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. 2022 Jul 4;61(27):e202202238.
doi: 10.1002/anie.202202238. Epub 2022 May 9.

Efficient Triplet-Triplet Annihilation Upconversion Sensitized by a Chromium(III) Complex via an Underexplored Energy Transfer Mechanism

Cui Wang  1   2 , Florian Reichenauer  3 , Winald R Kitzmann  3 , Christoph Kerzig  3 , Katja Heinze  3 , Ute Resch-Genger  1
Affiliations

Efficient Triplet-Triplet Annihilation Upconversion Sensitized by a Chromium(III) Complex via an Underexplored Energy Transfer Mechanism

Cui Wang et al. Angew Chem Int Ed Engl. .

Abstract

Sensitized triplet-triplet annihilation upconversion (sTTA-UC) mainly relies on precious metal complexes thanks to their high intersystem crossing (ISC) efficiencies, excited state energies, and lifetimes, while complexes of abundant first-row transition metals are only rarely utilized and with often moderate UC quantum yields. [Cr(bpmp)2 ]3+ (bpmp=2,6-bis(2-pyridylmethyl)pyridine) containing earth-abundant chromium possesses an absorption band suitable for green light excitation, a doublet excited state energy matching the triplet energy of 9,10-diphenyl anthracene (DPA), a close to millisecond excited state lifetime, and high photostability. Combined ISC and doublet-triplet energy transfer from excited [Cr(bpmp)2 ]3+ to DPA gives 3 DPA with close-to-unity quantum yield. TTA of 3 DPA furnishes green-to-blue UC with a quantum yield of 12.0 % (close to the theoretical maximum). Sterically less-hindered anthracenes undergo a [4+4] cycloaddition with [Cr(bpmp)2 ]3+ and green light.

Keywords: Chromium; Dexter Energy Transfer; Doublet-Triplet Energy Transfer; Triplet-Triplet Annihilation; Upconversion.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
a) Structures and excited state energies of [Cr(bpmp)2]3+ and DPA, b) normalized absorption and emission spectra of [Cr(bpmp)2]3+ and DPA, c) Jablonski diagram illustrating the excitation of the sensitizer with green light, intersystem crossing (ISC) to its doublet states, vibrational relaxation (VR), doublet‐triplet energy transfer (DTET), the reverse process triplet‐doublet energy transfer (TDET), triplet‐triplet annihilation (TTA) and delayed fluorescence, and d) illustration of the Dexter energy transfer between the excited [Cr(bpmp)2]3+ sensitizer and ground state DPA (DTET) and the reverse process TDET using relevant microstates. The multiplicities (spin degeneracies) of the involved states are highlighted.
Figure 2
Figure 2
Transient absorption studies of the formation of 3DPA upon sensitizer excitation with a 532 nm laser in Ar‐saturated acidified DMF containing 1 mM of DPA. a) TA spectra of [Cr(bpmp)2]3+ (middle panel) and [Ru(bpy)3]2+ (bottom panel) in the presence of DPA integrated over 100 ns as well as control experiments with [Cr(bpmp)2]3+ (upper panel). b) Comparative TA traces monitoring the formation and decay of 3DPA with [Cr(bpmp)2]3+ and [Ru(bpy)3]2+ under these conditions. The measurement of the 3[Ru(bpy)3]2+ TTET quenching efficiency is shown in the inset. The concentrations of [Cr(bpmp)2]3+ and [Ru(bpy)3]2+ were adjusted to matching absorbances of 0.027±0.001 at 532 nm. The detection windows used for the measurement of the 3DPA spectra are indicated with black arrows.
Figure 3
Figure 3
a) UCL spectrum (532 nm, cw, 1.5 W cm−2) and b) UCL decay (532 nm, 250 Hz, pulse width 500 μs) of [Cr(bpmp)2]3+/DPA (blue/red). The red emission trace in (b) corresponds to the concomitant [Cr(bpmp)2]3+ phosphorescence decay. Inset: Photograph of the sample under 532 nm laser excitation (laser power ≈40 mW). The sensitizer and acceptor concentrations in deoxygenated acidified DMF were 50 μM and 1 mM.
Figure 4
Figure 4
a) UCL spectra of [Cr(bpmp)2]3+/DPA (50 μM/1 mM in deoxygenated acidified DMF) as a function of excitation power density of a 520 nm laser (cw, ≈8 W cm−2), b) excitation power density dependence of the integrated UCL (I 400–500) of DPA, c) relatively determined Φ UC of the [Cr(bpmp)2]3+/DPA pair as a function of excitation power density of the 520 nm laser; the red and black symbols denote two independent experiments.
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