DiICz MR-TADF emitters as potent energy transfer photocatalysts

Abstract

Photoinduced energy transfer (PEnT) reactions are a subset of photochemical reactions that involve the indirect photoactivation of substrates following an energy transfer from a photoexcited sensitizer/photocatalyst. Examples of PEnT reactions include E/Z isomerizations, [2 + 2] cycloadditions, and sigmatropic shifts. Here we introduce a family of diindolocarbazole (DiICz) multi-resonant thermally activated delayed fluorescent (MR-TADF) photocatalysts (PCs), DiICztBu₄, DiICzMes₄, DiICztBuCz₄, and DiICztBuDPA₄, which have relatively high triplet energies (E_T). We cross-compare their photocatalytic behavior and that of relevant literature reference PCs' in five distinct PEnT reactions. We demonstrate that the use of the DiICz PCs consistently leads to more rapid reaction rates and higher yields compared to the widely used 4CzIPN. DiICztBu₄, DiICzMes₄, and DiICztBuCz₄ possess similar E_T but decreasing singlet–triplet energy gaps, ΔE_ST, enabling for the first time a comparison of the dependency of both the reaction kinetics and the final yield on this photophysical parameter. We observed that when the reaction kinetics are fast, there is little sensitivity to quenching of the excited PC by oxygen, implying that Dexter Energy Transfer (DET) to the substrate is competitive with DET to oxygen. Importantly, this means that some of the DET reactions using these PCs can be performed in air without adversely affecting reaction yield.

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Introduction

One major class of photochemical reactions consists of those that proceed via a photoinduced energy transfer (PEnT) mechanism, which typically involves an electronically excited photocatalyst (PC*) transferring its energy to a substrate (sub). The substrate in its electronically excited state (sub*) then undergoes a photochemical transformation.^1–3 The majority of PEnT reactions proceed via Dexter Energy Transfer (DET) reactions. DET involves the simultaneous double electron exchange between the PC* and the sub to generate a PC in its S₀ state and a sub*. This can take place from the singlet state of the ¹PC* to generate ¹sub* or more commonly from the triplet state of ³PC* to generate ³sub* (Fig. S1) and does not involve a change in the overall multiplicity.¹ DET processes occur over shorter distances, typically less than 10 Å,⁴ as orbital overlap between the PC and the sub is required, which implies a collisional interaction is needed for DET to occur intermolecularly.

In cases where DET occurs from the ³PC* to ³sub, it is the spectral overlap of the phosphorescence of the ³PC* and the spin-forbidden absorption of the substrate that is relevant to generate ³sub*. Notably, as the spin-forbidden absorption spectrum is of negligible intensity in most organic substrates, the low-energy onset of the low-temperature phosphorescence spectrum of the sub is used as a surrogate estimation of the energy at which spectral overlap no longer occurs, which is none other than the energy of the T₁ state (E_T) of the sub. It is generally accepted that DET will occur when E_T(PC*) > E_T(sub*), and the closer these two values are, the more likely DET is to occur.⁵ Given that the ³sub* has a biradical-like character, this enables a diversity of reactions such as E/Z isomerization,^2,6–8 cycloadditions such as [2 + 2],^9–11 sensitization of metal complexes,^12–14 homolytic bond cleavages such as N₂ release from benzoyl azides, scission of S–S bonds, and N–O dissociation of oxime esters to instigate the formation of carbon-centered and nitrogen-centered radicals in concert with the loss of CO₂.^15–17

Of the PCs typically employed in PEnT reactions, organic carbonyl-based photosensitizers have the highest E_T. For instance, xanthone and acetophenone (Fig. 1) have similar E_T values of 3.22 eV (74.2 kcal mol⁻¹, 310.5 kJ mol⁻¹) and 3.21 eV (74.0 kcal mol⁻¹, 309.6 kJ mol⁻¹).^1,2,18 Organometallic PCs are particularly popular for PEnT reactions in part because the presence of the heavy metal center ensures an essentially quantitative triplet yield as a result of fast intersystem crossing (ISC) mediated by its large spin–orbit coupling (SOC).^19,20 Two of the most commonly used organometallic PCs for PEnT reactions include [Ir(dF(CF₃)ppy)₂(dtbbpy)](PF₆) (E_T = 2.68 eV)⁵ for reactions requiring a relatively high E_T and [Ru(bpy)₃](PF₆)₂ (E_T = 2.12 eV)²¹ for those that do not (Fig. 1).

Fig. 1 Chemical structures and E_T for selected literature PCs used for DET reactions and for the four TADF PCs investigated in this work. Quoted E_T values for 4CzIPN, DiICztBu₄, DiICzMes₄, DiICztBuCz₄ and DiICztBuDPA₄ were estimated from E_T = E_S − ΔE_ST, with E_S being the onset of the steady-state emission in DCM at room temperature and ΔE_ST determined in 2-MeTHF glass at 77 K. E_T for fac-Ir(ppy)₃ was taken from the onset of the room temperature emission in 2-MeTHF. E_T for [Ru(bpy)₃](PF₆)₂ was taken from the emission maximum at 77 K in 4 : 1 MeOH/EtOH glass.²²E_T for [Ir(dF(CF₃)ppy)₂(dtbbpy)]PF₆ was taken from the room temperature emission maximum in MeCN.²³ For the organic PCs, E_T is measured from the phosphorescence spectra at 77 K.⁵

