Extending Fluorescence of meso-Aryldipyrrin Indium(III) Complexes to Near-Infrared Regions via Electron Withdrawing or π-Expansive Aryl Substituents

The absorption and fluorescence spectra of 14 In(III) dipyrrin-based complexes are studied using time-dependent density functional theory (TDDFT). Calculations confirm that both heteroatom substitution of oxygen (N2O2-type) by nitrogen (N-4-type) in dipyrrin ligand and functionalization at the meso-position by aromatic rings with strong electron-withdrawing (EW) substituents or extended pi-conjugation are efficient tools in extending the fluorescence spectra of In(III) complexes to the near-infrared (NIR) region of 750-960 nm and in red-shifting the lowest absorption band to 560-630 nm. For all complexes, the emissive singlet state has pi-pi* character with a small addition of intraligand charge transfer (ILCT) contributing from the meso-aryl substituents to the dipyrrin ligand. Stronger EW nitro group on the meso-phenyl or meso-aryl group with extended pi-conjugation induces red-shifted electronic absorption and fluorescence. More tetrahedral geometry of the complexes with N-4-type ligands leads to less intensive but more red-shifted fluorescence to NIR, compared to the corresponding complexes with N2O2-type ligands that have a more planar geometry.

Automated summary

This study investigates the absorption and fluorescence spectra of 14 In(III) dipyrrin-based complexes using TDDFT and finds that nitrogen substitution and aromatic ring functionalization can extend the fluorescence spectra and induce red-shifted electronic absorption and fluorescence. The stronger electron-withdrawing substituents result in the red-shifted fluorescence of the complexes to the NIR region.