New thermotropic azomethine–naphthalene diimides for optoelectronic applications

A new type of thermotropic liquid crystalline compounds containing azomethine linkages and naphthalene diimide moieties were synthesized via condensation of novel N,N′-bis(4-amino-2,3,5,6-tetramethylphenyl)naphthalene-1,4,5,8-dicarboximide with 4-(4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecyloxy)benzaldehyde and 4-octadecyloxybenzaldehyde. The structures of compounds are characterized by means of FTIR, NMR spectroscopy and elemental analysis; the results show an agreement with the proposed structure. Optical properties of the obtained azomethine–naphthalene diimides (AZ-NIs) in solution and in solid state as thin films on the quartz substrate were tested by UV–vis spectroscopy. The electrochemical behavior of AZ-NIs was studied by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels, and electrochemical (E′gE′g) and optical (Eg) band gap values were calculated using the results of electrochemical and UV/vis measurements, respectively. The electrical properties of the azomethine–naphthalene diimides were investigated by current–voltage (I–V) measurements. I–V characteristics were performed on ITO/compound/Al, and ITO/compound:PC61BM/Al devices in the dark and during irradiation with light (under illumination 1000 W/m2). Additionally, the azomethine–naphthalene diimides films were tested using AFM technique. The mesomorphic behavior of the AZ-NIs was investigated via differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). First time, to the best of our knowledge, compounds with both azomethine and naphthalene diimide units with liquid crystalline properties were described in this article.