Over the last decade or so, organic thermally activated delayed fluorescence (TADF) compounds have been increasingly used in the academic community as alternatives to organometallic complexes, initially as emissive materials for organic light-emitting diodes (OLEDs),²⁴ and later, as PCs.^25,26 This is because TADF compounds have accessible triplet states owing to the small energy gap (ΔE_ST) between their lowest excited singlet (S₁) and triplet (T₁) states, which allows for relatively fast ISC and reverse ISC (RISC) to take place, despite the small SOC between these states in the absence of heavy atoms.^27,28 Molecules with small ΔE_ST are those where the donor (D) and acceptor (A) moieties are sufficiently electronically decoupled such that there is minimal orbital overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), i.e., the exchange energy is small. This spatial separation also produces an emissive excited state having charge transfer (CT) character. The most widely employed molecular design is one that adopts a strongly twisted D–A structure, exemplified in 4CzIPN and 4DPAIPN (Fig. S2).²⁶ Owing to the distance between D and A units, the emissive S₁ state possesses long-range charge transfer (LRCT) character.²⁹ D–A TADF PCs based on carbazoyl dicyanobenzenes (CDCBs) have been extensively employed in myriad photocatalytic reactions. Examples from this family of TADF PCs cover a broad range of E_T. For example, the E_T values of 4DPAIPN, 4CzTPN, 2CzIPN, 4CzIPN, and 3,5-2CzBN increase from 2.30 eV ³⁰ to 2.34,³¹ 2.72,³¹ 2.73,³² and 3.03 eV,³¹ respectively. These values overlap with those of, for example, [Ir(dF(CF₃)ppy)₂(dtbbpy)]PF₆ (E_T = 2.64 eV ²³) and benzophenone (E_T = 3.00 eV ³³). Beyond CDCBs, other D–A TADF PEnT PCs include pDTCz-DPmS³⁴ and DI-PF³⁵ (Fig. S2). pDTCz-DPmS (E_T = 2.93 eV) possesses a suitable E_T to photocatalyze the E/Z isomerization of diisopropyl fumarate (E_T = 2.7 eV),³⁶ producing 81% of diisopropyl maleate while 4CzIPN only yields 6%, owing to its lower E_T of 2.68 eV.³⁷DI-PF (E_T = 2.38 eV) yielded 66% Z-stilbene in the E/Z isomerization of E-stilbene (E_T = 2.2 eV) despite its lower triplet energy, while 4CzIPN affords 87% of the desired product.³⁵ Another class of TADF PCs contains an imidazo-phenothiazine (IPTZ) acceptor unit connected with different donor groups. The PCs ACR-IMAC, ACR-IPTZ, and SACR-IPTZ have similar triplet energies of E_T = 2.76 eV,³⁸ 2.76 eV,³⁹ and 2.77 eV,³⁹ respectively. Their use in a range of reactions, such as the [2 + 2] cycloaddition of an indole and dimethylphenylvinylsilane, afforded near quantitative product yields.^38,39 A second class of TADF compounds is the so-called multi-resonant TADF (MR-TADF) emitters. These are rigid polycyclic aromatic hydrocarbons that are typically doped with both electron-rich and electron-deficient groups. Suitable regiochemistry of these p- and n-dopants produces an alternating pattern of increasing and decreasing electron density in the excited state as compared to the ground state, leading to the necessary small exchange energy that turns on TADF. Given the short distance between D and A motifs, the emissive excited state is of short-range CT (SRCT) character.⁴⁰ Compared to D–A TADF compounds, MR-TADF emitters have more intense low energy absorption bands, narrower emission spectra, smaller Stokes shifts, and their emission spectra show only minimal positive solvatochromism—all potentially attractive properties for their use as PCs.⁴⁰ Our group has demonstrated the broad utility of two families of MR-TADF PCs in a range of photoinduced electron transfer (PET) and PEnT reactions.^41,42 These two families contain nitrogen donor atoms and either boron or carbonyl groups as n-dopants within the polycyclic aromatic hydrocarbon framework.

We, as well as Lee and co-workers, recently showed that the acceptor moiety is not required in MR-TADF emitter design for OLEDs.^43–47 Indeed, when the nitrogen atoms in diindolocarbazoles are para-disposed, there is very weak TADF as ΔE_ST is moderately large, resulting in slow k_RISC. DiICztBu₄ and DiICzMes₄ have similar E_T of 2.55 (in DCM)⁴³ and 2.57 eV (in toluene),⁴⁴ respectively, while their reported ΔE_ST values are 0.29 and 0.26 eV. Recognizing their potential as PCs, herein we investigated a series of four structurally related DiICz-based TADF PCs, DiICztBu₄, DiICzMes₄, DiICztBuCz₄, and DiICztBuDPA₄, in DET reactions (Fig. 2); the first two have previously been reported, while DiICztBuCz₄ and DiICztBuDPA₄ were, coincident with this work, recently reported by Lee and co-workers.⁴⁸ Across the first three compounds in this series, there is a progressive stabilization of the S₁ state, while the T₁ energy is largely unaffected; both S₁ and T₁ are stabilized in DiICztBuDPA₄. Thus, the impact of ΔE_ST on the performance in DET is probed for the first time. While we did not observe a dependency of the efficiency of the PEnT reactions as a function of the ΔE_ST of the PC across five different PEnT reactions, four involving direct DET to the substrate and one nickel-sensitized dual photocatalysis reaction, we did discover that these PCs initiate fast PEnT reactions that show remarkably little sensitivity to the presence of O₂, despite it being a competitive triplet quencher. In the four PEnT reactions with direct energy transfer to the substrate, DiICztBu₄, DiICzMes₄, and DiICztBuCz₄ consistently outperform 4CzIPN both in terms of NMR yield and reaction rate.

Fig. 2 Chemical structures of DiICztBu₄, DiICzMes₄, DiICztBuCz₄, and DiICztBuDPA₄.

Results and discussion

The syntheses of DiICztBu₄, DiICzMes₄, DiICztBuCz₄, and DiICztBuDPA₄ are detailed in the SI. The UV-vis absorption spectra of these compounds in DCM, along with that of the reference PC 4CzIPN, are shown in Fig. 3a. DiICztBu₄, DiICzMes₄, DiICztBuCz₄, and DiICztBuDPA₄ all exhibit low-energy absorption bands in the visible region that are more intense than that of 4CzIPN. The presence of the mesityl (Mes) groups in DiICzMes₄ leads to a very slight red shift in the absorption spectrum compared to DiICztBu₄, while the substitution of the diindolocarbazole with progressively stronger carbazole (Cz) and diarylamine donors results in a larger red shift of these bands (Fig. 3a and S37). A similar trend is observed in their photoluminescence (PL) spectra (Fig. 3b), where the PL maximum in DCM shifts from λ_PL of 440 to 442, 466, and 532 nm for DiICztBu₄, DiICzMes₄, DiICztBuCz₄, and DiICztBuDPA₄, respectively (Fig. 3b and Table 2). Their emission is more narrowband compared to the broad PL of 4CzIPN, reflecting their more rigid structures.

Fig. 3 (a) Absorption and (b) steady-state PL spectra in DCM of DiICztBu₄ (λ_exc = 340 nm), DiICzMes₄ (λ_exc = 380 nm), DiICztBuCz₄ (λ_exc = 340 nm), DiICztBuDPA₄ (λ_exc = 400 nm), and 4CzIPN (λ_exc = 378 nm).

