Triplet State Production Research Articles

Impact of chlorination and ozonation of dissolved organic matter on its photo-induced production of long-lived photooxidants and excited triplet states

Recent studies suggested that long-lived photooxidants (LLPO), which are reactive intermediates formed during irradiation of dissolved organic matter (DOM), may consist of phenoxyl radicals derived from phenolic moieties of the DOM. Besides the well-studied excited triplet states of chromophoric DOM (3CDOM*), LLPO presumably are important photooxidants for the transformation of electron-rich contaminants in surface waters. The main objective of this study was to further test the potential role of phenoxyl radical as LLPO. Suwannee River fulvic acid (SRFA) as a model DOM was pre-oxidised using the phenol-reactive oxidants chlorine and ozone, followed by its characterization by the specific UV absorption at 254 nm (SUVA254), the ratio of absorbance at λ = 254 nm and λ = 365 nm (E2:E3), and the electron donating capacity (EDC). Subsequently, the photoreactivity of pre-oxidized SRFA was tested using 3,4-dimethoxyphenol (DMOP) as a LLPO probe compound at two initial concentrations ([DMOP]0 = 0.1 and 5.0 μM). Linear inter-correlations were observed for the relative changes in SUVA254, E2:E3, and EDC for increasing oxidant doses. Pseudo-first-order transformation rate constants normalized to the changing SRFA absorption rate (i.e., k0.1obs/rCDOMabsand k5.0obs/rCDOMabs, for 0.1 and 5.0 µM, respectively) exhibited the following distinct trends: The LLPO-dominated k0.1obs/rCDOMabsratio decreased with increasing oxidant dose and with decreasing SUVA254 and EDC, while the 3CDOM*-dominated k5.0obs/rCDOMabsratio positively correlated with E2:E3. Finally, it was concluded that precursors of 3CDOM* and LLPO are chemically modified differently by pre-oxidation of DOM, and LLPO precursors likely consist of phenolic moieties of DOM, suggesting phenoxyl radicals as LLPO.

Intersystem crossing via charge recombination in a perylene–naphthalimide compact electron donor/acceptor dyad

Efficient triplet state production (singlet oxygen quantum yield: 80%) by SOCT-ISC was observed for a perylene–naphthalimide compact electron donor/acceptor dyad.

Mapping the Complete Reaction Path of a Complex Photochemical Reaction.

We probe the dynamics of dissociating CS_{2} molecules across the entire reaction pathway upon excitation. Photoelectron spectroscopy measurements using laboratory-generated femtosecond extreme ultraviolet pulses monitor the competing dissociation, internal conversion, and intersystem crossing dynamics. Dissociation occurs either in the initially excited singlet manifold or, via intersystem crossing, in the triplet manifold. Both product channels are monitored and show that, despite being more rapid, the singlet dissociation is the minor product and that triplet state products dominate the final yield. We explain this by a consideration of accurate potential energy curves for both the singlet and triplet states. We propose that rapid internal conversion stabilizes the singlet population dynamically, allowing for singlet-triplet relaxation via intersystem crossing and the efficient formation of spin-forbidden dissociation products on longer timescales. The study demonstrates the importance of measuring the full reaction pathway for defining accurate reaction mechanisms.

Precise Control of the Electronic Coupling Magnitude between the Electron Donor and Acceptor in Perylenebisimide Derivatives via Conformation Restriction and Its Effect on Photophysical Properties

Perylenebisimide (PBI)–phenothiazine (PTZ) and PBI–diphenylamine (DPA) dyads were prepared, with the PTZ or DPA as the electron donor and the 6-subsituted PBI unit as the electron acceptor. The dyads were designed in such a way that electronic coupling (magnitude is the matrix elements, VDA and VDA*) between the electron donor and acceptor is controlled by conformation restriction. The effects of the electronic coupling on UV–Vis absorption and emission were studied. More significant charge-transfer (CT) absorption and CT fluorescence emission bands were observed for dyads with smaller dihedral angles between the electron donor and acceptor, thus stronger electronic coupling (VDA = 3290 cm–1 and VDA* = 4317 cm–1) was observed than those with larger dihedral angle, showing weaker coupling effect (VDA = 1210 cm–1 and VDA* = 2770 cm–1). Triplet state production was observed only for PBI–DPA but not for PBI–PTZ dyads. With an intermolecular triplet photosensitizing method, the triplet state of PBI–PTZ dyads w...

