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

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 3DPA with close‐to‐unity quantum yield. TTA of 3DPA 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.

UV/Vis absorption spectra were recorded with the Varian Cary 5000 spectrometer using 1.0 cm cells. Particularly, as the absorbance of the low-concentrated chromium complex is very low, absorption spectra for the Φ UC determination were collected with very low measuring speed (0.1 nm/0.2 s) to suppress the influence from the noise and background. 6 ] 3 (sensitizer) were obtained with a calibrated spectrofluorometer FSP 920 from Edinburgh Instruments. For the measurement of the emission spectra, a CW xenon lamp was used as the excitation light source, while the time-resolved luminescence measurements were performed with a µs xenon flashlamp (100 Hz) and single photon counting detection in a multi-channel scaling mode. Fluorescence spectra and decays of the anthracene derivatives (acceptors) were measured on a calibrated spectrofluorometer FLS 920 from Edinburgh Instruments. The emission spectra were obtained with a CW xenon lamp as the excitation light source, while the decay kinetics were measured under direct excitation of DPA (395 nm), APA (375 nm), ACA (375 nm), and An (368 nm) with a supercontinuum laser (NKT FIU 15) (9.7 MHz) as the light source and a microchannel plate photomultiplier tube (MCP-PMT; R3809U50) ( Figure S1). UC luminescence measurements: UCL emission spectra were obtained with the calibrated spectrofluorometer FLS 920 equipped with a 532 nm CW laser (62 mW, Sunshine electronics, Shenzhen, CN) and a 520 nm CW laser (800 mW, Roithner Lasertechnik GmbH, Austria). One 532 nm notch filter was placed between the sample holder and the detector to suppress the excitation signal observed on the detector for the UC measurements using the 532 nm laser, while a metal filter (3926B, 9.12% transmission) was set between the sample and detector to reduce the too strong UC signal excited by the 520 nm laser. To take into account the various responses of the detector in different spectral regions, the emission of the UC signal (380-500 nm) and of the sensitizer (650-800 nm) were obtained with the corrective curve under strictly identical conditions (exc. and em. slit of 6 nm, using polarizers in the excitation and emission channel set to 0° and 54.7°, respectively). For excitation power density (P) dependent UCL emission measurements, the laser power was varied with a tunable OD filter and determined with a power meter (Newport 841-PE Powermeter) during the measurements. The laser spot size at the sample position was determined to be ca. 4 mm 2 (532 nm laser, basically identical spot sizes at different powers) and 5-8 mm 2 (520 nm laser, spot size differs a bit at different power) with a laser beam profiler (Newport, LBP2-HR-VIS). To exclude the different size effect on the calculated power density, the 520 nm-laser spot size was measured at each used laser power ranging from 0.3 mW to ca. 590 mW. The laser power density was calculated by dividing the measured laser power by the determined spot size.

Luminescence measurements (direct excitation): Phosphorescence spectra and decays of the [Cr(bpmp) 2 ][PF
UC luminescence decays of the annihilators were obtained on a calibrated spectrofluorometer FS5 from Edinburgh Instruments equipped with the same 532 nm laser (62 mW) and single photon counting detection in a multi-channel scaling mode. The laser was modulated in a pulsed mode (250 Hz, pulse width 500 µs) by a function generator (SRS, model DS345). Φ PL and Φ F measurements: The photoluminescence quantum yields of the sensitizer (50 µM, in deaerated DMF containing 0.1 M HClO 4 ) and the annihilators (10-15 µM, in air-saturated DMF containing 0.1 M HClO 4 ) were determined absolutely using a calibrated Ulbricht integrating sphere setup (Quantaurus-QY C11347-11, Hamamatsu). These measurements were carried out by direct excitation of [Cr(bpmp) 2 ][PF 6 ] 3 (462 nm), DPA (395 nm), APA (375 nm), ACA (375 nm), and An (368 nm), respectively, with the excitation wavelengths being given in brackets. For DPA, fluorescence quantum yield measurements in air-saturated and deoxygenated acidified DMF revealed only a very small influence of oxygen under these conditions that was not further quantified. The relative uncertainties of these measurements are estimated to be ±5 %. [2] Φ UC determination: Φ UC of UC samples containing 50 µM [Cr(bpmp) 2 ][PF 6 ] 3 and 1 mM anthracenes was determined relatively using a [Cr(bpmp) 2 ][PF 6 ] 3 (50 µM) solution without annihilator as reference. [3] Φ UC was calculated according to Eq. S1. [4] In Eq. S1, A Ref and A UC stand for the absorbances of the reference solution and the UC sample at the excitation wavelength. The absorbance readouts at 532 and 520 nm were averaged by taking values from 533 to 531 nm and from 517 to 520 nm, respectively. I UC and I Ref represent the integrated intensities of the UCL and the reference emission, respectively. Φ Ref , which equals the phosphorescence quantum yield of [Cr(bpmp) 2 ][PF 6 ] 3 in deaerated acidified DMF solution at room temperature, is 9.2%. Both reference and UC samples were prepared and measured twice and independently on different days.
To confirm the linear power density dependence of the [Cr(bpmp) 2 ][PF 6 ] 3 phosphorescence, the corrected emission spectra of the reference solution were measured as a function of increasing power densities of the 520 nm and the 532 nm laser used as excitation light sources, respectively ( Figure S13). UC luminescence decays of the annihilators were obtained on a calibrated spectrofluorometer FS5 from Edinburgh Instruments equipped with the same 532 nm laser (62 mW) and single photon counting detection in a multi-channel scaling mode. The laser was modulated in a pulsed mode (250 Hz, pulse width 500 µs) by a function generator (SRS, model DS345). Stern-Volmer studies with DPA and other anthracenes were performed by measuring the phosphorescence intensity (I 709 ) and lifetime ( 709 ) of the deaerated [Cr(bpmp) 2 ][PF 6 ] 3 solutions (0.5 mM in acidified DMF) in the presence of the anthracene annihilators of different concentrations (0 -1 mM), respectively.
Laser flash photolysis (LFP) experiments were carried out with an LP980KS apparatus from Edinburgh Instruments, monitoring either transient absorption or emission signals. A frequencydoubled Nd:YAG laser from Litron (Nano LG 300-10) was used for selective excitation of the sensitizers with laser pulses of 5 ns duration. The excitation intensity at 532 nm was adjusted by a step-motor driven attenuator. Typical laser pulse energies were 40 mJ and the laser output stability was confirmed by several control experiments during each series of experiments. Detection of transient absorption and time-gated emission spectra occurred on an iCCD camera from Andor with precisely adjustable detection delay time (relative to the laser pulse) and integration time. Kinetic traces at selected wavelengths were recorded using a photomultiplier tube. All LFP experiments were performed at 293 K with a cuvette holder allowing temperature control. The DMF solutions for these experiments were prepared in long neck cuvettes with an air-tight teflon screwcap in an Ar-filled glove box (MBraun Unilab Eco, <5 ppm oxygen).
NMR spectra were recorded on a Bruker Avance DRX 400 spectrometer at 400.31 MHz ( 1 H). Resonances are reported in ppm versus the solvent signal as internal standard.  Figure S1: a) Absorption (dotted lines) and emission spectra (solid lines) of [Cr(bpmp) 2 ][PF 6 ] 3 (50 µM) and the annihilators DPA, APA, ACA, and An in acidified DMF (10 -15 µM) at room temperature; The broad red-shifted emission spectrum of ACA observed in this solvent is ascribed to the formation of a linear-type ACA dimer bridged by hydrogen-bonds involving the COOH groups; [5] b) corresponding luminescence decays and quantum yields obtained for direct excitation at 462 nm, 395 nm, 375 nm, 375 nm, and 368 nm, respectively.

