The slow photo-induced CO2 release of N-phthaloylglycine.

Carboxylic acids and carboxylates may release CO2 upon oxidation. The oxidation can be conducted electrochemically as in the Kolbe synthesis or by a suitable oxidant. In N-phthaloylglycine (PG), the photo-excited phthalimide chromophore acts as an oxidant. Here, the photo-kinetics of PG dissolved in acetonitrile is traced by steady-state as well as time-resolved UV/vis and IR spectroscopy. The experiments provide clear evidence that, contrary to earlier claims, the photo-induced CO2 release is slow, i.e. it occurs on the microsecond time range. The triplet state of PG is, therefore, the photo-reactive one.

Influence of the substitution position on spin communication in photoexcited perylene-nitroxide dyads.

By virtue of the modularity of their structures, their tunable optical and magnetic properties, and versatile applications, photogenerated triplet-radical systems provide an ideal platform for the study of the factors controlling spin communication in molecular frameworks. Typically, these compounds consist of an organic chromophore covalently attached to a stable radical. After formation of the chromophore triplet state by photoexcitation, two spin centres are present in the molecule that will interact. The nature of their interaction is governed by the magnitude of the exchange interaction between them and can be studied by making use of transient electron paramagnetic resonance (EPR) techniques. Here, we investigate three perylene-nitroxide dyads that only differ with respect to the position where the nitroxide radical is attached to the perylene core. The comparison of the results from transient UV-vis and EPR experiments reveals major differences in the excited state properties of the three dyads, notably their triplet state formation yield, excited state deactivation kinetics, and spin coherence times. Spectral simulations and quantum chemical calculations are used to rationalise these findings and demonstrate the importance of considering the structural flexibility and the contribution of rotational conformers for an accurate interpretation of the data.

References for Small Fluorescence Quantum Yields.

Three compounds with fluorescence quantum yields in the range of 10- 5 to 10- 4 and emission spectra covering the UV/Vis spectral range are suggested as new references for the determination of small fluorescence quantum yields. The compounds are thymidine (dT) in water, dibenzoylmethane (DBM) in ethanol, and malachite green chloride (MG) in water, representing the blue, green, and red regions of the spectrum, respectively. All compounds are easily handled, photostable, and commercially available. Furthermore, these compounds exhibit a mirror-image symmetry between their absorption and fluorescence spectra. This symmetry, along with closely aligned fluorescence excitation and absorption spectra, confirms that the observed emissions originate from the compounds themselves. The fluorescence quantum yields were determined via a relative approach as well as Strickler-Berg analysis in conjunction with time resolved fluorescence spectroscopy. Within the respective error margins, the two approaches yielded identical results.

Spectroscopic view on the interaction between the psoralen derivative amotosalen and DNA.

Psoralens are eponymous for PUVA (psoralen plus UV-A radiation) therapy, which inter alia can be used to treat various skin diseases. Based on the same underlying mechanism of action, the synthetic psoralen amotosalen (AMO) is utilized in the pathogen reduction technology of the INTERCEPT® Blood System to inactivate pathogens in plasma and platelet components. The photophysical behavior of AMO in the absence of DNA is remarkably similar to that of the recently studied psoralen 4'-aminomethyl-4,5',8-trimethylpsoralen (AMT). By means of steady-state and time-resolved spectroscopy, intercalation and photochemistry of AMO and synthetic DNA were studied. AMO intercalates with a higher affinity into A,T-only DNA (KD = 8.9 × 10-5 M) than into G,C-only DNA (KD = 6.9 × 10-4 M). AMO covalently photobinds to A,T-only DNA with a reaction quantum yield of ΦR = 0.11. Like AMT, it does not photoreact following intercalation into G,C-only DNA. Femto- and nanosecond transient absorption spectroscopy reveals the characteristic pattern of photobinding to A,T-only DNA. For AMO and G,C-only DNA, signatures of a photoinduced electron transfer are recorded.

PDI-trityl dyads as photogenerated molecular spin qubit candidates.

Owing to their potential applications in the field of quantum information science, photogenerated organic triplet-radical conjugates have attracted an increasing amount of attention recently. Typically, these compounds are composed of a chromophore appended to a stable radical. After initialisation of the system by photoexcitation, a highly spin-polarised quartet state may be generated, which serves as a molecular spin qubit candidate. Here, we investigate three perylene diimide (PDI)-based chromophore-radical systems with different phenylene linkers and radical counterparts by both optical spectroscopy and transient electron paramagnetic resonance (EPR) techniques. Femtosecond transient absorption measurements demonstrate chromophore triplet state formation on a picosecond time scale for PDI-trityl dyads, while excited state deactivation is found to be slowed down considerably in a PDI-nitroxide analogue. The subsequent investigation of the coherent spin properties by transient EPR confirms quartet state formation by triplet-doublet spin mixing for all investigated dyads and the suitability of the two studied PDI-trityl dyads as spin qubit candidates. In particular, we show that using tetrathiaryl trityl as the radical counterpart, an intense spin polarisation is observed even at room temperature and quartet state coherence times of 3.0 µs can be achieved at 80 K, which represents a considerable improvement compared to previously studied systems.

