Boron subphthalocyanine axial groups: a comprehensive set for studying the tuning of photophysical and electrochemical properties

Eighteen boron subphthalocyanines (BsubPcs) axial derivatives were synthesized through axial exchange reactions with Br-BsubPc under relatively mild conditions to systematically study the influence of a structurally diverse array of axial group derivatives on the physical properties of the BsubPcs. The photophysical and electrochemical properties of BsubPcs were investigated through solution-state UV-vis absorbance and fluorescence spectroscopy, relative fluorescence quantum yield (QY), cyclic voltammetry (CV), and differential pulse voltammetry (DPV), as these properties are crucial for the application of BsubPcs in the field of organic electronics. The impact of the axial groups on photophysical properties was evaluated by taking measurements in both toluene and alpha,alpha,alpha-trifluorotoluene as the solvent, and referencing QY to two compounds. The axial group has a minimal impact on the absorbance and fluorescence peak shifts, with alpha,alpha,alpha-trifluorotoluene causing a slight blueshift. The axial group had a significant impact on QY, with values ranging from <1% to >70%, and the majority falling in the 30-60% range, depending on the experimental conditions. Although the trends remained consistent, the solvent and reference compound both had notable impacts on QY. CV revealed some BsubPcs have one reversible reduction and one irreversible or quasi-reversible oxidation, others displayed unique reversibility and/or additional redox processes. The axial groups also influenced the redox potentials, with first oxidation potentials spanning a 194 mV range and first reduction potentials covering a 266 mV range. Electron-withdrawing or electron-donating axial groups impacted the redox behaviour of BsubPcs, suggesting an electronic connection between the axial group and the BsubPc core occurs. This study leads to insights into the axial substituents that should be targeted to be used for other peripherally functionalized BsubPc derivatives for further studies.