Han Li,Cui Wang,Felix Glaser,Narayan Sinha,Oliver S. Wenger
Metal–Organic Bichromophore Lowers the Upconversion Excitation Power Threshold and Promotes UV Photoreactions
Sensitized triplet–triplet annihilation upconversion is a promising strategy to use visible light for chemical reactions requiring the energy input of UV photons. This strategy avoids unsafe ultraviolet light sources and can mitigate photo-damage and provide access to reactions, for which filter effects hamper direct UV excitation. Here, we report a new approach to make blue-to-UV upconversion more amenable to photochemical applications. The tethering of a naphthalene unit to a cyclometalated iridium(III) complex yields a bichromophore with a high triplet energy (2.68 eV) and a naphthalene-based triplet reservoir featuring a lifetime of 72.1 μs, roughly a factor of 20 longer than the photoactive excited state of the parent iridium(III) complex. In combination with three different annihilators, consistently lower thresholds for the blue-to-UV upconversion to crossover from a quadratic into a linear excitation power dependence regime were observed with the bichromophore compared to the parent iridium(III) complex. The upconversion system composed of the bichromophore and the 2,5-diphenyloxazole annihilator is sufficiently robust under long-term blue irradiation to continuously provide a high-energy singlet-excited state that can drive chemical reactions normally requiring UV light. Both photoredox and energy transfer catalyses were feasible using this concept, including the reductive N–O bond cleavage of Weinreb amides, a C–C coupling reaction based on reductive aryl debromination, and two Paternò–Büchi [2 + 2] cycloaddition reactions. Our work seems relevant in the context of developing new strategies for driving energetically demanding photochemistry with low-energy input light.
Metal–Organic Bichromophore Lowers the Upconversion Excitation Power Threshold and Promotes UV Photoreactions

Sensitized triplet–triplet annihilation upconversion is a promising strategy to use visible light for chemical reactions requiring the energy input of UV photons. This strategy avoids unsafe ultraviolet light sources and can mitigate photo-damage and provide access to reactions, for which filter effects hamper direct UV excitation. Here, we report a new approach to make blue-to-UV upconversion more amenable to photochemical applications. The tethering of a naphthalene unit to a cyclometalated iridium(III) complex yields a bichromophore with a high triplet energy (2.68 eV) and a naphthalene-based triplet reservoir featuring a lifetime of 72.1 μs, roughly a factor of 20 longer than the photoactive excited state of the parent iridium(III) complex. In combination with three different annihilators, consistently lower thresholds for the blue-to-UV upconversion to crossover from a quadratic into a linear excitation power dependence regime were observed with the bichromophore compared to the parent iridium(III) complex. The upconversion system composed of the bichromophore and the 2,5-diphenyloxazole annihilator is sufficiently robust under long-term blue irradiation to continuously provide a high-energy singlet-excited state that can drive chemical reactions normally requiring UV light. Both photoredox and energy transfer catalyses were feasible using this concept, including the reductive N–O bond cleavage of Weinreb amides, a C–C coupling reaction based on reductive aryl debromination, and two Paternò–Büchi [2 + 2] cycloaddition reactions. Our work seems relevant in the context of developing new strategies for driving energetically demanding photochemistry with low-energy input light.

