Laser-induced Transient Absorption Research Articles

Photooxidation by a long-lived photoexcited state of a triflic acid-bound Mn(IV)-oxo complex with the highest quantum efficiency

The photoexcited state of high-valent metal-oxo complexes binding protons would show the much enhanced oxidizing reactivity, providing a new way to achieve a photochemical oxidation of thermally inert substrates. However, there has been only one example for the long-lived photoexcited state of a MnIV-oxo complex binding scandium ion that is a rare-earth metal element. We report herein the formation of a long-lived photoexcited state of a MnIV-oxo complex binding triflic acid, composed of only earth-abundant elements, which exhibits the highest oxidizing reactivity among the high-valent metal-oxo species, including the excited state, reported so far. Photoexcitation of a triflic acid-bound MnIV-oxo complex (1, [(N4Py)MnIV(O)]2+–(HOTf)2) in a deaerated trifluoroethanol/acetonitrile resulted in the formation of a long-lived photoexcited state with a lifetime of 21 μs, which oxidized toluene to produce benzyl alcohol that was further oxidized to benzaldehyde. The driving force dependence of the rate constant of the photoinduced electron transfer from electron donors to the long-lived 2E excited state of 1 afforded a small reorganization energy of electron transfer (λ = 0.53 eV), since the 2E excited state of 1 may be composed of the ligand-to-metal charge-transfer (LMCT) state and the efficient electron transfer to the excited sate of 1 occurs at the ligand center. The quantum efficiency of 200% was achieved for the photochemical four-electron oxidation of toluene by 1, being the highest among various photooxidation reactions reported so far. The photodynamics were measured by laser-induced transient absorption spectroscopy to clarify the photooxidation mechanism of 1.

Generation of synchronized x-rays and mid-infrared pulses by Doppler-shifting of relativistically intense radiation from near-critical-density plasmas

It is shown that when relativistically intense ultrashort laser pulses are reflected from the boundary of a plasma with a near-critical density, the Doppler frequency shift leads to generation of intense radiation in both the high-frequency (up to the x-ray) and low-frequency (mid-infrared) ranges. The efficiency of energy conversion into the wavelength range above 3 µm can reach several percent, which makes it possible to obtain relativistically intense pulses in the mid-infrared range. These pulses are synchronized with high harmonics in the ultraviolet and x-ray ranges, which opens up opportunities for high-precision pump–probe measurements, in particular, laser-induced electron diffraction and transient absorption spectroscopy.

A Mononuclear Non-Heme Manganese(III)–Aqua Complex in Oxygen Atom Transfer Reactions via Electron Transfer

Metal-oxygen complexes, such as metal-oxo [M(O2-)], -hydroxo [M(OH-)], -peroxo [M(O22-)], -hydroperoxo [M(OOH-)], and -superoxo [M(O2•-)] species, are capable of conducting oxygen atom transfer (OAT) reactions with organic substrates, such as thioanisole (PhSMe) and triphenylphosphine (Ph3P). However, OAT of metal-aqua complexes, [M(OH2)]n+, has yet to be reported. We report herein OAT of a mononuclear non-heme Mn(III)-aqua complex, [(dpaq)MnIII(OH2)]2+ (1, dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate), to PhSMe and Ph3P derivatives for the first time; it is noted that no OAT occurs from the corresponding Mn(III)-hydroxo complex, [(dpaq)MnIII(OH)]+ (2), to the substrates. Mechanistic studies reveal that OAT reaction of 1 occurs via electron transfer from 4-methoxythioanisole to 1 to produce the 4-methoxythioanisole radical cation and [(dpaq)MnII(OH2)]+, followed by nucleophilic attack of H2O in [(dpaq)MnII(OH2)]+ to the 4-methoxythioanisole radical cation to produce an OH adduct radical, 2,4-(MeO)2C6H3S•(OH)Me, which disproportionates or undergoes electron transfer to 1 to yield methyl 4-methoxyphenyl sulfoxide. Formation of the thioanisole radical cation derivatives is detected by the stopped-flow transient absorption measurements in OAT from 1 to 2,4-dimethoxythioanisole and 3,4-dimethoxythioanisole, being compared with that in the photoinduced electron transfer oxidation of PhSMe derivatives, which are detected by laser-induced transient absorption measurements. Similarly, OAT from 1 to Ph3P occurs via electron transfer from Ph3P to 1, and the proton effect on the reaction rate has been discussed. The rate constants of electron transfer from electron donors, including PhSMe and Ph3P derivatives, to 1 are fitted well by the electron transfer driving force dependence of the rate constants predicted by the Marcus theory of outer-sphere electron transfer.

