Abstract
Photoactive metal complexes employing Earth‐abundant metal ions are a key to sustainable photophysical and photochemical applications. We exploit the effects of an inversion center and ligand non‐innocence to tune the luminescence and photochemistry of the excited state of the [CrN6] chromophore [Cr(tpe)2]3+ with close to octahedral symmetry (tpe=1,1,1‐tris(pyrid‐2‐yl)ethane). [Cr(tpe)2]3+ exhibits the longest luminescence lifetime (τ=4500 μs) reported up to date for a molecular polypyridyl chromium(III) complex together with a very high luminescence quantum yield of Φ=8.2 % at room temperature in fluid solution. Furthermore, the tpe ligands in [Cr(tpe)2]3+ are redox non‐innocent, leading to reversible reductive chemistry. The excited state redox potential and lifetime of [Cr(tpe)2]3+ surpass those of the classical photosensitizer [Ru(bpy)3]2+ (bpy=2,2′‐bipyridine) enabling energy transfer (to oxygen) and photoredox processes (with azulene and tri(n‐butyl)amine).
Highlights
A strongly growing interest in chromium(III) complexes, especially with polypyridyl ligands, arises from two perspectives, namely from the ambiguity of the ground state electronic structures of their reduced congeners[1,2] and their—for first row transition metal complexes—outstanding luminescent properties with long-lived spin-flip emission from doublet states.[3,4,5] The type of polypyridine ligand determines both, redox and photophysical properties of chromium(III) complexes
We exploit the effects of an inversion center and ligand non-innocence to tune the luminescence and photochemistry of the excited state of the [CrN6] chromophore [Cr(tpe)2]3+ with close to octahedral symmetry (tpe = 1,1,1-tris(pyrid-2-yl)ethane). [Cr(tpe)2]3+ exhibits the longest luminescence lifetime (t = 4500 ms) reported up to date for a molecular polypyridyl chromium(III) complex together with a very high luminescence quantum yield of F = 8.2 % at room temperature in fluid solution
The very long excited state lifetime and ligand-centered reduction of [Cr(tpe)2]3+ enable both, energy and electron transfer processes with suitable substrates such as oxygen, azulene and tri(n-butyl)amine. This excited state reactivity paves the way for employing this specific [CrN6] chromophore architecture in energy transfer schemes such as singlet oxygen formation,[26] triplet sensitizing[88] or lanthanide-based energy transfer upconversion[34b] as well as in photoredox catalysis.[27,28,29,30]
Summary
A strongly growing interest in chromium(III) complexes, especially with polypyridyl ligands, arises from two perspectives, namely from the ambiguity of the ground state electronic structures of their reduced congeners (redox noninnocence)[1,2] and their—for first row transition metal complexes—outstanding luminescent properties with long-lived spin-flip emission from doublet states.[3,4,5] The type of polypyridine ligand determines both, redox and photophysical properties of chromium(III) complexes. Bis(terpyridine)chromium(III) [Cr(tpy)2]3+ and other classical pyridine complexes are weakly emissive (Table 1).[7,8,9,10,11,12,13] electron donating substituents at the tpy ligands enhance absorption in the visible spectral region by intraligand charge transfer absorptions, luminescence quantum yields and lifetimes remain poor (Table 1).[14,15]. According to Laportes rule for dd transitions in centrosymmetric complexes, the inversion center should affect the absorption and emission properties.[49] tpe could be susceptible to ligand-based redox chemistry (ligand non-innocence) similar to tbpy, tpy or MePDP2À,[1,2] contrasting ddpd as a redox-innocent spectator ligand.[6]. Single crystal X-ray diffraction,[50,51,52,53,54] NIR luminescence quantum yields[55] and lifetimes, variable temperature luminescence and step-scan FT-IR spectroscopy,[56,57,58] electrochemistry and spectroelectrochemistry, Stern–Volmer analyses as well as quantum chemical calculations[59,60,61,62,63,64,65,66,67,68,69] confirm the proposed design guidelines
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