Most photoredox reactions rely on monophotonic excitation, usually implying a linear dependence of reaction rates on excitation power. Over the last decade, biphotonic processes received increasing attention in photoredox catalysis. Biphotonic reactions typically rely on the consecutive absorption of two photons per catalytic turnover and therefore feature a quadratic dependence on excitation power, but at the so-called excitation power density threshold (Ith), they can crossover into a linear power dependence regime. The development of new concepts to lower this threshold seems desirable for making biphotonic processes more amenable to photoredox catalysis. Many different types of biphotonic processes have been uncovered, among which sensitized triplet–triplet annihilation upconversion (sTTA-UC) is particularly promising. While upconversion has been intensively investigated from a photophysical perspective for applications in lighting and solar energy harvesting, its use in photoredox catalysis is a relatively recent development. Several non-catalytic photochemical reactions driven by upconversion have furthermore drawn interest. Blue-to-UV upconversion seems particularly attractive for photochemical applications because UV excitation sources are less available, typically less powerful, and less safe to handle than visible light-emitting diodes. UV irradiation can lead to photo-damage resulting from direct absorption by substrates rather than photocatalysts, and sufficiently deep penetration into reaction vessels can be hampered. Consequently, blue-to-UV upconversion is currently receiving substantial attention, though applications in photoredox catalysis have yet remained scarce. For photochemical applications, the upconversion system should be robust and operate with sufficient efficiency, for which there are several figures of merit. Besides the maximum upconversion efficiency reachable under high irradiance, the excitation power density threshold (Ith), at which the upconversion changes from the initial quadratic power dependence to a linear regime, is of key interest for lowering the needed irradiation power. A long triplet excited-state lifetime of the sensitizer is beneficial to lower the excitation power density threshold for upconversion. To elongate the respective lifetimes in metal-based sensitizers, organic chromophores with energetically close-by triplet states have been covalently attached to many different metal complexes, often resulting in reversible triplet–triplet energy transfer. This so-called “triplet reservoir effect” is a well-known strategy for obtaining very long-lived emissive triplet states, and in a handful of prior studies, such metal–organic bichromophores have already been successfully used for upconversion. However, we are unaware of prior studies that have investigated systematically their impact on the excitation power density threshold with different annihilators, and so far, blue-to-UV upconversion has not been investigated with metal–organic bichromophores. Pyrene is a well-suited triplet reservoir for RuII polypyridine complexes with triplet energies around 2.1 eV, but when aiming at blue-to-UV upconversion, a higher triplet energy is advantageous. Cyclometalated IrIII complexes such as Ir ref (Figure 1a) fulfill this requirement with triplet energies near 2.7 eV, but this calls for a different triplet reservoir. Naphthalene (Naph) was identified as an energetically matching partner to obtain a substantially longer-lived triplet state in the bichromophore of Figure 1b (Ir Naph). At the same time, all benefits offered by the IrIII complex remain exploitable, in particular, its absorption tailing into the blue spectral range, high photostability, and high efficiency for intersystem crossing. In combination with the three annihilators shown in Figure 1c, the Ir Naph bichromophore consistently gives lower excitation power density thresholds (Ith) than the Ir ref compound. This finding supports our hypothesis that bichromophores and the triplet reservoir effect can be beneficial for lowering Ith and represents one key novelty of our study. Ir Naph sensitizes the lowest triplet states of the three annihilators shown in Figure 1c with energies between 2.40 and 2.62 eV (lower half of Figure 1d), resulting in fluorescent singlet-excited states with energies between 3.68 and 3.96 eV after upconversion. The combination of Ir Naph with PPO emerged as the optimal sensitizer/annihilator couple for photochemical investigations. The singlet-excited state of PPO formed after upconversion engaged in both electron and energy transfer processes in three fundamentally different types of chemical reactions (upper half of Figure 1d). The Paternò–Büchi reaction between two benzaldehydes and furan usually relies on 365 nm (3.40 eV) irradiation, whereas here it was accomplished with 447 nm light (2.77 eV), owing to energy transfer from the upconverted PPO state at 3.68 eV. N–O bond cleavage reactions of three Weinreb amides, normally requiring UV activation, and one thermodynamically demanding C–C coupling reaction were driven by blue light, as a result of the strongly reducing character of the formed PPO singlet-excited state. The proof-of-concept that several different chemical reaction types, some relying on energy transfer and others functioning on the basis photoinduced electron transfer, become accessible after blue-to-UV upconversion represents another key novelty of our study. In combination with the bichromophore strategy to lower excitation power density thresholds, this represents a significant conceptual advance of upconversion-driven UV photocatalysis.

