Excited Singlet Research Articles

Singlet O2 Reactions with Radical Cations of 8-Bromoguanine and 8-Bromoguanosine: Guided-Ion Beam Mass Spectrometric Measurements and Theoretical Treatments.

8-Bromoguanosine is generated in vivo as a biomarker for early inflammation. Its formation and secondary reactions lead to a variety of biological sequelae at inflammation sites, most of which are mutagenic and linked to cancer. Herein, we report the formation of radical cations of 8-bromoguanine (8BrG•+) and 8-bromoguanosine (8BrGuo•+) and their reactions toward the lowest excited singlet molecular oxygen (1O2)─a common reactive oxygen species generated in biological systems. This work aims to investigate synergistic, oxidatively generated damage of 8-brominated guanine and guanosine that may occur upon ionizing radiation, one-electron oxidation, and 1O2 oxidation. Capitalizing on measurements of reaction product ions and cross sections of 8BrG•+ and 8BrGuo•+ with 1O2 using guided-ion beam tandem mass spectrometry and augmented by computational modeling of the prototype reaction system, 8BrG•+ + 1O2, using the approximately spin-projected ωB97XD/6-31+G(d,p) density functional theory, the coupled cluster DLPNO-CCSD(T)/aug-cc-pVTZ and the multireference CASPT2(21,15)/6-31G**, probable reaction products, and potential energy surfaces (PESs) were mapped out. 8BrG•+ and 8BrGuo•+ present similar exothermic oxidation products, and their reaction efficiencies with 1O2 increase with decreasing collision energy. Both single- and multireference theories predicted that the two most energetically favorable reaction pathways correspond to 1O2-addition to the C8 and C5-positions of 8BrG•+, respectively. The CASPT2-calculated PES represents the best quantitative agreement with the experimental benchmark, in that the oxidation exothermicity is close to the water hydration energy of product ions and, thus, is able to eliminate a water ligand in the product ions.

Theoretical Design of Blue-Color Phosphorescent Complexes for Organic Light-Emitting Diodes: Emission Intensities and Nonradiative Transition Rate Constants in Ir(ppy)2(acac) Derivatives.

Theoretical calculations of phosphorescent spectra and nonradiative transition (NRT) rate constants for S1 ⇝ T1, T1 ⇝ S0, and S1 ⇝ S0 were carried out to determine the best candidate for a blue-color phosphorescent complex among several derivatives of bis(2-phenylpyridine)(acetylacetonate)iridium(III). The geometries of the ground state (S0), the lowest triplet state (T1), and the lowest excited singlet state (S1) were optimized at the levels of density functional theory, in which B3LYP functionals and SBKJC+p basis sets were used. The NRT rate constants were derived by using a generating function method within the displaced harmonic oscillator model. The results of the calculation for phosphorescence showed that the introduction of F and/or CN substituents at the 4'/6'-th and 5'-th sites in 2-phenylpyridinate (ppy) ligands, respectively, causes a blue shift of the emission spectra. They also suggest that Ir(5-CN,6-F-ppy)2(acac), denoted 3(56) in the text, is a good candidate for a blue-color phosphorescent complex because a blue shift of emission spectra and a moderate intensity are obtained for phosphorescence and, furthermore, this complex is calculated to have a large rate constant for S1 ⇝ T1 and relatively smaller rate constants for T1 ⇝ S0 and S1 ⇝ S0 based on the calculations of spin-orbit coupling and nonadiabatic coupling constants.

Rotationally resolved electronic spectroscopy of 6-methylindole: Structures, transition moments, and permanent dipole moments of ground and excited singlet states

The rotationally resolved electronic spectrum of the S1←S0 electronic origin band of 6-methylindole (6-MI) has been measured in a molecular beam and has been fit to rigid asymmetric rotor Hamiltonians in both electronic states using evolutionary strategies. Rotational constants, dipole moments in both electronic states, and orientation of the transition dipole moment in the inertial frame of the molecule have been determined and compared to the results of ab initio calculations. Both the values of the permanent dipole moments as of the transition dipole moment point to an Lb-state as lowest electronically excited state in 6-MI.

