polymers consisting of a naphthalene diimide core and imine linkages were synthesized. The azomethine-diimides were prepared from N,N′-bis(4-amino-2,3,5,6-tetramethylphenyl)naphthalene-1,4,5,8-dicarboximide (DANDI) which was condensed with 4-pyridinecarboxaldehyde or 1,3-benzothiazole-2-carboxaldehyde or 2-thiophenecarboxaldehyde or 2,2′-bithiophene-5-carboxaldehyde. Poly(azomethine-naphthaleneimide)s were obtained by high temperature polycondensation of DANDI with 2,5-thiophenedicarboxaldehyde or 2,2′-bithiophene-5,5′-dicarboxaldehyde. The materials do not show decomposition below 300 °C. The azomethine-diimides exhibited glass-forming properties with glass transition temperatures in the range of 80–195 °C. For the first time, to the best of our knowledge, amorphous molecular naphthalene diimide compounds were obtained. Optical properties of the prepared compounds were investigated by UV–vis and photoluminescence (PL) measurements. Most of the azomethine-diimides emitted green light with the highest intensity with emission maximum (λem) at ca. 519–535 nm contrary to polymers which λem was at 450 and 485 nm. All the compounds are electrochemically active and azomethine-diimides undergo quasi-reversible reduction and irreversible oxidation as evidenced by cyclic voltammetry (CV). In all studied molecules the electrochemically determined HOMO level is in the range of −5.5 to −5.13 eV whereas the LUMO level is close to −4.0 eV for azomethine-diimides and −3.8 eV for polymers as determined by differential pulse voltammetry (DPV). The investigated compounds exhibited low electrochemical band gap (1.21–1.5 eV) being promising for optoelectronic applications.