Photophysical, thermal, and DFT studies on a tetraaryl-azadipyrromethene ligand and its zinc(II) complex

An azadipyrromethene ligand (H1) and homoleptic zinc(II) (H1-Zn) complex were synthesized. The resulting structures were elucidated by NMR, FTIR, and HRMS techniques. The photophysical properties and effects of complexing the zinc(II) atom to azadipyrromethene ligands in solution were studied by means of UV-Vis absorption and fluorescence spectroscopy. Experimental findings were elucidated using density functional theory computations and interfragment charge transfer (IFCT) and electron-hole analyses. The fluorescence features were found to be negligible. The ligand molecule decayed at a rate of 3% while the complex decayed at 2% upon photoirradiation based on photostability experiments. The singlet oxygen quantum yields of the ligand and complex were calculated as 0.127 and 0.233, respectively, signifying low photodynamic activity. The charge transfer transitions were determined between reciprocal ligands responsible for the red shift of the main absorption band by IFCT and electron-hole analysis. Compounds in an inert N₂ atmosphere demonstrated high thermal stability. Although the thermogravimetric analysis (TGA) and derivative thermogravimetry curves of the complexes were similar, zinc(II) coordination and homoleptic complex formation reduced the degradation temperatures. These findings suggest that azadipyrromethene and the Zn(II) class of chromophores have beneficial features for use in the development of novel photo- and thermostable materials that combine charge transfer with low energy in the visible and near infrared regions.