Roger Jan Kutta

University of Regensburg | UR · Institute of Physical and Theoretical Chemistry

Three iridium(III) triazolato complexes of the general formula [Ir(triazolatoR,R’)(ppy)(terpy)]PF6 with ppy=2‐phenylpyridine and terpy=2,2’:6’,2’’‐terpyridine were efficiently prepared by iClick reaction of [Ir(N3)(ppy)(terpy)]PF6, with alkynes and alkynones, which allowed facile introduction of biological carriers such as biotin and cholic acid. I...

The determination of ligand–receptor binding affinities plays a key role in the development process of pharmaceuticals. While the classical radiochemical binding assay uses radioligands, fluorescence-based binding assays require fluorescent probes. Usually, radio- and fluorescence-labeled ligands are dissimilar in terms of structure and bioactivity...

The unimolecular heterolysis of covalent σ-bonds is integral to many chemical transformations, including SN1-, E1- and 1,2-migration reactions. To a first approximation, the unequal redistribution of electron density during bond heterolysis is governed by the difference in polarity of the two departing bonding partners1–3. This means that if a σ-bo...

Precious metal complexes remain ubiquitous in photoredox catalysis (PRC) despite concerted efforts to find more earth‐abundant catalysts and replacements based on 3d metals in particular. Most otherwise plausible 3d metal complexes are assumed to be unsuitable due to short‐lived excited states, which has led researchers to prioritize the pursuit of...

The photophysics of a helicene derivative in which two benzene units are replaced by thiophene units (thiohelicene, 6H) was studied by steady state and transient absorption and emission spectroscopies covering time ranges from femtoseconds to minutes. Efficient intersystem crossing (ISC) to the triplet state was observed, by far exceeding that of t...

Precious metal complexes remain ubiquitous in photoredox catalysis (PRC) despite concerted efforts to find more earth‐abundant catalysts and replacements based on 3d metals in particular. Most otherwise plausible 3d metal complexes are assumed to be unsuitable due to short‐lived excited states, which has led researchers to prioritize the pursuit of...

Di-tert-butyldiphosphatetrahedrane (tBuCP)2 (1) is a mixed carbon- and phosphorus-based tetrahedral molecule, isolobal to white phosphorus (P4). However, despite the fundamental significance and well-explored reactivity of the latter molecule, the precise structure of the free (tBuCP)2 molecule (1) and a detailed analysis of its electronic properti...

Photocatalytic reactions involving a reductive radical‐polar crossover (RRPCO) generate intermediates with carbanionic reactivity. Many of these proposed intermediates resemble highly reactive organometallic compounds. However, conditions of their formation are generally not tolerated by their isolated organometallic versions and often a different...

Photocatalytic reactions involving a reductive radical‐polar crossover (RRPCO) generate intermediates with carbanionic reactivity. Many of these proposed intermediates resemble highly reactive organometallic compounds. However, conditions of their formation are generally not tolerated by their isolated organometallic versions and often a different...

2,5‐Diketopiperazines are cyclic dipeptides displaying a wide range of applications. Their enantioselective preparation has now been found possible from the respective racemates by a photochemical deracemization (53 examples, 74 % to quantitative yield, 71–99 % ee). A chiral benzophenone catalyst in concert with irradiation at λ=366 nm enables to e...

2,5‐Diketopiperazines are cyclic dipeptides displaying a wide range of applications. Their enantioselective preparation has now been found possible from the respective racemates by a photochemical deracemization (53 examples, 74% to quantitative yield, 71‐99% ee). A chiral benzophenone catalyst in concert with irradiation at λ= 366 nm enables to es...

The syntheses of two novel, organic, and chiral photocatalysts are presented. By combining donor-acceptor cyanoarene-based photocatalysts with a chiral phosphoric acid, bifunctional catalysts have been designed. In preliminary proof-of-concept reactions, their use in both enantioselective energy transfer and photoredox catalysis is demonstrated.

Upon irradiation in the presence of a chiral benzophenone catalyst (5 mol %), a racemic mixture of a given chiral imidazolidine-2,4-dione (hydantoin) can be converted almost quantitatively into the same compound with high enantiomeric excess (80-99% ee). The mechanism of this photochemical deracemization reaction was elucidated by a suite of mechan...

Through the linkage of two muscarinergic M3 receptor ligands to fluorescent tetramethylrhodamine- and cyanine-5-type dyes, two novel tool compounds, OFH5503 and OFH611, have been developed. Based on the suitable binding properties and kinetics related to the M3 subtype, both ligand-dye conjugates were found to be useful tools to determine binding a...