Ir ref and the bichromophore Ir Naph were synthesized and characterized as described in the Supporting Information (Sections 2.3 and 2.4). The UV–vis absorption spectra of Ir ref and Ir Naph (Figure 2a,b) are very similar to one another and differ mainly by an additional band at 224 nm in Ir Naph, which is attributable to π–π* transitions on the naphthalene unit. At the excitation wavelength used for the upconversion experiments (447 nm), both compounds have a molar extinction coefficient of ∼350 M–1 cm–1. From the luminescence spectra at 77 K, an energy of 2.68 eV is estimated for the emissive triplet excited state located on the IrIII complex. The photoluminescence quantum yields in deaerated CH3CN at 293 K are roughly 80% for both Ir ref and Ir Naph (Supporting Information Section 7). Evidently, the presence of the naphthalene moiety does not negatively affect the luminescence behavior, likely because the lowest triplet excited state of naphthalene decays 2–3 orders of magnitude more slowly than the emissive excited state of the IrIII complex. The luminescent excited states of cyclometalated IrIII complexes of this type commonly have mixed metal-to-ligand charge transfer (MLCT) and intraligand character, but for simplicity, we use only the MLCT label shown in Figure 2f and throughout the entire article. The luminescence of Ir ref decays mono-exponentially with a lifetime of 3.1 μs in deaerated CH3CN at 293 K, whereas Ir Naph shows a bi-exponential luminescence decay with 3.1 μs (14%) and 18.5 μs (86%) under identical conditions. In addition to the spectral features seen for Ir ref, the transient absorption spectrum of Ir Naph contains the spectral signature of the lowest triplet-excited state of naphthalene (narrow bands at 419 and 397 nm in the inset of Figure 2d). This confirms that the anticipated intramolecular triplet–triplet energy transfer (iTTET) from the initially excited iridium(III) complex to the appended naphthalene unit occurs (Figure 2f). The transient absorption signal at 418 nm decays in a tri-exponential fashion, comprising the abovementioned decay components of 3.1 and 18.5 μs seen already in the luminescence decay of Ir Naph (Figure S5b), along with a third decay component featuring a lifetime of 72.1 μs. The latter is attributable to the inherent decay of triplet-excited naphthalene, for which lifetimes around 100 μs and longer have been reported, whereas the 3.1 μs decay component is evidently due to the emissive excited state of the iridium(III) complex. The remaining 18.5 μs decay component is attributed to repopulation of the emissive state on the IrIII complex from the dark 3π–π* state of the naphthalene unit, and it reflects both the kinetics of the inherent 3π–π* decay and the kinetics of reverse iTTET. Analogous results have been obtained previously for PtII-anthracene and RuII-perylene bichromophores, and the similarity of their multi-exponential luminescence decay behavior to that observed for thermally activated delayed fluorescence has been noted. The reaction rate for iTTET can be determined by transient absorption experiments with sub-nanosecond time resolution (Figure 2e). Following pulsed excitation of Ir Naph at 375 nm, the spectrum recorded with a time delay of 1 ps (red trace in Figure 2e) is rather nondescript but features an absorption maximum near 410 nm, attributable to the initially excited IrIII complex. At this point, population of the naphthalene 3π–π* excited state is minimal. After 20 ps (pink trace), the growth of a signal at 419 nm becomes apparent, reaching its maximum intensity after 3000 ps (turquoise trace). Together with the vibrational sideband at 397 nm, the absorption band at 419 nm represents the diagnostic spectral signature of the naphthalene-centered 3π–π* excited state, as noted above (inset of Figure 2d). From the kinetic trace at 419 nm (inset of Figures 2e and S7), time constants of 198 and 935 ps for iTTET were extracted. The bi-exponential nature of this kinetic trace is tentatively attributed to the presence of different conformers. Temperature-dependent luminescence experiments with Ir Naph demonstrate that reverse iTTET becomes increasingly important as the temperature rises (Figure S6 and Table S1), and from an Arrhenius-type analysis, it follows that the energy gap between the emissive excited state of the IrIII complex and the naphthalene 3π–π* state is roughly 0.1 eV. This finding is in line with the experimentally determined energy of the emissive excited state (2.68 eV) of Ir Naph and the known triplet energy (2.62 eV) of naphthalene.

