Mechanistic studies of the sensitized photoreduction of bis(acetylacetonato)copper(II) by phenylalkyl ketones

Bis(acetylacetonato)copper(II) (Cu(acac)₂) interacts with both the triplet excited state and the triplet biradical of phenylalkyl ketones which undergo the Norrish type II reaction. Mechanistic studies by static quenching methods show that the triplet biradicals interact with the paramagnetic copper species, leading to the preferential formation of cyclobutanols without the formation of new products; in the presence of Ph₃P the former interaction causes the known reduction of Cu(acac)₂) to Cu(acac)(Ph₃P)₂, with a rate constant of about 6 × 10⁹ M⁻¹ s⁻¹. It is shown that Ph₃P interacts with one reactive intermediate, the triplet excited state ketone. The results of extensive kinetic analysis strongly support the proposed reaction mechanism.     

A DFT Study on the Mechanism of Rh2II,II‐Catalyzed Intramolecular Amidation of Carbamates

The potential‐energy surfaces of the reactions of dirhodium tetracarboxylate (Rh₂^II,II) catalyzed nitrene (NR) insertion into C H bonds were examined by a DFT computational study. A pure Becke exchange functional (B88) rather than a hybrid exchange functional (B3, BHandH) was found to be appropriate for the calculation of the energy difference between the singlet and triplet Rh₂^II,II–NH nitrene species. Rh₂^II,II–NR¹ (R¹=(S)‐2‐methyl‐1‐butylformyl) is thermodynamically more favorable with a free energy lower than that of Rh₂^II,II–N(PhI)R¹. The singlet and triplet states of Rh₂^II,II–NR¹ have similar stability. Singlet Rh₂^II,II–NR¹ undergoes a concerted NR insertion into the C H bond with simultaneous formation of the N H and N C bonds during C H bond cleavage; triplet Rh₂^II,II–NR¹ undergoes H atom abstraction to produce a diradical, followed by subsequent bond formation by diradical recombination. The singlet pathway is favored over the triplet in the context of the free energy of activation and leads to the retention of the chirality of the C atom in the NR insertion product. The reactivities of the C H bonds toward the nitrene‐insertion reaction follow the order tertiary>secondary>primary. Relative reaction rates were calculated for the six reaction pathways examined in this work.     

Time-resolved EPR study of 3C60 in solid matrices

Using time-resolved cw EPR and pulsed EPR techniques, a study was made of the characteristics of the photoexcited triplet state of C₆₀ adsorbed in silica gel pores and embedded in a polymethylmethacrylate matrix. It was found that the time-resolved spectra from ³C₆₀ in these matrices at room temperature retain the absorption/emission features of spectra from ³C₆₀ in frozen solution (≤120 K). Apparently, the combination of molecular rotation and pseudo rotation (resulting from interconversion between Jahn-Teller states) is not fast enough, to lead to complete averaging of the dipole-dipole interaction between the unpaired electrons. The study explored the effect of introduction of solvent molecules in the silica gel pores on the ³C₆₀ spectrum and quenching of the triplet by electron donors.     

Long-lived Triplet Excited State of Perylenemonoimide (PMI) in a Diimine Pt(II) Bis-acetylide Complex: Preparation and Study of the Photophysical Property with Transient Spectra

We prepared a N^N Pt(II) bisacetylide complex that has strong absorption of visible light (molar absorption coefficients ϵ=6.7×10⁴ M⁻¹ cm⁻¹ at 570 nm), and the singlet oxygen quantum yield (Φ_Δ) is up to 78 %. Femtosecond transient absorption spectra show the intersystem crossing (ISC) of the complex takes 81.8 ps, nanosecond transient absorption spectra show the triplet excited state lifetime is 7.6 μs. Density functional theory (DFT) computation demonstrated that the S₁ and T₁ states are mainly localized on the perylenemonoimide (PMI) ligands, although the involvement of the Pt(II) centre is noticeable. The complex was used as triplet photosensitizer to generate delayed fluorescence with perylenebisimide (PBI) as the triplet state energy acceptor and emitter, via the intermolecular triplet-triplet energy transfer (TTET) and triplet-triplet annihilation (TTA), the delayed fluorescence lifetime is up to 52.5 μs under the experimental conditions.     

