Two-photon spectroscopy of tungsten(0) arylisocyanides using nanosecond-pulsed excitation

Abstract

The two-photon absorption (TPA) cross sections (δ) for tungsten(0) arylisocyanides (W(CNAr)6) were determined in the 800–1000 nm region using two-photon luminescence (TPL) spectroscopy.

Third-Generation W(CNAr)6 Photoreductants (CNAr = Fused-Ring and Alkynyl-Bridged Arylisocyanides)

“…Related studies by our group have established that the sterically encumbered tungsten(0) hexakis­(2,6-diisopropylphenylisocyanide) complex W­(CNDipp) 6 is also a robust, relatively long-lived (τ­( 3 MLCT) = 75 ns in THF) excited-state reductant ( E (W + /*W 0 ) = −3.0 V vs Fc [+/0] ) capable of triggering one-electron reduction of challenging substrates, including anthracene, cobaltocenium, benzophenone, and acetophenone, upon visible light excitation. − Notably, electronic modification of CNDipp by coupling of aryl substituents para to the isocyanide functionality (Figure ) leads to W­(CNDippAr) 6 oligoarylisocyanide complexes with greatly enhanced excited-state properties. Compared to W­(CNDipp) 6 , W­(CNDippPh) 6 , W­(CNDippPh OMe2 ) 6 , W­(CNDippPh OMe3 ) 6 , and W­(CNDippPh Ph ) 6 feature more intense and red-shifted MLCT absorption and emission profiles, τ­( 3 MLCT) in the microsecond range, and ϕ PL up to 0.4, while retaining comparable excited-state reduction potentials. , More recently, we also found that W­(CNDippAr) 6 complexes exhibit exceptionally high two-photon absorption cross sections (δ) in the range 1000–2000 GM (GM = Goeppert–Mayer; 1 GM = 10 –50 cm 4 s photon –1 molecule –1 ) at 812 nm, making them attractive platforms for two-photon imaging and near-IR two-photon redox catalysis …”

“…Compared to W(CNDipp) 6 , W(CNDippPh) 6 , W(CNDippPh OMe2 ) 6 , W-(CNDippPh OMe3 ) 6 , and W(CNDippPh Ph ) 6 feature more intense and red-shifted MLCT absorption and emission profiles, τ( 3 MLCT) in the microsecond range, and ϕ PL up to 0.4, while retaining comparable excited-state reduction potentials. 21,22 More recently, we also found that W-(CNDippAr) 6 complexes exhibit exceptionally high twophoton absorption cross sections (δ) in the range 1000− 2000 GM (GM = Goeppert−Mayer; 1 GM = 10 −50 cm 4 s photon −1 molecule −1 ) at 812 nm, 23 making them attractive platforms for two-photon imaging 24 and near-IR two-photon redox catalysis. 25 Because the photophysical properties of W(CNDippAr) 6 complexes are strongly dependent on the identity of the appended aryl group, we became interested in further examining the effects of extension of the ligand π-system on the excited-state properties of W(CNAr) 6 photoreductants.…”

Photoredox Catalysis Mediated by Tungsten(0) Arylisocyanides

“…We have extended this work to include TPA cross sections for all three generations of W­(CNAr) 6 complexes in toluene solution (810 nm excitation; Table ). As δ values for [Ru­(bpy) 3 ] 2+ (bpy = 2,2′-bipyridine; δ 812 = 7 GM) and fac -Ir­(ppy) 3 (δ 800 = 20 GM) are orders of magnitude smaller at similar wavelengths, our scalable tungsten complexes have the potential to displace ruthenium and iridium photosensitizers in two-photon biological imaging, photodynamic therapy, and photoredox catalysis applications ( vide infra ).…”

“…All investigated W­(CNAr) 6 complexes displayed photoluminescence upon femtosecond-pulsed 810 nm excitation; *W­(CNAr) 6 emission spectra obtained following one- or two-photon excitation were virtually identical (Figure ), confirming that the same long-lived excited triplet ( 3 MLCT) photoreductant was generated in both cases. In 2-methyl-THF solution, W­(CNDippPh OMe2/3 ) 6 and W­(CNDippPh Ph ) 6 exhibited very large two-photon absorption cross sections [δ = 1000–2000 GM (1 GM = 10 –50 cm 4 s photon –1 molecule –1 )] upon excitation at 812 nm . We have extended this work to include TPA cross sections for all three generations of W­(CNAr) 6 complexes in toluene solution (810 nm excitation; Table ).…”

“…In prior work we demonstrated that three W­(CNAr) 6 photoreductants exhibit extremely large two-photon absorption (TPA) cross sections (δ > 10 3 GM) . In the context of photoredox catalysis, near-infrared (NIR) two-photon excitation can be used to generate reactive W­(CNAr) 6 MLCT excited states without competitive absorption or photoreactivity by the substrate.…”