Maria‐Sophie Bertrams

Triplet–triplet annihilation‐based photon upconversion (TTA‐UC) expands the accessible spectral window for photochemical transformations by generating high‐energy photons from low‐energy excitation. Although TTA‐UC has been widely demonstrated in solution and in various solid‐state formats, examples of polymer materials capable of efficient visible‐to‐ultraviolet (vis‐to‐UV) upconversion under low excitation intensities remain scarce. Achieving vis‐to‐UV TTA‐UC in a porous solid is particularly challenging because it requires balancing chromophore proximity for TTA with the suppression of aggregation‐induced quenching, while simultaneously preserving substrate accessibility. Here, we introduce porous polymer monoliths that enable solid‐state TTA‐UC specifically designed for heterogeneous photochemistry. By embedding sensitizer‐annihilator dye pairs into polymer matrices with controlled co‐continuous porosity, we create monolithic architectures that display upconverted UV emission from blue light at excitation intensities below a few mW cm ^{− 2} , with the operational wavelength further extended to green light excitation. The porous architecture enhances energy transfer to reaction substrates, providing a versatile platform for heterogeneous photochemical processes. Owing to their robustness and tunability, these monoliths are promising candidates as heterogeneous photoreactors, opening new opportunities for scalable and sustainable photochemistry.

Triplet–triplet annihilation‐based photon upconversion (TTA‐UC) expands the accessible spectral window for photochemical transformations by generating high‐energy photons from low‐energy excitation. Although TTA‐UC has been widely demonstrated in solution and in various solid‐state formats, examples of polymer materials capable of efficient visible‐to‐ultraviolet (vis‐to‐UV) upconversion under low excitation intensities remain scarce. Achieving vis‐to‐UV TTA‐UC in a porous solid is particularly challenging because it requires balancing chromophore proximity for TTA with the suppression of aggregation‐induced quenching, while simultaneously preserving substrate accessibility. Here, we introduce porous polymer monoliths that enable solid‐state TTA‐UC specifically designed for heterogeneous photochemistry. By embedding sensitizer‐annihilator dye pairs into polymer matrices with controlled co‐continuous porosity, we create monolithic architectures that display upconverted UV emission from blue light at excitation intensities below a few mW cm ^{− 2} , with the operational wavelength further extended to green light excitation. The porous architecture enhances energy transfer to reaction substrates, providing a versatile platform for heterogeneous photochemical processes. Owing to their robustness and tunability, these monoliths are promising candidates as heterogeneous photoreactors, opening new opportunities for scalable and sustainable photochemistry.

A series of N , N ‐disubstituted buta‐2,3‐dienamides was prepared from 3‐butynoic acid and probed as substrates in a light‐induced photocyclization. It was found that xanthen‐9‐one (10 mol%) promotes the desired reaction to 3‐vinyl‐substituted 2‐azetidinones (β‐lactams) when performed at λ = 350 nm in acetonitrile as the solvent. Evidence was collected by transient absorption spectroscopy that the catalyst promotes excitation of the allene amide to its triplet state by Dexter energy transfer. Upon intramolecular hydrogen atom transfer from one of the nitrogen substituents, the ensuing 1,4‐diradical undergoes C─C bond formation to the lactam product. If the substituent at the nitrogen atom is a primary benzyl group, the product displays a stereogenic center in 4‐position and is formed exclusively as the trans ‐product (eleven examples, 18%–73% yield). If the substituent is secondary, 4,4‐disubstituted products are formed. If the buta‐2,3‐dienamide is substituted at the terminal carbon atom, the substituent at C3 in the 2‐azetidinone is an ( E )‐configured alkenyl group. Two alternative reaction pathways were observed, i.e. an intramolecular para photocycloaddition for N ‐phenyl substituted substrates and an elimination from the 1,4‐diradical intermediate. The vinyl group at C3 can serve as useful handle for consecutive transformations.

A series of N , N ‐disubstituted buta‐2,3‐dienamides was prepared from 3‐butynoic acid and probed as substrates in a light‐induced photocyclization. It was found that xanthen‐9‐one (10 mol%) promotes the desired reaction to 3‐vinyl‐substituted 2‐azetidinones (β‐lactams) when performed at λ = 350 nm in acetonitrile as the solvent. Evidence was collected by transient absorption spectroscopy that the catalyst promotes excitation of the allene amide to its triplet state by Dexter energy transfer. Upon intramolecular hydrogen atom transfer from one of the nitrogen substituents, the ensuing 1,4‐diradical undergoes C─C bond formation to the lactam product. If the substituent at the nitrogen atom is a primary benzyl group, the product displays a stereogenic center in 4‐position and is formed exclusively as the trans ‐product (eleven examples, 18%–73% yield). If the substituent is secondary, 4,4‐disubstituted products are formed. If the buta‐2,3‐dienamide is substituted at the terminal carbon atom, the substituent at C3 in the 2‐azetidinone is an ( E )‐configured alkenyl group. Two alternative reaction pathways were observed, i.e. an intramolecular para photocycloaddition for N ‐phenyl substituted substrates and an elimination from the 1,4‐diradical intermediate. The vinyl group at C3 can serve as useful handle for consecutive transformations.

Lower quencher concentrations favor triplet over singlet quenching, enhancing radical yield via higher cage escape efficiency. This “less is more” strategy suits TADF and conventional triplet-populating photocatalysts, boosting photoredox efficiency.

