Singlet-triplet Absorption Research Articles

Experimental investigations of diacetylene ice photochemistry in Titan’s atmospheric conditions

Context. A large fraction of the organic species produced photochemically in the atmosphere of Titan can condense to form ice particles in the stratosphere and in the troposphere. According to various studies, diacetylene (C4H2) condenses below 100 km where it can be exposed to ultraviolet radiation. Aims. We studied experimentally the photochemistry of diacetylene ice (C4H2) to evaluate its potential role in the lower altitude photochemistry of Titan’s atmospheric ices. Methods. C4H2 ice films were irradiated with near-ultraviolet (near-UV) photons (λ > 300 nm) with different UV sources to assess the impact of the wavelengths of photons on the photochemistry of C4H2. The evolution of the ice’s composition was monitored using spectroscopic techniques. Results. Our results reveal that diacetylene ice is reactive through singlet-triplet absorption, similar to the photochemistry of other organic ices of Titan (such as dicyanoacetylene C4N2 ice) that we investigated previously. Several chemical processes occurred during the photolysis: the hydrogenation of C4H2 to form other C4 hydrocarbons (vinylacetylene C4H4 to butane C4H10); the formation of larger and highly polymerizable hydrocarbons, such as triacetylene (C6H2); and the formation of an organic polymer that is stable at room temperature. Conclusions. The nondetection of diacetylene ice in Titan’s atmosphere or surface could be rationalized based on our experimental results that C4H2 is photochemically highly reactive in the solid phase when exposed to near-UV radiation that reaches Titan’s lower altitudes and surface. C4H2 may be one of the key molecules promoting the chemistry in the ices and aerosols of Titan’s haze layers, especially in the case of co-condensation with other organic volatiles, with which it could initiate more complex solid-phase chemistry.

Near-infrared light-triggered prodrug photolysis by one-step energy transfer

Prodrug photolysis enables spatiotemporal control of drug release at the desired lesions. For photoactivated therapy, near-infrared (NIR) light is preferable due to its deep tissue penetration and low phototoxicity. However, most of the photocleavable groups cannot be directly activated by NIR light. Here, we report a upconversion-like process via only one step of energy transfer for NIR light-triggered prodrug photolysis. We utilize a photosensitizer (PS) that can be activated via singlet-triplet (S-T) absorption and achieve photolysis of boron-dipyrromethene (BODIPY)-based prodrugs via triplet-triplet energy transfer. Using the strategy, NIR light can achieve green light-responsive photolysis with a single-photon process. A wide range of drugs and bioactive molecules are designed and demonstrated to be released under low-irradiance NIR light (100 mW/cm2, 5 min) with high yields (up to 87%). Moreover, a micellar nanosystem encapsulating both PS and prodrug is developed to demonstrate the practicality of our strategy in normoxia aqueous environment for cancer therapy. This study may advance the development of photocleavable prodrugs and photoresponsive drug delivery systems for photo-activated therapy.

Non-Palindromic C∧C∧P Platinum and Palladium Pincer Complexes Showing Intense Phosphorescence via Direct Spin-Forbidden S0 → T1 Excitation.

The synthesis of C∧C∧P pre-ligands based on a dicyclohexylphosphine-substituted biphenyl framework is reported. The pre-ligands form the respective non-palindromic pincer complexes of PtII and PdII via double oxidative addition and subsequent comproportionation or C-H activation. The complexes of PtII as well as PdII emit similar green phosphorescence efficiently in the solid state, the former also in solution albeit with less intensity. The most fascinating photophysical feature, however, is a direct singlet-triplet (S0 → T1) excitation of this phosphorescence in the spectral window between the emission and the major singlet-singlet UV absorption. The S0 → T1 excitation spectra show a rich vibronic pattern, which is especially pronounced for the solid samples at cryogenic temperatures. The molar extinction of the lowest-energy singlet-triplet absorption band of the homologous Pt and Pd complexes as well as that of the Pt complex with a different (NHC) ancillary ligand were determined in tetrahydrofuran solutions. Quantum efficiencies of triplet formation (by intersystem crossing) via the "standard" excitation pathway S0 → Sn → T1 were determined for the Pt complexes and found to be different in dependence of the ancillary ligand.