The S₁ and T₁ energies were determined from the onsets of the steady-state PL and delayed emission (gate time: 1–9 ms) spectra in 2-MeTHF glass at 77 K, respectively (Fig. S40); the ΔE_ST value is the difference in energy between these two (Fig. S40 and Table 1). DiICztBu₄, DiICzMes₄, and DiICztBuCz₄ have very similar E_T of 2.58, 2.57, and 2.55 eV, respectively. As their emission profiles are similar, so too will be their spectral overlap with the sub, and thus the k_DET should be comparable across these three PCs. The S₁ energy, E_S, however, progressively decreases from 2.91 to 2.84 and 2.77 eV (measured at 77 K in 2-MeTHF); thus, the ΔE_ST likewise narrows from 0.33 to 0.27 and 0.22 eV. The E_S and E_T for DiICztBuDPA₄ are 2.54 and 2.41 eV, resulting in the smallest ΔE_ST of 0.13 eV. The E_T values determined at cryogenic temperatures of these four derivatives are all lower than the 2.73 eV of 4CzIPN (Fig. S40e). DiICztBu₄ and DiICzMes₄ have previously been reported as MR-TADF emitters, and given the similar photophysical properties of DiICztBuCz₄ and DiICztBuDPA₄ to these two, the high molar absorptivity of the low-energy absorption band, the minimal positive PL solvatochromism, the narrowband emission and the small ΔE_ST, they can also be classified as MR-TADF (Table 2).

Table 1 Excited-state energies for DiICztBu₄, DiICzMes₄, DiICztBuCz₄, DiICztBuDPA₄, and 4CzIPN

Compound	77 K values^a			Room temperature ET^b/eV
	E S/eV	E T/eV	ΔEST/eV	Toluene	EtOAc	DCM	DMF
a 77 K values were measured in 2-MeTHF glass. ES was determined from the onset of the steady-state PL and ET from the onset of the gated emission acquired in a time window of 1–9 ms after excitation. ΔEST = ES − ET. λexc = 390 nm for DiICztBu4, DiICzMes4, and DiICztBuCz; λexc = 400 nm for DiICztBuDPA4 and λexc = 380 nm for 4CzIPN. b Room temperature ET values were estimated following our previously reported methodology,^49 where ET(RT) = ES(RT) − ΔEST(LT), with ES(RT) being the onset of the steady-state PL in the respective solvents and ΔEST(LT) being measured in 2-MeTHF at 77 K.
DiICztBu4	2.91	2.58	0.33	2.60	2.63	2.60	2.61
DiICzMes4	2.84	2.57	0.27	2.64	2.66	2.65	2.65
DiICztBuCz4	2.77	2.55	0.22	2.58	2.61	2.58	2.58
DiICztBuDPA4	2.54	2.41	0.13	2.44	2.45	2.40	2.46
4CzIPN	2.77	2.73	0.04	2.67	2.66	2.56	2.54

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Table 2 Solution-state photophysical properties of DiICztBu₄, DiICzMes₄, DiICztBuCz₄, and DiICztBuDPA₄ and the reference photocatalysts 4CzIPN and fac-Ir(ppy)₃

Compound	λ abs /nm (ε/10^3 M^−1 cm^−1)	λ PL /nm	E S /eV	E T /eV	ΔEST	τ PL /ns
a In DCM at 77 K, taken from the onset of steady-state PL spectrum for ES and the onset of the gated emission spectrum (1–9 ms) for ET. b In 2-MeTHF at 77 K, taken from the onset of steady-state PL spectrum for ES and the onset of the gated emission spectrum (1–9 ms) for ET. c Under degassed conditions, excited-state lifetimes in air in parentheses. d Lifetime is the average lifetime of 4CzIPN calculated as τavg = τ1 × w1 + τ2 × w2.
DiICztBu4	305 (58), 315 (67), 343 (29), 362 (54), 408 (13), 427 (16)	440	2.91	2.58	0.33	9 (8)
DiICzMes4	300 (84), 306 (88), 315 (81), 344 (29), 364 (53), 409 (13), 430 (16)	442	2.84	2.57	0.27	10 (8)
DiICztBuCz4	319 (152), 347 s (84), 385 s (20), 420 s (17), 444 (25)	466	2.77	2.55	0.22	10 (8)
DiICztBuDPA4	305 (141), 343 (104), 411 (7), 485 (14)	523	2.54	2.41	0.13	16 (13)
4CzIPN	313 (14), 325 (16), 377 (18), 450 s (7)	544	2.77	2.73	0.04	1896 (218)^d
fac-Ir(ppy)3	344 (10), 378 (13), 409 (8), 455 (3)	520	—	2.58	—	1391 (54)

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While the low temperature (LT) measurements, at 77 K, permit a robust estimation of ΔE_ST, noting that in the vast majority of TADF compounds the S₁ state has greater CT character than T₁, this measurement will not accurately capture E_S at room temperature (RT) in solution. This is because CT states are stabilized as a function of solvent polarity, and the greater the CT character, the stronger the stabilization of the state.⁵⁰ As triplet states tend to show lesser CT character than singlets, the estimated E_T = E_S(RT) − ΔE_ST(LT) represents the outer bound value for what E_T may be at room temperature. Thus, given the LRCT character of the S₁ state of 4CzIPN, the change in E_T at room temperature is more pronounced than for the four DiICz MR-TADF compounds (Table 1). This implies that in DET reactions, the spectral overlap between the PC* and the sub may change significantly, especially when the T₁ state possesses some CT character. For MR-TADF compounds, this stabilization effect is small given the SRCT character of the S₁ and T₁ states. So, at low temperature, the E_T values are 2.58, 2.57, 2.55, and 2.41 eV in 2-MeTHF for DiICztBu₄, DiICzMes₄, DiICztBuCz₄, and DiICztBuDPA₄, respectively, while in DCM at room temperature, the E_T values remain essentially the same at 2.60, 2.65, 2.58, and 2.40 eV, respectively (Table 1). This contrasts with the effect on the T₁ energy of 4CzIPN (E_T(LT) = 2.73 eV vs. E_T(RT) = 2.56 eV; where LT is low temperature, 77 K, and RT is room temperature). This is illustrated in a comparison between the T₁ energies in 2-MeTHF glass and DCM solutions, ΔE_T–T, which is 0.08 eV for DiICzMes₄ and 0.17 eV for 4CzIPN. Thus, these four MR-TADF compounds maintain their E_T in polar solvents, while for 4CzIPN, the E_T is much more stabilized. The time-resolved PL decays of DiICztBu₄, DiICzMes₄, DiICztBuCz₄, and DiICztBuDPA₄ in DCM are shown in Fig. S40 and S41 and are listed in Table 2. There is no delayed emission, and PL lifetimes range from 9–16 ns. Given the moderately large ΔE_ST, it is not uncommon for MR-TADF emitters to not show delayed emission in solution as non-radiative decay competes with RISC, while in the solid state, this is largely suppressed, and delayed emission becomes apparent.

