Till J. B. Zähringer

Sensitized triplet–triplet annihilation systems enable the efficient conversion of two low-energy photons from a low-intensity, noncoherent light source into one high-energy photon, opening avenues for diverse applications. The attachment of triisopropylsilylethynyl (TIPS-ethynyl) groups to aromatic compounds has led to the development of many annihilators with high upconversion quantum yields. Here, we synthesized a series of novel symmetrical biphenyls bearing silylethynyl substituents of varying sizes and evaluated them as annihilators in visible-to-UV upconversion systems. While all of these systems do not suffer from excimer formation issues, their upconversion performances differ significantly. Small substituents like trimethylsilylethynyl give similar upconversion quantum yields (up to ∼12%) as the TIPS-ethynyl groups, but the larger triphenylsilylethynyl substituents reduce the achievable quantum yields by half, which is likely due to additional nonradiative loss channels arising from altered energies of higher excited triplet states. The UV emission of the novel annihilators is hypsochromically shifted by about 20 nm relative to that of the TIPS-naphthalene benchmark UV annihilator, thereby reaching a highly attractive spectral region for bond activation photochemistry. Using the most efficient annihilator, bTIPS-BP, we achieved blue-to-UV upconversion-driven release of fluorescein from a photocage. In the greater context of photon upconversion and in view of other recent reports on substituent effects, our results indicate that several chromophore-specific effects must be understood for obtaining optimized systems.

Sensitized triplet–triplet annihilation systems enable the efficient conversion of two low-energy photons from a low-intensity, non-coherent light source into one high-energy photon, opening avenues for diverse applications. The attachment of triisopropylsilylethynyl (TIPS-ethynyl) groups to aromatic compounds has led to the development of many annihilators with high upconversion quantum yields. Here, we synthesized a series novel of symmetrical biphenyls bearing silylethynyl substituents of varying sizes and evaluated them as annihilators in visible-to-UV upconversion systems. While all of these systems do not suffer from excimer formation issues, their upconversion performances differ significantly. Small substituents like trimethylsilylethynyl gave similar upconversion quantum yields (up to ~ 12 %) as the TIPS-ethynyl groups, but the larger triphenylsilylethynyl substituents reduce the achievable quantum yields by half, which is likely due to additional non-radiative loss channels arising from altered energies of higher excited triplet states. The UV emission of the novel annihilators is hypsochromically shifted by about 20 nm relative to the TIPS-naphthalene benchmark UV annihilator, thereby reaching a highly attractive spectral region for bond activation photochemistry. Using the most efficient annihilator, bTIPS-BP, we achieved blue-to-UV upconversion-driven release of fluorescein from a photocage. In the greater context of photon upconversion and in view of other recent reports on substituent effects, our results indicate that several chromophore-specific effects have to be understood for obtaining optimized systems.

A benzene-based annihilator and a high triplet energy sensitizer were combined in an upconversion scheme, emitting photons with unprecedented energy.

Investigating the migration of excited-state energy in DNA is crucial for a deep understanding of protection mechanisms and light-induced DNA damage. While numerous reports focused on single electron transfer and Förster-type energy transfer in DNA, studies on the Dexter-type triplet-triplet energy transfer are scarce, in particular those with direct detection of photoexcited triplet states. Herein, we present direct measurements of the distance-dependent triplet-triplet energy transfer rates through DNA by using transient absorption spectroscopy. This was achieved through the synthetic incorporation of thioxanthone as energy donor and naphthalene as energy acceptor into a DNA double strand at defined positions. The energy transfer rates strongly depend on the number of A-T base pairs (up to four) separating the energy donor from the energy acceptor. We observed a fast energy transfer rate with a time constant of 17 ns for the DNA sample in which the donor and acceptor are directly adjacent in the DNA. By analyzing two additional donor-acceptor distances, a steep exponential distance dependence with an attenuation factor of 1.15 Å^−1 could be obtained. Our results demonstrate that DNA acts as a poor conductor of triplet energy when energy donors with triplet energies below 2.7 eV are used, complementing more indirect studies on sensitized DNA damage.

Norbornadiene‐based photoswitches have emerged as promising candidates for harnessing and storing solar energy, holding great promise as a viable solution to meet the growing energy demands. Despite their potential, the effectiveness of their direct photochemical conversion into the resulting quadricyclanes has room for improvement owing to (i) moderate quantum yields, (ii) poor overlap with the solar spectrum and (iii) photochemical back reactions. Herein, we present an approach to enhance the performance of such molecular solar thermal energy storage (MOST) systems through the triplet‐sensitized conversion of aryl‐substituted norbornadienes. Our study combines deep spectroscopic analyses, irradiation experiments, and quantum mechanical calculations to elucidate the energy transfer mechanism and inherent advantages of the resulting MOST systems. We demonstrate remarkable quantum yields using readily available sensitizers under both LED and solar light irradiation, significantly surpassing those achieved through direct excitation with photons of higher energy. In contrast to the conventional approach, light‐induced back reactions of the high‐energy products do not play any role, allowing quantitative switching within minutes. These results not only underscore the potential of triplet‐sensitized MOST systems to leverage the high energy storage capabilities of multistate photoswitches but they might also stimulate the broader usage of sensitization strategies in photochemical energy conversion.

