Lowest Triplet State Research Articles

Tunability of triplet excited states and photophysical behaviour of bis-cyclometalated iridium(III) complexes with imidazo[4,5-f][1,10]phenanthroline.

This is a comprehensive study of the photophysical behaviour of heteroleptic iridium(III) complexes with imidazo[4,5-f][1,10]phenanthroline (imphen) as an ancillary ligand, represented by the general formula [Ir(N∩C)2(imphen)]PF6. As cyclometalating ligands, 2-phenylpyridine (Hppy), 2-phenylquinoline (Hpquin), 2-phenylbenzothiazole (Hpbztz), and 2-(2-pyridyl)benzothiophene (pybzthH) were used. The impact of structural modifications of cyclometalating ligands was widely explored by a combination of steady-state and time-resolved optical techniques accompanied by theoretical calculations. We evidenced that the cyclometalating ligands induce essential changes in the nature of the emissive excited state and the emission characteristics of [Ir(N∩C)2(imphen)]PF6. While the complex [Ir(ppy)2(imphen)]PF6 (1) is a typical 3MLLCT emitter, the lowest triplet states of [Ir(pquin)2(imphen)]PF6 (2), [Ir(pbztz)2(imphen)]PF6 (3) and [Ir(pybzth)2(imphen)]PF6 (4) have a predominant 3LCN∩C character. The phosphorescence colour of the investigated Ir(III) complexes changes from greenish-yellow to red, their quantum yields vary from 56 to 2%, and their triplet excited-state lifetimes fall in the 743-3840 ns range. The highest photoluminescence quantum yield was revealed for 2 in CH2Cl2, while complex 3 in MeCN shows the most pronounced increase in the lifetime. Both complexes 2 and 3 show an increased efficiency of singlet oxygen generation. The herein discussed structure-property relationships are of high significance for controlling photoinduced processes in heteroleptic iridium(III) complexes with the imphen-based ancillary ligand, and making further progress in effectively tuning the emission energies, quantum yields and excited-state lifetimes of these systems by structural modifications of cyclometalating ligands, especially the π-conjugation, the position of the N-donor and the presence of sulfur heteroatoms.

The effect of heavy atoms on the deactivation of electronic excited states of dye molecules near the surface of metal nanoparticles.

The influence of a heavy atom and the plasmon field on the efficiency of populating the lowest triplet state (T1) and on the phosphorescence intensity has been studied for fluorescein, 2Br-fluorescein, eosin and erythrosine, which have an increasing number of substituted heavy atoms. We show that the heavy atoms affect not only the rate constant of intersystem crossing (kISC) but also the rate constant of internal conversion (kIC). The calculations show that the C-H bonds in the meso position are the primary acceptors of the excitation energy of the lowest excited electronic singlet state (S1). Substitution of the meso hydrogen atoms with I or Br leads to a smaller kIC rate constant of 1 × 108 s-1 for fluorescein to 8 × 106 s-1 for eosin. Substitution with heavy atoms also leads to a larger ISC rate constant (kISC) between the T2 and S1 states because the spin-orbit coupling matrix element 〈S1|HSO|T2〉 increases by two orders of magnitude from 0.36 cm-1 for fluorescein to 35.0 cm-1 for erythrosine. The phosphorescence rate constant increases by three orders of magnitude from 4.8 × 10 s-1 for fluorescein to 3.3 × 104 s-1 for erythrosine, which is supported by experimental data. The plasmon effect increases the intensity of the xanthene dye emissions. The intensity and the quantum yield of fluorescence increase in the series fluorescein < 2Br-fluorescein < eosin < erythrosine. The intensity of the delayed fluorescence and phosphorescence grows in the same way. The enhancement factor of the phosphorescence intensity increases from 1.8 to 5.6 in the series from fluorescein to erythrosine. The differences in the plasmon effect originate from intensity borrowing to the radiative triplet-singlet transition (T1 → S0) from the singlet-singlet transitions (Sn → S0), which is more efficient when molecules have heavy atoms in the meso position.

Simultaneous photoactivation of a fluoroquinolone antibiotic and nitric oxide with fluorescence reporting.