Excited-state dynamics of thiophene substituted betaine pyridinium compounds

This work deals with the photophysics of novel pyridinium betaine based on 2-pyridin-1-yl-1H-benzimidazole (SBPa) substituted symmetrically by mono- (Th2SBPa) and bi-thiophene fragments (Th4SBPa). The study is based on a combination of steady-state, femtosecond transient absorption spectroscopic measurements supported by PCM-(TD)DFT calculations. It is found that the two step ICT process (S0 → S2 excitation followed by S2(CT) → S1(CT) internal conversion) occurring for the parent molecule remains unaffected for Th2SBPa while the situation is less clear for Th4SBPa. Actually, for both molecules, a new decay route involving the π-electron system localized in thiophenic groups is responsible for the production of triplet states. Involvement of this new route in the parallel production of S1(CT) is strongly suspected.

Switching of the triplet excited state of rhodamine-C60 dyads

Acid-switching of the triplet excited state in rhodamine-C60 dyads was achieved. The rhodamine moiety acts as an acid-activated visible light-harvesting antenna and C60 as the singlet energy acceptor and the spin converter, and production of the triplet state was enhanced in the presence of acid.

Photochemical production of organic matter triplet states in water samples from mountain lakes, located below or above the tree line

The production of triplet states (T*) of chromophoric dissolved organic matter (CDOM), reacting with the probe molecule 2,4,6-trimethylphenol (TMP) was measured upon irradiation of water samples, taken from lakes located in a mountain area (NW Italy) between 1450 and 2750m above sea level. The lakes are located below or above the tree line and surrounded by different vegetation types (trees, alpine meadows or exposed rocks). The most photoactive samples belonged to lakes below the tree line and their fluorescence spectra and CDOM optical features suggested the presence of a relatively elevated amount of humic (allochthonous) material. The lowest (negligible) photoactivity was found for a lake surrounded by exposed rocks. Its CDOM showed an important autochthonous contribution (due to in-lake productivity) and considerably higher spectral slope compared to the other samples, suggesting low CDOM molecular weight and/or aromaticity. Among the samples, CDOM photoactivity (measured as the rate of TMP-reactive T* photoproduction) decreased with changing vegetation type in the order: trees, meadows, rocks. It could be connected with decreasing contribution from catchment runoff and increasing contribution from autochthonous processes and possibly precipitation.

Production of N2 Vegard–Kaplan and other triplet band emissions in the dayglow of Titan

Recently the Cassini Ultraviolet Imaging Spectrograph (UVIS) has revealed the presence of N2 Vegard–Kaplan (VK) band A3Σu+-X1Σg+ emissions in Titan’s dayglow limb observation. We present model calculations for the production of various N2 triplet states (viz., A3Σu+,B3Πg,C3Πu,E3Σu,W3Δu, and B3Σu) in the upper atmosphere of Titan. The Analytical Yield Spectra technique is used to calculate steady state photoelectron fluxes in Titan’s atmosphere, which are in agreement with those observed by the Cassini’s CAPS instrument. Considering direct electron impact excitation, inter-state cascading, and quenching effects, the population of different levels of N2 triplet states are calculated under statistical equilibrium. Densities of all vibrational levels of each triplet state and volume production rates for various triplet states are calculated in the model. Vertically integrated overhead intensities for the same date and lighting conditions as reported by the UVIS observations for N2 Vegard–Kaplan A3Σu+-X1Σg+, First Positive B3Πg-A3Σu+, Second Positive (C3Πu−B3Πg), Wu–Benesch (W3Δu−B3Πg), and Reverse First Positive bands of N2 are found to be 132, 114, 19, 22, and 22R, respectively. Overhead intensities are calculated for each vibrational transition of all the triplet band emissions of N2, which span a wider spectrum of wavelengths from ultraviolet to infrared. The calculated limb intensities of total and prominent transitions of VK band are presented. The model limb intensity of VK emission within the 150–190nm wavelength region is in good agreement with the Cassini UVIS observed limb profile. An assessment of the impact of solar EUV flux on the N2 triplet band emission intensity has been made by using three different solar flux models, viz., Solar EUV Experiment (SEE), SOLAR2000 (S2K) model of Tobiska (Tobiska, W.K. [2004]. Adv. Space Res. 34, 1736–1746), and HEUVAC model of Richards et al. (Richards, P.G., Woods, T.N., Peterson, W.K. [2006]. Adv. Space Res. 37 (2), 315–322). The calculated N2 VK band intensity at the peak of limb intensity due to S2K and HEUVAC solar flux models is a factor of 1.2 and 0.9, respectively, of that obtained using SEE solar EUV flux. The effects of higher N2 density and solar zenith angle on the emission intensity are also studied. The model predicted N2 triplet band intensities during moderate (F10.7=150) and high (F10.7=240) solar activity conditions, using SEE solar EUV flux, are a factor of 2 and 2.8, respectively, higher than those during solar minimum (F10.7=68) condition.

