Excited Singlet Research Articles

Ultrafast Spectroelectrochemistry of the Catechol/o‐Quinone Redox Couple in Aqueous Buffer Solution

AbstractEumelanin is a natural pigment found in many organisms that provides photoprotection from harmful UV radiation. As a redox‐active biopolymer, the structure of eumelanin is thought to contain different redox states of quinone, including catechol subunits. To further explore the excited state properties of eumelanin, we have investigated the catechol/o‐quinone redox couple by spectroelectrochemical means, in a pH 7.4 aqueous buffered solution, and using a boron doped diamond mesh electrode. At pH 7.4, the two proton, two electron oxidation of catechol is promoted, which facilitates continuous formation of the unstable o‐quinone product in solution. Ultrafast transient absorption (femtosecond to nanosecond) measurements of o‐quinone species involve initial formation of an excited singlet state followed by triplet state formation within 24 ps. In contrast, catechol in aqueous buffer leads to formation of the semiquinone radical Δt>500 ps. Our results demonstrate the rich photochemistry of the catechol/o‐quinone redox couple and provides further insight into the excited state processes of these key building blocks of eumelanin.

Photochemical surface modification of amorphous carbon thin film to transparency in atmosphere of excited singlet oxygen atoms O(1D) under 157 nm F2 laser irradiation

Amorphous carbon (a-C) thin films have various useful properties; thus, their applications are wide-ranging. These properties are characterized by the ratio of sp3 to sp2 carbon in the thin film, and many studies have been reported on changing this ratio. In this paper, we used an F2 laser with a wavelength of 157 nm to photoexcite the surface of an a-C thin film, and simultaneously photodecompose atmospheric oxygen molecules O2 into excited singlet oxygen atoms O(1D). The results showed that O(1D) atoms can photochemically etch sp2 carbon in thin films significantly better than triplet oxygen atoms O(3P). Thus, we found that the surface layer of the a-C thin film was photochemically modified to be transparent. Raman spectroscopy revealed no change in the chemical bonding state of the transparently modified a-C thin film. On the other hand, in X-ray photoelectron spectroscopy, the intensity of the C 1s peak originated to sp2 carbon decreased. The refractive index was also slightly lowered, probably due to lowering film density. In addition, droplets mainly composed of sp2 carbon generated during thin film deposition were significantly removed after F2 laser irradiation. The electrical resistivity slightly decreased upon F2 laser irradiation and remarkably decreased as the substrate was heated, exhibiting low bandgap properties. Therefore, we succeeded in discovering a new method for changing the ratio of sp3 to sp2 carbon in a-C to be transparent thin film by preferentially generating O(1D) atoms using the F2 laser, as a new elemental technology for fabricating transparent flexible electronic devices.

Solvent Effects on the Absorption and Emission Spectra of the [5-amino-1-bromoindolizin-3-yl](4-bromophenyl)methanone Molecule.

The effects of solvent on the absorption and emission spectra and dipole moments of the 5ABBM have been extensively studied in a series of solvents. The dipole moments in the excited state are observed to be greater than those in the ground-state in all the solvents studied for the chosen molecule. The dipole moment increase in the excited singlet state ranges from 2.42 to 24.14 D. The experimentally calculated ground state and excited state dipole moments were determined using the solvatochromatic shifts in the absorption and emission spectra as a function of dielectric constant (ɛ) and refractive index (n). These data are used to estimate the excited-state dipole moment using the experimentally determined ground-state dipole moment. A series of fifteen different organic solvents (toluene, methanol, n-butyl alcohol, ethyl acetate, DMS, acetonitrile, benzene, isopropyl alcohol, water, DMF, DCM, DIO, THF, ethanol, and octanol) were investigated at constant dye concentrations. Small changes in the fluorescence spectrum were observed for the different solvents; the highest fluorescence intensity was observed for DMS, and the lowest was observed for water. The Stokes shift in different solvents was studied for the 5ABBM molecule. This results in molecule being more polar in the excited state than in the ground state for the solvents used. The ground state dipole moments, HOMO-LUMO, and molecular electrostatic potential maps were also computed via ab initio calculations and evaluated via Gaussian 09W software.