The achievement of smart pharmaceuticals whose bioactivity can be spatiotemporally controlled by light stimuli is known as photopharmacology, an emerging area aimed at improving the therapeutic outcome and minimizing side effects. This is especially attractive for antibiotics, for which the inevitable development of multidrug resistance and the dwindling of new clinically approved drugs represent the main drawbacks. Here, we show that nitrosation of the fluoroquinolone norfloxacin (NF), a broad-spectrum antibiotic, leads to the nitrosated bioconjugate NF-NO, which is inactive at the typical minimum inhibitory concentration of NF. Irradiation of NF-NO with visible blue light triggers the simultaneous release of NF and nitric oxide (NO). The photouncaging process is accompanied by the revival of the typical fluorescence emission of NF, quenched in NF-NO, which acts as an optical reporter. This permits the real-time monitoring of the photouncaging process, even within bacteria cells where antibacterial activity is switched on exclusively upon light irradiation. The mechanism of photorelease seems to occur through a two-step hopping electron transfer mediated by the lowest triplet state of NF-NO and the phosphate buffer ions or aminoacids such as tyrosine. Considering the well-known role of NO as an "unconventional" antibacterial, the NF-NO conjugate may represent a potential bimodal antibacterial weapon activatable on demand with high spatio-temporal control.

Theoretical and experimental investigation of exciplex-forming and electroluminescent properties of 4-ethyl-3,5-bis[40-(N, N-diphenylamine)biphenyl-4-yl]-4H-1,2,4-triazole

The recently synthesized 4-ethyl-3,5-bis [40-(N, N-diphenylamine)biphenyl-4-yl]-4H-1,2,4-triazole (TPA-TZ) is in this work thoroughly studied as an efficient luminophore with an account of density functional theory. The intense fluorescence is explained as an intramolecular charge-transfer transition from the diphenylamine (donor) moiety to the central part of the dye with a characteristic electron redistribution in the triazole ring (acceptor). The photoluminescence (PL) and phosphorescence (Ph) spectra of TPA-TZ in the film and in THF solution at 77 K were studied together with the well-known 2,4,6-tris [3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T) dye. The latter compound was used as an electron-withdrawing component of the investigated TPA-TZ: PO-T2T exciplex, which is characterized by a relatively high energy level of the excited lowest triplet state. The films of a solid-state mixture of TPA-TZ and PO-T2T showed exciplex-type emission being slightly red-shifted compared to the PL of the individual donor and acceptor compounds. Two OLED devices with emitting layers of TPA-TZ and of the exciplex-forming molecular mixture of TPA-TZ and PO-T2T were prepared. The exciplex–based OLED demonstrated 4 103 cd/m2 brightness, a current efficiency of 22 cd/A, a power efficiency of 6.77 lm/W, and an external quantum efficiency (EQE) of 7%. This confirms the possibility of triplet harvesting in the molecular mixture of TPA-TZ and PO-T2T, since the theoretical EQE limit for OLED based on the prompt fluorescence is 5%.

Photoexcited triplet state and singlet oxygen generation of quinine, an antimalarial drug.

Energy transfer from the lowest excited triplet (T1) state of quinine (QN) to ground-state molecular oxygen produces singlet oxygen. In aqueous solutions, a neutral form QN, a singly protonated cation QNH+ and doubly protonated cation QNH22+ are present according to their pKa values. To the best of our knowledge, the pH dependence of QN-photosensitized singlet oxygen generation has not been reported. In the present study, the quantum yields of photosensitized singlet oxygen generation (ΦΔ) by QN, QNH+ and QNH22+ have been determined through the measurements of time-resolved near-IR phosphorescence. ΦΔ decreases in the following order: ΦΔ (QNH+) > ΦΔ (QNH22+) > ΦΔ (QN). The nature of the T1 states of QN, QNH+ and QNH22+ has been studied through the measurements of transient absorption, phosphorescence and EPR by changing the pH of the medium. This is the first report of EPR for the T1 state of QN. The photoexcited T1 state of 6-methoxyquinoline (6-MeOQL), a closely related component, has been studied for comparison. The observed zero-field splitting parameters, phosphorescence spectra and triplet lifetimes suggest that the nature of the T1 state of QN can be regarded as a locally excited 3ππ*state within 6-MeOQL. The two unpaired electrons localize mainly on 6-MeOQL. The nature of the T1 state of QN scarcely changes when the quinuclidine nitrogen site is protonated. Applying the Förster cycle to the T1 states of QN and its protonated cations, it was found that QNH+ becomes more basic when excited to its T1 state.