All three annihilators used in this work (Figure 1c) are known compounds with triplet energies between 2.40 and 2.62 eV (Table 1) and energies of the fluorescent singlet-excited states ranging from 3.68 to 3.96 eV. Consequently, bimolecular TTET from the emissive excited state of both Ir ref and Ir Naph (at 2.68 eV, Figure 2f) to all three annihilators is expected to be exergonic, but for 1-MN and TP, the driving force is small. All three annihilators fluoresce in the UV range with high quantum yields (ϕ), thus fulfilling a key prerequisite for blue-to-UV upconversion.

Transient absorption spectroscopy provides clear evidence for bimolecular TTET from Ir Naph to PPO (Figure 3a). A solution containing only Ir Naph (red trace) yields the same transient absorption spectrum as discussed above (inset of Figure 2d), with spectral features of the emissive IrIII excited state and the naphthalene-localized 3π–π* state. In the presence of excess PPO, these spectral signatures remain observable only at very short delay time (blue trace in Figure 3a), whereas at long delay times (orange trace), the diagnostic spectrum of triplet-excited 2,5-diphenyloxazole (3*PPO) is detected. Analogous experiments with 1-MN and TP (Figures S18a and S20a) provide equally clear evidence for bimolecular TTET. Luminescence quenching experiments and Stern–Volmer analyses were used to determine the rate constants for bimolecular TTET (kTTET) from Ir Naph or Ir ref to the three annihilators. Exemplary data are shown for the Ir Naph/PPO couple in Figure 3b; analogous data sets for all other sensitizer/annihilator combinations are in the Supporting Information (Section 5). Expectedly, the highest kTTET values are obtained with PPO (Table 2) because the driving force for TTET is largest in this case, roughly 0.28 eV, compared to only 0.09 eV for 1-MN and 0.06 eV for TP based on Figure 2f and Table 1. Steady-state luminescence quenching experiments provide similar kTTET values as the time-resolved luminescence studies (Table S3). The apparent rate constants for TTET determined from luminescence quenching experiments are consistently lower for Ir Naph than for Ir ref, and this difference becomes more obvious as the reaction free energy for TTET (ΔGTTET) gets less negative from PPO to 1-MN and TP (Table 2). This is likely attributable to the population of the naphthalene-based 3π–π* excited state at 2.62 eV (Figure 2f) in Ir Naph, from which TTET to the annihilator is less exergonic than from the emissive IrIII-localized MLCT state at 2.68 eV (second series of ΔGTTET values in Table 2). The 3MLCT and 3π–π* states of the bichromophore are in equilibrium with each other as discussed above, hence the luminescence quenching experiments monitor their joint depopulation. In principle, transient absorption spectroscopy could be used to probe bimolecular TTET from the naphthalene subunit to the annihilators, but such experiments are hampered by the fact that the absorptions of the respective triplet donors and acceptors overlap spectrally. The efficiency for TTET (ϕTTET) in the presence of excess annihilator, as relevant under upconversion conditions, can be pushed to nearly 100% in all cases (Table 2). In the presence of 1 mM PPO, time-resolved luminescence quenching experiments indicate that 94% of the emissive excited state of Ir ref are quenched by TTET, whereas for Ir Naph, quenching by TTET is 99% efficient under identical conditions. Despite similar kTTET values in these two specific cases, a higher TTET efficiency is reached for Ir Naph owing to the substantially longer lifetimes of the triplet reservoir (18.5 and 72.1 μs) in comparison to the luminescent 3MLCT state of the IrIII complex (3.1 μs). With 1-MN and TP, higher concentrations (5 mM instead of 1 mM) are needed to reach similar quenching efficiencies due to the lower driving forces for TTET.

