A Near-Infrared-II Luminescent and Photoactive Vanadium(II) Complex with a 760 ns Excited State Lifetime

Abstract

Ruthenium and iridium are key components in the most important applications of photoactive complexes, namely, light-emitting devices, photocatalysis, bioimaging, biosensing, and photodynamic therapy. Especially, near-infrared (NIR) emissive materials are required in fiber-optic telecommunications, anticounterfeit inks, night-vision readable displays, and bioimaging. Replacing rare and expensive precious metals with more abundant first-row transition metals is of great interest; however, photophysical properties and the chemical stability of 3d metal complexes are often insufficient. Here, we tackle these challenges with a nonprecious metal polypyridine vanadium(II) complex that shows emission above 1300 nm with excited state lifetimes of up to 760 ns. Strong light absorption in the visible spectral region and exceptional stability in the presence of oxygen enable photocatalysis in water and acetonitrile using green to orange-red light for excitation. This study unravels a new design principle for NIR-II luminescent and photoactive complexes based on the abundant first-row transition metal vanadium.

1. NIR-II Emissive Complexes with Earth-Abundant Metals

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Synthesis of vanadium­(II) complexes [V­(tpe)₂]­[X]₂ . b) Structure of the dication of [V­(tpe)₂]­[BPh₄]₂ in the solid state with thermal ellipsoids shown at 50% probability. Hydrogen atoms and counterions are omitted. c) IR and Raman spectra of [V­(tpe)₂]­[PF₆]₂ in the solid state. Asterisks denote bands of the counterion. IR/Raman spectra of other salts [V­(tpe)₂]­[X]₂ as well as DFT calculated vibrational spectra are depicted in Supporting Information, Figure S4.

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UV/vis absorption and emission spectrum of [V­(tpe)₂]­Cl₂ in H₂O. TD-DFT calculated transitions (vertical bars, shifted bathochromically by 923 cm^–1), electron density difference maps [B3LYP/Def2-TZVPP] of optimized [V­(tpe)₂]²⁺ showing electron density gain (blue) and depletion (red) in the ⁴MLCT­(5) and ⁴MLCT­(7) Franck–Condon states and spin density map of the optimized lowest energy doublet state (isosurfaces at 0.003 au, H atoms omitted for clarity). Photograph of the aqueous complex solution. b) Cyclic voltammogram of [V­(tpe)₂]­[PF₆]₂ in CH₃CN/[ ⁿ Bu₄N]­[PF₆]. c) Normalized emission spectra of [V­(tpe)₂]­[BPh₄]₂ in the solid state at temperatures between 293 K (red) and 77 K (blue).

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fs-Transient absorption spectra of [V­(tpe)₂]­Cl₂ in D₂O after excitation with 570 nm at 293 K. b) ns-transient absorption spectra of [V­(tpe)₂]­Cl₂ in D₂O after excitation with 570 nm at 293 K.

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Qualitative scheme of the excited state landscape showing the ⁴A₂ ground state (purple), the ⁴MLCT excited state (purple) and the adiabatic ²MC/²MLCT potentials (blue). Exemplary diabatic doublet potentials of hypothetical pure ²MC and ²MLCT states are shown to illustrate the doublet state mixing (dashed gray). At small-amplitude distortions in the Franck–Condon region, the character is largely metal-centered (²E/²T₁) while the adiabatic state gains more ²MLCT character at larger distortions. At the adiabatic energy minima, the two lowest-energy emissive states possess mixed ²E/²MLCT and ²T₁/²MLCT character, respectively. b) Exemplary qualitative orbital occupations of the quartet and diabatic doublet states for illustration.

1. Reactions of (a) 5-HMF and (b) 1-MCH with ¹O₂ Formed by [V­(tpe) ₂ ] ²⁺ , ³O₂ (Continuous Bubbling of Air through the Solution), and 560 or 625 nm Light in Water or Acetonitrile, Respectively