Encoding Hole-Particle Information in the Multi-Channel MolOrbImage for Machine-Learned Excited-State Energies of Large Photofunctional Materials.

We present a novel class of one-electron multi-channel molecular orbital images (MolOrbImages) designed for the prediction of excited-state energetics in conjunction with the state-of-the-art VGG-type machine-learning architecture. By representing hole and particle states in the excitation process as channels of MolOrbImages, the revised VGG model achieves excellent prediction accuracy for both low-lying singlet and triplet states, with mean absolute errors (MAEs) of <0.08 and <0.1 eV for QM9 molecules and large photofunctional materials with up to 560 atoms, respectively. Remarkably, the model demonstrates exceptional performance (MAE < 1 kcal/mol) for the T1 state of QM9 molecules, making it a non-system-specific model that approaches chemical accuracy. The general rules attained, for instance, the improved performance with well-defined MO energies and the reduced overfitting concern via the inclusion of physically insightful hole-particle information, provide invaluable guidelines for the further design of orbital-based descriptors targeting molecular excited states.

Exploring Thioxanthone Derivatives as Singlet Oxygen Photosensitizers for Photodynamic Therapy at the Near-IR Region.

In the lowest excited triplet state, the excited photosensitizer reacts with tissue oxygen and forms reactive oxygen species (ROS), which kills tissue cells in photodynamic therapy (PDT). Metal-free thio-based pure organic molecules and analogous nucleobases can be used as photosensitizers for PDT applications. Using quantum chemical methods, we studied one- and two-photon optical absorptions, fluorescence, and other excited-state properties of substituted thioxanthone derivatives for their potential as photosensitizers for PDT. Our calculated values were compared with the available experimental data. The calculation of the intersystem crossing rate constant for these photosensitizers explains the high quantum yield of the formation of ROS, as reported experimentally. The excited triplet-state population of the photosensitizer occurs through the 1π-π* → 3n-π* channel of intersystem crossing and increases in the presence of halogen substitution.

A Novel Characteristic of a Water-soluble UV-B Absorber, 2-Phenylbenzimidazole-5-sulfonic Acid: Suppression of Riboflavin-sensitized Singlet Oxygen Generation

Abstract 2-Phenylbenzimidazole-5-sulfonic acid (PBSA) is a water-soluble UV-B absorber used in cosmetic sunscreens. Riboflavin (RF) is a water-soluble vitamin B2. RF is an efficient singlet oxygen photosensitizer. The effects of PBSA on RF-photosensitized singlet oxygen generation have been studied through measurements of transient absorption and time-resolved near-IR phosphorescence in phosphate buffer (pH 7.4). PBSA suppresses the RF-photosensitized singlet oxygen generation. The observed suppression can be ascribed to the quenching of the lowest excited triplet state of RF by PBSA.

Photo-physical characterization of high triplet yield brominated fluoresceins by transient state (TRAST) spectroscopy