Diphosphapentalene-derived P-heterocyclic materials with two directly fused phospholes are fairly scarce, at least in comparison to their simpler congeners containing only one phosphole entity. To fill that void, π-conjugated naphtho-fused phospholo[3,2-b]phosphole dications were prepared via in situ oxidation of the corresponding diphosphines. In the case of one specific naphtho-annulation pattern, a hitherto unprecedented bis-(Δ²-phosphetene) dication was formed selectively and isolated as a colorless powder. DFT modelling studies revealed that this bis-(Δ²-phosphetenium) salt is produced via single electron transfer steps, while all the phospholo[3,2-b]phosphole salts may either be generated via their P-diylidic counterparts or via similar radical mechanisms. Exploiting this knowledge, the dicationic phospholo[3,2-b]phosphole isomer of the bis-(Δ²-phosphetenium) salt was isolated as well. In view of the high fluorescence quantum yields of these naphtho-fused phospholo[3,2-b]phosphole salts in aqueous solution, linearly π-extended anthraceno-fused derivatives were developed in order to bathochromically shift their emissions into the biological window. While detailed optoelectronic studies confirmed our expectations, the utmost remarkable observation is that even the anthraceno-fused materials were found to be sufficiently soluble in water, despite the fact that these fluorophores comprise up to eight fused rings.

In a recent communication, Collins and coworkers presented a Cu(I) complex with photocatalytic activity under red light LED conditions, mainly for singlet oxygen‐driven reactions. Guided by steady‐state emission measurements with 800 nm excitation, the authors suggested that the underlying mechanism for the generation of the photoexcited key species is a simultaneous two‐photon absorption via a virtual state. However, such a mechanism requires pulsed laser excitation and cannot compete when a conventional one‐photon excitation is also feasible with the selected excitation wavelength range. Using several spectroscopic techniques and reactivity assays under different light color and intensity conditions, we unambiguously demonstrate that a conventional one‐photon excitation followed by rather inefficient singlet oxygen generation (quantum yield <5%) is responsible for the observed photoreactivity of the Cu(I) complex. In addition, we briefly summarize general mechanistic considerations, estimate typical photon densities required for a variety of two‐photon mechanisms, highlight the importance of optical filters and impurities to avoid artifacts in the emission spectra, and present some guidelines for the differentiation between one‐ and two‐photon mechanisms.

In a recent communication, Collins and coworkers presented a Cu(I) complex with photocatalytic activity under red light LED conditions, mainly for singlet oxygen‐driven reactions. Guided by steady‐state emission measurements with 800 nm excitation, the authors suggested that the underlying mechanism for the generation of the photoexcited key species is a simultaneous two‐photon absorption via a virtual state. However, such a mechanism requires pulsed laser excitation and cannot compete when a conventional one‐photon excitation is also feasible with the selected excitation wavelength range. Using several spectroscopic techniques and reactivity assays under different light color and intensity conditions, we unambiguously demonstrate that a conventional one‐photon excitation followed by rather inefficient singlet oxygen generation (quantum yield <5%) is responsible for the observed photoreactivity of the Cu(I) complex. In addition, we briefly summarize general mechanistic considerations, estimate typical photon densities required for a variety of two‐photon mechanisms, highlight the importance of optical filters and impurities to avoid artifacts in the emission spectra, and present some guidelines for the differentiation between one‐ and two‐photon mechanisms.

The effects of backbone amine functionalization in three new homoleptic C^N^C type ruthenium(II) complexes bearing a tridentate bis‐imidazole‐2‐ylidene pyridine ligand framework are characterized and studied by single crystal diffraction, electrochemistry, optical spectroscopy and transient absorption spectroscopy in combination with ab initio DFT calculations. Functionalization by dimethylamine groups in 4‐position of the pyridine backbone significantly influences the properties of the complexes as revealed by comparison with the unfunctionalized references. As a result of the amine functionalization, a higher molar absorption coefficient of the MLCT bands, a decreased photoluminescence quantum yield at room temperature together with a shortened excited state lifetime but an improved photostability is observed. Introduction of electron donating and withdrawing groups at the NHC unit modifies the electronic and optical properties, such as the oxidation potential, absorption and emission properties, and the lifetimes of the excited states.

Metal complex – arene dyads typically act as more potent triplet energy donors compared to their parent metal complexes, which is frequently exploited for increasing the efficiencies of energy transfer applications. Using unexplored dicationic phosphonium-bridged ladder stilbenes (P-X²⁺) as quenchers, we exclusively observed photoinduced electron transfer photochemistry with commercial organic photosensitizers and photoactive metal complexes. In contrast, corresponding pyrene dyads of the tested ruthenium complexes with the very same metal complex units efficiently sensitize the P-X²⁺ triplets. The long-lived and comparatively redox-inert pyrene donor triplet in the dyads thus provides an efficient access to acceptor triplet states that are otherwise very tricky to obtain. This dyad-enabled control over the quenching pathway allowed us to explore the P-X²⁺ photochemistry in detail using laser flash photolysis. The P-X²⁺ triplet undergoes annihilation producing the corresponding excited singlet, which is an extremely strong oxidant (+2.3 V vs. NHE) as demonstrated by halide quenching experiments. This behavior was observed for three P²⁺ derivatives allowing us to add a novel basic structure to the very limited number of annihilators for sensitized triplet-triplet annihilation in neat water.