The rates of non-adiabatic processes in large molecular systems: Toward an effective full-dimensional quantum mechanical approach.

Two computational approaches for computing the rates of internal conversions in molecular systems where a large set of nuclear degrees of freedom plays a role are discussed and compared. One approach is based on the numerical solution of the time-dependent Schrödinger equation and allows us to include almost the whole set of vibrational coordinates, thanks to the employment of effective procedures for selecting those elements of the Hilbert space which play a significant role in dynamics. The other approach, based on the time-dependent perturbation theory and limited to the use of the harmonic approximation, allows us to include the whole Hilbert space spanned by the vibrational states of the system. The two approaches are applied to the photophysics of azulene, whose anti-Kasha behavior caused by anomalous internal conversion rates is well assessed. The calculated rates for the decays of the first two excited singlet states are in very good agreement with experimental data, indicating the reliability of both methodologies.

Multireference Correlated Oscillator Strengths from Adiabatic Connection Approaches Based on Extended Random Phase Approximation.

We show that accurate oscillator strengths can be obtained from adiabatic connection (AC) approaches based on the extended random phase approximation (ERPA) combined with multireference (complete active space, CAS) wave functions. The oscillator strengths calculated using the perturbation-corrected ERPA transition density matrices, proposed in this work, and the excitation energies calculated with recently introduced AC correlation energy methods, AC0 and AC0D, compete with accuracy in the perturbational CASPT2 approach and require less computational effort. AC0 and AC0D methods scale more favorably with the number of active orbitals than multiconfigurational perturbation approaches like CASPT2 and NEVPT2 thanks to their dependence on reduced density matrices up to the order of 2. Importantly, the newly developed approach for computing correlated transition dipole moments does not entail any additional costs, as all intermediate quantities become available when AC0 energies are being computed. We also test the performance of the recently proposed AC method corrected for the negative-transition contributions to the correlation energy, AC0D, for triplet excitation energies. Similarly, as for the singlet excitations, the correction improves the performance of the AC0 method, particularly for the low-lying excited states.

Positronium Negative Ion Embedded in Non-ideal Classical Plasmas: Doubly Excited Singlet S States

Positronium Negative Ion Embedded in Non-ideal Classical Plasmas: Doubly Excited Singlet S States

Static and resonant properties of decorated square kagomé lattice KCu[formula omitted](TeO[formula omitted])(SO[formula omitted])[formula omitted]Cl

The magnetic subsystem of nabokoite, KCu7(TeO4)(SO4)5Cl, is constituted by copper ions forming a buckled square kagomé lattice decorated by quasi-isolated ions. This combination determines peculiar physical properties of this compound evidenced in electron spin resonance (ESR) spectroscopy, dielectric permittivity ɛ, magnetization M and specific heat Cp measurements. At lowering temperature, the magnetic susceptibility χ=M/H passes through a broad hump inherent for low-dimensional magnetic systems at about 150 K and a sharp peak at antiferromagnetic phase transition at TN=3.2 K. The Cp(T,H) curves demonstrate additional peak-like anomaly at Tpeak=5.7 K robust to magnetic field. The latter can be ascribed to low-lying singlet excitations filling the singlet-triplet gap in magnetic excitation spectrum of the square kagomé lattice (Richter et al., 2022). ESR spectroscopy provides indications that antiferromagnetic structure below TN is non-collinear. Separate issue is the observation of antiferroelectric-type behavior in ɛ at low temperatures, which tentatively reduces the symmetry and partially lifts frustration of magnetic interactions of decorating copper ions with buckled square kagomé lattice. These complex thermodynamic and resonant properties signal the presence of two weakly coupled magnetic subsystems in nabokoite, namely a spin-liquid in square kagomé lattice layers and an antiferromagnet represented by decorating ions.

The photochemistry of Rydberg-excited cyclobutanone: Photoinduced processes and ground state dynamics.