Controlled emission of platinum(II) terpyridyl complexes with poly-l-glutamic acid

Poly-l-glutamic acid, P(Glu), bearing multiple negatively charged side chains served as a polymeric spatially aligned scaffold for the aggregation of positively charged platinum(II) complexes [Pt(trpy)CCR](OTf) (trpy = 2,2′,6′,2″-terpyridine; R = Ph (PtH), PhC₁₂H₂₅-p (PtC₁₂)) through electrostatic interaction, resulting in tunable emission properties. PtC₁₂ was found to exhibit gradual increase in the emission intensity based on the triplet metal-metal-to-ligand charge transfer (³MMLCT) in a tris/HCl buffer (pH 7.6)/MeOH (v/v = 1/14) solution with concomitant decrease in the emission intensity based on the triplet metal-to-ligand charge transfer (³MLCT)/the triplet ligand-to-ligand charge transfer (³LLCT) as the amount of P(Glu) was increased. Such synergistic effect was not observed in the case of PtH, wherein the emission intensity based on ³MLCT/³LLCT was increased by the increase in the amount of P(Glu), indicating that alkyl long chain of PtC₁₂ is considered to play an important role in the aggregation of the platinum(II) terpyridyl moieties to show Pt(II)-Pt(II) and π-π interactions.     

Energy dependence of SF6 dissociation by pulsed CO2 laser radiation

The fractional dissociation,³²Γ, of ³²SF₆ by the output of a pulsed TEA CO₂ laser has been found to depend upon Φ, the total integrated laser intensity, as ³²Γ ∞ Φ^m for both the P(20) and P(16) laser lines, where m ≈ 3 in the range 2 < Φ < 7.2 J/cm² and increases from 3 to ≈ 4 as Φ is reduced from 2 to 0.9 J/cm².     

Kinetics of the uptake of SO2 on mineral oxides: Improved initial uptake coefficients at 298 K from pulsed Knudsen cell experiments

The uptake of SO₂ on γ‐Fe₂O₃, γ‐Al₂O₃, and Saharan dust has been studied at T = 298 K using a Knudsen cell reactor operated in a steady‐state as well as in a pulsed mode. The initial uptake coefficients determined in the steady‐state mode have been found to be affected by surface saturation as well as bulk diffusion of SO₂ resulting in an apparent dependence of the initial uptake coefficients on the sample mass of the mineral oxides. However, by operating the Knudsen cell in the pulsed mode with shorter response time, these effects could be suppressed. Initial uptake coefficients of γ_ini (Fe₂O₃) = (8.8 ± 0.4) × 10^?2, γ_ini (Al₂O₃) = (7.4 ± 0.9) × 10^?3, and γ_ini (Saharan dust) = (7.6 ± 0.5) × 10^?2 were derived. This suggests that Fe₂O₃ is an important component in determining the reactivity of mineral dusts. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 242–249, 2006     

Triplet Acceptors with a D-A Structure and Twisted Conformation for Efficient Organic Solar Cells

Triplet acceptors have been developed to construct high-performance organic solar cells (OSCs) as the long lifetime and diffusion range of triplet excitons may dissociate into free charges instead of net recombination when the energy levels of the lowest triplet state (T₁) are close to those of charge-transfer states (³CT). The current triplet acceptors were designed by introducing heavy atoms to enhance the intersystem crossing, limiting their applications. Herein, two twisted acceptors without heavy atoms, analogues of Y6, constructed with large π-conjugated core and D-A structure, were confirmed to be triplet materials, leading to high-performance OSCs. The mechanism of triplet excitons were investigated to show that the twisted and D-A structures result in large spin–orbit coupling (SOC) and small energy gap between the singlet and triplet states, and thus efficient intersystem crossing. Moreover, the energy level of T₁ is close to ³CT, facilitating the split of triplet exciton to free charges.     