Simulation of Low-Lying Singlet and Triplet Excited States of Multiple-Resonance-Type Thermally Activated Delayed Fluorescence Emitters by Delta Self-Consistent Field (ΔSCF) Method.

The delta self-consistent field (ΔSCF) method was applied to the simulation of low-lying singlet and triplet excited states of multiple-resonance (MR)-type thermally activated delayed fluorescence (TADF) molecules, which form a promising group for organic light-emitting diode (OLED) emitters. A comparison with the experimental values of 13 emitters from the literature showed that ΔSCF gave fairly accurate S1 and T1 excitation energies (mean absolute errors (MAEs) of 0.092 and 0.055 eV, respectively) as well as quite accurate ΔEST (S1-T1 gap, MAE of 0.041 eV), which could not be calculated with sufficient accuracy by the conventional time-dependent density functional theory (TDDFT). ΔSCF also demonstrated its utility for the analysis of photophysical properties through a simulation of the reverse intersystem crossing (RISC) process of an MR-type emitter.

Benchmark Calculations of the Energy Spectra and Oscillator Strengths of the Beryllium Atom

In this work, we present a series of benchmark variational calculations for the ground and 19 lowest bound excited singlet S and P states of the beryllium atom. The nonrelativistic wave functions of the states that represent the motion of the nucleus and the four electrons around the center of mass of the atom are expanded in terms of up to 17 000 all-particle explicitly correlated Gaussians. The Gaussians are optimized independently for each state. The leading relativistic corrections to the energy levels are computed in the framework of the perturbation theory and they explicitly include the nuclear recoil effects. We also calculate the leading quantum electrodynamics (QED) corrections for each considered state. Using the obtained energy levels and the corresponding wave functions, we compute the transition frequencies, transition dipole moments, and oscillator strengths. A comparison with the available experimental data shows very good agreement. The results of this most comprehensive set of calculations of spectroscopic accuracy for Be to date may open up new applications pertinent to the precision tests of QED, determination of the nuclear charge radius, and modeling matter-radiation equilibria of the beryllium gas that has relevance to the physics of interstellar media.

Evolution of non-Kramers doublets in magnetic field in PrNi2Cd20 and PrPd2Cd20

Praseodymium-based 1-2-20 cage compounds Pr$T_2X_{20}$ ($T$ is generally Ti, V, Nb, Ru, Rh, Ir; and $X$ is either Al, Zn or Cd) provide yet another platform to study non-trivial electronic states of matter ranging from topological and magnetic orders to unconventional multipolar orders and superconductivity. In this paper, we report measurements of the electronic heat capacity in two Pr-based 1-2-20 materials: PrNi$_2$Cd$_{20}$ and PrPd$_2$Cd$_{20}$. We find that the lowest energy multiplet of the Pr $4f^2$ valence configuration is a $\Gamma_3$ non-Kramers doublet and the first excited triplet is assumed to be a magnetic $\Gamma_5$. By analyzing the dependence of the energy splitting between the ground and first excited singlet states on external magnetic field, we found that the maximum in the heat capacity corresponding to the Schottky anomaly in PrPd$_2$Cd$_{20}$, unlike PrNi$_2$Cd$_{20}$, shows pronounced linear dependence on external magnetic field at higher field values. This effect is associated with the exchange interactions between the field-induced magnetic dipole moments.