Ultra-short optical pulse generation in micro OLEDs and the perspective of lasing

We report experimental and theoretical investigations with an in-house fabricated tris(8‐hydroxyquinoline)‐aluminum (Alq3)-based high-speed micro-organic light-emitting diodes (μ-OLEDs) and demonstrate very promising optical pulse responses as short as 400 ps. With a model for an electrically pumped OLED, we simulate the emission of sub-nanosecond optical pulses. The model includes field-dependent (Poole-Frenkel) Langevin recombination and reabsorption of photons by singlets (Stokes-shifted), and by triplets (TA). The good agreement between the measurement and the simulation is the basis for further study of the prospects on the ps time scale, i.e. stimulated emission and conditions for laser operation. For an Alq3-based micro-OLED with high-Q optical cavity, we predict pulsed laser operation with damped relaxation oscillations in the GHz regime and several orders of magnitude linewidth narrowing, but only during 3 ns at most. The simulated current density threshold values vary from 10 kA cm−2 for Q= 500 to less than 1 kA cm−2 for Q = 1000. No continuous-wave (CW) lasing is found due to the accumulation of triplet excitons for t > 5 ns, which has two deteriorating effects, (a) suppression of the gain-providing singlet excitons due to singlet-triplet absorption and (b) suppression of the photon density due to photon absorption by triplets (TA). Prospects for CW-lasing with other organic molecules are discussed.

Enhancement of Near-Infrared Singlet–Triplet Absorption of Ru(II) Sensitizers for Improving Conversion Efficiency of Solar Cells

Enhancement of Near-Infrared Singlet–Triplet Absorption of Ru(II) Sensitizers for Improving Conversion Efficiency of Solar Cells

Luminescence properties of 5-Bromoindole in PVA films at room temperature: Direct triplet state excitation

We studied spectral properties of 5-Bromoindole (5-BrI) in poly (vinyl alcohol) (PVA) film. The fluorescence of 5-BrI in PVA films is similar to indole in PVA film but strongly quenched compared to unsubstituted indole. The quantum yield of 5-BrI is only 0.0034 comparing to 0.26 of indole. In contrast, phosphorescence of 5-BrI in PVA is comparable to indole phosphorescence in magnitude and lifetime but its emission shifted towards longer wavelengths by 50 nm for a maximum at 530 nm. The phosphorescence of 5-BrI in PVA can be effectively excited at longer wavelengths (450–500 nm). The maximum of phosphorescence excitation spectrum of the 5-BrI in PVA is at about 480 nm. This direct excitation to the triplet state results in high positive phosphorescence anisotropy of 0.2. Although the S0 → T1 absorption transition is only about 800 fold weaker from the main 280 nm UV absorption band yielding measurable trace of absorption for 5-BrI in PVA film in the visible/blue range. We believe this is a first report of singlet-triplet absorption (S0 → T1) at room temperature, not involving external heavy atoms or oxygen presence. The comparison of phosphorescence of 5-BrI in PVA film to Rhodamine 6G in ethanol fluorescence allows us to estimate the phosphorescence quantum yield of 5-BrI in PVA to be ~0.02. We measured independently fluorescence and phosphorescence lifetimes and quantum yields as well as phosphorescence excitation spectrum across all wavelengths from UV to the visible range. These measurements were used to determine rate constants for all processes involved the in 5-BrI in PVA excited state deactivation.

Is Nitrate Anion Photodissociation Mediated by Singlet-Triplet Absorption?

Photolysis of the nitrate anion is involved in the oxidation processes in the hydrosphere, cryosphere, and stratosphere. While it is known that the nitrate photolysis in the long-wavelength region proceeds with a very low quantum yield, the mechanism of the photodissociation remains elusive. Here, we present the quantitative modeling of singlet-singlet and singlet-triplet absorption spectra in the atmospherically relevant region around 300 nm, and we argue that a spin-forbidden transition between the singlet ground state and the first triplet state contributes non-negligibly to the nitrate anion photolysis. We further propose that the nitrate anion excited into the first singlet excited state relaxes nonradiatively into its ground state. The full understanding of the nitrate anion photolysis can improve modeling of the asymmetric solvation in the atmospheric processes, e.g., photolysis on the surfaces of ice or snow.