Photo-induced dark transient states of fluorophores can pose a problem in fluorescence spectroscopy. However, their typically long lifetimes also make them highly environment sensitive, suggesting fluorophores with prominent dark-state formation yields to be used as microenvironmental sensors in bio-molecular spectroscopy and imaging. In this work, we analyzed the singlet–triplet transitions of fluorescein and three synthesized carboxy-fluorescein derivatives, with one, two or four bromines linked to the anthracence backbone. Using transient state (TRAST) spectroscopy, we found a prominent internal heavy atom (IHA) enhancement of the intersystem crossing (ISC) rates upon bromination, inferred by density functional theory calculations to take place via a higher triplet state, followed by relaxation to the lowest triplet state. A corresponding external heavy atom (EHA) enhancement was found upon adding potassium iodide (KI). Notably, increased KI concentrations still resulted in lowered triplet state buildup in the brominated fluorophores, due to relatively lower enhancements in ISC, than in the triplet decay. Together with an antioxidative effect on the fluorophores, adding KI thus generated a fluorescence enhancement of the brominated fluorophores. By TRAST measurements, analyzing the average fluorescence intensity of fluorescent molecules subject to a systematically varied excitation modulation, dark state transitions within very high triplet yield (>90%) fluorophores can be directly analyzed under biologically relevant conditions. These measurements, not possible by other techniques such as fluorescence correlation spectroscopy, opens for bio-sensing applications based on high triplet yield fluorophores, and for characterization of high triplet yield photodynamic therapy agents, and how they are influenced by IHA and EHA effects.

Significance of Nonadiabatic Effects on Efficient Triplet Generation in Lumazines.

The photophysics of lumazines leading to triplet formation and the effect of thionation are explored in the presence of near-degenerate electronic states. Wave packet simulations are performed on model potential energy surfaces to understand the nonadiabatic population transfer among close-lying excited states. Ultrafast population transfer among singlets opens up new intersystem crossing channels from the higher states. An increased spin-orbit coupling strength originating from thionation enhances intersystem crossing and populates the higher triplets first. The rapid internal conversion in the triplet manifold eventually brings the molecules to their respective low-lying long-lived triplet state.

Gas phase Elemental abundances in Molecular cloudS (GEMS)

Context. Carbon monosulphide (CS) is among the few sulphur-bearing species that have been widely observed in all environments, including in the most extreme, such as diffuse clouds. Moreover, CS has been widely used as a tracer of the gas density in the interstellar medium in our Galaxy and external galaxies. Therefore, a complete understanding of its chemistry in all environments is of paramount importance for the study of interstellar matter. Aims. Our group is revising the rates of the main formation and destruction mechanisms of CS. In particular, we focus on those involving open-shell species for which the classical capture model might not be sufficiently accurate. In this paper, we revise the rates of reactions CH + S → CS + H and C2 + S → CS + C. These reactions are important CS formation routes in some environments such as dark and diffuse warm gas. Methods. We performed ab initio calculations to characterize the main features of all the electronic states correlating to the open shell reactants. For CH+S, we calculated the full potential energy surfaces (PESs) for the lowest doublet states and the reaction rate constant with a quasi-classical method. For C2+S, the reaction can only take place through the three lower triplet states, which all present deep insertion wells. A detailed study of the long-range interactions for these triplet states allowed us to apply a statistic adiabatic method to determine the rate constants. Results. Our detailed theoretical study of the CH + S → CS + H reaction shows that its rate is nearly independent of the temperature in a range of 10–500 K, with an almost constant value of 5.5 × 10−11 cm3 s−1 at temperatures above 100 K. This is a factor of about 2–3 lower than the value obtained with the capture model. The rate of the reaction C2 + S → CS + C does depend on the temperature, and takes values close to 2.0 × 10−10 cm3 s−1 at low temperatures, which increase to ~ 5.0 × 10−10 cm3 s−1 for temperatures higher than 200 K. In this case, our detailed modeling - taking into account the electronic and spin states – provides a rate that is higher than the one currently used by factor of approximately 2. Conclusions. These reactions were selected based on their inclusion of open-shell species with many degenerate electronic states, and, unexpectedly, the results obtained in the present detailed calculations provide values that differ by a factor of about 2–3 from the simpler classical capture method. We updated the sulphur network with these new rates and compare our results in the prototypical case of TMC1 (CP). We find a reasonable agreement between model predictions and observations with a sulphur depletion factor of 20 relative to the sulphur cosmic abundance. However, it is not possible to fit the abundances of all sulphur-bearing molecules better than a factor of 10 at the same chemical time.

Adaptive Aromaticity in 18e Metallapentalenes.