The phenomenon of photoacidity, i.e., an increase in acidity by several orders of magnitude upon electronic excitation, is frequently encountered in aromatic alcohols capable of transferring a proton to a suitable acceptor. A promising new class of neutral super-photoacids based on pyranine derivatives has been shown to exhibit pronounced solvatoch...

2,3,5-triphenyltetrazolium chloride (TTC) may convert into phenyl-benzo[c]tetrazolocinnolium chloride (PTC) and 1,3,5-triphenylformazan (TPF) under irradiation with light. The latter reaction, albeit enzymatically rather than photochemically, is used in so-called TTC assays indicating cellular respiration and cell growth. In this paper, we address...

Herein, the synthesis in conjunction with the structural, electrochemical, and photophysical characterization of a 5,5'-bisphenanthroline (phenphen) linked heterodinuclear RuPt complex (Ru(phenphen)Pt) and its light-driven hydrogen formation activity are reported. A single crystal X-ray diffraction (SC-XRD) analysis identified a perpendicular orien...

Calmodulin (CaM) is a highly dynamic Ca2+-binding protein that exhibits large conformational changes upon binding Ca2+ and target proteins. Although it is accepted that CaM exists in an equilibrium of conformational states in the absence of target protein, the physiological relevance of an elongated helical linker region in the Ca2+-replete form ha...

We report the first successful use of consecutive two-photon accumulation of visible light energy to obtain ultra-high oxidation potentials (> +3 V vs. saturated calomel electrode) enabled by a new class of rationally designed cyclic triarylamine photocatalysts. We demonstrate its practical and synthetic utility in a series of reactions of electron...

Deazaflavins are well suited for reductive chemistry acting via a consecutive photo‐induced electron transfer, in which their triplet state and semiquinone – the latter is formed from the former after electron transfer from a sacrificial electron donor – are key intermediates. Guided by mechanistic investigations aiming to increase intersystem cros...

Organic ligands comprising thiophene ring(s) afford complexes of transition metals, such as Pt(II) and Ir(III), with photoluminescence readily tunable via ligand modifications. In this work we demonstrate targeted design of N^C-C^N type ditopic ligand with central thiophene ring as the cyclometalating core and variation of nuclearity of the complex...

Room-temperature dual emission consisting of spectrally separated fluorescence and phosphorescence is highly attractive as a design principle for ratiometric sensing materials, for example, for ratiometric detection of dioxygen. Compounds susceptible...

The Cover Feature shows the photocatalytic oxidative cycloelimination of a coumarin dimer by a flavinium salt inspired by the light‐induced photolyase DNA repair reaction in nature. Light absorption and subsequent intersystem crossing (isc) enable diffusion‐controlled electron transfer (eT) from the coumarin dimer to the excited triplet flavinium s...

Flavinium salts are frequently used in organocatalysis but their application in photoredox catalysis has not been systematically investigated to date. We synthesized a series of 5‐ethyl‐1,3‐dimethylalloxazinium salts with different substituents in the positions 7 and 8 and investigated their application in light‐dependent oxidative cycloelimination...

The photochemical deracemization of spiro[cyclopropane‐1,3′‐indolin]‐2′‐ones (spirocyclopropyl oxindoles) was studied. The corresponding 2,2‐dichloro compound is configurationally labile upon direct irradiation at λ=350 nm and upon irradiation at λ=405 nm in the presence of achiral thioxanthen‐9‐one as the sensitizer. The triplet 1,3‐diradical inte...

Light-dependent and dark-operative protochlorophyllide oxidoreductases (LPORs and DPORs) are evolutionary and structurally distinct enzymes that are essential for the synthesis of (bacterio)chlorophyll, the primary pigment needed for both anoxygenic and oxygenic photosynthesis. In contrast to the long-held hypothesis that LPORs are only present in...

A chiral thioxanthone‐ or xanthone‐based sensitizer allows for the differentiation of cyclopropane enantiomers and for a photochemical deracemization (17 examples, up to 85 % ee). The key intermediate of this entropically disfavored process was identified by transient absorption spectroscopy to be a 1,3‐ diradical with a lifetime of τ=22 μs. Abstr...

Flavin-mediated photocatalytic oxidations are established in synthetic chemistry. In contrast, their use in reductive chemistry is rare. Deazaflavins with a much lower reduction potential are even better suited for reductive chemistry rendering also deazaflavin semiquinones as strong reductants. However, no direct evidence exists for the involvemen...