Long-Lived Photoexcited State of a Mn(IV)-Oxo Complex Binding Scandium Ions That is Capable of Hydroxylating Benzene.

Photoexcitation of a MnIV-oxo complex binding scandium ions ([(Bn-TPEN)MnIV(O)]2+-(Sc(OTf)3)2) in a solvent mixture of trifluoroethanol and acetonitrile (v/v = 1:1) resulted in formation of the long-lived photoexcited state, which can hydroxylate benzene to phenol. The photohydroxylation of benzene by [(Bn-TPEN)MnIV(O)]2+-(Sc(OTf)3)2 was made possible by electron transfer from benzene to the long-lived 2 E excited state of [(Bn-TPEN)MnIV(O)]2+-(Sc(OTf)3)2 to produce a benzene radical cation, which reacted with water as revealed by laser-induced transient absorption measurements.

Influence of Charge Transport Layers on Open-Circuit Voltage and Hysteresis in Perovskite Solar Cells

Summary Perovskite materials have experienced an impressive improvement in photovoltaic performance due to their unique combination of optoelectronic properties. Their remarkable progression, facilitated by the use of different device architectures, compositional engineering, and processing methodologies, contrasts with the lack of understanding of the materials properties and interface phenomena. Here we directly target the interplay between the charge-transporting layers (CTLs) and open-circuit potential ( V OC ) in the operation mechanism of the state-of-the-art CH 3 NH 3 PbI 3 solar cells. Our results suggest that the V OC is controlled by the splitting of quasi-Fermi levels and recombination inside the perovskite, rather than being governed by any internal electric field established by the difference in the CTL work functions. In addition, we provide novel insights into the hysteretic origin in perovskite solar cells, identifying the nature of the contacts as a critical factor in defining the charge accumulation at its interface, leading to either ionic, electronic, or mixed ionic-electronic accumulation.

PH Dependence of Photoinduced Electron Transfer with [Ru(TAP)3]2.

The quenching of the excited state of [Ru(TAP)3]2+ (TAP = 1,4,5,8-tetraazaphenanthrene) by guanosine-5'-monophosphate (GMP), N-acetyltyrosine (N-Ac-Tyr), and hydroquinone (H2Q) has been studied in aqueous solution over a wide range of pH values including, for the first time, strongly acidic media. This quenching by electron transfer was examined by steady-state 1H photochemically induced dynamic nuclear polarization (photo-CIDNP) as well as by more conventional techniques, among which are pulsed laser-induced transient absorption and emission experiments. A deeper knowledge of the photochemical behavior of [Ru(TAP)3]2+ has been gained thanks to the combined use of these two approaches, photo-CIDNP and electronic spectroscopies, highlighting their complementarity. In contrast to what was believed, it is found that the protonated excited state of [Ru(TAP)3]2+ may give rise to an electron transfer with N-Ac-Tyr and H2Q. Such a photoinduced electron transfer does not occur with protonated GMP, however. 1H photo-CIDNP experiments are expected to be particularly promising for characterization of the reductive quenching of excited-state ruthenium(II) polypyridyl complexes comprising several nonequivalent protonation sites.

Solvent-Free Photooxidation of Alkanes by Dioxygen with 2,3-Dichloro-5,6-dicyano-p-benzoquinone via Photoinduced Electron Transfer.

Photooxidation of alkanes by dioxygen occurred under visible light irradiation of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) which acts as a super photooxidant. Solvent-free hydroxylation of cyclohexane and alkanes is initiated by electron transfer from alkanes to the singlet and triplet excited states of DDQ to afford the corresponding radical cations and DDQ(⋅-) , as revealed by femtosecond laser-induced transient absorption measurements. Alkane radical cations readily deprotonate to produce alkyl radicals, which react with dioxygen to afford alkylperoxyl radicals. Alkylperoxyl radicals abstract hydrogen atoms from alkanes to yield alkyl hydroperoxides, accompanied by regeneration of alkyl radicals to constitute the radical chain reactions, so called autoxidation. The radical chain is terminated in the bimolecular reactions of alkylperoxyl radicals to yield the corresponding alcohols and ketones. DDQ(⋅-) , produced by the photoinduced electron transfer from alkanes to the excited state of DDQ, disproportionates with protons to yield DDQH2 .

