Alexander Hepp

This work describes the synthesis and photophysical characterization of a series comprised by four nonionic yet amphiphilic octa‐substituted Ni(II) phthalocyanines (NiPc) with peripheral or proximal decoration. The combination of MALDI–TOF mass spectrometry, ¹ H‐NMR, ¹³ C‐NMR, and UV–vis spectroscopy enabled an unambiguous structural elucidation. Notably, these NiPcs are soluble in both polar organic solvents and aqueous media. Since these complexes exhibit near‐infrared absorption ( λ _max = 650–800 nm), are non‐emissive, and display remarkable chemical stability, a new experimental setup was developed to investigate their photoacoustic efficiency by combining transient absorption spectroscopy and laser‐induced photoacoustic calorimetry. This approach revealed that all complexes are equally suited and stable as photoacoustic references in both aqueous and organic media.

Correction for ‘Copper( ii )-mediated base pairing involving the artificial nucleobase 3 H -imidazo[4,5- f ]quinolin-5-ol’ by Nikolas Sandmann et al. , Dalton Trans. , 2019, 48 , 10505–10515, https://doi.org/10.1039/C9DT02043H.

An organometallic phosphorescent Pt ^II complex is introduced into DNA duplexes via oxidative addition to a tailor‐made artificial nucleobase, acting as an N^N^C donor ligand. Its covalent attachment to the nucleic acid backbone localizes the metal ion in the center of the duplex. The platination exerts a stabilizing effect of up to 6.4 °C on the duplex, with the degree of stabilization depending on the identity of the nucleobase in the complementary position. No interstrand crosslink is formed, as indicated by mass‐spectrometry and corroborated by almost identical photoluminescence lifetimes ( τ ) in Ar‐purged solution. In air‐equilibrated solution, τ shows moderate dependence on the complementary nucleobase, suggesting that the latter influences oxygen accessibility to the luminophore. When two Pt ^II complexes are incorporated in neighboring positions, a redshifted luminescence from aggregates is observed in addition to monomer emission, indicating Pt···Pt interactions along the helical axis. The approach presented here allows the site‐specific incorporation of highly stable Pt ^II complexes as phosphorescent tags into nucleic acids.

In an attempt to broaden the scope of functional nucleic acids, phosphorescent platinum(II) complexes, resembling artificial metal-containing nucleobases, were attached covalently to DNA oligonucleotides via a deoxyribose moiety. The distance between the deoxyribose and the complex was varied by selecting three different linkers (propylene, ethylene, and methylene). Stable duplexes were obtained with any of the canonical nucleobases in the complementary position. When guanine was placed in this position, the most stable duplexes were obtained. No clear correlation was found between the identity of the linker and duplex stability. When two platinum(II) complexes were placed in adjacent positions within an oligonucleotide strand, photoluminescence spectra exhibited an additional broad low-energy band due to luminescence with excimeric character, indicating Pt···Pt interactions. The ratio of monomeric and excimeric emissions depends on the linker length and, interestingly, on the presence of dioxygen. Hence, a platinated oligonucleotide was developed into a ratiometric dioxygen sensor, capable of rapidly detecting dioxygen levels in volumes as small as 2 μL. The oligonucleotide proved to be nontoxic at relevant concentrations and could be transfected into cells, where it appeared to degrade so that further modification will be necessary to obtain an oligonucleotide-based ratiometric dioxygen sensor for intracellular measurements.

A bidentate ligand concept based on β-carbolines functionalized with a 1,2,4-triazolyl-moiety was designed and realized, enabling the development of a series of neutral rhenium(I) complexes. This new class of anionic ligands, incorporating either an unsubstituted 9H-pyrido[3,4-b]indole core (L_nHo) or a 9-methyl-substitued variant (L_Me-nHo), was developed towards tailored photofunctionality. Structural modification via methyl substitution at the indole moiety was found to enhance overall phosphorescence efficiency. Comparative studies of two monodentate auxiliary units revealed that 1,3,5-triaza-7-phosphaadamantane (PTA) significantly reduces the photoluminescence efficiency compared to pyridine (Py). Solvent-dependent photoluminescence studies indicated that a lowered polarity leads to an increase in photoluminescence quantum yields (Φ_L). The complex Re(L_Me-nHo)Py emerged as the most efficient emitter, displaying a Φ_L of 44% in dichloromethane (DCM). Notably, all complexes exhibited efficient quenching of excited triplet states by diffusional collision with triplet dioxygen (³O₂), yielding good singlet dioxygen (¹O₂) photoproduction efficiencies (Φ_Δ) with a maximum of 45% observed for Re(L_nHo)Py. These results highlight the suitability of these complexes for applications requiring efficient phosphorescence and oxygen photosensitization, such as bioimaging, and photodynamic therapy or photooxidation catalysis, while underscoring the central role of the tailored β-carboline-based chromoluminophores in enabling precise tuneability of photophysical properties.

