Thermally activated delayed fluorescence (TADF), involving reversible electron transfer between a fluorescing S1 state and a magnetically responsive charge-separated (CS) state, constitutes a sensitive probe of the spin-chemical dynamics in the CS state as well as of the local magnetic molecular environment. With (Cl)TAA-mB-PDI and SQA-pB-PDI-featuring a Cl-substituted triarylamine ((Cl)TAA) or a squaraine unit as electron donors, meta-benzene (mB) or para-benzene (pB) bridges, and a perylene diimide (PDI) unit as electron acceptor-we introduce two novel triads with sufficiently small S1-CS energy gaps to enable TADF. Their excited state properties were investigated by femtosecond and nanosecond time-resolved transient absorption spectroscopy as well as nanosecond time-resolved fluorescence spectroscopy at variable temperatures and in variable magnetic fields. The variable temperature data were used for a full kinetic and thermodynamic characterization of the reversible electron transfer in the excited state. The magnetic-field-dependent kinetic data were thoroughly analyzed using three models of increasing complexity that explicitly include the role of the S1 state: a purely classical model, a mixed quantum - classical model, and a fully quantum-dynamical model. It is shown that the magnetic-field-dependent decay of the total CS state population is well approximated by a mono-exponential function with a magnetic-field-dependent rate constant k CSS(B). Furthermore, the delayed fluorescence proves to be a reliable indicator of the spin-dependent dynamics of the CS state. Together with two structurally related triads lacking TADF, the magnetic field effects of the new systems are placed into a broader framework that categorizes such effects using three characteristic parameters: the resonance field B res = 2J, the resonance line width (fwhm), and the relative magnetically induced kinetic range R k of k CSS(B). We demonstrate how these characteristic parameters depend on the kinetic properties of the systems.

Thermally assisted delayed fluorescence (TADF) in electron-donor-bridge-acceptor triads has recently been shown to provide a new way of observing the spin dynamics of charge-separated states (CSS) corresponding to linked radical pairs. In this work, we present a theoretical approach for extending standard quantum-dynamical models to describe this system. Using a representative example that combines four electronic radical-pair spin states with three nuclear spin states of a single nitrogen nucleus, we extend the Hilbert space from 12 to 15 dimensions by including the excited singlet state S1. We derive Liouvillian operators that account for the kinetic coupling between S1 and the CSS, including decay, charge separation, and recombination, and illustrate the resulting dynamics with numerical examples.

This study presents a numerical simulation approach to investigate singlet-triplet interconversion effects in organic materials with rigid molecular structures that facilitate the photogeneration of charge-separated (CS) states, such as zwitterions resulting from intramolecular electron transfer. Our approach enables the detailed modeling of electron and nuclear spin-dependent observables, including magnetic field-affected reaction yields (MARY) and chemically induced dynamic nuclear polarization (CIDNP). The equilibrium solution of the stochastic Liouville equation can be obtained with simple algebraic manipulation by noting the relationship between the Laplace transform of the density operator and the time-domain representation of the same operator. Experimental MARY and CIDNP data are modeled as functions of key external and internal system parameters, such as magnetic field strength, hyperfine interactions, and exchange couplings. This allows for exploring processes that are otherwise experimentally inaccessible, providing deeper insights into the spin dynamics of the photoinduced CS state. Understanding these interconversion processes is not only essential for the fundamental photochemistry studies but also for the rational design and development of novel organic materials for photovoltaics and photocatalysis. Our results demonstrate the significant impact of singlet-triplet interconversion on the overall efficiency of charge separation and recombination processes, highlighting the importance of spin dynamics in the design of next-generation organic photovoltaic materials.

Four new donor-acceptor dyads, featuring triarylamine donors and perylene diimide acceptors, were synthesized to investigate the influence of the g-tensor on the magnetic field-dependent spin dynamics of the resulting radical pairs. These pairs are characterized by the exchange interaction 2J being larger than the effective isotropic hyperfine coupling aeff. To control the isotropic g-factor and g-tensor anisotropy of the radical anion generated via photoinduced electron transfer, the perylene diimides were functionalized with phenyl chalcogen ethers. In dyads containing oxygen, sulfur, and selenium ether substituents, not only was the characteristic 2J-resonance observed but also a pronounced high-field effect in the charge recombination kinetics, extending up to B = 10 T. Quantum dynamics simulations based on the stochastic Liouville equation revealed that this effect is primarily driven by g-tensor anisotropy-induced relaxation, which increases along the chalcogen ether series. Additionally, we derived an exact analytical solution describing the impact of g-factor differences and g-tensor anisotropy on spin relaxation in the high-field limit. These findings highlight the critical role of g-tensor-induced relaxation in radical pairs at high magnetic fields, offering new insights for the molecular design of materials with potential applications in quantum information science, where incoherent relaxation processes should be avoided.