Absorption and emission spectroscopic characterization of platinum-octaethyl-porphyrin (PtOEP)

The absorption and emission spectroscopic behaviour of the platinum complexed porphyrin PtOEP (platinum-octaethyl-porphyrin) is studied at room temperature. Liquid solutions, doped films, and a neat film are investigated. The absorption cross-section spectra including singlet–triplet absorption, the triplet–singlet stimulated emission cross-section spectra, the phosphorescence quantum distributions, the phosphorescence quantum yields, and the phosphorescence signal decays are determined. In the neat film a red-shifted phosphorescent excimer emission is observed. In diluted solid solution (polystyrene and dicarbazole-biphenyl films) as well as in de-aerated liquid solutions (tetrahydrofuran, toluene, chloroform) high phosphorescence quantum yields are obtained. In air-saturated liquid solutions, the phosphorescence efficiency is reduced by oxygen quenching. At intense short-pulse laser excitation the phosphorescence lifetime is shortened by triplet–triplet annihilation. No amplification of spontaneous emission was observed.

Absorption and emission spectroscopic characterization of Ir(ppy)3

Abstract The absorption and emission spectroscopic behaviour of cyclometalated fac -tris(2-phenylpyridine) iridium(III) [Ir(ppy) 3 ] is studied at room temperature. Liquid solutions, doped films, and neat films are investigated. The absorption cross-section spectra including singlet–triplet absorption, the triplet–singlet stimulated emission cross-section spectra, the phosphorescence quantum distributions, the phosphorescence quantum yields and the phosphorescence signal decays are determined. In neat films fluorescence self-quenching occurs, in diluted solid solution (polystyrene and dicarbazole-biphenyl films) as well as deaerated liquid solution (toluene) high phosphorescence quantum yields are obtained, and in air-saturated liquid solutions (chloroform, toluene, tetrahydrofuran) the phosphorescence efficiency is reduced by triplet oxygen quenching. At intense short-pulse laser excitation the phosphorescence lifetime is shortened by triplet–triplet annihilation. No amplification of spontaneous emission in the phosphorescence spectral region was observed indicating higher excited-state absorption than stimulated emission.

Spin-Dependent Recombination in PPV and Polyfluorene LEDs

AbstractIn a polymer light-emitting diode, the fraction of excitons formed as singlets is of crucial importance in determining the quantum efficiency. We have shown that it is possible to measure excited state absorptions due to triplet excitons and polarons in working polymer LEDs, and we are able to quantify the triplet generation rate by measuring the strength of the triplet absorption. Here, we show that by careful study of singlet emission and triplet absorption in an LED based on a poly(p-phenylenevinylene) derivative we can obtain an accurate value of 83±7% for the singlet formation probability, significantly higher than the value of 25% predicted by simple spin statistics. We extend these measurements to devices based on poly(dioctyl-fluorene), where we find similarly high values for the singlet formation probability. In devices using the polyfluorene copolymer F8BT, we find that the triplet absorption is extremely small, consistent with even higher singlet formation probabilities.

Vibronic analyses of the lowest singlet–singlet and singlet–triplet band systems of pyridazine

The first singlet–singlet and singlet–triplet band systems of the absorption spectrum of pyridazine vapour are analysed using ab initio and vibronic coupling calculations. The lowest singlet–triplet absorption involves a comparatively unperturbed (π ∗,n) 3B 1 state, and contrasts with the highly perturbed singlet–singlet spectrum. The major source of vibronic perturbation in the singlet–singlet absorption is attributed to coupling between near-resonant (π ∗,n) 1A 2 and (π ∗,n) 1B 1 states, with the former being slightly lower in energy. Many features of this complex and unusual spectrum, and its associated single vibronic level fluorescence spectrum, can be explained using a simple vibronic model. This provides experimental support for recent relaxed CASPT2 and EOM-CCSD calculations, but contrasts with earlier assignments of the spectrum. Theory and experiment suggest that the spacing between the lowest A 2 and B 1 states is larger in the triplet manifold, leading to a simpler spectrum.

Hund’s Case (a)-case (b) transition in the singlet-triplet absorption spectrum of pyrazine in a supersonic jet

An analytic expression is derived for calculating the intensities of individual spin-rovibronic lines in the fully resolved gas phase electronic spectrum of a polyatomic molecule, in which one of the zero-order electronic states is a triplet state. The expression is employed to calculate the effect of fine structure splitting on the singlet-triplet absorption spectrum of pyrazine using the parameters available from experiment. A transition from Hund’s coupling Case (a) to Case (b) on going from low J to high J rotational levels is predicted to occur at a moderate resolution of a few hundred MHz. The effect is more pronounced in pyrazine-d 4 and the pyrazine-argon van der Waals complex owing to their larger mass.