Upon selective excitation of Ir Naph at 447 nm in the presence of excess PPO, upconversion luminescence from the singlet-excited state of the annihilator and prompt MLCT emission from the sensitizer are both observable (Figure 4a). The maximum of the upconverted fluorescence is at 360 nm, corresponding to a pseudo anti-Stokes shift of 0.67 eV with respect to the excitation wavelength (447 nm). The delayed nature of the upconversion fluorescence manifests readily in a pulsed experiment (inset of Figure 4a). The decay of this delayed fluorescence is bi-exponential with time constants of 14.6 μs (65%) and 57.1 μs (35%) for 30 μM Ir Naph and 1 mM PPO in CH3CN at 293 K (Figure S16d). These time constants are roughly a factor of two shorter than those extracted from the decay of the lowest triplet excited state of the PPO annihilator [29.9 μs (53%) and 136 μs (47%)] under identical conditions. This approximate factor-of-two discrepancy between upconversion luminescence decays and annihilator triplet excited-state decays has been previously found by other investigators in different systems and is encountered here for all six sensitizer/annihilator couples (Figures S16–S21).

The upconverted annihilator fluorescence and the prompt sensitizer emission were recorded as a function of laser excitation power between 6.7 and 433.5 mW (Figure 4b,c), which corresponds to power densities ranging from roughly 0.2 to 16.0 W·cm–2 given the beam size of the employed laser (Supporting Information Section 1). For the Ir Naph/PPO couple, the integrated upconversion luminescence intensity depends quadratically on the excitation power density (red trace in Figure 4d) up to a value of 0.71 ± 0.05 W·cm–2, manifesting in a slope of 2.04 in the respective double-logarithmic representation. Beyond this threshold value (Ith, vertical red dotted line in Figure 4d), the slope decreases to 1.12, indicating that the strong annihilation limit is reached, where the upconversion luminescence intensity depends linearly on excitation power density. The prompt sensitizer MLCT emission intensity follows the expectable linear dependence over the full range of excitation power densities used (red trace in Figure 4e). When performing the same experiment with Ir ref instead of Ir Naph as the sensitizer (black-colored data sets in Figure 4d,e; raw data in Figure S22), the excitation power density threshold (Ith) is reached at 1.4 ± 0.1 W·cm–2, roughly double the value determined for the bichromophore sensitizer. When decreasing the PPO annihilator concentration from 1 to 0.2 mM while keeping the sensitizer concentration constant at 30 μM (Figure S23), the excitation power density thresholds increase because the concentration of triplet-excited annihilators becomes lower. However, the Ith value obtained with Ir Naph (1.6 ± 0.2 W·cm–2) remains substantially below that for Ir ref (2.2 ± 0.2 W·cm–2). When using 1-MN or TP instead of PPO as annihilators (Figures S24 and S25), the Ir Naph bichromophore also gives substantially lower power density thresholds than Ir ref (Table 3). The finding that the metal–organic bichromophore with its very long-lived triplet reservoir gives consistently lower Ith values than an ordinary sensitizer (Table 3) is a key result of this study. A quantitative analysis of the excitation power density threshold dependence on exact sensitizer/annihilator properties, as discussed previously, is hampered by the overall complexity of our upconversion systems (Supporting Information Section 9), particularly with 1-MN and TP, for which bimolecular reverse TTET from the annihilators to the sensitizer complicates the situation (see below and Figure S27). However, in the case of the PPO annihilator, the physical origin of the lower Ith values for Ir Naph in comparison to that for Ir ref can be rationalized by the considerably longer excited-state lifetime of the sensitizer (Figure 2d). The TTET rate constants are similar for both sensitizers (Table 2), but at any given annihilator concentration, the τ0/τ and I0/I ratios extracted from time-resolved and steady-state luminescence quenching experiments are a factor of 3.3–4.4 higher for Ir Naph than for Ir ref (Figures S11–S13). This indicates that considerably lower concentrations of PPO are needed in