DET reactions

We began by studying intramolecular DET reactions with photocatalysts of increasing triplet energy to focus on the interaction between the PC and a single molecule. We first investigated the E/Z isomerization of an alkene, E-cinnamyl acetate, with a T₁ energy similar to those of the MR-TADF emitters; notably, the structurally similar E-methyl cinnamate has a E_T = 2.38 eV ⁵¹ (Table S7). We expect the triplet energy of E-cinnamyl acetate, which contains a methylene group between the styrenyl moiety and the ester group, to be slightly higher compared to E-methyl cinnamate, which has a larger conjugation length. The geometric isomerization does not take place in the absence of an irradiated PC (Table 3, entries 1 and 2). After 24 h of irradiation under N₂, the use of DiICztBu₄, DiICzMes₄, and DiICztBuCz₄ as PCs yielded comparable E/Z ratios of 18/82, 19/81, and 19/81, respectively (Table 3, entries 4, 6, and 8), performing as well in this reaction as 4CzIPN (E/Z ratio of 14/86, Table 3, entry 12). Similar E/Z ratios are observed under air (E/Z = 19/81, 17/84, 20/80, and 15/85 for DiICztBu₄, DiICzMes₄, DiICztBuCz₄, and 4CzIPN, respectively, Table 3, entries 3, 5, 7, and 11). Amazingly, the end ratio obtained for this DET reaction is unaffected despite O₂ being a competitive triplet quencher. This is, however, not the case with DiICztBuDPA₄, which under N₂ yielded an E/Z ratio of 28/72, a lower ratio than those obtained with the other PCs, while there is effectively no isomerization observed in air (Table 3, entries 9 and 10).

Table 3 E/Z isomerization of cinnamyl acetate^a

Entry	Photocatalyst	Conditions	E/Z ratio
a Cinnamyl acetate (0.2 mmol) in DCM (0.2 M) with 1 mol% PC loading. E/Z ratios were determined via^1H NMR spectroscopy. Reported yields are the mean from at least two reactions with associated standard deviations.
1	None	Air	99/1 ± 0
2	None	N2	99/1 ± 0
3	DiICztBu4	Air	19/81 ± 0
4	DiICztBu4	N2	18/82 ± 0
5	DiICzMes4	Air	16/84 ± 1
6	DiICzMes4	N2	19/81 ± 0
7	DiICztBuCz4	Air	20/80 ± 0
8	DiICztBuCz4	N2	19/81 ± 0
9	DiICztBuDPA4	Air	98/2 ± 0
10	DiICztBuDPA4	N2	28/72 ± 0
11	4CzIPN	Air	15/85 ± 0
12	4CzIPN	N2	14/86 ± 0

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Fig. 4 shows the reaction progression for the E/Z isomerization of cinnamyl acetate in air at short times, while for DiICztBuDPA₄, the reaction under N₂ is shown. The conversion saturates after 30 min for DiICztBu₄, DiICzMes₄, and DiICztBuCz₄, with these PCs having similar reaction rates. There is no correlation observed between ΔE_ST and either the E/Z ratio or reaction rate for this transformation. The absence of any substantive change in the post-reaction absorption spectra of DiICztBu₄, DiICzMes₄, and DiICztBuCz₄ revealed that these compounds are photochemically stable to O₂ under the reaction conditions (Fig. S46a–c). Given that O₂ does not affect the E/Z ratio, we investigated the effect on the reaction kinetics as a function of the presence/absence of O₂ by comparing the E/Z ratios after 5 min. The reaction rates slightly increase in the absence of O₂, leading to slightly improved E/Z ratios of 48/52, 51/49, and 49/51 for DiICztBu₄, DiICzMes₄, and DiICztBuCz₄, respectively, from those in air (53/47, 61/39, and 56/44, respectively); it is not clear why the change is more dramatic with DiICzMes₄. Hence, O₂ acts as an ineffective yet competitive quencher of the triplet excited state of these PCs, as the change in the E/Z ratio at early times is marginal. The contrasting results with DiICztBuDPA₄ are discussed in the SI.

Fig. 4 Reaction progression for the E/Z isomerization of cinnamyl acetate in air unless otherwise noted. E/Z ratios are given in % Z isomer after x minutes (λ_exc = 440 nm). Cinnamyl acetate (0.2 mmol) in DCM (0.2 M) and PC (1 mol%). E/Z ratios were determined by ¹H NMR spectroscopy. Reactions with DiICztBuDPA₄ as the PC were performed under N₂.

The isomerization reaction with 4CzIPN is significantly slower than with DiICztBu₄, DiICzMes₄, and DiICztBuCz₄, only producing a 39/61 E/Z ratio after 60 min (Fig. 4). Thus, despite similar thermodynamic driving forces (i.e., similar E_T), the origin of the higher yields of the Z isomer and faster reaction kinetics could be due to either the higher molar absorptivity of these three MR-TADF compounds at the photoexcitation wavelength and/or an increased spectral overlap of the phosphorescence of the MR-TADF emitters with the spin-forbidden absorption of the sub. 4CzIPN is mostly photostable; however, a slight decrease in the absorbance of the CT band is observed in air, suggesting that some degradation is taking place if the reaction is carried out when O₂ is present (Fig. S46e). Furthermore, there is a significant dependency of the kinetics of the reaction with respect to the presence/absence of O₂. After 5 min, the E/Z ratios are 80/20 and 94/6 under N₂ and air, respectively. These results reveal that 4CzIPN photocatalyzes this reaction much more slowly than DiICztBu₄, DiICzMes₄, and DiICztBuCz₄ and that its triplet excited state is also more strongly quenched by O₂. The excited-state lifetimes of the PCs under aerated conditions are 8, 8, 8, and 13 ns for DiICztBu₄, DiICzMes₄, DiICztBuCz₄, and DiICztBuDPA₄, respectively. These are not affected by the presence of E-cinnamyl acetate (τ_PL of 8, 8, 8, and 12 ns for DiICztBu₄, DiICzMes₄, DiICztBuCz₄, and DiICztBuDPA₄, respectively, Fig. S47).