Response theory calculations of the singlet–triplet transition probabilities in the HOCl molecule

The role of the HOCl molecule in stratospheric photochemistry is highly dependent on its photolysis in the long wavelengthregion (λ 300 nm). In order to predict the possible importance of the singlet–triplet absorption in this molecule we haveperformed geometry optimizations of the ground singlet X1A′ and the excited triplet 3A″ and 3A′ states, and performed quadraticresponse theory calculations of the singlet–triplet transition probabilities at different bending angles and O–Cl distances, accounting for the complete form of the spin–orbit coupling operator. Based on the results of these calculations the importance of the 3A″ ← X1A′ and 3A′ ← X1A′ absorption in HOCl photolysis is discussed.

Character and spectra of triplet states in short polyenes

Multi-configuration quadratic response calculations with complete account of the spin-orbit coupling operator are performed to study the singlet-triplet absorption and emission spectra of the short ...

Response theory calculations of singlet-triplet transitions in molecular nitrogen

Abstract The probabilities and energetics of singlet-triplet absorption transitions in molecular nitrogen from the ground X 1 Σ g + state to all triplet states of ungerade symmetry in the range up to 13 eV, the A 3 Σ u + , W 3 Δ u , B ′ 3 Σ u − , C 3 Π u , C ′ 3 Π u , and D 3 Σ u + states, have been studied by multi-configuration quadratic response theory with complete account of spin-orbit coupling (SOC) perturbation. The vibrational Schrodinger equation has been solved for all states with the theoretical and RKR (where these are available) potentials, and vibrational averaged transition probabilities have been calculated and compared with experimental data for absorption from the lowest ν″ = 0 ground state to different ν′ vibronic upper-state levels. The Vegard-Kaplan (A3Σu+−X1Σg+) and the Ogawa-Tanaka-Wilkinson (B′3Σu−−X1Σg+) band intensities were calculated also in emission (ν′→ν″). Different types of correlating active spaces have been explored for the reference ground state. Although the use of single reference self-consistent field wavefunctions in the lowest level of response calculations (random phase approximation) in general is not sufficient, it is found that quite moderate complete active space (CAS) expansions of the valence orbitals lead to good transition probabilities and energies for the excited triplet states. Even the smallest active space gives a spin-averaged radiative lifetime of the Vegard-Kaplan phosphorescence band only twice as large as the experimental value (5.84 versus 2.4 s). The transition moment curves for the Vegard-Kaplan (A3Σu+) and for the Saum-Benesch (W3Δu) bands are quite similar, both changing sign in the region of the ground state internuclear equilibrium distance. The B′3Σu−−X1Σg+ Ogawa-Tanaka-Wilkinson system (B′ ← X) is predicted to be much more intensive (f = 1.6 × 10−8 for the most intensive vibronic bands (ν′=7–9)) than the A-X and W-X transitions, and the ratio of transition moments for the Ω=1 and Ω=0 sublevels has been qualitatively reproduced. The Tanaka band (the C3Πu ← X1Σg+ transition) is calculated as the most intensive singlet-triplet transition with an oscillator strength (f = 1.1 × 10−7) which is comparable with the spin-allowed, orbitally forbidden absorption in the Lyman-Birge-Hopfield band in the same wavelength region (105–112 nm). The highest in energy of the investigated states, the D3Σu+ state, has Rydberg character in the vicinity of re and is strongly mixed by SOC with Rydberg and valence 1Πu states. The calculated intensity of the D3Σu+−X1Σg+ transition as well as the hot nitrogen absorption C′3Πu ← X1Σg+ could be somewhat overestimated because of unrealistic S-T crossings. The relative vibronic transition intensities are well reproduced for a large variety of transitions for which experimental data are available.