(RuBpy3)2+-bisterpyridinyl triangle promoted singlet oxygen (1O2) photosensitization for fast oxidation of sulfur mustard simulant

(RuBpy3)2+-bisterpyridine-based metallacycle T photosensitizer has been prepared by a facile single-step self-assembly process. Supramolecular self-assembly strategy greatly improved metallacycle T’s photosensitized ability due to its enhanced light-harvesting capability, smaller energy gap (ΔEST) between lowest excited singlet state (S1) and lowest excited triplet state (T1) along with excellent solubility which exhibiting higher efficiency for singlet oxygen (1O2) production than the ligand L and its pendant [(RuBpy3)2+·2PF6–]. In the practical photo-driven degradation of sulfur mustard simulant (2-chloroethyl ethyl sulfide, CEES), full conversion of toxic CEES to nontoxic CEESO has been achieved by metallacycle T with an extremely fast lifetime of 90 s (half lifetime t1/2 = 25 s) and 100% selectivity (without formation of highly toxic CEESO2), while ligand L and [(RuBpy3)2+·2PF6–] needed 130 s and 330 s, respectively. This study demonstrated terpyridine-based novel supermolecules can serve as efficient scaffolds for effective enhancement of photosensitization.

Distance dependence of plasmon-enhanced fluorescence and delayed luminescence of molecular planar nanostructures

The distance dependence of the plasmon-accelerated decay of excited singlet and triplet states of Eosin molecules was studied. Layered films of dye were prepared using the Langmuir-Blodgett technology on the surface of silver island films. The plasmon effect manifests itself in an increase in the intensity and a decrease in the lifetimes of fluorescence, delayed fluorescence and phosphorescence of the dye films with metal nanoparticles. It is shown that the optimal distance at which the maximum enhancement of all luminescence types was achieved is equal to 6–8 nm and approximately coincides with the Förster radius of the nonradiative inductive-resonance process. A proposed mathematical model is in agreement with the experimental data. From the model it is follows that the distance dependences of the long-lived luminescence intensity are qualitatively similar to the distance dependence of the fluorescence of dye molecules on the metal island films.

Optical and electrochemical properties of peripherally substituted chromium and manganese phthalocyanines

New chromium phthalocyanine complex peripherally substituted with 8-quinolinoxy group was synthesized from modified precursor 4-(8-quinolinoxy) phthalonitrile in presence of CrCl3·6H2O in DMAE, using zinc dust as the reducing agent. The product μ-oxodimer of Cr(III)QPc [Cr(III)QPc-O-Cr(III)QPc] exhibited green florescence, in contrary to the typical red emissive behaviour of phthalocyanines. The detailed study on the emission spectrum of Cr(III)QPc-O-Cr(III)QPc revealed dual peaks with abroad band situated at 510 nm and a narrow peak situated at 686 nm, where the former broad band was found to be responsible for the intense green emission. The emission at 686 nm was the expected ligand based transition of Pc core from excited state singlet to ground state, S1 → S0, while the abnormal peak at 510 nm was found to be due to the combined effect of radiative relaxation of second excited singlet state to ground state (S2 → S0) and the LMCT of Pc2- to Cr(III) metal center. The emission spectra of Cr(III)QPc-O-Cr(III)QPc also exhibited a reciprocal relationship in the intensity of the dual peaks when the excitation wavelength was varied from 350 to 440 nm. The presence of an isoemission point at 648 nm indicated that two emission bands were brought about by two different excitonic transitions. Substituted chromium phthalocyanines are very less reported to date with remarkable fluorescent features, and in this work, the modified cyclotetramerization method applied by using zinc dust have paved new ways for their preparation. The preparation of unstable manganese phthalocyanine, MnQPc, was also rarely reported owing to its aerobic oxidation to the more stable and often synthesized analogue, MnQPc(CH3COO). Here, the synthesis of MnQPc was achieved under normal cyclization conditions and the effect of acetic acid and chlorinated solvents on the oxidative stability of MnQPc were also studied.

Structures, atomic charges, emission properties and DFT studies of biquinoline derivatives induced by protonation of a nitrogen atom