Owing to ring strain, cyclic ketones exhibit complex excited state dynamics with multiple competing photochemical channels active on the ultrafast timescale. While the excited state dynamics of cyclobutanone after π* ← n excitation into the lowest-energy excited singlet (S1) state has been extensively studied, the dynamics following 3s ← n excitation into the higher-lying singlet Rydberg (S2) state are less well understood. Herein, we employ fully quantum multiconfigurational time-dependent Hartree (MCTDH) simulations using a model Hamiltonian as well as "on-the-fly" trajectory-based surface-hopping dynamics (TSHD) simulations to study the relaxation dynamics of cyclobutanone following 3s ← n excitation and to predict the ultrafast electron diffraction scattering signature of these relaxation dynamics. Our MCTDH and TSHD simulations indicate that relaxation from the initially-populated singlet Rydberg (S2) state occurs on the timescale of a few hundreds of femtoseconds to a picosecond, consistent with the symmetry-forbidden nature of the state-to-state transition involved. There is no obvious involvement of excited triplet states within the timeframe of our simulations (<2ps). After non-radiative relaxation to the electronic ground state (S0), vibrationally hot cyclobutanone has sufficient internal energy to form multiple fragmented products including C2H4 + CH2CO (C2; 20%) and C3H6 + CO (C3; 2.5%). We discuss the limitations of our MCTDH and TSHD simulations, how these may influence the excited state dynamics we observe, and-ultimately-the predictive power of the simulated experimental observable.

Indole-imidazole hybrid Schiff base for the selective detection of the explosive picric acid via fluorescence “turn-off” process: Experimental and theoretical DFT-D3 study

A compound containing a heteroatom and Schiff base, 3-(4,5-Diphenyl-1H-imidazol-2-yl)-1H-indole (INBIL) has been synthesized through the one-pot condensation of indole-3-carbaldehyde with benzil in ethanol and characterized using various analytical parameters like FT-IR, NMR; its thermal and chemical stability also established. The synthesized INBIL showed a 'turn off' luminescent property that has been used to detect picric acid. This luminescent sensor has a broad emission band at 415 nm in acetonitrile due to the conjugation of its double bond. It is worth noting that INBIL can be recycled through TEA treatment. The chemosensor exhibited the highest sensitivity with Ksv = 1.51 × 10−6 M, and a detection limit of 2.91 × 10−7 M, with perfect linearity up to 22 µL. The binding stoichiometry ratio between INBIL and picric acid was predicted to be 1:1 using Job's plot, which was confirmed by 1H NMR analysis. Finally, DFT-D3 calculations were used to calculate the excited molecule's singlet and triplet energy states.

Density Functional Theory Study on the Mechanism of Nickel-Catalyzed 3,3-Dialkynylation of 2-Aryl Acrylamides Via Double Vinylic C-H Bond Activation.

The mechanisms of Ni-catalyzed 3,3-dialkynylation of 2-aryl acrylamide have been investigated by using density functional theory calculations. The result shows that this reaction includes double alkynylation, which involves sequential key steps of vinylic C-H bond activation, successive oxidative addition, and reductive elimination, with the second C-H bond activation being the rate-determining step. C-H and N-H bond activation occurs via the concerted metalation-deprotonation mechanism. The calculations show that no transition state exists in the first reductive elimination process, and a negative free energy barrier in the second reductive elimination process though a transition state is identified, indicating that the nickel-catalyzed vinylic C(sp2)-C(sp) bond formation does not require activation energy. Z-E isomerization is the prerequisite for the second alkynylation. In addition, our spin-flip TDDFT (SF-TDDFT) computational result discloses that the actual process of Z-E isomerization occurs on the potential energy surface of the first excited singlet state S1.

Dissolved organic matter isolates obtained by solid phase extraction exhibit higher absorption and lower photo-reactivity: Effect of components