S6
Possible explanations for the deviations between the lifetime-and intensity-based SV plots could be static energy transfer and/or aggregation induced emission quenching, which does not yield the desired photoproduct. To address the observed deviations between lifetime-and intensity-based SV plots, transient absorption spectroscopic measurements ( Figure S5 -S7) and luminescence lifetime measurements of the delayed phosphorescence of the sensitizer ( Figure S8, Table S1) were performed. These data confirm the purely dynamic nature of the DTET process for the [Cr(bpmp) 2 ][PF 6 ] 3 /DPA pair. More details follow in Sections 5 and 6.

Density functional theory calculations
All calculations were performed using the quantum computing suite ORCA 4.2.1. [6] Geometry optimization with DFT-methodology was performed using unrestricted Kohn-Sham orbitals DFT (UKS), the restricted Kohn-Sham (RKS) orbitals (for the anthracene derivatives in S 0 state), and the B3LYP functional [7] in combination with Ahlrichs' split-valence triple- basis set def2-TZVPP for all atoms. [8] Tight convergence criteria were chosen for DFT-(U)KS calculations (keywords tightscf and tightopt). All DFT-(U)KS calculations make use of the resolution of identity (Split-RI-J) approach for the Coulomb term in combination with the chain-of-spheres approximation for the exchange term (COSX). [9] The zero order relativistic approximation was used to describe relativistic effects in all calculations (keyword ZORA).

Delayed sensitizer phosphorescence measurements
increases. An increasing sensitizer concentration leads to an increasing number of DPA molecules in the triplet state via DTET. This subsequently feeds the TDET process, which accounts for the increase of A 2 . The shortened  2 of the delayed sensitizer phosphorescence is assigned to the deactivation via DTET, providing a clear hint for the excited state equilibrium [14] between the 2 E/ 2 T 2 states of [Cr(bpmp) 2 ][PF 6 ] 3 and the T 1 state of DPA.           enabling to reach saturation.

Photon upconversion
[d]: The delayed fluorescence lifetimes  UC from the anthracenes were fitted monoexponentially, while the sensitizer lifetimes  709 were fitted bi-exponentially; the respective relative amplitudes are given in brackets. The estimated uncertainty of the lifetimes amounts to ±5%.
The integrated UCL intensities of APA, ACA, and An showed an excitation power density dependence (532 nm laser, cw, 1.5 W·cm -2 ) with slopes of about 1.65 ( Figure S21), probably due to the involved photodimerization. In addition, the Φ UC values obtained with these annihilators are considerably lower than the [Cr(bpmp) 2 ][PF 6 ] 3 /DPA pair. The lower Φ UC values are ascribed to the lower Φ F of the anthracene annihilators ( Figure S1), the more efficient TDET due to their higher triplet energies ( Figure S15), the inherently less efficient TTA compared to DPA, [15] and the additional deactivation pathway of the UC-activated singlet state of the APA, ACA, and An annihilators via photodimerization (see below).