A series of triads consisting of a triarylamine electron donor and a perylene diimide electron acceptor which were attached to two different wings of a triptycene bridging unit was investigated...

Intramolecular rotations modulate exchange interaction ( J ) as well as electronic couplings through variable π-overlap, affecting the rate constants of photoinduced charge separation and recombination in electron-donor-bridge-acceptor triads.

Ladungsgetrennte Radikalpaare (Charge‐Separated Radical Pairs, CSRP) spielen eine zentrale Rolle in der photochemischen Energiekonversion, wie bei der Photosynthese oder in organischen Solarzellen. Mit einem neuen Verfahren lässt sich die Spinentwicklung solcher Paare optisch bestimmen.

A detailed experimental study on reversible photo-induced intramolecular charge separation is presented based on nuclear magnetic resonance detection of chemically induced dynamic nuclear polarization. From variation of such polarization with the external magnetic field, the coupling constants of isotropic and anisotropic hyperfine interactions at individual 13C sites are measured in the short-lived charge separated state of dyad molecules composed of donor-bridge-acceptor parts. The objects of study were rigid donor-bridge-acceptor dyads, consisting of triarylamine as a donor, naphthalene diimide as an acceptor, and a meta-conjugated diethynylbenzene fragment as a bridge. By systematic variation of side groups in the bridging moiety, their influence on the electron withdrawing strength is traced. In combination with similar data for the 1H positions obtained previously for the same compounds [I. Zhukov et al., J. Chem. Phys. 152, 014203 (2020)], our results provide a reliable basis for the determination of the spin density distribution in the charge separated state of such dyads.

Spin quantum beats prove the quantum nature of reactions involving radical pairs, the key species of spin chemistry. However, such quantum beats remain hidden to transient absorption–based optical observation because the spin hardly affects the absorption properties of the radical pairs. We succeed in demonstrating such quantum beats in the photoinduced charge-separated state (CSS) of an electron donor–acceptor dyad by using two laser pulses—one for pumping the sample and another one, with variable delay, for further exciting the CSS to a higher electronic state, wherein ultrafast recombination to distinct, optically detectable products of singlet or triplet multiplicity occurs. This represents a spin quantum measurement of the spin state of the CSS at the time instant of the second (push) pulse.

Conjugation of unprotected carbohydrates to surfaces or probes by chemoselective ligation reactions is indispensable for the elucidation of their numerous biological functions. In particular, the reaction with oxyamines leading to the formation of carbohydrate oximes which are in equilibrium with cyclic N-glycosides (oxyamine ligation) has an enormous impact in the field. Although highly chemoselective, the reaction is rather slow. Here, we report that the oxyamine ligation is significantly accelerated without the need for a catalyst when starting with glycosyl amines. Reaction rates are increased up to 500-fold compared to the reaction of the reducing carbohydrate. For comparison, aniline-catalyzed oxyamine ligation is only increased 3.8-fold under the same conditions. Glycosyl amines from mono- and oligosaccharides are easily accessible from reducing carbohydrates via the corresponding azides by using Shoda's reagent (2-chloro-1,3-dimethylimidazolinium chloride, DMC) and subsequent reduction. Furthermore, glycosyl amines are readily obtained by enzymatic release from N-glycoproteins making the method suited for glycomic analysis of these glycoconjugates which we demonstrate employing RNase B. Oxyamine ligation of glycosyl amines can be carried out at close to neutral conditions which makes the procedure especially valuable for acid-sensitive oligosaccharides.

Quantum chemical calculations and laser flash spectroscopy probe spin-dynamics of the photoinduced electron transfer mechanism in two donor–acceptor bisphenanthrolinecopper(i) species, marking the influence of spin on solar energy conversion schemes.

A series of triads consisting of a triarylamine donor, a naphthalenediimide acceptor, and a palladium photosensitizer bridge was investigated for the photoinduced electron transfer processes and the spin chemistry involved. In this series, the ligand in the palladium photosensitizer was varied from bis-dipyrrinato to porphodimethenato and to a porphyrin. With the porphyrin photosensitizer, no charge separated state could be reached. This is caused by the direct relaxation of the excited photosensitizer to the ground state by intersystem crossing. The bis-dipyrrinato-palladium photosensitizer gave only a little yield (7%) of the charge separated state, which is due to the population of a metal centered triplet state and a concomitant geometrical rearrangement to a disphenoidal coordination sphere. This state relaxes rapidly to the ground state. In contrast, in the porphodimethenato-palladium triads, a long lived (μs to ms) charge separated state could be generated in high quantum yields (66%-74%) because, here, the population of a triplet metal centered state is inhibited by geometrical constraints. The magnetic field dependent transient absorption measurement of one of the porphodimethenato triads revealed a giant magnetic field effect by a factor of 26 on the signal amplitude of the charge separated state. This is the consequence of a magnetic field dependent triplet-singlet interconversion that inhibits the fast decay of the charge separated triplet state through the singlet recombination channel. A systematic comparative analysis of the spin-dependent kinetics in terms of three classical and one fully quantum theoretical methods is provided, shedding light on the pros and cons of each of them.