We had previously assessed MR-TADF compounds DABNA-1, tDABNA, CzBN, and DtBuCzB as photocatalysts in the E/Z isomerization of cinnamyl acetate. Of these, CzBN produced the highest conversion to the Z isomer in a ratio of 19/81 after two hours in air (in THF); the ratio remained the same after 24 hours.⁴² In our previous study, we did not monitor the reaction kinetics at shorter times. Thus, CzBN yields a similar E/Z ratio as the DiICz photocatalysts (19/81 for CzBN after 2 h and 18/82 for DiICztBu₄ after 1 h). This is consistent with their comparable E_T values (2.58 eV for CzBN and 2.58 eV for DiICztBu₄, respectively, in 2-MeTHF at 77 K).⁴²

We next investigated the E/Z isomerization of diisopropyl fumarate, which has a higher E_T of 2.7 eV,³¹ to produce the corresponding maleate (E_T = 3.1 eV)³¹ (Table S8). Despite the E_Ts of the PCs being lower than 2.7 eV (Table 2), both DiICztBu₄ (E_T = 2.60 in DCM) and DiICzMes₄ (E_T = 2.65 in DCM) photocatalyzed the reaction, producing identical E/Z ratios of 13/87 after 24 h (Table 4, entries 4 and 6). DiICztBuCz₄ and 4CzIPN were less effective, producing E/Z ratios of 66/34 and 69/31, respectively (Table 4, entries 8 and 12). Surprisingly, it seems that the slightly lower E_T of DiICztBuCz₄ (2.58 eV in DCM) compared to DiICzMes₄ (E_T = 2.65 eV in DCM) and DiICztBu₄ (E_T = 2.60 eV in DCM) is responsible for these large changes in E/Z ratios; notably, DiICztBuCz₄ is unstable under the reaction conditions (Fig. S46c). These results also clearly illustrate the effect of solvent polarity on the magnitude of the spectral overlap between the PC and sub, and the corresponding efficiency to photocatalyze the isomerization. Despite 4CzIPN having the highest E_T of 2.73 eV in 2-MeTHF at 77 K, meaning that if these values were reflective of accessible triplet energies under the reaction conditions, then it should be able to photocatalyze the isomerization to a similar extent as the DiICz PCs. The triplet state of 4CzIPN, however, shows the greatest stabilization as a function of solvent polarity (E_T in DCM is 2.56 eV at room temperature, Table 1). This accounts for the poorer E/Z ratio of 69/31 (Table 4, entry 12). Notably, as 4CzIPN is photostable under these reaction conditions when performed under N₂, its poor performance cannot be attributed to PC degradation (Fig. S45e).

Table 4 E/Z isomerization of diisopropyl fumarate^a

Entry	Photocatalyst	Conditions	E/Z ratio
a Diisopropyl fumarate (0.2 mmol) in DCM (0.2 M) with 1 mol% PC loading. E/Z ratios were determined by ^1H NMR spectroscopy. Reported yields are the mean from at least two reactions with associated standard deviations.
1	None	Air	100/0 ± 0
2	None	N2	100/0 ± 0
3	DiICztBu4	Air	95/5 ± 1
4	DiICztBu4	N2	13/87 ± 0
5	DiICzMes4	Air	92/8 ± 2
6	DiICzMes4	N2	13/87 ± 0
7	DiICztBuCz4	Air	88/12 ± 0
8	DiICztBuCz4	N2	66/34 ± 4
9	DiICztBuDPA4	Air	100/0 ± 0
10	DiICztBuDPA4	N2	100/0 ± 0
11	4CzIPN	Air	91/9 ± 4
12	4CzIPN	N2	69/31 ± 4

---

In contrast to cinnamyl acetate (Table 3), the E/Z isomerization of diisopropyl fumarate essentially does not proceed to any appreciable extent in the presence of O₂, with E/Z ratios of 96/5, 93/8, 88/12, and 91/9 for DiICztBu₄, DiICzMes₄, DiICztBuCz₄, and 4CzIPN, respectively (Table 4, entries 3, 5, 7, and 11). DiICztBuDPA₄ could not photocatalyze the reaction (Table 4, entries 9 and 10), given its E_T of 2.41 eV.

To understand the divergence in the behavior of these two isomerization reactions in the presence of O₂, we interrogated the reaction rates of diisopropyl fumarate under N₂ (Fig. 5). If the rates are slower compared to those with cinnamyl acetate, then the lower E/Z ratios may be explained by competitive O₂ quenching of the T₁ state to that of the fumarate. Unlike the reaction with cinnamyl acetate, the reaction rates under N₂ differ between DiICzMes₄, DiICztBu₄, and DiICztBuCz₄, but still are all faster than 4CzIPN. The fastest conversion to the Z isomer uses DiICzMes₄, yielding an E/Z ratio of 22/79 ± 3 after 60 min (13 : 87 after 24 h, Table 4, entry 6), while DiICztBu₄ only produces an E/Z ratio of 60/41 ± 1 after 60 min, yet affords the same E/Z ratio after 24 h as DiICzMes₄ (13 : 87 after 24 h, Table 4, entry 4). The reaction reaches its steady-state E/Z ratio after 2 h for DiICzMes₄ (E/Z ratio of 12/88), while for DiICztBu₄ it takes 4 h to reach the 13/87 E/Z ratio. These rates are slower than those with cinnamyl acetate, where the reaction was completed after 30 min. Given that both DiICzMes₄ and DiICztBu₄ are stable during the reaction when conducted under N₂, photodegradation of the PC can be excluded as the origin of the slower reaction kinetics observed for DiICztBu₄. As the phosphorescence of both DiICztBu₄ and DiICzMes₄ is the same, the spectral overlap between the phosphorescence spectra of these two PCs and the spin-forbidden absorption spectrum of the sub will therefore be similar. A plausible conclusion is that the tert-butyl groups are bulkier than mesityl groups and impede the collisional interaction more, thus adversely affecting the reaction kinetics with DiICztBu₄. We observed that these two PCs do photodegrade when the reaction is carried out under air. Thus, there is a divergence in the photochemical outcome wherein the kinetics of the E/Z isomerization of cinnamyl acetate outcompetes photodegradation, while this is not the case with the E/Z isomerization of diisopropyl fumarate (Fig. S45a and b). Surprisingly, while the initial rate of isomerization using DiICztBuCz₄ is comparatively as fast as using DiICzMes₄, there is a significant off-cycle photodegradation both in air and N₂ that effectively caps the E/Z ratio at only 66/34 ± 4 (Fig. S45c). DiICztBuDPA₄ is photostable when the reaction is conducted under N₂, while there are changes in the absorption spectrum when the reaction is conducted in air (Fig. S46d). Hence, the fact that the reaction does not proceed is a consequence of there being no spectral overlap between the phosphorescence of DiICztBuDPA₄ and the spin-forbidden absorption of the substrate.

Fig. 5 Reaction progression for the E/Z isomerization of diisopropyl fumarate under N₂. E/Z ratios are given in % Z isomer after x minutes irradiated at 440 nm. Diisopropyl fumarate (0.2 mmol) in DCM (0.2 M) and PC (1 mol%). E/Z ratios were determined by ¹H NMR spectroscopy.