Collision induced intersystem crossing the photophysics of glyoxal vapor excited at 4358 Å

The yields, lifetimes and spectra of singlet ¹A_u (S₁) and triplet ³A_u (T₁) emissions from glyoxal vapor (0.003 to 10 torr) have been measured after initially pumping levels about 1000 cm^?1 above the S₁ zero-point level with the 4358 A Hg line and with flash excitation centered at 4345 A. Only S₁ emission is observed at the lowest pressures. The singlet fluorescence contains appreciable structure from the zero-point level even when the hard sphere collision interval exceeds the radiative lifetime calculated from the absorption coefficient. Implications of long lifetimes (due to S₁ - T₁ vibronic interactions) are not confirmed by pulsed excitation studies. Both S₁ and T₁ emissions are observed at pressures above about 0.1 tert and both are self-quenched. However, added gases such as cyclohexane, argon, and helium selectively quench only S₁ emission. This quenching is collision-induced S₁→T₁ intersystem crossing with cross sections of order 0.1 hard sphere for transitions from the S₁ zero-point level. The triplet yield in 0.2 torr of pure glyoxal is probably near unity, and the subsequent crossing T₁ → S₀, if it occurs, lies in the statistical limit. Indications of fast nonradiative decay from high triplet vibrational levels are seen in the phosphorescence yields. Self-quenching of the triplet state appears to be associated with the photochemical activity of glyoxal.     

Photoionization of tetrakis(pyrophosphito)diplatinum(II)

Hydrated electrons are produced by photoexcitation of aqueous solutions of a binuclear platinum(II) complex, Pt₂(POP)⁴⁻₄. Studies of yields of e⁻_aq as a function of laser energy indicate that the electrons are liberated by a two-photon process. The efficiency of production of e⁻_aq relative to that of the Pt(II) triplet can be as high as 0.15.     

Study of protonic diffusion in hydrogen uranyl phosphate using a pulsed field gradient NMR method

The diffusion of ¹H in the fast protonic conductor H(UO₂PO₄) · 4H₂O has been investigated as a function of temperature by employing pulsed field gradient NMR measurements. In the light of the present diffusion results and published conductivity observations on the same compound, it is likely that protonic diffusion and conductivity in this material occur by the same mechanism. Possible mechanistic processes for the proton motion in this material which are consistent with the activation energy for protonic diffusion found in this study are briefly discussed.     

X‐ray and DFT studies of a mono‐ and binuclear copper(II) ionic compound containing a Schiff base

In the structure of trans‐bis(ethanol‐κO)tetrakis(1H‐imidazole‐κN³)copper(II) bis[μ‐N‐(2‐oxidobenzylidene)‐D,L‐glutamato]‐κ⁴O¹,N,O^2′:O^2′;κ⁴O^2′:O¹,N,O^2′‐bis[(1H‐imidazole‐κN³)cuprate(II)], [Cu(C₃H₄N₂)₄(C₂H₆O)₂][Cu₂(C₁₅H₁₄N₃O₅)₂], both ions are located on centres of inversion. The cation is mononuclear, showing a distorted octahedral coordination, while the anion is a binuclear centrosymmetric dimer with a square‐pyramidal copper(II) coordination. An extensive three‐dimensional hydrogen‐bonding network is formed between the ions. According to B3LYP/6–31G* calculations, the two equivalent components of the anion are in doublet states (spin density located mostly on Cu^II ions) and are coupled as a triplet, with only marginal preference over an open‐shell singlet.     

Triplet Acceptors with a D‐A Structure and Twisted Conformation for Efficient Organic Solar Cells

Triplet acceptors have been developed to construct high‐performance organic solar cells (OSCs) as the long lifetime and diffusion range of triplet excitons may dissociate into free charges instead of net recombination when the energy levels of the lowest triplet state (T₁) are close to those of charge‐transfer states (³CT). The current triplet acceptors were designed by introducing heavy atoms to enhance the intersystem crossing, limiting their applications. Herein, two twisted acceptors without heavy atoms, analogues of Y6, constructed with large π‐conjugated core and D‐A structure, were confirmed to be triplet materials, leading to high‐performance OSCs. The mechanism of triplet excitons were investigated to show that the twisted and D‐A structures result in large spin–orbit coupling (SOC) and small energy gap between the singlet and triplet states, and thus efficient intersystem crossing. Moreover, the energy level of T₁ is close to ³CT, facilitating the split of triplet exciton to free charges.     

