Tuning Excited State Character in Iridium(III) Photosensitizers and Its Influence on TTA-UC

Abstract

A series of mixed ligand, photoluminescent organometallic Ir(III) complexes have been synthesized to incorporate substituted 2-phenyl-1H-naphtho[2,3-d]imidazole cyclometalating ligands. The structures of three example complexes were categorically confirmed using X-ray crystallography each sharing very similar structural traits including evidence of interligand hydrogen bond contacts that account for the shielding effects observed in the ¹H NMR spectra. The structural iterations of the cyclometalated ligand provide tuning of the principal electronic transitions that determine the visible absorption and emission properties of the complexes: emission can be tuned in the visible region between 550 and 610 nm and with triplet lifetimes up to 10 μs. The nature of the emitting state varies across the series of complexes, with different admixtures of ligand-centered and metal-to-ligand charge transfer triplet levels evident. Finally, the use of the complexes as photosensitizers in triplet-triplet annihilation energy upconversion (TTA-UC) was investigated in the solution state. The study showed that the complexes possessing the longest triplet lifetimes showed good viability as photosensitizers in TTA-UC. Therefore, the use of an electron-withdrawing group on the 2-phenyl-1H-naphtho[2,3-d]imidazole ligand framework can be used to rationally promote TTA-UC using this class of complex.

Scheme 1. General Synthetic Scheme for the Ligands and Complexes

Reagents and conditions: (i) Benzaldehyde EtOH, NH₄Cl, or Na₂S₂O₃, reflux; (ii) 0.5 eq. IrCl₃·xH₂O, 2-ethoxyethanol, water, heat; (iii) 2 eq. 2,2′-bipyridine, 2-ethoxyethanol, heat, NH₄PF₆.

Scheme 2. Family of Cyclometalated Iridium(III) Complexes Investigated in This Study

Figure 1

Structures obtained from the single crystal X-ray diffraction studies of (from left to right) Ir–H, Ir–Me, and Ir–OMe (H-atoms, solvent molecules and counteranions are omitted for clarity). Ellipsoids drawn at 50% probability.

Figure 2

(a) UV/vis absorption spectra of the Ir(III) complexes in methanol, c = 1.0 × 10^–5 M. (b) Phosphorescence emission spectra of the Ir(III) complexes in degassed dichloromethane (N₂ atmosphere). Optically matched solutions were used in each panel (each of the solutions gives the same absorbance at the excitation wavelength, A = 0.100), λ_ex = 410 nm, 20 °C.

Figure 3

Phosphorescence emission spectra of the (a) Ir–Me, (b) Ir–H, (c) Ir–CF₃, (d) Ir–Cl, and (e) Ir–OMe complexes in degassed solvents. Optically matched solutions were used in each panel (each of the solutions gives the same absorbance at the excitation wavelength, A = 0.100), λ_ex = 410 nm, 20 °C.

Figure 4

Overlay of the experimentally derived (beige) and calculated (DFT // B3LYP/6-31 G*) SDD optimized singlet (blue) structures of Ir-Me. RMSD = 0.253 Å.

Figure 5

A comparison of the calculated Kohn–Sham frontier molecular orbitals for Ir–H (left) and Ir–OMe (right).

Figure 6

Nanosecond time-resolved transient absorption spectra for (a) Ir–Me, (b) Ir–H, (c) Ir–CF₃, (d) Ir–Cl, and (e) Ir–OMe complexes in degassed dichloromethane under nitrogen atmosphere upon pulsed laser excitation, λ_ex = 420 nm, c = 3 × 10^–5 M.

Figure 7

(a) Time-resolved luminescence of Ir–CF₃ (c = 3.0 × 10^–5 M); (b) the decay traces of phosphorescence in different atmosphere; (c) delayed fluorescence with Ir–CF₃ (c = 3.0 × 10^–5 M) as the triplet photosensitizer and DPA (c = 1.0 × 10^–4 M) as the triplet acceptor; (d) the decay traces of the emission at 600 nm (T₁ → S₀) and 430 nm (¹DPA* → S₀); the spike in the delayed fluorescence traces is the scattered laser. Excited with nanosecond pulsed laser at 445 nm. In deaerated dichloromethane, 20 °C.