combination with Ir Naph to reach the same amount of quenching as with Ir ref, and this in turn is in line with a lower Ith value for the bichromophore. The excitation power density thresholds determined for our sensitizer/annihilator couples (Table 3) compare reasonably well to previously reported Ith values for other blue-to-UV upconversion systems (Table S5) and lie somewhere in the middle between the best and worst performers. The main point here is not to establish new record Ith values, but to demonstrate that bichromophores, which benefit from a triplet reservoir effect, are useful to lower the excitation power density thresholds. This proof-of-principle is now made, and we are unaware of previous reports that have explored and successfully demonstrated the usefulness of this specific concept. Optimizing Ith is of significant interest, for example, in the context of solar energy conversion, but also to use LEDs instead of lasers in photocatalysis. Aside from the excitation power threshold, the upconversion efficiency (ϕsTTA-UC) is a key figure of merit. To determine ϕsTTA-UC for our systems, the ratios of upconversion luminescence intensity and the prompt sensitizer emission intensity of a reference solution containing no annihilator were measured as a function of the excitation power density (Figures 4b,c and S22–S25), to provide data sets such as those in Figure 4f (see Supporting Information Section 7 for details). For the Ir Naph/PPO couple, ϕsTTA-UC is roughly 70% higher than for the Ir ref/PPO combination (Table 3), suggesting that the bichromophore concept and the triplet reservoir effect might be generally helpful to boost ϕsTTA-UC in addition to Ith. However, with the other two investigated annihilators (1-MN and TP), the upconversion efficiencies are lower with Ir Naph than with Ir ref (Table 3), which we attribute to reverse TTET from the annihilator to the sensitizer. Several observations support this conclusion, as discussed briefly in the following. First, the driving-forces for TTET to these annihilators are very small (ΔGTTET in Table 2), particularly for Ir Naph, hence reverse TTET is thermodynamically viable. Time-gated luminescence experiments provide direct evidence for reverse TTET (Figures S18–S21) in the form of delayed sensitizer MLCT luminescence. Bimolecular reverse TTET is likely responsible for the finding that the upconversion excitation power dependence deviates substantially from quadratic behavior already in the low-power regime, yielding slopes closer to 1.8 instead of 2.0 (Figures S24e/S25e). Given the presence of the naphthalene-based 3π–π* state at 2.62 eV in Ir Naph (Figure 2f), reverse TTET from the annihilator to this excited state is more facile than to the 0.06 eV higher-lying 3MLCT state of the IrIII complex. On this basis, the lower upconversion efficiencies obtained for Ir Naph in comparison to Ir ref when using 1-MN and TP as annihilators (Table 3) can be rationalized. With TP, an unusual excitation power dependence of the upconversion efficiency (ϕsTTA-UC) is obtained (Figure S25g), likely as a result of particularly prominent reverse TTET in this case. The maximally achievable upconversion quantum yield of the Ir Naph/PPO couple compares favorably to many previously reported blue-to-UV upconversion systems (Table S5), for which ϕsTTA-UC has often been in the range of 1–5% until now, though improved performance became possible recently in several cases. The Ir Naph/PPO couple emerged as the best upconversion system from our study both regarding ϕsTTA-UC and Ith, and furthermore, this combination turned out to be remarkably photostable (Figure S10). Over 6 h of irradiation at 447 nm with a 451 mW cw-laser, only a 25% decrease of the upconversion luminescence intensity was observed, making the Ir Naph/PPO couple promising for applications in UV photocatalysis. The inherent photostability in neat solution is often lower than under catalytic conditions, particularly when long-lived triplet excited states are involved. Strongly quenched systems avoid deleterious side reactions, and when using elevated sensitizer/annihilator concentrations, photo-irradiation periods longer than 6 h are expected to become unproblematic for our upconversion system (Supporting Information page S95).