Drosophila melanogaster cryptochrome functions as the primary blue-light receptor that mediates circadian photo entrainment. Absorption of a photon leads to reduction of the protein-bound FAD via consecutive electron transfer along a conserved tryptophan tetrad resembling the signalling state required for conformational changes and induction of sub...

Hydride transfers play a crucial role in a wide range of biological systems. However, its mode of action - concerted or stepwise - is still under debate. Light-dependent NADPH: protochlorophyllide oxidoreductase (POR) catalyzes the stereospecific trans addition of a hydride anion and a proton across the C17-C18 double bond of protochlorophyllide. W...

Hydride transfers play a crucial role in a wide range of biological systems. However, its mode of action - concerted or stepwise - is still under debate. Light-dependent NADPH: protochlorophyllide oxidoreductase (POR) catalyzes the stereospecific trans addition of a hydride anion and a proton across the C17-C18 double bond of protochlorophyllide. W...

All cryptochromes are currently classified as flavoproteins. In animals their best-described role is as components of the circadian clock. This circadian function is variable, and can be either light-dependent or -independent; the molecular origin of this difference is unknown. Type I animal cryptochromes are photoreceptors that entrain an organism...

The coenzyme B 12-dependent photoreceptor protein, CarH, is a bacterial transcriptional regulator that controls the biosynthesis of carotenoids in response to light. On binding of coenzyme B 12 the monomeric apoprotein forms tetramers in the dark, which bind operator DNA thus blocking transcription. Under illumination the CarH tetramer dissociates,...

The chromophores ethynyl pyrene as blue, ethynyl perylene as green and ethynyl Nile red as red emitter were conjugated to the 5-position of 2'-deoxyuridine via an acetylene bridge. Using phosphoramidite chemistry on solid phase labelled DNA duplexes were prepared that bear single chromophore modifications, and binary and ternary combinations of the...

LOV domains are the light sensitive parts of phototropins and many other light-activated enzymes that regulate the response to blue light in plants and algae as well as some fungi and bacteria. Unlike all other biological photoreceptors known so far, the photocycle of LOV domains involves the excited triplet state of the chromophore. This chromopho...

The coupling of photochemistry to protein chemical and structural change is crucial to biological light-activated signaling mechanisms. This is typified by cyanobacteriochromes (CBCRs), members of the phytochrome superfamily of photoreceptors that exhibit a high degree of spectral diversity, collectively spanning the entire visible spectrum. CBCRs...

We describe the setup and performance of an apparatus for simultaneous time- and spectrally resolved measurements of transient absorption. The key component in this apparatus is a streak camera, yielding, for every excitation of the sample, a two-dimensional data array with 512 × 512 data points. The apparatus covers the spectral range 350–750 nm a...

In this chapter1 we describe the setup and performance of an apparatus for simultaneous time-resolved and spectrally resolved measurements of transient absorption (TA). The key component in this apparatus is a streak camera, yielding, for every excitation of the sample, a two-dimensional data array with 512 × 512 data points. The apparatus covers t...

Flavin-mediated photooxidations have been described for applications in synthetic organic chemistry for some time and are claimed to be a route to the use of solar energy. We present a detailed investigation of the involved photophysical and photochemical steps in methoxybenzyl alcohol oxidation on a timescale ranging from sub-picoseconds to tens o...

Irradiation of the LOV1 domain from the blue-light photoreceptor phototropin of the green alga Chlamydomonas reinhardtii leads to the formation of a covalent adduct of the sulfur atom of cysteine 57 to the carbon C(4a) in the chromophore FMN. This reaction is not possible in the mutant LOV1-C57G in which this cysteine is replaced by glycine. Irradi...

Novel photoresponsive materials based on ruthenium(II) σ-acetylides coupled to an azobenzene moiety in the main π-conjugated chain have been synthesized. The introduction of a metal acetylide fragment in the same conjugated chain as the azobenzene induces the trans-cis-trans isomerization of the azo unit, while the rate of the thermal cis →trans ba...

The phototropin from Chlamydomonas reinhardtii is a 120 kDa blue light receptor that plays a key role in gametogenesis of this green alga. It comprises two light-sensing domains termed LOV1 and LOV2 (light oxygen and voltage) and a serine/threonine kinase domain. The post-translationally incorporated chromophore is flavin mononucleotide (FMN). Upon...