Complexes with aromaticity in both the lowest singlet state (S0) and the lowest triplet state (T1) (denoted as adaptive aromaticity) are rare because according to Hückel's and Baird's rules, a species could be aromatic in either the S0 or T1 state in most cases. Thus, it is particularly challenging to design species with adaptive aromaticity. Previous reports on adaptive aromaticity were mainly focused on 16e metallapentalenes. Here, we demonstrate that 18e metallapentalenes could possess adaptive aromaticity supported by a set of aromaticity indices when the nitrido and imido ligands are introduced via density functional theory calculations. Further investigation suggests that the metal-carbon bond strength plays an important role in the S0 state aromaticity and the T1 state aromaticity could be attributed to spin electron localization. All these findings could be useful for the development of metallaaromatic chemistry.

Identification of Unknown Inverted Singlet-Triplet Cores by High-Throughput Virtual Screening.

Molecules where the energy of the lowest excited singlet state is found below the energy of the lowest triplet state (inverted singlet-triplet molecules) are extremely rare. It is particularly challenging to discover new ones through virtual screening because the required wavefunction-based methods are expensive and unsuitable for high-throughput calculations. Here, we devised a virtual screening approach where the molecules to be considered with advanced methods are pre-selected with increasingly more sophisticated filters that include the evaluation of the HOMO-LUMO exchange integral and approximate CASSCF calculations. A final set of 7 candidates (0.05% of the initial 15 000) were verified to possess inversion between singlet and triplet states with state-of-the-art multireference methods (MS-CASPT2). One of them is deemed of particular interest because it is unrelated to other proposals made in the literature.

Multi-Mode Photoluminescence Regulation in a Zero-Dimensional Organic-Inorganic Hybrid Metal Halide Perovskite─[(CH3)4N]2SnCl6.

Metal ion-doped zero-dimensional halide perovskites provide good platforms to generate broadband emission and explore the fundamental dynamics of emission regulations. Recently, Sb3+-doped zero-dimensional halide perovskites have attracted considerable attention for the high quantum yield of yellow emission; however, the triplet state recombination is activated and the singlet state emission is usually absent. Herein, we fabricate an Sb3+-doped zero-dimensional [(CH3)4N]2SnCl6 perovskite that can induce singlet and triplet emission. Density functional theory calculation shows that there are some overlaps between the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals, which may induce a large energy separation between the lowest excited triplet states (T1) and the lowest excited singlet states (S1) [ΔE(S1 - T1)], impeding all the carriers' transfer from the singlet state to the triplet state. As a result, the reserved singlet emission together with the triplet emission can be regulated by excitation wavelength in situ. In addition, different Bi3+ ratios are co-doped into Sb3+@[(CH3)4N]2SnCl6, resulting in a photoluminescence ex situ regulation. Single-phase white light LED and optical anti-counterfeiting are developed further.

Effects of the change of isomers on room-temperature phosphorescence, thermally activated delayed fluorescence, and long persistent luminescence of organic hole-transporting materials with the selective potential for the application in electronic devices and optical sensors of oxygen

Competition of room-temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF) was investigated on the example of four newly synthesised organic isomeric compounds containing two electron-donating triphenylamine moieties and single electron-accepting dibenzothiophene-2-yl(phenyl)methanone unit by numerous theoretical and experimental approaches. The different emission origin (RTP or TADF) of the isomeric compounds was caused by the different energy gaps between the lowest singlet and triplet states locally exited and charge transfer in nature. Very different external quantum efficiencies ranging from 2.8 to 13.9% were observed for green phosphorescent organic light-emitting diodes fabricated using the isomeric compounds as the hosts. Such device efficiency differences were related to the different hole mobility values. The hole mobilities of ca. 1 × 10−3 cm2V−1s−1 were observed at an electric field of 6.4 × 105 V/cm for the derivatives having triphenylamine moieties at C-2, C-4, and at C-3, C-5 positions of the central benzene ring. Considerably lower hole mobilities of ca. 1 × 10−5 cm2 V−1 s−1 were recorded at the same electric field for the compounds with triphenylamine groups at C-2, C-3, and at C-2, C-6 positions. Depending to the great extent on RTP and TADF processes, different efficiencies of long persistent luminescence (LPL) of the exciplex-forming solid-state mixtures of the isomers and bis[2-(diphenylphosphine)phenyl] ether oxide (DPEPO) were detected. The compound with triphenylamine groups at C-3, and C-5 positions as RTP emitter and its molecular mixture with DPEPO as LPL emitter the active layers of optical sensors of oxygen were prepared. They showed the Stern–Volmer constant of 4.55 × 10−4 ppm in the range of oxygen concentrations up to 10000 ppm.