Long-lived long-distance photochemically induced spin-polarized charge separation in β,β′-pyrrolic fused ferrocene-porphyrin-fullerene systems

The exceptionally long lived charge separation previously observed in a β,β′-pyrrolic-fused ferrocene-porphyrin-fullerene triad (lifetime 630 μs) and related porphyrin-fullerene dyad (lifetime 260 μs) is attributed to the production of triplet charge-separated states. Such molecular excited-state spin polarization maintained over distances of up to 23 A is unprecedented and offers many technological applications. Electronic absorption and emission spectra, femtosecond and nanosecond time-resolved transient absorption spectra, and cyclic voltammograms of two triads and four dyads are measured and analyzed to yield rate constants, donor–acceptor couplings, free-energy changes, and reorganization energies for charge-separation and charge-recombination processes. Production of long-lived intramolecular triplet states is confirmed by electron-paramagnetic resonance spectra at 77–223 K, as is retention of spin polarization in π-conjugated ferrocenium ions. The observed rate constants were either first predicted (singlet manifold) or later confirmed (triplet manifold) by a priori semiclassical kinetics calculations for all conceivable photochemical processes, parameterized using density-functional theory and complete-active-space self-consistent-field calculations. Identified are both a ps-timescale process attributed to singlet recombination and a μs-timescale process attributed to triplet recombination.

Incorporation of methylene blue and nanogold into polyvinyl chloride catheters; a new approach for light-activated disinfection of surfaces

Methylene blue and 2 nm gold nanoparticles were incorporated into commercial PVC catheters by use of a simple “swell–encapsulation–shrink” method using acetone–water mixtures. Neither the methylene blue nor the nanogold leached into aqueous solution and the assemblage was stable to photodegradation upon laser irradiation. Exposure of the modified catheters to red laser light for 4–8 minutes induced the lethal photosensitisation of Staphylococcus epidermidis and Escherichia coli. Results from time-resolved EPR spectroscopy suggested that enhanced methylene blue triplet state production occurs in the presence of 2 nm gold nanoparticles. The implication being that the levels of reactive oxygen species are higher in these co-doped materials than with methylene blue alone.

Phase-reversed structures in superlattice of nonlinear materials

We present detailed description of so-called phase-reversed structures that are characterized by two grating wave vectors allowing simultaneously phase-match two parametric three-wave processes. The novelty is that the structure is realized as a definite assembly of nonlinear segments leading to detailed description of cascaded three-photon processes with the parameters of realistic structured nonlinear materials of finite length. We apply these results for analysis of the quasi-phase-matching in production of both photon triplet and four-photon states in cascaded down-conversion. The received results are matched with the experimental data.

Density functional theoretical study of the structure, bonding and electrochemistry of N 2S 2 nitrosyl complexes of iron and cobalt

Density functional calculations are used to study the structure, bonding and properties of the N 2S 2 nitrosyl complexes (bme-pda)M(NO) and (bme-dach)M(NO) M = Fe, Co. The complexes studied in this work have been previously shown to exhibit unusual structural and electrochemical properties. Calculated optimal geometries and vibrational frequencies are compared to experimental data and found to be in excellent agreement. A very small energy barrier for rotation of the NO ligand relative to the S-donor atoms of the N 2S 2 ligand is calculated. The similarity in reduction potentials of Fe and Co complexes is investigated using molecular orbital analysis and calculation of absolute reduction potentials. It is concluded that reduction of the Fe complexes may produce triplet-state products. Such products are determined to be of lower energy than singlet-state products, and also consistent with the observed reduction potentials of the Fe and Co complexes.

Photoexcited carrier relaxation dynamics in pentacene probed by ultrafast optical spectroscopy: Influence of morphology on relaxation processes

We present a comparative study of ultrafast photoexcited state relaxation in pentacene single crystals and in pure and C60-doped pentacene films using optical pump-probe spectroscopy. The photoinduced absorption spectra in pentacene crystals is consistent with a dominant singlet–triplet fission decay channel for above-gap excitation. This decay channel is suppressed in thin films and even further suppressed by electron trapping in C60-doped films. Thus we show that suppression of triplet state production, which is necessary for free carrier formation and thus photovoltaic and photodiode performance, is controllable via sample morphology.