Storing roughly 3.68 eV of excitation energy (Table 1), the fluorescent singlet-excited state of 2,5-diphenyloxazole (1*PPO) is a fairly strong reducing agent with an oxidation potential of −2.2 V vs SCE (Supporting Information Section 10). Consequently, it seemed attractive to explore whether 1*PPO, formed via upconversion of visible light, can catalyze chemical reductions that previously required UV irradiation, such as the recently reported reductive N–O bond cleavage reactions of Weinreb amides (Figure 5a). The reduction potential of these substrates is tunable through variation of the substituent R in para-position to the protected amide group (Table S7). For the three specific examples investigated here, the obtainable product yields correlated with the substrate reduction potential (Figure 5a, Table S7), suggesting that the initial photoinduced electron transfer step from 1*PPO to the substrate is indeed a performance-limiting factor. Following this initial elementary process enabled by upconversion, the deprotection of the Weinreb amide is expected to proceed along the same reaction pathway as postulated for direct UV excitation (Scheme S1). Substrate 1 is converted to product 1-P in 71% yield under 447 nm irradiation of 1 mol % Ir Naph as the sensitizer and 10 mol % PPO as the annihilator. Control experiments confirm that both upconversion components (sensitizer, annihilator) and light are indispensable for this reaction (Table S8 and Figures S30–S36), and only a minor background reactivity (17% yield of 1-P) is observed when the annihilator is missing (Supporting Information Section 12.1). Substrates 2 and 3 have slightly more negative one-electron reduction potentials than substrate 1 (Table S7), and the obtainable N–O bond cleavage yields are markedly lower (40% for 2-P, 20% for 3-P; Supporting Information Sections 12.2 and 12.3). For substrates 1 and 2, Ir Naph and Ir ref are similarly good sensitizers under the conditions used for these experiments, whereas for substrate 3, Ir ref performs substantially more poorly (<6% yield, Table S10). For the reductive debromination of substrate 4 (Figure 5b) followed by interception of the formed aryl radical by N-methylpyrrole (Supporting Information Section 12.4), the difference in product yield (4-P) obtained with Ir Naph and Ir ref is substantial (64% vs 15%, Table S11). In a control experiment performed in the absence of PPO, 21% yield of the C–C coupling product 5-P is obtained, and Stern–Volmer experiments indicate that substrate 4 quenches the Ir Naph emission with a rate constant of 3 × 106 M–1 s–1 (Figure S50), which is more than three orders of magnitude slower than TTET from Ir Naph to PPO (Table 2). Thus, it seems clear that the dominant reaction pathway in the complete upconversion system proceeds via TTET from Ir Naph to PPO, followed by upconversion to 1*PPO and electron transfer from that upconverted state to the substrate (Supporting Information Section 12.4). Aside from reacting via photoinduced electron transfer, 1*PPO can furthermore be expected to serve as an energy donor, which could open new perspectives for energy transfer catalysis. Photoredox catalysis via upconversion is an emerging field, whereas energy transfer catalysis via upconversion is yet very uncommon. Two Paternò–Büchi model reactions (Figure 5c) were chosen to explore the concept of converting a lower energy triplet-excited state into a higher energy triplet-excited state. The [2 + 2] cycloaddition reaction of benzaldehydes with furan usually requires UV irradiation at 365 nm (3.40 eV) because the energy of the reactive triplet-excited state of benzaldehydes is ca. 3.1 eV and light absorption commonly occurs via excitation of substantially higher-lying singlet-excited states. Here, we aimed to populate the reactive triplet-excited state of two different benzaldehyde molecules via an upconversion process, using an input energy of 2.77 eV (447 nm) instead of 3.40 eV, anticipating that the spectral overlap between the (delayed) 1*PPO fluorescence and the benzaldehyde absorption (Figure S29) would enable energy transfer from the upconversion system to the substrate (Figure 6). The respective spectral overlap between donor emission and acceptor absorption is markedly better with substrate 6 than with substrate 5. Consequently, it seems plausible that the higher yield of cycloaddition product 6-P (44%, 49% conversion) in comparison to 5-P (18%; 23% conversion) is largely attributable to this better spectral overlap and a higher molar extinction of 6-P in the relevant spectral range (Figure S29). Control experiments performed for the two reactions shown in Figure 5c confirm that both upconversion components (sensitizer and annihilator) must be present and that blue light drives the UV photoreactions via upconversion (Tables S12 and S13). Ir Naph outperforms Ir ref as the sensitizer in both cases (Tables S12 and S13). Both [2 + 2] cycloaddition products are obtained predominantly in their exo form, as expected (right part of Figure 6). The