On the photobehaviour of curcumin in biocompatible hosts: The role of H-abstraction in the photodegradation and photosensitization.

Curcumin (CUR) is a naturally occurring pigment extensively studied due to its therapeutic activity and delivered by suitable nanocarriers to overcome poor solubility in aqueous media. The significant absorption of CUR in the visible blue region has prompted its use as a potential phototherapeutic agent in treating infectious and cancer diseases, although the mechanism underlying the phototoxic effects is still not fully understood. This contribution investigates the photobehaviour of CUR within polymeric micelles, microemulsions, and zein nanoparticles, chosen as biocompatible nanocarriers, and human serum albumin as a representative biomolecule. Spectroscopic studies indicate that in all host systems, the enolic tautomeric form of CUR is converted in a significant amount of the diketo form because of the perturbation of the intramolecular hydrogen bond. This leads to intermolecular H-abstraction from the host components by the lowest excited triplet state of CUR with the formation of the corresponding ketyl radical, detected by nanosecond laser flash photolysis. This radical is oxidized by molecular oxygen, likely generating peroxyl and hydroperoxyl radical species, unless in Zein, reasonably due to the poor availability of oxygen in the closely packed structure of this nanocarrier. In contrast, no detectable formation of singlet oxygen was revealed in all the systems. Overall these results highlight the key role of the H-abstraction process over singlet oxygen sensitization as a primary photochemical pathway strictly dictated by the specific features of the microenvironment, providing new insights into the photoreactivity of CUR in biocompatible hosts that can also be useful for a better understanding of its phototoxicity mechanism.

Adaptive σ aromaticity in the rhenacyclopropene rings.

Species generally exhibit one-state aromaticity either in the lowest singlet state (S0 ) or the lowest triplet state (T1 ) according to the Hückel's and Baird's rules. Hence, it is rare for species exhibit two-state aromaticity in both the S0 and T1 states (termed as adaptive aromaticity), let alone adaptive σ aromaticity. Here, we report adaptive σ aromaticity in unsaturated rhenacyclopropene rings via density functional theory (DFT) calculations. Various aromaticity indices including NICS, ACID, EDDB together with isodesmic reactions support the adaptive σ aromaticity in these rhenacyclopropene rings. As the T1 state of these species is formed by the ππ* excitation, the σ-aromaticity of these three-membered rings in the S0 state could hold in the T1 state. In addition, the aromaticity effect of the fused rings is also examined. Our findings expand the family of adaptive σ aromaticity, enriching the metallaaromatic chemistry.

Spin-vibronic coherence drives singlet-triplet conversion.

Design-specific control over the transitions between excited electronic states with different spin multiplicities is of the utmost importance in molecular and materials chemistry1-3. Previous studies have indicated that the combination of spin-orbit and vibronic effects, collectively termed the spin-vibronic effect, can accelerate quantum-mechanically forbidden transitions at non-adiabatic crossings4,5. However, it has been difficult to identify precise experimental manifestations of the spin-vibronic mechanism. Here we present coherence spectroscopy experiments that reveal the interplay between the spin, electronic and vibrational degrees of freedom that drive efficient singlet-triplet conversion in four structurally related dinuclear Pt(II) metal-metal-to-ligand charge-transfer (MMLCT) complexes. Photoexcitation activates the formation of a Pt-Pt bond, launching a stretching vibrational wavepacket. The molecular-structure-dependent decoherence and recoherence dynamics of this wavepacket resolve the spin-vibronic mechanism. We find that vectorial motion along the Pt-Pt stretching coordinates tunes the singlet and intermediate-state energy gap irreversibly towards the conical intersection and subsequently drives formation of the lowest stable triplet state in a ratcheting fashion. This work demonstrates the viability of using vibronic coherences as probes6-9 to clarify the interplay among spin, electronic and nuclear dynamics in spin-conversion processes, and this could inspire new modular designs to tailor the properties of excited states.