Femtosecond Spectroscopy on a Dibenzophenazine-Cored Macrocycle Exhibiting Thermally Activated Delayed Fluorescence.

The photophysics of a thermally activated delayed fluorescence (TADF) emitting macrocycle consisting of two dibenzo[a,j]phenazine acceptor moieties bridged by two N,N,N',N'-tetraphenylene-1,4-diamine donor units was scrutinized in solution by steady-state and time-resolved spectroscopy. The fluorescence lifetime of the compound proved to be strongly solvent-dependent. It ranges from 6.3 ns in cyclohexane to 34 ps in dimethyl sulfoxide. In polar solvents the fluorescence decay is predominantly due to internal conversion. In non-polar ones radiative decay and intersystem crossing contribute. Contrary to the behaviour in polymer matrices (S. Izumi et al., J. Am. Chem. Soc. 2020, 142, 1482) the excited state decay is not predominantly due to prompt and delayed fluorescence. The solvent-dependent behaviour is analyzed with the aid of quantum chemical computations.

A Thermally Activated Delayed Fluorescence Emitter Investigated by Time-Resolved Near-Infrared Spectroscopy.

Emitters for organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) require small singlet (S1 )-triplet (T1 ) energy gaps as well as fast intersystem crossing (ISC) transitions. These transitions can be mediated by vibronic mixing with higher excited states Sn and Tn (n=2, 3, 4, …). For a prototypical TADF emitter consisting of a triarylamine and a dicyanobenzene moiety (TAA-DCN) it is shown that these higher states can be located energetically by time-resolved near-infrared (NIR) spectroscopy.

The photophysics of 2-cyanoindole probed by femtosecond spectroscopy.

The photophysics of 2-cyanoindole (2-CI) in solution (water, 2,2,2-trifluoroethanol, acetonitrile' and tetrahydrofuran) was investigated by steady-state as well as time resolved fluorescence and absorption spectroscopy. The fluorescence quantum yield of 2-cyanoindole is strongly sensitive to the solvent. In water the quantum yield is as low as 4.4 × 10-4. In tetrahydrofuran, it amounts to a yield of 0.057. For 2-CI dissolved in water, a bi-exponential fluorescence decay with time constants of ∼1 ps and ∼8 ps is observed. For short wavelength excitation (266 nm) the initial fluorescence anisotropy is close to zero. For excitation with 310 nm it amounts to 0.2. In water, femtosecond transient absorption reveals that the fluorescence decay is solely due to internal conversion to the ground state. In aprotic solvents, the fluorescence decay takes much longer (acetonitrile: ∼900 ps, tetrahydrofuran: ∼2.6 ns) and intersystem crossing contributes.

Accessing the triplet state of perylenediimide by radical-enhanced intersystem crossing.

Owing to their exceptional photophysical properties and high photostability, perylene diimide (PDI) chromophores have found various applications as building blocks of materials for organic electronics. In many light-induced processes in PDI derivatives, chromophore excited states with high spin multiplicities, such as triplet or quintet states, have been revealed as key intermediates. The exploration of their properties and formation conditions is thus expected to provide invaluable insight into their underlying photophysics and promises to reveal strategies for increasing the performance of optoelectronic devices. However, accessing these high-multiplicity excited states of PDI to increase our mechanistic understanding remains a difficult task, due to the fact that the lowest excited singlet state of PDI decays with near-unity quantum yield to its ground state. Here we make use of radical-enhanced intersystem crossing (EISC) to generate the PDI triplet state in high yield. One or two 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) stable radicals were covalently attached to the imide position of PDI chromophores with and without p-tert-butylphenoxy core substituents. By combining femtosecond UV-vis transient absorption and transient electron paramagnetic resonance spectroscopies, we demonstrate strong magnetic exchange coupling between the PDI triplet state and TEMPO, resulting in the formation of excited quartet or quintet states. Important differences in the S1 state deactivation rate constants and triplet yields are observed for compounds bearing PDI moieties with different core substitution patterns. We show that these differences can be rationalized by considering the varying importance of competitive excited state decay processes, such as electron and excitation energy transfer. The comparison of the results obtained for different PDI-TEMPO derivatives leads us to propose design guidelines for optimizing the efficiency of triplet sensitization in molecular assemblies by EISC.

Synthesis and Photophysics of Water-Soluble Psoralens with Red-Shifted Absorption.