Previous transient absorption measurements using the magnetically affected reaction yield (MARY) technique for a series of rigidly linked electron donor/electron acceptor dyads (D-X-A) consisting of a triarylamine donor, a naphthalene diimide acceptor, and a meta-conjugated diethynylbenzene unit as a bridge had revealed the presence of electronic exchange interaction, J, in the photoexcited charge separated (CS) state. Here, we present results obtained by photochemically induced dynamic nuclear polarization (photo-CIDNP) that allows for determining the sign of J. By variation of the magnetic field from 1 mT to 9.4 T, pronounced absorptive maxima of CIDNP were detected for more than 20 1H nuclei disregarding the sign of their hyperfine coupling constants in the transient charge separated state, with positions of maxima close to those found by the MARY technique. Quantitative comparison of the observed CIDNP signals for various D-X-A dyads reveals an increase in the CIDNP enhancement factor with increasing population of the triplet state determined by MARY spectroscopy at zero magnetic field. For CIDNP of the methyl groups of the TAA donor dyads, we found in all studies a good linear dependence between the CIDNP signal amplitude and the initial population of the CS triplet state. The linear relationship together with the absorptive CIDNP allows us to conclude that (i) the sign of the electronic exchange interaction Jex is positive, (ii) CIDNP is formed predominantly in the vicinity of level anticrossing between the T+ and S electronic levels, and (iii) coherent triplet-singlet transitions are induced by hyperfine interaction and accompanied by simultaneous electron and nuclear spin flip, T+β→Sα.

The magnetic-field dependent charge recombination kinetics upon ns-laser flash photolysis of a novel triarylamine/cyclometalated platinum complex/napthalenediimide triad (DPtA) have been measured in tetrahydrofuran (THF) and polytetrahydrofuran (pTHF) to study the spinchemical effect of a large increase of the macroviscosity of the solvent by a factor of about 1000. The magnetic-field dependence of the decay kinetics of the charge separated state has been accurately reproduced by a classical kinetic model, wherein the rate constants of transitions between spin substates of different Zeeman energy are represented by a single magnetic-field dependent rate constant k±. In THF, the magnetic-field dependence of k± is given by a double Lorentzian function which, in a double log plot, shows two characteristic steps that can be consistently assigned to the magnetic field inhibition of the coherent and incoherent i.e. relaxational spin mixing mechanism. In pTHF, the magnetic field effect on k± is a single step function decreasing to its limiting value at about 5 times lower fields than in THF. As shown by a quantum theoretical simulation, in this solvent coherent and incoherent processes are contributing equally at all fields. The nanoviscosity of pTHF appears to be much smaller than its macroviscosity.

Abstract Organisms use enzymes to ensure a flow of substrates through biosynthetic pathways. How the earliest form of life established biosynthetic networks and prevented hydrolysis of intermediates without enzymes is unclear. Organocatalysts may have played the role of enzymes. Quantitative analysis of reactions of adenosine 5’‐monophosphate and glycine that produce peptides, pyrophosphates, and RNA chains reveals that organocapture by heterocycles gives hydrolytically stabilized intermediates with balanced reactivity. We determined rate constants for 20 reactions in aqueous solutions containing a carbodiimide and measured product formation with cyanamide as a condensing agent. Organocapture favors reactions that are kinetically slow but productive, and networks, over single transformations. Heterocycles can increase the metabolic efficiency more than two‐fold, with up to 0.6 useful bonds per fuel molecule spent, boosting the efficiency of life‐like reaction systems in the absence of enzymes.

Charge recombination in the photoinduced charge separated (CS) state of a rigidly linked donor/bridge/acceptor triad with a triarylamine (TAA) donor, a 1,3-diethynyl-2,5-dimethoxy benzene bridge (OMe), and a perylenediimide (PDI) unit as an acceptor, represents a spin chemical paradigm case of a rigid radical ion pair formed with singlet spin and recombining almost exclusively to the locally excited PDI triplet state (3PDI). The magnetic field dependence of the CS state decay and 3PDI formation kinetics are investigated from 0 to 1800 mT by nanosecond laser flash spectroscopy. The time-resolved magnetic field affected reaction yields spectra of the CS state population and 3PDI population exhibit a sharp and deep resonance at 18.9 mT, indicating level crossing of the S and T+ levels separated by an exchange interaction of J = 18.9/2 mT at zero field. The kinetics are biexponential around the resonance field and monoexponential outside that range. The monoexponential behavior can be simulated by a classical kinetic model assuming a single field dependent double Lorentzian function for the energy gap dependence of all spin conversion processes. The full field dependence of the kinetics has been simulated quantum theoretically. It has been shown that incoherent and coherent hyperfine coupling contribute to S/T+ spin conversion at all fields and that the biexponentiality of the kinetics at resonance is due to a partitioning of the overall kinetics into 2/3 of the singlet hyperfine states exhibiting strong isotropic coupling to T+ and 1/3 of the singlet hyperfine states that do not or only weakly couple isotropically to T+.