We prepared a monoprotonated compound series comprising dmphenHCl ([1H]Cl), dmbpyHCl ([2H]Cl), dmbpyHPF6 ([2H]PF6), bqnHCl ([3H]Cl), bqnHPF6 ([3H]PF6), dmbqnHPF6 ([4H]PF6), naphthyridineHPF6 ([5H]PF6) and phenazineHPF6 ([6H]PF6) and elucidated the crystal structures of [1H]Cl, [2H]Cl, [2H]PF6, [4H]PF6, [5H]PF6 and [6H]PF6. Density functional theory (DFT) calculations of the protonated compounds involving 1, [1H]Cl, [1H]PF6, 2, [2H]Cl, [2H]+, 3, [3H]Cl, [3H]+, 4, [4H]+, 5, [5H]+, 6 and [6H]+ were subsequently performed. Further, time-dependent DFT (TD-DFT) analyses were carried out to elucidate the excited singlet structure. Moreover, the atomic charges of these compounds were examined in detail, and the 1H NMR chemical shifts were correlated to atomic charges in the molecule. Notably, the attachment of one proton to a nitrogen atom in these compounds imparted ionic properties, which resulted in strong red-shifted emissions compared to those of their non-protonated counterparts. In particular, the protonated compound [4H]PF6, with two methyl substituents, displayed the strongest emission.

Photodissociation dynamics of CF3CHO: C-C bond cleavage.

The photodissociation dynamics of jet-cooled trifluoroacetaldehyde (CF3CHO) into radical products, CF3 + HCO, was explored using velocity mapped ion imaging over the wavelength range 297.5nm ≤λ≤ 342.8nm (33613-29172 cm-1) covering the entire section of the absorption spectrum accessible with solar actinic wavelengths at the ground level. After initial excitation to the first excited singlet state, S1, the radical dissociation proceeds largely via the first excited triplet state, T1, at excitation energies above the T1 barrier. By combining velocity-mapped ion imaging with high-level theory, we place this barrier at 368.3 ± 2.4 kJ mol-1 (30780 ± 200 cm-1). After exciting to S1 at energies below this barrier, the dissociation proceeds exclusively via the ground electronic state, S0. The dissociation threshold is determined to be 335.7 ± 1.8 kJ mol-1 (28060 ± 150 cm-1). Using laser-induced fluorescence spectroscopy, the origin of the S1 ← S0 transition is assigned at 28903 cm-1. The S0 dissociation channel is active at the S1 origin, but the yield significantly increases above 29100 cm-1 due to enhanced intersystem crossing or internal conversion.

Photodissociation Dynamics of Methyl Hydroperoxide at 193 nm: ATrajectory Surface-Hopping Study.

The photodissociation of methyl hydroperoxide (CH3OOH) at 193 nm has been studied using a direct dynamics trajectory surface-hopping (TSH) method. The potential energies, energy gradients, and nonadiabatic couplings are calculated on the fly at the MRCIS(6,7)/aug-cc-pVDZ level of theory. The hopping of a trajectory from one electronic state to another is decided on the basis of Tully's fewest switches algorithm. An analysis of the trajectories reveals that the cleavage of the weakest O-O bond leads to major products CH3O(2E) + OH(2Π), contributing about 72.7% of the overall product formation. This OH elimination was completed in the ground degenerate product state where both the ground singlet (S0) and first excited singlet (S1) states become degenerate. The O-H bond dissociation of CH3OOH is a minor channel contributing about 27.3% to product formation, resulting in products CH3OO + H. An inspection of the trajectories indicates that unlike the major channel OH elimination, the H-atom elimination channel makes a significant contribution (∼3% of the overall product formation) through the nonadiabatic pathway via conical intersection S1/S0 leading to ground-state products CH3OO(X 2A″) + H(2S) in addition to adiabatic dissociation in the first excited singlet state, S1, correlating to products CH3OO(1 2A') + H(2S). The computed translational energy of the majority of the OH products is found to be high, distributed in the range of 70 to 100 kcal/mol, indicating that the dissociation takes place on a strong repulsive potential energy surface. This finding is consistent with the nature of the experimentally derived translational energy distribution of OH with an average translational energy of 67 kcal/mol after the excitation of CH3OOH at 193 nm.