4CzIPN is significantly slower and less efficient in this isomerization reaction, producing a steady-state E/Z ratio of only 69/31 ± 4 after 24 h (Table 4, entry 12); indeed, after 4 h, there is only an E/Z ratio of 95/5 ± 1 (Table S8). Under these conditions, 4CzIPN is photostable under N₂; however, in air, photodegradation of 4CzIPN is observed (Fig. S46e). Thus, the poor performance of 4CzIPN can be explained by the too-small spectral overlap with the fumarate substrate.

The E/Z isomerization of diisopropyl fumarate studies demonstrate that three of these PCs can catalyze PEnT reactions with substrates having E_T of 2.7 eV.³¹ Thus, we next explored the intramolecular [2 + 2] cycloaddition of norbornadiene (E_T = 2.7 eV ⁵²) to quadricyclane (Table S9). Schmid et al. had reported a 99% yield after one hour using an anionic Ir complex having an E_T of 2.99 eV (Table 5, entry 14).

Table 5 [2 + 2] cycloaddition of norbornadiene^a

Entry	Photocatalyst	Conditions	Yield/%
a Norbornadiene (0.2 mmol) and PC (1 mol%) in DCM (2 mL). Yields were determined by ^1H NMR spectroscopy using 1,3,5-trimethoxybenzene as the internal standard. b From ref. 52 in CD3CN, λexc = 415 nm after 1 h, norbornadiene (0.05 mmol), PC (0.3 mol%), CD3CN (0.6 mL). Reported yields are the mean from at least two reactions with associated standard deviations.
1	None	Air	0 ± 0
2	None	N2	0 ± 0
3	DiICztBu4	Air	64 ± 4
4	DiICztBu4	N2, 2 h	87 ± 1
5	DiICztBu4	N2	89 ± 2
6	DiICzMes4	Air	71 ± 6
7	DiICzMes4	N2	85 ± 5
8	DiICztBuCz4	Air	75 ± 2
9	DiICztBuCz4	N2	88 ± 0
10	DiICztBuDPA4	Air	0 ± 0
11	DiICztBuDPA4	N2	0 ± 0
12	4CzIPN	Air	0 ± 0
13	4CzIPN	N2	0 ± 0
14	[Ir(L)2(BCF)2]^−	Degassed	99^b

---

The reaction does not take place in the absence of a PC (Table 5, entries 1 and 2). After 30 min reaction time using DiICztBu₄, quadricyclane was obtained in an 89% yield under N₂ (Table 5, entry 5). In contrast to the E/Z isomerization of fumarate, DiICzMes₄ and DiICztBuCz₄ perform as well as DiICztBu₄, yielding 85, 88, and 89%, respectively, after 30 min (Table 5, entries 5, 7, and 9). The reaction is slightly quenched in the presence of O₂, producing yields of 64, 71, and 75% after 30 min when using DiICztBu₄, DiICzMes₄, and DiICztBuCz₄, respectively (Table 5, entries 3, 6, and 8). Given the fast reaction rates, there is thus significantly less quenching of the ³PC* by O₂ than what has been observed with diisopropyl fumarate. Again, it is not surprising that DiICztBuDPA₄ does not photocatalyze the reaction, given its too low E_T (Table 5, entries 10 and 11). Similarly, given that 4CzIPN did not photoisomerize diisopropyl fumarate to diisopropyl maleate after 30 min to any appreciable extent, this PC did not photocatalyze the [2 + 2] cycloaddition of norbornadiene within 30 min (Table 5, entries 12 and 13). These results not only demonstrate the value of three of these MR-TADF PCs to photocatalyze a demanding PEnT reaction but also that E_T as a parameter is too coarse when cross-comparing different reactions using substrates with similar E_T. We had previously demonstrated that DiKTa, another MR-TADF photocatalyst (E_T = 2.62 eV),⁴¹ could efficiently photoisomerize diisopropyl fumarate into diisopropyl maleate in a ratio of 10/90 in MeCN over 16 h, a similar ratio to that using DiICzMes₄ (8/92); in this earlier study, the reaction progression at shorter reaction times was not analysed.⁴¹

The photostability of the PCs was tested under the reaction conditions under N₂ and in air (Fig. S51). The absorption spectra of DiICztBu₄, DiICzMes₄, and DiICztBuCz₄ show only a minimal decrease in intensity under N₂, while there are greater changes with O₂ present that suggest more photodegradation, which is consistent with the less efficient performance when the reaction is carried out in air (Fig. S51a–c). Interestingly, the small degree of photodegradation of the MR-TADF PCs in the [2 + 2] cycloaddition in air contrasts with the significant photodegradation in the E/Z isomerization with diisopropyl fumarate in air and the associated absence of product formation, despite both substrates having the same reported E_T.

There is significant photodegradation observed for 4CzIPN under N₂, and the profile is similar to that in air (Fig. S51e). The absorption band at λ_abs = 377 nm and the shoulder at λ_abs = 450 nm disappear, while a more red-shifted, less intense band at λ_abs = 520 nm appears. These results imply that the substrate reacts with 4CzIPN and that the [2 + 2] cycloaddition is likely a radical stepwise process as opposed to a concerted mechanism (Table 5, entry 12 and 13).

We then explored a PEnT reaction with a substrate having a higher E_T, the sigmatropic shift of (S)-verbenone (E_T = 3.0 eV)⁵² to chrysanthenone (Table S10). This rearrangement has been used in the synthesis of the natural product xishacorene B, where verbenone was directly irradiated in the first step with UV light (λ_exc = 365 nm), yielding 67% chrysanthenone.⁵³ Schmid et al. showed that this transformation can be photocatalyzed using the same iridium isocyanoborato complex as was used in the [2 + 2] cycloaddition of norbornadiene,⁵² whereupon irradiating the solution at 415 nm produced the desired product in 80% yield after 180 min (Table 6, entry 10). With DiICztBu₄, DiICzMes₄, and DiICztBuCz₄ as the PCs, the yields were generally low after 24 h at 29, 22, and 18%, respectively, using 1 mol% PC (Table 6, entries 2, 4, and 6); evidently, DiICztBuDPA₄ cannot photocatalyze this reaction due to its too low E_T (Table 6, entry 8). The yields were improved by increasing the PC loading to 5 mol%, resulting in yields of 37, 38, and 21% for DiICztBu₄, DiICzMes₄, and DiICztBuCz₄, respectively (Table 6, entries 3, 5, and 7). Using 4CzIPN afforded only 5% product yield at 5 mol% PC loading (Table 6, entry 9).