A family of 16 platinum(II) complexes was synthesized with the aim of obtaining water-soluble luminescent complexes for guanine quadruplex (G4) binding. The complexes share a common tridentate N^N^C-donor ligand (based on 2-phenyl-6-(1H-1,2,3-triazol-4-yl)pyridine) bearing different substituents for solubilization, and an additional monodentate ancillary ligand (either phenylacetylide or 3-(trimethylammonium)prop-1-yne-1-ide). Single-crystal X-ray diffraction analyses confirm that the substituents do not interfere with the central planar core of the complexes required for π stacking interactions with the DNA. The interaction of the complexes with four DNA oligonucleotides that fold into various G4 topologies was evaluated using luminescence and circular dichroism spectroscopy as well as cryo-ESI mass spectrometry. The data indicate a complex correlation between type of substituent and ability of the complex to interact with G4 DNA.

A novel artificial nucleobase, 1‐(3‐(hydroxymethyl)phenyl)‐3‐phenylpropane‐1,3‐dione (Hdbm^OR), containing a β‐diketone designed to form inter‐strand complexes that resemble metal‐mediated base pairs, was synthesized and characterized towards its lanthanoid ion binding properties. Based on UV/vis spectra, a (labile) coordination of Eu^III and Sm^III by the benzyl‐functionalized model ligand is proposed, allowing the detection of distinct luminescence patterns upon coordination under alkaline conditions. The compatibility of the nucleoside analog with automated solid‐phase DNA synthesis was proven by the generation of a dinucleotide. NMR spectroscopic characterization of this dinucleotide revealed a temperature‐dependent equilibrium of two conformers, which likely involve hydrogen bonding of the β‐diketone moiety and the deoxyribose and/or phosphate moieties. The antenna effect of the new ligand promoting the luminescence of Eu^III is retained upon incorporation into DNA. Modification of a parallel‐stranded DNA duplex with a terminal homo base pair of Hdbm^OR allows, despite natural fraying of DNA, a thermal stabilization of about 3 °C upon coordination of Eu^III or Sm^III. Temperature‐dependent luminescence spectroscopy demonstrates a direct correlation between the thermal stability of the duplex and its luminescence intensity. The ligand extends the library of artificial nucleobases for lanthanoid‐mediated base pairs, combining the luminescence properties of lanthanoid complexes with the self‐assembly properties of DNA.

Transition metal complexes are archetypal luminescent probes that are widely used for various applications ranging from optoelectronics to biomedicine. However, they face significant challenges such as photobleaching and photooxidative stress, which limit their performance. Herein, we introduce a photosystem-inspired concept based on the use of a vitamin (ascorbic acid, Asc-Ac) to adaptively suppress photobleaching of molecular luminophores. As a proof-of-concept compound, we have selected a new bis-cyclometalated Pt(II) complex (Pt-tBu) and investigated its adaptive photoluminescence resulting from singlet dioxygen (¹O₂) photoproduction in the presence of Asc-Ac. Interestingly, the excited state quenching and subsequent photobleaching of Pt-tBu in aerated solutions is suppressed by addition of Asc-Ac, which scavenges the ¹O₂ photosensitized by Pt-tBu upon irradiation and results in an adaptive oxygen depletion with enhancement of luminescence. The adaptation is resilient for successive irradiation cycles with oxygen replenishment, until peroxidation overshooting leads to the degradation of Pt-tBu by formation of a dark Pt(IV) species. The complexity-related adaptation with initial overperformance (luminescence boost) relies on the external energy input and cascaded feedback loops, thus biomimicking inflammation, as the repeated exposure to a stressor leads to a final breakdown. Our antioxidative protection mechanism against photobleaching can be successfully extended to multiple coordination compounds (e.g., Ir(III), Ru(II) and Re(I) complexes), thus demonstrating its generality. Our findings broaden the scope of molecular adaptation and pave the way for enhancing the stability of molecular luminophores for multiple applications.