Phenanthrolines decorated with branched lipophilic chains and their yellow emitting heteroleptic iridium(III) complexes: Synthesis, photophysical and acidochromic behaviour, and computational analysis

To improve the solubility and to manipulate the optical properties of phenanthroline-based materials, a series of compounds with branched lipophilic chains Phen1-4 was prepared in three steps with the yield of 22–81%. In addition, a related phenanthroline-based bidental compound Phen5 was synthesised with blocked N sites on imidazolyl units to influence π-π* and n-π* transitions and to increase FPL. The optical properties were analysed by UV–vis absorption and fluorescence spectroscopy, showing a strong emission at 427–437 nm and improved FPL for the series (20–68%). Furthermore, Phen2-4 and bidental Phen5 exhibited a clear acidochromic behaviour with a dramatic shift in absorption spectra of ca. 30 nm achieved after the addition of 1 eq. and 2 eq. of an acid (TFA), respectively. Likewise, all compounds showed a loss of fluorescence under acidic conditions. Next, to test the complexation ability of these ligands, the heteroleptic iridium(III) complexes Ir1-4 were prepared in excellent yields (86–98%), along with the binuclear complex Ir5. Their optical properties revealed strong yellow emission centered at 539–571 nm with FPL of up to 7%; additionally, all complexes exhibited large Stokes shifts (220–290 nm).