Carbazole core derivatives and their photophysical and electrochemical investigations supported by the theoretical calculations

Abstract

The N-alkylcarbazoles with different aromatic fragments at the carbazole core’s 3, 6 and 8 positions were synthesized and characterized as potential compounds for optoelectronics. Their thermal, electrochemical and optical properties were determined by considering the effect of their chemical structure. The experimental results were supported by the theoretical calculations (DFT). The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) results showed high thermal stability in the range of 346 – 410 °C of the investigated N-alkylcarbazoles and the possibility of forming molecular glasses with the highest glass transition temperature (Tg = 123 °C) registered for the 1,3,6-tri[(4-diphenylamino)phenyl]− 9-(2-ethylhexyl)carbazole compound. The electrochemical activity was examined by the cyclic voltammetry (CV) and differential pulse voltammetry (DPV) methods. The electrodonating character of the investigated compounds was revealed with the EHOMO below − 5.66 eV. Only for the compound 1,3,6-tri[(4-diphenylamino)phenyl]− 9-(2-ethylhexyl)carbazole the oxidation process was registered at 0.39 V. The optical investigationsin the solutions (the five solvents with different polarity) and in the solid state (as the thin film and powder) were carried out. The quantum yield (Φ) was also registered and in the non-polar solvents the Φ was lower than in polar solvents. The 3,6-bis[(fluoren-9-ylidene)methyl]− 9-hexylcarbazole as the only was non-emissive in the solution. The xerographic discharge method was utilized to confirm the possibility of the hole charge mobility of the 1,3,6-tri[(4-diphenylamino)phenyl]− 9-(2-ethylhexyl)carbazole compound. This same compound was tested in the perovskite solar cell as the hole-transporting material (HTM) doped with the lithiumbis-(trifluoromethanesulfonyl)imide salt.