Mechanisms of Low-Power Noncoherent Photon Upconversion in Metalloporphyrin−Organic Blue Emitter Systems in Solution

The mechanisms of noncoherent photon upconversion that involve triplet-triplet annihilation (TTA) in solution have been investigated for two model systems. ZnTPP (meso-tetraphenylporphine zinc) is used as the model visible light-absorbing metalloporphyrin because its S(1) fluorescence intensity can be used to monitor the initial rate of porphyrin triplet state production and because its S(2) fluorescence intensity can be used as a direct measure of the rate of porphyrin TTA. When perylene, which has a triplet energy lower than that of ZnTPP, is added as a signaling blue emitter (BE), the mechanism of photon upconversion involves triplet energy transfer from the porphyrin to the BE followed by TTA in the BE to form the fluorescent perylene S(1) state. The kinetics of this process have been characterized and are unremarkable. When coumarin 343 (C343), which has photophysical properties similar to those of perylene except that it has a much higher triplet energy than ZnTPP, is added as the signaling BE, emission from the ZnTPP S(2) state is quenched and fluorescence from the C343 grows in. Contrary to previous suggestions, the mechanism of photon upconversion in this system does not involve singlet energy transfer from the porphyrin S(2) state to the BE. Instead, ground-state C343 complexes with the ZnTPP triplet to form a triplet exciplex, which then undergoes TTA with a second ZnTPP triplet to give the fluorescent state of the BE in a three-center process.

Infrared spectra and density functional calculations of triplet pnictinidene N÷ThF3, P÷ThF3 and As÷ThF3 molecules

Thorium atoms react with NF(3), PF(3), and AsF(3) to produce the first actinide triplet state pnictinidene molecules, N divided by ThF(3), P divided by ThF(3), and As divided by ThF(3), which are trapped in solid argon and identified from infrared spectra and comparison to computed vibrational frequencies. Density functional theory calculations for these lowest energy triplet state products converge essentially to C(3v) symmetry structures. Spin density calculations show that the two unpaired electrons are mostly localized in nitrogen 2p, phosphorus 3p, or arsenic 4p orbitals. Although thorium has a small spin density, the weak degenerate pi(alpha) molecular orbitals are populated entirely from the terminal N, P, or As based on DFT natural bond orbital analysis. This is in contrast with HC divided by ThF(3), which contains degenerate pi(alpha) molecular orbitals with 81% C and 19% Th character.

Photophysical characterization and photodynamic activity of metallo 5-(4-(trimethylammonium)phenyl)-10,15,20-tris(2,4,6-trimethoxyphenyl)porphyrin in homogeneous and biomimetic media

The photophysical properties and photodynamic effect of Zn(ii), Pd(ii), Cu(ii) and free-base 5-(4-(trimethylammonium)phenyl)-10,15,20-tris(2,4,6-trimethoxy phenyl)porphyrin (H2P) iodide have been studied in N,N-dimethylformamide (DMF) and in different biomimetic systems. The absorption, fluorescence, triplet state and singlet molecular oxygen production of the metal complexes were all referred to H2P. The photodynamic activity was first analyzed using 9,10-dimethylanthracene and guanosine 5'-monophosphate in N,N-dimethylformamide. The photooxidation processes were also investigated in benzene/benzyl-n-hexadecyldimethyl ammonium chloride/water reverse micelles. Photosensitization efficiency of these porphyrins was H2P approximately ZnP > PdP in homogeneous solution and ZnP > H2P > PdP in micelles, whereas no photooxidation effect was detected using the Cu(ii) complex. Human erythrocytes were used as a biological membrane model. The photohemolytic activity depended on irradiation time, sensitizer and concentration of the agent. When cells were treated with 1 microM sensitizer, the hemolytic activity was H2P > ZnP >> CuP. However, it was H2P > ZnP approximately CuP using 5 microM of the respective porphyrin. Although CuP could undergo a type I photoreaction, in all cases the photohemolytic effect considerably diminishes in anoxic conditions, indicating that an oxygen atmosphere is required for the mechanism of cellular membrane damage. The behavior of these amphiphilic metallo porphyrins provides information on the photodynamic activity of these agents in biomimetic microenvironments.