Notable differences in photo-physical and chemical properties were found between bulk water and solid phase extraction (SPE) isolates for dissolved organic matter (DOM). The moieties extracted using modified styrene divinylbenzene cartridges, which predominantly consist of conjugated aromatic molecules like humic acids, contribute mainly to light absorption but exhibit lower quantum yields of fluorescence and photo-produced reactive intermediates (PPRIs). Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) revealed lignin as the moieties displaying most significant variance in abundance. In Van Krevelen-Spearman plot, we observed molecules positively or negatively correlated with DOM's optical and photochemical properties (including SUVA254, steady-state concentrations of ·OH, 1O2 quantum yield, etc.) were confined to specific regions, which can be delineated using a threshold modified aromaticity index (AImod) of 0.3. Based on the relationships between optical properties and PPRI production, it is suggested that the energy gap between ground state and excited singlet state (△ES1→S0), governing the inner conversion rate, serves as a determinant for apparent quantum yield of PPRIs in DOM, with intra-molecular charge transfer (CT) interactions potentially playing a pivotal role. Regarding DOM's photoreactivity with pollutants, this study has revealed, for the first time, that protein/amino sugars/amino acids could act as antioxidant groups in addition to phenols on the photolysis of sulfadiazine. These findings provide valuable insights into DOM photochemistry and are expected to stimulate further research in this area.

On the Dependence of the Rate of Deactivation of Triplet Excitations of a Number of Molecules on Their Ionization Potential and the Charge of the Nucleus of Heavy Atoms of the Solvent

Purpose. Establish the nature and reasons for the dependence of the change in the radiative rate constant of deactivation of triplet excitations of a number of molecules, caused by the effect of an external heavy atom, on their ionization potential from the ground state and the energy of the first excited singlet state.Methods. Kinetic methods were used to determine the rate constant for the deactivation of triplet excitations of organic molecules in solid solutions at 77 K. The solvents were n-heptane (neutral), carbon tetrachloride and benzene bromide (containing heavy chlorine and bromine atoms). The molecules studied were coronene, triphenylene, phenanthrene, naphthalene and biphenyl.Results. A linear relationship has been established between an increase in the ionization potential, the energy value of the first excited singlet state, and an increase in the change in the rate of radiative deactivation of triplet excitations of the studied molecules in carbon tetrachloride. It is shown that the increase in the change in the radiative deactivation rate of these molecules with an increase in their ionization potential is due to a decrease in the difference between the ionization energies and the first excited singlet state.Conclusion. The results of the study showed that there is a linear relationship between the increase in the radiative deactivation rate of triplet excitations of the studied molecules and their ionization potential. A linear relationship is also observed between the increase in the rate of radiative deactivation of triplet excitations of these molecules and the first excited singlet state. As the ionization potentials of the molecules under study increase, the difference between the ionization energy and the energy of the first excited singlet state decreases. This is the reason for the increase in the rate of deactivation of triplet excitations of molecules with an increase in their ionization potential.

Effect of confinement on photoluminescence of MeLPPP/SBA-15 nanocomposite

Photoluminescence spectra and kinetics of methyl-substituted ladder-type poly(para-phenylene) (MeLPPP)/SBA-15 mesoporous silica (SBA-15) nanocomposite have been studied and compared with photoluminescence properties of the MELPPP film and solution. Relaxation processes of excited singlet states in the closed geometry of the nanocomposite and in the diluted solution compared to the film based on the temperature dependence of photoluminescence (5–220 K) were studied. It was shown that the nanocomposite has twice the photoluminescence lifetime and three times weaker temperature dependence of photoluminescence intensity in comparison with the film. These peculiarities are due to a decrease in the number of polymer chains in a low-dimensional structure and a reduced role of exciton diffusion.

Vibronic coupling and ultrafast relaxation dynamics in the first five excited singlet electronic states of bithiophene.

The vibronic structure and nuclear dynamics in the first five excited singlet electronic states of bithiophene (2T) are investigated here. Specifically, considerations are given to comprehend the first two structureless and broad electronic absorption bands and the role of nonadiabatic coupling in the excited state relaxation mechanism of 2T in the gas phase. Associated potential energy surfaces (PESs) are established by constructing a model vibronic coupling Hamiltonian using 18 vibrational degrees of freedom and extensive abinitio electronic structure calculations. The topographies of these PESs are critically examined, and multiple conical intersections are established. The nuclear dynamics calculations are performed by propagating wave packets on the coupled electronic manifold. The present theoretical results are in good agreement with the experimental observations. It is found that strong nonadiabatic coupling between the S1-S4 and S1-S5 states along totally symmetric modes is predominantly responsible for the structureless and broad first absorption band, and overlapping S2, S3, S4, and S5 states form the second absorption band. Photorelaxation from the highly excited S5 to the lowest S1 state takes place through a cascade of diabatic population transfers among the S1-S4-S5 electronic manifold within the first ∼100fs. Totally symmetric C=C stretching, C-S stretching, C-H wagging, ring puckering, and inter-ring bending modes collectively drive such relaxation dynamics.