Table 6 Sigmatropic shift of (S)-verbenone^a

Entry	PC	Loading/mol%	Yield/%
a (S)-verbenone (0.2 mmol) in DCM (0.1 M) with given PC loadings. Yields were determined by ^1H NMR spectroscopy using 1,3,5-trimethoxybenzene as the internal standard. b From ref. 52 in CD3CN with hv = 415 nm after 3 h. Reported yields are the mean from at least two reactions with associated standard deviations.
1	None	—	0 ± 0
2	DiICztBu4	1	29 ± 3
3	DiICztBu4	5	37 ± 1
4	DiICzMes4	1	22 ± 2
5	DiICzMes4	5	38 ± 6
6	DiICztBuCz4	1	18 ± 1
7	DiICztBuCz4	5	21 ± 2
8	DiICztBuDPA4	1	0 ± 0
9	4CzIPN	5	5 ± 1
10	[Ir(L)2(BCF)2]^−	0.3	80^b

---

There is essentially no change in the absorption profiles of DiICztBu₄, DiICzMes₄, and DiICztBuCz₄ before and after irradiation, suggesting that any photodegradation of the PCs is not the cause for the low yields in this reaction but rather the too low triplet energy (Fig. S53a–c). 4CzIPN and DiICztBuDPA₄ are stable under the reaction conditions (Fig. S53d and e), and the 5% yield for 4CzIPN and the 0% yield for DiICztBuDPA₄ can be attributed to each having a T₁ energy that is effectively too low to enable DET with any degree of efficiency.

Thus far, we have explored four intramolecular PEnT reactions. To expand the portfolio of available reactions, we next investigated an example of a bimolecular cross-coupling reaction. This is frequently achieved by combining the photocatalytic cycle with a Ni-mediated cross-coupling reaction. In such dual-catalyzed reactions, the PC can either be involved in a PET mechanism, a so-called metallaphotoredox reaction, or in a PEnT reaction to sensitize the Ni co-catalyst by DET after it has undergone oxidative addition and transmetallation steps. This photoactivation accelerates the reductive elimination of the product.³¹ One such cross-coupling reaction where the PC is purported to engage in DET is an esterification involving aryl halides being cross-coupled with carboxylic acids, where Welin et al. used fac-Ir(ppy)₃ as the PC (E_T(fac-Ir(ppy)₃) = 2.58 eV).¹³ In this cross-coupling reaction, an undesired side product is lactone 2 (Table 7). Several groups have employed organic PCs such as 4DPAPN or SACR-IPTZ for these reactions (Fig. S1).^31,394DPAPN, after optimization of the reaction conditions, yielded 91% of 4-(trifluoromethyl)phenyl benzoate in the cross-coupling of benzoic acid with 4-bromobenzotrifluoride as substrates.³¹ With the same substrates but under slightly different conditions, Welin et al. reported an 86% yield using fac-Ir(ppy)₃ as the PC. The reaction photocatalyzed using SACR-IPTZ yielded 99% of the coupled product between 5-bromophthalide and benzoic acid, and no protodehalogenated product was observed.³⁹

Table 7 Energy transfer-mediated Ni-catalyzed cross-coupling esterification^a

Entry	Photocatalyst	Conditions	Yield 1/%	Yield 2/%
a 5-Bromophthalide (0.188 mmol, 1.0 equiv.), benzoic acid (0.301 mmol, 1.6 equiv.), 2,2,6,6-tetramethylpiperidine (TMP) (0.375 mmol, 2.0 equiv.), Ni source (0.011 mmol, 6 mol%), 4,4′-di-tert-butyl-2,2′-bipyridine (dtbbpy) (0.013 mmol, 7 mol%) and PC (0.004 mmol, 2 mol%) in DCM (0.09 M). Yields were determined by ^1H NMR spectroscopy using 1,3,5-trimethoxybenzene as the internal standard.
1	None	Ni(COD)2	10 ± 1	11 ± 2
2	None	NiBr2·glyme	0 ± 0	0 ± 0
3	DiICztBu4	Ni(COD)2	59 ± 2	28 ± 0
4	DiICztBu4	NiBr2·glyme	31 ± 4	15 ± 1
5	DiICzMes4	Ni(COD)2	65 ± 4	28 ± 3
6	DiICzMes4	NiBr2·glyme	41 ± 0	19 ± 1
7	DiICzMes4	[Ni(dtbbpy)(OH2)4]Cl2	68 ± 1	18 ± 0
8	DiICztBuCz4	Ni(COD)2	60 ± 6	31 ± 2
9	DiICztBuCz4	NiBr2·glyme	20 ± 1	20 ± 1
10	DiICztBuDPA4	Ni(COD)2	47 ± 5	14 ± 2
11	DiICztBuDPA4	NiBr2·glyme	27 ± 3	11 ± 0
12	4CzIPN	Ni(COD)2	43 ± 2	36 ± 4
13	4CzIPN	NiBr2·glyme	31 ± 0	19 ± 0
14	fac-Ir(ppy)3	Ni(COD)2	65 ± 1	15 ± 1
15	fac-Ir(ppy)3	NiBr2·glyme	49 ± 5	16 ± 1

---

We first investigated the use of different Ni precursors in the presence of dtbbpy as an ancillary ligand under conditions similar to those reported by Hojo et al.³⁹ We changed the solvent from DMF to DCM, as the DiICz PCs are more soluble in the latter.³⁹ We observe a generally higher product yield in DCM when using Ni(COD)₂ than with NiBr₂·glyme, regardless of the choice of PC. The use of DiICzMes₄ yielded 65% in combination with Ni(COD)₂, while using NiBr₂·glyme only results in a 41% yield (Table 7, entries 5 and 6). In both cases, side product 2 is observed in 30 and 19% yield (Table 7, entries 5 and 6). DiICztBu₄, DiICzMes₄, and DiICztBuCz₄ perform similarly in combination with Ni(COD)₂ with yields of 59, 65, and 60% of 1, respectively, while 2 formed in 28, 28, and 31% yield, respectively (Table 7, entries 3, 5, and 8). Changing the Ni source to NiBr₂·glyme yielded 31/15 and 41/19% of 1 and 2 using DiICztBu₄ and DiICzMes₄, respectively, implying that the use of NiBr₂·glyme results in more undesired protodehalogenated product for these two complexes, while with DiICztBuCz₄, the reaction proceeds less readily, affording 20% of each of 1 and 2. These PCs produce higher yields of 1 in combination with Ni(COD)₂ than using 4CzIPN (43%, Table 7, entry 12), and this is correlated to a greater amount of 2 forming (36%) with this latter PC. When fac-Ir(ppy)₃ is employed as the PC in combination with Ni(COD)₂, 65% of 1 and 15% of 2 form (Table 7, entry 14). A comparable yield is observed with DiICzMes₄ (68/18% of 1 and 2, Table 7, entry 7), but the Ni source must be [Ni(dtbbpy)(OH₂)₄]Cl₂, implying that the kinetics of the formation of the active Ni species are suboptimally aligned with the kinetics of DET with this PC. After solvent optimization with the best performing DiICz PC, DiICzMes₄, the coupled product 1 was obtained in a 74% yield when using a 1 : 1 mixture of DMSO : toluene (Table 8, entry 4), while the control experiment with fac-Ir(ppy)₃ yielded a comparable yield of 77% (Table 8, entry 6). The 1 : 1 DMSO : toluene solvent system had previously been shown to be optimal with SACR-IPTZ as the PC, yielding 99% of 1 (Table 8, entry 8).³⁹ The reaction does not readily proceed in toluene, yielding only 10% product, while in DMSO an increase in the yield (64% of 1) compared to that in DCM is observed, ostensibly due to a partial suppression of the formation of 2 (Table 8, entry 1–3). Using 1 : 1 DMSO : toluene in combination with the preformed Ni complex [Ni(dtbbpy)(OH₂)₄]Cl₂ and DiICzMes₄ as the PC resulted in 81% yield of 1 and only 13% of 2 (Table 8, entry 7). Reducing the reaction time from 24 to 3 h resulted in a lower yield of 1 (54%) but not of 2 (13%) (Table 8, entry 5), while with fac-Ir(ppy)₃ as the PC the product was obtained in 68% yield after 3 h (Table 8, entry 7). While the reaction with fac-Ir(ppy)₃ is slightly faster than with DiICzMes₄, effectively similar yields of 74 and 77% were obtained after 24 h using DiICzMes₄ and fac-Ir(ppy)₃, respectively.