Red-Light-Photosensitized NO Release and Its Monitoring in Cancer Cells with Biodegradable Polymeric Nanoparticles.

The role of nitric oxide (NO) as an "unconventional" therapeutic and the strict dependence of biological effects on its concentration require the generation of NO with precise spatiotemporal control. The development of precursors and strategies to activate NO release by excitation in the so-called "therapeutic window" with highly biocompatible and tissue-penetrating red light is desirable and challenging. Herein, we demonstrate that one-photon red-light excitation of Verteporfin, a clinically approved photosensitizer (PS) for photodynamic therapy, activates NO release, in a catalytic fashion, from an otherwise blue-light activatable NO photodonor (NOPD) with an improvement of about 300 nm toward longer and more biocompatible wavelengths. Steady-state and time-resolved spectroscopic and photochemical studies combined with theoretical calculations account for an NO photorelease photosensitized by the lowest triplet state of the PS. In view of biological applications, the water-insoluble PS and NOPD have been co-entrapped within water-dispersible, biodegradable polymeric nanoparticles (NPs) of mPEG-b-PCL (about 84 nm in diameter), where the red-light activation of NO release takes place even more effectively than in an organic solvent solution and almost independently by the presence of oxygen. Moreover, the ideal spectroscopic prerequisites and the restricted environment of the NPs permit the green-fluorescent co-product formed concomitantly to NO photorelease to communicate with the PS via Förster resonance energy transfer. This leads to an enhancement of the typical red emission of the PS offering the possibility of a double color optical reporter useful for the real-time monitoring of the NO release through fluorescence techniques. The suitability of this strategy applied to the polymeric NPs as potential nanotherapeutics was evaluated through biological tests performed by using HepG2 hepatocarcinoma and A375 melanoma cancer cell lines. Fluorescence investigation in cells and cell viability experiments demonstrates the occurrence of the NO release under one-photon red-light illumination also in the biological environment. This confirms that the adopted strategy provides a valuable tool for generating NO from an already available NOPD, otherwise activatable with the poorly biocompatible blue light, without requiring any chemical modification and the use of sophisticated irradiation sources.

Electronic excitations of the charged nitrogen-vacancy center in diamond obtained using time-independent variational density functional calculations

Elucidation of the mechanism for optical spin initialization of point defects in solids in the context of quantum applications requires an accurate description of the excited electronic states involved. While variational density functional calculations have been successful in describing the ground state of a great variety of systems, doubts have been expressed in the literature regarding the ability of such calculations to describe electronic excitations of point defects. A direct orbital optimization method is used here to perform time-independent, variational density functional calculations of a prototypical defect, the negatively charged nitrogen-vacancy center in diamond. The calculations include up to 511 atoms subject to periodic boundary conditions and the excited state calculations require similar computational effort as ground state calculations. Contrary to some previous reports, the use of local and semilocal density functionals gives the correct ordering of the low-lying triplet and singlet states, namely {}^{3}A_2 &amp;lt; {}^{1}E &amp;lt; {}^{1}A_1 &amp;lt; {}^{3}E3A2&lt;1E&lt;1A1&lt;3E. Furthermore, the more advanced meta generalized gradient approximation functionals give results that are in remarkably good agreement with high-level, many-body calculations as well as available experimental estimates, even for the excited singlet state which is often referred to as having multireference character. The lowering of the energy in the triplet excited state as the atom coordinates are optimized in accordance with analytical forces is also close to the experimental estimate and the resulting zero-phonon line triplet excitation energy is underestimated by only 0.15 eV. The approach used here is found to be a promising tool for studying electronic excitations of point defects in, for example, systems relevant for quantum technologies.