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

Abstract

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 (N 2 O 2 -type) by nitrogen (N 4 -type) in dipyrrin ligand and functionalization at the meso-position by aromatic rings with strong electronwithdrawing (EW) substituents or extended π-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 π−π* 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 π-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 N 2 O 2 -type ligands that have a more planar geometry.

Improving Near-Infrared Emission of meso-Aryldipyrrin Indium(III) Complexes via Annulation Bridging: Excited-State Dynamics

“…For absorption spectra, 30 singlet optical transitions were computed and broadened by the empirical Gaussian function using a line width of 0.1 eV to reproduce the thermal broadening of the typical experimental spectra. This approach has previously provided reasonable agreement with experimental absorption spectra for many Ir­(III) complexes, and the result qualitatively agrees with the experimental data of 3a . , …”

“…Our previous TDDFT calculations have shown that substitution of oxygen in N 2 O 2 - m ADIPY with nitrogen in N 4 - m ADIPY with strong EW substituents at the meso -phenyl group on DIPY resulted in the most red-shifted fluorescence (725–961 nm) in In­(III) complexes. However, the oscillator strength of the optical transition contributing to the fluorescence of In­(III) N 4 - m ADIPY is weak, compared to that of the corresponding In­(III) N 2 O 2 - m ADIPY counterpart.…”

“…11 Our recent calculations based on linear response timedependent density functional theory (TDDFT) have predicted that both replacing oxygen in N 2 O 2 -mADIPY with nitrogen in N 4 -mADIPY and functionalization at the meso position by phenyl rings with strong electron-withdrawing (EW) substituents or extending π-conjugation of the meso-aryl group result in red-shifting of the fluorescence energy of the In(III) complexes to the NIR regions of 750−960 nm and the lowest absorption band to 560−630 nm. 12 This trend is rationalized by a stronger shift of the electron density on the meso-aryl group with an improvement in the EW ability of the substituent on it or by extending the π-conjugation, which reduces the energy of the optical transition. These computational findings are encouraging results, providing important insights into the effect of substituents at the meso-aryldipyrrin on the optical response of the In(III) complexes.…”

“…This approach has previously provided reasonable agreement with experimental absorption spectra for many Ir(III) complexes, 41 and the result qualitatively agrees with the experimental data of 3a. 12,11 To simulate fluorescence spectra, we used analytical gradient TDDFT (AG-TDDFT), 13,14 which provides optimization of excited state S 1 . To include vibronic features in the profile of absorption and fluorescence spectra, Franck−Condon (FC) progression 42,43 for the first optical transition was computed for each complex, using the same methodology that was used for TDDFT calculations.…”