Photooxygenation of alkanes by dioxygen with p-benzoquinone derivatives with high quantum yields.

Alkanes were oxygenated by dioxygen with p-benzoquinone derivatives such as p-xyloquinone in alkanes which are used as solvents to yield the corresponding alkyl hydroperoxides, alcohols and ketones under visible light irradiation with high quantum yields (Φ = 1000, 1600%). The photooxygenation is started by hydrogen atom abstraction from alkanes by the triplet excited states of p-benzoquinone derivatives as revealed by laser-induced transient absorption spectral measurements.

(Invited) Multiple Photosynthetic Reaction Centers of Porphyrinic Polypeptide-Li+@C60 Supramolecular Complexes

Extensive efforts have so far been devoted to mimic light harvesting and charge separation processes in natural photosynthesis.1 Multiporphyrin arrays have been employed as light-harvesting units. Light-harvesting and charge-separation units have been combined by coordination bonds between metalloporphyrins and electron acceptor moieties bearing Lewis base ligands. Thus, metal centres have been required for construction of supramolecular complexes between porphyrins acting as light harvesting units and electron acceptors containing Lewis base ligands for coordination to the metal centers. As such there has been only few report on supramolecular complexes of free base porphyrin arrays and electron acceptors without Lewis base ligands, in which the binding is rather weak. We report herein construction of supramolecular complexes of free base porphyrin polypeptides (P(H2P) n : n = 4, 8) with lithium ion-encapsulated C60 (Li+@C60)2,3 (Figure 1), in which binding is much stronger than the caes of C60 in benzonitirle (PhCN). The photodynaimcs were studied by femtosecond and nanosecond laser-induced transient absorption and fluorescence lifetime measurements.4 Upon mixing a PhCN solution of Li+@C60 with that of P(H2P)8, the intensity of the Soret band decreased with increasing concentration of Li+@C60. A Soret band was slightly red-shifted from 425 to 427 nm by the addition of Li+@C60 to the solution. From the net absorption change at 425 nm in which the absorption due to Li+@C60 was subtracted, a linear Benesi-Hildebrand plot was obtained, indiating that each porphyrin unit of P(H2P)8 forms a 1:1 supramolecular charge-transfer complex with Li+@C60 independently with approximately the same binding constant of the unit of M–1. The binding constant at 298 K was determined to be 2.1 ´ 104 M–1, which is significantly larger than that of C60 (5.3 ´ 103 M–1). The stronger binding of Li+@C60 as compared with C60 may result from the stronger elecron acceptor ability of Li+@C60, which facilitates the charge-transfer interaction as reported for the stronger charge-transfer binding of Li+@C60 with corranulerene. Similary the binding constant of the supramolecular complex of P(H2P)4 was determined from the spectral titration to be 6.2 × 103 M–1. In the case of P(H2P)2 and P(H2P)1, howerver, the spectral change was too small to be able to determine the binding constants accurately. Thus, multiporphyris may facilitate charge-transfer interactions through encapsulation of Li+@C60 by multiple porphyrins.4 Nanosecond laser-induced transient absorption spectra of P(H2P)8 with Li+@C60 (40 mM) at the excitation wavelength of 532 nm showed the transient absorption band at 730 nm due to the triplet excited state of Li+@C60 (3Li+@C60 *) observed together with the absorption band at 1035 nm due to Li+@C60 •–. The decay of the absorption at 730 nm coincides with the appearance of Li+@C60 •–. Thus, electron transfer from P(H2P)8 to 3Li+@C60 * occurs to produce the triplet charge-separate (CS) state of P(H2P)8 •+ and Li+@C60 •–. The transient absorption due to P(H2P)8 •+ is overlapped with that of 3Li+@C60 * in the 600-700nm region. The photoexcitation of 532 nm resulted in the excitation of Li+@C60 leading to the formation of 3Li+@C60 * via intersystem crossing. The decay of the absorbance at 1035 nm due to Li+@C60 •– obeyed first-order kinetics with the lifetime of 210 ms. Thus, back electron transfer from Li+@C60 •– to P(H2P)8 •+ occurs in the supramolecular complex. The lifetime is long because of the spin-forbidden back electron transfer in the triplet CS state.4 References [1] Fukuzumi, S.; Ohkubo, K.; Suenobu, T. Acc. Chem. Res. 2014, 47, 1455. [2] Kawashima, Y.; Ohkubo, K.; Fukuzumi, S. Chem. Asian J. 2015, 10, 44. (Review) [3] (a) N. Solladié , A. Hamel and M. Gross, Tetrahedron Lett., 2000, 41, 6075; (b) F. Aziat, R. Rein, J. Peon, E. Rivera and N. Solladié, J. Porphyrins Phthalocyanines, 2008, 12, 1232 [4] Ohkubo, K.; Hasegawa, T.; Rein, R.; Solladié, N.; Fukuzumi, S. Chem. Commun. 2015, 51. 17517. Figure 1