Structural changes of 1-(phenylethynyl)naphthalene upon electronic excitation from Franck–Condon fits of several fluorescence emission spectra

The structural changes of 1-(Phenylethynyl)naphthalene (1-PEN) upon excitation from the ground state to the lowest excited singlet state have been assessed from a combined Franck–Condon-Fit of fluorescence emission spectra after excitation of several vibronic bands of 1-PEN and ab initio calculations. An analysis of the infrared spectrum, the laser induced fluorescence (LIF) spectrum and the fluorescence emission spectra is given on the basis of the approximate coupled cluster singles and doubles model (CC2) within the resolution-of-identity approximation and taking spin-component scaling into account. The excited state structure of 1-PEN is found to be planar, with the major geometry changes upon excitation localized in the naphthalene moiety. Electron density shifts from the naphthyl ring to the phenyl ring upon electronic excitation, leading to an increase of the permanent dipole moment of 1-PEN. The lowest electronically excited state of 1-PEN can be labeled as 1Lb-state, with the 1La-state energetically quasi-degenerate. The geometry changes, determined from the Franck–Condon analysis are in agreement with this assignment.

Toward efficient photochemistry from upper excited electronic states: Detection of long S2 lifetime of perylene.

A long 0.9ps lifetime of the upper excited singlet state in perylene is resolved by femtosecond pump-probe measurements under ultraviolet (4.96eV) excitation and further validated by theoretical simulations of transient absorption kinetics. This finding prompts exploration and development of novel perylene-based materials for upper excited state photochemistry applications.

Ultraviolet photolysis of monochloro-p-benzoquinone (MCBQ) in aqueous solution: Theoretical investigation into the dechlorination

In this work, the UV-induced transformation of monochloro-p-benzoquinone (MCBQ) in aqueous solution has been systematically investigated through quantum chemical calculations. During the UV irradiation at 253.7 nm, the first triplet state of MCBQ (3MCBQ*) was from the intersystem crossing of its first excited singlet state (1MCBQ*). In aqueous solution, the nucleophilic attack of OH− on carbon atoms in 3MCBQ* was the central reaction. The addition of OH− to olefinic carbon atoms was much more kinetically feasible than that to carbonyl carbon atoms, even though the carbonyl carbon atoms were more positively charged. Moreover, OH− preferred to add to the ortho-position of C–Cl bond, where the unchlorinated atom was more negatively charged than the chlorinated one. The UV photolysis of the primary intermediate (HO-CBQ) was not the same as that of MCBQ. The attack of OH− on the para-position of C–Cl bond was the most efficient pathway. The addition of OH− to the chlorinated atom of 3HO-CBQ* was much more efficient than that in the case of 3MCBQ*, which reveals that more UV irradiation may promote the dechlorination. The findings in the present study may be helpful to enrich the understanding of the halobenzoquinones transformation in aqueous solution.

D−A−D based pyrido-pyrazino[2,3-b]indole amines as blue-red fluorescent dyes: Photophysical, aggregation-induced emission, electrochemical and theoretical studies

Herein donor−acceptor−donor (D−A−D) structured, pyridopyrazino[2,3-b]indole amines 2–6 were designed and synthesized via C–N coupled Buchwald-Hartwig amination reaction and characterized by spectroscopic methods. C–N coupling in molecules offers intramolecular charge transfer (ICT) transitions at longer wavelengths (λmax = 400–459 nm) in absorption spectra reminiscent D−A assembly. Dyes covers broad range of emission and emit in blue to red region with emission maxima 433–600 nm in solvent and solid film. Dyes 2, 5 and 6 explore the aggregation-induced emission (AIE) characteristics. Further C–N blending of various donor with pyridopyrazino[2,3-b]indole acceptor results in suppressed band gap (1.99–2.43 eV) and comparable HOMO (−5.17 to −5.71 eV) and LUMO (−3.18 to −3.28 eV) energies with reported small organic ambipolar materials. DFT and TDDFT calculations agree with the experimental opto-electrochemical data. Moreover, computed small singlet and triplet excitation energy difference (ΔEST (0.01–0.14 eV)) suggest thermally activated delayed fluorescent (TADF) emitting properties of dyes. Thus pyridopyrazino[2,3−b]indole amine dyes propose their potential application in optoelectronic devices.