Excitation transfer and quenching in photosystem II, enlightened by carotenoid triplet state in leaves.

Accumulation of carotenoid (Car) triplet states was investigated by singlet-triplet annihilation, measured as chlorophyll (Chl) fluorescence quenching in sunflower and lettuce leaves. The leaves were illuminated by Xe flashes of 4μs length at half-height and 525-565 or 410-490nm spectral band, maximum intensity 2mol quanta m-2s-1, flash photon dose up to 10μmolm-2 or 4-10 PSII excitations. Superimposed upon the non-photochemically unquenched Fmd state, fluorescence was strongly quenched near the flash maximum (minimum yield Fe), but returned to the Fmd level after 30-50μs. The fraction of PSII containing a 3Car in equilibrium with singlet excitation was calculated as Te = (Fmd-Fe)/Fmd. Light dependence of Te was a rectangular hyperbola, whose initial slope and plateau were determined by the quantum yields of triplet formation and annihilation and by the triplet lifetime. The intrinsic lifetime was 9μs, but it was strongly shortened by the presence of O2. The triplet yield was 0.66 without nonphotochemical quenching (NPQ) but approached zero when NP-Quenched fluorescence approached 0.2 Fmd. The results show that in the Fmd state a light-adapted charge-separated PSIIL state is formed (Sipka et al., The Plant Cell 33:1286-1302, 2021) in which Pheo-P680+ radical pair formation is hindered, and excitation is terminated in the antenna by 3Car formation. The results confirm that there is no excitonic connectivity between PSII units. In the PSIIL state each PSII is individually turned into the NPQ state, where excess excitation is quenched in the antenna without 3Car formation.

Understanding microplastic aging driven by photosensitization of algal extracellular polymeric substances

The aging of microplastics (MPs) is extremely influenced by photochemically-produced reactive intermediates (PPRIs), which are mediated by natural photosensitive substances. Algal extracellular polymeric substances (EPS) can produce PPRIs when exposed to sunlight. Nonetheless, the specific role of EPS in the aging process of MPs remains unclear. This work systematically explored the aging process of polystyrene (PS) MPs in the EPS secreted by Chlorella vulgaris under simulated sunlight irradiation. The results revealed that the existence of EPS accelerated the degradation of PS MPs into particles with sizes less than 1 µm, while also facilitating the formation of hydroxy groups on the surface. The release rate of dissolved organic matter (DOM) from PS MPs was elevated from 0.120 mg·L-1·day-1 to 0.577 mg·L-1·day-1. The primary factor contributing to the elevated levels of DOM was humic acid-like compounds generated through the breakdown of PS. EPS accelerated the aging process of PS MPs by primarily mediating the formation of triplet excited states (3EPS*), singlet oxygen (1O2), and superoxide radicals (O2∙-), resulting in indirect degradation. 3EPS* was found to have the most substantial impact. This study makes a significant contribution to advance understanding of the environmental fate of MPs in aquatic environments impacted by algal blooms.

Dual Emissive Zn(II) Naphthalocyanines: Synthesis, Structural and Photophysical Characterization with Theory-Supported Insights towards Soluble Coordination Compounds with Visible and Near-Infrared Emission.