Tailoring between network rigidity and nanosecond transient absorption in a-Ge(x)As(35-x)Se₆₅ thin films.

In this Letter, we report the first observation of dramatic decrease in nanosecond (ns) pulsed laser-induced transient absorption (TA) in a-Ge(x)As(35-x)Se₆₅ thin films by tuning the amorphous network from floppy to rigid. Our results provide the direct experimental evidence of a self-trapped exciton mechanism, where trapping of the excitons occurs through bond rearrangements. Taken together, a rigid amorphous network with more constraints than degrees of freedom are unable to undergo any such bond rearrangements and results in weaker TA. However, we also demonstrate that excitation fluence can be effectively utilized as a simple tool to lift up enough constraints to introduce large TA even in rigid networks. Apart from this, we also show that TA is tunable with network rigidity as it blueshifts when the mean coordination is increased from 2.35 to 2.6.

Direct Kinetic Study of OH and O3 Formation in the Reaction of CH3C(O)O2 with HO2

The reaction between HO2 and CH3C(O)O2 has three exothermic product channels, forming OH (R3a), peracetic acid (R3b), and acetic acid plus O3 (R3c). The branching ratios of the OH- and ozone-forming reaction channels were determined using a combination of laser-induced fluorescence (LIF, for time-resolved OH concentration measurement) and transient absorption spectroscopy (TAS, for time-resolved O3 concentration measurement) following pulsed laser generation of HO2 and CH3C(O)O2 from suitable precursors. TAS was also used to determine the initial concentration of the reactant peroxy radicals. The data were evaluated by numerical simulation using kinetic models of the measured concentration profiles; a Monte Carlo approach was used to estimate the uncertainties of the rate constants (k3) and branching ratios (α) thus obtained. The reaction channel forming OH (R3a) was found to be the most important with α3a = 0.61 ± 0.09 and α3c = 0.16 ± 0.08. The overall rate coefficient of the title reaction was found to be k3 = (2.1 ± 0.4) × 10(-11) cm(3) molecule(-1) s(-1) for both HO2 and DO2. Use of DO2 resulted in an increase in α3a to 0.80 ± 0.14. Comparison with former studies shows that OH formation via (R3) has been underestimated significantly to date. Possible reasons for these discrepancies and atmospheric implications are discussed.

Influence of Cation on Charge Recombination in Dye-Sensitized TiO2 Electrodes

The reaction of a dye cation recombining with an electron in TiO(2), in the presence of Li(+), Ca(2+), and TBA(+) cations, was studied with laser-induced transient absorption measurements. The active cations, Li(+) and Ca(2+), shorten the dye cation lifetime on sensitized TiO(2) but not ZnO electrodes. By combining the absorbance measurements of the dye cation with simultaneous measurements of the current transient, the contribution of the recombination reaction to the current is identified. Furthermore, classical porous electrode theory is used to quantify the behavior of the heterogeneous electrode, and in doing so, the processes contributing to photoinduced current are identified as Helmholtz layer charging, porous electrode charging, recombination reactions, and surface diffusion of the active cations. The rate of charge recombination is proportional to the concentration of initially deposited active cations. The effect of water on the recombination rate and the current is also observed.