Mechanism of C60 Photoreactivity in Water: Fate of Triplet State and Radical Anion and Production of Reactive Oxygen Species

The mechanism involved with (1) energy and electron transfer by C60 in the aqueous phase during UV irradiation and (2) subsequent production of reactive oxygen species (ROS) such as singlet oxygen and superoxide radical anion was investigated. Electron paramagnetic resonance (EPR) study showed that C60 embedded in micelles of nonionic surfactant (Triton X 100) or anionic surfactant (sodium dodecylbenzenesulfonate) produced ROS, but aggregated C60 did not, consistent with our earlier findings made using indicator chemicals. Nanosecond and femtosecond laser flash photolysis showed that the aggregation of C60 significantly accelerates the decay of excited triplet state C60, which is a key intermediate for energy and electron transfer, thus blocking the pathway for ROS production. This finding suggests that C60 clusters will not contribute to oxidative damage or redox reactions in natural environment and biological systems in the same way molecular C60 in organic phase reportedly does. In contrast, C60 embedded in surfactant micelles produces ROS and the evidence is presented for the formation of C60 radical anion as an intermediate.

Photoinduced Energy and Electron-Transfer Processes in Porphyrin−Perylene Bisimide Symmetric Triads

The photophysics of two symmetric triads, (ZnP)2PBI and (H2P)2PBI, made of two zinc or free-base porphyrins covalently attached to a central perylene bisimide unit has been investigated in dichloromethane and in toluene. The solvent has been shown to affect not only quantitatively but also qualitatively the photophysical behavior. A variety of intercomponent processes (singlet energy transfer, triplet energy transfer, photoinduced charge separation, and recombination) have been time-resolved using a combination of emission spectroscopy and femtosecond and nanosecond time-resolved absorption techniques yielding a very detailed picture of the photophysics of these systems. The singlet excited state of the lowest energy chromophore (perylene bisimide in the case of (ZnP)2PBI, porphyrin in the case of (H2P)2PBI) is always quantitatively populated, besides by direct light absorption, by ultrafast singlet energy transfer (few picosecond time constant) from the higher energy chromophore. In dichloromethane, the lowest excited singlet state is efficiently quenched by electron transfer leading to a charge-separated state where the porphyrin is oxidized and the perylene bisimide is reduced. The systems then go back to the ground state by charge recombination. The four charge separation and recombination processes observed for (ZnP)2PBI and (H2P)2PBI in dichloromethane take place in the sub-nanosecond time scale. They obey standard free-energy correlations with charge separation lying in the normal regime and charge recombination in the Marcus inverted region. In less polar solvents, such as toluene, the energy of the charge-separated states is substantially lifted leading to sharp changes in photophysical mechanism. With (ZnP)2PBI, the electron-transfer quenching is still fast, but charge recombination takes place now in the nanosecond time scale and to triplet state products rather than to the ground state. Triplet-triplet energy transfer from the porphyrin to the perylene bisimide is also involved in the subsequent deactivation of the triplet manifold to the ground state. With (H2P)2PBI, on the other hand, the driving force for charge separation is too small for electron-transfer quenching, and the deactivation of the porphyrin excited singlet takes place via intersystem crossing to the triplet followed by triplet energy transfer to the perylene bisimide and final decay to the ground state.

Top and Bottom at Threshold: Recent Results

Recent results obtained for the systems of two heavy quarks are discussed. In particular, we consider phenomenological applications for the third-order results for the energy levels and wave function. Special emphasis is put on the resummation of potentially large logarithms in the velocity of the heavy quarks. Predictions for the mass of the η b meson are dicussed which still has to be confirmed experimentally. Furthermore, the ratio of the photon mediated production or annihilation rates of spin triplet and spin singlet heavy quarkonium states are considered.

A DFT study of the generation of interstellar species XN (X = Cl, and Br) activated by molecular sieve clusters

A cluster DFT (B3LYP) calculation study of ClN 3 and BrN 3 dissociation pathways catalyzed by molecular sieve is reported. A three-tetrahedral molecular sieve cluster (Al(OHSiH 3)(OSiH 3)(OH) 2, called T3) has been chosen in this study. Triplet-state products ClN and BrN are more easily produced with T3 molecular sieve cluster. Although the potential energy surfaces with T3 molecular sieve cluster are similar with those without T3 molecular sieve cluster, the dissociation process of ClN 3 and BrN 3 with T3 molecular sieve cluster becomes easier, which can be attributed to smaller energy gaps from the ground state reactant to the intersystem crossing point.