key energy transfer step between 1*PPO and the benzaldehyde substrates can occur both via radiative energy transfer and FRET. In a recent study, an upconverting sensitizer-annihilator couple system for red-to-blue upconversion was spatially separated from a triplet photoreaction system, in which case radiative energy transfer involving delayed fluorescence from 9,10-diphenylanthracence and subsequent re-absorption of the upconverted light by a RuII complex occurred. Similar two-compartment setups have been used previously for upconversion. Here, we employed a simple one-pot reaction setup with all components present in the same solution, in which case FRET is possible as an important energy transfer mechanism between the upconversion system and the Paternò–Büchi photoreaction system. One possible pitfall in this strategy is reverse TTET from the photoreactive triplet-excited state of the benzaldehyde (3*ArCHO in Figure 6) to the annihilator (to form 3*PPO) or the sensitizer (to result in 3*Ir Naph). Using an excess of furan (Figure 5c), it has been possible to counteract this energy-wasting reaction pathway, such that a bimolecular C–C bond forming reaction has become possible, whereas previous related studies dealt mostly with simpler photo-isomerization or photo–fragmentation reactions. Paternò–Büchi and related cycloaddition reactions are regaining interest in current research, and our proof-of-concept [2 + 2] cycloadditions shown in Figure 5c demonstrate that blue-to-UV upconversion can enable these reactions with visible light. Metal–organic bichromophores as sensitizers are useful for this purpose. From a purely practical perspective, direct UV irradiation remains the most straightforward approach for the reactions shown in Figure 5. Advantages of the upconversion approach could result in systems, in which filter and photodegradation effects are prevalent, as discussed in the Introduction. These advantages will likely become more significant as increasingly shorter-wavelength UV radiation becomes accessible via upconversion.

Three key discoveries emerge from this study: (1) metal–organic bichromophores can lower the excitation power density threshold for upconversion due to the triplet reservoir effect and substantially elongated excited-state lifetimes, enabling a linear power dependency at decreased excitation densities compared to ordinary sensitizers. (2) This concept enables blue-to-UV upconversion and subsequent exploitation of the upconverted light in photoredox and energy transfer catalysis, whereby not only photocleavage but also constructive C–C bond forming reactions are achievable. (3) Upconversion of lower energy triplets into higher energy triplet excited states can enable classical UV-dependent photoreactions of organic molecules, such as the Paternò–Büchi [2 + 2] cycloaddition. The bichromophore strategy requires a fine energy-tuning between the triplet excited state of the emissive metal complex and the covalently attached triplet reservoir, in order for intramolecular TTET to be reversible. A cyclometalated heteroleptic IrIII complex and naphthalene are well-suited to create a long-lived triplet excited state in the deep blue region, roughly 0.6 eV higher in energy than most of the previously explored RuII-pyrene bichromophores, which is essential for sensitization of blue-to-UV upconversion. While bimolecular TTET from the bichromophore to the annihilator is ideally associated with minimal energy loss in order to upconvert as deeply into the UV as possible, a minimum driving force of 0.2 eV seems necessary, otherwise the overall upconversion efficiency is compromised. Two of the three investigated annihilators did not fulfill this requirement, and reverse TTET from these annihilators to the naphthalene triplet reservoir was identified as a detrimental process. The visible light-harvesting ability of the bichromophore is relatively modest, as often the case for cyclometalated iridium(III) complexes. Sensitized triplet–triplet annihilation creates higher-energy singlet-excited states, which can act as FRET donors. When using carbonyl compounds as FRET acceptors, those carbonyls can undergo efficient intersystem crossing to reactive triplet-excited states that lie at substantially higher energies than those of the initially excited sensitizer. Our work shows that energy-wasting TTET from these high-energy triplet-excited states back to the sensitizer-annihilator couple can be counteracted to the extent that bimolecular C–C bond-forming reactions become viable. This opens the door to classical carbonyl compound UV photochemistry with visible light, beyond simple Norrish type I and type II photofragmentations, but furthermore including diffusion-controlled bimolecular chemistry with substrates having high triplet energies. As traditional UV photochemistry of organic compounds is currently regaining interest in synthetically oriented research, we hope that our study can contribute to developing new conceptual approaches to high-energy photochemistry.