Laser-induced formation of pyrenyloxy radical from 1-hydoxypyrene and further oxidation: Micellar effects

Laser photolysis of POH was investigated in aqueous ethanol, three organic solvents with different polarity, SDS micellar and AOT reverse micellar solutions. The analysis of the laser-induced transient absorption (TA) spectra showed that the pyrenyloxy radical (PO ) was formed via a prompt deprotonation from the cation radical (POH) + . The observed micellar effects on the formation of the pyrenyloxy radical were consistent with this mechanism of formation. Based on the shift as well as the intensity of the TA peak attributable to the pyrenyloxy radical, information on the microenvironment surrounding POH (i.e., hydrophobic or hydrophilic, contact or no contact with water phase) was obtained. The secondary photochemical reaction after laser irradiation was investigated with UV/vis absorption spectroscopy. 1,6- and 1,8-pyrenediones were found to be produced as the main photoproducts.

Intra- and Intermicellar Triplet−Triplet Annihilation of Pyrenetetrasulfonate in an AOT Reverse Micellar Solution: Relation to the Electric Percolation Transition

Decay behaviors of triplets were investigated as a function of temperature in ternary (water/AOT/oil) and quaternary (water/AOT/cosurfactant/oil) reverse micellar solutions. The triplet species generated by the laser irradiation of tetrasodium pyrenetetrasulfonate dissolved in a reverse micelle were detected by monitoring the triplet-triplet absorption at 430 nm. The observed decay curves were analyzed with the kinetic model combining the intermicellar triplet-triplet annihilation (TTA) with the intramicellar TTA. The kinetic model for the intermicellar TTA process was proposed on the basis of the equilibrium between the monomer and dimer micelles. The obtained kinetic parameters and thermodynamic quantities are discussed in relation to electric percolation phenomena in these micellar systems. A fraction of the dimer micelles was found to be almost constant (0.27 ′ 0.03) at the percolation temperature for all of the reverse micellar systems that were investigated. Furthermore, an average number of triplets per micelle and the Gibbs free energy change for dimer micelle formation seem to be closely related to the percolation threshold. These results suggest that the laser-induced transient absorption spectroscopy of a probe is a useful method for studying dynamic behavior in micellar systems.

Photochemistry of naphthosultine and naphthosulfolene in acetonitrile solutions

The photochemistry of naphthosulfolene ( 1 ) and naphthosultine ( 2 ) was investigated using time-resolved laser flash photolysis, conventional photochemical steady-state techniques and product analysis. Excited by a laser pulse at 266 nm using laser-induced transient absorption spectroscopy, both compounds 1 and 2 exhibit the typical excited triplet state peaks with maxima at 410 nm and 430 nm in acetonitrile solution under deoxygenated conditions. Interestingly, upon direct photolysis in acetonitrile solution, compound 2 has three additional peaks located at 380 nm, 500 nm and 525 nm, generated by two distinct mechanisms: (1) the cleavage of the C—O bond in the sultine molecule produces a singlet biradical with absorption bands located at 500 nm and 525 nm, which decays at a much lower rate than does the excited triplet state which absorbs at 410 nm and 430 nm; (2) a simultaneous cleavage of the C—O bond and C—S bond in the sultine molecule generates a triplet biradical with absorption band located at 380 nm. Steady-state product studies indicate that reaction (1) is much more favorable than reaction (2), and the decay of the singlet biradical generates naphthosulfolene 1 in high yields in the absence or presence of quenching agents (electron-poor alkenes). The reactivity of the excited triplet states, the singlet biradical and the triplet biradical are discussed.