Franck–Condon analysis of the S→T1 absorption and phosphorescence spectra of biphenyl and bridged derivatives

The equilibrium geometry and the vibrational force field of the ground and the lowest triplet electronic states of biphenyl and three bridged derivatives−biphenylene, fluorene and phenanthrene−are computed by using an updated version of the QCFF/PI (Quantum Chemical Force Field/π electron) Hamiltonian. The displacement parameters between T1 and S0 are obtained and used to model the S0→T1 absorption and the phosphorescence spectra. The calculated Franck–Condon envelopes are found to be in excellent agreement with the vibrational structure of the observed spectra. The common features of the phosphorescence spectra of biphenyl and fluorene are related to the same orbital nature of the lowest triplet state. The observed asymmetry between the phosphorescence and singlet–triplet absorption spectra of biphenyl is reproduced when the twisted equilibrium geometry of S0 is considered. It is shown that evidence of the nonplanarity of the ground state of biphenyl is manifested by the lower intensity of the band observed in the phosphorescence at 747 cm−1 with respect to the intensity of the same band in fluorene. The increased vibrational activity calculated in the lower frequency region for biphenylene and phenanthrene agrees with the observed spectra and reflects the different orbital nature of the lowest triplet state of the two strongly perturbed bridged derivatives with respect to biphenyl and fluorene. From the analysis of the computed vibrational frequencies, it is suggested that the false origin of the symmetry forbidden phosphorescence of biphenylene is due to the lowest out-of-plane mode of au symmetry.

Phosphorescence Excitation Spectra in the Singlet–Triplet Absorption Region of Pyrazine and Pyrimidine Vapors

Abstract Phosphorescence excitation spectra in the S0→T1 absorption region of pyrazine and pyrimidine have been observed in the vapor phase at room temperature. In pyrimidine vapor, the origin of the S0→T1 transition has been determined to be at 350.42 nm, and the T1 state is assigned to the nπ* state.

New efficient laser dyes for operation in the near UV range

A series of substituted 2, 6-diphenyl-benzo[1.2d;4.5d′]bisoxazoles(I) and 2, 6-diphenyl-benzo[1.2d;5.4d′]bisoxazoles(II) were synthesized for evaluation as laser dyes. The photophysical properties such as singlet absorption, triplet absorption and fluorescence spectra, the fluorescence quantum yield and fluorescence decay time as well as the laser performance (tuning range, conversion effeciency, photochemical stability) were measured in ethanol or dioxane at room temperature. The success achieved in molecular engineering was demonstrated by its influence on the spectroscopic and photophysical data as well as on the laser performance. Increasing the Stokes' shift by introducing sterical hindrance in the parent compound makes the tuning ranges broader. The substituted 2, 6-diphenyl-benzo[1.2d;4.5d′]bisoxazoles(I) have proved to be a promising new class of photostable laser dyes operating in the near ultraviolet spectral region.

Fluorescence quenching of Y-4,9-dihydro-4,6-dimethyl-9-oxo-1 H-imidazo-1,2-a purine (Y t base) and the model compounds Y t(CH 2) nadenine by α-bromonaphthalene

The fluorescence quenching of Y-4,9-dihydro-4,6-dimethyl-9-oxo-1 H-imidazo-1,2-a purine (Y t base) and of some model compounds Y t(CH 2) n adenine ( n = 2, 3, 5) by α-bromonaphthalene was studied in various solvents. The dependence of the ratio k/ k d (where k is the rate constant for energy transfer from Y t to α-bromonaphthalene and k d is the diffusion rate constant) on the integrated overlap of the Y t fluorescence and the singlet-triplet absorption of α-bromonaphthalene was linear. The constant k for the model compounds varied with the length of the polymethylene chain and attained its lowest value for n = 3. This is probably due to an increased degree of base stacking interaction in the trimethylene-bridged compound.

Temperature Dependence of the Delayed Fluorescence Lifetime in Anthracene Crystals with Triplet Exciton Traps

The lifetime of delayed fluorescence of anthracene crystals excited by the chopped-light of a He-Ne laser is investigated as a function of temperature over the range 110∼480 K. As compared with the fluorescence intensity, the lifetime is not influenced much by the temperature dependence of triplet-triplet annihilation rate and singlet-triplet absorption coefficient. Considering a trap saturation effect, an analysis has been made of the temperature dependence of the lifetime. In defect-induced crystals, three kinds of defects are observed by X-ray irradiation. Two are 0.33 and 0.16 eV in triplet trap depth, and the trapped triplet decay rate is ∼5×106 s-1 for both; the other is 0.29 eV and ∼1 s-1. The latter is of a different type from the former, namely, it is almost a pure trapping center which hardly quenches triplet excitons.