8-Methoxypsoralen (8-MOP) serves as a PUVA (psoralen + UV-A) agent in the treatment of certain skin diseases. Derivatives of 8-MOP with cationic aromatic substituents at the five positions were synthesized and characterized by steady-state, femtosecond and nanosecond spectroscopy as well as cyclic voltammetry. The aromatic substituents' positive charge increases the water solubility and the affinity toward intercalation into DNA. The aromatic substituents were supposed to lower the psoralen S1 energy and thereby suppress a photo-induced electron transfer (PET) with guanine-bearing DNA. Such a suppression of this PET is expected to increase the propensity of psoralens to photo-addition to DNA. For derivatives bearing methylpyridinium residues, femtosecond spectroscopy revealed an intramolecular PET occurring on the picosecond time scale. This PET precludes the population of the triplet state. As triplet states are the precursor state for the photo-addition to DNA, their intermolecular PET renders these derivatives ineffective in terms of PUVA. For two derivatives bearing trimethylphenylammonium moieties, such an intramolecular PET does not occur and the triplet state is populated. Surprisingly, these compounds also exhibit no PUVA activity. Based on these findings, implications for further optimization of PUVA agents are discussed.

Acridones: Strongly Emissive HIGHrISC Fluorophores.

An acridone derivative (N-methyl-difluoro-acridone, NMA-dF) is characterized with respect to its utility as an emitter in organic light emitting diodes (OLEDs). Using steady-state and time-resolved spectroscopy as well as quantum chemistry, its ability to convert singlet and triplet excitons into light was scrutinized. NMA-dF emits in the deep blue range of the visible spectrum. Its fluorescence emission occurs with quantum yields close to 1 and a radiative rate constant of ≈5 × 108 s-1. So, it processes singlet excitons very efficiently. Using 1,4-dichlorobenzene as a sensitizer, it is shown that NMA-dF also converts triplet excitons into light. With the aid of quantum chemistry, this is related to a reverse intersystem crossing starting from a higher triplet state (HIGHrISC).

Excitation Energy Transfer and Exchange-Mediated Quartet State Formation in Porphyrin-Trityl Systems.

Photogenerated multi-spin systems hold great promise for a range of technological applications in various fields, including molecular spintronics and artificial photosynthesis. However, the further development of these applications, via targeted design of materials with specific magnetic properties, currently still suffers from a lack of understanding of the factors influencing the underlying excited state dynamics and mechanisms on a molecular level. In particular, systematic studies, making use of different techniques to obtain complementary information, are largely missing. This work investigates the photophysics and magnetic properties of a series of three covalently-linked porphyrin-trityl compounds, bridged by a phenyl spacer. By combining the results from femtosecond transient absorption and electron paramagnetic resonance spectroscopies, we determine the efficiencies of the competing excited state reaction pathways and characterise the magnetic properties of the individual spin states, formed by the interaction between the chromophore triplet and the stable radical. The differences observed for the three investigated compounds are rationalised in the context of available theoretical models and the implications of the results of this study for the design of a molecular system with an improved intersystem crossing efficiency are discussed.

Tracing the Photoaddition of Pharmaceutical Psoralens to DNA.

The psoralens 8-methoxypsoralen (8-MOP), 4,5',8-trimethylpsoralen (TMP) and 5-methoxypsoralen (5-MOP) find clinical application in PUVA (psoralen + UVA) therapy. PUVA treats skin diseases like psoriasis and atopic eczema. Psoralens target the DNA of cells. Upon photo-excitation psoralens bind to the DNA base thymine. This photo-binding was studied using steady-state UV/Vis and IR spectroscopy as well as nanosecond transient UV/Vis absorption. The experiments show that the photo-addition of 8-MOP and TMP involve the psoralen triplet state and a biradical intermediate. 5-MOP forms a structurally different photo-product. Its formation could not be traced by the present spectroscopic technique.

Optimized amplitude modulation in femtosecond stimulated Raman microscopy.

In femtosecond stimulated Raman microscopy, two laser pulses (Raman pump and probe) interact at the focus of a scanning microscope. To retrieve the Raman signature of the sample, an amplitude modulation of the pump pulses is necessary. Here, different methods to achieve this modulation are presented and compared.

The Photoaddition of a Psoralen to DNA Proceeds via the Triplet State.

Psoralens are natural compounds that serve in the light dependent treatment of certain skin diseases (PUVA therapy). They are DNA intercalators that upon photoexcitation form adducts with thymine bases. For one psoralen derivative, 4'-aminomethyl-4,5',8-trimethylpsoralen (AMT), the photoreactions are characterized here by nanosecond UV-vis and IR absorption spectroscopy. The triplet state of AMT is identified as the reactive one. On the 1-10 µs time scale this local triplet state transforms into a triplet biradical bearing one single bond between the addends. Within ∼50 µs this biradical forms the final adduct featuring a cyclobutane ring. This kinetic behavior is in stark contrast to the closely related photoaddition of two thymine moieties within the DNA. Origins of the differences are discussed.