Table 8 Reaction optimization of the Ni-co-catalyzed cross-coupling esterification reaction^a

Entry	Solvent	PC	Conditions	Yield 1/%	Yield 2/%
a 5-Bromophthalide (0.188 mmol, 1.0 equiv.), benzoic acid (0.301 mmol, 1.6 equiv.), TMP (0.375 mmol, 2.0 equiv.), Ni-source (0.011 mmol, 6 mol%), dtbbpy (0.013 mmol, 7 mol%) and PC (0.004 mmol, 2 mol%) in the respective solvent (0.09 M). Yields were determined by ^1H NMR spectroscopy using 1,3,5-trimethoxybenzene as the internal standard. b Literature yield taken from ref. 39 5-bromophthalide (0.188 mmol, 1.0 equiv.), benzoic acid (0.301 mmol, 1.6 equiv.), TMP (0.375 mmol, 2.0 equiv.), Ni-source (0.011 mmol, 6 mol%), dtbbpy (0.013 mmol, 7 mol%) and PC (0.004 mmol, 2 mol%) in the respective solvent (0.09 M), λexc = 400 nm.
1	DCM	DiICzMes4	Ni(COD)2	60 ± 3	30 ± 1
2	Toluene	DiICzMes4	Ni(COD)2	10 ± 3	—
3	DMSO	DiICzMes4	Ni(COD)2	64 ± 5	21 ± 1
4	DMSO : toluene 1 : 1	DiICzMes4	Ni(COD)2	74 ± 2	15 ± 1
5	DMSO : toluene 1 : 1	DiICzMes4	Ni(COD)2, 3 h	54 ± 0	13 ± 0
6	DMSO : toluene 1 : 1	fac-Ir(ppy)3	Ni(COD)2	77 ± 1	15 ± 0
7	DMSO : toluene 1 : 1	fac-Ir(ppy)3	Ni(COD)2, 3 h	68 ± 2	16 ± 2
8	DMSO : toluene 1 : 1	DiICzMes4	[Ni(dtbbpy)(H2O)4]Cl2	81 ± 1	13 ± 1
9	DMSO : toluene 1 : 1	SACR-IPTZ	Ni(COD)2	99	—

---

Conclusion

We have investigated four DiICz emitters as fast and efficient DET photocatalysts in five different energy transfer reactions, highlighting their ability to activate high triplet energy substrates and outperform literature reference PCs. There is scant solvent dependency of the T₁ state energies of these four MR-TADF PCs, unlike D–A TADF PCs such as 4CzIPN. Surprisingly, we observed a dependency of the sensitivity of O₂ quenching as a competitive triplet quencher as a function of the nature of the PEnT reaction, even when the E_T of the substrate was the same. Reactions that proceed sufficiently rapidly with these PCs show little to no O₂ dependency on the final yield, which is a benefit of these compounds as photocatalysts. This suggests that the quenching of the excited state by O₂ is not competitive with the DET kinetics to the substrate. Excitingly, DET to substrates with triplet energies as high as 3.0 eV is feasible with DiICztBu₄ and DiICzMes₄, both of which catalyze the sigmatropic shift of (S)-verbenone, yielding the product in around 38% yield. We concluded that there is no connection between either the yield or the reaction rate and the ΔE_ST of the PC. This study reveals the particularly valuable and wide utility of DiCzMes₄ as a PEnT photocatalyst for substrates possessing high triplet energies.

Author contributions

E. Z.-C. conceived and managed the project and supervised the work. D. H. synthesized DiICzMes₄ and DiICztBuCz₄, E. B. synthesized DiICztBuDPA₄ and L. H. synthesized DiICztBu₄. D. H. performed the steady-state emission and absorption measurements in toluene, T. H. measured the photoluminescence quantum yields of the four photocatalysts, and the emission lifetimes of the photocatalysts in presence of cinnamyl acetate, and L. H. performed steady-state emission and absorption measurements in DCM, DMF and EtOAc, the time-resolved photoluminescence measurements in DCM, and the determination of ΔE_ST in 2-MeTHF. L. H. carried out photocatalysis reactions and photostability studies. L. H., D. H. and E. Z.-C. contributed to the manuscript writing and discussion.

Conflicts of interest

The authors declare no competing interests.

Data availability

The research data supporting this publication can be accessed at https://doi.org/10.17630/22609279-98d6-4d3c-a1c1-b1f14b04b22d. Supplementary information: synthetic procedures, UV-vis absorption and photoluminescence spectra (room temperature steady-state, 77 K steady-state, and 77 K gated emission), time-resolved PL decays, photoluminescence quantum yield data, photocatalysis procedures, and photostability studies. See DOI: https://doi.org/10.1039/d5sc04014k.

Acknowledgements

We thank the European Union H2020 research and innovation program for the funding under the Marie S. Curie Grant Agreement (PhotoReAct, No. 956324). We thank Geena Zoubarev for the contribution to the initial photocatalytic assessments. We thank Dr Megan Bryden, Francis Millward, and Violaine Manet for providing samples of 4CzIPN and fac-Ir(ppy)₃. We thank EPSRC for financial support (EP/W522259/1, EP/W007517/1 and EP/M02105X/1).

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