Norbornadiene‐based photoswitches have emerged as promising candidates for harnessing and storing solar energy, holding great promise as a viable solution to meet the growing energy demands. Despite their potential, the effectiveness of their direct photochemical conversion into the resulting quadricyclanes has room for improvement owing to (i) moderate quantum yields, (ii) poor overlap with the solar spectrum and (iii) photochemical back reactions. Herein, we present an approach to enhance the performance of such molecular solar thermal energy storage (MOST) systems through the triplet‐sensitized conversion of aryl‐substituted norbornadienes. Our study combines deep spectroscopic analyses, irradiation experiments, and quantum mechanical calculations to elucidate the energy transfer mechanism and inherent advantages of the resulting MOST systems. We demonstrate remarkable quantum yields using readily available sensitizers under both LED and solar light irradiation, significantly surpassing those achieved through direct excitation with photons of higher energy. In contrast to the conventional approach, light‐induced back reactions of the high‐energy products do not play any role, allowing quantitative switching within minutes. These results not only underscore the potential of triplet‐sensitized MOST systems to leverage the high energy storage capabilities of multistate photoswitches but they might also stimulate the broader usage of sensitization strategies in photochemical energy conversion.

Sigma bond cleavage through electronically excited states allows synthetically useful transformations with two radical species. Direct excitation of simple aryl halides to form both aryl and halogen radicals necessitates UV-C light, so undesired side reactions are often observed and specific equipment is required. Moreover, only aryl halides with extended π systems and comparatively low triplet energy are applicable to synthetically useful energy transfer catalysis. Here we show the conceptual advantages of arylthianthrenium salts (ArTTs) for energy transfer catalysis with high energy efficiency compared to conventional aryl (pseudo)halides and their utility in arylation reactions of ethylene. The fundamental advance is enabled by the low triplet energy of ArTTs that may originate in large part from the electronic interplay between the distinct sulfur atoms in the tricyclic thianthrene scaffold, which is not accessible in either simple (pseudo)halides or other conventional sulfonium salts.

Sigma bond cleavage through electronically excited states allows synthetically useful transformations with two radical species. Direct excitation of simple aryl halides to form both aryl and halogen radicals necessitates UV-C light, so undesired side reactions are often observed, and specific equipment is required. Moreover, only aryl halides with extended pi systems and comparatively low triplet energy are applicable to synthetically useful energy transfer catalysis with visible light. Here we show the conceptual advantages of arylthianthrenium salts (ArTTs) for energy transfer catalysis with visible light in high quantum yield as compared to conventional aryl(pseudo)halides, and their utility in arylation reactions of ethylene. The fundamental advance is enabled by the low triplet energy of ArTTs that may originate in large part from the electronic interplay between the distinct sulfur atoms in the tricyclic thianthrene scaffold, which is neither accessible in simple (pseudo)halides nor other conventional sulfonium salts.

Organic photocatalysts frequently possess dual singlet and triplet photoreactivity and a thorough photochemical characterization is essential for efficient light-driven applications. In this article, the mode of action of a polyazahelicene catalyst (Aza-H) was investigated using laser flash photolysis (LFP). The study revealed that the chromophore can function as a singlet-state photoredox catalyst in the sulfonylation/arylation of styrenes and as a triplet sensitizer in energy transfer catalysis. The singlet lifetime is sufficiently long to exploit the exceptional excited state reduction potential for the activation of 4-cyanopyridine. Photoinduced electron transfer generating the radical cation was directly observed confirming the previously proposed mechanism of a three-component reaction. Several steps of the photoredox cycle were investigated separately, providing deep insights into the complex mechanism. The triplet-excited Aza-H, which was studied with quantitative LFP, is formed with a quantum yield of 0.34. The pronounced triplet formation was exploited for the isomerization reaction of (E)-stilbene to the Z-isomer and the cyclization of cinnamyl chloride. Catalyst degradation mainly occurs through the long-lived Aza-H triplet (28 µs), but the photostability is greatly increased when the triplet efficiently reacts in a catalytic cycle such that turnover numbers exceeding 4400 are achievable with this organocatalyst.

A new green-to-blue TTA system based on an asymmetric BODIPY and a perylene derived emitter was characterised by spectroscopic techniques. This couple has provided efficient photocatalytic activity in three different coupling reactions.