On the large apparent Stokes shift of phthalimides.

The photophysics of N-methylphthalimide (MP) in solution (cyclohexane, ethanol, acetonitrile, and water) was characterized by steady state as well as time resolved fluorescence and absorption spectroscopy. In all solvents the compound exhibits an unusually large Stokes shift of ∼10 000 cm-1. It is attributed to an ultrafast (<100 fs) depletion of the initially excited state, which results in the population of a weakly emitting state. Quantum chemical computations (DFT-MRCI) support this. They identify two energetically low-lying singlet ππ* excitations of different oscillator strength. Whereas the Stokes shift and thereby the ultrafast depletion of the initial excitation are hardly affected by the solvent later processes respond strongly. The fluorescence lifetime varies from ∼10 ps (cyclohexane) to ∼3 ns (water). This is attributed to a varying energetic accessibility of nπ* excitations.

Decay Pathways of Thymine Revisited.

The decay of electronically excited states of thymine (Thy) and thymidine 5'-monophosphate (TMP) was studied by time-resolved UV/vis and IR spectroscopy. In addition to the well-established ultrafast internal conversion to the ground state, a so far unidentified UV-induced species is observed. In D2O, this species decays with a time constant of 300 ps for thymine and of 1 ns for TMP. The species coexists with the lowest triplet state and is formed with a comparably high quantum yield of about 10% independent of the solvent. The experimentally determined spectral signatures are discussed in the light of quantum chemical calculations of the singlet and triplet excited states of thymine.

Impact of Mono-Fluorination on the Photophysics of the Flavin Chromophore.

Three mono-fluorinated derivatives of the flavin core system 10-methyl-isoalloxazine (MIA) were synthesized. Aqueous solutions of these compounds were characterized by steady-state and time-resolved spectroscopy. The positions for the fluorination (6, 7 and 8) were motivated by the nodal structure of the frontier orbitals of MIA. In comparison with MIA, the fluorination results in bathochromic (6F- and 7F-MIA) and hypsochromic (8F-MIA) shifts of the adiabatic excitation energy of the lowest allowed transition. Shifts of up to ~500 cm-1 were observed. These spectroscopic shifts go along with changes in fluorescence quantum yields and lifetimes. In addition, triplet yields are affected. For 7F-MIA, a 50% increase in the fluorescence quantum yield as well as a 50% decrease in triplet yield is observed rendering the compound interesting for fluorescence applications. The measured effects are discussed in terms of qualitative perturbation theory.

Triplet Harvesting with a Simple Aromatic Carbonyl.

The efficiency of organic light-emitting diodes crucially depends on triplet harvesters. These accept energy from triplet correlated electron hole pairs and convert it into light. Here, experimental evidence is given that simple aromatic carbonyls, such as thioxanthone, could serve this purpose. In these compounds, the emissive 1 ππ* excitation may rapidly equilibrate with an upper triplet state (3 nπ*). This equilibrium may persist for nanoseconds. Population of the 3 nπ* state via energy transfer from an electron hole pair should result in fluorescence emission and thereby triplet harvesting. To demonstrate the effect, solutions of 1,4-dichlorobenzene (triplet sensitizer) and thioxanthone (harvester) were excited at 266 nm with a nanosecond laser. The emission decay reveals a 100 ns decay absent in the thioxanthone only sample. This matches predictions for an energy transfer limited by diffusion and gives clear evidence that thioxanthone can convert triplet excitations into light.

Femtosecond Spectroscopy of Calcium Dipicolinate-A Major Component of Bacterial Spores.

Bacterial spores are rich in calcium dipicolinate (CaDPA). The role of this compound in the high UV resistance of spore DNA and their unique DNA photochemistry is not yet clarified. Here, the photophysical properties of CaDPA dissolved in water are studied by means of steady-state and time-resolved spectroscopy as well as quantum chemistry. Upon 255 nm excitation, a fluorescence emission with a yield of 1.7 × 10(-5) is detected. This low yield is in line with a measured fluorescence lifetime of 110 fs. Transient absorption experiments point to further transitions with time constants of 92 ps and 6.8 µs. The microsecond time constant is assigned to the decay of a triplet state. The yield of this state is close to unity. With the aid of quantum chemistry (TD-DFT, DFT-MRCI), the following transitions are identified. The primarily excited (1)ππ* state depletes within 110 fs. The depletion results in the population of an energetically close lying (1)nπ* state. An El-Sayed allowed intersystem crossing process with a time constant of 92 ps ensues. Implications of these findings on the interaction between photoexcited CaDPA and spore DNA are discussed.