Metal phthalocyaninates and their higher homologues are recognized as deep-red luminophores emitting from their lowest excited singlet state. Herein, we report on the design, synthesis, and in-depth characterization of a new class of dual-emissive (visible and NIR) metal naphthalocyaninates. A 4-N,N-dimethylaminophen-4-yl-substituted naphthalocyaninato zinc(II) complex (Zn-NMe2Nc) and the derived water-soluble coordination compound (Zn-NMe3Nc) exhibit a near-infrared fluorescence from the lowest ligand-centered state, along with a unique push-pull-supported luminescence in the visible region of the electromagnetic spectrum. An unprecedentedly broad structural (2D-NMR spectroscopy and mass spectrometry) as well as photophysical characterization (steady-state state and time-resolved photoluminescence spectroscopy) is presented. The unique dual emission was assigned to two independent sets of singlet states related to the intrinsic Q-band of the macrocycle and to the push-pull substituents in the molecular periphery, respectively, as predicted by TD-DFT calculations. In general, the elusive chemical aspects of these macrocyclic compounds are addressed, involving both reaction conditions, thorough purification, and in-depth characterization. Besides the fundamental aspects that are investigated herein, the photoacoustic properties were exemplarily examined using phantom gels to assess their tomographic imaging capabilities. Finally, the robust luminescence in the visible range arising from the push-pull character of the peripheral moieties demonstrated a notable independence from aggregation and was exemplarily implemented for optical imaging (FLIM) through time-resolved multiphoton micro(spectro)scopy.

Role of Halobenzene Guest Molecules in Modulating Room Temperature Phosphorescence of Benzophenone-Naphthalene Diimide Inclusion Crystals.

Materials exhibiting room temperature phosphorescence (RTP) have recently emerged as a subject of significant interest. In this study, we successfully created inclusion crystals by introducing halobenzenes as guests into a host molecule combining benzophenone with naphthalene diimide. This approach led to the creation of fascinating fluorescence and RTP properties dependent on the guest molecules. Notably, crystals containing chlorobenzene showed cyan fluorescence, while those with iodobenzene displayed red RTP. This difference highlights the impact of the guest molecule on the luminescent properties, with the significant external heavy-atom effect of iodobenzene playing a key role in promoting efficient intersystem crossing between the excited singlet and triplet states. Crystals with bromobenzene exhibited a unique blend of fluorescence and RTP, both from benzophenone and naphthalene diimide, highlighting the moderate heavy-atom effect. These findings reveal composite materials with remarkably diverse and interesting optical characteristics.

Effect of UV-LED Wavelength on Reactive Species Photogeneration from Dissolved Organic Matter

The photogeneration of reactive species from dissolved organic matter (DOM) plays a crucial role in the photochemical and photobiochemical processes in natural aquatic systems. However, the impact of the ultraviolet (UV) wavelength on the photogeneration of reactive species by different sources of DOM remains unclear. In this study, UV light at four wavelengths (365 nm, 310 nm, 280 nm, and 260 nm) provided by UV-LEDs were irradiated onto three types of DOM: humic acid (HA), fulvic acid (FA), and effluent organic matter (EfOM). Three reactive species produced by DOM, including excited triplet-state DOM (3DOM*), singlet oxygen (1O2), and hydroxyl radicals (•OH), were determined. UV365 proved to be the most efficient wavelength for generating 1O2 and •OH, with formation rates of 3.47 × 10−6 M s−1 and 1.67 × 10−8 M s−1, respectively, with the addition of FA and EfOM. The highest steady-state concentrations of all three reactive species were also generated under UV365, reaching 3.00 × 10−13 M (3DOM*) and 1.64 × 10−11 M (1O2) with the FA addition, and 1.44 × 10−10 M (•OH) with the EfOM. Across the different DOM sources, UV365 obtained the maximum quantum yields of reactive species, indicating the stronger effect of UV365 on inducing the photosensitization of DOM compared to the other shorter wavelengths. This study expands our understanding of the photochemistry of DOM in aquatic environments.

Temperature effect on spectral characters and dipole moment in short-lived excited singlet S1 state of 3HBI-7D-2HChromen-2-one laser dye in non-associative and highly viscous glycerol solvent

Steady state method is employed to study the effect of temperature on the absorption and emission characteristics of 3-(1H-benzo[d]imidazol-2-yl)-7-(diethylamino)-2H-chromen-2-one(3HBI-7D-2HChromen-2-one) laser dye in glycerol solvent. Temperature induces thermal effect on maxima absorption, emission wavelength, fluorescence intensity etc. Radiative and non-radiative transition probabilities mechanism, fluorescence lifetime, frequency assisted parameters decay rate constants, and activation energy is estimated. Advance thermochromic shift method is adopted to determine the ground and excited state dipole moments.