The photoinduced charge separation and charge recombination in a set of four molecular dyads consisting of a triarylamine donor and a naphthalene diimide acceptor were investigated by time resolved transient absorption spectroscopy with fs and ns time resolution. In these dyads the donor and acceptor are bridged by a meta-conjugated diethynylbenzene bridge whose electronic nature was tuned by small electron donating (OMe, Me) or electron withdrawing (Cl, CN) substituents. While the formation of the transient charge separated states is complete within tens of ps, charge recombination is biphasic with a shorter component of several hundred ns and a longer component of several microseconds. This behaviour could be rationalized by assuming an equilibrium of singlet and triplet charge separated states. Magnetic field dependent measurements showed a strong influence on the biphasic decay kinetics and also a pronounced level crossing effect in the magnetic field affected reaction yield (MARY) spectra caused by a significant exchange coupling. An analysis of the observed kinetics using classical kinetic rate equations yields rate constants for charge separation and charge recombination as well as the exchange interaction splitting in the radical ion pair, all of them showing a delicate dependence on the bridge substituents.

Magnetic field effects (MFEs) allow detailed insight into spin conversion processes of radical pairs that are formed, for example, in all charge separation processes, and are supposed to play the key role in avian navigation. In this work, the MFE of charge recombination in the charge-separated state of a rigid donor bridge acceptor dyad was analyzed by a classical and a quantum theoretical model and represents a paradigm case of understanding spin chemistry with unprecedented detail. The MFE is represented by magnetic field-affected reaction yield (MARY) spectra that exhibit a sharp resonance, resulting from S/T level crossing as the Zeeman splitting equals twice the exchange interaction. Although in the classical kinetic model, the spin conversion processes between the four singlet and triplet substates are shown for the first time to obey an identical generalized energy dependence, quantum theory proves that the MARY resonance line is composed of relaxation, coherent hyperfine induced spin mixing, and S/T dephasing contributions.

Five biotinylated photolabile compounds of the general structure Bt-L1 -NPPOC-X-L2 were synthesized, in which Bt represents a biotin unit, L1 is a 3,6-dioxa-n-octane or an n-hexane spacer, NPPOC is the photolabile protecting group 2-(2-nitrophenyl)propoxycarbonyl, and X is a thymidine unit as a representative nucleoside or a direct linkage to L2 , an ω-mercapto- or ω-aminohexoyl linker, for coupling to a substrate surface. These compounds served for testing the photocleavage kinetics in self-assembled monolayers on gold or glass by using surface plasmon resonance (SPR) on gold or reflectometric interference spectroscopy (RIfS) on glass, whereby the biotin moiety offered the possibility to increase the bulkiness of the leaving group by binding to streptavidin, which thereby largely enhanced the SPR or RIfS signals. The photokinetics, found to consist in a dominating fast stage and a less contributing slow stage, were quantitatively analyzed, and the quantum yield of the fast part reached values up to almost 1 in favorable cases. A direct comparison of the results from SPR and RIfS yielded almost identical results. The present investigations pave the way to in situ monitoring of the photolithographic synthesis of DNA chips.

Abstract In recent work from this group (J. H. Klein et al. J. Am. Chem. Soc. 2015, 137, 11011), the magnetic field dependent charge recombination kinetics in donor/Ir-complex/acceptor triads has been determined with outstanding accuracy and reproducibility. The field-dependent kinetics has been analyzed in terms of a classical reaction scheme including the field-independent rate parameters of singlet recombination (rate constant k S) and S/T0 mixing (rate constant k ST0) and the field-dependent rate constant k±(B) connecting central and outer Zeeman levels. In the present work, the extraction of k± from the experimental data is more precisely defined and the appearance of a “coherent” and “incoherent” regime of spin motion in a double log plot of k± vs. B is confirmed. The experimental decay curves have been reproduced by a full quantum dynamical model based on the stochastic Liouville equation, which was solved numerically, taking into account isotropic hyperfine coupling with five nuclear spins (1 N on donor radical, 4 H on acceptor radical) and anisotropic hyperfine coupling with the nitrogen nucleus at the donor radical. The results of the quantum calculations serve as a rigorous basis of interpreting the classical parameter k±. Furthermore, it is demonstrated that the incoherent part of spin motion is essential for a full understanding of the charge recombination kinetics even in the “coherent” regime.