Thermoelectric properties of Bi2Te3 films by constant and pulsed electrodeposition

Bi₂Te₃ films have been grown by constant and pulsed electrochemical deposition. The pulsed deposition was carried out by alternating between a constant potential (potentiostatic mode) and an open circuit potential (galvanostatic mode, where current density is fixed at 0 mA/cm²). The Harris texture analysis was performed to determine the degree of preferred orientation. The results showed that the films were strongly oriented along (1?1?0) direction. The morphology and compositions of the films were then analyzed. Finally, their Seebeck coefficient and electrical resistivity were measured and used to determine the thermoelectric Power Factor of the films for a temperature range between 57 and 107 °C.     

The lowest excited triplet state of triphenylboron and of the isoelectronic triphenylcarbenium ion

Triphenylboron BPh₃ and the triphenylcarbenium salts C⁺Ph₃/SbCl₆^? and C⁺Ph₃/BF₄^? have been investigated by ODMR and emission spectroscopic methods. The zero-field splitting (ZFS) parameters D and E and the decay rate constants of the triplet zero-field levels (ZFL) as well as the phosphorescence spectra were measured. The non-zero E values indicate a symmetry lower than D₃ for the Jahn-Teller unstable triplet state of all compounds. The radiative decay of T₁ shows a strong delocalization of the triplet wavefunction for C⁺Ph₃, but a strong localization on the benzene rings for BPh₃. This is in agreement with MO calculations.     

Phosphorescence of 1,1,1-trifluoroacetone using pulsed nitrogen laser excitation

Phosphorescence decay times of 1,1,1-trifluoroacetone in the vapour phase have been measured as a function of pressure of ketone with low-power excitation from a pulsed N₂ laser. A strong pressure dependence is observed, yielding a rate constant for self-quenching of (4.6 ± 0.5) × 10⁶ ? mol^?1 s^?1, and an extrapolated zero-pressure lifetime of 60 μs, considerably shorter than that measured earlier in a higher-energy flash photolysis experiment. Reasons for these differences are discussed.     

A theoretical comparison of the lowest-lying singlet and triplet states of H2CBe and of HCBeH

The low-lying singlet and triplet states of H₂CBe and HCBeH are examined using ab inito molecular orbital theory. In agreement with earlier results, the lowest-lying structure of H₂CBe has C_2v symmetry and is a triplet with one π electron (³ B₁). The results presented here suggest that the lowest-energy singlet structure is the (¹B₁) open-shell singlet, also with C_2v symmetry, at least 2.5 kcal/mol higher in energy. The singlet C_2v structure with two π electrons (¹A₁) is 15.9 kcal/mol higher than ³B₁. All of these structures are bound with respect to the ground state of methylene and the beryllium atom. In HCBeH, linear equilibrium geometries are found for the triplet (³Σ) and singlet (¹Δ) states. The triplet is more stable than the singlet (¹Δ) by 35.4 kcal/mol, and is only 2.9 kcal/mol higher in energy than triplet H₂ CBe. Since the transition structure connecting these two triplet molecules is found to be 50.2 kcal/mol higher in energy than H₂ CBe, both triplet equilibrium species might exist independently. The harmonic vibrational frequencies of all structures are also reported.     

Study of kinetics of luminescence and triplet-triplet absorption of a mixture of europium and gadolinium chelate complexes in finely porous glass

Spectral and kinetic characteristics of luminescence of the Eu³⁺ complex EuTTA₃Phen (where TTA is thenoyltrifluoroacetone and Phen is 1,10-phenanthroline) and the triplet state ³TTA of the nonluminescent complex GdTTA₃ Phen, as well as mixtures of these complexes, have been studied in a CH₂Cl₂ solution, in finely porous glass (PG) filled with the solution, in a CH₂Cl₂ monolayer on the PG surface, and in the adsorbed state on the dry PG surface. The yields of Eu³⁺ luminescence and ³TTA for mixtures of the complexes in a solution, including the solution located in PG, correspond to those expected from the portion of the absorbed light. However, in the monolayer and on a dry PG surface, the luminescence yield observed is substantially higher and the ³TTA yield is lower than those expected, which is due to static energy transfer from the triplet states of TTA in the complex with Gd³⁺ to TTA in the complex with Eu³⁺ in mixed clusters of the complexes on the surface.     