Excited-State Dynamics of a Substituted Fluorene Derivative. The Central Role of Hydrogen Bonding Interactions with the Solvent.

Substituted fluorene structures have demonstrated unusual photochemical properties. Previous reports on the substituted fluorene Schiff base FR0-SB demonstrated super photobase behavior with a ΔpKb of ∼14 upon photoexcitation. In an effort to understand the basis for this unusual behavior, we have examined the electronic structure and relaxation dynamics of the structural precursor of FR0-SB, the aldehyde FR0, in protic and aprotic solvents using time-resolved fluorescence spectroscopy and quantum chemical calculations. The calculations show three excited singlet states in relatively close energetic proximity. The spectroscopic data are consistent with relaxation dynamics from these electronic states that depend on the presence and concentration of solvent hydroxyl functionality. These results underscore the central role of solvent hydrogen bonding to the FR0 aldehyde oxygen in mediating the relaxation dynamics within this molecule.

Vibronic coupling in serotonin studied by rotationally resolved electronic spectroscopy

Vibronic couplings between the two lowest excited singlet states of the neurotransmitter serotonin are studied both experimentally and theoretically. Using rotationally resolved electronic spectroscopy, we observe a mode-dependent rotation of the S1 transition dipole moment (TDM). This is rationalized as a coupling of the S1 (1Lb) to the S2 (1La) state, lying about 3300 cm−1 higher in energy.

Kinetics of near-infrared-to-visible upconversion in rubrene: An initial excitation of triplets

Upconversion (UC) in a molecular system is a process in which excitons produced by a multiple absorption of low-energy photons at long wavelengths undergo fusion to produce high energy excitons that consequently recombine to emit anti-Stokes shifted photons. Molecular systems for UC typically require a sensitizer. However, recent experiments show that UC in rubrene occurs even without the presence of the sensitizer. In this system, intermediate states are assumed to absorb photons at near-infrared wavelengths, which either absorb additional photons to populate the emissive singlet state or undergo fusion to generate triplets. The triplets can again undergo fusion to populate the excited singlet state. The final emission is around 600 nm. These models have been tested against the intensity dependence of the UC emission. Here, we have measured the kinetics of UC in rubrene by using intensity modulated photoexcitation at 800 nm to better understand the underlying mechanism. The models of UC that have been proposed so far do not agree with our measurements. Our results show that the yield of UC lags behind excitation significantly, indicating that triplet states are directly excited from the ground state, and their fusion, which depends on the population, becomes prominent after a certain build up time. While the intermediate states could form dynamically after the UC has been initiated and enhance the process, further sensitive absorption measurements are necessary to understand the role of the intermediate states in the process. Our results are important in finding new routes to enhance UC in pristine organic semiconductors for applications in photovoltaics, lasers, bioimaging, optical devices, and lighting.

High-Speed and Continuous-Wave Programmable Luminescent Tags Based on Exclusive Room Temperature Phosphorescence (RTP).

Most materials recently developed for room temperature phosphorescence (RTP) lack in practical relevance due to their inconvenient crystalline morphology. Using amorphous material systems instead, programmable luminescent tags (PLTs) based on organic biluminescent emitter molecules with easy processing and smooth sample shapes are presented recently. Here, the effective quenching of the emitter's RTP by molecular oxygen (O2) and the consumption of the excited singlet O2 through a chemical reaction represent the central features. With customized activation schemes, high‐resolution content can be written and later erased multiple times into such films, providing a versatile yet simple photonic platform for information storage. However, two important limitations remain: The immutable fluorescence of the emitters outshines the phosphorescent patterns by roughly one order of magnitude, allowing readout of the PLTs only after the excitation source is turned off. The programming of these systems is a rather slow process, where lowest reported activation times are still >8 s. Here, a material‐focused approach to PLTs with fast activation times of 120 ± 20 ms and high‐contrast under continuous‐wave illumination is demonstrated, leading to accelerated programming on industry relevant time scales and a simplified readout process both by eye and low cost cameras.