Moving Beyond Cyanoarene Thermally Activated Delayed Fluorescence Compounds as Photocatalysts: An Assessment of the Performance of a Pyrimidyl Sulfone Photocatalyst in Comparison to 4CzIPN

Carbazolyl dicyanobenzene (CDCB) derivates exhibiting thermally activated delayed fluorescence (TADF) have shown themselves to be excellent photocatalysts over recent years, particularly 4CzIPN, although investigation into organic TADF compounds as photocatalysts outside of the CDCB group has been limited. Herein, we report an alternative donor–acceptor TADF structure, 9,9′-(sulfonylbis(pyrimidine-5,2-diyl))bis(3,6-di-tert-butyl-9H-carbazole), pDTCz-DPmS, for use as a photocatalyst (PC). A comparison of the electrochemical and photophysical properties of pDTCz-DPmS with 4CzIPN in a range of solvents identifies the former as a better ground state reducing agent and photoreductant, while both exhibit similar oxidation capabilities in the ground and excited state. The increased conjugation of pDTCz-DPmS relative to 4CzIPN presents a more intense CT band in the UV–vis absorption spectrum, aiding in the light absorption of this molecule. Prompt and delayed emission lifetimes are observed for pDTCz-DPmS, confirming the TADF nature, both of which are sufficiently long-lived to participate in productive photochemistry. These combined properties make pDTCz-DPmS useful in photocatalysis reactions, covering a range of photoredox oxidative and reductive quenching reactions, as well as those involving a dual Ni(II) cocatalyst, alongside energy transfer processes. The higher triplet energy and increased photostability of pDTCz-DPmS compared with 4CzIPN were found to be advantages of this organic PC.


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Photophysical measurements. Optically dilute solutions of concentrations on the order of 10 -5 or 10 -6 M of the photocatalysts were prepared in spectroscopic or HPLC grade solvents for absorption and emission analysis. Absorption spectra were recorded at room temperature on a Shimadzu UV-2600 double beam spectrophotometer and a Varian Cary 50 BIO spectrophotometer with a 1 cm quartz cuvette or a Hellma ultra-micro cuvette with 3 mm optical path length. Molar absorptivity determination was verified by linear regression analysis of values obtained from five independent solutions at varying concentrations with absorbance ranging from 4.12 × 10 -6 to 2.06 × 10 -5 M. For emission studies, aerated solutions were bubbled by compressed air for 5 minutes and spectra were taken using the cuvette for absorption analysis.
Degassed solutions were prepared via four freeze-pump-thaw cycles and spectra were taken using home-made Schlenk quartz cuvette. Steady-state emission, excitation spectra and timeresolved emission spectra were recorded at 298 K using an Edinburgh Instruments F980 or a Perkin Elmer LS55 spectrofluorometer, equipped with a Hamamatsu R928 phototube. Samples were excited at 360 nm or 420 nm for steady-state measurements and at 378 nm or 340 nm for time-resolved measurements.
The singlet-triplet splitting energy ∆EST was estimated by recording the prompt fluorescence spectra and phosphorescence emission at 77 K. An open Dewar was used for solution samples.
The samples were photoexcited using the third harmonic emission (343 nm) from a femtosecond Nd:YAG laser, which originally emits at 1030 nm (Orpheus-N, model: PN13F1).
Emission from the samples was focused onto a spectrograph (Chromex imaging, 250is spectrograph) and detected on a sensitive gated iCCD camera (Stanford Computer Optics, 4Picos) having subnanosecond resolution. Phosphorescence spectra were measured 1 ms after the excitation of the Nd:YAG laser with iCCD exposure time of 8.5 ms. Prompt fluorescence spectra were measured 1 ns after the excitation of the femtosecond laser with iCCD exposure time of 100 ns.
To an oven dried flask were added tCz-BrPm (0.67 g, 1.54 mmol, 1 equiv.), NaI (0.915 g, 6.14 mmol, 4 equiv.) and CuI (0.029 g, 0.154 mmol, 0.1 equiv.). The flask was degassed by three cycles of vacuum-nitrogen purging and 24 mL of dry DMF was injected alongside trans-1,2cyclohexanediamine (0.037 mL, 0.307 mmol, 0.2 equiv.). The mixture was stirred at 145 °C using an oil bath for 17 h under a nitrogen atmosphere. The reaction mixture was allowed to cool before being poured onto H2O (40 mL) and extracted with DCM (3 × 50 mL). The combined organic layers were dried over MgSO4 and the organic solvent was removed under reduced pressure. The crude product was purified by washing with acetone to obtain a white solid which was a mix of the tCz-IPm and tCz-BrPm. The product was used for the next step without further purification.

Photocatalysis
Photocatalysis experiments were conducted using a custom-built photoreactor, as shown in Figure S20, allowing for up to 8 parallel photochemical reactions (7 mL) at a time. The photochemistry reaction chamber is filled with mirrors to evenly distribute light. The reactor is placed upon a magnetic stirrer plate allowing for reactions to be completed with stirring.
Reactions are irradiated using Kessil PR160 LED sources. For Kessil PR160-390 nm, the chosen LED source for photocatalysis reactions completed in this study, the power consumption maximum is 52 W, with the average intensity measured from 1 cm distance being 352 mW cm -2 . The intensity on each lamp is tuneable, with the maximum intensity selected for all photocatalytic reactions. A cooling fan is directed at the photoreactor to ensure the reaction mixture maintains at room temperature, which is further guaranteed by the presence of two fans on the photoreactor itself.
After the photoreactions were completed, the products were analysed by 1 H NMR spectroscopy with an internal standard, either 1,3,5-trimethoxybenzene or 1,4-(bis(trimethylsilyl)benzene). All yields shown represent the mean yield from at least two reactions with the associated standard deviation.    To an oven-dried vial was added benzaldehyde (0.020 mL, 0.  a Reaction conditions as stated in the procedure above unless otherwise noted.
2) Decarboxylative addition of N-Cbz-Pro to diethyl maleate To an oven-dried vial was added N-Cbz-Pro (50 mg, 0.  a Reaction conditions as stated in the procedure above.
Procedure for E/Z isomerisation reaction: To an oven-dried vial was added E-stilbene (36 mg, 0. a Reaction conditions as stated in the procedure above unless otherwise noted. Procedure for dual Ni(II) cross-coupling reaction:  a Reaction conditions as stated in the procedure above.