Metallofullerene photoswitches driven by photoinduced fullerene-to-metal electron transfer

We report on the discovery and detailed exploration of the unconventional photo-switching mechanism in metallofullerenes, in which the energy of the photon absorbed by the carbon cage π-system is transformed to mechanical motion of the endohedral cluster accompanied by accumulation of spin density on the metal atoms. Comprehensive photophysical and electron paramagnetic resonance (EPR) studies augmented by theoretical modelling are performed to address the phenomenon of the light-induced photo-switching and triplet state spin dynamics in a series of Y_xSc_3−xN@C₈₀ (x = 0–3) nitride clusterfullerenes. Variable temperature and time-resolved photoluminescence studies revealed a strong dependence of their photophysical properties on the number of Sc atoms in the cluster. All molecules in the series exhibit temperature-dependent luminescence assigned to the near-infrared thermally-activated delayed fluorescence (TADF) and phosphorescence. The emission wavelengths and Stokes shift increase systematically with the number of Sc atoms in the endohedral cluster, whereas the triplet state lifetime and S₁–T₁ gap decrease in this row. For Sc₃N@C₈₀, we also applied photoelectron spectroscopy to obtain the triplet state energy as well as the electron affinity. Spin distribution and dynamics in the triplet states are then studied by light-induced pulsed EPR and ENDOR spectroscopies. The spin–lattice relaxation times and triplet state lifetimes are determined from the temporal evolution of the electron spin echo after the laser pulse. Well resolved ENDOR spectra of triplets with a rich structure caused by the hyperfine and quadrupolar interactions with ¹⁴N, ⁴⁵Sc, and ⁸⁹Y nuclear spins are obtained. The systematic increase of the metal contribution to the triplet spin density from Y₃N to Sc₃N found in the ENDOR study points to a substantial fullerene-to-metal charge transfer in the excited state. These experimental results are rationalized with the help of ground-state and time-dependent DFT calculations, which revealed a substantial variation of the endohedral cluster position in the photoexcited states driven by the predisposition of Sc atoms to maximize their spin population.

Photoexcitation mechanism of Y_xSc_3−xN@C₈₀ metallofullerenes is studied by variable-temperature photoluminescence, advanced EPR techniques, and DFT calculations, revealing photoinduced rotation of the endohedral cluster.     

Polymerization of neat 2‐ethylhexyl acrylate induced by a pulsed electron beam

The bulk polymerization of 2‐ethylhexyl acrylate (2‐EHA), induced by a pulsed electron beam, was investigated with pulse radiolysis, gravimetry, and Fourier transform infrared spectroscopy. The roles of the dose rate, pulse frequency, and added acrylic acid (AA) in the polymerization of 2‐EHA were examined at ambient temperature. In the range of 12.6–71.2 Gy/pulse, the polymerization of 2‐EHA was dose‐rate‐dependent: at the same total dose, a lower dose rate yielded a higher conversion. Also, a lower pulse rate gave a higher conversion at the same total dose. The addition of up to 10 wt % AA showed no increase in the conversion of 2‐EHA at a low conversion (8 kGy), but at a higher conversion (16 kGy), a 20 wt % increase in the conversion of 2‐EHA was observed. The estimated values (1.6 ± 0.3) × 10^?3 (dm³ s)^3/2 mol^?1 s^?1/2 for k_p(G/2k_t)^1/2 and 2.6 ± 0.8 dm³ s J^?1 for 2k_tG (where k_p is the rate constant of propagation, k_t is the rate constant of bimolecular termination, and G is the yield of free radicals) were obtained at relatively low conversions. The reaction rate constant of the addition of 2‐EHA^· free radicals to the monomer was measured by pulse radiolysis and found to be 2.8 × 10² mol^?1 dm³ s^?1. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 196–203, 2003