Exploiting exciton coupling of ligand radical intervalence charge transfer transitions to tune NIR absorption

Abstract

We detail the rational design of a series of bimetallic bis-ligand radical Ni salen complexes in which the relative orientation of the ligand radical chromophores provides a mechanism to tune the energy of intense intervalence charge transfer (IVCT) bands in the near infrared (NIR) region. Through a suite of experimental (electrochemistry, electron paramagnetic resonance spectroscopy, UV-vis-NIR spectroscopy) and theoretical (density functional theory) techniques, we demonstrate that bimetallic Ni salen complexes form bis-ligand radicals upon two-electron oxidation, whose NIR absorption energies depend on the geometry imposed in the bis-ligand radical complex. Relative to the oxidized monomer [1˙]⁺ (E = 4500 cm^-1, ε = 27 700 M^-1 cm^-1), oxidation of the cofacially constrained analogue 2 to [2˙˙]²⁺ results in a blue-shifted NIR band (E = 4830 cm^-1, ε = 42 900 M^-1 cm^-1), while oxidation of 5 to [5˙˙]²⁺, with parallel arrangement of chromophores, results in a red-shifted NIR band (E = 4150 cm^-1, ε = 46 600 M^-1 cm^-1); the NIR bands exhibit double the intensity in comparison to the monomer. Oxidation of the intermediate orientations results in band splitting for [3˙˙]²⁺ (E = 4890 and 4200 cm^-1; ε = 26 500 and 21 100 M^-1 cm^-1), and a red-shift for [4˙˙]²⁺ using ortho- and meta-phenylene linkers, respectively. This study demonstrates for the first time, the applicability of exciton coupling to ligand radical systems absorbing in the NIR region and shows that by simple geometry changes, it is possible to tune the energy of intense low energy absorption by nearly 400 nm.

Fig. 1. Exciton coupling of the excited states leads to band shifting and splitting relative to the analogous monomeric transition depending on the molecular geometry. Solid and dashed red lines represent allowed and forbidden transitions, respectively. Small black arrows represent transition moment dipoles.

Fig. 2. Monometallic and bimetallic salen complexes studied. 1 and 3 – previous work;2, 4, and 5 – this work.

Fig. 3. ORTEPs of 1–5 (50% probability) generated using POV-Ray, excluding hydrogen atoms and solvent. See Tables S1 and S2 for metrical parameters and crystallographic details, respectively.

Fig. 4. Cyclic voltammograms of 1–5 (top to bottom). Conditions: 1 mM complex, 0.1 M ⁿBu₄ClO₄, scan rate 100 mV s^–1, CH₂Cl₂, 230 K.

Fig. 5. X-band EPR spectra (black) of [1˙]⁺ and [2–4˙˙]²⁺ recorded in frozen CH₂Cl₂ at 0.33 mM. Red lines represent simulations to the experimental data. Inset: weak half-field ΔM_s = 2 transition for [2˙˙]²⁺. Conditions: frequency = 9.38 GHz ([1˙]⁺ and [3˙˙]²⁺), 9.64 GHz ([2˙˙]²⁺ and [4˙˙]²⁺); power = 2 mW; modulation frequency = 100 kHz; modulation amplitude = 0.4 mT; T = 6 K. Asterisks denote the monoradical [2˙]⁺ impurity.

Fig. 6. Room temperature X-band EPR spectrum of [5˙˙]²⁺ (black) recorded in CH₂Cl₂ at 0.35 mM (simulation in red). Inset: saturation curve of the signal at g = 2.014 with increasing concentration suggesting aggregation above 0.5 mM. Conditions: frequency = 9.44 GHz; power = 2 mW; modulation frequency = 10 kHz; modulation amplitude = 0.15 mT; T = 298 K. Bottom: X-band EPR spectrum of [5˙˙]²⁺ oxidized at 77 K. Conditions: frequency = 9.38 GHz; power = 2 mW; modulation frequency = 100 kHz; modulation amplitude = 0.6 mT.

Fig. 7. NIR region in the absorption spectra of [1˙]⁺ and [2–5˙˙]²⁺. (A) Black line: [1˙]⁺, red line: [3˙˙]²⁺; (B) [2˙˙]²⁺; (C) [4˙˙]²⁺; (D) [5˙˙]²⁺. The dashed black line represents λ_max for [1˙]⁺. Colored dashed lines represent λ_max for the respective bis-ligand radical complex. Conditions: CH₂Cl₂, 0.33 mM complex (A–C), 0.08 mM complex (D), T = 195 K (A–C), 298 K (D).

Fig. 8. Spin density plots for [1˙]⁺ and the broken symmetry (S = 0) solution for [2–5˙˙]²⁺. See the experimental section for calculation details.

Fig. 9. Kohn–Sham molecular orbitals for the broken-symmetry solution (S = 0) of [2˙˙]²⁺ associated with the calculated NIR transitions at 4995 and 3590 cm^–1. The predicted low energy bands are symmetric and antisymmetric linear combinations of the αHOMO → αLUMO (black arrow), and βHOMO → βLUMO (red arrow) local transitions on the individual salen radicals.