Nonlinear optical response of Ta nanoparticles embedded in insulators by 60 keV negative ion implantation

Linear and nonlinear optical properties of Ta nanoparticle composites, fabricated by negative ion implantation, were studied in the visible range. Negative Ta ions of 60 keV were implanted into amorphous SiO 2 and crystalline TiO 2 with a total fluence of 3 × 10 16 ions/cm 2. Spheres of Ta particles of 10–40 nm in diameter are distributed near the surface in amorphous SiO 2. Optical absorption evidently indicated a surface plasmon peak around 2 eV and the peak resulted from formation of nanoparticles embedded in the matrices. The plasmon peak shifted depending on the refractive index of the substrates. Laser-induced transient absorption was measured with the technique of pump-probe femtosecond spectroscopy. The transient absorption of the Ta nanoparticle composite in SiO 2 recovered in several picoseconds due to energy transfer from the excited electrons to the lattice via the electron–phonon interaction. The transient response was comparable to that of the noble metals. The electron–phonon coupling was evaluated by the two-temperature model and the coupling constant yielded a value of g=3.1×10 18 W/m 3 K. The Ta nanoparticle composite has the advantage of greater thermal stability in comparison with Cu nanoparticle composites.

Transient Absorption Probe of Intermolecular Triplet Excimer of Naphthalene in Fluid Solutions: Identification of the Species Based on Comparison to the Intramolecular Triplet Excimers of Covalently-Linked Dimers

We report here the observation of the laser-induced transient absorption spectrum of intermolecular triplet excimers of naphthalene in fluid solution. This assignment is confirmed by comparison to the transient absorption spectra of the intramolecular triplet excimers of covalently linked dimers of naphthalene and quinoxaline.

Using the Decay of Incorporated Photoexcited Triplet Probes to Study Unilamellar Phospholipid Bilayer Membranes

The decay kinetics of triplet excited probes (meso-tetraphenylporphyrin or anthracene) in unilamellar phospholipid bilayer membranes was studied using nanosecond laser-induced transient absorption ...

Hydrogen atoms inKH2PO4crystals

Hydrogen atoms have been produced at 77 K in single crystals of potassium dihydrogen phosphate (${\mathrm{KH}}_{2}{\mathrm{PO}}_{4}$ or KDP) using either 60-kV x rays or the fourth harmonic (266 nm) of a pulsed Nd:YAG (yttrium aluminum garnet) laser. The electron-paramagnetic-resonance spectrum from these hydrogen atoms exhibits a characteristic 500-G hyperfine splitting and is easily saturated with microwave power. These atoms occupy interstitial sites and are electron traps. The corresponding hole trap is the well-known $({\mathrm{HPO}}_{4}{)}^{\ensuremath{-}}$ hole center identified by its 31-G phosphorus hyperfine splitting. Both the hydrogen atoms and the hole centers thermally decay in the temperature range between 80 and 200 K. The observed displacement of protons by 266-nm photons provides direct evidence in support of the proton-transport mechanism recently proposed by Davis, Hughes, and Lee [Chem. Phys. Lett. 207, 540 (1993)] to explain laser-induced transient optical absorption at room temperature in KDP.

TIME-RESOLVED SPECTROSCOPIC STUDIES ON PHOTOPHYSICAL PROPERTIES OF BENOXAPROFEN IN MICELLAR SOLUTIONS: AN INTRAMICELLAR FLUORESCENCE QUENCHING

The micellar dependencies of the photophysical properties of benoxaprofen (BXP), a 2-phenyl benzoxazole derivative, have been investigated using fluorescence spectroscopy and laser flash photolysis techniques. The fluorescence of BXP in aqueous solution has been observed to be remarkably quenched upon addition of a surfactant, cetyltrimethyl ammonium bromide (CTAB) or Triton X-100, in contrast to its enhancement in sodium dodecyl sulfate (SDS) micellar solution. Time-resolved fluorescence measurements show that the fluorescence decays biexponentially in the micellar solution, indicating the relaxation of micellar environments surrounding the excited BXP. The major component of fluorescence lifetimes in CTAB or Triton X-100 micellar phase is even shorter (330–427ps) than in SDS micellar phase (731 ps). The nonradiative decay constants are significantly larger (ca 3.0 times 109 s−1) in the CTAB or Triton X-100 micellar phase than in SDS micelles by a factor of ca 10. The major nonradiative decay is interpreted to be the internal conversion due to nuclear geometric change of BXP in the first excited singlet state. This is consistent with the observation that the quantum yields of intersystem crossing are very low (less than 0.01) in the micellar solutions as determined by the laser flash photolysis technique. The laser-induced transient absorption spectrum of BXP in CTAB or Triton X-100 micellar solution shows that the decay kinetics of the transients in CTAB or Triton X-100 are significantly different from first order kinetics in SDS.